GTG - Overfrequency protection 1

[RRaghunath]

Overfrequency protection is set at 52Hz and wired to trip the Generator circuit breaker in a Gasturbine power station, I know. The unit has 11kV generator circuit breaker as well as 132kV grid circuit breaker on the HV side of generator transformer. The unit is 30MW rated run by GE Frame-6 gas turbine.

I think this is meant to protect the GTG against governor malfunction and thus the scheme is appropriate. My colleagues argue that the overfrequency is problem of the grid and tripping the grid circuit breaker is appropriate as this ensures that the unit islands feeding its own auxiliaries.

Could you share the practices in your power stations please.

Thanks in anticipation.

 

[edison123]

Overfrequency is the back up protection for GTG due to governor malfunction and failure of mechanical overspeed protection and is normally set at 10%. What if the GTG races away before synchronizing due to governor failure? I would retain this protection and also wire it to trip not only the generator but also the GT, which is the source of overspeed.

 

[4654]

As long as your turbine control is perfect, this electrical protection is required, how ever the turbine shall have over speed protection at 105% to 100% based on the manufacturer's recommendation.

 

[X2K3]

RRaghunath,

Indeed we need over frequecy protection primarily for prime mover protection. However as edison has told, this will also have role while synchronising at higher frequencies. This I have ssen particularly in eastern grids in India.

The desicion to trip which CB is debatable. If you have over flux protection 24 for Generator Breaker (in fact this should cover your Gene X'mer also), you can use 81 only to trip Grid braker and continue to feed station load by keeping GCB closed. This scheme is useful for CPPs or the places where there is large station load equal to or more than spinning reserve of UNIT. You need your Turbine Control System to be set such that once isolated, it should return to speed control and machine comes back to rated speed. Unless you need synchronising set up with grid breaker, you will end with power interruption for synchronising again.

 

[4654]

A small correction

As long as your turbine control is perfect, this electrical protection is not required, how ever the turbine shall have over speed protection at 105% to 100% based on the manufacturer's recommendation.

 

[engr2003]

Generator is provided with over frequency protection to get itself isolated so that the system does not operate with over frequency. Many of the equipment(loads)in the system are rated for a particluar frequency and may get damaged due to operation at frequency beyond its range. When such a situation arises the generator protection does not know whether it is due to its own governer failure or due to some other governer failure,hence every unit will get isoalted(with self survivability if possible)at its trip value, the objective being to connect back again immediately. But the one which has its governer out of control(and caused over frequency)would then trip on its mechanical/electronic over speed protection generally set at 110%(normally two independent relays).

The bottom line is over frequency is only for isolation and over speed is for turbine trip.
The over frequency may be connected to trip turbine only if it for sure that the turbine can never control its speed from load throw off and would always touch over speed trip value.This is to avoid unnecessary over stressing of the turbine.