161 Automatic Transfering of Power Supllies in HV and LV Network

7/29/2019 161 Automatic Transfering of Power Supllies in HV and LV Network

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n 161

automatictransfering of

power suppliesin HV and LVnetworks

E/CT 161, first issued march 1996

photographie

Georges THOMASSET

An engineering graduate fromthe IEG in 1971, he proceeded toconduct research into the design ofcomplex industrial networks inMerlin Gerins TechnicalManagement Department.

Following his position as manager of

the medium voltage publicdistribution and hydraulicsengineering and design department,since 1984 he is head of thetechnical section of the industrialunit in the Engineering andContracting Department.


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Cahier Technique Merlin Gerin n 161 / p.2


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automatic transfering of power suppliesin HV and LV networks

contents

1. Introduction p. 4

2. Various types of transfer schemes Problems presented by switching p. 4and precautions to be taken

Presence of fault on the p. 4downstream network

Characteristics of the replacement p. 5source

Preparing the switching orders p. 5

Voltage loss during non- p. 6synchronous switching

Mechanical interlocking of p. 6switching devices in LV an HV

Dielectric withstand of switching p. 6devices in HV

3. Synchronous switching Example n 1 p. 7

Example n 2 p. 7

Example n 3 p. 8

4. Interrupted circuit transfer In low voltage p. 9Example p. 9

In high voltage p. 10

Example n 1 p. 10

Example n 2 p. 10

5. Pseudo-synchronous switching Principle p. 12

Area of application p. 12

Difficulties p. 12

Ultra-rapid switching p. 13with phase shift monitoring

6. Summarising table p. 15

7. Conclusion p. 15

Appendix: brief description of a phase comparator p. 16

Nowadays even temporary loss ofelectrical power is a major handicap forfirms whose production processes

brook no interruption as well as forhigh-rise buildings whose safety circuitsmust be operational at all times.

Consequently, transfering of mainsupply sources onto replacement oremergency sources has proved to be afunction used with increasing regularityin electrical distribution, public andprivate alike.

This Cahier Technique begins withan examination of the difficulties ofimplementing such switching devices,together with the possible technicalsolutions. This section is followed by a

presentation of the various switchingtypes backed up by practical examples.Finally, a table summarises the aboveand gives the main areas of application.


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v complete manufacturing lines whoseprocess brooks no interruption, eventemporary, of any element in the line(iron and steel, petrochemical, etc...);c in medium voltage public distributionfor:v switching of HV lines andtransformers in source substations,v supply of HVA/LV-HV level A(U i 50 kV)/Low Voltage (U i 1 kV)dual incomer substations.

The devices using these power

transfers are highly varied.For example, in power circuits,switching devices are electromechnicalor static contactors, circuit-breakersand switches, in high and low voltagealike.

These devices can be operated:c manually: such devices are themost elementary and economic.They require intervention of anoperator, and the time needed tochange from the defective source to thereplacement or backup source can bevery long (the operator has to moveplace);c automatically: these devices arethe fastest and the most commonlyused.

Nevertheless the basic configurationcan, in most cases, be simplya main power supply, a replacementor backup source and a busbar whichis the point common to both sourcesfrom which the loads are supplied.

1. introduction

problems presented byswitching and precautionsto be takenTo construct an installation containingsource transfers complying with thecontinuity of service requirements of itsusers, a number of factors, calling forspecial precautions, must be examinedin the design stage:

c presence of a fault on thedownstream network;

c characteristics of the replacementsource;c preparing the switching orders;c voltage loss during switching (fornon-synchronous switching);

c mechanical interlocking of switchingdevices in LV and HV;c dielectric withstand of switchingdevices (in HV).

presence of a fault on thedownstream networkWhen failure of the main powersupply is caused by a fault downstreamof the supply transfer location, werecommend you do not switchthe supply sources. Rather, the circuitcontrolling the switching deviceshould be blocked by an order fromthe downstream network protectionsystem.

There are three main source switchingtypes, namely:

c synchronous:transfer time: zero(e.g. generator coupling);

c interrupted circuit transfertransfer time: 0.2 to 30 s(e.g. main/emergency functionin LV);

c pseudo-synchronous

transfer time: 100 to 300 ms(e.g. reacceleration of asynchronousmotors).

For switching, a certain numberof prior conditions must be met, someof which call for special precautions.

