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C h

isto

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Generator Protection, AEG

History is the tutor of life

Reverse-Power

Leakage Suppression Winding for Differential

CTs, Bütow, 1925

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 generators were equipped with at least two-phase or better three-phase reverse-current tripping device with sensitive setup: the relays should only trip in case of internal faults. Backup protection was realized by high-current tripping devices with a long delay. In case of tripping of the generator circuit-breaker the generator had to be de-excited. This avoids fire in the winding of the generators.

Only one power relay was used at this time because the engineers thought that only in case of a failure a power in that direction could occur. Such a "reverse power" is possible in case of power swing, bad synchronization or during a short circuit with up to 15% of the nominal power of the generator. Such reverse relays should not endanger normal operation. The setting should be above the value mentioned or with a time longer than the power swing (1.5 s). Of course in that case the efficiency 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-circuit because the voltage collapses and an active power of more than 10% could not be measured anymore. This "dead zone" could be avoided particularly with a directional relay used in a 30°- or

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

Reverse-Power ProtectionIn the first years a reverse power has been indicated by

an annunciation only. H&B produced a reverse-current and direction-of-current indicator in 1894 (Fig. 1). The rotating red disk in front of white plate showed the irregularity.

Directional relays have been used to distinguish between short-circuits at busbar or in feeders and failures in the generator. They could detect if the current flows from the generator 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 trip without time delay.

A combined overcurrent and reverse-power relay for generators was shown by AEG in 1903 (Fig. 5). An aluminum-disk was driven by a magnetic three-leg core. The outer legs have been excited by the voltage, the middle one by current. At normal direction of current even in case of a huge overcurrent the relay is delayed, in case of reverse current

BiographyWalter Schossig

(VDE) was born

in Arnsdorf (now

Czech Republic) in

1941. He studied

electrical engi-

neering in Zittau

(Germany), and

joined a utility in

the former Eastern

Germany. After the

German reunion

the utility was

renamed as TEAG,

now E.ON Thuer-

inger Energie AG in

Erfurt. There he re-

ceived his Masters

degree and worked

as a protection

engineer until his

retirement. He was

a member of many

study groups and

associations. He is

an active member

of the working

group “Medium

Voltage Relaying”

at the German

VDE. He is the

author of several

papers, guidelines

and the book

“Netzschutztechnik

[Power System Pro-

tection]”. He works

on a chronicle

about the history

of electricity sup-

ply, with emphasis

on protection and

control.

HistoryProtection

Overvoltage, Differential, Turn-to-Turn-Fault

Generator Protection

Reverse-Power

2 Direction of power relays RR2, AEGapproximately 1925

1 Reverse-current & direc-tion-of-current indicationV&H, 1894

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

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

On Ascension Day 1924 a disaster occurred in a steam station in Erfurt (Germany) during the taking of a generator out of service. The bolt of the trip valve was full of salt and could not interrupt completely the steam supply. Now the rotor was accelerated and the new installed generator was destroyed completely.

In the 1920s AEG developed the RR2 power relays. See Fig. 2. They consist of two induction driving elements with a common Ferraris-disk. Springs hold them in the middle position. The driving elements work in Aaron-circuit. An arm moved according to the amount and direction of power. It was more or less a wattmeter with a contact. The switching capacity was poor and an auxiliary relays was necessary.

Reverse power protection was later used for protection of steam turbines.

The turbine operates as a synchronous motor and it could be damaged.

Circuit and view of a 2-pole reverse-power protection CG90c (BBC) is shown in Fig. 3 and Fig. 10.

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

ZPA produced the reverse-power relay GSCT12 (Fig. 9) in the early 1970s. For measuring a ferrodynamic relay SW in Figure 8 was used. An advantage of this device was the sensitivity for harmonics because it trips on the mean value of the products of voltage and current. A torque was produced only 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 case of a leak valve. Another usage was for huge changes of load which could cause an out-of-step of the generator. All these conditions could be supervised and evaluated with a counter.

