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.

Contract No. DE-AC36-08GO28308

Grid Simulator for Testing a 

Wind Turbine on Offshore 

Floating Platform 

V. Gevorgian

Technical Report

NREL/TP-5500-53813

February 2012 
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.

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Grid Simulator for Testing a 

Wind Turbine on Offshore 

Floating Platform 

V. Gevorgian

Prepared under Task No. WTY7.1000

Technical Report

NREL/TP-5500-53813

February 2012
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

owned rights. Reference herein to any specific commercial product, process, or service by trade name, 

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 

or favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy 

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/help/ordermethods.aspx

Cover Photos: (left to right) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste. 
3 

Contents

1. Introduction.................................................................................................................................. 4

2. Grid Simulator Design Challenges .......................................................................................... 5

3. V27 Wind Turbine Characteristics ........................................................................................... 5

4. Overview of Grid Simulator Topologies .................................................................................. 9

4.1 Battery/inverter topology............................................................................................................. 9

4.2 Diesel generator topology ........................................................................................................ 11

4.3 Diesel generator/synchronous condenser topology............................................................. 12

5. Transient Modeling of Grid Simulator Topologies............................................................... 13

5.1 Transient modeling of battery/inverter option........................................................................ 13

5.2 Transient modeling of diesel generator option...................................................................... 17

5.3 Synchronous condenser option............................................................................................... 19

5.4 Transient simulation conclusions ............................................................................................ 20

6. Continuous Steady State Simulations for Inverter/Battery Topology ...............................21

6.1 Inverter efficiency....................................................................................................................... 21

6.2 Battery models ........................................................................................................................... 21

6.3 Wind turbine steady-state model............................................................................................. 22

6.4 Battery Simulation Results ....................................................................................................... 23

Appendix A........................................................................................................................................ 27

A.1 Technical Specifications........................................................................................................... 27

A.2 One-line Electrical Diagram..................................................................................................... 29
4 

1. Introduction

The DeepCwind Consortium's mission is to establish the State of Maine as a national leader in 

deepwater offshore wind technology through a research initiative funded by the U.S. Department of 

Energy, the National Science Foundation, and others. The University of Maine-led consortium includes 

universities, nonprofits, and utilities; a wide range of industry leaders in offshore design, offshore 

construction, and marine structures manufacturing; firms with expertise in wind project siting, 

environmental analysis, environmental law, composites materials to assist in corrosion-resistant material 

design and selection, and energy investment; and industry organizations to assist with education and tech 

transfer activities. One of the tasks in the DeepCwind research is to develop a complete design of a 

floating turbine platform, capable of supporting a wind turbine rated for up to 250 kW for deployment at 

the University of Maine Deepwater Offshore Wind Test Bed. 

An important aspect of such offshore testing of a wind turbine floating platform is electrical loading of 

the wind turbine generator. An option of interconnecting the floating wind turbine with the onshore grid 

via submarine power cable is limited by many factors such as costs and associated environmental aspects 

(i.e., an expensive and lengthy sea floor study is needed for cable routing, burial, etc). It appears to be a 

more cost effective solution to implement a standalone grid simulator on a floating platform itself for 

electrical loading of the test wind turbine. Such a grid simulator must create a stable fault-resilient 

voltage and frequency bus (a micro grid) for continuous operation of the test wind turbine. Such an 

onboard grid simulator has its own technical and design challenges due to space, weight, and 

environmental limitations. 

In March 2011, the National Renewable Energy Laboratory (NREL) entered into contractual agreement 

with University of Maine (TSA-22-361) to perform a modeling task and develop a design specification 

for such a grid simulator for operating a V-27 225-kW wind turbine to be installed on a floating platform 

off the coast of Maine. 

Authors would like to thank Steve Drouilhet of Sustainable Power Systems, LLC for his valuable input to

this work. 
5 

2. Grid Simulator Design Challenges

The University of Maine proposed deployment of a V-27 floating wind turbine to meet the primary 

objective of testing the one-third scale floating system, and to obtain motion and structural response data 

to compare and validate numerical models developed by NREL that predict how turbines and supporting 

platforms would perform under various meteorological and oceanographic conditions. To achieve these 

research goals, the wind turbine electrical loading must be provided to replicate characteristics of the 

onshore power grid under varying wind speed conditions. Such characteristics include amplitude and 

frequency of voltage on turbine terminals, as well as power quality issues. 

In such a standalone offshore application, there are essentially two main grid-simulator configuration 

options that can be considered:

• Battery and inverter-based grid simulator topology 

• Diesel generator-based grid simulator topology 

Both topologies are possible from an engineering point of view. The main challenge is to select a 

topology that provides stable, unattended turbine operation during the whole deployment period at 

minimum hardware costs and least weights. Weight minimization is of most importance since heavy 

hardware will affect weight distributions, and therefore the dynamic performance of the floating wind 

turbine platform. 

