1540-7977/07/$25.00©2008 IEEE may/june 2008

40 IEEE power & energy magazine may/june 20081540-7977/07/$25.00©2008 IEEE

©A

RT

VIL

LE, L

LC

may/june 2008 IEEE power & energy magazine 41

DDISTRIBUTED ENERGY RESOURCES (DER), INCLUDING DISTRIBUTED GENERATION(DG) and distributed storage (DS), are sources of energy located near local loads and can pro-vide a variety of benefits including improved reliability if they are properly operated in theelectrical distribution system. Microgrids are systems that have at least one distributed energyresource and associated loads and can form intentional islands in the electrical distributionsystems. Within microgrids, loads and energy sources can be disconnected from and recon-nected to the area or local electric power system with minimal disruption to the local loads.Any time a microgrid is implemented in an electrical distribution system, it needs to be wellplanned to avoid causing problems.

For microgrids to work properly, an upstream switch must open (typically during an unaccept-able power quality condition), and the DER must be able to carry the load on the islanded section.This includes maintaining suitable voltage and frequency levels for all islanded loads. Dependingon switch technology, momentary interruptions may occur during transfer from grid-connected toislanded mode. In this case, the DER assigned to carry the island loads should be able to restartand pick up the island load after the switch has opened. Power flow analysis of island scenariosshould be performed to insure that proper voltage regulation is maintained and to establish that theDER can handle inrush during “starting” of the island. The DER must be able to supply the realand reactive power requirements during islanded operation and to sense if a fault current hasoccurred downstream of the switch location. When power is restored on the utility side, the switchmust not close unless the utility and “island” are synchronized. This requires measuring the volt-age on both sides of the switch to allow synchronizing the island and the utility.

Microgrids’ largest impact will be in providing higher reliability electric service and betterpower quality to the end customers. Microgrids can also provide additional benefits to the localutility by providing dispatchable power for use during peak power conditions and alleviating orpostponing distribution system upgrades.

Microgrid TechnologiesMicrogrids consist of several basic technologies for operation. These include DG, DS, intercon-nection switches, and control systems. One of the technical challenges is the design, acceptance,and availability of low-cost technologies for installing and using microgrids. Several technologiesare under development to allow the safe interconnection and use of microgrids (see Figure 1).

Digital Object Identifier 10.1109/MPE.2008.918718

Distributed GenerationDG units are small sources of energy located at or near thepoint of use. DG technologies (Figures 2–5) typicallyinclude photovoltaic (PV), wind, fuel cells, microturbines,and reciprocating internal combustion engines with genera-tors. These systems may be powered by either fossil orrenewable fuels.

Some types of DG can also provide combined heat andpower by recovering some of the waste heat generated by thesource such as the microturbine in Figure 2. This can signifi-cantly increase the efficiency of the DG unit. Most of the DGtechnologies require a power electronics interface in order toconvert the energy into grid-compatible ac power. The powerelectronics interface contains the necessary circuitry to con-vert power from one form to another. These converters mayinclude both a rectifier and an inverter or just an inverter. Theconverter is compatible in voltage and frequency with theelectric power system to which it will be connected and con-

tains the necessary output filters.The power electronics interfacecan also contain protective func-tions for both the distributedenergy system and the localelectric power system that allowparalleling and disconnectionfrom the electric power system.These power electronic inter-faces provide a unique capabilityto the DG units and can enhancethe operations of a microgrid.

Distributed StorageDS technologies are used inmicrogrid applications wherethe generation and loads of themicrogrid cannot be exactlymatched. Distributed storage

provides a bridge in meeting the power and energy require-ments of the microgrid. Storage capacity is defined in termsof the time that the nominal energy capacity can cover theload at rated power. Storage capacity can be then categorizedin terms of energy density requirements (for medium- andlong-term needs) or in terms of power density requirements(for short- and very short-term needs). Distributed storageenhances the overall performance of microgrid systems inthree ways. First, it stabilizes and permits DG units to run at aconstant and stable output, despite load fluctuations. Second,it provides the ride-through capability when there are dynam-ic variations of primary energy (such as those of sun, wind,and hydropower sources).Third, it permits DG toseamlessly operate as adispatchable unit. More-over, energy storage canbenefit power systems bydamping peak surges inelectricity demand, coun-tering momentary powerdisturbances, providingoutage ride-through whilebackup generators respond,and reserving energy forfuture demand.

