Gas turbine static starting and generator protection relay 1

[obut4]

Hello,

We want to select a generator protection relay for a gas turbine starting with a Static Frequency Converter.
This relay must protect the generator during startup and normal operation.
The generator is used as a motor to start the turbine.
Once the turbine is started the generator operates as a generator.

What criteria should we use to select the generator protection relay?
The "Power System Relay Committee" gives some guidelines:
"Typical generator protection may be limited during the initial starting sequence due to poor operation or non-response at low frequencies. Multifunctional digital relay functions may deviate outside of specifications due to inability to track the lowest frequencies. Thus, they should be evaluated for performance below their specified frequency range."

IEEE standards C37.102 says:

"Curves of element performance versus frequency are often supplied by manufacturers down to some value
of frequency. Multifunction relays provide protection down to a low frequency value but if a protective
function characteristic is such that it may falsely operate at low frequencies, then it should be disabled
during starting. In some installations single function relays, with good performance at low frequency, have
been applied to provide additional protection during start-up."

There are different relays on the market some relays are able to track frequency from 3 Hz to 70 Hz other from 10 Hz to 125Hz for example.

What is the minimum frequency the relay should be able to track?
Is 10 Hz acceptable or do we need to go down to 3 Hz?
Which relays do you typically see for this application?

Can we use a standard protection relay by considering that the SFC will provide phase overcurrent, phase unbalance protection, and ground protection at low frequency?

Thanks for your help!

 

[DHambley]

obut4,
The selection of such a relay requires knowledge of several things which are not in your post:
- the motor type (PMSM?, induction?)
- control scheme (sensorless?, resolver? other?)
- start-up sequence (low frequency until phase-lock? closed loop with shaft sensor? total open-loop start-up?)
If you are the lead engineer on this project, I assume you know these factors.


Darrell Hambley P.E.
SENTEK Engineering, LLC

 

[davidbeach]

I would expect the drive to provide the "motor" protection during the starting process. Most of the best generator protections are either unavailable or meaningless during that time. Your first line of generator protection is differential, but the drive will be connected inside the differential zone so that you'll get a differential trip unless that's blocked. During run up you'll be way outside the frequency/time windows, so that has to be turned off. With no field current, your loss of excitation protection won't do you much good. CTs on the neutral end of the generator are perfectly placed to detect generator faults with the breaker open, but won't do any good for detecting "motor" faults during start up.

You'll be far better off with two systems, one protecting the generator while it is being motored and the other protecting it once it is being driven by the turbine, at some reasonable fraction of nominal speed, and the field applied. As a driven (rather than driving) machine, there's not a lot you're going to see based simply on the residual magnetism of the field. In that range where the combustion turbine is taking it from start speed to operate speed, your best protections are the vibration sensors. Once you're at speed no load the generator protection is active and ready the connection to the system.

 

[obut4]

Davidbeach thanks for your answer!

I was not sure if the generator protection relay has also to provide protection of the synchronous generator during the time it is used as a motor.

The reason for this is that GE has a technical note explaining that their generator protection relay can track frequency down to 3 hz and can therefore be used to provide overcurrent and overvoltage protection during stating starting at low frequency.

You said:
"You'll be far better off with two systems, one protecting the generator while it is being motored and the other protecting it once it is being driven by the turbine, at some reasonable fraction of nominal speed, and the field applied"

Do you know what type of electrical protection can be used to protect the generator at low frequency?

IEEE standards says:

"Curves of element performance versus frequency are often supplied by manufacturers down to some value
of frequency. Multifunction relays provide protection down to a low frequency value but if a protective
function characteristic is such that it may falsely operate at low frequencies, then it should be disabled
during starting. I[highlight #FCE94F]n some installations single function relays, with good performance at low frequency, have
been applied to provide additional protection during start-up."[/highlight]

Thanks again

Obut4

 

[davidbeach]

You'll probably find that all those references to low frequency protection during "start-up" have nothing to do with generators being used as motors to "start" their prime mover, but rather are concerned with the much more common occurrence of a prime mover slowly winding up to operating speed and the generator simply being along for the ride. Once the generator is spinning, any residual magnetism on the rotor will produce a voltage in the windings; once you have a voltage you can have fault current. That's what "start-up" protection is for.