Var sharing instead of voltage control


Thread Starter


This is related to an island grid. There are four identical engines. Each is about 3 MW supplying suppling power to a small (finite grid). The generators are supplying a country. All engines are operating in iscochronous loadsharing mode using woodward 2301 D. The operators sometimes has to adjust the excitation in order to balance the load equally on all gensets. The plant manager would like to have balance the var automatically. Actually would like to reduce operator intervention on the control side as much as possible. The voltage regulator that is in place is the Cosimat N+. There also is a QPF (reactive current and cos phi regulator and balancer). The QPF and the Cosimat N+ are interconnected. Thus the possibility exist to do the balancing of the VAR. But so far I can read an fixed Var setpoint need to be set at the QPF. Since that generators are te only generators on the grid and that the VAR is determined by the load, can we still do the var balancing by setting a fixed var setpoint? I think this is not a good idea but appreciate your feedback. Since I do not have experienced doing this on finite grid I do not know how this may be affect the voltage in transient conditions. I assume that in stable conditions the affect on the VAR balancing on the voltage can be be very small, but would appreciate feedback on this to.

Thx in advanced for your feedback

Ramiro Woodward

you should install a CT in the B phase of every genset (dropp CT), and install a cross current compensation circuit between voltage regulators in this configuration they'll share reactive power.

Another way is to configure the voltage regulators in droop mode and set the same droop percentage in every voltage regulator (5% is a good choice).

These are good ways used since a lot of time ago to control KVAR in automatic mode

Ramesh Kapur

Understand that the 3 phase Alternators are from AvK. AVR is COSIMAT N+ and additional module QPF.

While cross compensation works for most of AVR system but not for COSIMAT N+. Reason being that kVAR sensing of COSIMAT N+ works on principle of voltage input (proportional to current), hence Droop CT of AVK are internally burdened with approx. 15 Ohm resistance. Hence if connected in cross - current compensation, it wont work.

QPF supplied by AVK can be used in different modes/ application. Most common is "Mains Coupling Mode" protecting alternator from not exceeding its rated kVAR or Power Factor when it is connected in Infinite grid (Small Generating set in large grid). This is not the case in this application as this is the only power plant for power supply.

Other application is to use QPF regulator to automatically balance kVAR between different DG Sets. For this please refer to Clause 7, page 13 of AVK QPF manual "BV QPF.1 - 01093 E". you may find this on google search or ask me I can scan and send the copies. Pl note the CTs have to be in Line V of AVK alternators and burdened to give 7 to 10V, AC RMS at rated load.

Ramesh kapur
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