Current Interruption Transients Calculation, 2nd Edition

6Inductive Load Switching

6.1 General

The inductive load switching cases to be considered are the switching of shunt reactors and unloaded transformers. Shunt reactor loads are 90° lagging in a number of load circuit configurations and represent an onerous switching duty for circuit breakers. Unloaded transformer switching also presents a lagging power factor, but less than 90°, and is generally not a significant test duty for circuit breakers. However, particularly in North America, air‐break disconnectors (also known as disconnect switches) are used to switch out unloaded transformers and this represents a special application case.

For the vast majority of shunt reactor installations, the circuit breaker is applied on the source side of the shunt reactor and variations with respect to the reactor neutral earthing are as shown in Figure 6.1. The variation in the shunt reactor neutral earthing is related to system voltages but also to individual utility requirements:

• Shunt reactors with isolated neutrals are most common at system voltages below 72.5 kV but are also in use up to 170 kV in some countries.
• Shunt reactors with earthed neutrals are in use at all system voltage levels.
• Shunt reactors with neutrals earthed through a neutral reactor are mostly in use at voltages of 300 kV and above in order to enable single‐phase fault‐clearing and reclosing.

Each of the above variations represents a unique case from a transient analysis perspective. The variations can, however, ...