Power Plant Tripping

What are the settings of the <...GCB...> and the <...DEH...>?

GCB trip at 51.5Hz in 1 sec. DEH trip is OPC at 110%( this comes out to be 55Hz or 3300 rpm) . turbine rated speed is 3000 rpm.

An increase in power drawn as the frequency increases is understandable. It looks like the <...DEH...> couldn't respond to the frequency increase fast enough and the <...GCB...> tripped afterwards.

why did power decreasefrom 22MW to 0MW immediately after reaching 51.5Hz?

Because either the <...GCB...> or the <...DEH...> tripped. At this distance, it is impossible to say which happened first. One might try looking at plant records, telemetry data, or asking eye-witnesses, perhaps.

GCB was not tripped until power decreased to zero and steam valve was not tripped. actually when power was decreasing from 22MW , the GCB tripped at 48HZ and caused the MSV to shut.this is the sequence that happened.

That would, indeed, SDTP.

buy why power dropped from 22MW to zero when the GCB was still closed?

The answer can be found on site, in the <...DEH...>. It looks as though the plant unloaded while running near the desired frequency.

i will check the DEH response but it looks that DEH failed to regulate the speed by closing the GV completely (thus causing 22MW to 0MW).i think the DEH should have shifted automatically from power control mode to speed control mode after sensing the national grid breaker tripping.

Exactly. See #3⇑. The <...DEH...> set-up is NBG.

ok i checked the GV valves response today.upon sensing an overspeed of 51.5HZ , the DEH completley shutdown the GVs (80% opening to 0% opening) and this caused underfrequency .the underfrequency caused GCB to trip which inturn shutdown MSV thus tripping whole plant. thus we can assume that DEH failed to regulate GV's on overspeed. but the question now is :why DEH closed GVs completely on a minor overspeed of 103%(51.5Hz) instead on regulating it?the trip setting of OPC is 110%.������

<...why DEH closed GVs completely....instead on regulating it?...>

Because it has been set to do something other than what was intended to happen. Go back to the CRS and check that the settings in the <...DEH...> now are the same as when it was first commissioned. If not, then find out why not.

This thread is now beyond what can be done without visibility of the plant and its documentation, and runs the risk of turning circular. If the matter cannot be addressed locally by those present then maybe it's time to hire someone else who can.

<unsubscribes>

as mentioned earlier ,the GCB tripped at 48Hz underfrquency when power decreased rapidly from 22MW to 0MW.

<...as mentioned earlier...> ...then there's no need to repeat the information.

we expected our plant to operate in isolated mode to supply our industrial load of 20MW because the earth fault was temperoray and not in our plant system.

Given your previous posts I would hazard a guess you were involved in setting up the protection scheme and you got it wrong some how. This is not something anyone here can help you with, or arguably should due to serious safety concerns about your worksite.

Why is it so hard for some people to seek professional help locally rather than asking an online forum for free advice.

Because:

1. It's free.

2. It allows them to take credit for someone else's knowledge, hard work and experience without any effort on their part.

3. It shields them from having their superiors learn of their incompetence.

4. Laziness.

5. It doesn't come out of anyone's budget, so no one will ever know.

6. They keep getting others to do their work for them, out of pity, I guess.

We briefly interrupt this 'battle of the acronyms' to say,, DEH is NBG , OMG ...and to ponder the difference between acronyms and initialisms and what categories each shall be assigned to....The general public is invited to join in with prudish and meandering comments....

Your SLD with protective relay details with settings are required to professionally analyze and submit report.

SLD for a DEH trip? you are confusing things sir.

i could tell that because GVs tripped after 3 seconds.simple

Well, that's NBG.

I am not sure whether the Speed governor of your turbine has 'Speed control' mode. When the generator is isolated from the grid, the governor control shall shift from 'Droop' mode to 'Speed control' mode.

Could you check whether the control scheme is designed to sense the isolation from the grid and transfer the control!

To me, it seemed that the turbine remained in droop control mode even after isolation from the grid and thus was not fast enough in its response.

very logical point . yes our DEH has speed control mode but the auto-switching function from power control to speed control mode is only on GCB trip and not on grid isolation. we are now planning to implement this feature in DEH. however our concern is why the DEH closed the GVs completely on a 103% (51.5Hz) overspeed even if it was operating in power control mode after isolated from grid.

That corresponds to your '% Droop setting'.

If the turbine has come from 100% to 0% when frequency went from 50Hz to 51.5Hz, that makes the droop to be 3%.

Power Control Mode - you never mentioned that before, Coolyaar!!

