Voltage control and Trafo position 1

[niilaryea]

I run a power station with generator rating 110MW 0.8 pf, 14.4KV which is connected to the grid at 161KV through a 165MVA 14.4/161kv transformer and synchronize at 14.4kv.
During peak hours the corridor experiences low voltages although we try to keep up our vars to improve voltages the system voltage still remain low however the low side 14.4kv side is able to achieve voltages of 15kv and above but the system voltages still remain low at 156-7kv. My tap position is at tap 5 14.4/161KV. I have been asked to change my tap position to tap3 at 14.4/165kv. My opinion on this is during peak hours the var demand exceeds the var supplied hence what is required is additional var supply to keep up the voltage and not a cosmetic tap change. however I have been meet with very stiff opposition and i have no choice but to change the taps.
I want to know if this is the right way to go and what are the implications?

 

[rcw retired EE]

Changing to tap 5 raises the GSU transformer output voltage 2.5%. Since the utility voltage doesn't change much, the effect will be to lower the generator bus voltage 2.5% for the same output.

If the generator MVAR output was previously voltage limited, the generator will be able to deliver more MVAR. By voltage limited I mean the generator maximum voltage of 105% was reached before the maximum MVAR output on the reactive capability curve was achieved. If that was the case, the tap change will provide more room to raise generator voltage and produce MVARs.

The tap change may cause low voltage problems on the plant auxiliaries, especially during startup when the generator is not on-line. The auxiliary bus voltage will be 2.5% lower.

 

[niilaryea]

Thanks for the information,the other explanation i needed is , why do I normally achieve high voltages on the generator side but on the system the voltage still remains low, for example usually for the current tap position 14.4/161kv i achieve 15kv but the system voltage is still low about 156/7kv.
Do I also have to do any changes on the protection because the design calculations were based on a 14.kv/161kv.

 

[waross]

Voltage drop in the GSU.

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

 

[niilaryea]

Thank you again, I guessed so initially too, what baffles me is during off peak periods the drop is very small,and the slightest increase in MWARs result in a significant change in system voltage, however during peak periods the drop turns be very big. Any reason for this?

 

[rcw retired EE]

Look at the Thevanin equivalent circuit of the utility system for your two operating points: light load versus heavy load. Your system does not change, but the utility does.

At light system load, your generator is probably a larger % of total system and can push enough MVAr's to raise the local system voltage. During heavy system loads, your plant is not big enough to budge the system voltage.

Try modeling it in Easy Power, Etap, or other program, with your transformer connected to an infinite bus. No matter what GSU tap you set or what voltage the generator puts out, the system voltage never changes. Only the MVAr flow changes. This is the extreme example of a small generator on a large system.

Part of what you are seeing is the system impedance changing between you and the imaginary infinite bus.

There are other effects due to MVAR/Voltage/Load interactions that are occurring but I need to leave that explanation to someone who understands them.

 

[m3ntosan]

If the system operator will coordinate a simultaneous action, that would help to bring the volts down/up. I don't know how it works in your network, if you change taps on GSU and wait for AVR to respond or change AVR set point directly, to achieve MVAR instructions or voltage targets, but if you agree under system operator's coordination, each power station to take one tap down/up (if that's the case) at 10h00, you'll see a significant change in voltage on the HV network without using so many MVARs on the machines.
If this is a known problem maybe they need to push the volts high in the morning before the pick up, to get the most from the system gain, BV2, works better with underground cable networks.

Consider a 400/132 kV step down transformer in a very weak 400 kV system, when raising 132 kV volts, you would expect 400 kV volts to go down as well, in a strong 400 kV network you'll see no change at all, only if there are no MVAR reserves available on the 400 kV side, and all ATCC will try to tap at the same time and crash the 400 kV system, like in France 1978 blckout.

Regarding the protection adjustments I would say the deviation from nominal voltage is not that large, if you have any protection using impedance elements, maybe that needs attention since depends on squared voltage

May you grow up to be righteous, may you grow up to be true...

 

[niilaryea]

rcwilson i appreciate your explanation , i will still be interested in the effects due to MVAR/Voltage/Load interactions that are occurring, i want to really understand this , any links will also be appreciated.

 

[rcw retired EE]

Look for articles on voltage collapse, such as

http://www.pes-psrc.org/Reports/Voltage Collapse Mitigation.pdf.