Load Shedding - Danfoss AK-PC 781A User Manual

Copeland Stream compressor
The PWM signal can also be used to control one stream compres-
sor with one unloader valve (Stream 4) or one with two unloaders
(Stream 6).
Stream 4: The compressor capacity is distributed by up to 50% for
one relay and the remaining 50-100% for the unloader.
Stream 6: The compressor capacity is distributed by up to 33% for
one relay and the remaining 33-100% for the unloader.
Bitzer CRII Ecoline
CRII 4: The pulse signal can also be used to control one CRII with
two unloaders (4-cylinder version).
The compressor capacity can be controlled from 10 to
100%, depending on the pulsation of the unloaders. The
compressor start signal is connected to a relay output, and
the unloaders are connected to solid state output fx DO1
and DO2.
Unloader 2 follows unloader 1,
but it remains displaced by a
half period.
CRII 6: The pulse signal can also be used to control one CRII with
three unloaders (6-cylinder version).
The compressor signal is connected to one relay output.
The two unloaders are connected to solid state output fx
DO1 and DO2. The third is connected to a relay output.
The compressor capacity can be controlled from 10 to 67%,
depending on the pulse of the unloaders.
The relay is then connected to the third unloader. When
this relay is off, the capacity will be controlled between 33
and 100%.
Individual Sd monitoring
When regulating with Sd monitoring, one of the three compressor
types will increase capacity if the temperature nears the Sd limit.
This will result in better cooling of the unloaded compressor.
102
Unloader 1
Unloader 2
Capacity controller RS8HE202 © Danfoss 2017-07

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.
This limitation can be activated in the following way:
• Via signal from the network
• Via signal on one DI input + signal via the network
• Via signal on two DI inputs + signal via the network
The signal via the network will result in the same function as if the
signal were received on DI 1.
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.
The threshold value may not be set lower than the compressor's
lowest capacity step/"Start speed".
When both load-shedding signals are active, the lowest limit value
for the capacity will be the one that is applicable.
Max. time
A max. period with low compressor capacity can be set. When the
period expires, the system switches to normal regulation until the
suction pressure is once again in place. Load shedding will then
be permitted.
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 P0
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 781A