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how to calculate restraing current and operating current
for differential protection if 20 mva transformer is OLTC
33 kv/11 kv ? and what should be the type and ratio of CT?
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Answer Posted / raghavendran.r
It requires the HT and LT side current ratings to be calculated.The current on the HT side comes around 606A(line current) and on the LT side it is 1818 A(line current), Considering that HT and LT are Delta and Star type wounded respectively.
But for Merz-Prize protection, the CT on Delta side should be Star wound and for the CT on the Star side of Power Trans,it should be delta Wound.
So calculating the primary currents of the CT1 and CT2 we get 350A and 1818A respectively.
Now we have a standard CT ratio of 400/5A and 2000/5A in the market.Which can be used here.
Now the formula for Operating current is mod[Is1 - Is2] = K[Is1+Is2]/2 = K * Iavg.Where K = .1 a constant.
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