QUESTION 3 a) A 30 MVA 132/33 kV star delta power...

QUESTION 3 a) A 30 MVA 132/33 kV star delta power transformer is to be protected by a bias differential protection system. The transformer is supplied from 132 kV grid as shown in Figure Q3 (a). The current ratios of the current transformers on the 132kV side are 250/5A while those on the 33kV are 600/5A. If the impedance of the transformer is 9% and a three phase fault occurs on the 33 kV side of the transformers outside its zone of protection, and if the bias is set at 20%, check the relay operation condition. (13 marks) b) Please suggest MVA fault required if the operating point of the relay falls in the trip region. (12 marks) [25 marks]

Transcript

00:01 To check the relay operation condition for the bias differential protection system and the determine the required mvf fault, we are following here three steps.

00:11 So the first, we are checking the relay operation condition, relay operation condition.

00:21 So calculating the restarting current and bias differential current.

00:25 The restating current is the sum of the current transformer, secondary's current and the 132 kv side when the fault is outside the predicted zone.

00:37 So we are first seeing that i, restating, so it is 250 divided by 5, it is in ampere, plus 250 divided by 5 because we have 3 and plus this is 250 divided by 5.

01:00 So we will get here, we calculated this is 150 ampere.

01:05 So since there is three phase fault.

01:09 Now we are seeing the bias current.

01:11 So the bias differential current is the restraining current multiplied by the bias percentage.

01:17 So this is i bias is b multiplied by the i restraining current.

01:26 So this is the value of b that is bias percentage given to us 0 .20 and we will multiply it by the i restraining.

01:35 So this is when we calculate this, this will be 30 ampere.

01:39 Now we will calculate the differential current.

01:42 So i differential, so the differential current is the sum of the ct secondary's current and the 33 kv side when the fault occurs.

01:55 So this will be 600 divided by 5, this is an ampere plus 600 divided by 5 plus 600 divided by 5 because here is three phase fault and when we will calculate this, we will get 360 ampere, the differential current.

02:21 Now we will check the relay operation condition.

02:24 So for the relay to operate and re -transferred, the following condition must be made.

02:30 So it is i differential minus i biased.

02:37 So magnitude of it should be greater than the i bias, difference in magnitude of this will be i bias, should be greater than i bias.

02:50 So let's see it.

02:50 So differential current 360 minus bias current it is 30 and if we see it should be greater than 30 ampere.

03:00 So when we will calculate this, this is 330 ampere and it is greater than 30 ampere...

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