Review of PREPA Technical 

Requirements for 

Interconnecting Wind and Solar 

Generation 

Vahan Gevorgian and Sarah Booth 

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Office of Energy Efficiency & Renewable Energy 

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Technical Report 

NREL/TP-5D00-57089 

November 2013 

Contract No. DE-AC36-08GO28308 
National Renewable Energy Laboratory 

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Review of PREPA Technical 

Requirements for 

Interconnecting Wind and 

Solar Generation 

Vahan Gevorgian and Sarah Booth 

Prepared under Task Nos. IDPR.1002, IDPR.1003, and 

ID14.1000 

NREL is a national laboratory of the U.S. Department of Energy 

Office of Energy Efficiency & Renewable Energy 

Operated by the Alliance for Sustainable Energy, LLC. 

This report is available at no cost from the National Renewable Energy 

Laboratory (NREL) at www.nrel.gov/publications. 

Technical Report 

NREL/TP-5D00-57089 

November 2013 

Contract No. DE-AC36-08GO28308 
 

 

 

 

 

 

 

 

 

 

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Abstract 

The U.S. Department of Energy (DOE) and the National Renewable Energy Laboratory (NREL) 

have partnered with the Government of Puerto Rico to assist in addressing barriers to the 

deployment of energy efficiency and renewable energy in Puerto Rico. This document provides 

an overview of the minimum technical requirements (MTR) for interconnection of wind power 

and photovoltaic generation developed by the Puerto Rico Electric Power Authority (PREPA). 

Integrating a large amount of variable renewable generation such as wind and solar into an 

electrical grid presents several potential challenges for operating a power system, particularly 

with small island grids like the Puerto Rico electrical system. Establishing valid technical 

requirements for interconnection of variable renewable generation to the electric grid is an 

important step in overcoming such challenges. The interconnection requirements need to address 

several important aspects of renewable integration such as safety, impacts on power system 

reliability and performance, and costs. Being an isolated island system, PREPA does not fall 

under Federal Energy Regulatory Commission (FERC) jurisdiction, and therefore FERC 

interconnection standards are not required to be applied to generators in Puerto Rico. In this 

regard, PREPA has developed its own set of interconnection requirements with which all 

transmission-level wind and solar PV generators shall comply. In this report, NREL provides an 

impartial third-party review of PREPA’s MTRs, drawing comparisons with other similar 

requirements and best engineering practices. Over the course of the collaboration on the MTR 

review (2012-2013), PREPA revised the MTRs based on the draft review. As such, PREPA’s 

responses to the recommendations are also included in this report. The PREPA MTRs are 

anticipated to constantly evolve as the level of renewable penetration increases, modeling tools 

are improved, and experience with system performance increases. This review is intended to 

assist PREPA in this process. 

Acknowledgements 

The authors would like to acknowledge the substantial contributions to this report provided by 

staff at PREPA, the Governor’s Office, and the Puerto Rico Energy Affairs Administration 

(PREAA). NREL worked closely with staff from PREPA, PREAA, and the Governor’s Office 

throughout the technical review of PREPA’s MTRs to ensure accurate understanding of Puerto 

Rico’s electricity grid and the status of renewable energy integration. The authors would like to 

particularly thank Martin Pérez, Luis Francis, Alvin Román (PREPA) and Efraín O’Neill 

(Governor’s Energy Affairs Advisor) for their significant contributions and support of this effort, 

especially for reviewing the recommendations and providing a summary of PREPA’s responses. 

The authors would also like to thank Jennifer DeCesaro and Steve Lindenberg from DOE who 

sponsored this work, this effort could not have been completed without their support and 

involvement. 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) iv 

at www.nrel.gov/publications. 
 

 

 

 

 

List of Abbreviations and Acronyms
 AGC automatic generation control 

DER distributed energy resources 

DFIG double-fed induction generator 

ERCOT Electric Reliability Council of Texas 

FESTIV Flexible Energy Scheduling Tool for Integration of Variable Generation 

FRT fault ride-through 

HECO Hawaiian Electric Company 

HVRT high-voltage ride-through 

Hz hertz 

IEC International Electrotechnical Commission 

IEEE Institute of Electrical Engineers & Electronics Engineers 

kV kilovolt 

LVRT low-voltage ride-through 

ms millisecond 

MTR minimum technical requirements 

MVA mega volt ampere 

MVAR mega volt ampere reactive 

MW megawatt 

NERC North American Electric Reliability Corporation 

PFR primary frequency response 

PSLF Positive Sequence Load Flow 

POI point of interconnection 

PQ power quality 

PPA power purchase agreement 

PRC protection and control 

PREPA Puerto Rico Electric Power Authority 

PSCAD power systems computer aided design 

PSS/E power system simulation for engineering 

PV photovoltaic 

RR ramp rate 

SCADA supervisory control and data acquisition 

SCR short circuit ratio 

STATCOM static synchronous compensator 

SVC static VAR compensator 

VAR volt ampere reactive 

VRS voltage regulation system 

WPP wind power plant 

WTG wind turbine generator 

ZVRT zero-voltage ride-through 

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Table of Contents 

Abstract ....................................................................................................................................................... iv
 

Acknowledgements ................................................................................................................................... iv
 

List of Abbreviations and Acronyms ........................................................................................................ v
 

1 Introduction ........................................................................................................................................... 1
 

2 Voltage Fault Ride-Through ................................................................................................................ 2
 

3 Voltage Regulation System, Reactive Power, and Power Factor Requirements ........................... 6
 

4 Short Circuit Ratio .............................................................................................................................. 13
 

5 Frequency Ride-Through ................................................................................................................... 14
 

6 Frequency Response ......................................................................................................................... 15
 

7 Ramp Rate Control ............................................................................................................................. 19
 

8 Power Quality ...................................................................................................................................... 20
 

9 Other Considerations ......................................................................................................................... 21
 

10 Conclusions ........................................................................................................................................ 23
 

References ................................................................................................................................................. 24
 

Appendix A: Minimum Technical Requirements of Interconnection of PV Facilities (ver. June 14,
 

2012)..................................................................................................................................................... 25
 

Appendix B: Minimum Technical Requirements of Interconnection of Wind Facilities (ver. June 14,
 

2012)..................................................................................................................................................... 34
 

Appendix C: Minimum Technical Requirements of Interconnection of PV Facilities (ver. Aug. 15,
 

2012)..................................................................................................................................................... 46
 

Appendix D: Minimum Technical Requirements of Interconnection of Wind Facilities (ver. Aug. 15,
 

2012)..................................................................................................................................................... 55
 

List of Figures 

Figure 1. Comparison of LVRT and HVRT requirements ........................................................................ 2
 

Figure 2. E-ON reactive current droop ...................................................................................................... 3
 

Figure 3. Envelope of international LVRT/ZVRT/HVRT requirements ................................................... 4
 

Figure 4. Example of PV plant interconnection with STATCOM on PV collector bus ......................... 8
 

Figure 5. Example of PV plant interconnection with STATCOM on transmission bus ........................ 8
 

Figure 6. Types of reactive power capability curves for wind generators .......................................... 10
 

Figure 7. Comparison of reactive power curves .................................................................................... 11
 

Figure 8. Two types of DFIG WTG reactive power capabilities [3] ...................................................... 12
 

Figure 9. LVRT sensitivity to SCR [4]...................................................................................................... 13
 

Figure 10. Comparison of frequency FRT requirements ...................................................................... 14
 

Figure 11. Various frequency droop characteristics ............................................................................. 15
 

Figure 12. Non-symmetrical frequency droop by wind turbines.......................................................... 16
 

Figure 13. Inertial response by Type 3 WTG at lower wind speed (NREL model) .............................. 18
 

Figure 14. Inertial response by Type 3 WTG at rated wind speed (NREL model) .............................. 18
 

Figure 15. Wind power participation in AGC (NREL model) ................................................................. 21
 

Figure 16. Examples of wind power operations with reserves ............................................................ 21
 

National Renewable Energy Laboratory (NREL) 

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1 Introduction 

The Puerto Rico Electric Power Authority (PREPA) has established minimum technical 

requirements (MTR) for interconnection of wind turbine generation and photovoltaic (PV) power 

plants (see Appendices A and B). During a stakeholder workshop conducted by the U.S. 

Department of Energy (DOE), the National Renewable Energy Laboratory (NREL), and the 

Government of Puerto Rico in May 2012, barriers to renewable energy deployment were 

discussed. The stakeholders, including PREPA, the Puerto Rico Energy Affairs Administration, 

and representatives from the Governor’s Office, requested DOE and NREL assistance in a 

technical review of the MTRs for variable renewable energy generation. 

NREL has conducted a review of these requirements based on generic technical aspects and 

electrical characteristics of wind and PV power plants, and on existing requirements from other 

utilities in both the United States and Europe. The purpose of this review is to analyze each 

aspect of the PREPA MTRs, comparing and contrasting with interconnection requirements from 

similar power systems, identifying areas of concern, and generating recommendations and 

suggestions for improvements or additional study. 

Over the course of the collaboration on the review of the MTRs (2012-2013), PREPA staff 

reviewed the draft recommendations and incorporated some of the recommendations into the 

2012 revisions of the MTRs (see Appendices C and D). In addition, PREPA engineers responded 

to the NREL recommendations by providing a technical rational based on their operational 

experience or previous modeling results for not modifying some of the requirements. Each 

section of this report provides the recommendations for revisions to be made to the original 

MTRs based on industry best practices as well as PREPA’s response to how the 

recommendations were addressed in the MTRs revised in August 2012. For the sake of 

convenience, in this report we shall refer to the June 14, 2012, version of the PREPA MTR 

document as the “original” MTR, and to the August 15, 2012, version as the “modified” MTR. 

This report is available at no cost from the 

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at www.nrel.gov/publications. 
 

 

 

 

2 Voltage Fault Ride-Through 

In the original MTR document, PREPA’s low-voltage ride-through (LVRT) requirements for 

both wind and PV are similar. Wind and PV plants are supposed to remain online and ridethrough zero-voltage faults for up to 600 milliseconds (ms); this is essentially a zero-voltage 

ride-through (ZVRT) requirement. The no-trip zone is bounded by linear voltage recovery for up 

to 3 seconds. PREPA sets high-voltage ride-through (HVRT) requirements for maximum high 

voltage at 140% of nominal for up to 1 second. A comparison between LVRT and HVRT 

requirements for PREPA, North American Electric Reliability Corporation (NERC), and other 

islands systems such as the Hawaiian Electric Company (HECO) and EirGrid are shown in 

Figure 1. 

Figure 1. Comparison of LVRT and HVRT requirements 

The draft NERC Protection and Control (PRC) Standard PRC-024-1 requires 150 ms ZVRT. 

Such ZVRT is a typical requirement for transmission-connected wind power plants (WPP) in the 

United States. The Federal Energy Regulatory Commission Order 661A also requires 150 ms 

ZVRT, but recovery time is determined on a site-by-site basis depending on the local grid 

characteristics and protection schemes. Longer ZVRT requirements are necessary for WPPs 

connected to weaker lines (typical for smaller island grids). A 600 ms ZVRT requirement is 

implemented in the HECO system. EirGrid also requires 600 ms ride-through but at 15% voltage 

and with longer recovery time. Such longer zero-voltage requirements and slower rate of 

recovery suggested by PREPA is justified for weaker island systems to avoid tripping of the 

wind generators. 

Currently, most U.S. utilities have adopted the Institute of Electrical and Electronics Engineers 

(IEEE) Standard 1547 for interconnecting distributed resources on the distribution system. IEEE 

1547 covers interconnection of all types of distributed energy resources (DER) up to 10 mega 

volt ampere (MVA) at the point of common coupling. IEEE 1547 clearing times are also shown 

in Figure 1. 