There are several formsof energy storage availablethat can be used in micro-grids; these include batter-ies, supercapacitors, andflywheels. Battery systemsstore electrical energy inthe form of chemical ener-gy (Figure 6). Batteries aredc power systems that

42 IEEE power & energy magazine may/june 2008

Distribution Feeder from Substation

Open for a Utility Microgrid

DSDG Load Load

DG Load

Interconnection Switch

Distributed Generation

Distributed Generation

Distributed Storage


Open for a Industrial/Commerical Microgrid

PossibleControl Systems

figure 1. Microgrids and components.

figure 3. Wind turbine.

figure 2. Microturbines with heat recovery.

may/june 2008 IEEE power & energy magazine

require power electronics to convert the energy to and fromac power. Many utility connections for batteries have bi-directional converters, which allow energy to be stored andtaken from the batteries. Supercapacitors, also known asultracapacitors, are electrical energy storage devices that offerhigh power density and extremely high cycling capability.Flywheel systems have recently regained consideration as aviable means of supporting critical load during grid powerinterruption because of their fast response compared to elec-trochemical energy storage. Advances in power electronicsand digitally controlled fields have led to better flywheeldesigns that deliver a cost-effective alternative in the powerquality market. Typically, an electric motor supplies mechani-cal energy to the flywheel and a generator is coupled on thesame shaft that outputs the energy, when needed, through aconverter. It is also possible to design a bi-directional systemwith one machine that is capable of motoring and regenerat-ing operations.

Interconnection SwitchThe interconnection switch (Figure 7) ties the point of connec-tion between the microgrid and the rest of the distribution sys-tem. New technology in this area consolidates the variouspower and switching functions (e.g., power switching, protec-tive relaying, metering, and communications) traditionally pro-vided by relays, hardware, and other components at the utilityinterface into a single system with a digital signal processor(DSP). Grid conditions are measured both on the utility andmicrogrid sides of the switch through current transformers(CTs) and potential transformers (PTs) to determine opera-tional conditions (Figure 8). The interconnection switches aredesigned to meet grid interconnection standards (IEEE 1547and UL 1741 for North America) to minimize custom engi-neering and site-specific approval processes and lower cost. Tomaximize applicability and functionality, the controls are alsodesigned to be technology neutral and can be used with a cir-cuit breaker as well as faster semiconductor-based staticswitches like thyristors and integrated gate bipolar transistortechnologies and are applicable to a variety of DG assets withconventional generators or power converters.

Control SystemsThe control system of a microgrid is designed to safely oper-ate the system in grid-connected and stand-alone modes. Thissystem may be based on a central controller or imbedded asautonomous parts of each distributed generator. When theutility is disconnected the control system must control thelocal voltage and frequency, provide (or absorb) the instanta-neous real power difference between generation and loads,provide the difference between generated reactive power andthe actual reactive power consumed by the load; and protectthe internal microgrid.

In stand-alone mode, frequency control is a challengingproblem. The frequency response of larger systems is basedon rotating masses and these are regarded as essential for the

inherent stability of these systems. In contrast, microgridsare inherently converter-dominated grids without or with

43

figure 4. Fuel cell.

figure 5. PV array.

figure 6. Large lead-acid battery bank.

very little directly connect-ed rotating masses, likeflywheel energy storagecoupled through a convert-er. Since microturbines andfuel cells have slowresponse to control signalsand are inertia-less, isolat-ed operation is technicallydemanding and raises load-tracking problems. Theconverter control systemsmust be adapted to providethe response previouslyobtained from directly con-nected rotating masses.The frequency controlstrategy should exploit, ina cooperative way, thecapabilities of the microsources to change theiractive power, through fre-quency control droops, theresponse of the storagedevices, and load shedding.

Appropriate voltageregulation is necessary for

local reliability and stability. Without effective local voltagecontrol, systems with high penetration of distributed energyresources are likely to experience voltage and/or reactivepower excursions and oscillations. Voltage control requiresthat there are no large circulating reactive currents betweensources. Since the voltage control is inherently a local prob-lem, voltage regulation faces the same problems in bothmodes of operation; i.e., isolated or interconnected. In thegrid-interconnected mode, it is conceivable to consider that

DG units can provide ancillary services in the form of localvoltage support. The capability of modern power electronicinterfaces offers solutions to the provision of reactive powerlocally by the adoption of a voltage versus reactive currentdroop controller, similar to the droop controller for frequen-cy control.

Microgrid Testing ExperienceAround the world, there are several active experiments in themicrogrid area covering an array of technologies. As part ofthis research, microgrid topologies and operational configu-rations are being defined and design criteria established forall possibilities of microgrid applications.

Testing Experience in the United States

Consortium for Electric Reliability Solutions (CERTS) TestbedThe objective of the CERTS microgrid testbed is to demon-strate a mature system approach that allows for high penetra-tion of DER equipment by providing a resilient platform forplug-and-play operation, use of waste heat and intermittentsources, and enhancement of the robustness and reliability ofthe customers’ electrical supply. The CERTS microgrid hastwo main components: a static switch and autonomoussources. The static switch has the ability to autonomouslyisland the microgrid from disturbances such as faults, IEEE1547 events, or power quality events. After islanding, thereconnection of the microgrid is achieved autonomously afterthe tripping event is no longer present. This synchronizationis achieved by using the frequency difference between theislanded microgrid and the utility grid. Each source canseamlessly balance the power on the islanded microgrid usingreal power versus frequency droop and maintain voltageusing the reactive power versus voltage droop. The coordina-tion between sources is through frequency, and the voltage

controller provides local stability.Without local voltage control, systemswith high penetrations of DG couldexperience voltage and/or reactivepower oscillations. Voltage controlmust also insure that there are nolarge circulating reactive currentsbetween sources. This requires a volt-age versus reactive power droop con-troller so that, as the reactive powergenerated by the source becomesmore capacitive, the local voltage setpoint is reduced. Conversely, as reac-tive power becomes more inductive,the voltage set point is increased.

