Generator stator earth fault.pdf

FEED BACK – 22 , 98-99

PROJECT

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RE-ENGINEERING

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JTPCL/TORANAGALLU(2X130MW) GENERATOR STATOR EARTH FAULT RELAY 6462 - WIRING RE-ENGINEERED TO INCREASE 3*° HARMONIC VOLTAGE THEREBY E/F PROTECTION WORKING PROPERLY.

EXPLANATION; For protection of Generator from heavy damage due to the earth fault between winding to ground. Generator neutral is grounded through neutral grounding transformer which limits the ground fault current between winding to ground to a low value and protects the winding and core from burning. Normally primary fault current is limited by NGT to about 10A, which is safe for protecting the core and winding from excessive damage. For detection of E/F on stator, voltage across the neutral grounding resistor is monitored by a relay (64G1) and initiates tripping of generator at voltage value of 5.4 V and above which corresponds to approximately >10A fault current. This scheme works well for detection of 95% of generator winding from terminal side. If any fault occurs in winding zone of first 5% from neutral side, voltage on winding is not sufficient to drive 10A fault current and the E/F cannot be detected by 64G1. Hence for detection of faults in first 5% of winding from neutral side, relay 64G2 is used which is 3rd harmonic relay (150 Hz). For any generator a small 3rd harmonic voltage of the order of 0.15 V to .5 Volts is present on neutral i.e. across NGT at normal working line voltage. This 3rd harmonic voltage is sensed by 64G2 relay.

During ground faults on generator in first 5% zone from neutral side this voltage drops and hence the relay also drops initiating tripping of generator. As 3rd harmonic voltage present on neutral depends on line voltage, at lower terminal voltages 3rd harmonic voltage also will be lour and relay 6462 will not pick-up. Hence tripping contact of 6462 relay is wired in series with an under voltage relay contact to block the tripping whenever gererator voltage is below 70% (below normal working level). As per principal scheme (refer fig.) 3rd harmonic voltage of Relay 6462 was approximately 0.12V © Generator voltage of 10.5 KV.Relay6462rangeis O.1 V to 0.45 V. Hence relay 6462 was not picking up even about 70% of generator terminal voltage, giving a tripping command. Reason for low voltage on relay 6462 was the step-down transformer 220/110V which was stepping down the third harmonic voltage to < 0.15V. RE-ENGINEERING ADOPTED: The relay 6462 wiring modified as shown in Fig. at site. After modification, 3rd harmonic voltage on relay 6462 was approximately 0.3 V at 70% of generator terminal voltage. Hence with U/V relay setting of 70% and 3rd harmonic relay setting of 0.3V. E/F protection is working properly. This modification was carried out at JTPCL/Toranagallu Unit No.1.