Voltage during the fault is measured at the plant point of interconnection (POI), which is 

typically at the transmission interface, and is separated from wind turbine generators (WTG) by 

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two transformers (turbine transformer and wind plant transformer). Combined impedances of 

two transformers result in voltages at individual turbine terminals to be 15% to 20% higher than 

transmission voltage. From this perspective, no zero-voltage conditions exist at turbine terminals 

even during a ZVRT event if the turbine stays online and produces fault current. The fault 

voltage is normally interpreted as the positive sequence component at the POI. For unbalanced 

faults the positive sequence voltage may differ from the average phase voltage. 

It is important to note that International Electrotechnical Commission (IEC) 61400-21 for power 

quality testing of grid-connected wind turbines requires testing of individual WTG exposed to 

rectangular balanced and unbalanced voltage faults at turbine terminals with durations of 200 to 

500 ms, and voltage magnitudes of 20% to 90% of nominal (see Table 1). This means that some 

WTGs that have been certified to this standard will not by default meet PREPA’s LVRT 

requirements shown in Figure 1. Some hardware modifications might be needed for certain wind 

turbine models to enhance their LVRT performance and meet PREPA’s requirements. However, 

major wind turbine manufacturers (Siemens, General Electric, etc.) have turbine designs capable 

of performing accordingly. 

Table 1. IEC 61400-21 LVRT Test Matrix for Wind Turbines 

Although it is not necessary for the converter to produce fault current during the fault in the 

United States, fault voltage support is required by European grid codes. The turbine converter 

injects reactive current into the fault as a function of percentage of voltage drop. For example, 

German utility operator E-ON requires reactive current injection according to the droop line 

shown in Figure 2 (a minimum of 2% positive sequence reactive current for every percent of 

voltage drop below 100%). This implies that for faults with voltage drops of 50% or lower, the 

reactive component of current goes to 100% or higher (depending on the current rating of the 

converter–usually 1.1-1.2 p.u.). 

Figure 2. E-ON reactive current droop 

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In case of unbalanced faults, the general approach is to inject reactive current support as a 

positive sequence current. 

Recommendation #1: 

We recommend PREPA further examine the reactive current injection requirement and consider 

implementation of a similar droop. Currently, PREPA requires that “during the fault conditions, 

the wind generation facility shall operate on maximum reactive current injection mode.” 

In some cases, injecting maximum reactive current may have the opposite effect and cause 

voltage instability in WPP or PV feeder/collector systems. Implementing reactive fault current 

droops will help avoid potential instabilities. 

Figure 3. Envelope of international LVRT/ZVRT/HVRT requirements 

Situations with over-voltages may arise due to sudden load loss, unbalanced faults, and other 

conditions. The resulting over-voltages may have different magnitudes and durations depending 

on the scenario. PREPA HVRT requirements (as shown in Figure 1) are the highest compared to 

various grid codes that can be found in literature [1]. The highest HVRT requirements in some 

power systems are limited at 130% of rated voltage. In other island systems (such as HECO or 

EirGrid) the HVRT is limited at 115% to 120%. There are some grids that require 130% HVRT. 

Boundaries of fault ride-through (FRT) requirements from several European utilities listed in [1] 

are shown in Figure 3 for reference purposes. 

PREPA’s Response to Recommendation #1: 

In accordance with NREL’s recommendation to further examine the reactive current 

injection requirement and consider implementation of a similar droop, PREPA revised the 

MTRs to include the following in the section 1.a.iv: 

“During the low voltage fault conditions, the wind generation/PV facility shall operate on 

reactive current injection mode. This mode of operation shall be implemented with a 

reactive current droop characteristic which shall have an adjustable slope from 1 to 5%. A 

dead band of 15 % is required.” 

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Recommendation #2: 

The universal requirement of 1-second 140% HVRT by PREPA seems to be excessively strict 

and may not be necessary for all renewable project locations. Such requirements may cause 

unnecessary increases in project costs since project developers will have to cover expenses for 

system upgrades or additional VAR support devices. We recommend PREPA look closely at 

HVRT requirements and consider also the option of setting them on a site-by-site basis for both 

wind and PV systems as an alternative. 

PV plants using inverters that are IEEE 1547/UL 1741 compliant will not contribute fault 

current. A resolution of this issue is expected later in 2013 from the UL/IEEE 1547.8 working 

group, with cooperation from the NERC Integration of Variable Generation Task Force (Task 

Force 1.7). 

PREPA’s Response to Recommendation #2: 

Based on NREL’s recommendation to review the HRVT requirements, PREPA revised 

the MTRs by reducing the 140% overvoltage ride-through (OVRT) time to 150 ms, and 

also included a new OVRT range of 130%-125%. The modified section 1.b.i incorporates 

this new requirement into the following table: 

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3 Voltage Regulation System, Reactive Power, and 

Power Factor Requirements 

IEEE 1547/UL 1741 compliant PV inverters typically do not have reactive power capability and 

operate with a unity power factor. To meet PREPA requirements for reactive power and voltage 

control, a substation level reactive power compensation system will need to be applied [static 

synchronous compensator (STATCOM)] for IEEE 1547 inverters. 

Voltage regulation can be achieved by either regulating the inverter’s terminal voltage on the low 

side of the transformer or by adjusting reactive output according to a droop characteristic. For the 

purposes of voltage regulation, PREPA requires deployment of a voltage regulation system 

(VRS) with adjustable set point between 95% and 105% of rated POI voltage. The VRS employs 

an adjustable 0% to 10% reactive droop with accuracy of control within +/-0.5% of set point. 

Recommendation #3: 

Such reactive droop controls have been implemented by some wind turbine manufacturers on 

individual wind turbine and plant levels. The reactive droop capabilities by PV inverters are also 

offered by some inverter manufacturers, but still need large-scale demonstrations of control 

schemes in the field. Reactive droops are usually in the range of 2% to 10%. PREPA’s 

requirement of reactive droop below 2% is essentially a “bang-bang” voltage control that may 

introduce some stability problems and deplete reactive reserves for contingencies. Such small 

droops are usually not recommended and should generally be avoided for this reason. They still 

might be justified in weak systems, but require careful evaluation so voltage stability and 

reliability is not compromised. 

PREPA’s Response to Recommendation #3: 

PREPA decided to keep the complete voltage droop range (0% to 10%) based on the
 

experience with actual renewable projects operating in Puerto Rico and to permit more
 

interconnection site options (from “strong” to “weak” system sites). For example, one 

existing PV facility is actually regulating with a voltage droop of 0% and the performance
 

of their voltage regulator system (VRS) has been as expected. The MTR language was
 

revised by removing “... and must also be adjustable by PREPA’s Energy Control Center
 

via SCADA” from 2.d. 

The new version of item 2.d now reads: 

“The VRS controller regulation strategy shall be based on proportional plus integral (PI)
 

control actions with parallel reactive droop compensation. The VRS Droop shall be 

adjustable from 0 to 10%.”
 

Recommendation #4: 

Reactive droops usually have voltage dead bands (+/-1% for example) that are not specified in 

PREPA’s requirements. Having voltage dead bands may affect the ability to control voltage with 

+/- 0.5% accuracy. In some case, slower communications may also limit the reactive power 

response time and affect required accuracy. 

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PREPA’s Response to Recommendation #4: 

Following NREL’s recommendation to specify voltage dead bands, PREPA revised 

Section 2 in the MTRs by adding a new subsection 2.h as follows: 

“The VRS dead band shall not exceed 0.1%.” 

Recommendation #5: 

Some examples of closed-loop voltage regulation for PV power plants are shown in Figure 4 and 

Figure 5 where PV inverters are connected to a 13.8 kilovolt (kV) collector bus, and then to 

PREPA’s 115 kV transmission system via medium voltage (MV) feeder and step-up transformer. 

The closed loop voltage regulation is performed by a PV plant controller that senses remotely the 

voltage at the POI. This control loop allows for the best mitigation of voltage fluctuations since it 

allows eliminating voltage impacts in one location. In the case of other existing feeder voltage 

regulation (switched capacitors, tap-changing transformers, etc.), there may be some difficulties 

coordinating PV inverter controls with existing feeder voltage regulation. Care must be taken for 

PV not to carry an excess share of reactive demand of other feeder loads, avoiding interactions 

between controls and addressing hunting regulator stability issues. 

In cases where voltage control capability of PV inverters is absent (IEEE 1547 compliance) or 

limited, additional devices such as STACOM or Static VAR Compensator (SVC) can be 

implemented on both PV plant collector (Figure 4) or transmission bus (Figure 5) levels. We 

think that switched capacitor banks or tap changer transformers (in case they already exist in that 

particular feeder) can be accepted, and in some cases can eliminate a need for additional 

STATCOM devices. Different regulation strategies can be implemented with these schemes. For 

example, fast voltage fluctuations can be mitigated with PV inverters, and slower ones can be 

addressed by the existing feeder equipment. Appropriate time constants must be chosen to 

coordinate these various controls. 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 7 

at www.nrel.gov/publications. 
Figure 4. Example of PV plant interconnection with STATCOM on PV collector bus 

Figure 5. Example of PV plant interconnection with STATCOM on transmission bus 

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PREPA’s Response to Recommendation #5: 

PREPA’s MTR’s are specific to transmission (115 kV) and sub-transmission (38 kV) level
 

systems. In PREPA’s actual applications, the feeders used by the PV and wind facilities are 

internal; excess share of reactive demand of other feeder loads is not expected. NREL’s
 

comments regarding this issue seem to be more applicable to distribution system feeders. 

The MTR’s actual reactive power requirements provide flexibility for different types of
 

technologies at the renewable energy facility. These technologies include combinations of
 

different equipment like inverters, STATCOMs, SVCs and capacitor banks. As a result, no 

changes were made to the MTRs.
 

Recommendation #6: 

PREPA requires VRS to operate only in a voltage set point control mode and does not permit 

power factor control mode. Note, however, that in some cases plant level power factor control 

can be an effective tool for mitigating both steady-state and transient voltage impacts of PV, 

minimizing control interactions with other feeder devices, and is IEEE 1547 compliant. The 

voltage variations at the POI shown in Figure 4 and Figure 5 depend on real/reactive power 

variations (∆P and ∆Q) and feeder parameters R and X: 

∙ R − ∆Q ∙ X

∆V ≈ ∆P

V

In order to maintain ∆V ≈ 0, a constant ratio between active and reactive power variations must 

be maintained: 

∆Q

X = const ∆P ≈ R

Reactive power needed to maintain constant voltage is at an approximately constant ratio to real 

power at typical short-circuit ratios. Operating at fixed Q/P ratio essentially means operating at 

constant power factor. Of course, this method has its disadvantages due to some nonlinearities of 

the P-Q curve. Also, the desired power factor may change if the feeder is reconfigured. This 

method is not feasible with feeders that have other large loads connected to it since feeder 

reactive demand may increase during high-load conditions. 

Wind and PV power converters have reactive capabilities that are different from those of 

conventional synchronous generators. Unlike power-limited synchronous generators, the power 

converter-based generation is also limited by internal voltages, temperatures, and currents. 

Different types of reactive power capabilities can be implemented for power converter-based 

generation depending on power system requirements, inverter limits, etc. 

Both double-fed induction generators (DFIG) and full-converter WTGs may come with a variety 

of V-shape, rectangular, D-shape, or U-shape reactive power characteristics as shown in Figure 

6. In some cases, reactive power characteristics can be non-symmetric. One advantage of WTGs 

with rectangular and D-shape reactive capabilities is the possibility of providing voltage 

regulation service during no-wind periods. The same is true for PV inverters because they can 

provide voltage regulation at night by operation in a STATCOM mode. 

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PREPA’s Response to Recommendation #6 (similar to #5 above): 

PREPA’s MTR’s are specific to transmission (115 kV) and sub-transmission (38 kV) level
 

systems. In PREPA’s actual applications, the feeders used by the PV and wind facilities are 

internal; excess share of reactive demand of other feeder loads is not expected. NREL’s
 

comments regarding this issue seem to be more applicable to distribution system feeders. 

