Voltage control and Trafo position
Voltage control and Trafo position
(OP)
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?
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?






RE: Voltage control and Trafo position
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.
RE: Voltage control and Trafo position
Do I also have to do any changes on the protection because the design calculations were based on a 14.kv/161kv.
RE: Voltage control and Trafo position
Bill
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"Why not the best?"
Jimmy Carter
RE: Voltage control and Trafo position
RE: Voltage control and Trafo position
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.
RE: Voltage control and Trafo position
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...
RE: Voltage control and Trafo position
RE: Voltage control and Trafo position
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