The CERTS microgrid has no“master” controller or source. Eachsource is connected in a peer-to-peerfashion with a localized control

IEEE power & energy magazine may/june 2008

DSP Control Board

figure 7. Interconnection switchand control board.

Circuit Breaker

DSP: Relay + Comm+ Monitoring/Diagnostics

DGCT

PTPTLoad

3

3

UtilityGrid

3

CTMeasurement

Communication

figure 8. Schematic diagram of a circuit breaker based interconnection switch.

44


scheme implemented with each component. This arrange-ment increases the reliability of the system in comparison to amaster–slave or centralized control scheme. In the case of amaster–slave architecture, the failure of the master controllercould compromise the operation of the whole system. TheCERTS testbed uses a central communication system to dis-patch DG set points as needed to improve overall systemoperation. However, this communication network is not usedfor the dynamic operation of the microgrid. This plug-and-play approach allows expansion of the microgrid to meet therequirements of the site without extensive re-engineering.

The CERTS testbed (Figure 9) is located at AmericanElectric Power’s Walnut test site in Columbus, Ohio. It con-sists of three 60-kW converter based sources and a thyristorbased static switch. The prime mover in this case is an auto-mobile internal combustion engine converted to run on natu-ral gas. It drives a synchronous generator at variable speeds toachieve maximum efficiencies over a wide range of loads.The output is rectified and inverted to insure a constant acfrequency at the microgrid. To insure that the converter canprovide the necessary energy demanded by the CERTS con-trols there is storage on the dc bus. This also insures that thedynamics of the permanent magnet and generator are decou-pled from the dynamics of the converter. This insures that avariety of energy sources can have the same dynamicresponse as the sources used at the testbed.

The testbed has three feeders, two of which have DG unitsconnected and can be islanded. One of these feeders has twosources separated by 170 m of cable. The other feeder has asingle source, which allows for testing parallel operation ofsources. The third feeder stays connected to the utility but canreceive power from the micro sources when the static switch

is closed without injecting power into the utility. The objec-tive of the testing is to demonstrate the system dynamics ofeach component of the CERTS microgrid. This includessmooth transitions from grid-connected to islanded operationand back, high power quality, system protection, speed ofresponse of the sources, operation under difficult loads, andautonomous load tracking.

Figure 10 is an example of islanding dynamics between twosources on a single feeder at the CERTS testbed. Initially, themicrogrid is utility connected with unit A and unit B output at6 kW and 54 kW, respectively. The load is such that the gridprovides 42 kW. Upon islanding, unit B exceeds 60 kW andquickly settles at its maximum steady-state operating point of60 kW with a reduced frequency of 59.8 Hz due to the powerversus frequency droop. Unit A increases to 42 kW and con-verges to the same islanded frequency. The smoothness andspeed of the transition is seen in the invert currents and themicrogrid voltages. The loads do not see the islanding event.

Figure 11 shows voltage across the switch and the phasecurrents through the static switch during autonomous

45

DG Container

Loads

Distribution Circuits


figure 9. CERTS/AEP microgrid testbed.

Δ : 382 ms@: 312 ms

C1 RMS1.550 V

C2 RMS2.006 V

C3 RMS277.0 V

C4 RMS409 mV

24

3

PowerFrequencyInverter CurrentVoltage

Tek Stop Single Seq 5.00 kS/s Tek Stop Single Seq 10.0 kS/s

Δ : 2 Hz@: 5 Hz

Ch2

C1 RMS2.444 V

C2 RMS1.875 V

C3 RMS273.6 V

C4 RMS735 mV

Ch3Ch1

250 V1.00 V

Ch4Ch2

1.00 V1.00 V M100 ms Ch1 1.18 V

Ch3Ch1

250 V1.00 V

Ch4Ch2

1.00 V1.00 V M100 ms Ch1 2.36 V

Zoom 1.0X Vert 0.05X Horz

3

(a) (b)

figure 10. Operation of two 60-kW sources using CERTS autonomous controls during an islanding event.

synchronization. This synchronization is achieved by usingthe frequency difference between the islanded microgrid andthe utility grid. This results in a low-frequency beat voltageacross the switch. When the two voltages come in phase dueto this frequency difference the switch will close. The phasecurrents display a smooth transition due to closing at zerovoltage phase difference. The unbalanced currents are drivenby a utility voltage unbalance of around 1% and a balancedvoltage created by the DG source. All loads see balancedvoltages provided by the DG sources. The neutral third har-monic current and phase current distortion are due to trans-former magnetization currents.

