Current Interruption Transients Calculation, 2nd Edition

Appendix FShunt Reactor Switching – First‐Pole‐to‐Clear Circuit Representation

A shunt reactor, quite apart from its function, is an inductive load on the system. The shunt reactor Mvar rating is generally very much less than the system mega volt amps (MVA) short circuit level at its location. For the overvoltage calculations of Chapter 6, the system was therefore treated as being “infinite” and the purpose of this Appendix is to justify that approach using symmetrical components.

The circuit for the general shunt reactor switching case – or in fact any balanced load with neutral impedance earthing – is shown in Figure F.1. Each shunt reactor phase is represented by an impedance Z and the neutral is earthed through an impedance Z_e. V_a, V_b, and V_c are phase voltages to earth and V_e is the neutral voltage to earth. Prior to first pole clearing the circuit is balanced and V_e is floating at earth potential. After current interruption in a‐phase, we can write the “fault” equations:

and V_a is the voltage we wish to calculate.

Calculating first the sequence currents:

giving

showing that the sequence networks are connected in parallel.

Figure F.1 General circuit for shunt reactor switching ...