Fault analysis is very important part of power system study. Without fault analysis, we cannot determine the exact specifications of different protective devices in the system. Accordingly, fault analysis can be more tedious when dealing with unbalanced system. The calculation of fault currents in an unbalanced system starts from the development of sequence network.
For this reason, the method of symmetrical components are widely used to carry out the calculations. Although there are already computer software that can perform in behalf of the manual calculation, still, it is necessary to understand the fundamental concept of fault analysis.
From the previous article of symmetrical components, it is explained how an unbalanced vectors are converted into three sets of equal vectors namely: the positive sequence, negative sequence and zero sequence components.
Accordingly, these values have also equivalent sequence networks such as the sequence network of transformers, and others.
There are three types of unbalanced fault in three phase system:
- Single line to ground fault - this fault happens when one phase of the system is accidentally connected to the ground.
- Line-to- line fault - this fault happens when two live conductors accidentally connected with each other.
- Double- line- to ground fault - this happens when two phases are accidentally connected to the ground.
Once the sequence networks for the system are defined, the way they are connected is dependent on the type of fault. Sequence network connections for common shunt fault types are shown.
1. Single line to ground fault: In this type of fault, the positive, negative and zero sequence components are connected in series. It follows that, all sequence currents have similar value i.e. I1 = I2 = I0.
2. Line-to- line fault or Phase to Phase Fault: In this type of fault, the zero sequence network is non-existent. The only networks involved are the positive and negative sequence respectively and the two networks are connected in parallel. As we can see in the figure below, the negative sequence current is opposite t the positive sequence current. I1 = - I2.
3. Double Line- to- round fault: In this type of fault, all sequence networks are connected in parallel. By simple Kircchoff's Current Law in the top node we can find that I1 + I2 + I0 = 0.
The rest of the analysis can be done using circuit analysis with complex quantities.