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Collection Technique

Cahier technique no. 196

Integration of local powergeneration in industrial sitesand commercial buildings

T. Hazel


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"Cahiers Techniques" is a collection of documents intended for engineersand technicians, people in the industry who are looking for more in-depthinformation in order to complement that given in product catalogues.

Furthermore, these "Cahiers Techniques" are often considered as helpful"tools" for training courses.They provide knowledge on new technical and technological developmentsin the electrotechnical field and electronics. They also provide betterunderstanding of various phenomena observed in electrical installations,systems and equipments.Each "Cahier Technique" provides an in-depth study of a precise subject inthe fields of electrical networks, protection devices, monitoring and controland industrial automation systems.

The latest publications can be downloaded from the Schneider Electricinternet web site.Code: http://www.schneider-electric.comSection: Experts' place

Please contact your Schneider Electric representative if you want either a"Cahier Technique" or the list of available titles.

The "Cahiers Techniques" collection is part of the Schneider Electrics"Collection technique".

ForewordThe author disclaims all responsibility subsequent to incorrect use ofinformation or diagrams reproduced in this document, and cannot be heldresponsible for any errors or oversights, or for the consequences of usinginformation and diagrams contained in this document.

Reproduction of all or part of a "Cahier Technique" is authorised with theprior consent of the Scientific and Technical Division. The statement"Extracted from Schneider Electric "Cahier Technique" no. ....." (pleasespecify) is compulsory.


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no. 196

Integration of local powergeneration in industrial sitesand commercial buildings

ECT 196 (e) first issue, January 2000

Terence HAZEL

Terry Hazel received his BSc in Electrical Engineering from theUniversity of Manitoba Canada in 1970. He then worked in Perth

Australia for a year as power coordination engineer, and in FrankfurtGermany as a consulting engineer until he joined Merlin Gerin in 1980.For 15 years was the technical team leader for several majorinternational projects involving process control and power distribution.He has since been with the tendering section of the industrial projectsdepartment and often meets with clients during the front endengineering stage to discuss and compare the various possibleelectrical distribution systems. He is an active member of IEEE and haspresented papers dealing with electrical power distribution at Industry

Applications Society conferences.


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Cahier Technique Schneider Electric no. 196 / p.2

Lexicon

Black start: The capability of starting generatorsets without the presence of a utility supply.

Damage curve: A current-versus-time curveshowing the allowable limit without permanentdamage to equipment.

Equipment commissioning: Performance ofthe testing and adjustment at site leading up toand including the energization of a piece ofequipment. An example would be the operationof one generator set.

Frequency droop: The absolute change infrequency between steady state no load and

steady state full load, typically 4%. An increasein power output results in a decrease offrequency for generator sets operating alone inthis mode.

Isochronous speed governing: Governing withsteady-state speed regulation of essentially zeromagnitude.

Load sharing: Centralized elaboration andsending of set points for generator set loading.This ensures that all sets will share the load inan equal manner proportional to their powerrating.

Load shedding: Voluntary disconnection of low

priority loads when the available power isinsufficient to supply the total plant load.

Residual voltage: The voltage on a busbar afterdisconnection from the supply. This voltage isgenerated by rotating machines which remainconnected to the busbar.

Spinning reserve: The difference between thetotal available capacity of all generating setsalready coupled to the system and their actualloading.

Static switch: A fast acting switch normallyconsisting of a power electronics device whichwill transfer the load from the power conversionmodule of a UPS to another supply without delayor unacceptable transients.

Synchronism-check relay: A verification relaywhose function is to operate when two inputvoltage phasors are within predetermined limits.

Synchroscope: An instrument embodying a

continuously rotatable element whose position isa measure of the instantaneous phase differencebetween the voltages across a circuit-breaker.

System commissioning: Performance ofadditional testing and adjustments at site ofequipment which have been commissioned toensure correct operation of the systemcomprised of the equipment. An example wouldbe the parallel operation of several generatingsets including synchronizing, and load sheddingfeatures.

System stability: A system is considered stable

if bounded input disturbances result in boundedoutput disturbances. For an electrical distributionsystem, changes in load, faults, switchingoperation, etc. will not cause wide fluctuations involtage or frequency if it is stable.

Unit substations: A substation containing theelectrical distribution equipment necessary forsupplying the loads of a particular plantproduction unit. It typically contains mediumvoltage switchgear, power and distributiontransformers, low voltage switchgear and MCC.

Voltage restrained overcurrent relay: Anovercurrent protection relay having a voltageinput which opposes the typical response of the

relay to the current inputs. This is used forgenerators since they deliver much lower short-circuit currents than utility connections havingthe same capacity.

Voltage waveform distortion: The differencebetween the actual voltage waveform and a puresinusoidal waveform, often expressed as totalharmonic distortion,

THDh

U

U=

2

1

where Uh is the harmonic voltage and U1 is thefundamental of the voltage waveform.

X/R ratio: The ratio of the electrical distributionsystem inductance to the resistance. This ratiodetermines the time constant of the d.c.component of the short-circuit current which isan important factor in defining the rating of high-voltage circuit-breakers.


