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2.9.2 Cold Load Protection
The cold load function modifies the overcurrent protection settings for a period after energising the
system. This feature is used to prevent unwanted protection operation when closing on to the type of
load which takes a high level of current for a period after energisation.
In normal operation, the load current on the distribution line is smaller than the sum of the rated
loads connected to the line. But it amounts to several times the maximum load current for a moment
when all of the loads are energised at once after a long interruption, and decreases to 1.5 times
normal peak load after three or four seconds.
To protect those lines with overcurrent element, it is necessary to use settings to discriminate the
inrush current in cold load restoration and the fault current.
This function modifies the overcurrent protection settings for a period after closing on to the type of
load that takes a high level of load on energisation. This is achieved by a 'Cold Load Settings
Group', in which the user can use alternative settings of measuring elements in other setting group.
Normally the user will choose higher current settings and/or longer time delays and/or disable
elements altogether within this group. The 'Cold Load Settings' can be set in any of the four setting
groups provided for protection and the group is specified by the scheme switch [CLSG] setting.
2.9.2.1
Scheme Logic
A state transition diagram and its scheme logic are shown in Figure 2.9.3 and Figure 2.9.4 for the
cold load protection. Note that the scheme requires the use of two binary inputs, one each for CB
OPEN and CB CLOSED.
Under normal conditions, where the circuit breaker has been closed for some time, the scheme is in
STATE 0, and the normal default settings group is applied to the overcurrent protection.
If the circuit breaker opens then the scheme moves to STATE 1 and runs the Cold Load Enable
timer TCLE. If the breaker closes again while the timer is running, then STATE 0 is re-entered.
Alternatively, if TCLE expires then the load is considered cold and the scheme moves to STATE 2,
and stays there until the breaker closes, upon which it goes to STATE 3.
In STATE 2 and STATE 3, the 'Cold Load Settings Group' is applied.
In STATE 3 the Cold Load Reset timer TCLR runs. If the circuit breaker re-opens while the timer
is running then the scheme returns to STATE 2. Alternatively, if TCLR expires then it goes to
STATE 0, the load is considered warm and normal settings can again be applied.
Accelerated reset of the cold load protection is also possible. In STATE 3, the phase currents are
monitored by overcurrent element ICLDO and if all phase currents drop below the ICLDO
threshold for longer than the cold load drop off time (TCLDO) then the scheme automatically
reverts to STATE 0. The accelerated reset function can be enabled with the scheme switch
[CLDOEN] setting.
Cold load protection can be disabled by setting [CLSG] to "Off".
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- Inverse Time Overcurrent Protection
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