Question

1-Power Systems: Line Diagrams

(a) Sketch a single line diagram for the following power system. There are three 3-phase alternators supplying an 11 kV busbar and one 3-phase alternator supplying a 33 kV busbar. There are two parallel transmission lines fed from the 11 kV line and supplying a 33kV busbar. Each has a step-up and step-down transformer. One secondary feeders from the 33 kV bus bar feeds a further kV busbar through a stepdown transformer. This is further stepped down through a transformer to supply a number of distribution points on a 415 V busbar. One of the distributors has a fault point. Indicate this on your diagram.

(b) Give a short paragraph describing what is a fault current, what the worst case scenario for fault currents might be and what protection might be put in place. We are not looking for extended explanations but enough attempts in your own words to demonstrate understanding.

Answer 1

a.) Single line diagram representation of a power system indicates the original 3 phase power system woking under balance consitions.

The single line diagram of the given question is shown in FIgure (1).

b.) Short circuit- It is a low voltage, high current, low power factor and highly lagging phenomenon and the current which flows in the circuit during short circuit is called fault current.

Basically, fault current flows when there is some unintentional fault occurs in the circuit.

Three phase short circuit produces most severe fault current so this is the worst case scenario for fault currents whichg we study under fault analysis. It is a kind of symmetrical fault.

We can use following instruments for protection of system from fault currents,

1. Auto reclosing circuit breaker.

2. Relays (Mainly over current relays).

3. Instruments transformer.