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Integration of local power generation inindustrial sites and commercial buildings

Contents

1 Types of engine generator sets p. 4

2 Rated power for generator set applications p. 5

3 Typical applications 3.1 Stand-by generator sets p.7

3.2 Production generator sets p. 9

4 Operation of generator sets 4.1 Starting and stopping of generator sets p. 11

4.2 Stand alone operation p. 12

4.3 Parallel operation with utility supply p. 12

4.4 Parallel operation with other generator sets p. 12

5 Transfer schemes and synchronization 5.1 Automatic transfer on loss of supply p. 14

5.2 Maintenance transfer p. 14

5.3 Synchronization of generator circuit-breaker p. 14

5.4 Synchronization of bus-tie, bus coupler, or utility incomingcircuit-breakers p. 15

6 Generator set protection 6.1 General protection philosophy p. 16

6.2 Electrical protection p. 17

6.3 Machine protection p. 18

7 Connection of generators to electrical network 7.1 Connection to generator circuit-breaker p. 19

7.2 Connection of generator neutral point p. 19

8 Load shedding p. 20

9 Interfacing generator with electrical 9.1 Typical split of supply between generator setdistribution system manufacturer and switchgear manufacturer p. 21

9.2 Information to be exchanged p. 21

9.3 Integration of generator set into electrical distributionsupervisory system p. 22

10 Installation of engine generator sets 10.1 Location p. 23

10.2 Air intake and exhaust p. 23

10.3 Compliance with local regulations p. 23

10.4 Special tools and spare parts p. 24

11 Conclusion p. 24

Bibliography p. 24

Engine driven alternating current generator sets are often used in remoteindustrial sites as a prime source of electrical energy. They are alsoextensively used in both industry and commercial buildings as a source ofback-up power. This cahier technique discusses most of the subjectswhich have to be handled when implementing engine driven alternatingcurrent generator sets having rated powers up to 20 MW.


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The main types of prime movers used inengine driven generator sets for industrial sitesand commercial buildings are Diesel engines,gas turbines, and steam turbines. Turbines areused mainly for production sets whereas Dieselengines can be used for both production andstandby sets.

Most of the topics covered in this cahiertechnique are not dependant on the type of

1 Types of engine generator sets

prime mover used, and therefore the generalterm generator set will be used.

The choice of the prime mover is determined bysuch considerations as the availability and typeof fuel and is not covered in this cahiertechnique.

Since Diesel engines are very often used somespecific information about Diesel generator sets

will be given.

Fig. 1: different configurations of local generation

Above is an example of a

combined oil treatment and power

plant. It incorporates two gas

turbine generator sets with an

output of approx. 100 MW.

(Courtesy of GE Energy Products France S.A.).

In most industrial plants, however,

power generation is not the main

purpose. The plant may have one

or several Diesel generator units to

produce the necessary electrical

power, mainly for stand-by, and

possibly for local consumption

requirements. The picture shows a

1 MW Diesel generator unit.

(Courtesy of Houvenaghel/Hennequin S.A.).


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The power output requirement for the generatorset is probably the most important criterion to bedefined. The output of a generator set is typically

defined on the active/reactive power graph asrepresented in figure 2.

The active power output depends on the type of

fuel used, and on site conditions including

ambient temperature, cooling medium

temperature, altitude, and relative humidity. It

also depends on load characteristics such as

possible overloading and load variations over

time. The ISO 3046-1 standard for Diesel

engines defines three different types of power

ratings, and a standard definition of overloadcapability.

The different power ratings are:

c continuous power rating: The engine cansupply 100% rated power for an unlimited time.This rating is normally used for production sets.

c prime power rating: The engine can supply a

base load for an unlimited time, and 100% rated

power for a limited time. The base load andacceptable time for 100% rated power are

different for each manufacturer. Typical valuesare a base load of 70% of the rated power, and

100% rated power during 500 hours per year.

Fig. 2: active/reactive power graph showing operating limits

c standby power rating: This is the maximumpower that the engine can deliver and is limitedin time, typically less than 500 hours per year.This rating should only be applied to generatorsets which are used exclusively for emergencypower. Since the engine is incapable ofsupplying more power, a security factor of atleast 10% should be used when defining thestandby power rating.

The standard overload capacity is defined as10% more power during 1 hour for every 12hours of operation. There is no overload capacitywith a standby power rating. Most manufacturersallow the standard overload capacity with thecontinuous power rating and the prime powerrating, but since there are exceptions, theoverload capacity should always be specifiedtogether with the type of power rating used.A typical example is a Diesel engine having acontinuous power rating of 1550 kW, a primepower rating of 1760 kW, and a standby powerrating of 1880 kW.

2 Rated power for generator set applications

Active poweraxis

Reactive poweraxis

Rated engine power

Normal operation point, cos = 0.8

Over-excited machineUnder-excited machine

Stator current limit

Stability limit

Pn

Qn

Excitation current limit


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When generator sets are used as a prime sourceof electrical energy the following points shouldbe considered:

c provide for parallel operation with other setsand/or with utility,

c allow for long maintenance periods (overhaul),c ensure black-start capabilities,

c use low speed equipment for long life(maximum 750 rpm for Diesel engines).

When used as a standby source:

c ensure quick and reliable start-up and loading,

c implement reliable load shedding to avoidoverloading or stalling,

c allow for periodic testing under load,

c provide for parallel operation with utility if set isused during peak loads,

c supply magnetizing current for distributiontransformers.

One common application for standby generatorsis to supply UPS (uninterrupted power supply)equipment during power outages. Since the

generator has a relatively high impedance ascompared to a utility supply, voltage waveformdistortion can occur due to harmonic currentsgenerated by the UPS. Generator manufacturersnormally derate their machines by up to 60% toensure correct voltage waveforms when loadsare UPS equipment without harmonic currentfiltering. The engine must also be able to supplythe power absorbed by the UPS which isdetermined by

P =UPS output kW+battery recharge kW

UPS efficiency+ auxil. load

For preliminary generator set sizing wheredetailed UPS information is unavailable, thebattery charger kW can be estimated to be 25%of the UPS output kW, and the UPS efficiencycan be estimated to be 90%. Final determinationof the generator set should be based onspecified values of acceptable voltage distortion,and the actual UPS data such as efficiency, andharmonic currents.


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The typical supply of essential loads forcommercial buildings, small industrial sites or foremergency power to unit substations in a largersite, is shown in figure 3.

Under normal operating conditions the essentialload is supplied from the utility supply. Upon lossof this supply the bus-tie circuit-breaker Q3 istripped, the generator set is started, and thenload is supplied by the standby generator set byclosing the generator circuit-breaker Q2.

