history summer09

8/6/2019 History Summer09

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PAC.SUMMER.2009

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PA

Chistory

Generator Protection, AEG

History is the tutor of life

Reverse-Powe

Leakage SupprWinding for Diffe

CTs, Btow


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Special protection functions have been de-

veloped for bigger generators.

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by Walter Schossig

it operates more or less immediately. In 1920 generatorswere equipped with at least two-phase or better three-phasereverse-current tripping device with sensitive setup: the relaysshould only trip in case of internal faults. Backup protectionwas realized by high-current tripping devices with a longdelay. In case of tripping of the generator circuit-breaker thegenerator had to be de-excited. This avoids fire in the windingof the generators.

Only one power relay was used at this time because theengineers thought that only in case of a failure a power in thatdirection could occur. Such a "reverse power" is possible in caseof power swing, bad synchronization or during a short circuitwith up to 15% of the nominal power of the generator. Such

reverse relays should not endanger normal operation. Thesetting should be above the value mentioned or with a timelonger than the power swing (1.5 s). Of course in that case theefficiency of the protection was quite poor.

The clearing time was long (for a generator protection)and the relay operates only in case of a terminal short-circuitbecause the voltage collapses and an active power of more than10% could not be measured anymore. This "dead zone" couldbe avoided particularly with a directional relay used in a 30- or

The first developments in generator protectionhave been discussed in the last issue of PACW. Since themachines became bigger special protection functions havebeen developed and will be discussed in this article.

Reverse-Power Protection

In the first years a reverse power has been indicated byan annunciation only. H&B produced a reverse-current anddirection-of-current indicator in 1894 (Fig. 1). The rotatingred disk in front of white plate showed the irregularity.

Directional relays have been used to distinguish betweenshort-circuits at busbar or in feeders and failures in thegenerator. They could detect if the current flows from thegenerator into the grid or in reverse direction. These relays

used current transformers in the generator circuit breakers;this location was the border where the overcurrent should tripwithout time delay.

A combined overcurrent and reverse-power relayfor generators was shown by AEG in 1903 (Fig. 5). Analuminum-disk was driven by a magnetic three-leg core. Theouter legs have been excited by the voltage, the middle one

by current. At normal direction of current even in case of ahuge overcurrent the relay is delayed, in case of reverse current

BiograpWalter Scho

(VDE) was b

in Arnsdorf

Czech Repu1941. He st

electrical e

neering in Z(Germany),

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now E.ON Tinger Energ

Erfurt. Ther

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study group

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control.

History

Protection

Overvoltage, Differential, Turn-to-Turn-Fault

GeneratorProtection


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Direction ofwer relays

R2, AEGroximately 1925

Reverse-rrent &direc-

on-of-currentdicationH, 1894

BBC, 1943 BBC, 1968 AEG, 1927BBC, 1978

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istory

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50-scheme. Now the relay starts up even in case of inductivereactive currents during unequal excitation.

On Ascension Day 1924 a disaster occurred in a steamstation in Erfurt (Germany) during the taking of a generator

out of service. The bolt of the trip valve was full of salt andcould not interrupt completely the steam supply. Now therotor was accelerated and the new installed generator wasdestroyed completely.

In the 1920s AEG developed the RR2 power relays. SeeFig. 2. They consist of two induction driving elements witha common Ferraris-disk. Springs hold them in the middleposition. The driving elements work in Aaron-circuit. Anarm moved according to the amount and direction of power.It was more or less a wattmeter with a contact. The switchingcapacity was poor and an auxiliary relays was necessary.

Reverse power protection was later used for protection ofsteam turbines.

The turbine operates as a synchronous motor and it couldbe damaged.

Circuit and view of a 2-pole reverse-power protectionCG90c (BBC) is shown in Fig. 3 and Fig. 10.

To avoid a tripping of the protection in case of turbine bladesalt deposits, the tripping signal is active only if the valve 2 isclosed (Fig.4).

ZPA produced the reverse-power relay GSCT12 (Fig. 9)in the early 1970s. For measuring a ferrodynamic relay SWin Figure 8 was used. An advantage of this device was thesensitivity for harmonics because it trips on the mean valueof the products of voltage and current. A torque was producedonly in case of equal fundamental or harmonic.

These relays could be used for earth-fault detection too.The successor was the static relay GSCT12X in 1981.

BBC produced a static PPX110/111 (Fig. 6) in the 1970s.This relay was used for supervision and tripping of generators,but it also could detect if a generator still receives energy in caseof a leak valve. Another usage was for huge changes of loadwhich could cause an out-of-step of the generator. All theseconditions could be supervised and evaluated with a counter.

A definite time reverse power relay, type WCG, produceby GEC in 1988 is shown in Fig. 11.

Differential Protection

Effective short-circuit protection became possible with thintroduction of differential protection. First developmenand the usage for transformer and line protection have beecovered in the last issues of this magazine. The most commobasic connections in the 1930s are shown in Fig. 15.

