Loadflow Study - Where do instantaneously required VARs come from ?
Loadflow Study - Where do instantaneously required VARs come from ?
(OP)
I am using SKM's DAPPER loadflow software to calculate loadflow voltages when a 1.5MW motor is DOL started.
The plant has local steam turbine generators ( I have modelled as PQ ) and an interconnection to a Utility ( modelled as Swing Bus ).
The plant does not have pf correction capacitors or static VAR compensator.
Loadflow results show MVAr export/import to Utility as -
Before Motor Start - Export 1153 MVAr
At Instant of Motor Start - Import 7803 MVAr.
These imported MVAr's must instantaneously be available at the instant of motor start for the loadflow to converge.
The kinetic energy of the Utility's generators will supply the MW before the Utility generator governor responds, however the Utility's AVR can not respond instantaneously to produce the VARs required at the instant of motor start.
Where do instantaneous VARs required by the motor at the instant of starting come from before AVR response ?
How do loadflow software programs deal with it ?
The plant has local steam turbine generators ( I have modelled as PQ ) and an interconnection to a Utility ( modelled as Swing Bus ).
The plant does not have pf correction capacitors or static VAR compensator.
Loadflow results show MVAr export/import to Utility as -
Before Motor Start - Export 1153 MVAr
At Instant of Motor Start - Import 7803 MVAr.
These imported MVAr's must instantaneously be available at the instant of motor start for the loadflow to converge.
The kinetic energy of the Utility's generators will supply the MW before the Utility generator governor responds, however the Utility's AVR can not respond instantaneously to produce the VARs required at the instant of motor start.
Where do instantaneous VARs required by the motor at the instant of starting come from before AVR response ?
How do loadflow software programs deal with it ?






RE: Loadflow Study - Where do instantaneously required VARs come from ?
The var (yes, SI says var, it is a unit of its own) is nothing but current multiplied with voltage. If both are in phase, then you have watts, if they are 90 degrees out of phase, then you have vars.
The phase angle is not determined by the utilities generator or its AVR. It is determined by the load. So, if - during the start - the motor draws a lot of current and most of that current is consumed by a reactive (inductive) motor winding, you will have a high var consumption.
The current needed is always there, ready to be consumed. No matter what the load's phase angle is.
So, the answer to your question is: The vars are delivered by the utility. Also during the start. There is nothing magic or confusing about that.
RE: Loadflow Study - Where do instantaneously required VARs come from ?
The angle of the load current is determined only by the load.
The angle of the current determines VAR = VA sin angle.
The generator AVR would eventually correct the voltage drop at its reference bus due to the increased current.
However the software corrects the utility's voltage instantaeously to the per unit setting.
RE: Loadflow Study - Where do instantaneously required VARs come from ?
RE: Loadflow Study - Where do instantaneously required VARs come from ?
Bung
Life is non-linear...
RE: Loadflow Study - Where do instantaneously required VARs come from ?
RE: Loadflow Study - Where do instantaneously required VARs come from ?
RE: Loadflow Study - Where do instantaneously required VARs come from ?
If you are analyzing your system for a motor impact study to determine the minimum voltage while starting (locked rotor) a large motor; I would recommend that you determine the minimum power system configuration and model it using a Thevenin's equivalent using the system's equivalent transient reactance. I would also recommend that the "voltage behind the transient reactance" should be the load flow voltage established prior to starting the motor.
RE: Loadflow Study - Where do instantaneously required VARs come from ?
RE: Loadflow Study - Where do instantaneously required VARs come from ?
The motor loadflow power on starting is 667kW + j 6632 kVAr.
Yes Mstrvb19, my original posting erroneously stated plant import of 7803 MVAr instead of 7803 kVAr.
Timohearn, why do you recommend using the generator transient reactance and not sub-transient reactance ?
The first instant would be voltage behind sub-transient reactance.
RE: Loadflow Study - Where do instantaneously required VARs come from ?
RE: Loadflow Study - Where do instantaneously required VARs come from ?
When using steady state loadflow software for motor starting studies,and when the motor starting kVA is significant in comparison with the generator kVA, using the generator transient reactance is appropriate because it presents a larger reactance for a longer time than the sub-transient reactance. eg. 0.1-0.3 seconds before full AVR response.
RE: Loadflow Study - Where do instantaneously required VARs come from ?
RE: Loadflow Study - Where do instantaneously required VARs come from ?
I think that the assumed power factor of 0.1 is way too low - something between 0.3 and 0.4 might be better. The 0.1 pf might be OK for the first cycle or two, but once the field is established and the motor begins its run-up 0.3 to 0.4 is better.
Bung
Life is non-linear...
RE: Loadflow Study - Where do instantaneously required VARs come from ?
On smaller generators 1.5 MW or less the electronic voltage regulator monitors all 3 phases and can sense a voltage dip in 1/6th of a cycle. On better 60 Hertz generators the voltage regulator gets its power from a 300 Hertz single phase generator and the brushless exciter that supplies rotor current generally runs at 120 Hertz 3 phase. The bandwidth of the voltage regulator is high enough that voltage recovery could occur in 1/2 cycle but in actuality is deliberately slower for stability reasons. The high bandwidth does make it easier to tune out instabilities much like how electronic servo drives are definitely easier to tune than amplidyne servo drives.
On old fashioned generators the voltage regulator is electromechanical which slow things down a lot.
Larger generators have voltage regulation that is deliberately slower that for smaller generators. One reason why electrical utilities still keep smaller generators on line with larger generators is that the voltage regulators for the smaller generators tend to be more nimble - the faster response helps with small rapid changes allowing the larger units to cope with slower changes to which they are more suited.
Mike Cole
RE: Loadflow Study - Where do instantaneously required VARs come from ?
But this is all the more reason why a loadflow study is inappropriate for this task. None of the possible generator / system dynamics effects (and possible problems) will show unless a transient study is done.
Bung
Life is non-linear...