1. You need to explain why site load jumped 20 to 22MW before blaming the governor. Do you have a system which transfers genset/site auxiliary power from Grid side to direct feed from genset when grid disconnects?
2. Is 20MW really your maximum site load or did some extra loads cut-in spuriously, like a standby drive (while main drive actually still running)?
3. 20 to 22MW on a 25MW unit is a load increase & frequency drop event, not a frequency increase. Was that 2 MW instantaneous - or is it just the inherent load versus frequency response of your plant happening as frequency increased? 22MW is within capacity of 25MW unit so should be sustained in theory - but response of coal units can be limited by speed/demand response of coal "powdering"/delivery system.
4. Have you ever tested your genset running your site at 20 MW isolated, or tried disconnecting Grid with various site load and export/import MW? What was result? Any modifications since those tests?
5. I can envisage a power control mode using signal from Grid connection MW to regulate export/import power to zero. MW transducers often deliver 4 mA @ full scale import, 12 mA @ zero & 20 mA @ full scale export. Power loss to transducer could cause output to decay from 12 mA to 0 mA - looking like a big import & requiring loop to raise governor setting [frequency] to compensate. Or an offset in the transducer could cause a -0.2 MW import when real input is zero - the system could have zero deadband & be OK for applications for which the genset is just tripped when Grid fails.
6. Some gensets may not be very good at rejecting full-load and have a ramp close of fuel valve initiated by a moderate overspeed. Did you specify to genset maker that it was required to maintain isolated load after Grid disconnection & consequent transients?
7. Have you asked the turbine/governor makers how the unit is supposed to behave for disconnection from Grid at 20MW generator load, with instant increase to 22MW, under [presumably] droop control.
8. Reference item 4 above, just because unit is stable connected to Grid with droop control does not ensure it is stable disconnected from Grid if it is under a different "speed control" normally before connecting to Grid. The inertia and MW versus Hz parameters can be different between isolated & Grid operation.

dear , i have explained in previous posts what happened time wise.

1: the DEH was set at 20MW target load in power control mode. the grid breakers tripped at load of 20MW at time 0sec.this caused frquency to ramp from 50 hz to 51.5hz .the speed increase caused power increase from 20MW to 22MW .this is logical.

2: when freency was at 51.5hz and generation at 22MW at 3sec (relative time to grid breakers tripping ,the GVs complety closed which caused frequecy to drop 51.5HZ to 47Hz. this caused the generator underfrequency to pickup and trip the GCB (setting : 81u:47Hz ,trip time:0.5sec).

3: regarding testing of generator during commissioning stage , the contractor refused to perform the partial and full load rejection tests on generator. it was a long dispute . the contractor said that the plant is not designed for isolated opertation and additional equipments are required to do so. contractor did not exactly mention,howerver, what additional equipment are those. finally we penalized the contractor for defaulting and expelled them. so end of story.

Dear Coolyaar,

Replying to your post #36,

Your original post only wrote that generator power & frequency increased "suddenly" with no timescale.

Your 1: It would be logical for your system to stay steady at 20 MW, because with negligible power through Grid breaker & your generator in droop control, the Grid breaker could open without anything in your system needing to react. What you appear to have is closed loop control of Grid Tie MW - it should be obvious this loop should be turned off when Grid disconnects & be replaced by a "droop" or "constant frequency" speed [frequency] governor. The trouble is that there may be a frequency transient if its setpoint does not maintain the frequency at which the Grid disconnected.

Did your set specification require this or require that the whole system should be able to disconnect from a Grid fault and continue supplying your loads without interruption??

Your 2: It must be that the "DEH" you have has a programme in it which closes valves at 51.5 Hz, effectively stopping the set, but not instantly, since there is no turbine fault. Does your supplier confirm this? The event can be avoided if frequency stays <51.5 Hz.

Your 3: I infer that the contract did not require site load rejection tests, did not require Grid disconnection & transfer into isolated operation while maintaining your loads within tolerable frequency/voltage excursions & that you did not accept the contractors price for supply, installation & commissioning of additional equipment to do that. "End of story" is that they will not help with remedies for nothing, although accepting their price would "un-end" story.

I guess all this control stuff is buried in a password-protected DEH programme, for which you do not have a listing. That is the trouble with "integrated" equipment run by micro-controllers, especially when the plant is under a warranty conditional on no modification.

I would attempt to switch the generator from "power control" into manual "droop" control when Grid breaker opens - a smooth transfer, however, requires that the droop setting is tracking the set MW out & frequency.

What facilities do you have for manual "droop" control & how can they be selected??

I think part of your answer is in your initial information. When the plant isolated from the grid, you had a 20MW generator attempting to drive the 22MW load (10% overload as best case scenario). The 2MW difference you describe had to be somewhere in the system load at that instant. This overload situation caused your prime mover to slow very quickly.

If at the instant of disconnection your plant was only supplying 15MW, then the slowing would have been even more pronounced.

This overload condition (power overload) was probably detected by your plant controller and unloaded your generator from the system as protection. The three seconds delay would account for that.

Two separate events where the second is an outcome of transient conditions created by the first.

Just a guess.