Interconnect Protection of Dispersed Generators

Charles J. MozinaBeckwith Electric Co., Inc.

Applications Manager, Protection Products and Systems

I. INTRODUCTION

A significant portion of the new generation capacity in-stalled during the next millenium may be accomplished throughthe construction of dispersed generation facilities. This pa-per discusses the protection requirements to interconnect thesegenerators to utility systems, as well as methods to recon-nect these generators after interconnect protection tripping.The paper also discusses the limitations of present-day in-terconnection protection methods in addressing issues suchas system generation support during major utility systemdisturbances.

Dispersed generators need to be protected not only fromshort circuits, but from abnormal operating conditions. Manyof these abnormal conditions can be imposed on the dis-persed generator by the utility system. Examples of suchabnormal conditions are: overexcitation, overvoltage, un-balanced currents, abnormal frequency and shaft torque stressdue to utility breaker automatic reclosing. When subjectedto these conditions, damage or complete generator failurecan occur within seconds. Machine damage due to thesecauses are a major concern of dispersed generator owners.

Utilities, on the other hand, are generally concerned thatthe installation of an dispersed generator will result in dam-age to their equipment or to the equipment of their custom-ers. Small dispersed generators are connected to the utilitysystem at the distribution and subtransmission level. Theseutility circuits are designed to supply radial loads. The in-troduction of generation provides an unwanted source forredistribution of both load and fault current, as well as apossible source of overvoltage. Islanded operation of dis-persed generation with utility loads external to the dispersedgenerator site is generally not allowed for two major rea-sons:

1. The utility needs to restore the outaged circuits and thiseffort is greatly complicated by having islanded gen-erators with utility loads. Automatic reclosing is uni-versally the first method attempted to restore power tocustomers. Having islanded generators complicates bothautomatic reclosing, as well as manual switching whichrequires synchronizing the generator/load islanded tothe utility system.

2. Power quality (voltage and frequency levels, as well asharmonics) may not be maintained by the islanded dis-persed generators within the level provided by the util-ity and could result in damage to the customers equip-ment.

Properly designed interconnection protection should ad-dress the concerns of both the dispersed generator owner, aswell as the utility, at the lowest possible cost. The major func-tions of interconnect protection is to prevent system islandingby detecting asynchronous dispersed generator operationin other words, determining when the generator is no longeroperating in parallel with the utility system. This detectionand tripping must be rapid enough to allow automatic reclosingby the utility.

The technology available to provide dispersed generatorprotection has evolved from single-function electromechanicalrelays to static (electronic) relays and now to digital relays.The development of low-cost microprocessor technology hasmade possible the development of the multifunction digitalrelays which combine many relaying functions into a singlerelay package. This relay technology offers significant ad-vantages over older electromechanical and static relays. Theuse of this technology to provide interconnect protection ishighlighted in this paper. Other specific topics to be ad-dressed are:

Brief History of Dispersed Generation in the U.S.Influence of PURPACurrent state of dispersed generationMicroturbine technology

Interconnection versus Generator Protectionfor Dispersed Generation Units

Major impact of interconnection transformer connectionson protection requirements

Transient overvoltages caused by dispersed generatorson utility distribution systems and mitigating measures

Detection methods for asynchronous dispersed generatoroperations with utility systems

Limitations of current practices to allow dispersedgeneration to provide generator support during majorsystem disturbances

Automatic restoration practices and automatic reclosingby utilities

Dispersed Generator Interconnection ProtectionMethods and Practices

Detection of loss of parallel operation with utilityFault backfeed detectionDetection of damaging system conditionsAbnormal power flowRestoration

Use of Digital Technology for Interconnection ProtectionAdvantages of the technologyUser-selectable functionalitySelf-diagnosticsCommunications capabilityOscillographic capability

II. BRIEF HISTORY OF DISPERSED GENERATION IN THE U.S.

Until the late 1970s, utilities were not required to pur-chase the electric power generated by non-utility generatingentities within their service areas. However, there were in-dustries such as pulp and paper, steel mills, as well as petro-chemical facilities that had internal generation within theirelectrical facilities that operated in parallel with the utilitysystem. These cogenerators produced electricity from heatsources such as process steam. Typically, these generatorsserved a portion of the load at these industrial facilities andprovided emergency power to the industrial facility duringutility outages.

