SYMMETRICAL FAULT CALCULATIONS – PART – 01 – IMPORTANCE OF FAULT CALCULATIONS

AIM OF SHORT CIRCUIT STUDY

The aim of short circuit study is to determine fault current, bus voltages and line currents under the fault condition. It is also used to determine ratings of CB and Power transformers and other power systems equipment (CT and PT).

CLASSIFICATION OF FAULTS

1. Shunt fault (short circuits) – Shunt faults involve power conductor or conductors-to-ground or short circuit between conductors. Shunt faults are characterized by the increase in current and fall in voltage and frequency.

2. Series faults (open circuits) – When circuits are controlled by fuses or any device which does not open all three phases, one or two phases of the circuit may be opened while the other phases or phase is closed. Series faults are characterized by the increase in voltage and frequency and fall in current in the faulted phases.

CAUSES OF FAULTS

Lightning, heavy winds, trees falling across lines, vehicles colliding with towers or poles, line breaks, failure of insulation etc. are the causes of faults.

ILL EFFECTS OF SHORT-CIRCUIT FAULTS

Most of the faults on the power systems lead to a short-circuit condition. When such a conditions occur, a heavy current flows through the equipment, causing considerable damage to the equipment and interruptions of service to the consumers.

IMPORTANCE OF STUDYING FAULT CALCULATIONS

Fault calculations involve finding the voltage and current distribution throughout the system during the fault condition so that the protective devices may be set to detect the fault and isolate the faulty portion of the system so as to minimize the harmful effects of such contingencies.

SYMMETRICAL FAULT

The fault on the power system which gives rise to symmetrical currents (i.e. equal fault current in the lines with 1200 displacement) is called a symmetrical fault.

UNSYMMETRICAL FAULTS

Those faults on the power system which give rise to symmetrical currents (i.e. unequal fault currents in the lines with unequal displacement) are known as unsymmetrical faults. 

Generally, the fault currents are much larger than load current and, therefore, the load currents can be neglected during fault calculation.

ASSUMPTIONS ARE MADE IN FAULT CALCULATIONS

1. The emf of all the generators is assumed to be 1 per unit

2. Shunt elements in the transformer model that account for magnetizing current and core loss is neglected.

3. Shunt capacitances of the transmission lines are neglected.

4. System resistance is neglected only inductive reactance of the system is taken into account.

THEOREM USED IN FAULT CALCULATIONS

The fault calculation of the fault current can be very easily calculated by applying Thevenin’s theorem. It is only necessary to find the open circuit emf and network impedance as seen from the fault point. In most of the calculations, the open circuit emf can be assumed to be 1pu.

STEPS INVOLVING IN FAULT CALCULATIONS

1. Draw a single line diagram of the system

2. Select a common base and find out the per unit reactance of all generators, transformers, lines etc., as referred to the common base.

3. Indicate all the reactance values in the reactance diagram.

4. Find the reactance of the system as seen from the fault point.

5. Find the fault current and fault MVA in per unit values.

6. Convert the per unit values into actual values.

7. Retrace the steps in the calculation to find the current and voltage distribution throughout the network.