Load Shedding - Danfoss ADAP-KOOL AK-HP 780 Design Manual

Example:
- Two speed-regulated compressors with a nominal capacity of
20 kW and frequency range 25-60 Hz
- Two one-step compressors, each of 20 kW
Cyclical operation
For cyclical operations, both speed-regulated compressors will
have the same size and operating hours will be equalized be-
tween the compressors in accordance with the First-in-First-Out
Principle (FIFO). The compressor with the least operating hours
will be the first to start. The following speed-regulated compressor
will be cut in when the first compressor runs at max. speed and
the desired capacity has reached a value that allows the cut-in of
the next speed-regulated compressor at start speed. Afterwards,
both compressors will be cut in together and they will run in
parallel. The following one-step compressors will be cut in and out
in accordance with First-In-First-Out principle in order to equalise
operating hours.
Example:
- Two speed-regulated compressors with a nominal capacity of
20 kW and frequency range 25-60 Hz
- Two one-step compressors, each of 20kW
Best fit
During best-fit operations, the speed-regulated compressors can
have different sizes and they will be handled in such a way that
the best possible capacity adjustment is achieved. The smallest
compressor will be started first, then the first will be cut-out and
the second compressor will cut in. Finally, both compressors will
be cut in together and will run in parallel.
The following one-step compressors will, in every case, be
handled in accordance with the best-fit coupling pattern.
Example:
- Two speed-regulated compressors with a nominal capacity of
10 kW and 20 kW respectively
- Frequency range of 25-60 Hz
- Two one-step compressors of 20 and 40 kW respectively
AK-HP 780 Heat pump controller RS8GF202 © Danfoss 2015-08

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.
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