Fault Analysis in Power Systems Part 4a

Fault Analysis and Power Systems Part 4A: Reflecting Fault Currents and Voltage Values on the High Voltage Side of the Transformer. This is the first video in Part 4 of our series on fault analysis and power systems. In this video, we will explore how faults are analyzed from the high voltage side, as opposed to the low voltage side of the transformer, which we have covered in previous videos. This step, number seven in our fault analysis methodology, serves as an additional bonus concept.

Up until now, we have focused on calculations and methodologies for faults on the low voltage side of the 13.8 kV transformer in our example. But let’s consider a scenario where the fault still occurs on the 13.8 kV side of the bus, and we need to determine the same fault current and voltage values from the high voltage side, or the 115 kV side of the bus. This analysis is useful in power system protection and analysis, and therefore, we will reflect the fault current and voltage quantities from the low voltage side to the high voltage side.

To do this, we need to account for two changes in sequence current values. The first change involves the magnitude due to the change in voltage level, and the second change involves the angle. Since we have a delta-Y transformer connection, the angle will be subject to change. It’s important to note that per unit values of sequence voltage and current quantities will remain the same, thanks to the benefits of the per unit system.

To account for the change in magnitude when analyzing the fault on the high voltage side, we simply change the base value of the voltage from 13.8 kV to 115 kV. This allows us to convert the per unit sequence current values calculated earlier to sequence ampere values reflected on the 115 kV side. The table provided summarizes the changes in magnitude concepts on the high voltage side.

Since the transformer is a delta-Y transformer, there will be no zero sequence current on the high voltage side, as the delta winding traps the zero sequence current within itself. This was discussed in detail in Part 2C of this series.

Now, let’s shift our focus to the angle adjustment, considering the specific delta-Y transformer connection where the low voltage line current lags the high voltage line current by 30 degrees. We need to consider this transformation when reflecting the low voltage positive sequence and negative sequence currents on the high voltage side. The figure provided clearly shows the 30-degree lag for the positive sequence line current and the 30-degree lead for the negative sequence line current.

To calculate the high voltage positive sequence line current value, we multiply the low voltage per unit line current value by one angle 30 degrees. Similarly, for the high voltage negative sequence line current value, we multiply the low voltage negative sequence current per unit value by one angle negative 30 degrees.

With these adjustments for sequence component currents based on the delta-Y transformer connection, we can now move forward with reflecting the fault current on the high voltage side. From there, we can calculate the voltage quantities, which are a bit more complicated. In the next video, we will demonstrate how to apply these concepts in calculation examples for specific types of faults on the low voltage side.

Thank you.