［ A Case Study of the Installed System ］

Japan-U.S. New Mexico Smart
Grid Collaborative Demonstration
ProjectKeywords

Smart grid, Photovoltaic generation, FRT (Fault Ride Through)

Abstract

The smart grid has been gaining attention as a technology that enables agrid-connection of large amounts of renewable energy resources such as
Photovoltaic (PV) generation. The New Energy and Industrial Technology
Development Organization of Japan is promoting the “Japan-U.S. New Mexico
Smart Grid Collaborative Demonstration Project” in cooperation with theGovernment of the State of New Mexico, U.S.A. As part of this demonstration
project for the smart building demonstration in Albuquerque, we have developed
and manufactured the photovoltaic Power Conditioning Subsystem (PCS)
equipped with the Fault Ride Through (FRT) function and power output restraining
function. Such functions improved the robustness during the grid-connection
and islanding operation.

1 Preface
In many countries around the world, introduction of renewable energy resources is promoted in
response to global environmental issues and energy security. In the case of the grid-connections witha large volume of renewable energy resources suchas Photovoltaic (PV) resources, (which is widelychangeable with weather conditions) there aremany challenges such as a voltage rise in distribution feeders due to reverse power ﬂows from PV
power generation, the occurrence of surplus power,
and necessity for taking measures to secure the frequency adjustment power. As a technology to solve
such challenges, a technical concept by the nameof smart grid has been recently noted where powerﬂows are efﬁciently managed by using IT expertiseand based on information acquired from both gridand electricity user sides.
In consideration of future worldwide businessdevelopment of Japanese grid-connection technologies, the New Energy and Industrial Technology
Development Organization (NEDO) is now undergoing the “Japan-U.S. New Mexico Smart Grid
Collaborative Demonstration Project” in cooperation with the Government of the State of New
Mexico, U.S.A. This smart grid project is in cooperation with the Green Grid Initiative project conductedby the State of New Mexico.

10Yoshinobu Ueda,Yu Sasaki

MEIDEN REVIEW Series No.162 2014 No.3

This demonstration research aims at the building of a MicroGrid in a distribution line scale wherethe effect from power yield changes of renewableenergy resources can be minimized through thecooperative control using electricity user side equipment such as lead-acid batteries and thermal storage tanks. The demonstration project promoted in
North America is based on the following missions:( 1) Veriﬁcation is carried out on the techniques difﬁcult to use in Japan because of regulatory restrictions, and on the performance of various equipment
under circumstances different from those in Japan.
The acquired data and expertise are fed back tofuture research and development of smart grids inJapan.
( 2 ) Through the introduction and demonstration ofJapanese energy-related equipment and facilities
into real power grid systems there, the results of thedemonstration project will serve as a springboard to
help the Japanese suppliers develop their businessin the U.S. and world markets.
( 3 ) Through effectively utilizing various data obtained
from this demonstration project, the establishedexpertise is used for smart grid standardization programs which are expected to increase sharply inthe future.
As part of this demonstration project, we built aMicroGrid system capable of islanding operation
in a commercial building Albuquerque City. ThisCommercial building
Building
load

Thermalstorage
tank

100
90
Air-cooled
chiller70USRT

Absorption
chiller
20USRT

Cooling
tower

Voltage (%)

Distribution
systemDistribution
feeder70

Sectionundertakenby thecompany

PV
array
50kW

Fig. 1

Dummy
load
100kW

Lead-acidbatteries
160kWhShort-time disconnection
allowedContinued operation
not required

0 150

1500

700
Time (ms)Fig. 2PVPCS
50kWContinuousoperation(conditional)

300BEMS

Continuous operation

Fuelcell80kW

Gasengine
generator
240kW

BEMS: Building Energy Management System

Configuration of the Demonstration System

This diagram shows the MicroGrid conﬁguration built for thisdemonstration project. We manufactured the PV PCS.

