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Power Generation Riddle No.9 - Island paralleled diesel generators with inductive loads
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.
Author : Safwat - From: Egypt
Mon, November 28th, 2011 - 11:33
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 “reactive droop compensation.”
Common droop settings for frequency and voltage can be different and are typically in the range of 3–5% from no load to full load. The voltage variations that occur due to droop operation at this level are not significant in isolated bus systems, but the frequency variations that occur can be disruptive to operation of some loads, especially in emergency/standby systems where the load can vary considerably over time.
Droop load sharing is common on attended prime power applications.
Droop governing can generally be used for generator loading control in single generator set-to-utility paralleling systems because the utility frequency is usually very constant. However, reactive droop is not effective for utility paralleling due to the greatly varying voltage level at any point in a utility distribution system as the load on the system changes. Var/power factor controllers should be used when generators are paralleled to a utility source.

Isochronous kW and kVAR load sharing
Isochronous load sharing control systems are active control systems that calculate the percentage of real and reactive load on a specific generator set, compare those values to the percentage of real and reactive load on the system, and then provide control to the fuel and excitation system of the generator to drive the percentage of load on the generator to the same value as the percentage of load on the system. Load sharing is critical to paralleling compatibility because the load sharing communication is the only point where generator controls interact with each other when operating on an isolated bus.
To provide load sharing functions, each generator set in the system must have controls that will calculate the total percent of kW and kVAR load on the machine, and then have a means to compare that value to that of the system as a whole. Several approaches are available in the marketplace to provide this interface. In general, they can be broken into two large groups:
-Systems that use analog signals for load sharing
-Systems that use digital communication signals for load sharing (such as CAN, RS485 or Ethernet)

Analog control systems are often faster responding than digital communication/control systems and can often be made to be compatible between different manufacturers. Cummins manufactures an analog load sharing interface module (ILSI) that can be used for interface of Cummins Power Command  systems with other load sharing controls that use analog signals.
Digital communication/control systems generally have less susceptibility to external interference than analog signals, but every supplier has a different communication algorithm that makes it difficult to add dissimilar equipment to an existing system.

Cross current compensation
Cross current is a flow of electrical current between generator sets that is caused by dissimilar excitation levels in those sets. Cross current compensation is a term describing operation of paralleled generator sets without intentional voltage droop. This is achieved by insertion of a droop current transformer (CT), usually on
“B” phase of each generator, and interconnecting the
CTs to provide an identical voltage bias to each AVR in the system. The system works best when the voltage regulators are all of the same manufacturer and model. Not all voltage regulators work together in this mode, so the best planning practice is to make sure that all the voltage regulators in a system that uses cross current compensation are all identical.
If system operation in isochronous condition is desired, the best practice is to arrange the equipment so that all generators in the system use the same load sharing control system.

Reference: Power topic #9015 | Technical information from Cummins Power Generation

Author : Hamid - From: Iran - Firoozabad Fars
Mon, November 28th, 2011 - 19:57
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? 
Author : Safwat - From: Egypt
Tue, November 29th, 2011 - 09:40
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. 
Author : Hamid - From: Iran- Firoozabad Fars
Sun, December 4th, 2011 - 12:54

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.

Author : Safwat - From: Egyptian
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