Station After BlackStart 2

[Mbrooke]

(Wanted to add to an older thread but its locked)


Say you've got a multi turbine station running after a blackout. By how much would frequency dip if 220MW of load was dumped off the station while it was producing 2,500MW output prior? Would is shoot up afterwards and by how much? Would the pumping system and AC equipment survive the voltage sag if the fault took place near the plant? What happens when 220MW plus its inrush are closed back in? Can simple IAC inverse/definite time relays suffice in clearing a faulted outgoing line anywhere alone its (shortish length) or would high speed 3 cycle clearing be essential?

I have coal and nuclear in mind, other types are open to discussion as well.

 

[xnuke]

These questions are impossible to answer with the information given. They also require transient analysis.

For an open-loop frequency control system, you could get a rough answer for the load shed action using the swing equation if you knew the plant's overall inertia constant. However, plant frequency control isn't open loop, so you'd need the governor model and its tuning values along with plant parameter model values to determine the resulting frequency transient. Same for the load add action.

The voltage sag question also require exciter model and parameter values to be known for transient analysis to see what the resulting voltage sag would be. Fault location matters also.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Mbrooke]

Can't we just assume typical values, like the IEEE (AC1A or the like) exciter model?

In the least is it even physically possible for a typical plant consisting of 250-500MW output alternators to even survive such a fault without tripping off line or is there a good chance all will remain stable?

 

[FacEngrPE]

Generating plants may be required to pass a load rejection test during commissioning. Depending on the goals of the test the remaining load after trip could be house load, or no load. To pass, the speed control system needs to be able to arrest the speed overshoot before the generator safety trip settings are reached (for many plants the control action needs to extend beyond the generator throttle). Some plants are required to be able to recover to ready to synchronize within a specified time.

I don't know if a typical plant will pass load rejection testing, but some plants do.

https://en.wikipedia.org/wiki/Load_rejection

https://www.eng-tips.com/viewthread.cfm?qid=297755

https://www.babcockpower.com/wp-con...n-on-fossil-fuel-utility-steam-generators.pdf

Fred

 

[Mbrooke]

Thanks, in particular the PDF.

Do plant motor control centers drop out from the brief dip in voltage?

 

[waross]

Do plant motor control centers drop out from the brief dip in voltage?

That depends on the MCC control circuits.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Mbrooke]

So it is possible. I always wondered how power stations don't shut down after a 3 phase line fault nearby. In theory the voltage would drop to zero for at least 3-5 cycles dropping out every AC relay and interlock circuit?

 

[davidbeach]

There are mandatory ride through requirements that allow normal clearing of close-in line faults. Critical systems can’t use standard AC motor contactors.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mbrooke]

What type of contactors and controls do power plants use? I'll admit I know little about what goes on, on the other side of the yard.

 

[davidbeach]

Some combination of DC control, UPS based AC control, and fast transfer to a standby station service source that is independent of the plant’s transmission outlet.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mbrooke]

Got it.

So do all those MCCs use DC starters?

 

[davidbeach]

Depends on the plant. Using critical AC (plus an undervoltage relay) on normal AC motor control may be more common than DC. But like so many other things "it depends".

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

Large motors may be started with a power circuit breaker.
These may use DC to close, DC to open and DC to wind the stored energy spring.
MCCs typically have 120 Volt coils.
A 120 Volt UPS may be used to source 120 Volts for the entire MCC.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

Regarding the voltage sag few factors help the generator stay online even with very low transmission voltage:
1) For many faults types, the delta-wye GSU moderates the voltage sag. For example a line-to-ground fault on the transmission terminals would have >60% voltage on the generator terminals.
2) The voltage drop through the GSU will result in the generator terminals being at a higher PU voltage than the transmission terminals. I looked at one particular generator that showed >0.30 PU voltage with a 3 phase fault at the transmission terminals.
3) The exciter action will attempt to raise the generator voltage during a fault. This increases the voltage drop mentioned in item 2.

 

[crshears]

In the coal-fired generating station where I worked the "critical loads" [ condensate extraction pumps, boiler feed pumps, FD and ID fans, motor-generator exciters, pulverizers etc. ] were supplied via maintained-position breakers, viz., there were no low-voltage dropout provisions. This enabled the equipment to survive the brief transient under-voltage conditions associated with AC station service transfers.

IIRC the control voltage was 250VDC.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

That makes sense. I forgot that breakers were DC open/close, but now I can see all the more reasons why.

 

[crshears]

I now realize my submission referred more to system contingencies than black starts, but I think it's apropos anyway; numerous were the times there was stuff happening on the grid that would separate units from the system, and it was gratifying to see how often the auxiliaries on grid supply would auto-transfer onto the unit station service supply / supplies and survive the contingency. Of course this left the unit operators scrambling to hurriedly match the boiler firing rate to the almost minimal steam consumption of the unit carrying only its own auxiliaries, but a very high percentage of the time they were successful and the unit would be stabilized in preparation for subsequent re-synchronization and loading at the request of the IESO.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

Sounds like fun. Can I ask, what was auto transferring on the unit substation?

 

[crshears]

It was a few years ago [ 38 or so ] and I no longer have access to any of that documentation, not to mention that plant was demolished some 20 years ago now, but IIRC the "critical loads" as described previously, viz., condensate extraction pumps, boiler feed pumps, FD and ID fans, motor-generator exciters, pulverizers etc. were diversely supplied when the unit was in normal operation.

Some loads were fed from the "reserve station service" which was supplied directly from the grid via a 230 kV > 4.16 kV step-down transformer with dual secondary windings, one for each pair of units. Others of them were fed from [unit] station service trafos; supply to them was from the isolated phase bus of the respective generator at from ~10 to 18 kV.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]