The MTR’s actual reactive power requirements provide flexibility for different types of
 

technologies at the renewable energy facility. These technologies include combinations of
 

different equipment like inverters, STATCOMs, SVCs and capacitor banks. As a result, no 

changes were made to the MTRs.
 

Recommendation #7: 

It is important to note that PV inverters have historically been predominantly designed for 

deployment in distribution systems in accordance to IEEE 1547, which does not allow voltage 

regulation. The wind generation was predominantly designed to operate at transmission level, so 

it is more common for WTGs to come with standard reactive power capabilities similar to ones 

shown in Figure 6. For PV systems such functionality is not standard and must be explicitly 

specified in the design stage of the project and in the power purchase agreement (PPA). Also, 

during periods of no wind or solar resource, both wind and PV generators can still provide 

reactive power support to PREPA’s grid. It makes sense to take this fact into consideration when 

specifying the reactive power capability since wind and PV inverters may be disconnected from 

the grid during the periods of no production. 

Figure 6. Types of reactive power capability curves for wind generators 

In its minimum interconnection requirements PREPA specifies a rectangular reactive power 

capability curve for both dynamic and total ranges. A comparison of reactive power capabilities 

between HECO, PREPA, and EirGrid is shown in Figure 7. Similar rectangular reactive power 

curves are required by HECO in their model PPA document [2]. EirGrid implemented a Ushaped reactive capability curve that uses a reduced power factor range [or a permissive mega 

volt ampere reactive (MVAR) range] below 50% or rated power. 

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PREPA’s Response to Recommendation #7: 

A complete description of the voltage regulation system (VRS) requirement is specified in
 

section 2 of the MTRs. As part of this requirement, a complete and detailed description of
 

the VRS control strategy shall be submitted by the project designer for PREPA’s
 

evaluation. 

Recommendation #8: 

We agree with PREPA that rectangular reactive capability is more beneficial. However, a 

requirement to maintain reactive power range at full output by PV inverters represents a change 

with respect to historical industry practice and has some cost impact associated with it. This cost 

impact could be substantial if the PV plant relies on inverters to provide all required reactive 

power since additional inverter capacity will be needed. For example, PV inverters would need 

11.1% of additional capacity to meet PREPA’s dynamic reactive power capability and 17.6% of 

additional capacity to meet the steady-state reactive power capability. Otherwise, the plant has to 

be de-rated or external reactive power support needs to be installed. 

Figure 7. Comparison of reactive power curves 

For some DFIG wind turbine topologies the reactive power capability curves are not 

symmetrical, as shown in Figure 8 (option 2). This is why permissive MVAR ranges as shown in 

Figure 7 for EirGrid make sense for systems with DFIG-dominated wind power. 

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Figure 8. Two types of DFIG WTG reactive power capabilities [3] 

Source: University Duisburg-Essen 

PREPA’s Response to Recommendation #8: 

The requirement to maintain reactive power range at full output (rectangular reactive 

capability) is based mainly on the significant displacement of conventional generation units 

and their corresponding regulation capabilities due to the expected high penetration levels 

of renewable projects. As a result, no changes were made to the MTRs. 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 12 

at www.nrel.gov/publications. 
 

 

 

 

4 Short Circuit Ratio 

Short circuit ratio (SCR) is one of the important limiting factors for wind power installations and 

has a great influence on the power quality impacts from variable renewable generation. SCR has 

an influence on LVRT characteristics of wind turbine generators. 

Recommendation #9: 

PREPA limits SCR=5 as the minimum permissible level for interconnecting both wind and PV 

projects. We think this level is adequate. However, we’d like to point out that in some cases even 

lower SCR might be reasonable assuming LVRT requirements set by PREPA as shown in Figure 

1. An example of LVRT sensitivity to SCR for a DFIG wind farm is shown in Figure 9. The 

turbine speed of recovery degrades as SCR is decreased, but the LVRT requirement is still met 

even at SCR=2. 

PREPA’s Response to Recommendation #9: 

PREPA maintained the SCR limit on 5. Studies indicate that SCR levels below 5 can cause 

VRS control instabilities in some applications. 

Figure 9. LVRT sensitivity to SCR [4] 

Source: IEEE 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 13 

at www.nrel.gov/publications. 
5 Frequency Ride-Through 

PREPA’s frequency ride-through (FRT) requirements align with standards in other island 

systems. A comparison between PREPA, HECO, NERC, and IEEE requirements is shown in 

Figure 10. PREPA allows adequate frequency range and clearing time typical for weaker 

islanded systems. 

Figure 10. Comparison of frequency FRT requirements 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 14 

at www.nrel.gov/publications. 
 

6 Frequency Response 

PREPA requires 5% frequency droop for primary frequency control from both wind and PV 

plants for up and down regulation similar to conventional synchronous generators. This is a 

symmetrical droop requirement, and PREPA does not specify any dead bands or type of droop 

characteristic for wind and PV generation. We would like to point out that there are several types 

of droop characteristics that can be implemented as shown in Figure 11. The Smooth Droop 1 

with dead bands is the most common droop used in conventional synchronous generation. The 

droop curve with power step change is also possible but may cause some stability issues during 

certain contingency scenarios. 

Figure 11. Various frequency droop characteristics 

Recommendation #10: 

We think that more detailed specifications on the exact type of droop and dead bands would be 

necessary to remove any ambiguities and misinterpretations for project developers. Primary 

frequency response (PFR) is one of the most expensive services for renewable generation 

operators since power reserves must be constantly maintained for up regulation by curtailment 

(loss of production) or by the use of energy storage devices (extra investment cost). 

We would also like to point out that both WTG and PV power plants can reduce their power 

outputs very effectively so non-symmetric droop characteristics similar to the one shown in 

Figure 12 can be implemented. Both positive and negative droops and frequency dead bands can 

be controlled and set by a wind or PV power plant SCADA system to provide aggregate plant 

PFR that meets power system needs. We recommend PREPA consider the possibility of 

requesting an option of setting PFR parameters (dead bands, droops, reserve margins) remotely. 

PFR responses by wind and PV power are additional controls that can be tuned to provide 

optimum performance and maximum reliability to the PREPA power system and can become a 

source of additional control flexibility for PREPA system operators. 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 15 

at www.nrel.gov/publications. 
 

 

 

 

 

Figure 12. Non-symmetrical frequency droop by wind turbines 

It is important to note that there are no active requirements in the United States for wind and PV 

generation to provide PFR. This is partly due to low penetration levels and lack of incentives (no 

ancillary service market structures for variable generation to provide PFR). The Electric 

Reliability Council of Texas (ERCOT) has started implementing a PFR requirement for their 

inverter based generation. Some of the ERCOT PFR requirements include [5]: 

•	 Minimum 5% and maximum 2% droop performance with maximum allowable dead-band 

of ±0.0166 Hertz (Hz) 

•	 Governors must remain in service and be allowed to respond 

•	 70% of response has to be delivered within 16 seconds after disturbance to meet the PFR 

requirements 

•	 Full response must be deployed within an additional 30 seconds after an initial 16
 

seconds of a total 46 seconds after disturbance.
 

PREPA’s Response to Recommendation #10: 

Per NREL’s recommendation, PREPA added the following clarifications to Section 6: 

•	 The frequency response dead band shall not exceed 0.02%. 

•	 If energy storage systems are utilized to comply with the frequency regulation 

requirements, and during a disturbance the system frequency stays below 59.7Hz, 

the facility frequency response shall be maintained for at least 10 minutes. 

Recommendation #11: 

For large frequency deviations (more than 0.3 Hz), PREPA requires an immediate increase in 

active power production of at least 10% of alternating current-rated capacity of both wind and 

PV power plants for at least 10-minute time periods. The response time of these services should 

be no more than 1 second. This requirement is somewhat ambiguous: Does this mean that a 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 16 

at www.nrel.gov/publications. 
 

 

 

renewable plant must provide a 10% increase in power production at any time when frequency 

declines by 0.3 Hz? What if this frequency decline happens during periods of no wind for wind 

generation, or during nights for PV? Do they still need to provide this 10% increase? If so, then 

at least 10-minute energy storage will be required. This requirement definitely needs more 

clarification since it is the most important requirement for wind and PV integration. 

PREPA’s Response to Recommendation #11: 

In accordance with NREL’s recommendation, PREPA included the following clarification 

in the fourth paragraph of section 6: 

“The operational range of the frequency response and regulation system shall be from 10% 

to 100% of the maximum AC active power capacity (established in the contract).” 

Recommendation #12: 

Further, PREPA requires WTG controls to provide inertial response. This requirement also needs 

more detailed explanation. The main questions here are: 

1.	 What is the priority of inertial response over 10% real-power increase during large 

frequency declines (more than 0.3 Hz)? 

2.	 What are the dead bands for inertial response for smaller frequency declines? 

3.	 Transition from inertial response to PFR must be specified. 

Inertial response by wind power is highly dependent on initial pre-fault conditions of each 

individual wind turbine (revolutions per minute and wind speed). The aggregate inertial response 

of the whole WPP will be different than individual WTGs. 

Inertial response by wind power is “energy neutral.” Initial periods of overproduction triggered 

by utilizing rotor inertia are followed by a period of underproduction (someone else needs to 

meet the load during this period) as shown in Figure 13 and Figure 14. Nevertheless, the overall 

benefit of such inertial response is significant because it helps the power system in the form of 

arresting initial rate of change of frequency and “gaining time” for a slower PFR of conventional 

generation. 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 17 

at www.nrel.gov/publications. 
 

Figure 13. Inertial response by type 3 WTG at lower wind speed (NREL model) 

The recovery period and return to normal operations after providing inertial response can be 

different depending on the available wind resource. At rated wind speed (Figure 14) the return to 

pre-disturbance level is much faster than at lower wind speed (Figure 13) because of more 

favorable conditions (there is power available from wind to provide incremental electric power). 

Figure 14. Inertial response by type 3 WTG at rated wind speed (NREL model) 

PREPA’s Response to Recommendation #12: 

PREPA understands the value and importance of inertial control by wind. While revisions 

relating to this recommendation have not been made to the MTRs, this requirement will be 

considered in the future. 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 18 

at www.nrel.gov/publications. 
 

 

 

 

 

 

7 Ramp Rate Control 

Recommendation #13: 

PREPA requires a 10% of rated capacity limit on 1-minute ramp rates (RR) by both wind and PV 

generation. Some clarification is needed on how PREPA calculates these 1-minute RRs since 

there are several ramp calculation methods described in technical literature (difference between 

two endpoints of any 60-second interval, minimum/maximum values between two endpoints, 

etc.). A 10% per minute RR limit seems to be reasonable. For reference, EirGrid limits positive 

ramps up to 30 MW/minute. HECO limits its ramps at +/- 2 MW per minute during all times 

(except for 12:00–4:00 a.m. and 4:00–8:00 p.m. when ramps are limited to +1 MW and -1 MW 

per minute respectively for wind projects less than 50 MW). In Germany, transmission system 

operators require a similar 10% limit of rated power per minute for positive ramps. Some grid 

operators set limits not only for 1-minute RRs, but also on instantaneous power fluctuations 

calculated at every scan. For example, HECO sets 1 MW/2-second limits for every 2-second 

scan. 

PREPA’s Response to Recommendation #13: 

Based on NREL’s recommendation, PREPA modified section 7 by adding the following 

statement: 

“The ramp rate control tolerance shall be +10%.” 

Regarding the ramp rate calculation methods, the Agreed Operating Procedure (AOP) of 

the project will include the ramp rate calculation method used by PREPA. 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 19 

at www.nrel.gov/publications. 
 

