Faults in Power System

In this section we will discuss regarding faults in power system networks. This is the more important fundamental in power system protection since this will be the basis on the study of protection. As part of the study of faults, we need to understand fault calculation. This is the analysis of the behavior of the electrical network under fault conditions. The current and voltages at different part of the network for different types of faults, location of the faults and the various configurations of the power system network.

There are different types of faults that can occur in a network, it can be categorize into three major groups:

1. Short circuit faults
2. Open circuit faults
3. Simultaneous faults

 We shall define each of them to clearly understand what these faults are and how it affects the system.

1. Short circuit faults - a short circuit is an accidental connection between conductors by zero or non-zero impedance. It is referred to as internal if it is located within equipment or external if it occurs on links. The causes of a short circuit can be mechanical, electrical or human. 

           a. Three phase faults 

           b. Two phase faults 

           c.Single phase to earth faults

Effects of short circuit currents:

·         Under short circuit conditions, equipment and connections are subjected to high mechanical stress that can cause breaks and thermal stress that can melt conductors and destroy insulation.

·         Equipments and connections require prompt disconnection or isolation from the fault so as to avoid human casualty and equipment damage.

2. Open circuit faults – this type of fault is not normally considered in fault studies but yet this is still very important. Open circuit faults will not cause overcurrent or overvoltages and therefore normally not dangerous to the network. This type of fault cause heating in rotating machines due to the negative sequence current that will flow in the system. Therefore, the rotating machines are equipped with negative sequence current protection. 

               a. Single phase open circuit 

               b.Two phase open circuit. 

               c.Three phase open circuit.

3. Simultaneous faults are combinations are the combination of short circuit faults and open circuit faults. Say, if one conductor of an overhead line is broken and one end of the line falls down. In this scenario there is both one single phase to earth fault and one single phase open circuited fault in the system.

Generally, the system is protected against phase and earth faults by protection relays. The severity or magnitude of the fault current is dependent on what type of fault occurs in the network. 3 Phase faults normally generate the highest short circuit current that is why short circuit for three phase faults is normally used.

At earth fault, the magnitude of the fault current is depending on the earthing resistance or reactance in the fault. The fault resistance for a phase fault is much smaller than that for an earth fault. For two phase fault, it normally generates low fault currents than three phase faults. However, two phase fault calculation can be necessary to check the minimum fault current level to verify the sensitivity for the back-up protection.

Now we already have the basic information regarding faults, there is still a lot of reading and studying to do to fully understand Faults and Short circuit calculations. You can find few free books online regarding fault study. We will try to discuss in detail if we have still time. Just put your comments and we’ll try to go into details.