Fault Analysis in Power Systems Part 3d-2

In this, we will consider the sequence voltage components and phase voltage quantities for a two-line-to-ground fault on the 13.8 kV side of the delta-y transformer. This is a continuation of Part 3D-1.

Looking back at our faulted sequence network and paying close attention to where the positive, negative, and zero sequence voltage quantities are calculated, we can see that all three sequence networks are connected to the same node. Therefore, by applying Thevenin’s theorem and basic circuit analysis, we know that all sequence voltages will be exactly the same. Let’s calculate the positive sequence voltage and proceed from there.

The positive sequence voltage is calculated by subtracting the voltage drop across the sum of the impedances in the positive sequence network from the source voltage. The sum of the impedances is J0.15, and the current flowing through the positive sequence network is -J4.7619 per unit. This calculation gives us 0.285 per unit at an angle of 0 degrees.

Now, we know that all sequence component voltages will be equal to 0.285 per unit. Multiplying this value by the base value of 13.8 kV divided by the square root of 3, we obtain the voltage quantity of 2.27 kV at an angle of 0 degrees.

As a final step, the sequence voltage quantities need to be transformed into phase voltage quantities using the legacy equation. Since the positive, negative, and zero sequence voltages are all equal, we know that the phase A voltage will equal three times the positive sequence voltage. Plugging in the values, we get 6.829 kV or 6.8 kV at an angle of 0 degrees for the phase A voltage. The phase B and phase C voltage quantities reduce to zero, which makes sense based on the earlier diagram we drew for a two-line-to-ground fault on the 13.8 kV system. Phase B and phase C were the faulted phases, and the voltage at the point of the fault is driven to zero.

There you have it! We have now calculated both the fault current values and the voltage values for a two-line-to-ground fault. In previous videos, we calculated line-to-ground faults, line-to-line faults, three-phase faults, and more. In the next video, Part 4 of our series, we will explore how to reflect fault calculations over the high voltage side when the fault occurred on the low voltage side of the transformer. This can be quite unique and sometimes confusing, as there are special considerations to analyze fault values on the high voltage side from a sequence components point of view. We will cover this in Part 4.

Thank you.