 

8 Power Quality 

The power quality test report, in accordance with IEC 61400-21 ed.2, must present wind turbine 

test data conducted on 60-Hz grids (some European wind turbine manufacturers still do not have 

60-Hz reports). NREL is performing Power Quality (PQ) certification testing for some 

manufacturers (Alstom, Gamesa, etc.) to produce 60-Hz reports. NREL is accredited by the 

American National Standard Institute and can perform the whole suite of IEC PQ testing for 

wind turbine testing, with the exception of LVRT tests. LVRT capability has been developed and 

commissioned at NREL in 2013. 

IEC 61400-21 requires active and reactive power set point tests that demonstrate ability of 

individual WTG to participate in load-frequency or voltage control. This standard does not 

require testing of the whole wind farm for the same purposes, so wind farm level control data 

must be provided separately by manufacturers. The same plant level control data must be 

provided for PV projects as well. 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 20 

at www.nrel.gov/publications. 
 

9 Other Considerations 

Recommendation #14: 

One important question that might affect the MTRs is if PREPA envisions wind and PV 

participation in automatic generation control (AGC) at some point in the future. If so, then such 

capability must be requested in the requirements. An example of wind power participation in 

AGC is shown in Figure 15, where a wind turbine is able to follow AGC signals by having 

available reserve margins for up regulation. 

Figure 15. Wind power participation in AGC (NREL model) 

There are other active power control schemes for wind power rather than inertial/PFR and RR 

limiting. If required, wind power can operate with spinning reserves as shown in Figure 16. It 

can provide continuous reserves as a percentage of available power or absolute reserves in 

megawatts. 

Figure 16. Examples of wind power operations with reserves 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 21 

at www.nrel.gov/publications. 
 

Such advanced active power control schemes might be beneficial to PREPA under certain 

circumstances. During periods of high wind production at light loads, fossil fuel units may need 

to be forced offline due to the excessive wind power, which leads to another problem of 

inadequate unit dispatch capability in system operations. The excessive wind power could serve 

as a dispatchable reserve rather than just curtailing for security concerns. In addition, such 

capability could release the regulation of diesel and other combustion generation when a high 

concentration of wind power is installed in an isolated island grid. 

PREPA’s Response to Recommendation #14: 

PREPA asked for the minimum known necessary technical requirements that were 

important for optimizing the power system performance under high levels of renewables
 

penetration, while providing the necessary mitigation to the impact of these resources on 

the electric system. Recognizing the level of complexity that the implementation of such
 

advanced active power control schemes might have on the projects, PREPA has decided
 

not to require renewables to provide or participate in AGC at this point in time. However, 

PREPA understands the benefits of these services by renewables, and may require them in
 

the future if necessary. 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 22 

at www.nrel.gov/publications. 
 

 

 

 

 

 

 

 

10 Conclusions 

In general, PREPA’s MTR documents are well written and cover all interconnection-related 

topics for wind and PV generation. It is important to note that both MTR documents for wind 

and PV are very similar to each other with some minor differences specific to each technology. It 

may make sense to combine them both into one general MTR document that includes a few 

provisions specific to wind or PV. 

As mentioned throughout this document, some aspects of the existing MTRs suggest further 

clarifications and better explanations are needed. Such “fine tuning” of interconnection 

requirements is important to avoid ambiguities in interpretation of the MTRs and help potential 

vendors and project developers to better meet PREPA’s reliability criteria. 

One important aspect that still needs to be included in the MTRs is the “borderline” for projects 

that need to be compliant with MTR requirements. Such a borderline can be formulated either in 

megawatt capacity of the project or voltage level at the POI. 

Additionally, PREPA may find it useful to evaluate various aspects of interconnection technical 

requirements, including: 

•	 Using transient modeling of wind and PV plants for the purposes of FRT analyses 

•	 Conducting sub-hourly modeling and optimization of the power system 

•	 Developing sub-hourly solar power output data sets with realistic spatial and temporal 

correlations 

•	 Improving wind and solar resource forecasting 

•	 Increasing the internal capacity to incorporate dynamic models for wind generators and 

PV inverters into the current power system simulations conducted to model power 

flow/stability and frequency response. 

PREPA made many revisions to the MTRs in August 2012 based on the initial recommendations 

provided during discussions with DOE and NREL. It is anticipated that PREPA will continue to 

revise the MTRs as technology evolves and PREPA’s experience with renewable energy on the 

Puerto Rico grid increases. PREPA is in a unique position as 64 renewable energy PPAs and 

master agreements have been signed between 2009 and 2012, totaling approximately 2,200 MW. 

If completed, this installed capacity would represent approximately 70% of the system peak load 

(3159 MW) of Puerto Rico. 

The utility and other government officials are currently reviewing many of these agreements and 

PREPA is working to determine how to safely incorporate large amounts of renewable energy 

onto the grid. As the amount of renewable energy on the grid increases, PREPA will gain 

valuable experience in understanding how high penetrations of renewables impact the grid. The 

increased experience operating the power system at different levels of renewable penetration will 

likely inform future revisions to the MTRs. Hopefully, the recommendations, based on industry 

best practices, will also be useful to PREPA when making future revisions to the MTRs. 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 23 

at www.nrel.gov/publications. 
 

 

References 

[1] Generator Fault Ride-Through (FRT) Investigation - Stage 1. (2009). Transpower New 

Zealand Limited. http://www.nerc.com/docs/pc/ivgtf/GEN-FRT-Stage1-Lit-review%20(2­

4Task%20Force).pdf. 

[2] Model Power Purchase Agreement for Renewable Energy Projects. (2008). HECO. 

http://www.heco.com/vcmcontent/GenerationBid/HECO/FinalRenewableModelPPA.pdf. 

[3] Erlich, I., et al. Reactive Power Generation by DGIF Based Wind Farms with AC Grid 

Connection. (2007). University Duisburg-Essen. http://www.unidue.de/ean/downloads/papers/wilch2007a.pdf. 

[4] Abbey, C.; Joos, G. Effect of Low Voltage Ride-Through (LVRT) Characteristics on Voltage 

Stability. (2005). IEEE. http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=01489659. 

[5] Real Power Balancing Control Performance. (August 2010). Texas Reliability Entity. 

http://texasre.org/CPDL/BAL-001-TRE-1%20Second%20Draft%20-%20Clean.pdf. 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 24 

at www.nrel.gov/publications. 
Appendix A: Minimum Technical Requirements of 

Interconnection of PV Facilities (ver. June 14, 2012) 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 25 

at www.nrel.gov/publications. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

1

MINIMUM TECHNICAL REQUIREMENTS FOR 

INTERCONNECTION OF PHOTOVOLTAIC (PV) FACILITIES 

The proponent shall comply with the following minimum technical requirements: 

1.	�VOLTAGE RIDE-THROUGH: 

Figure 1 Low Voltage Ride-Through Requirements 

a.	
 PREPA’s Low Voltage Ride-Through (LVRT) Requirements: 

i.	
 From Figure 1, PREPA requires all generation to remain online and 

be able to ride-through three phase and single phase faults down 

to 0.0 per-unit (measured at the point of interconnection), for up 

to 600 ms. 

ii.	
 All generation remains online and operating during and after 

normally cleared faults on the point of interconnection. 

iii.	
 All generation remains online and operating during backupcleared faults on the point of interconnection. 

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iv.	
 During the fault conditions, the PV facility shall operate on 

maximum reactive current injection mode. 

b.	
 PREPA’s Overvoltage Ride-Through (OVRT) Requirements: 

i.	
 PREPA requires all generation to remain online and able to ridethrough symmetrical and asymmetrical overvoltage conditions 

specified by the following values: 

Overvoltage (pu) Minimum time to remain 

online (seconds) 

1.4 – 1.25 1 

1.25 – 1.15 3 

1.15 or lower indefinitely 

2.	�VOLTAGE REGULATION SYSTEM (VRS) 

Constant voltage control shall be required. Photovoltaic System 

technologies in combination with Static Var Controls, such as Static Var 

Compensators (SVCs), STATCOMs, DSTATCOMs are acceptable options to 

comply with this requirement. A complete and detailed description of 

the VRS control strategy shall be submitted for evaluation. 

a)	
 Photovoltaic Facilities (PVF) must have a continuously-variable, 

continuously-acting, closed loop control VRS; i.e. an equivalent to the 

Automatic Voltage Regulator in conventional machines. 

b)	
 The VRS set-point shall be adjustable between 95% to 105% of rated 

voltage at the POI. The VRS set-point must also be adjustable by 

PREPA’s Energy Control Center via SCADA. 

c)	
 The VRS shall operate only in a voltage set point control mode. 

Controllers such as Power Factor or constant VAR are not permitted. 

d)	
 The VRS controller regulation strategy shall be based on proportional 

plus integral (PI) control actions with parallel reactive droop 

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compensation. The VRS Droop shall be adjustable from 0 to 10% and 

must also be adjustable by PREPA’s Energy Control Center via SCADA. 

e)	
 At zero percent (0%) droop, the VRS shall achieve a steady-state 

voltage regulation accuracy of +/- 0.5% of the controlled voltage at 

the POI. 

f)	
 The VRS shall be calibrated such that a change in reactive power will 

achieve 95% of its final value no later than 1 second following a step 

change in voltage. The change in reactive power should not cause 

excessive voltage excursions or overshoot. 

g)	
 The generator facility VRS must be in service at any time the PVF is 

electrically connected to the grid regardless of MW output from the 

PVF. 

3.	�REACTIVE POWER CAPABILITY AND MINIMUM POWER 

FACTOR REQUIREMENTS 

The total power factor range shall be from 0.85 lagging to 0.85 leading at 

the point of interconnection (POI). The reactive power requirements 

provide flexibility for many types of technologies at the Renewable 

Energy Facility. The intent is that a PVF can ramp the reactive power from 

0.85 lagging to 0.85 leading in a smooth continuous fashion at the POI. 

The +/- 0.90 power factor range should be dynamic and continuous at the 

point of interconnection (POI). This means that the PVF has to be able to 

respond to power system voltage fluctuations by continuously varying 

the reactive output of the plant within the specified limits. The 

previously established power factor dynamic range could be expanded if 

studies indicate that additional continuous, dynamic compensation is 

required. It is required that the PVF reactive capability meets +/- 0.85 

Power Factor (PF) range based on the PVF Aggregated MW Output, which 

is the maximum MVAr capability corresponding to maximum MW 

Output. It is understood that positive (+) PF is where the PVF is producing 

MVAr and negative (-) PF is where the PVF is absorbing MVAr. 

This requirement of MVAr capability at maximum output shall be 

sustained throughout the complete range of operation of the PVF as 

established by Figure 2. 

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4

Figure 2 Reactive Power Capability Curve 

4. SHORT CIRCUIT RATIO (SCR) REQUIREMENTS: 

Short Circuit Ratio values (System Short Circuit MVA at POI/PV Facility MVA 

Capacity) under 5 shall not be permitted. The constructor shall be 

responsible for the installation of additional equipment, such as synchronous 

condensers, and controls necessary to comply with PREPA’s minimum short 

circuit requirements. 

5. FREQUENCY RIDE THROUGH (FRT): 

• 57.5 - 61.5 Hz No tripping (continuous) 

• 61.5 - 62.5 Hz 30 sec 

• 56.5 - 57.5 Hz 10 sec 

• < 56.5 or > 62.5 Hz Instantaneous trip 

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6. FREQUENCY RESPONSE/REGULATION: 

PV facility shall provide an immediate real power primary frequency 

response, proportional to frequency deviations from scheduled 

frequency, similar to governor response. The rate of real power response 

to frequency deviations shall be similar to or more responsive than the 

droop characteristic of 5% used by conventional generators. PV facility 

shall have controls that provide both for down-regulation and upregulation. PV technologies, in combination with energy storage systems 

such as, but not limited to BESS, flywheels, hybrid systems are acceptable 

options to comply with PREPA’s frequency response and regulation 

requirements. 