Unlike transformer differential the same transforme(type, construction, ratio) could be used in star poin

3 Reverse-powerprotection CG90c

5 Combined overcurrent and reverse-powerelay, AEG, 1903

6 Power relays PPX110-14 Circuit of reverse-power protection

7 Differentirent transfor


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power rela

GEC, 1988

wiring diagram, ZPA, 1976 ZPA, 1976

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matching transformers and tap changers were not needed.Due to missing no-load current a more sensitive setup waspossible.

The differential relay produced by AEG in 1925 (DR inFig. 14) worked without timing element. It operates withFerraris' principle and tripped with a time delay for smallcurrents and instantaneously with large currents. Accordingto the requirements from the customers two- and three-phasedevices with a time range of 1 up to 6 seconds have beenproduced.

An appropriate circuit was the magnetic differential(Byrd-transformer as shown in Fig. 15c).

An iron core was connected at the beginning and the end ofevery leg. The secondary winding was connected to differentialrelays (in that case sensitive overcurrent relays). The neutralpoint is created beyond the transformer group. Other devicessuch as overcurrent relays and measuring devices (not shownin the figure) could be connected too.

Conventional differential schemes use six CTs transformingthe nominal current of the transformer (e.g. 1000 A) to 5 A.Here the winding D was connected in a special manner toachieve highest sensitivity. The impedance of the winding wasselected the same as the relay's. Instead of a ratio 1000 A/5 A= 200, ratio of 25 has been used and so the sensitivity (or thesafety against disturbances) was 8 times higher.

If every iron core got only one winding, false currentsoccurred even in case of equal primary currents due to

non-symmetrical configuration of the conductors. Dr. W.Btow proposed in 1925 (DRP 456202 and 480371) aleakage suppression winding.

The iron core (shown on the spread) carried 18 coils,

couples connected in series. These groups have been connectedin parallel to the differential relays (clamp D). In case of oneelectromagnetic force bigger than the other one (because itwas near to a primary conductor and thats why in a strongerfield) the equalizing current flows to the coil with the smallerelectromagnetic force.

The magnetic field of the equalizing current superimposesthe field of the primary currents, the magnetic flux in allcross-sections of the core was equal as long as the primarycurrents are equal. The flux has been moved from a strongermagnetic point in the core to a lighter magnetized one. Withsuch CTs differential currents as low as 0.1 % of the nominalcurrent could be safely detected.

For instance the four 30000 kVA generators in theVermuntwerk (Austria) and the two 40000 kVA generatorsin the pump-storage power station Herdecke (Germany) havebeen equipped with such a protection.

These CTs never became popular because the customerspreferred standard transformers.

In connection with stator earth fault protection BBCrecommended in 1945 a simplified differential protectionusing single pole differential relays (Fig. 16). In case ofphase-to-phase short circuit this protection was quite fast,while during two phase-to-earth faults it operates only insome cases.

BBC introduced their TG generator differential relays in1943. Figure 18 shows the further development TG3. The

8 Reverse-powerrelays GSCT12-S1

Effective and fast short-circuit protection

became possible with theintroduction of differentialrelays.

11 Dtime re

10 Circuit of Reverse-Power ProtectionBBC, 1943

9 Reverse-powerrelays GSCT-S1


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BBC, 1945 Delta-Connection, BBC, 1952

C

baT

S

R

W

S

R

U V

0

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istory

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3-pole one, for every phase an overcurrent relays RRID workas a differential element with rectifier and a series impedanceThe supply of the relays was with silicon rectifiers. ThRRIDs had no possibility for setup, nevertheless differenvalues could be achieved by changeover of the connectors.

In 1965 Oerlikon proposed a solution for coarse and findifferential protection (Fig. 21). According to the rules iSwitzerland, Austria and Germany current transformers ihigh voltage switchgears have to be earthed on the secondarside. That is why the interposing transformers 8 were realizein an wye/delta-circuit. In 1989 AEG produced the statgenerator protection SQG. The choice of the characteristcurve of the error-current stabilization was performe

by solder bridges. (Fig. 19). High impedance differentiprotection such as the FAC produced by GEC in 1988 is quitpopular in the Anglo-Saxon language area (Fig. 20).

Turn-to-Turn Fault Protection

In case of interturn faults it is essential to switch othe generator immediately due to local overload caused bequalizing currents in the windings, especially in case oseveral conductors in a single slot.

In transformers such a failure could be detected by thBuchholz-protection, however this is not that easy to detecin generators. The faulty line operates as a primary winding oa transformer with short-circuited secondary winding. If thphase of the transformer is equipped with 500 windings antwo of them are short-circuited the current is 250 times highethan the normal current flowing through this phase (leackag

connection for generators in delta-connection and the requiredcircuit for primary and secondary transformers are shown inFigure 17.