A key condition for stable operation of any power system is to maintain a constant balance between 

generation and load. In this standalone floating mode of operation, continuous load matching is necessary 

to dissipate the real power produced by the wind turbine at any wind speed. This condition sets a 

requirement for using a controllable load bank as part of the grid simulator setup. Such a load bank must 

be part of the system independent of the grid simulator electrical topology (battery/inverter or diesel). The 

difference is that in diesel topology the load bank, along with the diesel generator governor controller, are 

the primary means of maintaining voltage frequency (unlike the battery/inverter system, where grid 

frequency is set by the inverter controls). Another important aspect is the capability of the system to 

provide a sufficient amount of reactive power for voltage support and transient stability of the system. 

There are significant technical, cost, and operational differences in reactive power and frequency support 

approaches that are specific to battery/inverter or diesel topologies. 

3. V27 Wind Turbine Characteristics

Selection of V-27 wind turbine was based on the needs of the testing program, including the following: 

power control method (variable pitching or stall controlled blades), lead time, costs, suitability for use on 

a one-third scale platform (i.e., mass, geometry, and power output), structural capacity, availability of 

design information for numerical modeling, and the ability to publish turbine information as part of 

research publications. 

The Vestas V-27 wind turbine has been selected by the University of Maine for the testing program and 

the analysis presented in this report, and is based on specifications of the onshore Vestas V-27. The 
6 

Vestas V-27 turbine has a generating capacity of 225 kilowatts. 

The turbine rotor diameter is 88.6 feet, and the tower height would 

be approximately 100 feet for a maximum height of approximately 

144 feet for the proposed project. The turbine is designed with a 

wind cut-in speed of 8 miles-per–hour, and when the wind speed 

exceeds 55 miles per hour, the turbine automatically cuts out by 

slowing the rotor speed using the control system and feathering 

the turbine blades. The turbine also includes an emergency disk 

brake. The turbine is designed to withstand wind gusts up to 120 

miles-per-hour. The maximum rotational speed of the turbine rotor 

is 44 revolutions-per-minute.

The V-27 wind turbine is a pitch-regulated constant-rotational 

speed wind turbine. The blade pitch angles are controlled 

continuously to provide optimum power production. The active 

pitch brings additional operational benefits, such as:

• Output power limited at 225 kW;

• No need for motoring starts;

• Generator cut-in at synchronous speeds;

• Possibility of stopping the turbine without engaging the mechanical brakes. 

The turbine power (for sea level) and aerodynamic power coefficient Cp curves are shown in Figure 2. 

Figure 2: V-27 power and Cp curves

The electrical topology of the V-27 (480 VAC/60Hz) wind turbine is based on a two-winding squirrelcage induction generator. Each winding uses a different number of poles, so two-speed operation becomes 

possible. The electrical characteristics of the generator are shown in Table 1: 

Figure 1: Vestas V-27 wind turbine
7 

Table 1: Generator electrical characteristics

Winding 2 Winding 1

Number of poles 6 8 

Synchronous speed (rpm) 1200 900

Rated speed (rpm) 1209 906

Winding configuration Δ Δ

Power rating (kW) 225 50

Full load current (A) 335 85

No load current (A) 143 49

 

The wind turbine uses a thryristor-based soft starter for in-rush current limiting during generator start ups. 

Two sets of capacitor banks are used for power factor correction (PFC). The PFC capacitors are switched 

off during start up transients when the soft starter is active. The reactive power / power factor 

characteristics for the generator and the total wind turbine power factors are show in Table 2: 

Table 2: Reactive power / power factor characteristics

Winding 2 Winding 1

Generator power factor:

100% load

75% load

50% load

25% load

0.81

0.75

0.65

0.41

0.71

0.65

0.54

0.30

Generator reactive power, no 

load (kVAR)

118.9 40.7

PFC capacitor rating (kVAR) 112.5 37.5

Total wind turbine power factor:

100% load

75% load

50% load

25% load

0.98

0.98

0.98

0.96

0.97

0.99

1.00

0.98

Total wind turbine current at full 

load current (A) 277 62

Calculated steady-state power vs. rpm characteristics of V-27 turbine are shown in Figure 3: 
8 

Figure 3: V-27 steady state operation

The general layout of V-27 components is shown in Figure 4. A two-stage 443-kW gearbox is used in the 

V-27 wind turbine to step up the wind rotor speed. There are two 0.66-kW motors used in turbine’s yaw 

system. A disk brake unit is capable of emergency stopping of the wind turbine in case of grid loss. 

