Fault Analysis in Power Systems part 1a
This is the first part in the series called Fault Analysis in Power Systems. In this series, we will analyze the various types of faults that occur in power systems while intuitively understanding the hand calculations involved. When we refer to hand calculations, we specifically mean symmetrical components: positive, negative, and zero sequence components. These components are crucial for calculating fault currents.
Before delving into the analysis of different fault types, let’s discuss them in more general terms. The first type is a three-line to ground fault or three-line fault. These two types of faults are quite similar. In three-line-to-ground faults, phases A, B, and C are shorted together and grounded. In three-phase faults, phases A, B, and C are shorted together without ground involvement. The analysis and approach for both these fault types are the same, as they are symmetrical faults that produce a balanced fault output in power systems. The probability of a three-phase fault involving ground is 2 to 3%, and the probability of a three-phase fault without ground is less than 1%. Combined, these probabilities are very small, indicating that such faults are not common in power systems.
On the other hand, a single line to ground fault (represented as 1LG) occurs when one phase (A, B, or C) is shorted to ground. This is an unsymmetrical fault type, resulting in an unbalanced power system. The probability of this type of fault occurring is between 70 and 80%, making it quite common. Factors like trees or animals coming into contact with the power lines can cause single line to ground faults. For example, a tree branch brushing against the line or an animal touching the line and the ground simultaneously can result in a line to ground fault. These types of faults commonly occur in power systems.
The next type of fault is a line to line fault, abbreviated as 1LL. This fault occurs between phase A and phase B shorted together, phase B and phase C shorted together, or phase A and phase C shorted together. It involves two phases and is an unsymmetrical fault type, producing unbalanced power system characteristics. The probability of this type of fault occurring is only 15 to 20%.
Lastly, the double line to ground fault, abbreviated as 2LG, involves two lines and ground. It can occur between phases A and B with ground, phases B and C with ground, or phases A and C with ground. This is also an unsymmetrical fault type, but it occurs less frequently, accounting for less than 10% of faults in power systems.
The one line to ground, line to line, and two line to ground faults are all unsymmetrical faults that make the power system unbalanced during the faulted condition. If you’re unfamiliar with the terms “unbalanced” or “balanced,” there is a video tutorial available in the description section for further reference. Unbalance in power systems gives rise to positive, negative, and zero sequence components in three-phase systems. Therefore, a solid understanding of sequence components, or symmetrical components, is crucial for calculating unbalanced fault types. GeneralPAC has created an excellent symmetrical components video tutorial (1a and 2c), which are highly recommended. Links to those specific parts can be found in the description section below.
In the next video, we will discuss the general procedure used to calculate fault current and voltages for power systems.
Thank you.