In the 1960s ASEA produced the differential protectionRYDHA with high-impedance-stabilization. The differentialmeasuring elements have been equipped with big seriesimpedance working as a surge voltage protector. Choosing asuitable operating point could avoid undesired tripping dueto saturation of current transformers without balanced-beamrelays or delays. Operating time was 15 ms (without thetripping time of the auxiliary relays). Primary pickup-valuewas 2% of the nominal current of the CT. The device was a

15 Basic Connections of generator-differential, 1936

14 Simple differential protection, AEG,1925

3 Interturnult protectionBauch, SSW)

Interturn faults require theimmediate switch-off of the

generator in order to preventfurther damage

16 Simplifieddifferential protection

12 Interturnhort-circuitrotectionA2c withain of reactors b

emens, 1936)

EM - Excitation Machine; FA - Field Surpression; RR - Reverse Relay;S - Oil-Breaker; Sp - connected to Voltage Transformer; St - Current Transformer;UMZ - definite time.overcurrent relays; DR single phase differential

17 Differential protion for generators w


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19Generator differential protection,AEG, 1989

Interturn S

Protection

Siemens, 1

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is not considered). It is not possible to detect this fault by adifferential protection because the currents at the beginningand at the end of the winding are equal.

B. Bauch (SSW) patented in 1925 (DRP 432837) the

circuit shown in Fig. 13. The generator to be protected (G) isconnected to the auxilliary inductor (S). This is an imageof the generator and consists of a transformer with a primarywinding in star connected to the neutral of the generator.The neutral points are connected via the relays R. At the

beginning these relays have been simple overcurrent devices.The inductor was equipped with a delta connection. In caseof a turn-to-turn fault the phase-voltage decreased. The starpoint of the generator moves, the star point of the inductormoves with the impact of the delta-winding into the triangleof the voltages. Since a third harmonic current flows in theconnection of the neutrals , R. Bauch used the wattmetricrelay R in Fig. 13 with two coupled systems connected to thesinusoidal voltages U12 and U23.

Siemens used a circuit for interturn short-circuit protectionin 1936. The RA2 (c) worked with a chain of reactors (b). SeeFig. 12. Details and characteristic of frequency (current limitingas a function of the frequency) is shown in Fig. 24, it was usedto keep off the third harmonic. A combined differential- andinterturn short-circuit protection (5 and 6) for generators withtwo parallel windings 1 is shown in Fig. 26. Interturn faultscause equalizing currents between the neutral points, flowingthough the equalizing winding 4 to the relays 6.

In the 1950s SSW used the circuit shown in Fig. 25. Theopen delta winding was used for the interturn short-circuitprotection, connected to moving-coil relays with a rectifierand a filter network (for the 3rd harmonic). The secondary wasrealized as a wye connection. Measuring devices and relayshave been connected to the supporting coil.

For generators with two windings instead of the coils the"double phantom circuit" was used (Fig. 28).

Over-Voltage Protection

An increase of voltage was dangerous especially at hydrogenerators since it may result in huge increases of speed.

In 1936 Siemens produced an increase-of-voltage-relay(RV5, Fig. 23). It worked properly for increases up to 200% ofthe nominal voltage. In 1984 Siemens produced a static relaywith two stages 7RE21-Z1 (Fig. 29).

When the short-circuit currents in the high voltagegrids became bigger this caused especially problems ineffectively grounded systems due to high fault currents forphase-to-ground faults. A limitation was possible withisolation of different neutral points of transformers. This

became common at unit transformers. In case of opening thecircuit breaker between the unit and the grounded grid (e.g. incase of load-shedding) dangerous over-voltages could occur.The first nuclear power stations in Switzerland (Beznau I andII which NOK put into operation in 1969 and 1971) havebeen equipped with a star-point breaker developed by AEG (4in Fig. 27).

The effective power of both power stations together was700 MW. Generators operated as one unit (1 and 2); four

18Differential Relays TG3, BBC, 1952

20 High impedancedifferential protec-tion, GEC, 1988

The choi

of the

characte

curve is

performe

solder br

21 Coarse and finedifferential protec-tion, Oerlikon, 1965

22 Chreactors


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for generators with parallel windings, BBC, 1945

Siemens, 1984Interturn short-circuits for genera-tors with parallel windings per Ph.

a Supporting Reactance b Relays

GS

5 & 6 -A combined differential- and interturn short-circuit protection

1 - Generators with two parallel windings 4 - Equalizing winding 6- Relay

1 & 2 -Generator and Transformer 3 - Lightning arresters4 - Neutral earthing switch 5 - Circuit breaker

120

100

80

60

40

20

0

0 20 40 60 80 100 120 140 160 180 Hz

Penetrabilityofcurrent%

50 150

G Generator S Open delta winding

2

3

1

1

5

4

4

2

ba

3

65

7

8

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istory

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three-phase transformers (220 MVA, 15,5/250 kV) suppliedinto the 220-kV-grid. Neutral points on the high side have

been protected by lightning arresters (3) and could be earthedby neutral earthing switch (4). They have been tripped at thesame time with the circuit breaker. With their operating timeof 19 ms they have been earthed before the contact separationof the circuit breaker (Fig. 27).

Earth fault protection and other devices for generatorprotection will be covered in a later issue of PAC World.

[email protected]

24 Circuit and characteristic of frequency,Siemens

25Interturn short circuit protection, SSW,um 1950

28Double phantomcircuit for detection of

23Increasef voltageelay RV5

mens, 1936

26 Combined differential and interturnshort-circuit protection

27Neutral earthing Switch, AEG, 1969

29 Increase ofvoltage relay 7RE2

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