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A 20 MVA, 6

Electrical Engineering

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A 20 MVA, 6.6KV, 3phase alternator is connected to a 3-phase transmission line. The whole transmission line has a two-machine power system. Two ground faults take place at two different locations. The neutral of the alternator is connected to ground, the alternator is connected with unloaded synchronous generator operating at it's rate, where the fault occurs at the terminals, the sequence component of the fault current are as follows- ?Ipositive?=j1.5pu? , ?Inegative?? = -j0.5 pu.

Determine all the types of the faults occured in the circuit. Calculate value of the fault occured. Calculate the resistance and the shunt susceptances of the system. Calculate the voltage and the current at the point of the fault. Determine the types of the relays engage in the whole system. Design the whole system of circuit and also suitable spots for circuit breaker.

An 11/ 0.4 Kv, 25 Hz, three-phase transformer has ohmic, hysteresis and eddy current losses of 1.8, 0.8 and 0.3% respectively. Calculate the efficiency at ¼, ½, 1 and 1 ¼ of full load at 0.8 p.f. Compute the voltage regulation and the secondary terminal voltage under rated load at power factors of (i) 0.8 lagging and (ii) 0.8 leading. Determine the p.u. value of the equivalent resistance of the transformer.

A time grading is provided in following relay system, an alternator rated at 60kV, protected by balanced circulating current system, parallerly a delta -star connected alternator of 6.6kV, 5MVA, specification is connected. Merz-Price protection scheme is used which operates when relay current exceed 50% of armature current. The neutral of the generator is grounded through a resistance of 8 ohms.

(I) Determine the proportion of the winding which is remain unprotected against faulty conditions.

(II) Determine the percent winding which remain unprotected.

(III) Assume a CT ratio will be 40/120; determine whether the relay will operate or not?

(IV) What should be the specification of the differential protection scheme according to the given system?

(V) Draw the threshold characteristic of the system?

(VI) Determine the average rate of rise of restriking voltage?

(VI) Discuss how the rotar will be protected against the earth fault?