MicroGrid is grid-connected with an existing U.S.
grid power distribution system as shown in Fig. 1.
For this project, we built the demonstration system
during Fiscal 2010 to Fiscal 2011, and the demonstration test was conducted during Fiscal 2012 toFiscal 2013. Among the demonstration items of thisproject, we have been in charge of veriﬁcation of theimproved robustness (stability against system turbulence) in regard to the PV Power Conditioning
Subsystem (PCS). This paper introduces the development and manufacturing of PV PCS and the resultsof functional veriﬁcation.

2 Contents of Demonstration SystemDevelopment
2.1 Functions of PV PCS
2.1.1 Improvement of Robustness for PV PCSGenerally speaking, the PV PCS has highly
sensitive protection features when viewed from thestandpoint of equipment protection and system protection. The PV PCS tends to respond sharply toturbulence occurring in the grid system, thus making unwanted disconnections.
When Distributed Energy Resources (DERs)such as PV generation are introduced to a grid
power system in a large quantity, and if amounts of
DERs are disconnected as a result of unnecessary
disconnection, such a situation is anticipated toadversely affect the grid power system. For thisreason, the technologies of Fault Ride ThroughScope of Continuous Operation at a Voltage Sag

This diagram shows the scope of continuous operation at a voltage sag deﬁned for this demonstration project.

(FRT) or Low Voltage Ride Through (LVRT) continue
operation even in the case of grid power system turbulence are getting recent attention and the standardization programs are gaining traction.
Fig. 2 shows the scope of continuous operation deﬁned for this demonstration project. The deﬁned
scope has been stipulated in the grid-connection
guidelines (1) issued by Bundesverband der Energieund Wasserwirtschaft (BDEW). The scope of continuous operation denotes the capability of continuous operation to be accomplished by PV PCS. Even
though operation is maintained within this scope,
the PV PCS will stop working if the set point of gridinterconnection protection is exceeded.
In order to increase the added value of theMicroGrid, coordinated operation of multiple DERs
including renewable energy sources is carried out.
During islanding operation of MicroGrid, however,system voltage and frequency (phase) are likely tochange and a phenomenon like voltage balance isalso likely to occur. Consequently, unnecessary disconnection tends to take place as compared withthe grid-connection operation.
For example, when a disconnection takes
place when the PV power yield is high, there can besubstantial inﬂuence upon the stability of islanding
operation. In addition, an active unintentional islanding
detection can be a cause of disturbing the power
quality during islanding operation. In our demonstration research under this project, whenever thecondition of the MicroGrid islanding operation issensed through the Building Energy Management
System (BEMS), an auto-changeover function has
been devised so that operation is changed over tothe more robust operation and control system byrelieving set point for grid-connection protection andby suspending the effect of unintentional islanding
detection feature.
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2.1.2 Output Restraining Function

2.2 Manufacturing Speciﬁcations for PV PCS
Table 1 shows the major speciﬁcations for thePV PCS manufactured for this project. In order toensure that the DC voltage range of PV PCS coincides with that of PV battery modules, this DC voltage range is lowered to the level of 50kW class,based on the 100kW machine of the SP100 Series

12MEIDEN REVIEW Series No.162 2014 No.3

MPP

Max. power
Output power (kW)