Figure 4: V-27 major components
9 

4. Overview of Grid Simulator Topologies

It was pointed out earlier in this document that two main grid-simulator configurations are the inverter/ 

battery and diesel generator-based solutions. The third possible option is a modification of the diesel 

generator option that uses a synchronous condenser for voltage control. This solution also requires a 

controlled load bank for frequency regulation. Functional and control aspects of each topology are 

described in this section.

4.1 Battery/inverter topology 

Latest advances in energy storage technologies led to lower cost and improved performance battery 

solutions designed specifically for renewable energy and electric vehicle applications. The improvements 

include higher energy capacities at lower weights, faster charge/discharge rates, and increased reliability 

and battery life. Battery technologies such as advanced lead-acid, lithium, Ni-Cd and others are being 

adapted in many renewable energy projects including operations with wind generation. The main 

application of such batteries in wind related projects is a short-term balancing of wind power variability, 

participation in frequency regulation, and providing voltage support. All these functions are also made 

possible by employing advances in the inverter technologies. 

A general diagram of a possible battery/inverter solution for a floating platform grid simulator is shown in 

Figure 5. It consists of a battery bank and voltage source grid-forming inverter. The main purpose of the 

inverter is to set both the voltage and frequency in the system’s 480VAC/60Hz bus, making possible for

the wind turbine to operate. The V-27 wind turbine is represented by two generators (equivalent of 2-

winding topology) with individual capacitor banks for power factor correction, and a soft starter. 

Figure 5: General diagram of battery/inverter option

Platform secondary loads and the instrumentation battery charging system are separate from the grid 

simulator battery and inverter. 
10

The dynamic operation and control of the grid simulator is possible by means of employing a fastresponse resistive-load bank. Such load banks with controllers have been used in many wind-diesel and 

mini-hydro power systems for load and frequency controls. Element switching in such load banks can be 

achieved using several methods. It appears that element switching using a thyristor solid-state (SSR) zeroswitching technique is the most feasible, especially in a standalone offshore application. All SSR 

switching is done at zero voltage, which eliminates the current waveform distortion and the poor power 

factor associated with phase-controlled SCR power controllers. In SSR zero-switching controllers, all of 

the thyristors can be directly bonded to integral heat sinks rated to handle their respective electrical 

loading. By switching on various combinations of the heating elements, any power level from zero to full 

load may be achieved, with a resolution close to 1% of full load. Also, element switching can be updated 

at fast rates, allowing smooth continuous control of electric power.

Such smooth and fast power control is essential for reliable continuous operation of the battery/inverterbased grid simulator. The electrical power flowing to/from the battery bank is essentially the difference 

between the turbine power production and the ability of the load bank to absorb the same amount of 

power. Ideally, there will not be any power flow into/out of the battery (except for to compensate for 

losses) if load bank consumption is fully matched with turbine power production. Also, power flow to 

battery (battery charging) can be controlled by adjusting the load bank power at any desired level. 

The inverter system shown in Figure 5 has a harmonic filter and line reactor for improved voltage/current 

waveforms. An isolation transformer can also be used instead of a line reactor to eliminate possible 

ground loops, create a local ground-bonded neutral, and reduce harmonic currents. 

Another important function of the inverter is to allow battery charging from an external power source 

such as portable diesel generator that can be transported to the platform on a service boat. This would be 

for situations when battery bank is completely discharged due to contingencies. 

The inverter power rating must be sufficient to provide the reactive power needed by the wind turbine 

generator during start-up transients and switching between generator windings.

The grid simulator supervisory controller dispatches the overall system operation and performs the 

following tasks:

• Determines the required amount of power sent to the load bank at any given instance in time;

• Allows / inhibits wind turbine operation as necessary;

• Determines state of the charge (SOC) of the battery bank, and determine required amount of 

power to be sent to the battery for maintaining target SOC during whole period of operation;

• Sets inverter into battery charging mode when connected to portable diesel generator;

• Shuts down the inverter to preserve battery charge during the periods of no wind, or in case of 

wind turbine or other component failure; 

• Performs data logging;

• Detects faults and provides protection; and

• Provides for remote access via wireless connection. 
11

The supervisory controllers are generally based on an industrial-type panel-mount PC computer or 

programmable logic controller (PLC). The operator interface should have flexibility to change the 

operating strategy through the variety of user-settable parameters. 

Detailed results for battery/inverter option are described in the modeling sections of this document. 

4.2 Diesel generator topology

Wind-diesel hybrid systems have been used worldwide in remote standalone power-system applications. 

Many successful wind-diesel projects were deployed in various parts of the world (including offshore 

marine applications), so both control and operational aspects of such systems are well understood. The 

dynamic behavior of a wind-diesel micro-grid in a floating offshore-platform grid- simulator application 

is similar to a typical wind-diesel hybrid system. However, it has specific challenges mainly from the 

operational and fuel supply perspectives due to its location on a floating offshore platform.

