Fault Calculations

INTRODUCTION

Whenever there is insulation failure of equipment in a power system or flashover of the lines initiated by a lightning stroke or accidental faulty operation of the system, a heavy symmetrical short circuit current flows in the system. The system must be protected against flow of that heavy short circuit currents by disconnecting the faulty part of the system by means of circuit breakers operated by protective relays. The main objective of fault analysis is to determine the fault level or the fault MVA at the fault location. This fault MVA is the breaking capacity of the circuit breaker that has to be kept at the point of fault location.

There are two types of fault occurring in the power system, namely “symmetrical fault” and “unsymmetrical fault”. The majority of fault occurring in power system are faults involving one line to ground occasionally two lines to ground which are termed as “unsymmetrical fault”. The faults involving all the three phase to ground or all the three phase short-circuited belongs to “symmetrical fault”. The symmetrical faults generally leads to most severe fault current which need to be interrupted. Though the operating conditions at the time of fault are important, the loads can be neglected during fault, as voltage dip very low so that currents drawn by loads can be neglected in comparison to fault current. The most common and dangerous fault, that occur in a power system, is the short circuit or shunt faults which involves power conductor or conductors-to-ground or short-circuit between conductors and causes a heavy current, called the “short-circuit current”. Short circuit calculation is very important since they provide data, which is necessarily required for designing the protective scheme for the power system.”

Type of Faults in a Power System

There are basically two types of faults occurring in a power system namely:

1. Symmetrical Faults : Those faults on the power system which gives rise to symmetrical fault currents (i.e., equal fault currents in the lines with 120° displacement) is called a symmetrical fault. In other word system remains in balance during pre and post fault condition. e.g.,

• Three phase to earth fault (LLLG fault)
• Three phase fault (LLL fault)

2. Unsymmetrical faults : Those faults on the power system which give rise to unsymmetrical fault currents (i.e., unequal fault currents in the lines with unequal phase displacement) are known as unsymmetrical faults. e.g.,

• Single line-to-ground faults (L–G)
• Line-to-line fault (L–L)
• Double line-to-ground fault (L–L–G)

Note:

• Line to Ground (LG) fault is the most frequently fault followed by LLG and LLL fault.
• The calculation of symmetrical short-circuit current or symmetrical short-circuit KVA at a certain point in power system is, therefore, very important for the purpose of determination of circuit breaker ratings.
• The different kinds of faults in order of decreasing severity are :
  Three phase fault (3-φ fault) > Double line to ground (LLG) fault > Line to line (LL) fault > Single line to ground (LG) fault.

Assumptions made in Fault Analysis

• Transformer are represented by series reactance only, the winding resistance, magnetizing branch and connection induced phase shift present in Star-Delta transformer will be ignored.
• Synchronous machine are represented as constant induced emf in series with appropriate reactance. Winding resistance and salience of the machine is ignored.
• All static loads will be ignored.
• Pre-fault loading conditions present in the system is generally ignored.
• Shunt capacitance and series resistance in transmission are neglected.

NOTE: Fault is a high current, low voltage, high frequency, low power factor, highly lagging phenomena.