For large frequency deviations (for example in excess of 0.3 Hz), the PV 

facility shall provide an immediate real power primary frequency 

response of at least 10% of the maximum AC active power capacity 

(established in the contract) for a time period not less than 10 minutes. 

The time response (full 10% frequency response) shall be less than 1 

second. During disturbances or situations that provoke the system 

frequency to stay below 59.7 Hz for 10 minutes or more, after the ninth 

minute the real power primary frequency response shall not decrease at 

a ramp rate higher than 10% of the maximum AC active power capacity 

per minute. For smaller frequency deviations (for example less than 0.3 

Hz), the PV facility response shall be proportional to the frequency 

deviation, based on the specified 5% droop characteristic. The 

operational range of the frequency response and regulation system shall 

be from 10% to 100% of the maximum AC active power capacity 

(established in the contract). The PV facility power output at the POI 

shall never exceed the maximum AC active power (established in the 

contract). 

7. RAMP RATE CONTROL: 

Ramp Rate Control is required to smoothly transition from one output 

level to another. The PV facility shall be able to control the rate of change 

of power output during some circumstances, including but not limited to: 

(1) rate of increase of power, (2) rate of decrease of power, (3) rate of 

increase of power when a curtailment of power output is released; (4) 

rate of decrease in power when curtailment limit is engaged. A 10 % per 

minute rate (based on AC contracted capacity) limitation shall be 

enforced. This ramp rate limit applies both to the increase and decrease 

of power output and is independent of meteorological conditions. 

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8. POWER QUALITY REQUIREMENTS: 

The developer shall address, in the design of their facilities potential 

sources and mitigation of power quality degradation prior to 

interconnection. Design considerations should include applicable 

standards including, but not limited to IEEE Standards 142, 519, 1100, 

1159, and ANSI C84.1. Typical forms of power quality degradation 

include, but are not limited to voltage regulation, voltage unbalance, 

harmonic distortion, flicker, voltage sags/interruptions and transients. 

9. SPECIAL PROTECTION SCHEMES: 

PV facility shall provide adequate technology and implement special 

protection schemes as established by PREPA in coordination with power 

management requirements. 

10. GENERAL INTERCONNECTION SUBSTATION 

CONFIGURATION: 

An interconnecting generation producer must interconnect at an existing 

PREPA switchyard. The configuration requirements of the 

interconnection depend on where the physical interconnection is to 

occur and the performance of the system with the proposed 

interconnection. The interconnection must conform, at a minimum, to 

the original designed configuration of the switchyard. PREPA, at its sole 

discretion, may consider different configurations due to physical 

limitations at the site. 

11. MODELING AND VALIDATION 

The Contractor shall submit to PREPA a Siemens - PTI certified PSS/E 

mathematical model and data related to the proposed PV facility. When 

referred to the PV facility model, this shall include but is not limited to PV 

inverters, transformers, collector systems, plant controllers, control 

systems and any other equipment necessary to properly model the PV 

facility for both steady-state and dynamic simulation modules. It is 

required that the Contractor submits both an aggregate and detailed 

version of the PV facility model. At a later stage in the process, it is also 

required that the Contractor submits as-built PSS/E mathematical models 

of the PV Facility. 

The Contractor shall be required to submit user manuals for both the PV 

inverter and the PV facility models including a complete and detailed 

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description of the voltage regulation system (VRS) and frequency 

regulation system model implementation. The mathematical models 

shall be fully compatible with the latest and future versions of PSS/E. It is 

preferred that the models are PSS/E standard models. In the case that 

the Contractor submits user written models, the Contractor shall be 

required to keep these models current with the future versions of the 

PSS/E program until such time that PSS/E has implemented a standard 

model. The Contractor shall submit to PREPA an official report from 

Siemens - PTI that validates and certifies the required mathematical 

models, including subsequent revisions. The data and PSS/E model shall 

also be updated and officially certified according to PREPA requirements 

when final field adjustments and parameters measurements and field 

tests are performed to the facility by the contractor. The mathematical 

model (either PSS/E standard or user written model) of the PV facility 

shall be officially certified by Siemens - PTI before a specific and validated 

PSS/E mathematical model of the complete PV facility be submitted to 

PREPA. The Contractor shall be responsible of submitting the official 

reports and certifications from Siemens – PTI, otherwise the 

mathematical model shall not be considered valid. 

The Contractor shall be responsible to submit Siemens – PTI certified 

PSSE mathematical models of any kind of compensation devices (ie. SVC, 

STATCOMs, DSTATCOMs, BESS, etc.) used on the PV facility. It is 

preferred that the models are standard models provided with PSS/E. In 

the case that the Contractor submits user written models, the PV facility 

Contractor shall be required to keep these models current with the 

future versions of the PSS/E program until such time that PSS/E has 

implemented a standard model. In its final form, the mathematical 

model shall be able to simulate each of the required control and 

operational modes available for the compensation device and shall be 

compatible with the latest and future versions of PSSE. Final adjustments 

and parameters settings related with the control system commissioning 

process shall be incorporated to the PSSE mathematical model and tested 

accordingly by the PV facility Contractor and PREPA system study groups. 

The Contractor shall also perform on-site field tests for the identification, 

development, and validation of the dynamic mathematical models and 

parameters required by PREPA for any kind of compensation devices 

used at the PV facility. The mathematical models of the PV facility and its 

required compensation devices shall be officially certified by Siemens -

PTI before a specific and validated PSS/E mathematical model of the 

complete PV facility be submitted to PREPA. The Contractor shall be 

responsible of submitting the official reports and certifications from 

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Siemens – PTI, otherwise the mathematical models shall not be 

considered valid. 

PV facility Owners that provide user written model(s) shall provide 

compiled code of the model and are responsible to maintain the user 

written model compatible with current and new releases of PSS/E until 

such time a standard model is provided. PREPA must be permitted by the 

PV facility Owner to make available PV Facility models if required to 

external consultants with an NDA in place. 

12. TRANSIENT MATHEMATICAL MODEL 

The Contractor shall be responsible of providing a detailed transient 

model of the PV facility and to show that it is capable of complying with 

PREPA’s transient Minimum Technical Requirements. 

13. DYNAMIC SYSTEM MONITORING EQUIPMENT 

The developer of the PV facility shall be required to provide and install a 

dynamic system monitoring equipment that conforms to PREPA’s 

specifications. 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Photovoltaic Generation (PV) Projects 

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Appendix B: Minimum Technical Requirements of 

Interconnection of Wind Facilities (ver. June 14, 2012) 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 34 

at www.nrel.gov/publications. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1

MINIMUM TECHNICAL REQUIREMENTS FOR INTERCONNECTION OF WIND 

TURBINE GENERATION (WTG) PROJECTS 

The proponent shall comply with the following minimum technical requirements: 

1.	�VOLTAGE RIDE-THROUGH 

Figure 1 Low Voltage Ride-Through Requirements 

a.	
 PREPA’s Low Voltage Ride-Through (LVRT) Requirements: 

i.	
 From Figure 1, PREPA requires all generation to remain online and 

be able to ride-through three phase and single phase faults down 

to 0.0 per-unit (measured on the point of interconnection), for up 

to 600 ms. 

ii.	
 All generation remains online and operating during and after 

normally cleared faults on the point of interconnection. 

iii.	
 All generation remains online and operating during backupcleared faults on the point of interconnection. 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Wind Turbine Generation (WTG) Projects 

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iv.	
 During the fault conditions, the wind generation facility shall 

operate on maximum reactive current injection mode. 

b.	
 PREPA’s Overvoltage Ride-Through (OVRT) Requirements: 

i.	
 PREPA requires all generation to remain online and able to ridethrough symmetrical and asymmetrical overvoltage conditions 

specified by the following values: 

Overvoltage (pu) 

Minimum time to remain online 

(seconds) 

1.4 – 1.25 1 

1.25 – 1.15 3 

1.15 or lower Indefinitely 

2.	� VOLTAGE REGULATION SYSTEM (VRS) 

Constant voltage control shall be required. Wind Turbine Generation 

(WTG) technologies in combination with Static Var Controls, such as 

Static Var Compensators (SVC), STATCOMs, DSTATCOMs are acceptable 

options to comply with this requirement. A complete description of the 

VRS control strategy shall be submitted for evaluation. 

a)	
 Wind Generation Facilities (WGF) must have a continuously-variable, 

continuously-acting, closed loop control VRS; i.e. an equivalent to the 

Automatic Voltage Regulator in conventional machines. 

b)	
 The VRS set-point shall be adjustable between 95% to 105% of rated 

voltage at the POI. The VRS set-point must also be adjustable by 

PREPA’s Energy Control Center via SCADA. 

c)	
 The VRS shall operate only in a voltage set point control mode. 

Controllers such as Power Factor or constant VAR are not permitted. 

d)	
 The VRS controller regulation strategy shall be based on proportional 

plus integral (PI) control actions with parallel reactive droop 

Puerto Rico Electric Power Authority 

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3

compensation. The VRS Droop shall be adjustable from 0 to 10% and 

must also be adjustable by PREPA’s Energy Control Center via SCADA. 

e)	
 At zero percent (0%) droop, the VRS shall achieve a steady-state 

voltage regulation accuracy of +/- 0.5% of the controlled voltage at 

the POI. 

f)	
 The VRS shall be calibrated such that a change in reactive power will 

achieve 95% of its final value no later than 1 second following a step 

change in voltage. The change in reactive power should not cause 

excessive voltage excursions or overshoot. 

g)	
 The generator facility VRS must be in service at any time the WGF is 

electrically connected to the grid regardless of MW output from the 

WGF. 

Examples of possible VRS configurations: 

The following examples serve as possible solutions for REF voltage 

regulation and reactive power compensation. 

Figure 2 Voltage Regulation at WGF 

Puerto Rico Electric Power Authority 

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4

Figure 3 Voltage Regulation at single collector bus 

Figure 4 Voltage regulation on a multiple collector bus injection at collector bus 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Wind Turbine Generation (WTG) Projects 

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Figure 5 Shared Voltage Regulation for multiple WGF at transmission system bus 

3.	�REACTIVE POWER CAPABILITY AND MINIMUM POWER FACTOR 

REQUIREMENTS 

The total power factor range shall be from 0.85 lagging to 0.85 leading at the 

point of interconnection (POI). The reactive power requirements provide 

flexibility for many types of technologies at the Renewable Energy Facility. The 

intent is that a WGF can ramp the reactive power from 0.85 lagging to 0.85 

leading in a smooth continuous fashion at the POI. 

The +/- 0.90 power factor range should be dynamic and continuous at the point 

of interconnection (POI). This means that the WGF has to be able to respond to 

power system voltage fluctuations by continuously varying the reactive output of 

the plant within the specified limits. The previously established power factor 

dynamic range could be expanded if studies indicate that additional continuous, 

dynamic compensation is required. It is required that the WGF reactive 

capability meets +/- 0.85 Power Factor (PF) range based on the WGF Aggregated 

MW Output, which is the maximum MVAr capability corresponding to maximum 

MW Output. It is understood that positive (+) PF is where the WGF is producing 

MVAr and negative (-) PF is where the WGF is absorbing MVAr. 

This requirement of MVAr capability at maximum output shall be sustained 

throughout the complete range of operation of the WGF as established by Figure 

6. 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Wind Turbine Generation (WTG) Projects 

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6

Figure 6 Reactive Power Capability Curve 

4. SHORT CIRCUIT RATIO (SCR) REQUIREMENTS: 

Short Circuit Ratio values (System Short Circuit MVA at POI/WGF MVA Capacity) 

under 5 shall not be permitted. The constructor shall be responsible for the 

installation of additional equipment, such as synchronous condensers, and 

controls necessary to comply with PREPA’s minimum short circuit requirements. 

