Protection History: Generator Protection

Author: Walter Schossig, Germany

Generator Protection - Rotor Earth Fault Protection

Failures in the rotor, caused by low exciting voltage do not occur that often and single earth faults are not that dangerous. But it was obvious, that in case of a second breakdown of isolation, the turn-to-turn-fault exerts a force on the axle. The detection of turn-to-turn faults is difficult. With a low exciting voltage they occur in case of operation of the machine only. Centrifugal forces and heating utilize the winding mechanically and thermally.

This explains why a turn-to-turn fault occurs during special load conditions and not if the generator was out of service. Measurements are difficult due to a low resistance of the rotor winding. Only a careful assembly of the winding was a sufficient protection because the isolation was aged by the de-excitation.

Earth Fault Protection

Max Pohontsch, Berlin, patented “Earth Fault Supervision for DC Circuits” in 1928 (DRP 457323). See Figure 3.

Since rotor earth faults are not that dangerous, a simple indication was considered sufficient. One terminal of the DC circuit, supplied by a battery or a dynamo, was earthed via a series connection with a measurement device or an overcurrent relay, a capacity as DC barrier and the secondary side of a small transformer. The primary side was connected to a stable AC, e.g. the station transformer. AEG proposed a simple scheme (Figure 4) in 1940. The whole rotor circuit was shifted to an electrical potential of 30 up to 50 V (using AC and a capacitor). In case of normal operation the current was small, while in case of an earth fault the magnitude increases and could be measured or trip the relay.

Rotor Earth Fault Protection SSW ("Siemens-Schuckert-Werke") measures the currents with a current transformer and a moving coil instrument operated with a rectifier  (Figure 5). The setup range was between 10 and 50 mA and alerts if the resistance to earth was less than 1000 Ohms. Auxiliary AC voltage of 30…50 V was connected with a voltage divider to a DC generator.

The auxiliary voltage had to be limited to avoid a danger for the staff e.g. during checking of the brushings. That is why Bütow used a wattmetric devices (for measurement or relaying) as shown in Figure 1. The secondary side of the transformer was earthed with a voltage coil (shown with dashed lines in the circuit).  The DC circuit to be supervised was connected with the resistance R (200 Ω) and a capacity C (20 µF) on one tap of the transformer. The voltage between the tap and the earth was appr. 40 V. The fixed coil with 5000 windings was connected with a capacity of 4 µF and a resistance in parallel to the secondary winding of the transformer (voltage 120 V). With the capacity and the resistance in parallel the current could be set up. The current was constant as long as the primary voltage of the transformer was constant. The current delivered the field for the electro dynamic device (required 170 mA). BBC produced a rotor earth fault device RBV in 1947 Figure 13. A scheme used in the United States in 1948 is shown in Figure 12.

In the 50s of the last century rotor earth fault detection with alternating measurement of the voltage of the plus and the minus pole to earth (Figure 7) was the common method used.

Rudolf Ulbricht (GDR) proposed in 1951 a relay implementation of this method (Patent 5278 Figure 8). The idea was used in the REG5 of EAW. BBC produced the rotor earth fault protection as shown in Figure 11.

If the generators have not been excited by rotating machines but by the more and more used static exciting systems, the protection was not sensitive enough. That is why improving the system  or development of other systems were required.

The reason was that the rectifier exciting systems caused a ripple in the rotor current. This ripple caused in the capacitance C in the excitation circuit currents with harmonics which were responsible for false tripping. To avoid this, a Ferraris measuring element has been used as rotor earth fault protection PUM20. Polarized by the generator voltage, it was stable against the ripple of the rectifier exciting currents. This allows the detection of earth faults with earth contact resistance of up to 1000 Ohms. Utilizing the PUM20, BBC delivered the system “Compatrol” (Figure 14).

OERLIKON (CH) also used in the 1960s capacitors for blocking. They provided the AC in the bridge circuit (Figure 6). The zone of protection was also 100% of the whole rotor circuit; the winding itself and all coupled circuits as brushings and (if existing) an excitation system. Leopold Ferschl and Franz Hofer at SSW in Vienna patented in 1964 the circuit as shown in Figure 15 (AR 235933).