A general diagram of possible diesel generator solution for a floating platform grid-simulator is shown in 

Figure 6. It consists of a diesel generator and resistive AC load bank. The purpose of the load bank 

(similar to above described battery/inverter case) is to provide smooth load matching for wind turbine 

power. However, in this case the load bank is controlled based on measured electrical frequency rather 

than wind turbine power production.

Figure 6: General diagram of diesel generator option

Fast load bank controllability is also critical to operational stability of this system. The purpose of the 

diesel generator is to set up the voltage and frequency in the 480 VAC bus. It must run continuously 

during the whole period of wind turbine operation. The power rating of the diesel generator must be 

sufficient to provide reactive power needed by the wind turbine generator during start-up transients. The 

supervisory controller performs similar tasks as in the battery/inverter option except for battery related 

functions:

• Determines the required amount of power sent to the load bank at any given instance in time

• Allows / inhibits wind turbine operation as necessary;

• Starts and stops diesel engine;
12

• Performs data logging;

• Detects faults and provides protection; and

• Provides for remote access via wireless connection. 

The main disadvantage of this system is the need for periodic refueling of the diesel generator tank, and 

excessive weight associated with a storing large amount of fuel onboard the floating platform. 

4.3 Diesel generator/synchronous condenser topology

Continuous diesel operation can be avoided if a synchronous condenser is added to the grid simulator 

topology. Significant reduction in fuel consumption can be achieved this way also. A general diagram of 

synchronous condenser grid-simulator topology is shown in Figure 7. 

Figure 7: General diagram of diesel generator/ synchronous condenser option

The synchronous condenser allows a high level of control over the reactive power in the micro grid. It can 

generate or absorb reactive power to support the system’s voltage level (or to maintain a specified power 

factor). The synchronous condenser has its own custom controls and is started using a small induction 

motor (pony motor). First, the diesel generator is started to set up the system voltage and frequency. Then 

the synchronous condenser is brought up to speed and synchronized with the diesel generator. Upon 

synchronization, the condenser is connected to the main bus. The diesel generator can be turned off after 

turbine starts, and resistive load is controlled to maintain stable system frequency. 

The main advantage of this topology is that a smaller diesel generator with significantly reduced run-time 

can be used. Therefore, lower fuel consumption and smaller fuel tank weights can be achieved this way. 

The main disadvantages are more complex control and single speed operation. Unlike previous 

topologies, this topology won’t allow switching between large and small generator windings. Both 

voltage and electrical frequency will collapse immediately after the generator winding is disconnected 

from the main bus. 
13

5. Transient Modeling of Grid Simulator Topologies

Transient modeling results for all three grid-simulator topologies are described in this section. The 

purpose of transient modeling is to determine the power ratings of grid simulator components, identify 

potential controllability issues and ways of improving system transient performance. The most severe 

transient case for any grid simulator topology is a start up of the wind turbine using the large generator 

winding. The criteria for transient performance were based on V-27 wind turbine protection settings as 

shown in Table 3: 

Table 3: V-27 protection settings

Action Condition

Both generator and PFC will be cut off if: Voltage above 528 VAC for 60 sec

Voltage below 432 VAC for 60 sec

Frequency above 62 Hz for 0.2 sec

Frequency below 57 Hz for 0.2 sec

Only PFC will be cut off if: Voltage above 540 VAC for 0.2 sec

Only generator will be cut off if: Voltage is above 540 VAC for 0.5 sec 

All transient modeling was done using PSCAD simulation software. 

5.1 Transient modeling of battery/inverter option

The one-line diagram of the model is shown in Figure 8 (simplified version of Figure 5). The platform 

auxiliary loads are simulated by a single 3-kW (0.8 power factor) load. Wind turbine auxiliary loads 

(mainly yaw motors) are simulated by a single 2-kW induction motor. 

Figure 8: Simplified one-line diagram of battery/inverter option

Transient simulations were conducted during large generator cut in at constant wind speed. The resistive 

load bank with different response times (5, 15 and 50 ms) was controlled to match the wind turbine power 

output. It was mentioned earlier that the load bank response time is a critical parameter in providing stable 

operation and protecting battery from excessive currents. 
14

Simulation results for the battery/inverter system with fast load-bank response time (5 ms) are shown in 

Figure 9. The initial turbine RPM in this simulation is at 0.9 pu. The inverter sets 480 AC voltage and 

frequency from the beginning of the simulation. The turbine accelerates for about 1 sec until the generator 

reaches synchronous speed of 1 pu. At this moment, the soft starter becomes active and starts conducting 

current to the generator winding. A lookup table is used for soft starter firing-angle control. The firing 

angle is controlled to gradually increase generator voltage, thus limiting in-rush start-up currents. 

