The busbars of a 400 kV double busbar substation are protected by high-impedance circulating current protection...

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The busbars of a 400 kV double busbar substation are protected by high-impedance circulating current protection arranged to detect phase and earth-faults. The connections for the check zone are shown in Fig 1. The resistance of the c.t. secondary windings is 1.7 ohms at 75°C. Wiring comprises one 7/0.067 conductor per phase. The switchgear bays for outgoing circuits are at 19.4 metre centres and the multicore cables from the c.t.'s to the ring main buswires are 5 metres long. Other dimensions are shown on Fig 1. The per circuit and duplicate per zone relays have a fixed setting of 33 mA and a burden of 0.6 VA at setting. (The resistance of a 7/0.67 mm conductor is 7.41 ohms per 1000 metres). a b Calculate the value of the stabilising resistors required if the maximum through fault currents are 50,000 amps 3-phase and 60,000 amps single phase to earth. Assume that fault current fed in through circuit L and out through circuit A gives the worst case when calculating stability voltage. What minimum value of c.t. knee-point voltage would be satisfactory? Assume that, up to the knee-point voltage, the c.t. exciting current characteristic is a straight line through the origin. The knee-point is at 800 V where the exciting current is 80 mA. The current through the non-linear resistor at the relay setting voltage is negligible and the resistance of the alarm relay is 4000 ohms. Calculate the setting resistor required to give the check zone a fault setting of 2000 A when the future bays are equipped. What will the fault setting be if this value of resistor is used while the spare bays exist? SR 10 m AR ALARM RELAY PCR PER CIRCUIT RELAY PZR PER ZONE RELAY NLR NON LINEAR RESISTOR STABILISING RESISTOR SR CT TURNS RATIO 1/2000 SHUNT SETTING RESISTOR T MOLL RSH PCR 0 22 m FUTURE FEEDER FUTURE TRANSFORMER 10m PCR w his SRO NLRORSH SROO PZR Y PHASE ONLY SHOWN IN DETAIL BUT ALL OUTGOING CIRCUITS EQUIPPED AS CIRCUIT L. The busbars of a 400 kV double busbar substation are protected by high-impedance circulating current protection arranged to detect phase and earth-faults. The connections for the check zone are shown in Fig 1. The resistance of the c.t. secondary windings is 1.7 ohms at 75°C. Wiring comprises one 7/0.067 conductor per phase. The switchgear bays for outgoing circuits are at 19.4 metre centres and the multicore cables from the c.t.'s to the ring main buswires are 5 metres long. Other dimensions are shown on Fig 1. The per circuit and duplicate per zone relays have a fixed setting of 33 mA and a burden of 0.6 VA at setting. (The resistance of a 7/0.67 mm conductor is 7.41 ohms per 1000 metres). a b Calculate the value of the stabilising resistors required if the maximum through fault currents are 50,000 amps 3-phase and 60,000 amps single phase to earth. Assume that fault current fed in through circuit L and out through circuit A gives the worst case when calculating stability voltage. What minimum value of c.t. knee-point voltage would be satisfactory? Assume that, up to the knee-point voltage, the c.t. exciting current characteristic is a straight line through the origin. The knee-point is at 800 V where the exciting current is 80 mA. The current through the non-linear resistor at the relay setting voltage is negligible and the resistance of the alarm relay is 4000 ohms. Calculate the setting resistor required to give the check zone a fault setting of 2000 A when the future bays are equipped. What will the fault setting be if this value of resistor is used while the spare bays exist? SR 10 m AR ALARM RELAY PCR PER CIRCUIT RELAY PZR PER ZONE RELAY NLR NON LINEAR RESISTOR STABILISING RESISTOR SR CT TURNS RATIO 1/2000 SHUNT SETTING RESISTOR T MOLL RSH PCR 0 22 m FUTURE FEEDER FUTURE TRANSFORMER 10m PCR w his SRO NLRORSH SROO PZR Y PHASE ONLY SHOWN IN DETAIL BUT ALL OUTGOING CIRCUITS EQUIPPED AS CIRCUIT L.