The purpose of installing sourceswitching devices is to guarantee thecontinuity of power supply of certainpriority loads, for example to ensureprotection of persons or to maintainproduction cycles. Such devices areput into operation either by failure of themain power supply or by voluntaryoperator action.

Switching devices are particularly used:c to supply:v computers,

v high-rise buildings,v lighting and safety systems: airfieldlighting, premises receiving generalpublic, etc,v main auxiliaries of thermal powerplants,

2. various types of transfer schemes


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characteristics of thereplacement sourceThe rated power, short-circuit power,

connection impedances and earthingsystem of the replacement source mayvary considerably from the mainsource. Thus, for example, the mainsource may be an 800 kVA, 380 V,50 Hz, Isc = 20 kA transformer, whereas the replacement source is an200 kVA generator set, having atransient short circuit current of 1 kA.The devices protecting against phase-to-phase and phase-earth faults in thebacked-up network may thus cease tofunction in certain conditions when thisnetwork is supplied by the replacement

(or backup) source.Extreme care must be taken whenchoosing and setting the protectivedevices in order to find a protectionsystem compatible with the electricalcharacteristics and the operating andmaintenance modes of both supplysources. Two cases should beemphasised at this point: thereacceleration of certain motors and theresumption of supply of several loadedstep-down transformers:c the network load includes a numberof motors. When the replacement

source is a low power one, after themain source has switched to thereplacement source, the inrush currentand the permanent working currentmust be limited by:v partially shedding loads,v staggering restarting of prioritymotors in the event of an interruption.If these measures are not taken, inview of the low power replacementsource, voltage drops would beextremely serious and motorreacceleration impossible (drivingtorque less than mechanical loadtorque).c energising of a number of step-downtransformers in the distribution network.When switching takes place in HV,allowance must be made for the inrushcurrents of HV/LV transformers whichare some 10 to 15 times their

rated current. In actual fact, if thereplacement source is an LV generatorset, its generator cannot supplycurrents that high at rated voltage and

acts as though it were supplying ashort-circuit. It thus delivers a very lowvoltage for the first few moments afterswitching which does not simplify motorrestarting. Consequently, it ispreferable to trip all the step-downtransformers on the HV side beforeswitching, and then to re-energise themone after the other.

preparing the switchingordersThe input for these orders comes from

voltage monitoring:c if the replacement source is agenerator set: loss of voltage on themain power supply to initiate startup ofits motor;c presence of stabilised voltage ofreplacement source to authorizetransfer to the replacement source;c presence of voltage on the mainsupply source to return to the normalposition.

The switching orders

c to transfer from the main to thereplacement source.Loss of or drop in main supply voltagemay be:v permanent, due to:- tripping of the upstream protectivedevice,- excessive overloading of the networkcausing a large voltage drop,- etc,v but also temporary, due to:- operation of the rapid or slowreclosing controllers of the electricityboards overhead lines,- a short-circuit between phases,normally eliminated by the protectiondevices,- etc.

The action of the main sourceundervoltage detector will thusgenerally be delayed so as not toswitch sources due to temporary drop

in or loss of voltage. Moreover, if thereplacement source is a generator setwhose starting order is given by loss ofmain source voltage, the sets voltage

must first stabilise before a switchingorder is given (a few seconds).

c to transfer from the replacement tothe main source.Resumption of the main supply may bepreceded by attempts to re-energisethe main line, in order to:v locate a fault,v perform source looping further to anincident,v carry out tests following repair ormodification of the main line.

The action of the main source voltage

presence detector will thus be amplydelayed (a few dozen seconds to a fewminutes).

Note:a) devices used to switch from the mainto the replacement source withoutreturning to the main source when it isonce more present, are normally knownas changeover switches;b) devices automatically returning tothe main source are known as main/emergency changeover switches.

The difficulties in detecting loss ofmain supply voltage

c residual distribution network voltageon loss of supply source.Although the supply source has failed,distribution network voltage can besustained by:v the residual voltage generated byasynchronous motors duringdeceleration for a period ofapproximately 0.3 to 1 s,v the voltage induced at the terminalsof synchronous motors duringdeceleration,v the voltage due to discharging ofcapacitors connected to the network.In the case of rapid source switching,this sustained voltage prevents rapiddetection of loss of main supplyvoltage by ordinary conventionaldevices such as threshold voltagerelays.


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c temporary loss of voltage duringwhich the transfer must be blocked.These voltage losses are caused bynetwork controllers such as rapid

and/or slow reclosers and switchingof HV transformers or lines inHV transformer substations, etc.