5. FREQUENCY RIDE THROUGH (FRT): 

• 57.5 - 61.5 Hz No tripping (continuous) 

• 61.5 - 62.5 Hz 30 sec 

• 56.5 - 57.5 Hz 10 sec 

• < 56.5 or > 62.5 Hz Instantaneous trip 

6. FREQUENCY RESPONSE/REGULATION AND INERTIAL RESPONSE 

WTG facility shall provide an immediate real power primary frequency response, 

proportional to frequency deviations from scheduled frequency, similar to 

governor response. The rate of real power response to frequency deviations 

shall be similar to or more responsive than the droop characteristic of 5% used 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Wind Turbine Generation (WTG) Projects 

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by conventional generators. WTG facility shall have controls that provide both 

down-regulation and up-regulation. Wind turbine technologies, in combination 

with energy storage systems such as BESS, flywheels, hybrid systems are 

acceptable options to comply with PREPA’s frequency regulation requirements. 

For large frequency deviations (for example in excess of 0.3 Hz), the WGF shall 

provide an immediate real power primary frequency response of at least 10% of 

the maximum AC active power capacity (established in the contract) for a time 

period not less than 10 minutes. The time response (full 10% frequency 

response) shall be less than 1 second. During disturbances or situations that 

provoke the system frequency to stay below 59.7 Hz for 10 minutes or more, 

after the ninth minute the real power primary frequency response shall not 

decrease at a ramp rate higher than 10% of the maximum AC active power 

capacity per minute. For smaller frequency deviations (for example less than 0.3 

Hz), the WGF response shall be proportional to the frequency deviation, based 

on the specified 5% droop characteristic. The operational range of the frequency 

response and regulation system shall be from 10% to 100% of the maximum AC 

active power capacity (established in the contract). The WGF power output at 

the POI shall never exceed the maximum AC active power (established in the 

contract). 

The WTG controls shall have the capability to provide inertial response. 

7.	�RAMP RATE CONTROL 

Ramp Rate Control is required to smoothly transition from one output level to 

another. The WTG facility shall be able to control the rate of change of power 

output during some circumstances, including but not limited to: (1) rate of 

increase of power (2) rate of decrease of power, (3) rate of increase of power 

when a curtailment of power output is released; (4) rate of decrease in power 

when curtailment limit is engaged. A 10 % per minute rate (based on nameplate 

capacity) limitation shall be enforced. This limit applies both to the increase and 

decrease of power output and is independent of meteorological conditions. 

8.	�POWER QUALITY 

The developer shall address, in the design of their facilities potential sources and 

mitigation of power quality degradation prior to interconnection. Design 

considerations should include applicable standards including, but not limited to 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Wind Turbine Generation (WTG) Projects 

41
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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IEEE Standards 142, 519, 1100, 1159, ANSI C84.1, IEC 61400-21, IEC 61000-3-7 

and IEC 61000-3-6. Typical forms of power quality degradation include, but are 

not limited to voltage regulation, voltage unbalance, harmonic distortion, flicker, 

voltage sags/interruptions and transients. 

The developer shall submit the Power Quality Tests Result Report of the wind 

turbines as described in the IEC 61400-21 standard. This report includes: general 

wind turbine data, wind turbine rated data at terminals, voltage fluctuations 

coefficients (flicker coefficients), current harmonics components, current 

interharmonics components, current high frequency components, response to 

voltage drops, active power data, reactive power data, grid protection data and 

reconnection time. The wind turbines shall not exceed the flicker emission limits 

established by the IEC 61000-3-7 standard and the harmonics emission limits of 

IEC 61000-3-6. 

9. WIND POWER MANAGEMENT 

WTG facility shall provide adequate technology (communicating technology and 

the corresponding control equipment) and implement wind power management 

requirements (ramp rate limits, output limits, curtailment) as established by 

PREPA. 

10. SPECIAL PROTECTION SCHEMES 

WTG facility shall provide adequate technology and implement special 

protection schemes as established by PREPA in coordination with wind power 

management requirements. 

11. WIND GENERATION FORECASTING SYSTEMS 

WTG facility shall provide adequate technology to support wind generation 

forecasting systems (short term and day-ahead) as established by PREPA. 

Individual turbine’s availability shall be included. 

12. GENERAL INTERCONNECTION SUBSTATION CONFIGURATION 

An interconnecting generation producer must interconnect at an existing PREPA 

switchyard. The configuration requirements of the interconnection depend on 

where the physical interconnection is to occur and the performance of the 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Wind Turbine Generation (WTG) Projects 

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9

system with the proposed interconnection. The interconnection must conform, 

at a minimum, to the original designed configuration of the switchyard. PREPA, 

at its sole discretion, may consider different configurations due to physical 

limitations at the site. 

13. MODELING AND VALIDATION 

The Contractor shall submit to PREPA a Siemens - PTI certified PSS/E 

mathematical model and data related to the proposed WTG facility. When 

referred to the WTG facility model, this shall include but is not limited to wind 

generator, transformers, collector systems, plant controllers, control systems 

and any other equipment necessary to properly model the WTG facility for both 

steady-state and dynamic simulation modules. It is required that the Contractor 

submits both an aggregate and detailed model of the WTG facility model. At a 

later stage in the process, it is also required that the Contractor submits as-built 

PSS/E mathematical models of the WTG facility. 

The Contractor shall be required to submit user manuals for both the Wind 

Turbine Generator and WTG Facility models including a complete and detailed 

description of the voltage regulation system (VRS) and frequency regulation 

system model implementation. The mathematical models shall be fully 

compatible with the latest and future versions of PSS/E. It is preferred that the 

models are PSS/E standard models. In the case that the Contractor submits user 

written models, the Contractor shall be required to keep these models, as well as 

its corresponding user manual, current with the future versions of the PSS/E 

program until such time that PSS/E has implemented a standard model. The 

Contractor shall submit to PREPA an official report from Siemens - PTI that 

validates and certifies the required mathematical models, including subsequent 

revisions. The data and PSS/E model shall also be updated and officially certified 

according to PREPA requirements when final field adjustments and parameters 

measurements and field tests are performed to the facility by the contractor. 

The mathematical model (either PSS/E standard or user written model) of the 

WTG facility shall be officially certified by Siemens - PTI before a specific and 

validated PSS/E mathematical model of the complete WTG facility be submitted 

to PREPA. The Contractor shall be responsible of submitting the official reports 

and certifications from Siemens – PTI, otherwise the mathematical model shall 

not be considered valid. 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Wind Turbine Generation (WTG) Projects 

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The Contractor shall be responsible to submit Siemens – PTI certified PSSE 

mathematical models of any kind of compensation devices (ie. SVC, STATCOMs, 

DSTATCOMs, BESS, etc.) used on the WTG facility. It is preferred that the models 

are standard models provided with PSS/E. In the case that the Contractor 

submits user written models, the WTG facility Contractor shall be required to 

keep these models current with the future versions of the PSS/E program until 

such time that PSS/E has implemented a standard model. In its final form, the 

mathematical model shall be able to simulate each of the required control and 

operational modes available for the compensation device and shall be 

compatible with the latest and future versions of PSSE. Final adjustments and 

parameters settings related with the control system commissioning process shall 

be incorporated to the PSSE mathematical model and tested accordingly by the 

WTG facility Contractor and PREPA system study groups. The Contractor shall 

also perform on-site field tests for the identification, development, and 

validation of the dynamic mathematical models and parameters required by 

PREPA for any kind of compensation devices used at the WTG facility. The 

mathematical models of the WTG facility and its required compensation devices 

shall be officially certified by Siemens - PTI before a specific and validated PSS/E 

mathematical model of the complete WTG facility be submitted to PREPA. The 

Contractor shall be responsible of submitting the official reports and 

certifications from Siemens – PTI, otherwise the mathematical models shall not 

be considered valid. 

WTG facility Owners that provide user written model(s) shall provide compiled 

code of the model and are responsible to maintain the user written model 

compatible with current and new releases of PSS/E until such time a standard 

model is provided. PREPA must be permitted by the WGF Owner to make 

available WGF models if required to external consultants with an NDA in place. 

14. TRANSIENT MATHEMATICAL MODEL 

The contractor shall be responsible of providing a detailed transient model of the 

WTG facility and to show that it is capable of complying with PREPA’s transient 

Minimum Technical Requirements. 

15. DYNAMIC SYSTEM MONITORING EQUIPMENT 

Puerto Rico Electric Power Authority 

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11

The developer of the Renewable Energy Facility shall be required to provide and 

install a dynamic system monitoring equipment that conforms to PREPA’s 

specifications. 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Wind Turbine Generation (WTG) Projects 

45
Appendix C: Minimum Technical Requirements of
 

Interconnection of PV Facilities (ver. Aug. 15, 2012)
 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 46 

at www.nrel.gov/publications. 
 

 

 

 

 

 

 

 

 

 

 

 

1

MINIMUM TECHNICAL REQUIREMENTS FOR 

INTERCONNECTION OF PHOTOVOLTAIC (PV) FACILITIES 

The proponent shall comply with the following minimum technical requirements: 

1.	�VOLTAGE RIDE-THROUGH: 

Figure 1 Voltage Ride-Through Requirements 

a.	
 PREPA’s Low Voltage Ride-Through (LVRT) Requirements: 

i.	
 From Figure 1, PREPA requires all generation to remain online and 

be able to ride-through three phase and single phase faults down 

to 0.0 per-unit (measured at the point of interconnection), for up 

to 600 ms. 

ii.	
 All generation remains online and operating during and after 

normally cleared faults on the point of interconnection. 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Photovoltaic Generation (PV) Projects 

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2

iii.	
 All generation remains online and operating during backupcleared faults on the point of interconnection. 

iv.	
 During the low voltage fault conditions, the PV facility shall 

operate on reactive current injection mode. This mode of 

operation shall be implemented with a reactive current droop 

characteristic which shall have an adjustable slope from 1 to 5%. 

A dead band of 15 % is required. 

b.	
 PREPA’s Overvoltage Ride-Through (OVRT) Requirements: 

i.	
 PREPA requires all generation to remain online and able to ridethrough symmetrical and asymmetrical overvoltage conditions 

specified by the following values illustrated in Figure 1: 

Overvoltage (pu) Minimum time to remain 

online 

1.4 – 1.3 150 ms 

1.3 – 1.25 1 s 

1.25 – 1.15 3 s 

1.15 or lower indefinitely 

2.	�VOLTAGE REGULATION SYSTEM (VRS) 

Constant voltage control shall be required. Photovoltaic System 

technologies in combination with Static Var Controls, such as Static Var 

Compensators (SVCs), STATCOMs and DSTATCOMs are acceptable 

options to comply with this requirement. A complete and detailed 

description of the VRS control strategy shall be submitted for evaluation. 

a)	
 Photovoltaic Facilities (PVF) must have a continuously-variable, 

continuously-acting, closed loop control VRS; i.e. an equivalent to the 

Automatic Voltage Regulator in conventional machines. 

b)	
 The VRS set-point shall be adjustable between 95% to 105% of rated 

voltage at the POI. The VRS set-point must also be adjustable by 

PREPA’s Energy Control Center via SCADA. 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Photovoltaic Generation (PV) Projects 

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3

c)	
 The VRS shall operate only in a voltage set point control mode. 

Controllers such as Power Factor or constant VAR are not permitted. 

d)	
 The VRS controller regulation strategy shall be based on proportional 

plus integral (PI) control actions with parallel reactive droop 

compensation. The VRS Droop shall be adjustable from 0 to 10%. 

e)	
 At zero percent (0%) droop, the VRS shall achieve a steady-state 

voltage regulation accuracy of +/- 0.5% of the controlled voltage at 

the POI. 

f)	
 The VRS shall be calibrated such that a change in reactive power will 

achieve 95% of its final value no later than 1 second following a step 

change in voltage. The change in reactive power should not cause 

excessive voltage excursions or overshoot. 

g)	
 The generator facility VRS must be in service at any time the PVF is 

electrically connected to the grid regardless of MW output from the 

PVF. 

h)	
 The VRS dead band shall not exceed 0.1%. 

