Distribution transformer and peak load demand

[SKEDL78]

Hi all
im a new starter and this question i'm about to ask is probably very basic. The question is focused on providing peak load in a small gas fired power station via a black start diesel generator.
The main source of power generation are 5 units of 800MW gas gensets at 11kV connected to a 11kV switchboard. The 11kV switchboard feeds a 415V LV switchboard via a 11/0.433kV aux transformer.There is also a 500kVA Diesel set for black starting on this 415V LV switchboard. i was told that this Diesel set is also used for power station peak loading. What i dont get my head around is how does the 11/0.433kV aux transformer pushes voltage the opposite way to meet the peak load when the primary of this very transformer is simultaneously fed by the 11kV bus.

please help me to understand and thanks heaps

 

[waross]

Actually, what you don't understand is how power sources share the load.
When black starting, the combination of the 415 Volt generator and the 0.433kV/11kV is an 11 kV power source.
That 11 kV power source is used to start an 800 MW gas gen set which becomes another 11 kV power source.
You don't seem to have a problem with 5 800MV power sources working together to share the load.
Why is there a problem with one more 11 kV power source sharing the load?
Hint: Power sharing between parallel sets;
Power sharing depends on throttle settings on the prime movers.
Consider the case where all sets including the diesel set are sharing the load at the same PU loading.
eg: All 6 sets are 80% loaded.
Now as the trottle is opened on the diesel set, it will increase the share of the load that it supplies.
As the throttle is closed on the diesel set, it will supply less percentage of the load.
The point will be reached where the set is idling at synchronous speed and is neither supplying power nor consuming power.
At this point the real current will be zero Amps. (There may be some reactive current flowing but that is another subject.)
If the throttle is closed further the set will attempt to slow down but it will become a synchronous motor and will continue to rotate at synchronous speed.
The current will now increase as the throttle is closed, but the current will now be in the opposite direction.

Anecdote: An operator had a problem at a small diesel plant. (2.2MW total capacity.)
The load was dropping and it was time to drop a set off-line. He shut one set down but it wouldn't stop.
With the fuel completely cut off the set was still turning at synchronous speed.
He called and awakened his boss in the middle of the night.
His boss was not impressed but didn't have the budget to hire a smarter operator.
Turns out he had forgotten to trip the set main breaker and the set was being driven as a motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

... What i dont get my head around is how does the 11/0.433kV aux transformer pushes voltage the opposite way to meet the peak load when the primary of this very transformer is simultaneously fed by the 11kV bus.

Are you perhaps thinking that the transformer is some sort of one-way valve? Because it is not. If you put a voltage on one side, you get a voltage on the other side, different by the turns ratio. So 415 in on the LV side gets you 11kV on the HV side.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Parchie]

What peak loading are you talking about? 500kW is nothing compared to 5 X 800 MW. Even in your 500 "kW" is connected to the system, it means nothing when a 50MW block load is started.

 

[ppedUK]

Is there a mistake in your numbers? 5 x 800MW is not a small station by any stretch! 800MW @ 11kV = 42,000A, that is some big 11kV switchboard. Do you mean 5 x 800kW gas engines perhaps?

I doubt that a 500kVA genset could start an 800MW Gas Turbine. A 500kVA genset used as a peak lopping set on 4.0MW (total) plant would make more sense!

Have i missed something?

 

[waross]

ppedUK, My thought also.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Parchie]

@ppedUK,

I doubt that a 500kVA genset could start an 800MW Gas Turbine. A 500kVA genset used as a peak lopping set on 4.0MW (total) plant would make more sense!

IMO, we have a 500HP Detroit generator used as a blackstart source! It runs the starting air compressors (10 of them) and also runs the fuel oil/ lube pumps of a 11.25MW diesel generator which in turn starts up the 40MW TG unit and so on . . .

 

[ppedUK]

@Parchie,

You have an 11.25MW DG to start a 40MW GTG, the OP states that he has 5 x 800MW??? gas sets and a 500kVA DG, no mention of any other sets!

 

[waross]

ppedUK, the point is:
There seems to something wrong with the OP's numbers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

A 500 KVA set has no chance of starting a 800 MW GT and would not be felt as a "peaking" set in that lineup.
A 500 KVA set could start an 800 KVA GT set and would be useful for "peaking" as well.

Bill
--------------------
"Why not the best?"
Jimmy Carter