The fundamental and third-harmonic frequency compo-nent from the transformer magnetization is apparent. As theloading of the transformer increases, the distortion becomes asmaller component of the total current.

Interconnection Switch TestingThe National Renewable Energy Laboratory has worked witha variety of U.S. interconnection switch manufacturers on thedevelopment of advanced interconnection technologies thatallow paralleling of distributed generators with the utility foruninterrupted electrical service and the ability to parallel andsell electricity back to the utility. This research promotes thedevelopment of new products and technologies that enablefaster switching, greater reliability, and lower fault currentson the electrical grids, thereby providing fewer disruptionsfor customers while expanding capabilities as an energy-intensive world becomes more energy efficient in the future.

Testing of the various switch technologies includes typi-cal protective relay function tests such as detection andtripping for over–and undervoltage, over- and underfre-quency, phase sequence, reverse power, instantaneous over-

current, and discrete event trip tests.To evaluate the switches’ intercon-nection requirements, conformancetests to the IEEE 1547.1 standard areconducted. These tests evaluate ifthe unit detects and trips for over-and undervoltage, over- and under-frequency, synchronization, uninten-tional islanding, reconnection, andopen-phase tests. To evaluate thepower quality functions of theswitch, tests are performed to verifythat the switch responded as expect-ed, which was to disconnect the gridand DG terminals when a powerquality event occurred. Figure 12shows results from the power qualitytesting done on a circuit-breaker-based switch. This testing showedthat there is a minimum trip time for


300

200

100

Vss

(V)

0

−100

−200

−300−0.3 −0.2 −0.1 0 0.1

Time (s)

100

50

0

−50

−100−0.05 0

Time (s)

(a) (b)

0.05

I abc

ns(A

)

IaS

IbS

IcS

InS

figure 11. Synchronization of the microgrid to the utility

00.0001

50

100

150

200

250

300

350

400

450

500

Time (s)

% N

omin

al V

olta

ge (

V)

CBEMA3-Phase ChangeVa ChangeVb ChangeController Settings

Single-Phase PLL Response Error

0.001 0.01 0.1 1 10 100

figure 12. Testing of a circuit breaker-based microgrid switch versus the ITI curve.


the breaker (0.005 s) and that the control logic for thebreaker needs to be more accurately tuned to stay withinthe Information Technology Industry (ITI) Council curve.

Testing Experience in JapanThe New Energy and Industrial Technology DevelopmentOrganization (NEDO) is currently supporting a variety ofmicrogrid demonstration projects applying renewable anddistributed generation. The first group of projects, calledRegional Power Grids with Various New Energies, wasimplemented at three locations in Japan: Expo 2005 Aichi,recently moved to the Central Japan Airport City (Aichi proj-ect), Kyoto Eco-Energy project (Kyotango project), andRegional Power Grid with Renewable Energy Resources inHachinohe City (Hachinohe project). In these three projects,control systems capable of matching energy demand and sup-ply for microgrid operation were established. An importanttarget in all of the projects is achieving a matched supply anddemand of electricity. In each project, a standard for the mar-gin of error between supplied energy and consumed energyover a certain period was set as a control target.

In the Aichi project, a power supply system utilizing fuelcells, PV, and a battery storage system, all equipped withconverters, was constructed. A block diagram of the supplysystem for the project is shown in Figure 13. The fuel cells

adopted for the system include two molten carbonate fuelcells (MCFCs) with capacities of 270 kW and 300 kW, one25-kW solid oxide fuel cell (SOFC), and four 200-kW phos-phoric acid fuel cells (PAFCs). The total capacity of theinstalled PV systems is 330 kW, and the adopted cell typesinclude multicrystalline silicon, amorphous silicon, and asingle crystalline silicon bifacial type. A sodium-sulfur(NaS) battery is used to store energy within the supply sys-tem and it plays an important role in matching supply anddemand. In the Aichi project, the load-generation balancinghas been maintained at 3% for as short as ten-minute inter-vals. The Aichi project experienced a second grid-independ-ent operation mode in September 2007. In this operationalmode, the NaS battery converter controls voltage and balanc-ing of the load.