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

Differential ProtectionEffective short-circuit protection became possible with the

introduction of differential protection. First developments and the usage for transformer and line protection have been covered in the last issues of this magazine. The most common basic connections in the 1930s are shown in Fig. 15.

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

3 Reverse-power protection CG90c

5 Combined overcurrent and reverse-power relay, AEG, 1903

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

7 Differential cur-rent transformer

power relay WCG,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 was possible.

The differential relay produced by AEG in 1925 (DR in Fig. 14) worked without timing element. It operates with Ferraris' principle and tripped with a time delay for small currents and instantaneously with large currents. According to the requirements from the customers two- and three-phase devices with a time range of 1 up to 6 seconds have been produced.

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 of every leg. The secondary winding was connected to differential relays (in that case sensitive overcurrent relays). The neutral point is created beyond the transformer group. Other devices such as overcurrent relays and measuring devices (not shown in the figure) could be connected too.

Conventional differential schemes use six CTs transforming the nominal current of the transformer (e.g. 1000 A) to 5 A. Here the winding D was connected in a special manner to achieve highest sensitivity. The impedance of the winding was selected 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 the safety against disturbances) was 8 times higher.

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

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

The iron core (shown on the spread) carried 18 coils, couples connected in series. These groups have been connected in parallel to the differential relays (clamp D). In case of one electromagnetic force bigger than the other one (because it was near to a primary conductor and that’s why in a stronger field) the equalizing current flows to the coil with the smaller electromagnetic force.

The magnetic field of the equalizing current superimposes the field of the primary currents, the magnetic flux in all cross-sections of the core was equal as long as the primary currents are equal. The flux has been “moved” from a stronger magnetic point in the core to a lighter magnetized one. With such CTs differential currents as low as 0.1 % of the nominal current could be safely detected.

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

These CTs never became popular because the customers preferred standard transformers.

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

BBC introduced their TG generator differential relays in 1943. Figure 18 shows the further development TG3. The

8 Reverse-power relays GSCT12-S1

Effective and fast short-circuit protection became possible with the introduction of differential relays.

11 Definite time reverse

10 Circuit of Reverse-Power Protection BBC, 1943

9 Reverse-power relays GSCT-S1

BBC, 1945 Delta-Connection, BBC, 1952

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3-pole one, for every phase an overcurrent relays RRID works as a differential element with rectifier and a series impedance. The supply of the relays was with silicon –rectifiers. The RRIDs had no possibility for setup, nevertheless different values could be achieved by changeover of the connectors.

In 1965 Oerlikon proposed a solution for coarse and fine differential protection (Fig. 21). According to the rules in Switzerland, Austria and Germany current transformers in high voltage switchgears have to be earthed on the secondary side. That is why the interposing transformers 8 were realized in an wye/delta-circuit. In 1989 AEG produced the static generator protection SQG. The choice of the characteristic curve of the error-current stabilization was performed by solder bridges. (Fig. 19). High impedance differential protection such as the FAC produced by GEC in 1988 is quite popular in the Anglo-Saxon language area (Fig. 20).

Turn-to-Turn Fault ProtectionIn case of interturn faults it is essential to switch off

the generator immediately due to local overload caused by equalizing currents in the windings, especially in case of several conductors in a single slot.

In transformers such a failure could be detected by the Buchholz-protection, however this is not that easy to detect in generators. The faulty line operates as a primary winding of a transformer with short-circuited secondary winding. If the phase of the transformer is equipped with 500 windings and two of them are short-circuited the current is 250 times higher than the normal current flowing through this phase (leackage

connection for generators in delta-connection and the required circuit for primary and secondary transformers are shown in Figure 17.

In the 1960s ASEA produced the differential protection RYDHA with high-impedance-stabilization. The differential measuring elements have been equipped with big series impedance working as a surge voltage protector. Choosing a suitable operating point could avoid undesired tripping due to saturation of current transformers without balanced-beam relays or delays. Operating time was 15 ms (without the tripping time of the auxiliary relays). Primary pickup-value was 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

13 Interturn fault protection (R. Bauch, SSW)

Interturn faults require the immediate switch-off of the

generator in order to prevent further damage.