After the oil embargo of the early 1970s, the federalgovernment decided that conventional energy sources, es-pecially oil and gas, needed to be conserved to reduce ourreliance on foreign sources. The federal government wantedto promote the generation of electricity through renewablefuel sources by non-utility generators. This prompted thepassage of the Public Utility Regulatory Policies Act (PURPA)of 1978. PURPA required utilities, for the first time, to in-terconnect with qualified IPPs and purchase electricity at acost that reflects the cost avoided by the utility by not hav-ing to generate an equivalent amount of electricity itself.

To receive the benefits of PURPA, the dispersed genera-tion facility had to qualify the proposed generation site ei-ther through self-qualification or FERC (Federal Energy Regu-latory Commission) certification. Self-qualification was theeasier approval method because it required only a letter toFERC documenting that the proposed facility meet the PURPAeligibility requirements. Even though the intent of PURPAwas to conserve oil and gas resources, certain clauses withinthe law allowed these fuels to be used as primary fuel bythe Qualified Facilities (QF). Natural gas in particular, be-came a widely-used fuel for QF facilities.

PURPA also created a second type of dispersed genera-tion, the non-utility generator, that was in business solely tosell power to the utility for a profit. In the 1980s and 90s,as reserve margins within utilities dwindled, some utilitiesbegan inviting dispersed generator facilities to provide ca-pacity for their systems. They view dispersed generators asa viable alternative to building their own generating plantsthereby avoiding a large commitment of utility capital withuncertain returnsdifficulties caused by the regulatory pro-cess.

As natural gas costs have fallen, small dispersed micro-turbines have received renewed interest. Some of these high-speed permanent magnet generators use advanced technol-ogy turbines originally developed for military vehicles. Theyare currently manufactured in the range of 20 to 85 KVAand are connected to the electrical system of commercialcustomers to peak share the utility. Peak sharing allowsthe commercial facility to reduce demand charge. Some in-dustry experts believe that micro-turbines will play a sig-nificant role in meeting load demands in the next millenium.A number of utilities, taking advantage of new deregulationlaws, are involved in marketing dispersed micro-turbine gen-erators. Only time will tell if these types of dispersed gen-erators will find a place as a viable power source to com-mercial or even residential customers.

Interconnection protection requirements have also evolvedover the years. In the 1980s, the IEEE became involved indevelopment of recommendations and guidelines for the in-terconnection of IPP generators. ANSI/IEEE Standard 1001-1988 [1] provided the basic guidelines adopted by manyutilities. By 1990, most U.S. utilities published specific guide-lines for the connection of smaller dispersed generators (usu-ally less than 5 MW) to their systems. These guidelines al-most universally specified voltage and frequency relayingand required the relays to be utility-gradethey meet IEEE/ANSI C37.90 design standards. Over the years, these relayshave evolved from electromechanical to static, and finallyto digital protection devices.

III. INTERCONNECT VERSUS GENERATOR PROTECTION

Interconnect protection provides the protection that al-lows the dispersed generators to operate in parallel with theutility grid. Typically, protection requirements to connect adispersed generator to the utility grid are established by eachindividual utility. These standards generally cover smallergenerators. Larger generators are reviewed on a case-by-case basis and are usually connected to the utilitys trans-mission system. These larger dispersed generators do nottypically employ specific interconnection protection becausethey are integrated into the utility protection system itself.Dispersed generators (5 MW or smaller) are usually con-nected to the utilitys sub-transmission and distribution sys-tems. These utility circuits are designed to supply radialload. Thus, the introduction of generation provides a sourcefor redistributing the feeder circuit load and fault current aswell as a potential source of overvoltage. Typically, inter-connection protection for these generators is established atthe point of common coupling between the utility and theIPP. This can be at the secondary of the interconnectiontransformer as illustrated in Fig. 1a, or at the primary of thetransformer as illustrated in Fig. 1b, depending on owner-ship and utility interconnect requirements.