In the Kyotango project, the energy supply facilities anddemand sites are connected to a utility grid and are integratedby a master control system. The energy supply system func-tions as a “virtual microgrid.” A management system formatching the demand and supply of electricity is beingdemonstrated and a reduction in imbalances to within 3% ofexpected demand for five-minute intervals was achieved.Several criteria related to power quality (outages, voltagefluctuations, and frequency fluctuations) are being monitoredduring the demonstration period to determine if the system

47

PVs (330 kW)

NaSStorage

Battery(500kW)

Methane Fermentation System

Garbage (4.8 t/day)

(Derived from EXPO Site)

Wooden Chip WastePlastic Waste

(20kg/h)

(Derived form EXPO SiteConstruction)

Gasification System(City Gas)

MCFC+MGT(300 kW)

MCFC(270kW)

(TransformerConsole)

EXPO Site

Power Grid

COMMON 5

(Utility’s Grid)

Micro Grid

Government ExhibitBuilding

SOFT(25 kW)

PAFC(800 kW)

Refrigerator

Refrigerator

Cold Water(for Air

Conditioning)

COMMON5(New Energy Exhibition Room)

Refrigerator

Refrigerator

Waste Heat

figure 13. Diagram of Aichi Microgrid project.

can achieve and maintain the samepower quality level as a utility net-work. In the plant, gas engines with atotal capacity of 400 kW wereinstalled together with a 250-kWMCFC and a 100-kW lead-acid bat-tery. In remote locations, two PV sys-tems and one 50-kW small windturbine were also installed. Thepower generation equipment and end-user demand are managed by remotemonitoring and control. One of theinteresting features of the system isthat it is managed not by a state-of-the-art information network systembut by conventional information net-works, which are the only networksystems available in rural areas.

The Hachinohe project (Figure 14)features a microgrid system construct-ed using a private distribution linemeasuring more than 5 km. The privatedistribution line was constructed totransmit electricity primarily generated


Private D

istribution Line(E

lectricity and Com

munication)

The Sewage Plant

Steam Boiler

Exhaust Heat

Digestion Gas

Steam Boiler(4t/h)

KonakanoJunior High School (60 kW)

KonakanoElementary School (46 kW)

KouyoJunior High School (53 kW)

KouyoJ Elementary School (46 kW)

Hachinohe Regional WaterSupply Authority (40 kW)

WT8 kWPV

10 kW

To Existing Grid

WT8 kWPV

10 k W

PV10 kW

WT4 kW(2 kW×2)

Hachinohe City Hall(360 kW)

WoodWast

DigestionChamber

Gas Engines

PV Panels

DigestionGas Tank

ControlSystem Battery

figure 14. Overview of the Hachinohe project.

Trial Distribution Line

SVC

1 km

PowerSupplyRoom

DummyDemand

DummyDemand

DummyDemand

PowerSupplyRoom

Sensor 5

300 kVA SVC

500 kVA LBC.,10,000 kVA LBC

R+jX

PowerSupplyRoom Power

SupplyRoom

ControlRoom

SVC

SVRLBC

Sensor

300 kVA SVC

300 kVA SVR

Sensor

Sensor

Sensor

Substation

ModelLine 2

ModelLine 1

0+j0

0+j2

300~500 m

2+j3Model Line 3

figure 15. Structure of test network at CRIEPI.


by the gas engine system. Several PV systems and smallwind turbines are also connected to the microgrid. At thesewage plant, three 170-kW gas engines and a 50-kW PVsystem have been installed. To support the creation of diges-tion gas by the sewage plant, a wood-waste steam boiler wasalso installed due to a shortage of thermal heat to safeguardthe bacteria. Between the sewage plant and city office, fourschools and a water supply authority office are connected tothe private distribution line. At the school sites, renewableenergy resources are used to create a power supply that fluc-tuates according to weather conditions in order to prove themicrogrid’s control system’s capabilities to match demandand supply. The control system used to balance supply anddemand consists of three facets: weekly supply and demandplanning, economic dispatch control once every three min-utes, and second-by-second power flow control at intercon-nection points. The control target is a margin of errorbetween supply and demand of less than 3% for every six-minute interval. During testing, a margin of error rate of lessthan 3% was achieved during 99.99% of the system’s opera-tional time. The Hachinohe project experienced one week ofgrid-independent operation in November 2007. In this opera-tional mode, imbalance among the three phases was compen-sated by the PV converter.

The New Power Network Systems project is evaluatingnew test equipment installed on a test distribution network(Figure 15) constructed at the Akagi Test Center of the Cen-tral Research Institute of the Electric Power Industry(CRIEPI). This equipment includes a static var compensator(SVC), a step voltage regulator (SVR), and loop balance con-trollers (LBCs). The SVC and SVR are used for controllingthe voltage on a distribution line, and they are sometimesapplied on an actual utility network. In this project, theeffects of integrated control of this equipment are beingexamined. LBCs are a new type of distribution networkequipment that can control the power flow between two dis-tribution feeders by means of a back-to-back (BTB) type con-verter. The LBCs allow connections of two sources withdifferent voltages, frequencies, and phase angles by providinga dc link.

A final microgrid project is evaluating the possibility thatgrid technology can create value for consumers and variousenergy service levels. In Sendai City a microgrid consisting oftwo 350-kW gas engine generators, one 250-kW MCFC, andvarious types of compensating equipment is being evaluated todemonstrate four levels of customer power. Two of the servicelevels will have compensating equipment that includes an inte-grated power quality backup system that supplies high-quality

49

BC HydroUtility Grid

7.0 MW IndependentPower Producer

Run-of-River Hydro Plant

69 kV

60-kV T Line

14 MVA69/25 kV

Feeder 1

Feeder 2

Feeder 3

3 MWPeak Load

8.75 MVA4.16/25 kV

4.32 MVA 4.32 MVA

Boson Bar Substation

figure 16. System configuration for the Boston Bar IPP and BC Hydro planned islanding site.

