Question

Generally, in power-system short-circuit studies, for calculating sub-transient fault currents, transformers are represented by their _____, transmission lines by their equivalent _____, and synchronous machines by _____ behind their sub-transient reactances.

Short answer

Generally, in power-system short-circuit studies, for calculating sub-transient fault currents, transformers are represented by their leakage reactance, transmission lines by their equivalent series reactance, and synchronous machines by constant voltage source behind their sub-transient reactances.

Step-by-step solution

Determine the assumptions to calculate sub transient fault current.

In order to calculate the sub transient fault current in a three-phase short circuit power system, the following assumptions are made:

1. Transformers are represented by leakage reactances. The shunt admittance, phase shifts and winding resistance are neglected.

2. The transmission lines are represented by series reactance by neglecting series resistance and shunt admittance.

3. Neglect all non-rotating loads.

4. Synchronous machines are represented by constant voltage source behind the sub transient reactance by neglecting armature resistance, saliency and saturation.

Determine the answer.

From assumptions 1, 2 and 4, the solution is,

In power-system short-circuit studies, for calculating sub-transient fault currents, transformers are represented by their leakage reactance, transmission lines by their equivalent series reactance, and synchronous machines by constant voltage source behind their sub-transient reactances.