The same applies to networkundervoltage due to excessive voltagedrops caused by transient overcurrents(phase-to-phase or phase-to-earth faulteliminated by discriminating networkprotection devices, startup of largemotors, etc...).

c choice and cabling of detectors with:v one single-phase relayNormally, only one single-phasedetection relay is connected between

two phases of the main incomer.In this case, the third phase may failwithout being seen by the relay.Switching therefore will not take place,and power supply to the loads remainsdefective.This system is satisfactory only if thethree-phase power supply is unable tooperate for any length of time with twophases only. This calls for the use ofprotective devices such as three-polecircuit-breakers or fuses with striking-pindelivering a multi-pole breaking order.Otherwise, to prevent this problem,

either two relays connected betweendifferent phases or three relays in adelta connection must be fitted.

v three single-phase relaysThe above connection (three relays ina delta connection) may however provetricky when relay thresholds are setbetween 20 and 30 % of rated voltage.In point of fact, if only one phase fails,the two relays with a terminalconnected to this phase are thenseries-connected and are supplied bythe two remaining sound phases. Thevoltage at the terminals of these two

relays equals half their rated voltage, avoltage greater than the setting value(0.2 Un). No switching order is thensent. For this reason it is preferable touse three star-connected relays orthree delta-connected relays which areset to 60 % of Un or, better still, arotating field three-phase voltage relay.

v a single three-phase voltage relayThis type of relay does not allowdetection of supply phase failure atbusbar level if the consumer network

contains asynchronous three-phasemotors, since these motors sustainthe voltage of the broken phase atbusbar level. A three-phase rotating

field overcurrent relay must then beconnected to the main incomer.

c detector mountingInstantaneous electromagnetic relaysare vulnerable to impacts which causetheir contacts to bounce and to sendfalse switching orders. This isparticularly true when relays aremounted on doors; particular care mustthus be taken with this type of mountingto avoid all vibrations likely to interferewith equipment operation.

voltage loss during non-synchronous switchingSuch voltage loss, although temporary,is normally quite sufficient tode-energise all the contactors whosecoils are supplied by the power circuit.Automatic transfer systems may losetheir efficiency to a large extent sincethe loads controlled by thesecontactors, now open, cease to besupplied. However, manual staggeredresumption of motor operation ispossible using the on push buttons.In preference to manual resumption,

contactor coils can be supplied by areliable auxiliary source (battery orrotating set with an inertia flywheel), ora slow release relay can be used.Another possibility is a capacitorconnected in parallel with the coil andcharged by the power supply via arectifier. In this case, the energyrequired to keep the contactor in theclosed position is provided by thecapacitor for the brief instant of voltageloss. However, to avoid stretching thecapacity of the buffer capacitor, theundervoltage time must be relatively

short (a few hundred milliseconds) andcoil consumption low.Note that if these solutions are to beimplemented, the replacement sourcemust be able to take up all the loadsand particularly all the motors in thereacceleration.

Note:When a coil control circuit opens, thehigh voltages induced at the coilterminals must be withstood by therectifier and the capacitor.

mechanical interlocking ofswitching devicesin LV and HVWith the exception of synchronousswitching devices for which the twoswitching elements (main andreplacement source) may besimultaneously closed, mechanicalinterlocking of the devices andelectrical interlocking preventingsimultaneous supply of the two devicescontrol circuits are recommended inall installations and are normallya standard requirement of electricityboards.

dielectric withstand ofswitching devices in HVThe dielectric withstand of the switchingdevice of the replacement source usedin such synchronous and pseudo-synchronous switching systems mustbe particularly appropriate. In point offact, during coupling conditions,the poles of these devices may besubmitted, between input and output,to twice the phase-to-neutral voltage ofthe network (voltage of the two sourcesto be coupled in opposition of phases).



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example n 2Substitution of a generator inoperation by a standby device in aself-powered electricity producingplant consisting of generator sets.These generator sets require frequentperiodical servicing; the plant is runwith n - 1 sets, the nth being either onstandby or undergoing servicing.A set in operation is replaced by thestandby set as follows: the generator of

the standby set is brought up tosynchronous speed and to ratedvoltage; the circuit-breakers energisingorder is sent once coupling conditionshave been checked (frequency isequal, voltages have same magnitudeand are in phase).In order to obtain such conditions forcoupling and to maintain them aftercoupling, the generator and the thermalmotor are equipped with a voltage andspeed regulator, respectively.The coupling conditions are obtained:c either by intervention of an operatorgiving, according to readings on thedifferential voltmeter and frequency

3. synchronous switching

Both the main source and thereplacement source can besynchronized, namely:c their voltage vectors are in phase;c and their frequency and amplitudesare identical.