Critical loads which cannot accept any poweroutage are supplied by the UPS. The UPS isequipped with a static switch which willimmediately bypass the rectifier/inverter module

3.1 Standby generator sets

3 Typical applications

Fig. 3: typical emergency supply for small industrial sites

in case of an internal fault and thus ensure a

continuous supply of electrical power.

Typical generator set sizes for this scheme are250 kVA to 800 kVA.

The advantage of this scheme is its simplicity

and clarity. All essential loads are connected tothe same busbar as the generator set and

therefore no load shedding is required. UPS

backup time can normally be limited to 10

minutes since the UPS will be supplied by theemergency supply.

Both the normal and the backup supply to the

UPS should be taken from the essential busbar.

G

Emergency

supply

Q2Q1Q3

Normal loads Emergency

loads

Critical loads

Utility supply

~ ~

Staticswitch

UPS


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For large industrial sites a centralizedemergency power supply system as shown infigure 4 is often used.

The main emergency switchboard is normallysupplied from the utility, although in some sites

Fig. 4: typical emergency supply for large industrial sites

GG

Essentialloads

Essential loadsNormal loads

Emergency

switcboard -main substation

Typical unit substation - medium voltage switchboard

To essential busbarof other unit substations

Essentialloads

ATS

Utility incoming feeders

33 kV

6 kV

6 kV

(ATS: Automatic transfer system)

one of the generator sets may be in constantoperation. The emergency switchboard isdesigned to allow generator sets to operate inparallel and also to be connected to the utilitysupply.


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3.2 Production generator sets

c fewer generator sets for the site (normallymaximum of 2),

c permanently energized emergency supplyallowing fast transfer schemes to be used,

c no loss of emergency supply due to

maintenance of one generator set.Generator sets for such systems are normally inthe 1-4 MW range.

Fig. 5: industrial site without utility supply

For remote sites having no utility supply, severalgenerator sets are used. A typical distributionsystem is shown in figure 5.

The number of sets N will depend on the powerrequired, but since generator sets requireperiodic maintenance, plant power should be

able to be supplied by N - 1 sets without anyload shedding.

The generator set size should be such that theyare loaded at least 50%. A poor load factor can

be detrimental to the sets. For example Dieselengines loaded at less than 30% will not achievea good operating temperature resulting in poorcombustion and degrading of lubrication oil.

Plant operation at N - 2 sets should also beconsidered, this case occurring when one set is

being maintained and there is a loss of anadditional set.

The highest initial load factor F that can be usedwith N installed generators such that load

The automatic transfer from the utility to theemergency supply is performed in each unitsubstation. Since the emergency switchboard isnormally energized, fast transfers (described insection 5.1) without loss of plant load can beused.

The use of a centralized emergency supply hasthe following advantages:

Typical unit substation

GG G GG G

To othersubstations

To othersubstations

Generator undermaintenance

Yt

Yt

Earthingtransformer


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shedding is not required for N - 2 operation canbe determined from:

FN

N=

2

1

For example the highest load factor for N = 6 will

be 80%.

Bus-tie circuit-breakers are often used formaintenance purposes. During normal plantoperation all bus-tie circuit-breakers are normallyclosed. Short-circuit calculations should alwaystake operation with N generators into accountsince it is normal to connect standby sets prior toswitching off sets for maintenance.

A power supply using local generation is generallymuch weaker than a utility supply and therefore itis probable that load shedding will be required tomaintain system stability during fault conditions.

Determination of how much load must be shed

requires dynamic simulation of the network fordifferent fault conditions such as a loss of agenerator or a short-circuit. Prior to the study it is

necessary to determine which operatingconfigurations are to be considered. Operatingconditions with the bus-tie circuit-breaker both inthe open and the closed positions will greatlyincrease the complexity of the load sheddingsystem since each busbar can be operated

independently and will require specific loadshedding criteria. For most plants it isrecommended that only the standard operatingconfiguration be used for the dynamicsimulations and definition of the load sheddingstrategy.

Figure 5 shows each generator having its owntransformer. The use of generator transformershas several advantages:

c provides flexibility in the choice of generatorvoltage,

c reduces peak short-circuit current at mainboard,

c allows use of high impedance generatorgrounding (reduces possible damage togenerator).


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should the frequency and voltage tolerance besmall.

When stopping a generator set, the power outputshould be reduced to zero by transferring theload to other sources, and the circuit-breakerthen tripped. The generator set should be run forseveral minutes to allow it to cool down prior toshutdown. In some cases the cooling systemshould continue to operate after shutdown inorder to remove latent heat from the machine.Manufacturers recommendations for shutdownshould be followed.

Generator set start and stop sequences should behandled by the generator set control equipment.

Generator sets should be operated periodically.For installations where short power outages arenot critical, opening the normal incoming circuit-breaker will cause the set to start andautomatically pick up the emergency load.After the required minimum operating time, thegenerator circuit-breaker can be tripped and thenormal source circuit-breaker closed.

For plants where power outages meanunacceptable production losses, it must be

possible to test generator sets without firstswitching off the supply. This is normally done byusing a maintenance transfer. The generator setis started, and after it is ready to take load, it issynchronized to the incoming supply (seesection 5.3 below).

The generator circuit-breaker (or bus-tie circuit-breaker depending on the scheme) will then beclosed and the generator will thus be paralleledwith the incoming supply. The closing of thecircuit-breaker will cause tripping of the incomingsupply and the loads will be supplied by thegenerator. The transfer to the normal incomingsupply is done in the same manner withoutpower interruption. Since the supplies areparalleled only for a few hundred milliseconds, itis not necessary to dimension the switchboardfor the combined short-circuit power of both thenormal incoming supply and the generator.

