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.

How does Cold Load Pickup (CLP) function work

The user specifies a multiplier and a time. The controller detects when load current is zero and starts a timer called the Operational Cold Load Time. Using this timer, an Operational Cold Load Multiplier is calculated using the following formula:

The Operational Cold Load Multiplier is used to modify the phase, earth, and Negative Phase Sequence (NPS) Threshold Current Multipliers.
Therefore the phase, earth, and NPS protection thresholds will increase at a rate specified by the customer when the load is turned off – but only up to the User Set Cold Load Multiplier. The controller calculates the new thresholds every minute.

Example

For example, if the User Set Cold Load Time is 2 hours, the User Set Cold Load Multiplier is x2 and the current has been off for 1 hour, then the Operational Cold Load Time is 1 hour.
Consequently the phase, earth, and NPS thresholds are increased to equal the Operational Cold Load Multiplier of 1.5.
Once load current is restored the Operational Cold Load Timer starts to count down. This means that the Operational Cold Load Multiplier reverts to 1 and hence the phase, earth, and NPS threshold currents also revert to their values. Note that the rate of increase and decrease of threshold currents is the same.
In this way, lost load diversity is automatically compensated for. It doesn't matter where the current was turned off (e.g. at the substation or at the recloser) the compensation will still work.