The possibilities offered by thisswitching type are numerous in thatsource changeover takes place beforevoltage loss of the source in operation;a major advantage is that loads

experience no loss of power supply.The following examples illustrate thistype of transfer.

example n 1 (see fig. 1)Operating an EHV/HV interconnectionsubstation with double busbar.The 2 busbars are supplied by theEHV transmission lines of theinterconnection network, the couplingcircuit-breaker is open and the twobusbars are synchronized. The line andtransformer feeders are connected to

either busbar A or B. To change thesupply of a feeder (changing thebusbar), assuming that it is currentlysupplied from A, simply:c close the bus coupling circuit-breaker without checking couplingconditions as both busbars aresynchronized;c close the second disconnectingswitch for the feeder in question;c open the first disconnectingswitch ;c open the bus coupling circuit-breaker .

The feeder is then supplied by the otherbusbar B.

Note:Throughout switching all incomersare connected in parallel on thetwo coupled busbars; short-circuitpower is then high, and theequipments electrical characteristicsmust be sufficient for this mode ofoperation if the likelihood of a faultoccurring during the transfer is notnegligible.

meter and on the synchroscope,the speed orders to the speedregulator, the excitation orders to thevoltage regulator and the energisingorder to the circuit-breaker whencoupling conditions are met.In this case a controller, known as acoupler, can be used. This device isdesigned to check coupling conditionsand to give the energising order;regulators are always ajustedmanually.c or using a synchrocoupler which is aspecial controller associated with avoltage regulator. It sends the speedorders to the motor and the energisingorder to the generator circuit-breaker.The regulator sends the excitationorders to the generator.Coupling is then fully automatic.

After coupling, the set to be placed out ofoperation is discharged (using the speedregulator) and disconnected from thenetwork by opening its coupling circuit-breaker. Substitution is thus completedwithout disturbing the distributionnetwork and without voltage loss.


1

1

2b

2a

fig. 1: diagram showing an EHV/HV interconnection substation with double busbar.

incomers

bus coupling

busbar A

2b

busbar B

1

feeders

2a


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disappearance (accidental ordeliberate) of UPS voltage.To obtain this result, the UPSpermanently ensures its voltage is in

phase with the mains voltage.However, switching is only possible ifthe mains voltage level is correct.

The operating sequences are asfollows:c the UPS, its voltage regulated, itsfrequency controlled and in phase withthe mains, supplies the load. The SS isopen and the mains delivers no power;c the UPS-mains changeover takesplace:

v on an UPS shutdown:- either due to an internal fault,- or operator-initiated,v on detection of an overload.

The switching order alwaysinstantaneously closes the staticswitch. In the case of overload, the twosources run for a short instant inparallel before the UPS is disconnected.c the mains-UPS changeover isoperator-initiated.

Following UPS startup, the automaticcycle is as follows:

v the UPS is synchronised with themains,

v the mains and the UPS areconnected in parallel,

v the static switch opens,

v the UPS is then permanentlysynchronised on the mains and alonesupplies the load.

Importance of the static switch

The ac SS ensures:c permanent supply of the load,comparable in terms of reliability to thesolution with two parallel-connectedUPS, one backing up the other, but atlesser cost;c in the event of overload, an ultra-rapid device controls the static switchconnecting the mains in parallel to

the UPS.The mainss short-circuit powercan thus be profitably used toeliminate the downstream faultswithout taking any special precautionsother than standard discrimination rules.

fig. 2: diagram showing a circuit supplied by two sources with automatic no break switching

using a static switch.

example n 3 (see fig. 2)Automatic no break transfer withoutbreaking of an UPS (Uninterruptible

Power Supply) on the public network(or mains) using an SS (StaticSwitch).

This requirement is a common one:supply of computers, computermanagement centres, measurement,process control, etc.

The SS is a device enabling the mainsto be used to back up the UPS. Itstands out by the fact that no voltageloss, even transient, occurs on

load

UPS

mainsource

rectifier

mains

replacementsource

staticswitch

inverter


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exampleDiagram

The most common diagram forLow Voltage source switching withautomatic load shedding is presented infigure 3.