3.	�REACTIVE POWER CAPABILITY AND MINIMUM POWER 

FACTOR REQUIREMENTS 

The total power factor range shall be from 0.85 lagging to 0.85 leading at 

the point of interconnection (POI). The reactive power requirements 

provide flexibility for many types of technologies at the Renewable 

Energy Facility. The intent is that a PVF can ramp the reactive power from 

0.85 lagging to 0.85 leading in a smooth continuous fashion at the POI. 

The +/- 0.90 power factor range should be dynamic and continuous at the 

point of interconnection (POI). This means that the PVF has to be able to 

respond to power system voltage fluctuations by continuously varying 

the reactive output of the plant within the specified limits. The 

previously established power factor dynamic range could be expanded if 

studies indicate that additional continuous, dynamic compensation is 

required. It is required that the PVF reactive capability meets +/- 0.85 

Power Factor (PF) range based on the PVF Aggregated MW Output, which 

is the maximum MVAr capability corresponding to maximum MW 

Output. It is understood that positive (+) PF is where the PVF is producing 

MVAr and negative (-) PF is where the PVF is absorbing MVAr. 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Photovoltaic Generation (PV) Projects 

49
 

 

 

 

 

 

 

 

 

 

 

4

This requirement of MVAr capability at maximum output shall be 

sustained throughout the complete range of operation of the PVF as 

established by Figure 2. 

Figure 2 Reactive Power Capability Curve 

4. SHORT CIRCUIT RATIO (SCR) REQUIREMENTS: 

Short Circuit Ratio values (System Short Circuit MVA at POI/PV Facility MVA 

Capacity) under 5 shall not be permitted. The constructor shall be 

responsible for the installation of additional equipment, such as synchronous 

condensers, and controls necessary to comply with PREPA’s minimum short 

circuit requirements. 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Photovoltaic Generation (PV) Projects 

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5

5. FREQUENCY RIDE THROUGH (FRT):
�

• 57.5 - 61.5 Hz No tripping (continuous) 

• 61.5 - 62.5 Hz 30 sec 

• 56.5 - 57.5 Hz 10 sec 

• < 56.5 or > 62.5 Hz Instantaneous trip 

6. FREQUENCY RESPONSE/REGULATION: 

PV facility shall provide an immediate real power primary frequency 

response, proportional to frequency deviations from scheduled 

frequency, similar to governor response. The rate of real power response 

to frequency deviations shall be similar to or more responsive than the 

droop characteristic of 5% used by conventional generators. PV facility 

shall have controls that provide both for down-regulation and upregulation. PV technologies, in combination with energy storage systems 

such as, but not limited to BESS, flywheels and hybrid systems are 

acceptable options to comply with PREPA’s frequency response and 

regulation requirements. 

For small frequency deviations (for example less than 0.3 Hz), the PV 

facility response shall be proportional to the frequency deviation, based 

on the specified 5% droop characteristic. The frequency response dead 

band shall not exceed 0.02%. For large frequency deviations (for example 

in excess of 0.3 Hz), the PV facility shall provide an immediate real power 

primary frequency response of at least 10% of the maximum AC active 

power capacity (established in the contract). The time response (full 10% 

frequency response) shall be less than 1 second. 

If energy storage systems are utilized to comply with the frequency 

regulation requirements, and during a disturbance the system frequency 

stays below 59.7 Hz, the facility frequency response shall be maintained 

for at least 9 minutes. After the ninth minute the real power primary 

frequency response shall not decrease at a ramp rate higher than 10% of 

the maximum AC active power capacity per minute. 

The operational range of the frequency response and regulation system 

shall be from 10% to 100% of the maximum AC active power capacity 

(established in the contract). The PV facility power output at the POI 

shall never exceed the maximum AC active power (established in the 

contract). 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Photovoltaic Generation (PV) Projects 

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6

7. RAMP RATE CONTROL:
�

Ramp Rate Control is required to smoothly transition from one output 

level to another. The PV facility shall be able to control the rate of change 

of power output during some circumstances, including but not limited to: 

(1) rate of increase of power, (2) rate of decrease of power, (3) rate of 

increase of power when a curtailment of power output is released; (4) 

rate of decrease in power when curtailment limit is engaged. A 10 % per 

minute rate (based on AC contracted capacity) limitation shall be 

enforced. This ramp rate limit applies both to the increase and decrease 

of power output and is independent of meteorological conditions. The 

ramp rate control tolerance shall be +10%. 

8. POWER QUALITY REQUIREMENTS: 

The developer shall address, in the design of their facilities potential 

sources and mitigation of power quality degradation prior to 

interconnection. Design considerations should include applicable 

standards including, but not limited to IEEE Standards 142, 519, 1100, 

1159, and ANSI C84.1. Typical forms of power quality degradation 

include, but are not limited to voltage regulation, voltage unbalance, 

harmonic distortion, flicker, voltage sags/interruptions and transients. 

9. SPECIAL PROTECTION SCHEMES: 

PV facility shall provide adequate technology and implement special 

protection schemes as established by PREPA in coordination with power 

management requirements. 

10. GENERAL INTERCONNECTION SUBSTATION 

CONFIGURATION: 

An interconnecting generation producer must interconnect at an existing 

PREPA switchyard. The configuration requirements of the 

interconnection depend on where the physical interconnection is to 

occur and the performance of the system with the proposed 

interconnection. The interconnection must conform, at a minimum, to 

the original designed configuration of the switchyard. PREPA, at its sole 

discretion, may consider different configurations due to physical 

limitations at the site. 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Photovoltaic Generation (PV) Projects 

52
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

7

11. MODELING AND VALIDATION 

The Contractor shall submit to PREPA a Siemens - PTI certified PSS/E 

mathematical model and data related to the proposed PV facility. When 

referred to the PV facility model, this shall include but is not limited to PV 

inverters, transformers, collector systems, plant controllers, control 

systems and any other equipment necessary to properly model the PV 

facility for both steady-state and dynamic simulation modules. It is 

required that the Contractor submits both an aggregate and detailed 

version of the PV facility model. At a later stage in the process, it is also 

required that the Contractor submits as-built PSS/E mathematical models 

of the PV Facility. 

The Contractor shall be required to submit user manuals for both the PV 

inverter and the PV facility models including a complete and detailed 

description of the voltage regulation system (VRS) and frequency 

regulation system model implementation. The mathematical models 

shall be fully compatible with the latest and future versions of PSS/E. It is 

preferred that the models are PSS/E standard models. In the case that 

the Contractor submits user written models, the Contractor shall be 

required to keep these models current with the future versions of the 

PSS/E program until such time that PSS/E has implemented a standard 

model. The Contractor shall submit to PREPA an official report from 

Siemens - PTI that validates and certifies the required mathematical 

models, including subsequent revisions. The data and PSS/E model shall 

also be updated and officially certified according to PREPA requirements 

when final field adjustments and parameters measurements and field 

tests are performed to the facility by the contractor. The mathematical 

model (either PSS/E standard or user written model) of the PV facility 

shall be officially certified by Siemens - PTI before a specific and validated 

PSS/E mathematical model of the complete PV facility be submitted to 

PREPA. The Contractor shall be responsible of submitting the official 

reports and certifications from Siemens – PTI, otherwise the 

mathematical model shall not be considered valid. 

The Contractor shall be responsible to submit Siemens – PTI certified 

PSSE mathematical models of any kind of compensation devices (ie. SVC, 

STATCOMs, DSTATCOMs, BESS, etc.) used on the PV facility. It is 

preferred that the models are standard models provided with PSS/E. In 

the case that the Contractor submits user written models, the PV facility 

Contractor shall be required to keep these models current with the 

future versions of the PSS/E program until such time that PSS/E has 

implemented a standard model. In its final form, the mathematical 

model shall be able to simulate each of the required control and 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Photovoltaic Generation (PV) Projects 

53
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

8

operational modes available for the compensation device and shall be 

compatible with the latest and future versions of PSSE. Final adjustments 

and parameters settings related with the control system commissioning 

process shall be incorporated to the PSSE mathematical model and tested 

accordingly by the PV facility Contractor and PREPA system study groups. 

The Contractor shall also perform on-site field tests for the identification, 

development, and validation of the dynamic mathematical models and 

parameters required by PREPA for any kind of compensation devices 

used at the PV facility. The mathematical models of the PV facility and its 

required compensation devices shall be officially certified by Siemens -

PTI before a specific and validated PSS/E mathematical model of the 

complete PV facility be submitted to PREPA. The Contractor shall be 

responsible of submitting the official reports and certifications from 

Siemens – PTI, otherwise the mathematical models shall not be 

considered valid. 

PV facility Owners that provide user written model(s) shall provide 

compiled code of the model and are responsible to maintain the user 

written model compatible with current and new releases of PSS/E until 

such time a standard model is provided. PREPA must be permitted by the 

PV facility Owner to make available PV Facility models if required to 

external consultants with an NDA in place. 

12. TRANSIENT MATHEMATICAL MODEL 

The Contractor shall be responsible of providing a detailed transient 

model of the PV facility and to show that it is capable of complying with 

PREPA’s transient Minimum Technical Requirements. 

13. DYNAMIC SYSTEM MONITORING EQUIPMENT 

The developer of the PV facility shall be required to provide and install a 

dynamic system monitoring equipment that conforms to PREPA’s 

specifications. 

Puerto Rico Electric Power Authority 

Minimum Technical Requirements for Photovoltaic Generation (PV) Projects 

54
Appendix D: Minimum Technical Requirements of 

Interconnection of Wind Facilities (ver. Aug. 15, 2012) 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 55 

at www.nrel.gov/publications. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1

MINIMUM TECHNICAL REQUIREMENTS FOR INTERCONNECTION OF WIND 

TURBINE GENERATION (WTG) PROJECTS 

The proponent shall comply with the following minimum technical requirements: 

1.	�VOLTAGE RIDE-THROUGH 

Figure 1 Voltage Ride-Through Requirements 

a.	
 PREPA’s Low Voltage Ride-Through (LVRT) Requirements: 

i.	
 From Figure 1, PREPA requires all generation to remain online and 

be able to ride-through three phase and single phase faults down 

to 0.0 per-unit (measured on the point of interconnection), for up 

to 600 ms. 

ii.	
 All generation remains online and operating during and after 

normally cleared faults on the point of interconnection. 

iii.	
 All generation remains online and operating during backupcleared faults on the point of interconnection. 

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iv.	
 During the low voltage fault conditions, the wind generation 

facility shall operate on reactive current injection mode. This 

mode of operation shall be implemented with a reactive current 

droop characteristic which shall have an adjustable slope from 1 

to 5%. A dead band of 15 % is required. 

b.	
 PREPA’s Overvoltage Ride-Through (OVRT) Requirements: 

i.	
 PREPA requires all generation to remain online and able to ridethrough symmetrical and asymmetrical overvoltage conditions 

specified by the following values illustrated in Figure 1: 

Overvoltage (pu) Minimum time to remain online 

1.4 – 1.3 150 ms 

1.3– 1.25 1 s 

1.25 – 1.15 3 s 

1.15 or lower Indefinitely 

2.	� VOLTAGE REGULATION SYSTEM (VRS) 

Constant voltage control shall be required. Wind Turbine Generation 

(WTG) technologies in combination with Static Var Controls, such as 

Static Var Compensators (SVC), STATCOMs and DSTATCOMs are 

acceptable options to comply with this requirement. A complete 

description of the VRS control strategy shall be submitted for evaluation. 

a)	
 Wind Generation Facilities (WGF) must have a continuously-variable, 

continuously-acting, closed loop control VRS; i.e. an equivalent to the 

Automatic Voltage Regulator in conventional machines. 

b)	
 The VRS set-point shall be adjustable between 95% to 105% of rated 

voltage at the POI. The VRS set-point must also be adjustable by 

PREPA’s Energy Control Center via SCADA. 

