question about paraleling on

[dcset]

I want to connect two generators to the same step up transformer and then, get 0,90 cos on the connection to the grid...this means each gen set with the same amount of reactive, does somebody do it?

 

[rbulsara]

I can't see why it cannot be done. Gens would be paralleled with each other to a bus before connecting to the transformer. They can be set to export certain amount of VAR or set to constant power factor mode when in parallel with the grid.

The task would be easier if both the gens are identical. This assumes that the grid can absorb the amount of VAR your gens would produce.

Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

It's done fairly frequently in the utility world using a transformer with dual LV windings. Each generator synchronises to an LV winding. 0.9pf is easily achieved; achieving it at rated output might take a little more doing as Rafiq has hinted.


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If we learn from our mistakes I'm getting a great education!

 

[dcset]

I explain, this is a power plant with 2 units 4,6 MW, and the gen sets are connected to the low voltage side of the step up transformer, with a circuit breaker each one, and then the VT has other circuit breaker for the grid... the customer wants to get a power factor of 0,9 cause the electrical grids ask for it.... at this moment they are using a excitation control where a mA signal go through the two AVR + PF control in order to control reactive current and obtain the desired power factor.... ( the PF control is controlling generators in VAR mode), but the problem is that in some moment, one of the units get all the reactive in example 0,80 inductive and the other o,90 capacitive, but the power factor out of the VT is quite perfect... around 0,9, this used to be on relation with temperature on site, but i still have doubts, if in example, excitation is suitable for this application? or if there is some problem with two gen sets working that way.... At this moment i´m thinking on changing the analogical AVR + reactive controller for a DECS 200 from Basler this is a digital unit and i think it will works...
But i still thinking about if there are some problems in the fact of two units working together..
thanks a lot

 

[waross]

Do a search on quadrature compensation. Generally a circuit is used to bias the AVR so as to share the VARs properly.
With balanced loads, the voltage from A phase to B phase is at 90 degrees to the current on C phase. However, reactive current on C phase will be in phase with the Voltage from A to B.
The the current from the C phase CT is dropped across a resistor and the resulting voltage added to the sensing input of the AVR.
Real current produces a voltage at right angles to the sensed voltage and it has little effect on the voltage sensed by the AVR.
Reactive current produces a voltage that is in phase with the sensed voltage and it is significant. This feature is built in to many AVRs, but it is basically a resistor and a pair of terminals for the connection of the CT output.
You should be able to find a diagram for a cross current compensation circuit or a quadrature compensation circuit in many textbooks. The instructions with some AVRs include a quadrature compensation circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

dcset:

but the problem is that in some moment, one of the units get all the reactive in example 0,80 inductive and the other o,90 capacitive, but the power factor out of the VT is quite perfect.

This indication of VAR exchange or circulating currents between the generators. You need to ensure that on no-load condition (with grid side breaker open and gen breaker closed), there is no circulating current between the two gens while paralleled with each other.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

When a generator is islanded the excitation controls the voltage.
When a generator is paralleled with the grid, the grid controls the voltage, the excitation controls the flow of VARs.


Bill
--------------------
"Why not the best?"
Jimmy Carter