Adding a second captive power plant 2

[sudiptoghosh]

We have a Captive Power plant of 18 MW Capacity.The generation is at 11KV and after catering to our plant requirements the excess power is being sent to the 66 KV grid through a YNd1 66KV/11KV,20 MVA transformer bank.When the CPP is under maintenance or not operational the grid power,stepped down through the same transformer is being used to supply our plant load.
Now we intend to install a second CPP of 8 MW Capacity and a seperate transformer of YNd1 66KV/11KV,10 MVA Transformer.The two transformers will be connected to the 66 KV grid through a common bus but at 11 KV side they will not be connected.Both the Captive power plants will be sepperately connected to the grid through their respective transformers so in fact the transformers will never be operated in parallel though both are connected to a common bus at the HV Side.My question is:
1) Will there be any circulating currents between the transformers at the 66 KV Side.The neutral is solidly grounded and the percentage impedence of the transformers are exactly equal.
2)The generators and transformers are not paralled at 11KV.So the possibility of circulating current between the generators(star connected with low resistance NGR) can be ruled out?
3)We intend to run the generators in parallel only when the 10 MVA Transformer is not in line ie.So in this case the generators are operating in parallel and synchronised with the grid through a single transformer only ie..20MVA.In this case please let me know if we need to isolate the neutral of one generator.Both the generators are star connected with low resistance neutral grounding resistor along with motorised isolator.Both the generators are 2/3 rd .pole pitched .

 

[waross]

When power is fed into the wye side of a YNd transformer there is a possibility of circulating currents in the transformer itself if the wye side voltages and phase angles are not equal. The issues may be avoided by floating the wye point but this may introduce switching voltage surges. There are no such issues when feeding the delta side. This is a characteristic of the transformers and has nothing to do with the generators. A YNd transformer bank tries to correct voltage and phase angle errors on the supply side.
Circulating currents are generally the result of unmatched waveforms. With the same winding pitch the wave forms should be close.
Remember that power generation depends on throttle setting or steam valve opening.
Reactive power sharing depends on excitation. The grid will set the voltage. Excitation level will control the generation of reactive power. You need some reactive power for the magnetic field. Under excitation and the machine will import VARs from the grid. The proper level of excitation and there will be no flow of VARs into or out of the machine. Over-excitation and VARs will flow out of the machine.
Now comes the interesting part. When the machines are islanded the load controls the flow of VARs out of the machine and the excitation strength controls the voltage. Loosing the grid connection may result in a voltage jump, up or down.
If the excitation is set to supply the VARs demanded by your plant load, so that the power factor presented to the grid is unity, there will be the least or no voltage change if the grid connection is lost. There may be some transients until the machine settles at the new load.

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

 

[inpran]

Sudiptogosh,
Can you please supplement your question with a SLD? I feel confused with your questions, at one point you say the two generators aren't connected at 11 kV and that the two CPP operates independent. If that is the case, your question 3 is confusing where you speak about parallelling the two generators.
As for your question 1 is concerned, unless there is a voltage difference between the two trafo, there won't be any circulating current.

 

[sudiptoghosh]

inpran,
Sorry for the confusion

2) Both the generators have their own independent generator transformers.When both the transformers are in operation ,the two generators are connected to the grid through their respective GT.So it is similiar to two power plants supplying power to a common grid independently and in this case the generators are not in parallel
3) In this case one transformer is not operational.So we intend to parallel both the generators at 11 kv and also synchronise with the grid.So this is a situation when two generators are operating in parallel and are synchronised with the grid to supply excess power to the grid through a singlr transformer.

In short We are avoiding the parallel opearion of transformers

Hope this clarifies the matter

 

[sudiptoghosh]

Kindly find the SLD

 

[inpran]

Sudiptogosh,
2. You are right. In independent operation mode, there will be little or no circulating currents.
3. If you have not envisaged parallelling the two generators then I suppose you will have your switchgear designed accordingly. Also, you have a 18 MW generator. You are going to add 8 MW generator. Assuming you are going to export all this power put together, you don't have a GT (20 MVA) that can evacuate the full power.
Further, If you want to operate both your Generators in parallel, you have to have your 52-4 and 52-5 VCBs closed. Please ensure that there is synch check relays are in place.(SLD do not show it.)