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c)	
 The VRS shall operate only in a voltage set point control mode. 

Controllers such as Power Factor or constant VAR are not permitted. 

d)	
 The VRS controller regulation strategy shall be based on proportional 

plus integral (PI) control actions with parallel reactive droop 

compensation. The VRS Droop shall be adjustable from 0 to 10%. 

e)	
 At zero percent (0%) droop, the VRS shall achieve a steady-state 

voltage regulation accuracy of +/- 0.5% of the controlled voltage at 

the POI. 

f)	
 The VRS shall be calibrated such that a change in reactive power will 

achieve 95% of its final value no later than 1 second following a step 

change in voltage. The change in reactive power should not cause 

excessive voltage excursions or overshoot. 

g)	
 The generator facility VRS must be in service at any time the WGF is 

electrically connected to the grid regardless of MW output from the 

WGF. 

h)	
 The VRS dead band shall not exceed 0.1%. 

Examples of possible VRS configurations: 

The following examples serve as possible solutions for REF voltage 

regulation and reactive power compensation. 

Figure 2 Voltage Regulation at WGF 

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ation on a multiple collector bus injection at collector bus

 

 

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Figure 3 Voltage Regulation at single collector bus 

Figure 4 Voltage Regulation on a multiple collector bus injection at collector bus 

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Figure 5 Shared Voltage Regulation for multiple WGF at transmission system bus 

3.	�REACTIVE POWER CAPABILITY AND MINIMUM POWER FACTOR 

REQUIREMENTS 

The total power factor range shall be from 0.85 lagging to 0.85 leading at the 

point of interconnection (POI). The reactive power requirements provide 

flexibility for many types of technologies at the Renewable Energy Facility. The 

intent is that a WGF can ramp the reactive power from 0.85 lagging to 0.85 

leading in a smooth continuous fashion at the POI. 

The +/- 0.90 power factor range should be dynamic and continuous at the point 

of interconnection (POI). This means that the WGF has to be able to respond to 

power system voltage fluctuations by continuously varying the reactive output of 

the plant within the specified limits. The previously established power factor 

dynamic range could be expanded if studies indicate that additional continuous, 

dynamic compensation is required. It is required that the WGF reactive 

capability meets +/- 0.85 Power Factor (PF) range based on the WGF Aggregated 

MW Output, which is the maximum MVAr capability corresponding to maximum 

MW Output. It is understood that positive (+) PF is where the WGF is producing 

MVAr and negative (-) PF is where the WGF is absorbing MVAr. 

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This requirement of MVAr capability at maximum output shall be sustained



throughout the complete range of operation of the WGF as established by figure 6.



Figure 6 Reactive Power Capability Curve 

4. SHORT CIRCUIT RATIO (SCR) REQUIREMENTS: 

Short Circuit Ratio values (System Short Circuit MVA at POI/WGF MVA Capacity) 

under 5 shall not be permitted. The constructor shall be responsible for the 

installation of additional equipment, such as synchronous condensers, and 

controls necessary to comply with PREPA’s minimum short circuit requirements. 

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5. FREQUENCY RIDE THROUGH (FRT):
�

• 57.5 - 61.5 Hz No tripping (continuous) 

• 61.5 - 62.5 Hz 30 sec 

• 56.5 - 57.5 Hz 10 sec 

• < 56.5 or > 62.5 Hz Instantaneous trip 

6. FREQUENCY RESPONSE/REGULATION AND INERTIAL RESPONSE 

WTG facility shall provide an immediate real power primary frequency response, 

proportional to frequency deviations from scheduled frequency, similar to 

governor response. The rate of real power response to frequency deviations 

shall be similar to or more responsive than the droop characteristic of 5% used 

by conventional generators. WTG facility shall have controls that provide both 

down-regulation and up-regulation. Wind turbine technologies, in combination 

with energy storage systems such as BESS, flywheels and hybrid systems are 

acceptable options to comply with PREPA’s frequency regulation requirements. 

For small frequency deviations (for example less than 0.3 Hz), the WGF response 

shall be proportional to the frequency deviation, based on the specified 5% 

droop characteristic. The frequency response dead band shall not exceed 0.02%. 

For large frequency deviations (for example in excess of 0.3 Hz), the WGF shall 

provide an immediate real power primary frequency response of at least 10% of 

the maximum AC active power capacity (established in the contract). The time 

response (full 10% frequency response) shall be less than 1 second. 

If energy storage systems are utilized to comply with the frequency regulation 

requirements, and during a disturbance the system frequency stays below 59.7 

Hz, the facility frequency response shall be maintained for at least 9 minutes. 

After the ninth minute the real power primary frequency response shall not 

decrease at a ramp rate higher than 10% of the maximum AC active power 

capacity per minute. 

The operational range of the frequency response and regulation system shall be 

from 10% to 100% of the maximum AC active power capacity (established in the 

contract). The WGF power output at the POI shall never exceed the maximum 

AC active power (established in the contract). 

The WTG controls shall have the capability to provide inertial response. 

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7. RAMP RATE CONTROL
�

Ramp Rate Control is required to smoothly transition from one output level to 

another. The WTG facility shall be able to control the rate of change of power 

output during some circumstances, including but not limited to: (1) rate of 

increase of power (2) rate of decrease of power, (3) rate of increase of power 

when a curtailment of power output is released; (4) rate of decrease in power 

when curtailment limit is engaged. A 10 % per minute rate (based on nameplate 

capacity) limitation shall be enforced. This limit applies both to the increase and 

decrease of power output and is independent of meteorological conditions. The 

ramp rate control tolerance shall be +10%. 

8. POWER QUALITY 

The developer shall address, in the design of their facilities potential sources and 

mitigation of power quality degradation prior to interconnection. Design 

considerations should include applicable standards including, but not limited to 

IEEE Standards 142, 519, 1100, 1159, ANSI C84.1, IEC 61400-21, IEC 61000-3-7 

and IEC 61000-3-6. Typical forms of power quality degradation include, but are 

not limited to voltage regulation, voltage unbalance, harmonic distortion, flicker, 

voltage sags/interruptions and transients. 

The developer shall submit the Power Quality Tests Result Report of the wind 

turbines as described in the IEC 61400-21 standard. This report includes: general 

wind turbine data, wind turbine rated data at terminals, voltage fluctuations 

coefficients (flicker coefficients), current harmonics components, current 

interharmonics components, current high frequency components, response to 

voltage drops, active power data, reactive power data, grid protection data and 

reconnection time. The wind turbines shall not exceed the flicker emission limits 

established by the IEC 61000-3-7 standard and the harmonics emission limits of 

IEC 61000-3-6. 

9. WIND POWER MANAGEMENT 

WTG facility shall provide adequate technology (communicating technology and 

the corresponding control equipment) and implement wind power management 

requirements (ramp rate limits, output limits, curtailment) as established by 

PREPA. 

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10. SPECIAL PROTECTION SCHEMES 

WTG facility shall provide adequate technology and implement special 

protection schemes as established by PREPA in coordination with wind power 

management requirements. 

11. WIND GENERATION FORECASTING SYSTEMS 

WTG facility shall provide adequate technology to support wind generation 

forecasting systems (short term and day-ahead) as established by PREPA. 

Individual turbine’s availability shall be included. 

12. GENERAL INTERCONNECTION SUBSTATION CONFIGURATION 

An interconnecting generation producer must interconnect at an existing PREPA 

switchyard. The configuration requirements of the interconnection depend on 

where the physical interconnection is to occur and the performance of the 

system with the proposed interconnection. The interconnection must conform, 

at a minimum, to the original designed configuration of the switchyard. PREPA, 

at its sole discretion, may consider different configurations due to physical 

limitations at the site. 

13. MODELING AND VALIDATION 

The Contractor shall submit to PREPA a Siemens - PTI certified PSS/E 

mathematical model and data related to the proposed WTG facility. When 

referred to the WTG facility model, this shall include but is not limited to wind 

generator, transformers, collector systems, plant controllers, control systems 

and any other equipment necessary to properly model the WTG facility for both 

steady-state and dynamic simulation modules. It is required that the Contractor 

submits both an aggregate and detailed model of the WTG facility model. At a 

later stage in the process, it is also required that the Contractor submits as-built 

PSS/E mathematical models of the WTG facility. 

The Contractor shall be required to submit user manuals for both the Wind 

Turbine Generator and WTG Facility models including a complete and detailed 

description of the voltage regulation system (VRS) and frequency regulation 

system model implementation. The mathematical models shall be fully 

compatible with the latest and future versions of PSS/E. It is preferred that the 

models are PSS/E standard models. In the case that the Contractor submits user 

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written models, the Contractor shall be required to keep these models, as well as 

its corresponding user manual, current with the future versions of the PSS/E 

program until such time that PSS/E has implemented a standard model. The 

Contractor shall submit to PREPA an official report from Siemens - PTI that 

validates and certifies the required mathematical models, including subsequent 

revisions. The data and PSS/E model shall also be updated and officially certified 

according to PREPA requirements when final field adjustments and parameters 

measurements and field tests are performed to the facility by the contractor. 

The mathematical model (either PSS/E standard or user written model) of the 

WTG facility shall be officially certified by Siemens - PTI before a specific and 

validated PSS/E mathematical model of the complete WTG facility be submitted 

to PREPA. The Contractor shall be responsible of submitting the official reports 

and certifications from Siemens – PTI, otherwise the mathematical model shall 

not be considered valid. 

The Contractor shall be responsible to submit Siemens – PTI certified PSSE 

mathematical models of any kind of compensation devices (ie. SVC, STATCOMs, 

DSTATCOMs, BESS, etc.) used on the WTG facility. It is preferred that the models 

are standard models provided with PSS/E. In the case that the Contractor 

submits user written models, the WTG facility Contractor shall be required to 

keep these models current with the future versions of the PSS/E program until 

such time that PSS/E has implemented a standard model. In its final form, the 

mathematical model shall be able to simulate each of the required control and 

operational modes available for the compensation device and shall be 

compatible with the latest and future versions of PSSE. Final adjustments and 

parameters settings related with the control system commissioning process shall 

be incorporated to the PSSE mathematical model and tested accordingly by the 

WTG facility Contractor and PREPA system study groups. The Contractor shall 

also perform on-site field tests for the identification, development, and 

validation of the dynamic mathematical models and parameters required by 

PREPA for any kind of compensation devices used at the WTG facility. The 

mathematical models of the WTG facility and its required compensation devices 

shall be officially certified by Siemens - PTI before a specific and validated PSS/E 

mathematical model of the complete WTG facility be submitted to PREPA. The 

Contractor shall be responsible of submitting the official reports and 

certifications from Siemens – PTI, otherwise the mathematical models shall not 

be considered valid. 

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WTG facility Owners that provide user written model(s) shall provide compiled 

code of the model and are responsible to maintain the user written model 

compatible with current and new releases of PSS/E until such time a standard 

model is provided. PREPA must be permitted by the WGF Owner to make 

available WGF models if required to external consultants with an NDA in place. 

14. TRANSIENT MATHEMATICAL MODEL 

The contractor shall be responsible of providing a detailed transient model of the 

WTG facility and to show that it is capable of complying with PREPA’s transient 

Minimum Technical Requirements. 

15. DYNAMIC SYSTEM MONITORING EQUIPMENT 

The developer of the Renewable Energy Facility shall be required to provide and 

install a dynamic system monitoring equipment that conforms to PREPA’s 

specifications. 

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