Cos phi regulation 2

[slavag]

Dear All.
I try found some way/ways for solve "problem".
We have industrial factory with co-generation.
1. Two OH line 161kV feeded plant SS.
2. 4 ( four) transfomers feeded small SS's in the factory.
3. 2 ( two) 60MVA 161/6.6.KV transformers are connected to power plant with 2 (two) generators 20MW.

AVR of generators set on the reactive power control.
OLTC of transfromers with auto mode.
From one hand we need keep voltage on the 6.6kV bus, all importnat loads of factory are connected to this bus. from other hand we need keep cos(phi) on the OH lines ( penation to utilities).

Before few months 6.6kV was same bus and both generators worked in parallel with joint control of reactive power. Now, bus is separated to two, but both generators are operated in parallel on the 161kV level.

Now, start some games of opeartion personall with AVR, becouse cos phi and OLTC control, becouse voltage on the 6.6kV bus.

Q: are possible some optimization and auto control of all parameters in system , what is a options?

Thanks in advance.
Best Regards.
Slava

 

[esee135]

I'm surprised you have a VAR limit versus a voltage schedule for your high side. In the ideal world you would have a sync-condense machine on each 6.6kV bus.

Sounds like your issue is your high side operating guidelines are different from your low side.

Are your 6.6kV station service transformers on both 6.6kV buses? I am guessing that you want stable 6.6kV voltage because of motor loads during bus under-voltage.

It almost sounds like you need a second form of reactive power control (cap bank/sync). In utility generation, we swing units until the high side condition is met, and that's with DTC's only. In other words, we don't have a second agenda for the low side.

 

[ScottyUK]

Hi Slava,

I disagree that you need separate Var compensation on the 6600V bus because the bus voltage will behave similarly regardless of the source of the Vars, but I can well imagine the difficulties which the operations guys are having.

It sounds like the scheme is configured normally with the OLTC regulating bus voltage and the AVR controlling reactive output. What problems are you seeing - is the AVR operating at one extreme of its range with either the UEL or OEL active?


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi.
Thanks a lot for your answers.
Im now work on the retrofit of OLTC control and transformer protection. Im not worker of this factory. Im start learn what happend and check all according event list of SCADA, what is a number of OLTC operation and manual operation on the AVR, etc..
esee135 and Scotty, you are right in your suggestions.
1. Station transformers on both busses and with additional back up from utilities.
2. Scheme is configured normally with the OLTC regulating bus voltage and the AVR controlling reactive output.

Scotty, I will check what is range of AVR.
Problem is ( as I seeing):
Low cosphi on the HV, opeartion guys add reactive power via AVR, voltage on the bus is increase and OLTC is down this voltage to normal and again cosphi is decrease..and again, and again.. tens time per few hours .

From my point of view, isn't normal situation. I don't like exisisting setting of OLTC, 0.6% ( tap is 1.25%) and time 100sec. I was recommned change setting to 1.5-1.7% and time to 40-60sec, but customer not agree with me. I can block OLTC logic in this situation or move it to manual mode. Lot of options, I try found something right in this situation.

Thanks again.
Best Regards.
Slava

 

[esee135]

I had mentioned separate VAR compensation since the generator AVR's I work on are normally targeted to catering a high side voltage control (reactive compensation or high side bus voltage control) and the LV is flexible.

It sounds like your operators are forcing the controllers to fight each other.

What your description sounds like is the bus voltage is low or (VAR deficient), operators use the unit to raise VAR output, then the OLTC controls taps down due voltage bandwidth exceeded, and the process repeats. How do your OLTC controls work?

 

[ScottyUK]

The scheme certainly 'works' in principle: as the OLTC taps down the bar voltage is lowered, and the AVR boosts the excitation to restore the bar voltage. The secondary effect of doing this is that the reactive export increases.

I think the most likely cause of the problem is the tight OLTC band. Open it up to a value greater than one tap as you have already suggested and see how the scheme behaves.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi.
Thanks again.
Yep, Scotty, I will open band and see.
Actually, I don't want any additional logics, VAR compenstors.

esee135, OLTC control base on the voltage limit, I will add also paralleling work of two transformers on the MCC priciple.


Best Regards.
Slava