Where equipment has been designed to operatein parallel on a permanent basis, it is notnecessary to trip the incoming supply afterconnection the generator to the load. For thiscase, however, the switchboard must bedesigned for the combined short-circuit power ofthe incoming supply and the generator.

Since Diesel generator sets are often used foremergency power, it is necessary that steps betaken to ensure that the set will start correctlyand quickly when required.

An example of measures to be taken islubrication, and heating of the cooling waterwhen the set is not operating. The Dieselgenerator set manufacturer should list all suchmeasures and the design should take intoaccount the availability of all auxiliary suppliesnecessary during times when set is notoperating.

A starting time of 15 seconds from the start orderto the closing of the generator circuit-breaker canbe guaranteed by manufacturers. Specifyingshorter starting times should be avoided sincethe decrease in starting time will be small andcould increase the cost of the set. Criticalequipment must be supplied by an UPS in anycase.

Two techniques are commonly used for starting.These are compressed air and battery,compressed air generally being used for largersets. The starting equipment should be designed

for a minimum of 3 consecutive starts. It shouldbe carefully monitored in order to enablepreventive maintenance to be carried out prior toa failure during an attempted start. Failure tostart is most often due to a problem with thestarting battery. Where reliable starting isessential, consideration should be given to usingcompressed air.

When a generator is operating in parallel withanother source, it will be synchronized asdescribed in section 5.3 hereafter, and graduallyloaded.

When a generator set is operating alone, theload will be applied in one or more steps. The

variation in frequency and voltage will dependupon the size of the step loads. As an example,step loads of 90% can be applied to a Dieselgenerator set without the frequency varying morethan 10% and the voltage more than 15%.

Should specific limits on frequency and voltagevariations be required, they should be specifiedtogether with the type of load which is to beconnected. This information should includemotor starting characteristics such as the startingcurrent, and the type of starting (direct-on-line,wye-delta). Several steps may be required

4.1 Starting and stopping of generator sets

4 Operation of generator sets


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4.2 Stand alone operation

4.3 Parallel operation with utility supply

4.4 Parallel operation with other generator sets

Generator sets are often designed to operate

independently (isochronous mode).

In such cases the system frequency will be

controlled by the engine governor. Overloadsexceeding the maximum power output (standbypower rating for Diesel engines as describedin 2) of the set will cause the system frequency

to decrease and this can be used for initiatingload shedding.

The generator voltage regulator will determine

the system voltage. Generators can normallyoperate at a power factor of 0.8 and thereforesupply most industrial loads without additionalpower factor compensation equipment.

In some cases permanent operation of thegenerator set in parallel with the utility supply isrequired. Since the utility supply is muchstronger it will determine the system frequencyand the system voltage.

The governor will therefore be used to controlthe active power output of the engine, and thevoltage regulator will control the reactive poweroutput of the generator.

The generator set must know in whichconfiguration it is operating in order to be able toswitch the governor and voltage regulatoroperation from frequency and voltage control(isochronous operation) to active and reactive

power control (parallel operation). Auxiliarycontacts from the switchboard are normally usedto provide the necessary information to thegenerator sets.

In this case generator sets are operated inparallel with other generator sets ofapproximately the same size. There are threebasic schemes used.

a) All generator sets but one have fixed active

and reactive power output settings. Onegenerator set is in the iscochronous mode andwill provide the active and reactive powernecessary to keep the system frequency andvoltage within the allowable limits. Anysynchronizing instructions for frequency orvoltage changes will be sent to the generator setin the isochronous mode. Since all powerfluctuations will be absorbed only by thisgenerator set, this scheme cannot be easily usedwhere there are large variations in load.

b) All generator sets operate in the droop mode.The active and reactive power is then sharedequally among the sets or in proportion to their

rated power if sets with different ratings are

used. Variations in load will cause voltage andspeed fluctuations due to the droopcharacteristic which is normally 4% from zero to100% load. Since synchronizing of the sets withanother source can only be done by adjustingthe droop setting, this scheme is normally not

used when parallel operation with anothersource is required.

c) All generator sets are interfaced in order toshare the active and reactive power. An exampleof how this is done is shown in figure 6. Eachengine governor receives the active power setpoint from the active load dispatcher which alsoprovides frequency regulation.

Similarly each excitation regulator receives thereactive power set point from the reactive powerdispatcher which also provides voltageregulation.

This scheme allows for large load variation

without changes in frequency or voltage.


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Fig. 6: parallel operation using a load dispatcher

Load

GG G

kW sharing andfrequency regulation

Governer

Excitationregulator

kvar sharing andvoltage regulation


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An automatic transfer normally occurs whenthere is a loss of the normal supply and the loadis to be supplied from the back-up supply with aminimum outage time. The transfer is blockedshould the reason for the loss of supply be afault on the busbar. Closing the emergencysupply circuit-breaker onto a busbar fault willresult in loss of the emergency supply and couldresult in damage to the equipment.

Two techniques for transferring are generallyused, their choice being based on whether or not

the plant can accept a brief loss of supply.Residual voltage transfer

This is the most commonly used automatictransfer scheme and has the following basic steps:

c trip the incoming breaker to isolate the loadfrom the supply

c start the generator set

c shed any loads which cannot be supplied fromthe generator set

c close the generator circuit-breaker after thegenerator set is able to be loaded, and theresidual voltage on the busbar is less than 30%.

Fast transfer

A fast transfer scheme is used when the processcannot accept any power outages.

Such a system requires that the backup supplybe permanently available and that the load istransferred to the backup supply before driveshave had time to slow down. The time window forsuch switching is about 150 ms.

In order to avoid the mechanical stresses andlarge currents due to out-of-phase switching, it isnecessary to give the closing order to theemergency supply circuit-breaker such that thevoltage generated by the decelerating motors isclose to being in phase with the emergencysystem voltage when the circuit-breaker closes.