GG

Fig. 1a Typical Interconnection Protection

GG

Fig. 1b Typical Interconnection Protection

Interconnection protection satisfies the utilitys require-ments to allow the generator to be connected to the grid. Itsfunction is three-fold:

1. disconnects the generator when it is no longer operat-ing in parallel with the utility system;

2. protects the utility system from damage caused by con-nection of the generator, including the fault current sup-

plied from the generator for utility system faults and tran-sient overvoltages;

3. protects the generator from damage from the utility sys-tem, especially through automatic reclosing.

Generator protection is typically connected at the termi-nals of the generator as shown in Fig. 2.

G G

Fig. 2 Typical Generator Protection

Generator protection provides detection of:

1. generator internal short circuits;

2. abnormal operating conditions (loss of field, reversepower, overexcitation and unbalanced currents).

For smaller dispersed generators, most U.S. utilities leavethe responsibility to the IPP owners and their consultants toselect the level of generator protection they believe is ap-propriate. Utilities, however, become very involved in speci-fying interconnect protection. Typically, the following in-terconnection areas are specified by the utility:

1. winding configuration of the interconnection trans-former;

2. general requirements of utility-grade interconnectionrelays;

3. CT and VT requirements;4. functional protection requirementsi.e., 81O/U, 27 and

59;5. settings of some interconnection functions;6. speed of operation.

IV. SMALLER, DISPERSED GENERATORS

A. Types of Small GeneratorsThere are two traditional types of smaller dispersed gen-

erators which operate interconnected with the utility sys-tem. They are induction and synchronous generators. In-duction machines are typically smallless than 500 KVA.These machines are restricted in size because their excita-tion is provided by an external source of VArs as shown inFig. 3a. Induction generators are similar to induction mo-tors and are started like a motor (no synchronizing equip-ment needed). Induction generators are less costly than syn-chronous generators because they have no field windings.Induction machines can supply real power (watts) to theutility but require a source of reactive power (VArs) whichin some cases is provided by the utility system.

Synchronous generators have a dc field winding to pro-vide a source of machine excitation. They can be a sourceof both watts and VArs to the utility system as shown inFig. 3b and require synchronizing equipment to be paral-leled with the utility. Both types of machines require inter-connection protection. Interconnection protection associatedwith induction generators typically requires only over/un-der voltage and frequency relaying.

G

!" #!$%&'&&(#!)**#*

#

#

Fig. 3a Induction Generator

G

+,!" *-#

.

#

/'

Fig. 3b Synchronous Generator

Non-traditional small dispersed generators, especially thenew micro-turbine technologies, are being talked about morefrequently as an energy source for the next millenium. Mostof these machines are asynchronously connected to the powersystem through Static Power Converters (SPCs). These SPCsare solid-state microprocessor-controlled thyristor devicesthat convert AC voltage at one frequency to 60 Hz systemvoltage. Digital electronic control of the SPC regulates thegenerators power output and shuts down the machine whenthe utility system is unavailable. The need for independentprotection to avoid system islanding has not yet been deter-mined and is being addressed through Standards Coordinat-ing Committee 21 (SCC21), Working Group P1547, withinthe IEEE. As the size of these machines increase, there maybe a need to consider independent interconnection protec-tion. Fig. 3c shows a typical one-line diagram for these typesof generators.

G

+,!0"%)"'(#(#'*#!0

*#

Fig. 3c Asynchronous Generator

B. Major Impact of Interconnection TransformerConnections on Interconnection Protection

As mentioned in the previous section, the major functionof interconnection protection is to disconnect the generatorwhen it is no longer operating in parallel with the utilitysystem. Smaller IPPs are generally connected to the utilitysystem at the distribution level. In the U.S., distribution systemsrange from 4 to 34.5 KV and are multi-grounded 4-wiresystems. The use of this type of system allows single-phase,pole-top transformers, which typically make up the bulk ofthe feeder load, to be rated at line-to-neutral voltage. Thus,on a 13.8 KV distribution system, single-phase transform-ers would be rated at 13.8 KV/3~8 KV. Fig. 4 shows atypical feeder circuit.

