Photovoltaic power plants: inverter controls

[Alex68]

Dear all,
sorry for the long post.

During the design of two big photovoltaic power plants, the first one connected to the national HV grid and the second one isolated, manufacturers of inverters were not able to give some answers about the behaviour of their devices in steady state conditions and during transients.
Let me remark that I’m not interested in knowing what happens exactly inside an inverter. I have to make some power system studies (load flow, short circuit, and dynamics), so I will treat it as a black box.

Indication of papers or books will be appreciated.

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Power plant in parallel with the HV grid

Steady state operation

AC side
1. Are voltage and frequency imposed by HV grid?
2. Is the inverter synchronised with the grid voltage?
3. What happens if the grid is disconnected?
4. What are the criteria for the production and regulation of P and Q?
5. Is a capability curve (like that of a synchronous generator) available?

DC side
6. What are the quantities regulated by the inverter on this side?
7. Operating in MPPT, does the inverter regulate the V to obtain the desired I?
8. From the point of view of the PV field, can the inverter be considered as a (variable) resistor?
9. AC and DC regulations can not be independent, since the inverter controls a power flux from DC to AC side. How do these regulations affect each other? Does the MPPT prevail on the P and Q production on the AC side?

Transients
1. Short circuit on the AC side. What are the inverter contributions in terms of symmetrical and direct components as defined in IEC 60909-0?
2. Short circuit on the DC side. What is the inverter contribution as defined in IEC 61660?
3. In case of opening of a remote circuit breaker whose state is unknown, the power system is isolated without any direct information about the grid connection. What happens?
4. What happens in case of a sudden connection or disconnection of a load? What is the behaviour of the inverter and how fast is its reaction to the transient?
5. What happens in case of loss of a string? What is the behaviour of the inverter and how fast is its reaction to the transient?
6. Similar to the AVR models described in the IEEE Std 421.5, Is there any kind of mathematical model of the inverter regulator as Laplace transfer functions?

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Isolated Power Plant

Steady state operation

AC side
1. Are voltage and frequency imposed one inverter? Are the other possible inverter synchronised to this one?
2. What are the criteria for the production and regulation of V and f?
3. Can I consider a proportional (droop) or proportional + integral (isochronous) regulation of P and Q?
4. Is a capability curve (like that of a synchronous generator) available?

DC side
5. What are the quantities regulated by the inverter on this side?
6. Can the inverter regulate in MPPT or does the AC side regulation prevail?
7. From the point of view of the PV field, can the inverter be considered as a (variable) resistor?
Transients
8. Short circuit on the AC side. What are the inverter contributions in terms of symmetrical and direct components as defined in IEC 60909-0?
9. Short circuit on the DC side. What is the inverter contribution as defined in IEC 61660?
10. What happens in case of a sudden connection or disconnection of a load? What is the behaviour of the inverter and how fast is its reaction to the transient? Does the frequency vary? (In case of rotating generators, the frequency decreases because generators slow down, but I suppose that in this case frequency should be fixed by an oscillating circuit internal to the inverter)
11. What happens in case of loss of a string? What is the behaviour of the inverter and how fast is its reaction to the transient?
12. Similar to the AVR models described in the IEEE Std 421.5, Is there any kind of mathematical model of the inverter regulator as Laplace transfer functions?

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Thank you very much
Best Regards
Alex68

 

[redfurry]

I can't comment on a lot of the DC stuff, but I have been involved in some studies of solar farms connected to the distribution system (e.g. 10MW composed of 20x500kW inverters).
Note that the behavior of these devices is completely dependent on the specific device/manufacturer, so if you can't get information from them (which is strange) you're probably out of luck.

In my experience with a couple of manufacturerers I can give some examples of answers to your first few questions:

1) Frequency is imposed by the grid.
2) Voltage at the PCC will depend on system voltage, system stiffness and generation levels just like any other type of generation.
3) Most units (the ones I dealt with) have active anti-islanding features which will disconnect if the grid goes down. This is usually implemented by some type of frequency injection which 'pushes' against the grid frequency.
4,5) The units I dealt with were fixed at unity power factor, although an inverter theoretically has the ability to control power factor (have seen this on wind turbines with AC-DC-inverter generation). Ask the manufacturer. Real power is just the output of the solar cells minus any losses.

Under transient conditions things get a little murky. I've seen listed infeeds of 1.2 or 1.0 FLA in manufacturers literature, and you won't get this in terms of sequence impedances. However, I have heard of a report from Toronto Hydro (don't have it) where they indicated from tests that fault infeed from one manufacturers units was virtually zero. Either way, it's substantially less than a typical rotating machine.

If you are connecting to the HV grid, you are going to need a lot more information from the manufacturers because you're going to have all sorts of requirements for ride-through and power factor control capabilities. I only dealt with distribution where things are simpler.

 

[Alex68]

Thanks Redfurry
I asked several manufacturers in my country but I had only the possibility to speak with commercial people.
It seems that nobody have made power system studies including PV systems and asked similar questions.
Thank you