Distributed storage technologies are used in microgrid applicationswhere the generation and loads of the microgrid cannot be exactly matched.

power that reduces interruptions and voltage drops. In one ofthese cases, the wave pattern is guaranteed. Two additionallower service levels have only short-term voltage drops com-pensated by a series compensator. This work will evaluate thepossibility of providing various service levels to customerslocated in the same area. Since summer of 2007, the Sendai sys-tem has been in operation and has improved the power qualityat the site. Before starting actual operation, the compensationequipment was tested by using a BTB power supply system tocreate artificial voltage sag.

In addition to the NEDO-sponsored projects, there are sev-eral private microgrid projects. Tokyo Gas has been evaluatinga 100-kW microgrid test facility since September 2006 at theYokohama Research Institute, consisting of gas-engine com-

bined heat and power (CHP), PV, wind power, and battery-incorporated power electronics. Shimizu Corp. has developed amicrogrid control system with a small microgrid that consistsof gas engines, gas turbines, PV, and batteries. The system isdesigned for load following and includes load forecasting andintegrated control for heat and power.

Testing Experience in CanadaPlanned microgrid islanding application, also known asintentional islanding, is an early utility adaptation of themicrogrid concept that has been implemented by BC Hydroand Hydro Quebec, two of the major utility companies inCanada. The main objective of planned islanding projects isto enhance customer-based power supply reliability on ruralfeeders by utilizing an appropriately located independentpower producer (IPP), which is, for instance, located on thesame or adjacent feeder of a distribution substation. In onecase, the customers in Boston Bar town, part of the BCHydro rural areas, which is supplied by three 25-kV medi-um-voltage distribution feeders, had been exposed to poweroutages of 12 to 20 hrs two or three times per year. Thisarea, as shown in Figure 16, is supplied by a 69/25-kV distri-bution substation and is connected to the BC Hydro high-voltage system through 60 km of 69-kV line. Most of theline is built off a highway in a canyon that is difficult toaccess with high potential of rock/mud/snow slides. Theimplemented option to reduce sustained power-outage dura-tions is based on utilizing a local IPP to operate in an inten-tional island mode and supply the town load on one or morefeeders of the substation. The Boston Bar IPP has two 3.45-MW hydro power generators and is connected to one of thethree feeders with a peak load of 3.0 MW. Depending on thewater level, the Boston Bar IPP can supply the communityload on one or more of the feeders during the islanding oper-ation. If the water level is not sufficient, the load on onefeeder can be sectioned to adequate portions.

Based on the BC Hydro islanding guideline, to performplanned islanding, an IPP should be equipped with additionalequipment and control systems for voltage regulation, fre-quency stabilization, and fault protection. In addition, theisland-load serving capability of an IPP needs to be testedprior to and during the project commissioning to ensure thatthe IPP can properly respond to load transients such as a stepchange in load and still sustain the island.

The functional requirements added to the Boston Bar IPPto support planned islanding are as follows:

✔ governor speed control with fixed-frequency (isochro-nous) mode for single-unit operation and speed-droopsettings for two-unit operation in parallel

✔ engineering mass of generators and hydro turbines toincrease inertia and improve transient response

✔ excitation system control with positive voltage fieldforcing for output current boost during the feeder faultto supply high fault current for proper coordination ofprotection relays


L

10, 230 V/50 Hz

Wind Turbine1 kW

ControllableLoads

4.5 kVA

Batteries250 Ah, 60 V

PV1.1 kW

Grid

G

L

DessalinationUnit Load

4.5 kVA

Batteries250 Ah, 48 V

PV1.0 kW

Grid+−

+−

(a)

(b)

figure 17. Laboratory microgrid facility at NTUA, Greece:(a) single-line diagram and (b) view of one pole.


✔ automatic voltage regulation control to regulate volt-ages at the point of common coupling

✔ two sets of overcurrent protection set-points for thegrid-connected and the islanding operating modes

✔ real-time data telemetry via a leased telephone linebetween the IPP remote control site and the utility areacontrol center

✔ black start capability via an onsite 55-kW dieselgenerator.

In addition to the above upgrades, the auto-recloser on theconnecting IPP feeder is equipped with a secondary voltagesupervision function for voltage supervisory close and block-ing of the auto-reclosing action. Remote auto-synchroniza-tion capability was also added at the substation level tosynchronize and connect the island area to the 69-kV feederwithout causing load interruption. When a sustain power out-age event, such as a permanent fault or line breakdown,occurs on the utility side of the substation, the main circuitbreaker and feeder reclosers are opened (Figure 16). Then,the substation breaker open position is telemetered to the IPPoperator. Subsequently, the IPP changes the control and pro-tection settings to the island mode and attempts to hold theisland downstream of the feeder 2 recloser. If the IPP fails tosustain the island, the IPP activates a black-start procedureand picks up the dead feeder load under the utility supervi-sion. The island load may be supplied by one generator orboth generators in parallel.