Five transformer connections are widely used to inter-connect dispersed generators to the utility system. Each ofthese transformer connections has advantages and disadvan-tages. Fig. 5 shows a number of possible choices and someof the advantages/problems associated with each connec-tion.

GG

/++11

1

!#

/#

#

Fig. 4 Typical 4-Wire Distribution Feeder Circuit

Delta (Pri)/Delta (Sec), Delta (Pri)/Wye-Grounded (Sec) andWye-Ungrounded (Pri)/Delta (Sec) Interconnect TransformerConnections

The major concern with an interconnection transformerwith an ungrounded primary winding is that after substationbreaker A is tripped for a ground fault at location F1, themulti-grounded system is ungrounded subjecting the L-N(line-to-neutral) rated pole-top transformer on the unfaultedphases to an overvoltage that will approach L-L voltage.This occurs if the dispersed generator is near the capacityof the load on the feeder when breaker A trips. The result-ing overvoltages will saturate the pole-top transformer whichnormally operates at the knee of the saturation curve as shownin Fig. 6.

G

+11

/2

/3

/4

5

A

Can supply the feedercircuit from anungrounded source aftersubstation breaker Atrips causingovervoltage.

Provides an unwantedground current forsupply circuit faults atF1 and F2.

Allows source feederrelaying at A to respondto a secondary groundfault at F3.

0'%)

,'*'%)

6

Provide no groundfault backfeed forfault at F1 & F2. Noground current frombreaker A for a faultat F3.

No ground currentfrom breaker A forfaults at F3. Noovervoltage for groundfault at F1.No overvoltage forground fault at F1.

"'

7

7

7

Fig. 5 Interconnection Transformer Connections

3&89!&3&8!&

!

*

#

Fig. 6 Saturation Curve of Pole-Top Transformers

Many utilities use ungrounded interconnection transformersonly if a 200% or more overload on the generator occurswhen breaker A trips. During ground faults, this overloadlevel will not allow the voltage on the unfaulted phases torise higher than the normal L-N voltage, avoiding pole-toptransformer saturation. For this reason, ungrounded primarywindings should be generally reserved for smaller dispersedgenerators where overloads of at least 200% are expectedon islanding.

Wye-Grounded (Pri)/Delta (Sec) Interconnect TransformerConnections

The major disadvantage with this connection is that itprovides an unwanted ground fault current for supply cir-cuit faults at F1. Fig. 7a and Fig. 7b illustrate this point fora typical distribution circuit.

VS

X

VIPP

F1 (L-G Fault)

XL1

XL2

Xsystem

UtilitySubstation

Xd",X2,X0

DG

XTSub.

Sub.A

XTDG

Fig. 7a Single-Line Diagram for Wye-Grounded (Pri) / Delta(Sec) Interconnection Transformer

7

"(&

XL1 XsystemXTsubXL2XTDGX"dgen

#6&

7

'"(&

XsystemXTsubX"2gen

#6&

7

:(&

X0L1

XTsub

X0L2X"0gen

#6&XTDG

/3

A

A

A

XL2XL1XTDG

/3

/3

VSVDG

Fig. 7b Symmetrical Component Circuit for Wye-Grounded (Pri)/Delta (Sec) Interconnection Transformer

Analysis of the symmetrical component circuit in Fig. 7balso shows that even when the dispersed generator is off-line (the generator breaker is open), the ground fault cur-rent will still be provided to the utility system if the dis-persed generator interconnect transformer remains connected.This would be the usual case since interconnect protectiontypically trips the generator breaker. The transformer at thedispersed generator site acts as a grounding transformer withzero sequence current circulating in the delta secondary wind-ings. In addition to these problems, the unbalanced loadcurrent on the system, which prior to the addition of thedispersed generator transformer had returned to ground throughthe main substation transformer neutral, now splits betweenthe substation and the dispersed generator transformer neutrals.This can reduce the load-carrying capabilities of the dis-persed generator transformer and create problems when thefeeder current is unbalanced due to operation of single-phaseprotection devices such as fuse and line reclosers. Even thoughthe wye-grounded/delta transformer connection is univer-sally used for large generators connected to the utility trans-mission system, it presents some major problems when usedon 4-wire distribution systems. The utility should evaluatethe above points when considering its use.