Figure 9: Large generator start up (load controller response – 5 ms)

Immediately after cut in, the wind turbine generator starts producing real power (labeled as Pwtg). At the 

same time, the load bank controller starts engaging the resistive elements to match the turbine power. The 

difference between wind turbine power (Pwtg) and load bank power (Pload) is the power that flows 

through the inverter into battery bank (Pinv). The peak DC current into the battery is at around 70 A.

The peak inverter apparent power (Sinv) is slightly over 350 kVA. This peak essentially determines the 

power rating of the inverter. It is important to note that inverters usually have short term over-current 

capability (for example, 150% for 3 sec, or similar). This means that there is a potential for using an 

inverter with a smaller power rating if all over-current (peak and duration) limitations are met. 
15

It also can be seen from Figure 9 that the AC bus voltage drops to about 430 V during start-up transients, 

and then quickly recovers back to 480 VAC in about 1 sec. This recovery time will be different depending 

on inverter type, implemented controls, etc. 

Figure 10: Large generator start up (load controller response - 15 ms)

A case with 15-ms load-bank controller response time is shown in Figure 10. In this case, a slower load 

bank controller creates a larger difference between Pwtg and Pload, which in turn causes a higher DC 

current spike in to the battery bank (up to 100A). The peak apparent power of the inverter is still about 

350 kVA and is not affected by the slower load bank control. 

A case with an even slower controller response (50 ms) is shown in Figure 11. In this case, there is a 

larger difference between Pwtg and Pload, so peak DC current is higher at about 150 A.
16

Figure 11: Large generator start up (load controller response - 50 ms)

For all three above cases, there was little impact of load controller response-time on peak apparent power 

(or rating) of the inverter. However, there was a significant impact on battery current. These simulations 

stress the importance of fast load controller response for performance of the battery bank. 

A more detailed view of the generator voltage and current waveforms during start up transients are shown 

in Figure 12. The effect of a soft starter on voltage magnitude, and both current and voltage waveforms 

are shown. In general, as can be seen in Figure 9, Figure 10, and Figure 11, the voltage total harmonic 

distortion (THD) is far below 5% after the start up transients are over. Even during transients, the voltage 

THD is slightly above 5%. 
17

Figure 12: Wind turbine generator voltage and current

5.2 Transient modeling of diesel generator option

The one-line diagram of the model is shown in Figure 13 (simplified version of Figure 6). As in the 

previous case, the platform and turbine auxiliary loads are simulated by a single load and single induction 

motor respectively. 

Figure 13: Simplified one-line diagram of modeled diesel generator option

Transient simulations were conducted during large generator cut in. The diesel generator sets both AC 

frequency and voltage. The resistive load bank was controlled to maintain the AC voltage frequency at 60 

Hz. A 5-ms controller response time was used in these simulations. 
18

Figure 14: Simulation results with 400-kVA diesel generator

Simulation results of a large wind turbine generator start-up for a case with a 400-kVA diesel generator 

are shown in Figure 14. The wind turbine accelerates and generator cut in takes place as soon as 

synchronous speed is reached. The turbine soft starter is activated at the beginning of the transient. The 

load bank is engaged immediately by the load bank controller trying to stabilize the electrical frequency at 

60 Hz. The voltage in 480 VAC bus first drops and then swells before it settles back to steady state, but it 

stays within allowed limits during the whole transient process. The frequency peaks at about 60.1 Hz 

before it is forced back to 60 Hz. 

Many wind turbine generator start-up simulations were conducted for different kVA ratings of the diesel 

generator. It was found that 400 kVA was the minimum diesel rating that allowed reliable start up of the 

large wind turbine generator under various conditions. An example of a start-up simulation using a 300-

kVA diesel generator is shown in Figure 15. In this case, the AC voltage exceeds protection limits of V27 wind turbine, so both generator and PFC capacitor will be cut off under such conditions. 
19

Figure 15: Simulation results with 300 kVA diesel generator

5.3 Synchronous condenser option

The one-line diagram of the model is shown in Figure 16. A 50-kVA diesel generator is used to start the 

system. A 400-kVA synchronous condenser with a 15-kW pony motor is used in the simulation. 

Figure 16: Simplified one-line diagram of modeled synchronous condenser option

Simulations were conducted or this option as well. It was found that the minimum rating of a synchronous 

condenser that allows reliable wind turbine generator starts at various conditions was 400 kVA. The 
20

simulated real and apparent power time-series for the 400-kVA synchronous condenser option is shown in 

Figure 17. Smaller kVA ratings ended up either in unstable conditions or exceeded voltage limits. 

