Question

Consider Wye-connected, solidly grounded generator is being driven by a prime mover at synchronous speed. The generator has the following specifications:

Voltage: 13.8kV,10MVA
Impedances: X 1 =0.12pu;X 2 =0.15pu;X 0=0.05pu

What are the fault currents in terms of symmetrical components (per-unit), phase values (per-unit) and phase values (actual Amperes), for the following faults:
A. Bolted three phase faults
B. Line-to-Line fault
C. Line-to-Line-to-ground fault
D. Single Line-to-ground fault
E. Comment about the relative magnitudes of the possible generator fault current and the implications for calculation of available fault current for the purpose of sizing circuit breaker AIC ratings.|

Answer 1

In fault analysis of a generator, symmetrical components can be used to calculate fault currents. The fault currents in terms of symmetrical components for different types of faults in a Wye-connected generator are explained. The implications for calculation of available fault current for sizing circuit breaker AIC ratings are also discussed.

Step-by-step explanation:

In a fault analysis of a generator, it is common to use the method of symmetrical components to calculate the fault currents. In this method, the three-phase system is converted to a set of three symmetrical components: positive, negative, and zero sequence.

For the given faults:

1. A. Bolted three phase faults: The fault current in terms of symmetrical components is 1 pu for positive and negative sequence components, and 0 pu for the zero sequence component.
2. B. Line-to-Line fault: The fault current in terms of symmetrical components is 1 pu for positive and negative sequence components, and 0.87 pu for the zero sequence component.
3. C. Line-to-Line-to-ground fault: The fault current in terms of symmetrical components is 1 pu for positive sequence component, 0.5 pu for negative sequence component, and 0.87 pu for the zero sequence component.
4. D. Single Line-to-ground fault: The fault current in terms of symmetrical components is 1 pu for positive sequence component, 0 pu for negative sequence component, and 0.87 pu for the zero sequence component.

E. The fault currents in the generator can be significantly higher than the fault currents at the point of fault in the system. This is because generators typically have lower impedance compared to the system impedance. When calculating the available fault current for the purpose of sizing circuit breaker AIC ratings, it is important to consider the maximum fault current the generator can deliver at the point of fault by taking into account the generator impedance.