With a Ferraris system the CG30 relay (BBC) also detects contact resistances of up to 1000 Ohms (1965, Figure 19).

Further developments have been the PUM201 (1968) and the indication device ZUsw (1970) with ballast YZ/B2.

First it was just used to indicate the rotor earth faults and afterwards, during the next maintenance the problem is being fixed.

In the 1970 the first implementations have been used for tripping too. The introduction of electronics allowed a wider noise ratio and increased the tripping safety. SIEMENS produced an electronic rotor fault protection in 1972. Harmonics that occurred in thyristor controlled stations (150…300 Hz at 500 V) had no impact on the sensitivity anymore. The startup value could be set between 0.1 and 10 kΩ and detected earth faults without a “dead zone”.  See Figure 9.

Westinghouse  produced the Field Ground Detection Relay DGF in 1979 (Figure 17 and Figure 18).

Rotor earth fault protection 7RU21 produced by SIEMENS in 1984 is shown in Figure 21.

For big units and static excitation systems with huge harmonic distortion values the two-stages system 7UR22 with measurement of the resistance was provided (Figure 22).

To measure failures with high impedances a DC voltage has been used between the excitation circuit and the earth. The current IE was the measure for the contact resistance RE (Figure16). The DC was synchronized with a low-frequency clock with changing polarity.

AEG produced the static rotor earth fault protection SLG in 1992 (Figure 29).

Figure 25 and Figure 27 show the circuit and view of the MRSU 04 by GEC Alsthom (1993). It worked with frequency and voltage (4,75 Hz, 24 V).

The principle of the 7UM62 (SIEMENS) is shown in Figure 26, while the principle of REG216/316*4 with injection unit REX010 and injection transformer unit with auxiliary contactor REX011 (ABB) is in Figure 20.

NARI (China) uses an injection scheme according to Figures 28 and 32. The relay injects a square wave voltage between one end or the two ends of rotor windings and the rotor shaft, measures the earthing current and calculates the earthing resistance, performs rotor single point earth fault protection. During the dual-end injection, it can calculate the position of rotor earth fault.

For example, on 12th September 2007, in LongTan hydro-power plant, Guangxi of PRC, No.1 when a Generator’s rotor earth fault occurred, the relay alarmed properly and indicated correctly that the earth fault position was lying at the negative end of the rotor.

Rotor double earth fault protection

Double earth faults in the rotor are very dangerous. The fluxes in the rotor became different and radial forces occur, causing dangerous vibrations which could destroy the generator. Harms, Berlin, demonstrated in 1949, that at a 4-pole-rotor the bypass of one pole could cause a magnetic force on one side as high as 4-times the weight of the rotor. Especially the limitation of generation in Eastern Germany required an operation of generators during a single fault.  Rudolf Ulbricht, GDR, presented in 1951 his solution under Patent 5278. See Figure 30.

Precondition was a solid rotor earth fault. In the bridge connection the resistance R was defined in such a manner, that the double-earth-fault relay was without a current (mA-pointer). In case of a second earth fault, the relay A trips, switches off the generator and de-excites the machine. The bridge current was taken from the axle (“Welle”) directly. The relay itself was a sensitive polarized DC-device with contacts on both sides because the bridge current could be positive or negative (depending on the 2nd earth fault). If everything was dimensioned very well, double earth faults within 10…15% of the winding could be detected.

The sensitivity depends on the exciting voltage. The mA-pointer indicates starting failures already at their beginning. The device also detects earth faults in other equipments connected. The same scheme has been used in Western Germany, the United States and in the Soviet Union. In normal cases this protection was not a part of the standard configuration; it has been used after a rotor earth fault. The tuning was done during operation. Figure 23 shows the REG6 of EAW.

During the commissioning of the first 500-MW-unit in Eastern Germany (Hagenwerder, 1974) and in later installations (as Jänschwalde 1981) rotor double earth fault protection built in the Soviet Union was used, (the device   K3P-2Y4). See Figures 24 and 31. The sensitive measuring element has been two anti-parallel connected polarized relays ∏P1 and ∏P2. It was used as a built-in or mobile device (for several generators).

In most countries double earth fault protection was not an issue; in case of an earth fault the generator is no longer used.

The history of generator protection will be continued.

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

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