Relay Protection - How does Cold Load Pickup (CLP) function work

Why do we need Cold Load Pickup protection function

When a typical load has been without supply for a period of time (hours) it loses its diversity. When power is restored the load is higher than usual because all the heater, refrigerator or air conditioner thermostats have turned on. The longer the period without supply the greater the loss of diversity and the higher the load current when supply is restored.

Cold Load Pickup feature

The purpose of the Cold Load Pickup feature is to allow for this loss of diversity automatically and hold the load without tripping. It works by timing the loss of supply to the load and then raising the threshold current accordingly.

Introduction about auto-reclosing

Faults on overhead lines fall into one of three categories:

• transient
• semi-permanent
• permanent

80-90% of faults on any overhead line network are transient in nature. The remaining 10%-20% of faults are either semi-permanent or permanent.

Transient faults are commonly caused by lightning and temporary contact with foreign objects. The immediate tripping of one or more circuit breakers clears the fault. Subsequent re-energisation of the line is usually successful. A small tree branch falling on the line could cause a semi-permanent fault. The cause of the fault would not be removed by the immediate tripping of the circuit, but could be burnt away during a time-delayed trip. HV overhead lines in forest areas are prone to this type of fault. Permanent faults, such as broken conductors, and faults on underground cable sections, must be located and repaired before the supply can be restored.