Operating principle

Neither source can be coupled, andcircuit-breakers Jn and Jr aremechanically locked: their electricalcontrol mechanism is interlocked insuch a manner that a closing order sentsimultaneously to both circuit-breakerswill only close one of them.

A three-position selector controls thedevice:position 1 = off,position 2 = automatic,position 3 = voluntary closing ofthe replacement source circuit breakerand tripping of the coupling circuit-breaker.c position 1 = offAll the control circuits are de-energised

and all the circuit-breakers are open.c position 2 = automaticv main network voltage is present, andthe corresponding circuit-breaker andcoupling circuit-breaker are closed,v for a voltage loss exceeding0.4 second (up to 10 s), the switchingcontroller sends tripping orders to the

main and coupling circuit-breakers, anda starting order to the generator set,v on receipt of the message gen setvoltage OK, the controller sends anenergising order to the circuit-breakerof the replacement source;v on resumption of main voltage, andafter a time delay of 10 to 180 seconds:- circuit-breaker Jr opens,- circuit-breaker Jn closes,- the set receives a stopping order,- circuit-breaker Jc closes;c position 3 = voluntary energisingThis allows voluntary tripping of thecoupling circuit-breaker, with thebacked-up network supplied by thereplacement source.

A special case: reacceleration ofLV motors

A controller for restarting motors shouldbe provided when the backed-upnetwork supplies a large proportion ofmotors which must be reaccelerated asquickly as possible after loss of themain supply. This common need is

particularly required by the process andsafety of persons and property. Inactual fact, on loss of the main source,if no special measures have beentaken, all contactors will open. Whenthe supply is resumed, none of thecontactor-controlled loads are supplied.On the other hand, if the motors are

This type of source switching is themost common, in LV and HV alike, withan area of application extending fromindustry to the service sector. Switchingtimes normally vary from 0.5 to30 seconds, while not ruling out lowervalues for special cases.

in low voltageDevices must be extremely simple asthe electricians operating theLV networks are normally not specialists.

Switchgear

The type of switching device chosendepends on switching frequency:c large number of switchings:contactor;c small number of switchings (one aweek): circuit-breaker.

Control circuit

The control circuits of switching devicesare supplied either by a backed-upauxiliary source (e.g. battery) or directlyby the power circuit of the device to be

controlled.

Power supply

The main supply source is normally theLow Voltage public distribution network(or mains) or a private Low Voltagenetwork from an HV/LV transformersupplied on the HV side by the publicdistribution network.

The replacement source may be:c a second LV network separate fromthe first;c a no-break generator set for rapidresumption of operation;

c a generator set with manual orautomatic startup on undervoltage atthe main source;c an UPS;c etc.

These various replacement sources, inmost cases far less powerful than themain source, have a limited backuptime. When the backed-up network issupplied by the replacement source, itis thus often wise and sometimes evenvital to shed some loads and to restartonly ultra-priority motors (seechapter 2).

4. interrupted circuit transfer

fig. 3: diagram for low voltage source switching with automatic load shedding.

G

replacement source

generatorset

main source

Un

non-priorityloads

priority loads(backed-upnetwork)

Ur

Jr

Jc

Jn


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protected and controlled by circuit-breakers, they will simultaneouslyrestart when voltage is restored. If thecontactor coils of the motor feeders are

supplied by the ac network, use of slowrelease relays (see chapter 1) alsoenables their operating order to bemaintained during voltage loss.

Residual voltages of asynchronousmotors during deceleration normallypresent no risk on resumption of powersupply, since at that moment(t = + 500 ms), the amplitude of thesevoltages is less than 20 % of Un, avalue tolerated by manufacturers forresumption of supply in phaseopposition. However the main(overcurrent) protection device of the

distribution network may be tripped bythe sum of the motor restarting currents.

in high voltageSource switching taking place at highvoltage involves very high powers and,as such, must offer still greaterguarantees as to the overall operationaldependability of the device.

HV switching is particularly used whenthere is a large number of priority loadsor when these loads are not suppliedby a distribution sub-switchboard.

The switching device control orders areprepared by standard electroniccontrollers.

The two examples given belowillustrate this switching type.

example n 1Main-Emergency deviceThese devices have one switchingdevice per incomer and anRCV420 type controller (see fig. 4)

PrincipleThe controller manages the operations.Its function is to detect voltage loss onthe main source and to automaticallycontrol switching of the load to areplacement source when the followingtwo conditions have been met:c voltage is present on the replacementsource;

andc there are no faults in the installation.