Control gear for such transfer systems take intoaccount the closing time of the circuit-breaker inorder to anticipate the correct switching moment.If switching does not occur during the 150 mstime window, the fast transfer is blocked and aresidual voltage transfer is made including anyrequired load shedding.

5.1 Automatic transfer on loss of supply

5.2 Maintenance transfer

After the normal supply has returned, the load

should be transferred from the emergency

supply back to the normal supply. This is

normally initiated manually as described at theend of section 4.1 above.

5.3 Synchronization of generator circuit-breaker

Any time parallel operation of a generator set isrequired, it is necessary to be able tosynchronize it to the system. Synchronizationbasically consists in adjusting the generatorfrequency and voltage to values close to thesystem values. Since the system frequency andvoltage can vary within a few percent, it isnecessary that both the engine speed and thegenerator voltage be able to be adjusted forsynchronization purposes.

The engine speed and generator voltage arecontrolled by the governor and voltage regulator.Adjustments in the frequency and voltage arenormally achieved by momentarily closingcontacts connected to the governor and voltageregulator. When the generator voltage is almostin phase with the system voltage a closing orderis given to the generator circuit-breaker.

Synchronization is normally done automaticallyby means of relays which measure generatorand line voltages, frequencies, and phaseangles. The relay automatically adjusts thespeed and voltage of the generator set andcloses the circuit-breaker when the phase anglebetween the generator and line voltages issufficiently small. One set of automaticsynchronization equipment can be used forseveral generators by selecting the appropriatevoltage transformers and sending the voltage, speed as well as the closing order to theselected circuit-breaker.

Manual synchronizing should be provided in allcases, either as a back up to the automaticsynchronizing system, or for use in applicationswhere synchronization would only rarely occur.For manual synchronization the operator uses

5 Transfer schemes and synchronization


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push buttons to provide the voltage and speedadjustment signals.

A synchroscope will let the operator know whenthe line and generator voltages are sufficiently inphase to close the circuit-breaker. For manual

synchronization use of a synchronism checkprotection relay is recommended which will

inhibit closing of the circuit-breaker unless allconditions of frequency, voltage, and phaseangle have been satisfied.

Synchronization across the generator circuit-breaker is often included as a standard feature in

generator set control equipment.

5.4 Synchronization of bus-tie, bus-coupler, or utility incoming circuit-breakers

When several generator sets are used, they areoften connected to different busbars in order tofacilitate maintenance. It is therefore possible attimes to have generator sets supplying loads onbusbars which are not connected together. Inorder to have all busbars connected it will benecessary to synchronize groups of generator

sets across bus-tie or bus-coupler circuit-breakers.

Specific synchronization equipment is normallyrequired for such applications since thegenerator set normally allows synchronizingacross the generator circuit-breaker only.

A similar situation can occur when plant load isbeing supplied by generator sets and it isnecessary to connect the loads to the utility.Synchronization across the utility circuit-breakerwill be necessary.

Synchronization requires voltage and speedadjustments. As described in section 4.4 above,

synchronization of a group of generator sets ispossible when one set is in the isochronousmode, or when a load dispatcher is used whichwill change the power output (and therefore

speed) of all sets. When a set is in theisochronous mode, the voltage and speedadjustment signals will be sent to that set andthe others will follow according to their droopcharacteristic. When a load dispatcher is used,the frequency signal will be sent to the loaddispatcher which then sends appropriate signals

to the individual governors.The voltage regulators used in such cases aresometimes connected to the voltage transformerof the busbar to which they are to besynchronized and can therefore adjust theirexcitation accordingly without receiving aseparate voltage signal.

For both schemes, once the required frequency,voltage, and phase angle have been achieved,the circuit-breaker can be closed. Somemanufacturers of load dispatching systems offeradjustment of the voltage in addition toadjustment of the speed. Specifications for

synchronization equipment should thereforeclearly specify all the functional requirementsthereby allowing suppliers to choose the bestsolution.


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Since generators are a source of electrical power,the overcurrent protection relays should beconnected to current transformers on the neutralside of the stator windings in order to cover faultsoccurring in the windings.

Additional protection relays are required at thegenerator circuit-breaker only for applicationswhere generator sets will be operating in parallelwith other generator sets or with the utility, andwill pick up faults on the line side of thegenerator. The current transformers for these

protection relays are installed at the generatorcircuit-breaker in order to cover the wholeconnection to the generator.

Reverse active and reverse reactive powerrelays are normally connected to currenttransformers on the neutral side of thegenerator as shown in figure 7. They can alsobe connected to the current transformersassociated with the circuit-breaker. The locationwill depend on the split of works as described inchapter 9.1.

6 Generator set protection

25

51 67

67N

49T

E

64F

87G

27

59

59N

81

46 49 51 32P 32Q 51V

51G

Generator star point

Neutral earthing resistor

a

Fig. 7: recommended generator protection

6.1 General protection philosophy


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The recommended protection functions areshown in figure 7. Function reference numbersare the following:

c protection functions connected to generatorneutral current transformers:

v 32P : reverse active power

v 32Q : reverse reactive power serving as loss offield (for generators above 1 MVA)

v 46 : negative sequence (for generators above1 MVA)

v 49 : thermal image

v 51 : overcurrent

v 51G : earth fault

v 51V : voltage restrained overcurrent

v 87G : generator differential protection (for

generators above 2 MVA)

(Note: 46,49, 32P and 32Q can also beconnected to the line-side current transformers)

c protection functions connected to voltagetransformers:

v 25 : synchronism-check (for parallel operationonly)

v 27 : undervoltage

6.2 Electrical protection

v 59 : overvoltage

v 81 : overfrequency and underfrequency

c protection functions connected to line-side

current transformers (for parallel operation only):

v 67 : directional overcurrent (not required if 87G

is used)

v 67N : directional earth fault (on core balanceCT for better sensitivity)

c generator mechanical protection functions

connected to sensors

v 49T : stator temperature (recommended for


v 49T : bearing temperature (recommended for


v 64F : rotor earth fault protection

The following table ( see fig. 8) gives typicalsettings for each protection function, and whataction should be taken. This information shouldbe verified with the generator set manufacturerfor each application. A general shutdown meanstripping and locking out the generator circuit-breaker, switching off the excitation, and closingthe fuel supply to the engine.

