Protecting Islanded Generator

[cloudaltas]

We've recently had an application to upgrade a power plant to about 30 MW. The plant is tapped off of a 60kV line and as such, in power systems protection terminology, is classified as a three-terminal line. The existing protection clears faults in 10 cycles in zone 1, 30 cycles in zone 2 and 60 cycles at the plant. Hence for 30 cycles the plant can be islanded to the sole load which is also tapped off the same transmission line as the plant. We plan on putting overfrequency and overvoltage protection in place to trip the unit faster than the existing protection. The question is what sort of settings should we be looking at.

 

[RRaghunath]

The settings for overfrequency / voltage should take in to account your acceptable limits. So that the setting is not too conservative.

Further, in order to avoid frequent islanding you need look at the statistics of the grid voltage / frequency and ensure that the setting doesn't cause frequent islanding under transient over voltage / frequency conditions. Also, You need to always include a short time delay for this reason.

The islanding may be more effective if you consider rate of rise of voltage / frequency than just the absolute value.

 

[Aakhrinaam]

cloudaltas,
As rraghunath said df/dt and maybe even dv/dt could be useful.
Another method might be to use what is known as vector shift protection , which is specially geared up to detect islanding conditions.

 

[maypot]

Hi Cloudaltas,

I have been battling for years to sort out the problem of islanding on my 30 MW genset. Finally, I was advised to use the Alstom P341 protection relay with lots of functions but mainly dF/dT and vector shift and since then my problem was solved.Should you need additional infos plz feel free to ask.

Rgds.

Bob