Wye-Grounded (Pri)/Wye-Grounded (Sec) InterconnectTransformer Connections

The major concern with an interconnection transformerwith grounded primary and secondary windings is that italso provides a source of unwanted ground current for util-ity feeder faults similar to that described in the previoussection. It also allows sensitively-set ground feeder relaysat the substation to respond to ground fault on the second-ary of the dispersed generator transformer (F3). Fig. 8a andFig. 8b illustrate this point through the analysis of sym-metrical component circuitry.

C. ConclusionsThe selection of the interconnection transformer plays

an important role in how the dispersed generator will inter-act with the utility system. There is no universally acceptedbest connection. All connections have advantages and dis-advantages which need to be addressed by the utility in theirinterconnection guidelines to dispersed generators. The choicesof transformer connection also have a profound impact oninterconnection protection requirements.

X

VIPP

F3

XL

Xsystem

UtilitySubstation

X"d,X2,X0

DG

XTSub.

51N/50NASub.

XTDG (L-G Fault)

VS

Fig. 8a Single-Line Diagram for Wye-Grounded (Pri) /Wye-Grounded (Sec) Interconnection Transformer

7

:(&

X0L XTsubX"0gen

#6&

XTDG

/4

XTDG

/4

XTDG

/4

7

VDG

"(&

XsystemXTsubXLX"dgen

#6&

7

'"(&

XsystemXTsubXLX"2gen

#6&

VS

A

A

A

Fig. 8b Symmetrical Component Circuit for Wye-Grounded (Pri)/

Wye-Grounded (Sec) Interconnection Transformer

V. SMALLER, DISPERSED GENERATOR INTERCONNECTIONPROTECTION METHODS AND PRACTICES

The functional levels of interconnection protection varywidely depending on factors such as: generator size, pointof interconnection to the utility system (distribution orsubtransmission), type of generator (induction, synchronous,asynchronous) and interconnection transformer configura-tion (see previous section of this paper). As shown in Table1, specific objectives of an interconnection protection sys-tem can be listed, as well as the relay functional require-ments to accomplish each objective.

G46

metsysytilituhtiwnoitarepolellarapfossolfonoitceteD **TT,I95,95/72,*R18,U/O18

noitceteddeefkcabtluaF 12,76,V15:stluaFesahPN72,N95,N76,N15:stluaFdnuorG

snoitidnocmetsysgnigamadfonoitceteD 64,74

noitcetedwolfrewoplamronbA 23

noitarotseR 52

egnahcfoetaR*pirTrefsnarT**

Table 1 Interconnection Protection Areas

A. Detection of Loss of Parallel Operation with theUtility System

The most basic and universal means of detecting loss ofparallel operation with the utility is to establish an over/underfrequency (81O/U) and over/undervoltage (27/59) win-dow within which the dispersed generator is allowed tooperate. When the dispersed generator is islanded from theutility system, either due to a fault or other abnormal condi-tion, the frequency and voltage will quickly move outsidethe operating window if there is a significant difference be-tween load and dispersed generator levels.

In some cogeneration applications such as within the petro-chemical and pulp and paper industries, rate of change offrequency relays (81R) are used to more rapidly detect theloss of utility supply. The 81R function separates the plantfacility from the utility. In many cases, internal plantunderfrequency load shedding takes place and critical loadsare isolated and are supplied from the plants dispersed gen-erators.

