Solar PV penetration in power systems 3

[KojoSincal]

I recently came across; a GRID interconnection study that some engineers from out ETC Transmission utility did for a client. However; they quoted a formula from the book: "Jeurgen Schlabbach and Karl-Heinz Rafalki Power System Engineering" and indicated that; the PV penetration level is dependent on the 5% Rapid voltage change at the PCC; i.e (RVC 5% = MVA PV plant/ SC at PCC). I have not seen this reference in any of modelling guidelines and or standards.
i am confused because; i understood that safe connection of the PV Plant is how stiff the PCC is due to the Effective short circuit ratio is at PCC.
M question is; is their methodology correct? If not what determines the MW connection at a certain PCC?

 

[bacon4life]

Solar technology has been changing very rapidly, so I would be concerned about using limits from a book published in 2014. In parts of world that use IEEE standards, IEEE 1547-2018 was a paradigm shift in how distributed generation resources are supposed to operate.

 

[Parchie]

@KojoSincal,
Basically, that 5% is the usual acceptable limit that most statisticians use, a stochastic approach. Some call it HCVP or the hosting capacity violation probability. Bluntly, it is the figure that people assume would happen given the variable nature of distributed generation (PV and Wind).
In real life, any figure seen above 5% is considered significant. Those are figures that are kept as the norm whenever people can't get a fix on how things behave, IMHO.

 

[KojoSincal]

@Parchie,

Thanks for shedding some light on the 5% figure and its interpretation. It's helpful to understand the statistical approach behind it. I think this approach is should not be used because it doesn't have any no support by any standard document. However; in real life a figure above 5% could be significant; but the studies should be done for the full scale development. Thanks @bacon4life; I agree.

 

[KojoSincal]

Hello colleagues and Senior Engineers. Thanks very much for your help and insights. I am on a journey to Renewable energy and power system studies; so kindly bear with me.
1. When sizing transformers for a solar pv plant; should you consider sizing the step up transformers with the Grid MVAr + Transformer MVAr on the Inverter transformers or the MV transformer at Point of Connection (POC) or both?
2. Where would you consider the voltage control for a solar PV plant; on the immediate bus (LV_Inverter transformer) or at the remote bus (Point of Connection_POC)?

 

[stevenal]

Thanks for shedding some light...

.
Nice.

 

[Parchie]

Let me answer:
1) When sizing transformers for a solar pv plant; should you consider sizing the step up transformers with the Grid MVAr + Transformer MVAr on the Inverter transformers or the MV transformer at Point of Connection (POC) or both?
The measuring point in any interconnection will always be on the POCC. Sizing the output transformer will always be based on the inverter size plus any consideration for future expansion or utility requirements that may affect the transformer size (some utilities require DERs to export kVARS to support the grid). This is one of the reasons why most utilities only allow about a few hundred kilowatts as most cheap LV inverters don't supply kVArs making the system power factor degrade so much. Note that there may be 4 items separating your PV inverters from the output transformer: 1) the combiner circuits, 2) the isolator breakers, 3) the AC breaker, and 3)the lines -> anyone of those could change how your DER output reacts.
If the DER chooses to install switched capacitors to support the grid at LV side, additional capacity will be included in the transformer sizing.
2. Where would you consider the voltage control for a solar PV plant; on the immediate bus (LV_Inverter transformer) or at the remote bus (Point of Connection_POC)?
It could be done on either side. The edge when done on the inverter side is the abundance of ways plus you have a wider source of materials at the LV level. There are also newer inverters that incorporate grid-forming logic in them, making kVAr grid support easier. The high side would be just as simple as your choice will be synchronous condensers and switching capacitors.

 

[prc]

Your questions are well answered in this IEEE Standard C57.159-2016 Application Guide on Transformers for distributed
photovoltaic Power Generation systems.
Inverter transformer sized to the output of inverter+ load variation + harmonics, if nay. It has to meet only transformer reactive power

 

[Adam Morse]

You might refer to 7.2.2 rapid voltage changes in 1547-2018. You might explore using integrated smart inverter voltage regulation to do this mitigation--also described at length in the standard.