Load Shedding; Injection On - Danfoss ADAP-KOOL AK-PC 783 Design Manual

Load shedding

On some installations there is the desire to limit the cut-in com-
pressor capacity so that one can limit the total electrical load in
the store for periods.
There are 1 or 2 digital inlets available for this purpose.
For each digital inlet a limit value is attached for the maximum
allowable cut-in compressor capacity so that one can carry out the
capacity limitation in 2 steps.
When a digital inlet is activated, the maximum allowable compres-
sor capacity is limited to the set limit. This means that if the actual
compressor capacity upon activation of the digital inlet is higher
than this limit, then so much compressor capacity is cut-out that
it will then be on or under the set maximum limit value for this
digital inlet.
When both load-shedding signals are active, the lowest limit value
for the capacity will be the one that is applicable.
Overriding of load shedding:
To avoid load shedding leading to temperature problems for the
chilled products, an overriding function is fitted.
A overriding limit is set for the suction pressure as well as a delay
time for each digital inlet.
If the suction pressure during load shedding exceeds the set
overriding limit and the attached delay times for the two digital
inlets expire then load shedding overrides the signals so that the
compressor capacity can be increased until the suction pressure
is again under the normal reference value. The load shedding can
then be activated again.
Alarm:
When a load shedding digital inlet is activated, an alarm will be
activated to inform that the normal control has been bypassed.
This alarm can however be suppressed if so desired.
AK-PC 783 Capacity controller RS8GN202 © Danfoss 11-2013

Injection ON

The electronic expansion valves in the refrigeration appliances
must be closed when all the compressors are prevented from
starting. In this way the evaporators will not be filled with liquid
which is subsequently passed on to a compressor when regulation
is restarted.
One of the compressor control relays may be used for this func-
tion, or the function can be obtained via data communication
with relay
with data communication
The function is described based on the sequence of events below:
T1) The last compressor is cut-out
T2) The suction pressure has increased to a value corresponding to
Reference + NZ + "+Zone K" but no compressor can start due to
re-start timers or safety cut-out
T3) The time delay "Injection OFF delay" elapses and the injection
valves are forced to close via relay signal or via network signal.
T4) The first compressor is now ready to start. The forced closure
signal via the network is now cancelled.
T5) The time delay "Comp. Start delay" expires and the forced
closure signal via the relay switch is cancelled simultaneously
with the first compressor being allowed to start.
The reason why the forced closure signal via the network is
cancelled before the first compressor starts, is that it will take
some time to distribute the signal to all appliance controllers via
the network.
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