If the load and generator are near a balance at the timeof separation, voltage and frequency may stay within thenormal operating window and under/overfrequency and over/undervoltage tripping may not take place. If this is a possi-bility, then transfer trip (TT) using a reliable means of com-munication may be necessary. When induction generatorsare islanded with pole-top capacitors and the generator ca-pacity is near that of the islanded load, a resonant conditionthat produces a non-sinusoidal overvoltage can occur [5].For these cases, an instanteous overvoltage relay (59I) thatresponds to peak overvoltage can be used to detect this situ-ation.

When the loss of parallel operation is detected, the dis-persed generator must be separated from the utility systemquickly enough to allow the utility breaker at the substationto automatically reclose. High-speed reclosing from the utilitysystem can occur as quickly as 15 to 20 cycles after breakertripping. The utility needs to provide guidance to the dis-persed generator owner on the speed of separation required.

The use of underfrequency relays coupled with the needto separate the dispersed generator prior to utility breakerreclosing, precludes the ability of most smaller dispersedgenerators to provide power system support to the utilityduring major system disturbances. When frequency decreasesdue to a major system disturbance, these generators willtrip off-line. It may be possible to reduce underfrequencysettings to comply with regional Reliability Council require-ments, but the required trip time cannot generally be ex-tended to exceed automatic reclosing times. This system prob-lem will become more critical if the percentage of total sys-tem generation provided by smaller dispersed generators

increases over the next ten years as forecasted by some in-dustry experts.

The modification of substation reclosing using sourcevoltage supervision along with synchrocheck reclosing maybe needed if underfrequency trip times are extended. Thistype of scheme is illustrated in Fig. 9 and provides securityagainst reclosing prior to disconnection of the dispersed gen-erator.

*

A

* May require slip as well as angle detection

Utility Substation

DG

2527

Fig. 9 Utility Substation Scheme

Fig. 10 shows a typical basic over/undervoltage and over/underfrequency scheme for a small IPP installation. Theseprotection functions can be accommodated in a single mul-tifunction digital relay.

6#

81/O

81/U27 59

;##

GFig. 10 Typical Small Generator Interconnection Protection

B. Fault Backfeed DetectionOn many small dispersed generators, no specific fault

backfeed detection is generally provided. Induction genera-tors provide only two or three cycles of fault current toexternal faults similar to induction motors. Small synchro-nous machines are typically so overloaded after the utility

substation breaker trips that their fault current contributionis very small. For these small generators, the detection ofloss of parallel operation via 81O/U and 27/59 relays is allthe interconnection protection necessary.

The larger the dispersed generator, the greater is the chancethat it will contribute significant current to a utility systemfault. For this situation, fault backfeed detection in additionto loss of parallel operation protection is provided. It shouldbe recognized that the longer the generator is subjected to afault, the lower the current that the synchronous generatorprovides to the fault. Fig. 11 shows the generator decrementcurve. The level of fault current at various intervals afterthe fault occurs depends on the generator reactances (Xd,Xd). The decay rate depends on the open circuit field timeconstants (Tdo, Tdo).

%7

#6%7

6#

#6

51N

G

4

47 27 59 81/O81/U

51V 46 3267

Fault BackfeedRemoval

DamagingConditions

AbnormalPower Flow

Loss ofParallel

Transfer Trip **

;##

24=

67N*

Control

3

* or 21 Function** may be required depending on IPP size

Fig. 13 Alternate Typical Protection for Moderately-SizedDispersed Generator with Wye-Grounded (Pri)Interconnection Transformer

#!!#

#6

G

4

47 27 59 81/O81/U

51V 46 3267

Fault BackfeedRemoval

DamagingConditions

AbnormalPower Flow

Loss ofParallel

Transfer Trip ** ;##

24=

*

27N59N

34=

+ or

Control

* or 21 Function** may be required depending on IPP size

Fig. 14 Typical Protection for Moderately-Sized DispersedGenerator with Ungrounded (Pri) InterconnectionTransformer

C. Detection of Damaging System ConditionsUnbalanced current conditions caused by open conduc-

tors or phase reversals on the utility supply circuit can sub-ject the dispersed generator to a high level of negative se-quence current. This high negative sequence current resultsin rapid rotor heating causing IPP generator damage. Manyutilities provide the protection against these unbalanced cur-rents as part of the interconnection protection package us-ing a negative sequence overcurrent relay (46). To provide

protection for phase reversals caused by inadvertent phaseswapping after power restoration, a negative sequence voltagerelay (47) is also used. These functions are shown in Fig.12, Fig. 13 and Fig. 14.

D. Abnormal Power FlowSome interconnection contracts between cogenerating dis-

persed generators and the utility prohibit the dispersed gen-erator from providing power to the utility. The cogeneratingdispersed generator provides power solely to the local loadat the dispersed generator facility and reduces utility de-mand charges by peak shaving. It is the frequent practiceof utilities to install a directional power relay (32) to tripthe dispersed generator if power inadvertently flows intothe utility system for a predetermined time in violation ofthe interconnection contract. Fig. 12, Fig. 13 and Fig. 14illustrate this type of abnormal power flow detection.

E. Dispersed Generator Tripping/Restoration PracticesOnce the dispersed generator has been separated from

the utility system, after interconnection protection opera-tion, the intertie must be restored. Two dispersed generatortripping/restoration practices are widely used within the in-dustry. The first restoration method (case 1) is used in ap-plications where the generation at the dispersed generationfacility does not match the local load. In these cases, inter-connection protection typically trips the dispersed genera-tor breakers, as illustrated in Fig. 15. When the utility sys-tem is restored, the dispersed generators are typically auto-matically resynchronized. Many utilities require asynchrocheck relay (25) at the main incoming breaker tosupervise reclosing as a security measure to avoidunsynchronized closure. The synchrocheck relay is gener-ally equipped with dead bus undervoltage logic to allowreclosure from the utility system for a dead bus condition atthe dispersed generation facility.

G G

A

B C

29

#&&

!1?5

#!"'

;##'

5!6$1?@

*

*6$1?#

#&&

Fig. 15 Restoration after Interconnection TrippingCase 1

The second restoration method (case 2) is used where thedispersed generator roughly matches the local load. In thesecases, the interconnection protection trips the main incomingbreaker (breaker A) as illustrated in Fig. 16. In many cases,the dispersed generation facility may have internalunderfrequency load shedding as is the practice at petro-chemical and pulp and paper facilities to match the local loadto available dispersed generation after the utility separation.To re-synchronize the dispersed generation facility to the utilitysystem, a more sophisticated synchrocheck relay is requiredwhich not only measures phase angle () but also slip (F)and voltage difference (V) between the utility and dispersedgeneration systems. Typically, such relays supervise manualand supervisory reclosing.

G G

A

B C

29

!

;##'

529(#

*&

/!

'&

5

Fig. 16 Restoration after Interconnection TrippingCase 2

VI. USE OF DIGITAL TECHNOLOGYFOR INTERCONNECTION PROTECTION

Modern multifunction digital relays have a number of fea-tures which make them an ideal choice for interconnectionprotection of dispersed generators. The most important ofthese features are user-selectable functionality, self-diagnostics,communications capabilities and oscillographic monitoring.

A. User-Selectable (Pick and Choose) FunctionalityAs pointed out in this paper, interconnection protection

functionality varies widely with generator size, point of in-terconnection to the utility system, type of generator (in-duction or synchronous) and configuration of interconnec-tion transformer. These variables make user-selectable (pickand choose) functionality an important feature. This fea-ture allows the specific configuration of the multifunctiondigital relay to be controlled by the user rather than themanufacturer. The cost is proportional to the level of func-

tionality required. The user that purchases an expensive mul-tifunction interconnection package, only to disable numer-ous functions because they are not appropriate for this appli-cation, dilutes the economic advantage of multifunction pro-tection. By using a relay with the basic functions needed inmost applications and then selecting from a library of op-tional functions, the user configures the protection for thespecific application at the lowest cost. Fig. 17 shows a typicalinterconnection application using this approach.

B. Self-DiagnosticsSelf-diagnostics of a multifunction digital relay provides

immediate detection of relay failure. Without interconnec-tion protection, the dispersed generator, as well as the utilityssystem, may be subjected to damaging conditions such asundetected fault currents, overvoltages and high dispersedgenerator shaft torque due to automatic reclosing. For thesereasons, self-diagnostics takes on renewed importance. Manyutilities trip the dispersed generator on failure of the inter-connection protection package to avoid such damage. Self-diagnostics provide the utility with some assurance that theinterconnection protection is functional. This type of assur-ance was not available in older electronic or electromechanicaltechnologies.

C. Communications CapabilityAll multifunction digital relays have communication ports.

These are typically RS-232, RS-485 or in some cases, fi-ber-optic connections. Most moderate-to-large sized dispersedgenerators are required to provide continuous telemetry dataon generator operation to the utility. Information such asstatus (open or closed) of key interconnection and genera-tion breakers, as well as instantaneous MW and MVAr gen-erator output are typically required. Much of this informa-tion can be obtained from the interconnect relay package,eliminating the need for separate transducers and metering.Also, the ability to interrogate the interconnect protectionrelay from a remote location to determine the relay targetsthat operated provides information that is vital in restoringthe dispersed generator to service.

D. Oscillographic MonitoringOscillographic monitoring of relay inputs (currents and

voltages) provide information on the cause of the intercon-nect relays operation and if the relay has operated as planned.Since interconnection protection is applied at the point ofcommon coupling between the utility and the dispersed gen-erator facility, it provides valuable information as to whichsystem may have precipitated the tripping. Oscillographicinformation has resulted in settling a number of argumentsbetween utilities and dispersed generator owners as to thecause of a particular tripping event.

3

45

4

42

3

5'%)6*6$'A'#!'!!&

!"#

$%&'(#%

)**

&

3

#'#!'#

Fig. 17 One-Line Diagram for Digital Multifunction Interconnect Relay

VII. CONCLUSIONS

Interconnection protection will have renewed importancein the next millenium if the predictions of a number of indus-try experts become reality. Properly designed interconnec-tion protection should address the concerns of both the dis-persed generator owner as well as the utility. This paper hasattempted to outline the salient points that utilities and dis-persed generator owners need to consider when developinginterconnection requirements. One of the most important, andmost frequently overlooked, is the configuration of the inter-connection transformer. It plays a pivotal role in minimizingpotential utility system overvoltage and in determining inter-connection protection requirements.

Functional interconnection protection requirements varywidely. Factors that determine protection requirements in-clude: generator size, point of interconnection to the utilitysystem, type of generator (induction or synchronous), andfault backfeed levels. These variables make user-selectable,pick and choose functionality an important feature of modernmultifunction digital interconnection relays. In addition totripping logic, automatic restoration is also required and canbe incorporated within a digital interconnection relay pack-age. Hopefully, the issues highlighted in this paper will benefitutilities when they review their interconnection practices.

REFERENCES

[1] ANSI/IEEE Std. 1001-1988, Guide for InterfacingDispersed Storage and Generation Facilities with Elec-tric Utility Systems.

[2] Donahue, K.E., Relay Protection Interface andTelemetry Requirements for Non-Utility Generatorsand Electric Utilities, 1998 Power Generation Con-ference, Orlando, Florida.

[3] Mozina, C.J., Protecting Generator Sets Using Digi-tal Technology, Consulting/Specifying EngineerMagazine, EGSA Supplement, November 1997.

[4] Feero, Gish, Wagner and Jones, Relay Performancein DGS Islands, IEEE Transactions on Power De-livery, January 1999.

[5] ANSI/IEEE C37.102-1995, IEEE Guide for ACGenerator Protection.

[6] Yalla, Hornak, A Digital Multifunction Relay forIntertie and Generator Protection, Canadian Elec-trical Association Conference, March 1992.