Function Typical setting Action

27 0.75 Un, T 3 s General shut-downT > longest time of 51, 51V, 67

32P 1-5 % for turbine, 5-20 % for General shut-down

Diesel, T = 2 s

32Q 0.3 Sn, T = 2 s General shut-down

46 0.15 In, inverse time curve General shut-down

49 80% thermal capacity = alarm Trip breaker only, overload

120% thermal capacity = trip may be temporary

time constant 20 min operating

time constant 40 min standstill

51 1.5 In, 2 s General shut-down

51G 10 A, 1 s General shut-down

51V 1.5 In, T= 2.5 s General shut-down

59 1.1 Un, 2 s General shut-down

81 Overfrequency: 1.05 Fn, 2 s General shut-downUnderfrequency: 0.95 Fn, 2 s

87G 5 % In General shut-down

67 In, 0.5 s General shut-down

67N Is0 10 % of earth-fault current, General shut-down

0.5 s

25 Frequency < 1 Hz, Voltage < 5 %, Inhibit closing during

Phase angle


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Particularities of generator short-circuitcurrents

As shown in the above table, it is the duty of thegenerator circuit-breaker to effectively isolate thegenerator from the network. Due to the lowvalues of transient and permanent short-circuitcurrents, care must be taken in the choice andsetting of the protection relays. In addition, inorder to reduce losses in the generator, generatorstator resistance is normally kept low by themanufacturers. This will result in high X/R ratioswhich cause generator short-circuit currents tohave a d.c. component with a long time constant.

The IEC 60056 defines test conditions formedium-voltage circuit-breakers. The testconditions are based on short-circuit currentshaving a d.c. component with a time constant of45 ms. Since generator short-circuit currentsmay have time constants greatly exceeding thisvalue, the circuit-breaker manufacturer must

choose the adequate circuit-breaker anddemonstrate that it is suitable for the application.

Possible delaying of circuit-breakers

In addition to the significant d.c. component, thegenerator short-circuit current can also have

zero-axis crossings which occur only afterseveral periods resulting in unsuccessfulinterruption of the short-circuit current as shownin figure 9. This is due to the alternatingcomponent of the short-circuit current decreasingmuch more rapidly than the d.c. component.

Since medium-voltage circuit-breakers requirenatural zero-axis crossing of the short-circuitcurrent for successful interruption, it may benecessary to delay operation of the circuit-breaker until such time as zero-axis crossingsdo occur. Such delays must be taken intoaccount in the protection relay coordination study

and can also reduce the system stability.

Fig. 9: generator short-circuit current with delayed zero-axis crossing on phases 1 & 3 (phase 2 interrupts correctly

since short-circuit occurs here when voltage is at its peak on this phase, consequently short-circuit current, with

90 lag, starts at zero, without dc component).

The generator set will have mechanical protectionrelated to the prime mover. This typically includesoil level, oil temperature, water level, water

temperature, and exhaust temperature. Often therotor earth fault protection is provided as an

integral part of the set since it requires injectionof a d.c. current between the rotor and earth. Asignal should be sent to trip the generator circuit-

breaker without lockout should mechanicalprotection require a shutdown.

6.3 Machine protection

Circuit-breakercontact separationThree-phase short-circuit occurs

Voltage prior to fault Fault current


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Generators have a limited capacity to withstand

voltage impulses. When it is possible to operate

medium-voltage generators in parallel with the

utility supply it is recommended to provide surgeprotection at the generator incoming terminals.

This normally consists in connecting surge

capacitors (typical value of 0.3 F) and lightning

arresters between phase and ground in the

generator line-connection box. Such precautions

are not required for low-voltage generators since

they are shielded from impulses by the upstreamstep-down transformers.

When the generator line-connection box has

been designed for surge protection devices, it is

recommended to install the generator voltage

7.1 Connection to generator circuit-breaker

7 Connection of generators to electrical distribution network

transformers in it as well. The voltage

transformer can however be easily integrated

into the downstream switchgear should the

generator connection box not be sufficiently

large.

Current transformers should be installed in the

generator neutral point connection box. When

generator differential protection excludes the

generator line-side connection cable (or

busduct), current transformers are installed in

the generator line-side connection box. Whengenerator differential protection includes the line-

side connection cable (or busduct), the current

transformers are installed in the downstream

switchboard.

7.2 Connection of generator neutral point

Stand-alone generator set

A generator which does not operate in parallel

with any other source should be earthed by

means of a resistor connected between the starpoint and earth. The generator manufacturer can

provide a damage curve showing the allowableearth fault current as a function of time.

The earthing resistor and protection relay

settings should be determined based on this

curve. In general earth fault current for medium

voltage generators should be kept less than 30 A

in order to prevent any damage to the stator itself.

Operation in parallel with utility or other sets

When several sets operate in parallel or together

with the utility it is difficult to keep the earth fault

current within acceptable limits.

The maximum earth fault current will be the sum

of the earth fault current in all sources and this

can easily exceed the value given on the

damage curve mentioned previously. Reducing

this maximum value by limiting the earth fault

current to a small value for each source will

result in earth fault current being too small when

only one or two sets are in operation. It is

recommended to keep the star points unearthed

and to provide earthing transformers for eachbusbar as shown in figure 5.

When busbars are operated with the bus-tie

circuit-breakers closed, only one earthing

transformer should be connected. When the

bus-tie circuit-breakers are open, one earthing

transformer should be connected to each busbar

section. This will permit a constant value of earth

fault current independent of the type and number

of sources used, and greatly simplify the earth-

fault protection system.

