Help me calculate solar PV sizing with limited Available Fault Level at connection point.

[Tomas7]

Hi,
I am a Development Manager working in the energy industry in Australia. I want to learn more about what to do when there is a network issue or in this case work out what size of PV plant we can install. The company that I work for proposed a 10 MWac solar PV farm and the network company who performed the preliminary assessment of system strength came to the conclusion that the Available Fault levels became negative and the short circuit calculation indicates a shortfall. I have attached the preliminary assessment report and our selected Inverter information so that you can get all the information that I have. I have covered sensitive information because this is a live project.

What I would like to work out without using Digsilent Powerfactory or similar software is approximately what size PV plant we can connect without a shortfall in Available Fault levels at our connection point. [highlight #FCE94F]Is that possible?[/highlight] I just need an approximate figure.

The screening technique the network used was the Minimum Short Circuit Ratio method, also known as MSCR. They used the nameplate inverter capacity for this exercise which are listed in the attached report.

Information about our proposed solar PV plant. We intended to connect two different 4.95 MW solar plants to two different bus bars, one at 11 kV and the other at 22 kV, each plant was going to be 4.95 MW real power export capacity and 9.8 MVA of inverter capacity for reactive power support. There is another bigger solar farm upstream to our connection point which the network has included in their assessment, it is a 217.25 MVA solar plant.
Here is a link to the single line diagram for our two connections.

I would be happy to offer a consultancy fee if you like.

Appreciate your help thanks,
Tomas

 

[davidbeach]

I guess you need to explain the terminology as it is plainly obvious that there can not be negative fault current at any location on any real power system. All sources contribute to any fault; any fault on either red line will have fault current from both ends. So, what is the problem in terminology that doesn't rely on local usage? It may be that you're facing a regulatory problem rather than an electrical problem, in which case the answer may only be apparent to someone familiar with that regulatory environment.

 

[Tomas7]

Hi David,

In Australia we have a very long and not overly interconnected network. The more renewable that are installed into the network, especially at remote ends of the network the System Strength (SS) becomes very important. Sometimes System Strength is expressed in the terms of fault level because the fault level is highly correlated to the level of SS (the higher the current which will flow into a fault, the higher the retained voltage will be). Therefore, fault level has become a proxy measure for SS. The exit of large thermal synchronous generation, together with an increasing proportion of non-synchronous generation like wind and solar, has contributed to decreases in system strength in some areas of the power system. A ‘strong power system’ is one that is characterised by high fault levels.

There is now an obligation on Network Service Providers (NSP) to maintain minimum levels of system strength before they allow any further generators to connect.

If you look in the preliminary assessment report they calculate what the Available Fault Level is going to be, during system normal and N-1 scenario, with our new solar farm connected and the result is that there will be a short-fall which is not allowed. Table 2 shows what the minimum fault level needs to be under system normal conditions and table 6 for N-1 scenarios. They are summarised in table 10 at the end of the report. We are connecting at the end of a long radial feeder far away from synchronous generators and demand is low.

So I was wondering approximately what size PV plant we can connect without a shortfall in Available Fault levels at our connection point. Can this be worked out without using PSCAD models?

Hope this helps,

Tomas