Operation

The controller has two high impedanceinputs: a Main input, connected to acapacitive divider connected betweenone of the main networks phases andthe earth, and an Emergency inputconnected in like manner to one of thephases of the replacement oremergency network.

Voltage loss on the Main inputcontrols a time delay relay ,t1 (0.1 s to 1 s) which, at the end of thecycle, sends a temporary opening orderto the main network switching deviceand a temporary closing order to the

replacement (emergency) sourceswitching device.If voltage is lost on the replacement(emergency) source, the t1 time delayrelay is locked by the Emergencyinput and switching cannot take place.

Note that the RCV420 controller has asecond Emergency input in the formof a loop in which the contact of anexternal voltage relay can be insertedto prevent switching when it is open.If voltage is restored on the Mainnetwork, a second time delay relay,t2 (10 to 100 s) is energized and, at the

end of the cycle, sends a temporaryopening order to the Emergencynetwork switching device and atemporary closing order to the Mainnetwork switching device.

Faults in the installation must bedetected by an external device fittedwith a closing contact which preventsswitching using the fault input of thecontroller.

example n 2Dual incomer device

This device is extremely popular inFrance for the dual incomer supply ofHV/LV transformer substations directlyconnected to the HV public distributionnetwork. It consists of:c one switching device per incomer(HV switch);

c one electronic controller of theRVH type (see fig. 5).

PrincipleA dual incomer assembly(see fig. 6) can be supplied by eitherincomer as the system is completelyreversible.

Detection of voltage presence or loss isidentical to the Main-Emergencydevice.

Detection of phase-earth or phase-to-phase faults on the load network isperformed by the controller informed bytoroids current transformers (threetoroids per incomer).

In normal operation, switching isdependent on certain electrical

conditions and is performed aftercertain manual or automatic operationsdescribed below:c manual switchingThe operator manually opens switch Athen closes switch B, having firstchecked that the following conditionsare met:v no voltage on circuit A,v voltage present on circuit B,v no faults in the substation(downstream network).The conditions for resumption of themain supply are:v checking there are no faults in thesubstation,v manual opening of switch B,v manual closing of switch A.c automatic switchingThe controller opens switch A, thencloses switch B if the followingconditions are met:v no voltage on circuit A,v voltage present on circuit B,v no faults in the substation,v presence of this information for 5 or30 consecutive seconds. The mainpurpose of these 30 seconds is to waitfor the end of the cycles of the

automatic reclosers used on overheadnetworks.

Restoration of voltage on circuit Adoes not automatically causeswitching from circuit B to A, althoughmanual tripping by the operator ispossible.


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fig. 4: Main-Emergency RCV420 controller and its operating sequence (Merlin Gerin).

fig. 5: Dual incomer RVH215 controller and its operating sequence (Merlin Gerin).

fig. 6: diagram showing dual incomer distribution.

t1 t2

1

0

1

1

0 N R N

incomer

N

incomerR

networksuppliedby

0

t1 and t2 adjustable

t t

1

0

1

0

1

0A B A

incomerA

incomerB

networksuppliedby...

t = 5 or 25 s

P B A P

= changeover switch

HV/LV transformer

HVnetwork A

network B

P

HV/LV transformer


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The maximum amplitude of thisresidual voltage decreasesexponentially as a function of time, witha time constant depending on:c motor power;c operating state of the statorconnections:v open stator, case of a three-phasesupply failure,v short-circuited stator, case of a three-phase fault on the supply.

On the other hand, the rated supply

voltage of the motor only slightlymodifies the value of the time constant.

The table in figure 8 provides theapproximate values of the timeconstants for extinguishing the residualflow for medium cage asynchronousmotors.

If no special precautions are taken,rapid resumption of motor power supplyduring deceleration may lead tocoupling in phase opposition betweenthe replacement source and the load

network whose voltage is maintainedby the asynchronous motors.Only high voltage epoxy resin insulatedmotors can support resumption ofsupply in phase opposition. Note,however, that in this case the peakcurrent equals roughly 3 times motorstarting current, i.e. 15 to 20 In, with theresult that the entire distributionnetwork is seriously disturbed:c voltage drop and large, repetitiveelectrodynamic forces;

c nuisance tripping of circuit-breakersby full short-circuit protection;c etc.

For these reasons, ultra-rapidresumption of motor supply should notbe done without a prior comparison ofthe phases of the source voltage withresidual voltage. Using a voltage phaseshift comparison device, however,allows ultra-rapid switching.A brief description of a phasecomparator is given in the appendix.

5. pseudo-synchronous switching

principleThis type of source switching lastsroughly 150 ms.

The most common diagram is shown infigure 7.

In normal operation, the two half-busbars are supplied by the twoincomers respectively: the tie circuit-breaker is open.

Failure of either source triggers startup

of the rapid switching device which, inunder conditions, sends two orders:c a closing order to the tie circuit-breaker;c an opening order to the circuit-breaker of the faulty source.

The half busbar of the faulty source isthus re-energised.

area of applicationThe standard case is that of aninstallation connected to twoHV sources and mainly consisting of

asynchronous motors. The operatingimperatives of the machines driven bythe motors mean the latter cannotafford to stop, even temporarily, or slowdown during transfer from the main tothe replacement source.

This switching type is especiallypopular in chemical and oil plants and,more generally, in industries whosemanufacturing processes brook nointerruption, even temporary, of anelement in the line. It is also used tosupply thermal power plant auxiliaries.

difficultiesThe main problem of this switching typeis that a three-phase asynchronousmotor sustains at its terminals, duringdeceleration on loss of power, a three-phase ac voltage with decreasingfrequency and amplitude induced bythe motors residual flux.

motor power 10 kW 100 kW 200 kW 400 kW 800 kW

time constant,short-circuited stator 0.02 s 0.03 s 0.04 s 0.06 s 0.1 s

time constantopen stator 0.3 s 0.4 s 0.6 s 1.1 s 1.5 s

fig. 8: time constants for extinguishing residual flux for medium cage asynchronous motors.

fig. 7: diagram usually applied for pseudo-synchronous switching.

M M

source 1 source 2

J1 J2

M: asynchronous motors

HV busbar Jc


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ultra-rapid switchingwith phase shiftmonitoringThe ultra-rapid switchingoperations, which are possible,

are shown in the three diagrams in

figure 9.

Sequence A:

The switching order trips separator

circuit-breaker J1 or J2; after

opening, the phase comparator is put

into operation and, when switching

conditions are favourable, sends an

energising order to the tiecircuit-breaker Jc.

Sequence B:

The switching order trips separatorcircuit-breaker J1 or J2 andenergizes the phase comparator.When switching conditions arefavourable, the phase comparatorsends an energising order to the tiecircuit-breaker Jc.

Sequence C:

The switching order energizesthe phase comparator. When switching

conditions are favourable,

the comparator simultaneously sends

an energising order to the tie circuit-

breaker Jc and a tripping order to

separator circuit-breaker J1 or J2.

Note:

a) sequence C is the one normally

chosen as switching times are shortest.

b) certain problems may arise when

preparing the switching order, such as:

c detection of real loss of main

supply voltage in presence of residual

voltage;

c relay speed;

c etc.

fig. 9: possible ultra-rapid switching operations.

switching order

phase comparator

coupling circuit-breaker Jc

separator circuit-breaker J1 or J2

pending

synchro

energising

time

breaking

time

reclosing time

sequence A

switching order

phase comparator



end of switching

energising time

breaking

time

reclosing time

sequence B

switching order

phase comparator



energising time

breaking

time

reclosing

time

sequence C

pending

synchro

pending

synchro

end of switching

end of switching


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= voltage of source resupplying motors

(replacement source).

= voltage at motor terminals after

separation of their first source (residual

voltage).

= rated voltage of motor.

= acceptable residual voltage of motor.

Conditions be met by the networkfor rapid coupling

The first condition to be met whencoupling actually takes place is

expressed by the inequation:

Us

Um

< Un

+ Ur

(see fig. 10)

coupling meeting this inequation, thespeed of all motors is less than theirrated speed, meaning motors absorb acurrent exceeding their rated current.

To increase chances for successfulresumption of motor supply on rapidswitching, the following conditions arethus required:c the speed reached by motors, onresumption of supply, must be as highas possible. Speed depends on:v length of undervoltage time,v inertia of rotating masses,v load torque during deceleration;c the supply network voltage drop mustbe slight. This drop depends on:v impedance of the electrical circuits,v the current absorbed by the motors,

v the number of motors reaccelerated;c the value of the driving torque duringresumption must be (far) greater thanload torque.

Driving torque depends on:

v rated value of driving torque on fullvoltage,

v the form of torque in the speed range

between speed at which resumptiontakes place and rated speed,v the voltage applied at the motorterminals.

Note that if motor slip is considerableon resumption of power supply,throughout the restart period, thecurrent absorbed by the motorsis constant and approaches, in aninitial approximation, startingcurrent (the curve of current absorbedby an asynchronous motor as afunction of rotation speed, is relatively

flat).Diagram showing the rapidswitching device(see fig. 11)

Us

fig. 10: electrical variables and inequation

conditioning success of rapid coupling.

As a rule motors can support acoupling in phase opposition, afterseparation of their first source,provided that residual voltage at theirterminals does not exceed the value Urequal to 25 % Un.This initial condition, althoughnecessary, is not sufficient to ensuresuccessful reacceleration of motors. In

point of fact, despite monitoredfig. 11: diagram showing the rapid switching device to supply to MV networks.

Um

Un

Us

Um

< Un

+ Ur

IUnI+IUrI

Us-Um

Us

Um

Ur

M M

source 1 source 2

tripping

Us 1

Du Du

UM UM

energising

J1 J2cdp

Jc

HV busbar

tripping

Du: undervoltage detector

cdp: phase comparator

M: asynchronous motors

Us 2


15/16


switching type

synchronous interrupted circuit transfer pseudo-synchronous

LV HV

application - busbar switching, from industry to - supply with 2 switchable reacceleration ofexamples - substitution of one service sector: HV incomers, asynchronous motors

generator by another, - supply of pumps, - supply by one main- switching an UPS - supply of auxiliary circuits source and oneto the mains of a transformer substation, replacement source

- supply of hypermarkets,- etc

switching time zero 0.5 to 10 s 1 to 30 s 0.06 to 0.3 s

devices used - coupler, - automatic source - cubicle assembly rapid HV circuit-breaker(examples of - synchrocoupler, changeover switch with with changeover switch associated withMerlin Gerin - UPS unit with static circuit-breaker (VM6, DDM and NSM) a phase comparatorequipment) switch (Compact and

(EPS 2000 and 5000) Masterpact)

observations switching must take difficulties in preparingplace before main switching orders (presencesource voltage is of residual voltage)completely lost

6. summarising table

7. conclusion

This description of operating conditionsto be met and technical requirementsto be considered leads us to thefollowing practical conclusion.

Before choosing a switching device, it

is preferable and indeed vital to know:c the quality of the main andreplacement sources:v amplitude, duration and frequency ofvoltage drops,

v duration and frequency of voltageloss (whether or not service resumptioncontrollers are present upstream, e.g.rapid or slow reclosers),v available power;

c load requirements as regardscontinuity of supply:v no supply failure tolerated,v voltage loss tolerated (0.3 s, 1 s,30 s, etc).


16/16

appendix: brief description of a phase comparator

The phase shift of the two sources ismeasured by subtracting the twovoltage vectors, Us and Um, i.e.:

Us

Um

= U

Where

Us

= Us sin1t

(replacement source voltage)

Um

= Um sin2t

(residual voltage)Note: the writing

Us = Um = Ua

simplifies calculations

Using the trigonometric formula

sin sin = 2 cos +

2sin

2

the following can be written:

Us

Um

= 2 Ua cos1+2

2t sin

12

2t = U

The pulsation voltage envelope

1+2

2has a pulsation beat

1+2

2expressing phase shift

evolution in time between Us

and Um

(see fig. 12).

The voltage Us

Um

is rectified and

filtered. The instantaneous valuesof the envelope curve voltage, orbeating voltage, thus obtained are

used to determine phase shift

between the two sinusoidal voltages tobe compared: there is a zero phase

shift for the minimum instantaneous

value.

When the envelope curve voltage dropsbelow a set threshold, the comparatorsends a switching order until theenvelope curve voltage once againexceeds the threshold value.

If there is a great difference betweenthe two frequencies, the comparator

prevents switching since, in this case,the conditions for ultra-rapid switchingare not favourable.

However, if residual voltage is less thana preset value (e.g. 0.20 to 0.60 Un),switching takes place despite a largedifference in frequency.

Real. Illustration Technique - Lyon

Edition: DTE - Grenoble

a) U

= Us

Um

b) rectified U

c) comparaison between Isc rectified U

and threshold voltage

fig. 12: phase comparator analysis between two voltages: Us

andUm.

envelope curveU

t

U

t

U

t

rectified, filtered signal

threshold

possible switching order

2

Documents

161 Automatic Transfering of Power Supllies in HV and LV Network