Should a fault occur in the earthing transformer,

it should be tripped but the generator sets

connected to the busbar should be kept inoperation. There is no immediate danger to the

sets when operated on a temporarily unearthed

system.

The maintenance personnel should determine

the subsequent operation of the system.


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Load shedding is often required in order toensure that the essential parts of the process aresupplied with electrical energy during high loadconditions, or when system disturbances occur.Since the only additional energy available in anelectrical distribution system is the spinningreserve of rotating machinery, sites supplied bygenerator sets only have very limited reservesand are very susceptible to instability due todisturbances such as faults in the electricaldistribution system.

Three different scenarios requiring load shedding

can be considered:c gradual increase in load

c loss of a generator

c electrical faults

To ensure a reliable electrical supply to essentialprocess equipment, each of the above casesmust be studied to ensure that correct loadshedding is implemented. In general the loadshedding system must continually check thebalance between the load and the availablepower in order to switch off non-essential loadsrequired to maintain system stability. The effectsand remedial measures for each scenario aredescribed below.

Gradual increase in load

It is possible during certain periods that the totalload exceeds the rated power of the generatorsets. Due to the overload capacity of 10% forone hour normally provided with production sets,and the gradual increase of load, the loadshedding system can perform all calculations inreal time and generate load shedding signals totrip non-essential loads. The operators canswitch the non-essential loads back on after thepeak period has passed.

8 Load shedding

Loss of a generator

The loss of a generator can suddenly result inthe available power being much less than theload. It is necessary to shed non-essential loadsimmediately in order to ensure the stability of theelectrical distribution system. If this is not done,other generators will be tripped due to overload,undervoltage or underfrequency and the wholeelectrical supply could be lost. The loadshedding system normally prepares loadshedding tables based on the scenario of loss ofa generator so that when such an incident doesoccur, it can immediately send the trip signals.

Load shedding can be achieved in less than200 ms which is normally sufficient to preventloss of system stability which could lead to acomplete loss of the distribution system.

Electrical faults

When an electrical fault occurs, protection relayswill detect the fault and circuit-breakers willisolate the faulty equipment. During the timerequired to eliminate the fault, the voltage at thefault can be very close to zero which can causeall the motors in the plant to decelerate. Afterthe fault has been cleared, the motors will drawmore current since they must be brought back up

to speed. This can further reduce the voltage incertain portions of the network causing asnowball effect which can lead to tripping ofcircuit-breakers supplying healthy portions of thedistribution system. In order to prevent such aloss of stability, load shedding based on voltageand/or frequency should be implemented. Inorder to determine how much load should beshed, and at what value of voltage or frequency,a stability study of the electrical distributionsystem is required. This study will modelize thedynamic response of the system to disturbancesand enable the load shedding strategy to beprepared.


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It is very common for the generator set to besupplied by a different company than thecompany which supplied the switchgear to whichit is connected. It is therefore beneficial for allparties to reduce the interfaces between theequipment to a minimum. A coordinationmeeting between the switchgear and generatorset suppliers should be held prior to any detailedengineering. During this meeting the split ofworks, interfaces, information to be exchanged,and schedule should be determined. Correctdefinition should allow each supplier to doengineering, manufacturing, erection, testing,and equipment commissioning at site in anindependent manner. System commissioningcan then be done by both parties after allinterfaces have been made. Keeping interfacessimple also enables each manufacturersresponsibility to be clearly defined.

Each supplier should be responsible for theinstallation of all equipment in his supply.Installing components supplied by onemanufacturer in equipment supplied by the other

should be avoided. A typical example is thegenerator excitation module which should beinstalled in a panel supplied by the generatormanufacturer, and not in the switchgear.

When generators can operate in parallel it isnecessary to install protection gear in theswitchgear for eliminating faults occurring

9.1 Typical split of supply between generator setmanufacturer and switchgear manufacturer

9 Interfacing generator with electrical distribution system

between the generator and the switchgear.This protection gear should be in the switchgearmanufacturers scope. Protection gear for thegenerator itself can be supplied either by thegenerator set manufacturer, or the switchgearmanufacturer. Either solution is acceptable, andboth require exchanges of information sinceequipment data for setting the relays will comefrom the generator set manufacturer, whereasinformation for the integration into the overallplant protection scheme will come from theswitchgear manufacturer.

When generator differential protection is used, itis quite common for the line-side currenttransformer to be installed in the switchgear andthe neutral side current transformer to beinstalled in the generator neutral connection box.The supplier of the differential protection relayshould define the characteristics of the line andneutral current transformers and eachmanufacturer should supply the currenttransformer to be installed in his equipment. It isnot necessary nor for reasons mentioned

previously is it desirable that one manufacturersupply the current transformers to be installed inthe other manufacturers equipment.

The auxiliary supplies for the generator setshould be independent of those of theswitchgear. The generator set should have itsown battery backed d.c. supply.

9.2 Information to be exchanged

The information to be exchanged between thegenerator set and the switchgear should be kept

to a minimum. The information should beexchanged by means of potential-free contacts,and 4-20 mA analog signals.

The meaning of each signal (eg. close toactuate, closed for circuit-breaker open position)and the minimum duration of each signal (eg.closing signal duration: 500 ms) should beclearly stated on the interface documentation.

Fail-safe circuits should be used. Such circuitsuse contacts which close to actuate, andnormally open contacts which are maintainedclosed for authorization. These circuits arecalled fail-safe since a broken wire will not result

in undesired actuation or authorization.

The voltage to be applied to the potential-free

contacts, and the contact loading should also be

stated in order to ensure that the correct deviceshave been chosen.


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This type of interfacing enables each supplier to

design, manufacture, and test his equipmentindependently. Data exchanged directly via serial

links should be avoided since this is much more

difficult to define, commission, and trouble shoot.