Figure 17: Wind turbine start with 400 kVA synchronous condenser

5.4 Transient simulation conclusions

The main findings of transient simulation stage of the project are summarized below:

1. Transient simulation showed that the V-27 turbine may consume up to 350-400 kVAR of reactive 

power (peak during 1-2 sec) during large generator start up. For a battery/inverter option, a 300- 

KVA inverter seems to be an adequate match for the V-27 wind turbine, assuming it has enough 

short-term over current capability (i.e. 150% for 2-3 sec). The approximate weight of a 300-kVA 

inverter is on the order of 1500 lbs (including cabinet). Additional weight should be added if an 

isolation transformer is used instead of line reactors. The maximum power rating of the load bank 

is at about 275 kW, according to simulations. With a safety margin, it appears that a 300 to 325-

kW load bank will be sufficient for this grid simulator topology. 

2. For a diesel generator option, a 400-kVA diesel generator seems to be an adequate match for the 

V-27 wind turbine. The approximate weight of 400-kVA diesel genset is around 9,000 lbs (with 

fuel weight). The load bank must be rated for at least 350 kW in this application.

3. For a synchronous condenser option, a 400-kVA synchronous generator will provide enough 

reactive power capability for V-27 wind turbine start up. The synchronous condenser by itself 

will weigh around 4,000 lbs. In addition, a 50-kW diesel genset and 350-kW load bank will be 

needed. Only one wind turbine generator winding (single speed operation) can be used with this 

topology. 

Final comparison between the options can be done only after determining the capacity and weights of a 

battery bank for the battery/inverter grid simulator topology.
21

6. Continuous Steady State Simulations for Inverter/Battery Topology

6.1 Inverter efficiency

In the steady-state modeling case, the individual system components are modeled by their efficiencies and 

control strategies. In particular, inverter efficiency and losses are most important when modeling the longterm performance of battery/inverter option. A typical inverter efficiency curve is shown in Figure 18 and 

was used in this modeling stage.

Figure 18: Typical inverter efficiency

6.2 Battery models

The following battery types were used in the simulations:

• Advanced valve-regulated lead-acid (VRLA) batteries of 600, 300, and 100 Ah capacities; 

• Fast charge-discharge lithium-titanate batteries of 100 and 50 Ah capacities.

Examples of modeled nominal discharge characteristics for such batteries are shown in Figure 19 and 

Figure 20 respectively. 

Figure 19: Discharge characteristics of 600-Ah lead-acid batteries

0 100 200 300 400 500 600 700 800 900

500

600

700

800

Nominal Current Discharge Characteristic at 0.5C (300A)

Ampere-hour (Ah)

Voltage

Discharge curve

Nominal area

Exponential area

0 100 200 300 400 500 600 700 800 900

600

700

800

Ampere-hour (Ah)

Voltage

E0 = 742.4789, R = 0.15, K = 0.0011324, A = 58.5211, B = 0.1

100 A

200 A

300 A
22

Figure 20: Discharge characteristic of 50-Ah lithium batteries

The steady state modeling was performed in the Matlab/Simulink environment. Generic Simulink battery 

models were used in the simulations. The main parameters used in these battery models are:

• Nominal voltage;

• Rated capacity;

• Initial state of charge (SOC);

• Fully charged voltage;

• Internal resistance;

• Exponential zone.

6.3 Wind turbine steady-state model

The V-27 turbine was modeled using its power curve as shown in Figure 2. Some results from transient 

simulations were used as an input for steady-state simulation. In particular, it was observed in transient 

simulations that up to 150-A short-term DC current spikes (worst case) will go into the battery during 

start-up transients due to load-bank control delay. The corresponding real power spikes were introduced 

into a steady-state model every time the wind turbine was starting up. 

Also, a wind turbine may occasionally go into the transitional motoring/generating mode during the 

period of low winds. If motoring, the wind turbine will consume real power from the battery bank. The 

periods of motoring operation can be minimized by adjusting the appropriate settings in the V-27 wind 

turbine controller. In this simulation, the turbine control was programmed to minimize the motoring 

possibility and shut down the turbine after 60 seconds of operating in such a transition between generating 

and motoring. Also, grid simulator control was allowed to shut down the inverter during periods when the 

wind turbine is off to avoid parasitic losses. The inverter is turned back on only if average wind speed 

within the last 10 minutes is above a certain threshold, for example 7 m/s. The simulated 1-sec real power 

time-series for V-27 wind turbine is shown in 

0 20 40 60 80 100 120 140 160 180

500

600

700

800

Nominal Current Discharge Characteristic at 3C (300A)

Ampere-hour (Ah)

Voltage

Discharge curve

Nominal area

Exponential area

0 20 40 60 80 100 120 140 160 180

600

700

800

Ampere-hour (Ah)

Voltage

E0 = 745.3359, R = 0.15, K = 0.014168, A = 83.6641, B = 0.6

100 A

200 A

300 A
23

Figure 21 for 1 week of operation. 

