Cold Load Pickup - GE 850 Instruction Manual

COLD LOAD PICKUP

Cold Load Pickup
8–44
The 850 relay provides one Cold Load Pickup (CLP) element.
The 850 can be programmed to block instantaneous overcurrent elements, to raise the
pickup level of time overcurrent elements, or change the setting group when a Cold Load
Pickup condition is detected.
Under normal operating conditions, the actual load on a feeder is less than the maximum
connected load, since not all consumers require maximum load at the same time. A Cold
Load condition can be caused by a prolonged outage of the load, by opening of the circuit
breaker, or by a loss of supply even if the breaker remains closed. Upon the return of the
source, the circuit experiences inrush current into connected transformers, accelerating
currents into motors, and simultaneous demand from many other loads because the
normal load diversity has been lost. During the Cold Load condition, the current level can
be above the Pickup setting of some protection elements, so the feature can be used to
prevent the tripping that would otherwise be caused by the normal settings.
Without historical data on a particular feeder, some utilities assume an initial Cold Load
current of about 500% of normal load, decaying to 300% after 1 second, 200% after 2
seconds, and 150% after 3 seconds. See the following figure for details.
Figure 8-22: Cold Load Pickup
A Cold Load condition can be initiated in two ways:
1. Automatically responding to a loss of source of the feeder, by detecting that all phase
currents drop below 5% of the nominal current for an amount of time greater than the
Outage Time Before Cold Load setpoint. This timer is set to an interval after which it is
expected that the normal load diversity has been lost. After this delay interval, the
output operand is set, the time overcurrent pickups are raised, the instantaneous
overcurrent elements are blocked, and the setting group is changed according to the
settings.
2. The cold load condition can also be immediately initiated by asserting the logic input
External Initiation if all phase currents are below 5% of the nominal current. If a Cold
Load condition is initiated, the output operand is set, the time overcurrent pickups are
raised, the instantaneous overcurrent elements are blocked, and the setting group is
changed according to the settings. This method is intended to respond to an event
that sets an input (for example, an operator initiated virtual input).
Once Cold Load Pickup is in operation, the output operand remains set until at least one
phase of the load has returned to a level above 5% of nominal current for the interval
programmed by the Cold Load Time Before Reset setting has expired. The Reset delay
850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 8: CONTROL