The amount of information to be exchangeddoes not justify this type of interface.

The information typically exchanged is:

c information from generator set:

v ready to start (information)

v ready for loading (information)

v trip on fault (order)

v general alarm (information)

v generator voltage (from voltage transformer,

for synchronizing)

c information to generator set:

v start (order)v circuit-breaker on/off status (information)

v busbar voltage (from voltage transformer, for

synchronizing)

v stand-alone operation, or parallel operation

(information)

v type of fault (information)

9.3 Integration of generator set into electrical distribution supervisory system

In order to prevent loss of supply preventative

maintenance is required.Preventative maintenance can be very effectiveprovided that the information needed to trigger itis available, thus ensuring that the maintenancewill be made prior to the fault occurring.

The required information can be collected anddisplayed to the operator by a powermanagement system. Such information caninclude running hours of generator sets,temperature measurements of generator

windings or bearings, and power consumption of

particular loads. The power management systemcan also be used to supply the informationrequired by the load shedding system describedin section 8 above to perform the power balancecalculations.

The operator can also reconfigure the powerdistribution system from the power managementsystem console. This is very convenient shouldan incident have occurred and switching berequired to reenergize equipment.


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10 Installation and maintenance of generator sets

The installation of generator sets requires closecooperation among several disciplines such aselectrical, construction, process, and mechanical.

The following information should be consideredwhen designing the installation of the sets.

The location should be chosen close to the loadcenter to reduce voltage drop and losses in theconnections. Due to the relatively large size ofthe equipment, adequate space must be allowedfor the transportation to and from the location.

The building housing the equipment must haveadequate space to allow maintenance includingoverhauling, and be provided with the necessaryoverhead cranes. The generator setmanufacturer should provide all information

concerning space and access requirements oncivil works guide drawings.In many locations noise emission will be aproblem. The solution consists in sound proofingthe generator set, the building, or a combination

of both. Sound proofing will have a significantimpact on cost and therefore must be definedprior to placing an order for equipment.Care must also be taken to avoid noisetransmission via the generator set base.

In the definition of the rated power of generatorsets, the length and configuration of the airintake ducts and the exhaust piping is important.In certain cases generator sets will be located inareas where long ducting and piping is required,and this is to be taken into account in thedefinition of the rated power of the engine.

Care must also be taken to ensure that the airintake is remote from the exhaust.

Generator sets used for emergency power must beable to operate in all site conditions. In desert areas

this can include sand storms. Special sand filtersare required at the air intake and can increase thefoot print and cost of the generator set.

10.1 Location

10.2 Air intake and exhaust

10.3 Compliance with local regulations

In many countries there are local regulations thatmust be met. In addition to requirements relatedto emissions, environmental considerations oftendictate the design of the fuel system. This caninclude the maximum capacity of day tanks and thetype of buried storage tanks (double walled, etc.).

Local regulations must also be respected for the

fire detection and protection equipment. Firedetection should be installed in all locationswhere generator sets are located. Automatic fireprotection equipment should also be providedwhere possible.

Fire protection is normally achieved by floodingthe building with inert gas. This type of systemrequires automatic shutting of ventilationopenings, air intake openings, and doors.

Local regulations cover many aspects such asthe number and location of warning signs, thelocation of the fire control panel, and the type of

inert gas which can be used.

The assistance of a local company familiar withsuch regulations to get all required approvals isvery useful and often indispensable.

10.4 Special tools and spare parts

Generator sets require periodic maintenance andalso overhauls after a certain number of years ofoperation. Special tools are normally required forperiodic maintenance, and additional specialtools are required for overhauls. The definition

and supply of tools should be made with thegenerator set manufacturer based on the type of

maintenance to be performed. The list of specialtools should be checked with the maintenancemanuals in order to ensure that all have beenprovided. Spare parts for the first overhaulshould be provided in addition to those required

for normal operation.


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11 Conclusion

Engine driven alternating current generating setsare often installed in industrial sites andcommercial buildings as main sources ofelectrical energy or for supplying essential loadsin case of loss of the utility supply.

A good understanding of the electrical andmechanical characteristics of the generator setsand the standards which define them is importantfor correct choice of the equipment.

The integration of the generator sets into theelectrical distribution system has a large impacton most of the electrical equipment. Thegenerators will contribute to the maximumavailable short-circuit current which must betaken into account in dimensioning theswitchgear. The plant electrical protection

system must take into account the particularitiesof generators in order to ensure correctprotection of persons and equipment but at thesame time avoid nuisance tripping which resultsin loss of the supply of electrical power. Thecontrol system must enable the electricaldistribution system to be operated in differentconfigurations required for ensuring a reliablesupply of power.

The engineer responsible for the correct designof the complete electrical distribution system isconfronted with many different types of problemsto solve. Being aware of the problems andknowing typical solutions to them is the first stepin ensuring that the final electrical distributionsystem will meet the requirements of theapplication.

Bibliography

Standards

c IEC 60056: High voltage alternating currentcircuit breakers.

c IEC 60255: Electrical relays.

c IEC 60298: A.C. metal enclosed switchgearand controlgear for rated voltages above 1 kVand up to and including 52 kV.

c IEC 60439-1: Low-voltage switchgear andcontrolgear assemblies.

c ISO 3046: Reciprocating internal combustionengines.

c ISO 8528: Reciprocating internal combustion

engine driven alternating current generating sets.

Schneider Electric Cahiers Techniques

c Electrical disturbances in LVCahier Technique no. 141R. CALVAS

c Active harmonic conditioners and unity powerfactor rectifiersCahier Technique no. 183E. BETTEGA, J-N. FIORINA

c Disjoncteurs au SF6 Fluarc et protection desmoteurs MTCahier Technique n 143J. HENNEBERT et D. GIBBSo


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