16 Simplified differential protection

12 Interturn short-circuit protection RA2c with Chain of reactors b (Siemens, 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 protec-tion for generators with:

19Generator differential protection, AEG, 1989

Interturn Short-Circuit Protection RA2, Siemens, 1936

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is not considered). It is not possible to detect this fault by a differential protection because the currents at the beginning and 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) is connected to the “auxilliary inductor” (S). This is an image of the generator and consists of a transformer with a primary winding 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 case of a turn-to-turn fault the phase-voltage decreased. The star point of the generator moves, the star point of the inductor moves with the impact of the delta-winding into the triangle of the voltages. Since a third harmonic current flows in the connection of the neutrals , R. Bauch used the wattmetric relay R in Fig. 13 with two coupled systems connected to the sinusoidal voltages U12 and U23.

Siemens used a circuit for interturn short-circuit protection in 1936. The RA2 (c) worked with a chain of reactors (b). See Fig. 12. Details and characteristic of frequency (current limiting as a function of the frequency) is shown in Fig. 24, it was used to keep off the third harmonic. A combined differential- and interturn short-circuit protection (5 and 6) for generators with two parallel windings 1 is shown in Fig. 26. Interturn faults cause equalizing currents between the neutral points, flowing though the equalizing winding 4 to the relays 6.

In the 1950s SSW used the circuit shown in Fig. 25. The open delta winding was used for the interturn short-circuit protection, connected to moving-coil relays with a rectifier and a filter network (for the 3rd harmonic). The secondary was realized as a wye connection. Measuring devices and relays have 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 ProtectionAn increase of voltage was dangerous especially at hydro

generators 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% of the nominal voltage. In 1984 Siemens produced a static relay with two stages 7RE21-Z1 (Fig. 29).

When the short-circuit currents in the high voltage grids became bigger this caused especially problems in effectively grounded systems due to high fault currents for phase-to-ground faults. A limitation was possible with isolation of different neutral points of transformers. This became common at unit transformers. In case of opening the circuit breaker between the unit and the grounded grid (e.g. in case of load-shedding) dangerous over-voltages could occur. The first nuclear power stations in Switzerland (Beznau I and II which NOK put into operation in 1969 and 1971) have been equipped with a star-point breaker developed by AEG (4 in Fig. 27).

The effective power of both power stations together was 700 MW. Generators operated as one unit (1 and 2); four

18 Differential Relays TG3, BBC, 1952

20 High impedance differential protec-tion, GEC, 1988

The choice

of the

characteristic

curve is

performed by

solder bridges.

21 Coarse and fine differential protec-tion, Oerlikon, 1965

22 Chain of reactors for:

for generators with parallel windings, BBC, 1945

Siemens, 1984 Interturn 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 protection1 - Generators with two parallel windings 4 - Equalizing winding 6 - Relays

1 & 2 - Generator and Transformer 3 - Lightning arresters 4 - Neutral earthing switch 5 - Circuit breaker

120

100

80

60

40

20

0 0 20 40 60 80 100 120 140 160 180 Hz

Pene

trab

ility

of c

urre

nt%

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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three-phase transformers (220 MVA, 15,5/250 kV) supplied into the 220-kV-grid. Neutral points on the high side have been protected by lightning arresters (3) and could be earthed by neutral earthing switch (4). They have been tripped at the same time with the circuit breaker. With their operating time of 19 ms they have been earthed before the contact separation of the circuit breaker (Fig. 27).

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

walter.schossig@pacw.org www.walter-schossig.de

24 Circuit and characteristic of frequency, Siemens

25 Interturn short circuit protection, SSW, um 1950

28 Double phantom circuit for detection of

23Increase of voltage relay RV5Siemens, 1936

26 Combined differential and interturn short-circuit protection

27 Neutral earthing Switch, AEG, 1969

29 Increase of voltage relay 7RE21