Figure 21: Simulated 1-sec wind power time series (1 week simulation period)

6.4 Battery Simulation Results

Simulation results for a VRLA battery bank for three different capacities (600, 300, and 100 Ah) are 

shown in Figure 22, Figure 23, and Figure 24 respectively. The secondary load controller adjusted power 

to the battery bank to maintain 70% SOC. The spikes in battery voltage were caused by current spikes 

during turbine start ups due to load-bank controller delay. The fully charged voltage for the selected 

battery configuration is 756 V (672-V nominal voltage). 

0 20 40 60 80 100 120 140 160 180

-100

0

100

200

300

Time (Hr)

Wind Power (kW)

0 20 40 60 80 100 120 140 160 180

-100

0

100

200

300

Time (Hr)

Wind Power (kW)
24

Figure 22: SOC for 600-Ah VRLA Battery

Figure 23: SOC for 300-Ah VRLA Battery

0 20 40 60 80 100 120 140 160 180

69.86

69.88

69.9

69.92

69.94

69.96

69.98

70

70.02

Time (Hr)

SOC(%)

0 20 40 60 80 100 120 140 160 180

69.75

69.8

69.85

69.9

69.95

70

70.05

Time (Hr)

SOC(%)
25

Figure 24: SOC for 100-Ah VRLA battery

The secondary load controller was able to control the battery SOC close to the target level during the 

whole period of operation. The modeling results showed that battery voltage was exceeding acceptable 

limits for the 100-Ah lead-acid battery during most of the simulation period. Both 600-Ah and 300-Ah 

batteries showed acceptable voltage performance. However, the 300-Ah battery’s performance was at the 

margin of voltage limits. So, the 300-Ah capacity lead-acid battery is not recommended. The total weight 

of a 600-AH VRLA battery system is estimated at 37,000 lbs. 

Modeling results for 100 and 50-Ah lithium-titanate batteries are shown in Figure 25 and Figure 26. A 

50-Ah battery string (644-V nominal voltage) was used in the modeling. Two parallel strings were used 

for 100-Ah case. The fully charged voltage is 784 V. Both 100 and 50-Ah strings showed acceptable 

voltage performance. Based on lithium-titanate battery simulation results, a 50-Ah lithium battery is 

recommended for the grid simulator application. However, it is also recommended to have room for a 

second 50-Ah string as a precaution. The total weight of 50-Ah lithitum-titanate battery bank is estimated 

at 2,200 lbs (4,400 lbs for two parallel strings). 

0 20 40 60 80 100 120 140 160 180 69.4

69.5

69.6

69.7

69.8

69.9

70

70.1

70.2

Time (Hr)

SOC(%)
26

Figure 25: SOC and voltage for 100-Ah lithium battery

Figure 26: SOC and voltage for 50-Ah lithium battery

0 20 40 60 80 100 120 140 160 180

68.5

69

69.5

70

70.5

Time (Hr)

SOC(%)

0 20 40 60 80 100 120 140 160 180

700

710

720

730

740

Time (Hr)

Battery Voltage(V)

0 20 40 60 80 100 120 140 160 180

67

68

69

70

71

Time (Hr)

SOC(%)

0 20 40 60 80 100 120 140 160 180

680

700

720

740

760

Time (Hr)

Battery Voltage(V)
27

Appendix A

A.1 Technical Specifications

The technical specifications for the proposed battery/inverter-based grid simulator for V-27 225-kW wind 

turbine that will be installed on the DeepCwind floating platform were produced from transient and 

steady-state modeling described earlier in this report. It was also concluded earlier in this report that other 

possible grid-simulator solutions (such as a large diesel generator or small diesel genset with synchronous 

condenser) are not feasible for the application. 

The list of technical requirements for various system components that are part of the battery/inverter gridsimulator option is given below: 

Lithium-based batteries were found to be the best fit in terms of both performance and weight/size 

limitations. Recommended lithium battery capacity is 50 Ah, based on modeling results. In case of 

unexpected performance problems, it would be desirable to have capability of accommodating a second 

parallel string of 50-Ah batteries. 

Batteries

Technology Sealed lithium. 

Nominal battery bank voltage As required by the inverter. 

Nominal capacity Depends on battery type. 

Maximum continuous charge / discharge current 300 A

Short-term (5 sec) pulse charge / discharge current 450 A

Operating and storage temperature range -40.....+500C 

Minimum battery cut off voltage Up to bidder to determine. 

Maximum battery cut off voltage Up to bidder to determine. 

Battery arrangement Circular shelves around the perimeter of the 

column wall, see platform drawings for details. 

