Synchronizing of Photovoltaic power station 1

[lz5pl]

For current project I have to solve a problem which I never faced before:
Project is for substation 20/110 kV for large (50 to 150 MW) photovoltaic power station. My problem is how such substation have to be coupled to the grid. If generators are conventional rotating machines it is clear. But for PhV technology I suppose it should be different.
I wonder should I provide decoupling of the station in case of low frequency in power grid.
How feeders are synchronized with the grid, and on what level - 20 kV feeders(leading to inverter modules) or 110 kv connection with the grid?
Project is still on conceptual stage, main contractor have not choosen supplier of inverters, so I have no anybody to ask regarding the way of connection.
As you understand I have no any experience with PhV technology and will be gratefull for any help.

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[cranky108]

The inverters syncronize themselves. I doubt you will have one inverter that size, so there maybe several smaller units.

Why would you want to disconnect on low frequency?
1. low frequency could indicate an island, so a very low frequency might be worth while, provided the voltage is high or low.
2. You have no parts that maybe damaged by a low frequency, like a resonit frequency in a turben.

Also consiter the short circuit operation of the PV system, where most inverters my only put out 1.1 to 1.5 PU into a fault, or some may shut down.
And some countries may have a short circuit ride through requirment.

 

[lz5pl]

Yes cranky, the only point I know is that inverters synchronize themselves. Of course they will be not more than 1000 kVA each, so several of them will operate in parallel on one 20 kV feeder.
I don't worry about damage of inverters from low system frequency, but I wonder how they will react if system frequency start decreasing. Probably they will shut down from their own control system ?
Interesting case for me is decoupling in case of short circuit on 110 kV line. Relay protections should trip 110 kV circuit breaker, but should this trip shut down inverters too ? Accordinh to me they should be switched-off, because I cannot imagine control system which will synchronize 50 or more inverters in the same time for reclosing of the line.



------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[lz5pl]

Yes cranky, the only point I know is that inverters synchronize themselves. Of course they will be not more than 1000 kVA each, so several of them will operate in parallel on one 20 kV feeder.
I don't worry about damage of inverters from low system frequency, but I wonder how they will react if system frequency start decreasing. Probably they will shut down from their own control system ?
Interesting case for me is decoupling in case of short circuit on 110 kV line. Relay protections should trip 110 kV circuit breaker, but should this trip shut down inverters too ? According to me they should be switched-off, because I cannot imagine control system which will synchronize 50 or more inverters in the same time for reclosing of the line.
The same question is valid for short circuit on 20 kV feeders - in such case all inverters connected to the feeder should be switched-off too ?



------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[marks1080]

Will the utility allow you to island?

 

[lz5pl]

I don't think they will allow island operation.

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[marks1080]

They usually don't.

I think your specific questions about the inverters would be best answered by the manufacturer. The inverters probably won’t ever be exposed to excessively low frequencies unless the utility has some major problems of their own. You could expect 60Hz (or 50…) with a +/- range of no more than 1Hz or zero.

 

[cranky108]

I don't think you want to island, or most inverters that I know of, won't allow you to island them. This is not to say that should support an island with a utility generator.

You might check the utility for short circuit ride through. We require, or the power pool, require this, due to the concerns of loseing significent generation, and destablizing the grid.

 

[elecon]

whether the PV system is large or small, they synchronize to Utility at PCC (Point Of Common Coupling)....across 110kV breaker in your case. Since they do not make larger size inverters presently, you will be using about 50 iMVA inverters probably stepped-up to a 35kV level and then across a 110kV breaker using a 35/110kV power transformer. The utility do allow you to operate under a 81U/81U renge of 59.3hz - 60.5hz and usually all inverters shut down on low voltage and frequency by their own algorithm in software and internal protection. The main synchronization will occur across 110kV breaker and microprocessor protection Relaying has a 25 function along 81U/81O function to be coordinated by utility reclosure upstream.I have done couple of large PV interconnection with utilities and PV sync is the simplest system I have done. drop me an e-mail with specifics and I will guide you thru.

 

[lz5pl]

Thank you, elecon. It will be very kind of you to help me. Unfortunately rules of this forum don't allow direct posting of e-mail addresses. You can contact me via contact page of our website

http://www.triel.bg

My name is Plamen.
Just send me an empty e-mail and I will send you more information about the case.

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant