Fault in Indiana 34.5 kV. Pre-Fault current IA = IB = IC = 260A. During fault, I0 =4300A, I1 = 3900A, I2= 3800A. What is the fault type? Answer is ...

Fault in Indiana 34.5 kV. pre fault current IA = IB = IC = 260A. During fault, I0 =4300A, I1 = 3900A, I2= 3800A. What is the fault type? Answer is ...

Answer: Phase to Ground        

The answer to this poll is Phase to Ground. I attached the relay event screenshot to the actual event from Indiana Grid in this article. Let’s go into an explanation.

As I mentioned in the poll, it is 12: 16 am; see callout 1 in Figure 1. Everyone is sleeping, and a fault occurs; the phone rings and your facility loses some MW.

You need to react quickly and confirm the fault type. Obviously, you will have the phase values, but you also need to double-check your answer using Symmetrical Components.

As I also mentioned in the poll, during the pre-fault region, IA= IB = IC = 260A. You can see it in Callout 2. Obviously, the values are not precisely 260A, but they are close enough, so I round it to 260A.

You can also observe the phase angles, all 120 degrees apart; the system is healthy.

The top graph shows the voltage values. 34.5 kV and balanced.

Figure 1: Indiana grid relay pre-fault region during a fault. phase values

Figure 2 below shows the fault regions. Notice the top graph, only one voltage value dropped, the green curve Vb.

Callout 3 shows the symmetrical components; As I mentioned in the polls, I0 = 4300A, I1 = 3900A, I2= 3800A.

On the right side of picture 2, the relay also shows BG T. For those unfamiliar with SEL Relay, BG T means B to Ground Trip.?

Figure 1: Indiana grid relay fault region during a fault. Symmetrical components values

This poll aims to help you practice symmetrical components in real life. You probably will see the phase currents and the relay output BG T. Sometimes, the relay does not output the correct fault, and you must confirm it with symmetrical components.

So, what does the symmetrical component theory say about phase-to-ground fault? During a phase to ground, I0 = I1 = I2. This is the only type of fault that satisfies that equation.

Moreover, you must be practical and understand that those values will not be precisely equal. The differences will be higher than those present during the pre-fault region because the currents are much higher; CT readings and normal inaccuracies during high currents will produce discrepancies of up to hundreds of amps.

So, in practice, 4300A, 3900, and 3800A during fault means identical values, hence I0 = I1 =I2.

Theory is confirmed: I0 = I1 = I2

Phase voltage confirmed it: Vb is the only voltage dropping.

Relay event type confirmed it: BG T

You are now confident in your analysis. Phase to Ground Fault.

All the other options are incorrect because none of them satisfy the symmetrical theory equation I0 = I1 = I2.

The next poll will be about the catastrophe that occurred in February 2021. You must determine the correct relay communication between 2 distance relays installed in a critical transmission line that fed Houston. Are you excited? More to come.

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