Maximum weight of a single 50-Ah string 4400 lbs (including accessories)

Technology

Inverter 

3-phase grid forming, 480-VAC voltage source, 

PWM modulated. 

Minimum power rating 300 kVA continuous. 

Maximum weight 1800 lbs

DC bus voltage range Up to bidder to determine. 

DC bus active overvoltage protection DC chopper desirable. 

Over current protection (both hardware and 

software), L-to-G and L-to-L trips 

Circuit breakers on both AC and DC sides. 

Frequency control accuracy 60 Hz ± 0.1% (±0.01% preferred)

Short-term over-current capacity 150% of full load for 5 sec every 5 min. 

Enclosure Force ventilated cabinet. 

Inverter efficiency at rated load 97% minimum. 

Power quality (with filters) <5% voltage THD. 
28

Isolation transformer/ line reactors Transformer (480 VAC/325 kVA) or line reactors 

must be designed to comply with IEEE-519 on 

harmonic current injection (5% TDD max). In case 

of isolation transformer, a 1800-lbs max weight 

limit is applied. 

Voltage imbalance 1% max. 

Contingency battery charging Possibility to charge batteries from an external 

portable diesel genset. 

Power/voltage rating

Resistive (secondary) load bank 

300 kVA/480 VAC– delta connected. 

Maximum load step 1.5 kW (1kW preferred). 

Cooling Force ventilated cabinet. 

Resistor switching method Solid-state relays, zero crossing switching. 

Maximum weight 1500 lbs. 

Individual heating elements Flange-mount tubular elements, salt 

water/corrosion resistant. 

Main 480-AC bus, 600A 

Main Electrical Panel

Connection point for all equipment: inverter, wind 

turbine, secondary load, auxiliary transformer and 

umbilical contingency battery-charging equipment 

(such as a portable diesel genset for charging the 

main battery bank). 

Individual circuit breakers for each component 500-A molded case circuit breakers for a wind 

turbine, inverter, secondary load bank and 

umbilical (all remote trip capable) , 

TBD for auxiliary transformer and portable genset. 

Enclosure Industrial panel board cabinet, 1200 lbs. 

Ground bus In the case of an isolation transformer, the neutral 

point of wye winding must be grounded and tied 

up to the ground bus of the main electrical panel. 

In case of line reactors, the neutral point of the

inverter filter can be the grounding point. 

Secondary load bank control function (based on 

wind turbine power and battery SOC readings) 

Supervisory controller

Control the secondary load bank to match the 

wind generation and simultaneous control of 

battery SOC by adjusting real power flow to/from 

battery. Maintains the battery SOC at optimum 

setpoint (TBD by bidder). 

Max response time of the controller in this mode is 
29

a 15 ms. Better response times are highly 

desirable for better performance and battery 

longevity. (In fact, this response time is the most 

critical technical parameter of the grid simulator). 

Shut down the inverter during periods of no wind 

to preserve battery SOC (for example, shut down if 

turbine 10-min average power production is less 

than 10 kW – both time and kW thresholds must 

be selectable). Re-start inverter if 10-min average 

wind speed is greater than 7 m/s (both time and 

wind speed threshold must be selectable)

Signal from hub anemometer must be provided to 

the controller. 

Monitor and log data on all power system 

components 

List of channels TBD. 

Remote wireless and two-way communication 

capability 

Capability of two-way communication via existing 

satellite, Ethernet, or cell modem (part of platform 

DAS), time stamp synchronization capability with 

platform DAS, capability to send/receive data 

to/from platform DAS (using TCP/IP based 

communication protocol). 

Command inverter to operate in contingency 

battery charging mode 

Charge batteries from external portable diesel 

genset. 

Communication with wind turbine controller Send enable/disable command to wind turbine.

Capability to interrupt instrumentation battery 

charging process during periods of no wind (to 

preserve main battery state of charge)

Send enable/disable command to secondary 

charger circuit breaker. 

Power supply 24 VDC preferred to match with secondary battery 

bank nominal voltage. 

A.2 One-line Electrical Diagram

The one-line electrical diagram of proposed battery/inverter-based grid simulator is shown in this section. 

The main panel board is the point of interconnection of all major system components: V-27 wind turbine, 

inverter, secondary load, umbilical connection with portable diesel genset, and other platform loads. All 

these components are protected by the motor-operated remote-trip circuit breakers. All remote-trip 

circuits are controlled from the system supervisory controller. A choice of 777- kcmil AWG is based on 

flexible diesel locomotive (DLO) type RHH or RHW cables (777 kcmil allows about 545 A ampacity for 

three DLO conductors in a conduit at 300

C ambient temperature; correction factors must be applied for 

higher temperatures). DLO cables are preferred for their flexibility and lighter weights. 
30