Generator Reverse Power Protection

[eskim]

All,

Our 5,1128 MW's diesel engines have Reverse Power Prot. setting of 0.04 x Rated MW, it used both to stop the diesel engine and open main circuit breaker.
Can I just open the circuit breaker not stopping diesel engine ?

Could you please explain me of the following scenario:
* With existing setting:
- Breaker fail to open and generator stop
* With new setting:
- Is it save for the engine for a second before restarting it?

Thank you for your kind co

Idamus

 

[sykimk]

Reverse power protection is used for anti-motoring. This function is used for protection of prime mover not generator. Motoring occurs when prime mover outputs are less than no load loss of generator, and prime mover can be damaged due to overheating during motoring.
Due to this reason, 32 trips main breaker and field breaker as well as prime mover.

 

[Rodmcm]

To answer your question yes you can open the breaker without closing down the diesel, but you have to ask why you went into reverse power. If it was because you lost the diesel and the genset motored then close down the diesel. If it was because an operator failed to open the breaker on normal shut down then just open the breaker

If the breaker fails to open then you have a real problem. Modern relays have a bus zone trip output that operates for breaker fail. You can use this to trip upstream breakers and hence protect your diesel.

 

[powercomplex]

Shuting down the mover is safe, to avoide damage in motoring case, like in breaker failure situation. The reverse power protection should have a dual action i.e trip breaker and shut down fuel and air intake to mover too.
Usually reverse power not frequently operates, if your operator is operating actively.
So let the protection has command on both generator breaker openning as well as prime mover shut.

 

[dpc]

As sykimk says, the purpose of the reverse power relay is to protect your engine, not the generator. I would not recommend leaving the engine running after reverse power relay operation.

The setting seems quite low for a diesel engine, but for an engine this large, you should follow recommendation of engine supplier.

The reverse power relay also typically has a time delay function - you might be able to increase the time delay if you are experiencing nuisance tripping.

For what it's worth, normal shutdown on many steam turbines is to gradually close the control valves then allow the reverse power relay to trip the unit off-line. This is verify that the steam supply to the turbine is actually stopped and verify operation of the relay.

 

[eskim]

Thank you for your responses.

Actually, we often have Rev. Pwr trip caused our loads are very dynamic (SHOVELS) and other big motors. Also our 9 diesel engines are same location with 4 (120MW's) steam turbines which have slow response for any load reduction.

Diesel operator is little bit difficult to be fast acting if diesel tripped, they have to go outsite to reset engine etc. So my plan is to leave engine running if only rev pwr trip happened. It will be effisien for operator on emergency situation.

Again, Can I proceed with my plan?

Thnak you very much.

 

[rbulsara]

32R setting 3% or 4% of the rating usually apply to turbines which require very little power to 'motor'...

However diesel engines are far more robust and they will not motor with (reverse) power less than 10% of its rating.

 

[rbulsara]

What is the time delay on the tripping 32Rs?

 

[RRaghunath]

I think you can consider increasing the reverse power setting as the diesel engine is supposed to require as high as 25% of its rating for no-load operation.

Even if you keep the same power setting, time delay of 3seconds, if ensured before trip should help.

 

[eskim]

32R TD is 3.o seconds

 

[RRaghunath]

If the reverse power conditions stays for 3seconds before the trip occurs it cannot be due to nature of your plant loads (dynamic). There is bigger problem in parallel operation of DGs and STGs.

It is likely that the STG governors are not responding when there is load throw-off that leads to frequency shoot up. The DG governors must be closing the fuel valve completely on sensing the increased frequency and staying for 3seconds long to make the reverse power protection to operate.

I think you need to study the bigger problem of parallel operation of the plant generators to find a technically correct, 'safe to the plant' solution.

For your question whether generator breaker alone can be open on reverse power relay operation, I think it is safe for the DG set.

In the rare event of reverse power condition caused by Diesel engine governor problem, the engine will any way trip after the breaker opens (on other engine related protections).

Alternatively, you may consider having a over frequency relay (considering the inadequate response of the STG governors is causing the over frequency and then reverse power condition for DGs) trip the generator breaker alone with appropriate frequency setting and a short time delay of say 1second (just to allow time for governors to act).

Hope you find the above helpful.

 

[xabproject]

Dear eskim,
My sporadic thoughts-


a) Do you lose all the DEGs in every incident or just one black sheep or at random - lowest loaded?!
b) Are DEG and STG synchronized on the same bus with similar step up/direct arrangement?
c) Why operator cannot go to outsite? If you trip DEG breaker alone will you be closing it back from remote? What if closing is delayed - remember - long duration no load running can also be bad for the Engine.

Your first question:
There is nothing called as Generator stopping! If breaker fails to open when fuel is cut off to the engine and the excitation breaker has opened then the DG set enters Motoring mode. If local grid is sufficiently strong, then the DG set will MOTOR by consuming about 5 - 25 % of the rated active power (called motoring power - find the figure for your set) and nearly 100~300% reactive power. If excitation is still on then the reactive power can still be forward! but the set MOTORS. This may not go on for long since the piston rings will jam due to non-lubrication and subsequent overheat(remember, the fuel oil/carbon partially lubricate!) following which an explosion of unburnt fuel might take place, gearbox (if provided)/shaft might suffer due to 'increased torque', etc. Books and experts recommend a reverse power setting of 50% of the Motoring power (= 5 to 25% of rated power).
A LBB scheme, if active and thoughtfully applied, might disconnect that section of the bus and save the DEG set.

Second part: If the power excursions are of transient nature you might run the risk of overspeeding the Diesel Engine. So cut of fuel to the engine as well as trip the GCB if motoring is confirmed. The time delay for confirmation could be set with some operational experience 0.5 to 1 sec or even more. Take the advice from the set Vendor. Put an alarm contact from poer relay before timer to see how often are the power excursions?

Part 3: Think of adjusting the governors of both the DEGs and STGs to different droop settings so that the change of load does not impose more change on the DEG sets and least on STG. Change load bus if that is possible at all, sometimes very close to miracles do happen that can be explained later.

Best regards,