[Safwat]

I do have same power gensets different manufacturers working as standby giving power to inductive load, the two units having two AVR's: the first digital AVR the second is automatic (customer using PF correction units rated 2/3 of the total kva), …
I do have same power gensets different manufacturers working as standby giving power to inductive load, the two units having two AVR's: the first digital AVR the second is automatic
(customer using PF correction units rated 2/3 of the total kva), the first genset using auto. control PF correction capacitor while the other using fixed PF correction capacitor with total capacitors are on.
The first genset when reach 60% of its rated power, the reverse VAR cause its circuit breaker to trip.
Would the existing voltage regulators can share the VAR power?
What would be the max loading (KW, KVAR) @ 0.8, 0.99 PF.

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[Hamid]

It seems you face to problem for good paralleling two mentioned gensets and suitable load (KVAR) sharing, For general guidance you can refer to manufacturer recommendation, I rewrite some technical topic in this regard by using one of Cummins Co. white paper. In a paralleled arrangement, the voltage and frequency outputs of the generator sets are forced to exactly the same values when they are connected to the same bus. Consequently, generator set control systems cannot simply monitor bus voltage and speed as a reference for maintaining equal output levels. If, for example, one set operates at a higher excitation level than the other sets, the reactive load will not be shared equally. Similarly, if a generator set is regulated to a different speed than the others, it will not share kW load properly with other generator sets in the system. Each generator set in the system has two active control systems always in operation: the excitation control system regulating voltage and the fuel control system regulating engine speed. Generators can be sharing kW load and have problems sharing kVAR load, and vice versa. So real power sharing (expressed as kW) depends on speed matching between the generator sets and fuel rate control; reactive power (expressed as kVAR) is primarily dependent upon voltage matching and excitation system control between the generator sets. Although it is sometimes possible to integrate systems of different manufacturers, generator set governors and load sharing controls should be of the same manufacturer to avoid conflicts in responsibility for proper system operation. Several types of load sharing control are available: -Droop governing and voltage regulation (reactive droop compensation) -Isochronous kW load sharing -Cross current compensation for kVAR load sharing -Isochronous Voltage kVAR load sharing Droop load sharing Droop governing and voltage regulation systems have been historically used for isolated bus paralleling, because they allow proper load sharing between any two or more generators operating on an isolated bus, as long as the generators can be set up to droop frequency and voltage at the same rate, and can be controlled to stable frequency and voltage values. As illustrated in Figure below, droop governing or voltage regulation allows the engine speed (measured in Hz) or alternator voltage to decline by a predetermined percentage (typically 3%) of the output range as load increases. Identical speed and voltage drops result in equal load sharing between paralleled generator sets. The same practices as described here can be applied to the voltage regulation system of the generator sets in a system. Var load sharing via droop is often termed

[Safwat]

thanks dear the difference in the analogue and digital are in response time and the type of voltage signal would be sent from them, when i use a PLC like Deepsea or Easygen they can handle both signals coming from different AVR. the questions are still not answer: would both AVR the analogue and the digital have the ability to share the VAR load inspite of the difference in response in case of transient?, how much the power factor correction units would effect the performance?, should the gensets with the existing of the inductive load connected to the paralleled generators should be de-rated? should i change the analogue AVR to match that Digital AVR for better sharing the load?

[Hamid]

I think your problem is two incompatible AVR for fast and suitable VAR sharing, specially in transient conditions. As mentioned above, respond time of digital and analog control systems is not same and you should advise with manufacturer for the best modification of whole control systems. In fact delay response of one AVR (digital type) cause the transfer delay of loads from one genset to another which can cause mentioned VAR sharing limitation.

[Safwat]

Hamed…. The only problem with this system would be the late response of the analogue AVR and the KVAR will never equally shared between the identical generators (both have the same power). I proposed to the user to change the analogue AVR with digital one similar to the one with the same configuration this will help a lot to share equally KVAR, even at the transient states. Safwat

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