Grounding/Earthing Transformer for 1MVA Power Plant 1

[electroctopus]

Hi guys,

I need to size the grounding transformer and effective grounding for 1MVA solar farm (SLD + requirement attached).

As you can see in the attachment, there are 4 requirements. I was able to calculate requirements 1,3 and 4. However, I need help with Requirement #2. How do I obtain the value of R0,DG ? Can it be found using the transformer specifications or elsewhere?

Thanks in advance!

 

[Kiribanda]

Requirement # 2: X0(DG)/ R0(DG). To my knowledge R0 & X0 are the zero seq. reactance & resistance values for the generator (DG).
You should be able to get these values from the generator data sheets.
I have a question for you.
Per the SLD, I found that your main transformer is WYE(GND)/wye(gnd). That means your 480V system is solidly
grounded.In that case why do you need another grounding transformer for the 480V side?

 

[waross]

The grounding on the 480 Volt side may be to maintain a ground on the PV system in the event that the main 2000 A breaker opens.


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

 

[electroctopus]

Thanks for your input guys.

I wasn't able to find R0 & X0 data in the generator datasheets- generators in this case being the solar inverters. Do I need to look somewhere else?

Thanks and much appreciated.

 

[Parchie]

What is the size are your "generator"? I think you can find estimates based on sizes!

 

[electroctopus]

Hi, its a 1MW solar farm with 20x50kW inverters. And 1MWV transformer..

 

[Parchie]

@electroctopus,
AFAIK, inverters have very high zero-sequence impedances. The only equipment that could provide zero-sequence current path will be your main transformer (if the transformer configuration allows)

“The vast majority of three-phase grid-tied inverters are in a phase-to-phase configuration, with no path for zero sequence ac currents. In addition, three-phase inverters are often interconnected to the utility grid by transformers having open zero-sequence paths. Therefore, the generic zero-sequence model for three-phase inverters is an open circuit.” (PES - TR21 or IEEE 62.92.6)

 

[Kiribanda]

I was confused about your question at the begining. Now I can find that you are looking for the
X0 & R0 values to satisfy X0/R0>= 4 to fulfill XCEL requirement. This requirement should be at the
Point of Common Coupling (PCC). That means after getting the Thevenin's impedance data of EXCEL side,
you have to do a sc study by adding your 1MVA,Ynyn in paraell with Uitilty. The SC results
will give you the Z1, Z2 & Z0 values at the PCC.Then verify whether it satisfies X0/R0>=4.

 

[electroctopus]

@Kiribanda

Thanks for your reply. I'm guessing Thevenin's impedance data of XCEL side will include impedances of grid, transformer and cable?

Also, any recommendations on software for doing the SC study?

 

[Kiribanda]

I donot know what did you meant by "Thevenin's impedance data of XCEL side will include impedances of grid, transformer and cable?"
The Thevenin's impedance of the XCEL side will give the impedance looking from the PCC towrads the XCEL entire network.
But when you paraell your local impedance to XCEL impedance then obviously it is the equivalent resultant
driving point impedance at the PCC.

 

[waross]

When the solar disconnect switch is closed and the EXCEL transformer is in the circuit, the grounded wye/grounded wye EXCEL transformer will shunt out the Zig-zag transformer.
You should only have to be concerned with grounding the solar system when the main disconnect is open and only the solar system is connected.
You are feeding the Zig-zag with a 70 Amp breaker.
I see no need to size the zig-zag for more than 70 Amps.
But, I am not aware of any code or standard which allows a breaker or automatic switch in a grounding circuit. (Excepting mutiple, paralleled generators).
I would use a solid connection to the Zig-zag and resistance ground it.
Am I missing something here?

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