Two sets of tests were performed during the generatorcommissioning as follows:

1) grid parallel operation tests including a) the automaticand manual synchronization, and b) output load, volt-age and frequency controls, and load rejection tests

2) island operation tests comprising a) load pick-up anddrop-off tests in 350-kW increments, b) dead loadpick-up of 1.2 MW when only one of the two genera-tors is in operation, and c) islanded operation and loadfollowing capability when one unit is generating and/orboth units are operating in parallel.

The planned islanding operation of the Boston Bar IPP hasbeen successfully demonstrated and performed several timesduring power outages caused by adverse environmental effects.Building on the knowledge and experience gained from thisproject, BC Hydro has recently completed a second case ofplanned islanding and is presently assessing a third project.

Testing in EuropeAt the international level, the European Union has supportedtwo major research efforts devoted exclusively to microgrids:the Microgrids and More Microgrids projects. The Microgridsproject focused on the operation of a single microgrid, has suc-cessfully investigated appropriate control techniques, anddemonstrated the feasibility of microgrid operation throughlaboratory experiments. The Microgrids project investigated amicrogrid central controller (MCC) that promotes technicaland economical operation, interfaces with loads and micro

sources and demand-side management, and provides set pointsor supervises local control to interruptible loads and microsources. A pilot installation was installed in Kythnos Island,Greece, that evaluated a variety of DER to create a microgrid.

Continuing microgrid projects in Greece include a labora-tory facility (Figure 17) that has been set up at the National

51

0 100 200 300 400 500

0 100 200 300 400 500

0 100 200 300 400 500

49.9

49.95

50

50.05

50.1

50.15

fgrid

−200

−100

0

100

200

300

400

500

600Pload

220

222

224

226

228

230

Grid Tied

Island

Grid Tied

Island

Island

(b)

(c)

(a)

Grid Tied

Island

Vol

tage

(V

) F

requ

ency

(H

z)

Pow

er (

W)

fmg

Umg

Ugrid

Pbat

Ppv

Pgrid

figure 18. Changes of the microgrid operating mode fromisland to interconnected mode and vice-versa: (a) frequen-cy, (b) voltage, and (c) component powers.

Technical University of Athens (NTUA), with the objective totest small-scale equipment and control strategies for micro-grid operation. The system comprises two poles, eachequipped with local (PV and wind) generation and batterystorage, connected to each other via a low-voltage line aswell as to the main grid. Each pole may operate as a micro-grid via its own connection to the grid, or both poles may be

connected via the low-voltage line to form a two-bus micro-grid connected to the main grid at one end. The batteryconverters are the main regulating units in island mode, regu-lated via active power-frequency and reactive power-voltagedroops. Multi-agent technology has been implemented for thecontrol of the sources and the loads.

Figure 18 shows indicative test results demonstrating theseamless transition of the microgridfrom grid-connected to island modeand vice-versa (one-pole microgridoperation). The first diagram illus-trates the variation of the frequencyand the second of the voltage. Thechange of the component powerflows is shown in the third illustra-tion. While the load and the PV con-tinue operating at the same power,the output of the battery converterand the power flow from the gridchange to maintain the power equi-librium in the microgrid.

Testing on microgrid componentshas also been extensively conductedby ISET in Germany. Figure 19shows testing conducted to examinevoltage and current transient when


400

300

0

−300

100806040200

−60−80

−10010001000600400200

0−200−400−600−800

−1000

420 440 450 460 500 520 540 560 580 600

400 420 440 450 460 500

Time [ms]

520 540 560 580 600

Grid Connected Island

I Mor

eGrid

[A]

VM

oreG

rid[V

]

−20−40

I Grid

[A]

InterfaceGrid<->MicroGrid


Grid

Grid

Grid

Grid



MicroGrid

MicroGrid

MicroGrid

P>Pmax

isc

MicroGrid

SC: Short Circuit

SC: Short Circuit

figure 19. Voltage and current changes as the microgrid switches to islanded mode.

10 kV Utility Network

Dyn5400 kVA

630 A

200 ALV Feeder 1

200 ALV Feeder 2

200 ALV Feeder 3

200 ALV Feeder 4

BatteryUnit B

InverterB

BatteryUnit A

InverterA

AutomaticIslanding andReconnection

Switch

figure 20. Schematic for the Bronsbergen Holiday Park microgrid.


microgrids transfer from grid-connected to islanded mode.This figure shows that with proper design, there can be mini-mal load disruption during the transfer.

