Tutorial 10 - Bus zone, transformer and generator protection PDF

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Bus zone, transformer and generator protection...

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ELX303

Electrical Power

Tutorial 10

PART 3: Bus zone, transformer and generator protection Question 1 A 25MVA, 132/33 kV, resistance earthed star/delta connected transformer is protected overall by an unbiased circulating current scheme. The CT ratio on the low voltage side is 600/1A, and the relays have a setting of 40% of their rated current of 1A. If the connection of the star-point into true earth has a resistance of 5Ω, estimate the percentage of the high voltage winding protected against internal earth faults. You may assume that there is no infeed from the 132kV system and that only the earth resistance limits the fault current.

Following the current transformer, We can observe that:  Iset = (0.4)*(1)  = Vp,Ø 0.4= 33 kV (delta)  Vs,Ø = 132/sqrt(3) kV (star)  R=5Ω  Kp = 600 @ low voltage side

Question 2 A 10MVA, 66/11kV, solidly earthed star/delta connected transformer is protected overall by an unbiased circulating current scheme. The CT ratio on the low voltage side is 400/1A and the relays have a setting of 20% of their rated current of 1A. If the connection of the star point into true earth has a resistance of 3Ω, estimate the percentage of the high voltage winding protected against internal earth faults. You may assume that there is no infeed from the 66kV system and that only the earth resistance limits the fault current.

We can observe that: Following the current transformer,  Iset = (0.2)*(1)= 0.2 Vp,Ø = 11 kV (delta)  R=3Ω  Vs,Ø = 66/sqrt(3) kV (star)  Kp = 400 @ low voltage side

Question 3 A 15 MVA, 33 kV/11kV, resistance earthed star/delta connected transformer is protected overall by an unbiased circulating current scheme. The CT ratio on the low voltage side is 300/1 A and the relays have a setting of 30% of the rated current of 1A. If the connection of the star point into true earth has a resistance of 2 Ω, estimate the percentage of the high voltage winding protected against internal earth faults. Assume no infeed from the 33kV system and that only the earth resistance limits the fault current.

Following the current transformer, We can observe that:  Iset = (0.3)*(1) =0.3Vp,Ø = 11 kV (delta)  Vs,Ø = 33/sqrt(3) kV (star)  R=2Ω  Kp = 300 @ low voltage side

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ELX303

Electrical Power

Tutorial 10

Question 4 A 30MW, 37.5MVA, 33kV star connected alternator is protected by a differential protection scheme using 600/1A current transformers and unbiased relays set to operate at 10% of their rated current of 1A i.

If the earthing resistor is 90% based upon the machine’s rating, estimate the percentage of the stator winding that is not protected against an earth fault. If it is required to protect 90% of the winding, estimate the value of the earthing resistor in ohms and percentage based on the machine rating.

ii.

Question 5 A 5.5MW, 10MVA, 11kV star-connected alternator has a differential protection scheme using 500/1A current transformers and unbiased relays set to operate at 16% of their rated current of 1A. i.

Determine the percentage of the stator winding that is not protected against an earth fault, if the earting resistor is 85% based upon the machine’s rating. Determine the value of the earthing resistor in Ohms and percentage value based on the machine rating, if it is required to protect 95% of the winding.

ii.

Question 6 A 38MVA delta/star 66/6.6kV transformer with a reactance of 20% is fitted with restricted earth fault protection on both the high voltage and low voltage sides. The 6.6kV neutral is resistance earthed at 1.0 per-unit. i.

Sketch the diagram of the transformer showing the connection of the protection on both primary and secondary sides.

ii.

Calculate the 3-phase fault current for a fault on the secondary side of the transformer if the source fault level is 2800MVA.

iii.

The 1300/1 current transformers fitted on the secondary side of the transformer each have a secondary resistance of 2.2Ω. The relay has voltage settings of 10, 15, 20, 25 and 30V. Determine a suitable setting for the restricted earth fault protection from this list.

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