The generator supplies a motor through a Y-Delta connected transformer. The gene

Question

The generator supplies a motor through a Y-Delta connected transformer. The generator is connected to the Y-side of the transformer. A fault occurs between the motor terminals and the transformer. The symmetrical components of the sub transient current (in per units) in the motor flowing towards the fault are I_a^(1) = --j2.6: I_a^(2) = -j2.0 and I_a^(0) = j3.0. The symmetrical components of the sub transient current (in per units) in the transformer flowing towards the fault are I_a^(1) = -0.8 -j4.0: I_a^(2) = -j1.0 and I_a^(0) = 0. Assume X_d" = X_1 = X_2 for both the motor and the generator and answer the following (a) Describe the type of fault. (b) Find the prefault current, if any, in line (c) Find the sub transient fault current in per unit. (d) Find the transient current in each phase of the generator in per unit. Each of the machines connected to the two high voltage buses shown in the single-line diagram below is rated 100MVA, 345kV, 345Y/20 Delta kV, with leakage reactance of 8%. On a base of 100MVA, 345kV the reactance's of the transmission line are X_1 = X_2 = 15% and X_0 = 50%. If no prefault current is flowing in the network, find the sub transient current to ground for a double line to ground fault on lines B and C at bus 1. Repeat for a fault on bus 2. When the fault is at bus 2, determine the current in phase b of machine 2. Take T_1 = YNd1 and T_2 = Yd1.

Explanation / Answer

Open Circuit Faults:-
These faults occur due to the failure of one or more conductors.
The most common causes of these faults include joint failures of cables and overhead lines, and failure of one or more phase of circuit breaker and also due to melting of a fuse or conductor in one or more phases.
These are unsymmetrical or unbalanced type of faults except three phase open fault.

Short Circuit Faults:-
A short circuit can be defined as an abnormal connection of very low impedance between two points of different potential, whether made intentionally or accidentally.
These are the most faults, resulting in the flow of abnormal high currents through the equipment or transmission lines.
Short circuit faults are also called as shunt faults. These faults are caused due to the insulation failure between phase conductors or between earth and phase conductors or both.

Symmetrical Faults:-
A symmetrical fault gives rise to symmetrical fault currents that are displaced with 1200 each other. Symmetrical fault is also called as balanced fault. This fault occurs when all the three phases are simultaneously short circuited.

Unsymmetrical Faults
This kind of fault gives rise to unsymmetrical fault currents (having different magnitudes with unequal phase displacement). These faults are also called as unbalanced faults as it causes unbalanced currents in the system.

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