The More Microgrids project aims at the increase ofpenetration of microgeneration in electrical networksthrough the exploitation and extension of the Microgridsconcept, involving the investigation of alternative microgen-erator control strategies and alternative network designs,development of new tools for multimicrogrid managementoperation and standardization of technical and commercialprotocols, and field trials on actual microgrids and evalua-tion of the system performance on power system operation.

One of the More Microgrids projects is located at Brons-bergen Holiday Park, located near Zutphen in the Netherlands.It comprises 210 cottages, 108 of which are equipped withgrid-connected PV systems. The park is electrified by a tradi-tional three-phase 400-V network, which is connected to a 10-kV medium-voltage network via a distribution transformerlocated on the premises (Figure 20). The distribution trans-former does not feed any low-voltage loads outside of the hol-iday park. Internally in the park, the 400-V supply from thedistribution transformer is distributed over four cables, eachprotected by 200-A fuses on the three phases. The peak load isapproximately 90 kW. The installed power of all the PV sys-tems together is 315 kW. The objective of this project isexperimental validation of islanded microgrids by means ofsmart storage (coupled by a flexible ac distribution system)including evaluation of islanded operation, automatic isolationand reconnection, fault level of the microgrid, harmonic volt-age distortion, energy management and lifetime optimizationof the storage system, and parallel operation of converters.

Another More Microgrids project involves field test onthe transfer between interconnected and islanding modewith German utility MVV Energie. MVV Energie is plan-ning to develop an efficient solution to cope with theexpected future high penetration of renewable energysources and distributed generation in the low-voltage distri-bution grid. If integrated in an intelligent way, these newplayers in the distribution grid will improve independencefrom energy imports, reliability, and power quality at lowercost than the “business as usual” regarding replacement orreinforcement of the regional energy infrastructure. A suc-cessful transfer between interconnected and islanding modewould provide a substantial benefit for the grid operator.

This project will evaluate decentralized control in a resi-dential site in the ecological settlement in Mannheim-Wallstadt. The new control structures for the decentralizedcontrol with agents will be tested and allow the transitionfrom grid connection to islanding operation without interrup-tions. This would improve reliability of the grid and supportfor black start after failure of the grid.

The CESI RICERCA test facility in Italy will also be usedto experiment, demonstrate, and validate the operation of anactual microgrid field test of different microgrid topologies atsteady and transient state and power quality analysis. During

a transient state, the behavior during short-duration voltagevariation for single/three-phase ac faults, or dynamicresponse to sudden load changes and to conditions of phaseimbalance or loss of phase, the islanding conditions follow-ing interruption of the supply will be analyzed.

ConclusionsMicrogrids will provide improved electric service reliabilityand better power quality to end customers and can alsobenefit local utilities by providing dispatchable load for useduring peak power conditions and alleviating or postponingdistribution system upgrades. There are a number of activemicrogrid projects around the world involved with testing andevaluation of these advanced operating concepts for electricaldistribution systems.

For Further ReadingN. Hatziargyriou, A. Asano, R. Iravani, and C. Marnay,“Microgrids,” IEEE Power Energy Mag., vol. 5, no. 4, pp.78–94, July/Aug. 2007.

R. Lasseter, and P. Piagi, “MicroGrids: A conceptual solu-tion,” in Proc. IEEE PESC’04, Aachen, Germany, June 2004,pp. 4285–4290.

B. Kroposki, C. Pink, T. Basso, and R. DeBlasio, “Micro-grid standards and technology development,” in Proc. IEEEPower Engineering Society General Meeting, Tampa, FL, June2007, pp. 1–4.

S. Morozumi, “Micro-grid demonstration projects inJapan,” in Proc. IEEE Power Conversion Conf., Nagoya,Japan, Apr. 2007, pp. 635–642.

C. Abby, F. Katiraei, C. Brothers, L. Dignard-Bailey, andG. Joos, “Integration of distributed generation and wind ener-gy in Canada,” in Proc. IEEE Power Engineering GeneralMeeting, Montreal, Canada, June 2006.

BC Hydro (2006, June), “Distribution power generatorislanding guidelines,” [Online]. Available: http://www.bchy-dro.com/info/ipp/ipp992.html

BiographiesBenjamin Kroposki manages the Distributed Energy Sys-tems Integration Group at the National Renewable EnergyLaboratory and serves as chairman for IEEE P1547.4.

Robert Lasseter is a professor with the University ofWisconsin-Madison and leads the CERTS Microgrid project.

Toshifumi Ise is a professor with the Department ofElectrical Engineering, Faculty of Engineering, at OsakaUniversity in Japan.

Satoshi Morozumi leads research activities in micro-grids for the New Energy and Industrial Technology Devel-opment Organization in Japan.

Stavros Papathanassiou is an assistant professor withthe National Technical University of Athens, Greece.

Nikos Hatziargyriou is a professor with the NationalTechnical University of Athens and executive vice-chair anddeputy CEO of the Public Power Corporation of Greece.

53

p&e

1540-7977/07/$25.00©2008 IEEE may/june 2008

Documents