Where a large volume of PV power resource isintroduced to the grid, the possibility of adverse
inﬂuence over the stable grid operation is predictedbecause of the effect of the surplus power underlight-load conditions. Even in such a case, in orderto achieve effective utilization of energy resources,
the PV generation is expected to continue operation
for maximum power yield. We then store surplus
power in energy storage systems such as lead-acidbatteries or thermal storage tanks, and it is desired
to feed the power when demand for energy isincreased. On the contrary, the storage capacity ofenergy storage systems is limited and it becomes
impossible to operate the system if the upper orlower limits of storage capacity are exceeded. Inaddition, the energy storage systems are subject toa constant amount of losses at each time process ofin and out of energy. Therefore, according to the situation of operation, there may be a case whereenergy shift by storage does not always contributeto the high efﬁciency or reduction of CO2 emission.
For a solution in such a case, by adding the function
to accept an output reduction command from theBEMS, it allows to realize the stable management of
the MicroGrid for a long time. In addition, it also
allows for ﬂexible energy management by increasing options of management on the demand side. In
particular, for an islanding operation of the MicroGrid,by using PV PCS with the output restraining function, it is expected to contribute to stable grid powersystem operation.
The output restraining function of PV PCS
becomes effective when the operation point is intentionally moved from the Maximum Power Point(MPP) of the PV module output characteristics.
Fig. 3 shows the relationship between the MPP andthe working point during output restraint.
The required output restraining level is deﬁned on
BEMS side in consideration of loads in the MicroGrid,
power output from generation facilities, and total
capacity of energy storage systems (power storage
equipment, thermal storage tanks).

Output restraint level

Working point during
output restraint

DC voltage (V)Fig. 3

Output Restrained Operation

The relationship is shown between the MPP and the working
point during output restraint indicated on the output-voltage characteristic curve of PV generation.

Table 1 PV PCS Specifications
Major speciﬁcations are shown for the developed PV PCS.
Item

Specifications

Rated voltage

480V

Rated frequency

60Hz

Rated capacity50kWDC input voltage (MPPT )250-500V (275-425V )Power conversion efficiency

94.0% (Max. efficiency)

Installation environment

Outdoors※A PV PCS specified for indoor
use is accommodated in anoutdoor casing.

DC
voltage/current

DC power
sourceInverter
output current

ACvoltage/current

INV

Gridsimulator

PV PCS (photovoltaic PCS)

Fig. 4

Testing Circuit for the FRT Function

Conﬁguration of the testing circuit is shown, used to evaluate theFRT function. The grid simulator on AC side can be programmed
to cause any type of voltage drop.

that is for the export model PV PCS.
The PV PCS is accommodated in outdoor casings and installed beneath the PV array.

3 Result of FRT Function Evaluation
In order to evaluate the newly developed FRTfunctions, we made internal veriﬁcation tests using
a mini-model of 10kW PV PCS and a grid simulator.
Fig. 4 shows the testing circuit. The grid simulator
can be programmed to cause any type of voltagedrop.

DC voltage (V)DC current (A)Inverter output
current (A)AC line-to-linevoltage (V)AC current (A)tinued and a current supply is maintained while
residual voltage is 0V. In regard to the range ofduration shown in Fig. 2, we evaluated the duration
time and residual voltage under the conditions otherthan those of Fig. 5 and conﬁrmed that the goal
range of duration time is attained.

400350300
25050254 Postscript

0100
0－100
600
300
0－300－600
50
250－25－50

1.65

1.71.75

Time (s)Fig. 5Result of Continuous Operation Test witha Residual Voltage at 0V

As part of the MicroGrid demonstration facilities
constructed in a commercial building in Albuquerque,New Mexico, U.S.A., we have developed and manufactured the PV PCS that have FRT and output
restraining functions. As a result of our internal veriﬁcation testing, we could conﬁrm that the goal functions and specifications of the project are duly
attained.
For our future activities, we will continue toevaluate the effectiveness of the developed functions through veriﬁcation testing at the project site
so that we can contribute to activities for smart grid
standardization predicted to progress rapidly.
Lastly, this paper has introduced the result ofour activities as a contract research project for NEDO.

PV PCS operation is continued with a residual voltage of 0V andduration of 160ms.

・All product and company names mentioned in this paper areFig. 5 shows the result of FRT functional testing with a residual voltage of 0V and duration of160ms. It can conﬁrm that inverter operation is con-《Reference》

the trademarks and/or service marks of their respective owners.

( 1 ) BDEW: Technical Guideline “Generating Plants Connected tothe Medium-Voltage Network,” 2010.6

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