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70 PAC history History is the tutor of life (Moving the rotor Koepchenwerk, RWE, 1988) PAC.MARCH.2010 by Walter Schossig, Germany Protection (Moving the rotor of a 265-MVA- generator (station Waldeck II) 71 History Rotor Earth Fault Protection Biography Walter Schossig (VDE) was born in Arnsdorf (now Czech Republic) in 1941. He studied electrical engineering in Zittau (Germany), and joined a utility in the former Eastern Germany. After the German reunion the utility was renamed as TEAG,
  PAC.MARCH.2010 70     P    A    C    h    i   s   t   o   r   y (Moving the rotor Koepchenwerk, RWE, 1988)   History is the tutor of life   (Moving the rotor of a 265-MVA- generator(station Waldeck II) PAC.MARCH.2010 71 by Walter Schossig, Germany side of a small transformer. The primary side was connectedto 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 ACand a capacitor). In case of normal operation the current wassmall, 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 ACvoltage of 30…50 V was connected with a voltage divider to aDC generator. The auxiliary voltage had to be limited to avoid a danger for the staff e.g. during checking of the brushings. That is whyBütow used a wattmetric devices (for measurement or relaying) as shown in Figure 1. The secondary side of the transformerwas earthed with a voltage coil (  shown with dashed lines inthe circuit  ). The DC circuit to be supervised was connectedwith the resistance R (200 Ω) and a capacity C (20 μF) on onetap of the transformer. The voltage between the tap and the Failures in the rotor, caused by low excitingvoltage 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 forceon 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 duringspecial 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 simpleindication was considered sufficient. One terminal of theDC circuit, supplied by a battery or a dynamo, was earthedvia a series connection with a measurement device or anovercurrent relay, a capacity as DC barrier and the secondary Biography Walter Schossig(VDE) was bornin Arnsdorf (nowCzech Republic) in1941. He studiedelectrical engi-neering in Zittau(Germany), and joined a utility inthe former EasternGermany. After theGerman reunionthe utility wasrenamed as TEAG,now E.ON Thuer-inger Energie AG inErfurt. There he re-ceived his Mastersdegree and workedas a protectionengineer until hisretirement. He wasa member of manystudy groups andassociations. He isan active memberof the workinggroup “MediumVoltage Relaying”at the GermanVDE. He is theauthor of severalpapers, guidelinesand the book“Netzschutztechnik[Power System Pro-tection]”. He workson a chronicleabout the historyof electricity sup-ply, with emphasison protection andcontrol. History Protection Rotor Earth Fault Protection GeneratorProtection    2 Rotor earthfault protec-tion REGL (AEG) 1 Earth faultsupervision (Bütow, 1932) 1 . Voltage Transformer  2 . Capacitor  3 . Earth Fault Relay  GG _   213 ~     P    A    C    h    i   s   t   o   r   y PAC.MARCH.2010 72 earth was appr. 40 V. The fixed coil with 5000 windings wasconnected with a capacity of 4 μF and a resistance in parallelto 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 theprimary voltage of the transformer was constant. The current delivered the field for the electro dynamic device (required170 mA). BBC produced a rotor earth fault device RBV in1947 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 detectionwith alternating measurement of the voltage of the plus andthe minus pole to earth (Figure 7) was the common method used. Rudolf Ulbricht (GDR) proposed in 1951 a relayimplementation of this method (Patent 5278 Figure 8). Theidea 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, theprotection 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 causeda 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, aFerraris 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 (Figure6). The zone of protection was also 100% of the whole rotorcircuit; the winding itself and all coupled circuits as brushingsand (if existing) an excitation system. Leopold Ferschl andFranz 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) andthe 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 beingfixed. In the 1970 the first implementations have been usedfor tripping too. The introduction of electronics allowed awider noise ratio and increased the tripping safety. SIEMENS 3 Earth Fault Supervision, Pohontsch, 1928 4   Rotor earth fault protection, AEG, 1940 5   Rotor earth fault protection, SSW  Erarth Fault Detection with Alternating Measurement if the Voltages VRV Ulbricht-Principle   TRV   r ~ PAC.MARCH.2010 73 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 sensitivityanymore. The startup value could be set between 0.1 and10 kΩ and detected earth faults without a “dead zone”. See Figure 9. Westinghouse produced the Field Ground Detection RelayDGF in 1979 (Figure 17 and Figure 18).Rotor earth fault protection 7RU21 produced by SIEMENSin 1984 is shown in Figure 21. For big units and static excitation systems with hugeharmonic distortion values the two-stages system 7UR22 with measurement of the resistance was provided (Figure 22). To measure failures with high impedances a DC voltagehas been used between the excitation circuit and the earth.The current I E was the measure for the contact resistance R E  (Figure16). The DC was synchronized with a low-frequency clock with changing polarity.AEG produced the static rotor earth fault protection SLG in1992 (Figure 29). Figure 25 and Figure 27 show the circuit and view of the MRSU 04 by GEC Alsthom (1993). It worked with frequencyand 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 injectionunit REX010 and injection transformer unit with auxiliary contactor REX011 (ABB) is in Figure 20. OERLIKON, CH, 1966 EAW, 1966 6   Rotor earth fault protection, OERLIKON Rotor earth fault detectionwith alternating +/- pole toearth voltage measurementwas common in the 50s. 10   Rotor earth relay REG5 7   Earth fault detection,1950 9   Electronic rotor earth fault protection, 1972 11   Rotor earth faultprotection, BBC, 1940 8   Rotor earth faultprotection, 1951 Figure 9: Schematics of the ElectronicRotor EarthFault Protection,SIEMENS, 1972
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