A three-phase 33kV: 11kV power transformer is rated at 10 MVA. The 33kV winding is delta...

 

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A three-phase 33kV: 11kV power transformer is rated at 10 MVA. The 33kV winding is delta connected and the 11kV winding earthed star connected. The effective impedance of the transformer is j0.1 pu based on its own rating. All current transformers (CTS) have the ratio 500/5, a secondary winding resistance of 1.00 and a secondary voltage Vs versus excitation current le characteristic of:- a) The maximum resistance of the leads used to connect a CT to the relay is 0.50. Assume that the 11kV winding of the transformer is connected to a non-rotating load with no sources of generation. b) c) Vs le d) 20 40 60 80 100 120 0.01 0.02 0.04 0.08 0.16 0.32 A Sketch a diagram showing how a high impedance differential protection relay can be used to provide carth-fault protection for the star-winding of a transformer in which the neutral connection to earth is made external to the transformer tank. Determine the minimum voltage stability setting for the high impedance relay used in the above differential earth fault protection scheme; assume that the neutral is solidly earthed via a cable of resistance 0.10. If the operating voltage of the relay is 50V and the operating current is 25mA, calculate the resistance of the stabilising resistor used to ensure stability. Finally, evaluate the percentage of the winding protected by this relay for an earth fault if the winding is (i) solidly earthed using a cable of resistance 0.10 and (ii) impedance earthed using a single phase 6.35kV/240V distribution transformer with a secondary burden resistance of 0.10. A three-phase 33kV: 11kV power transformer is rated at 10 MVA. The 33kV winding is delta connected and the 11kV winding earthed star connected. The effective impedance of the transformer is j0.1 pu based on its own rating. All current transformers (CTS) have the ratio 500/5, a secondary winding resistance of 1.00 and a secondary voltage Vs versus excitation current le characteristic of:- a) The maximum resistance of the leads used to connect a CT to the relay is 0.50. Assume that the 11kV winding of the transformer is connected to a non-rotating load with no sources of generation. b) c) Vs le d) 20 40 60 80 100 120 0.01 0.02 0.04 0.08 0.16 0.32 A Sketch a diagram showing how a high impedance differential protection relay can be used to provide carth-fault protection for the star-winding of a transformer in which the neutral connection to earth is made external to the transformer tank. Determine the minimum voltage stability setting for the high impedance relay used in the above differential earth fault protection scheme; assume that the neutral is solidly earthed via a cable of resistance 0.10. If the operating voltage of the relay is 50V and the operating current is 25mA, calculate the resistance of the stabilising resistor used to ensure stability. Finally, evaluate the percentage of the winding protected by this relay for an earth fault if the winding is (i) solidly earthed using a cable of resistance 0.10 and (ii) impedance earthed using a single phase 6.35kV/240V distribution transformer with a secondary burden resistance of 0.10.

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a The diagram below shows how a high impedance differential protection relay can be used to provide ... View the full answer