Generator and Transformer in Power Plant 2

[UsmanSindhu]

Dear All,

I have a question that in our power plant we are using Siemens transformer of 175-MVA with Off-Load Tap Changer. Few days back, we have De-synchronized the machine and changed the tap position from 5 to 4 means we have increased the voltage from 533-KV to 545-KV. This we have done in order to meet the requirement for MVARs for the National Grid. I still can not understand how we are increasing the MVARs by doing this as the power output will be the same but the current will decrease and Voltage will increase.

Secondly, by changing the taps of Generator transformer what will be the effect on Generator. Is their anything that is causing problem for generator life?

Thanks

 

[waross]

For a grid connected generator:
Throttle position controls MW output.
Voltage setting controls MVAR output. The voltage setting may be varied with either the generator field or the transformer taps.
Starting from a low voltage setting; as the voltage is increased the power factor will increase towards unity and the current will decrease. As the voltage is increased further, the power factor will pass unity and then drop. As the voltage is increased the power factor will drop further and the current will increase further.
Unity power factor is the least current. As the power factor drops the current increases.
Greater current means more heating of the generator.
For islanded sets things are different.

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

 

[gerbii]

Hello
Changing taps on HV side does not affect LV side (generator). Power is still the same. There will be some increase in losses in transformer. Dielectrical strenght of transformer will be slightly lower (higher voltage) but short circuit withstand capability is increased (lower current on HV side). All this are minor changes, all included in design - it's safe.

As for the generator.. changing tap won't affect it at all. You can sleep well

Place for some MVAr savings is the grid. Higher voltage means higher line capacitance -> MVArs added. Higher voltage means lower current and Joule losses. I'm not specialist in grid issues.. that's all I know

"If an experiment works, something has gone wrong"

 

[mgtrp]

As for the generator.. changing tap won't affect it at all. You can sleep well

Changing the transformer tap will affect the generator. When trying to export more MVArs from a generator, aside from the generator limits shown in the generator capability curves, you will often be limited by the rated voltage of the equipment connected to the low-voltage side of the transformer - as the generator MVArs increase, so to does the generator bus voltage. Eventually, you will reach your voltage operating limit. An additional complication is that as the generator voltage increases, its ability to export MVArs decreases. Changing the transformer taps can lower the bus voltage back down again, so the generator MVArs can be increased further.

The similar situation occurs in reverse when trying to import MVArs, as the voltage on the generator bus decreases.

 

[rcw retired EE]

Vars flow from high voltage to low voltage. To increase MVAR output, raise the effective plant output voltage relative to the system by either changing the generator voltage or the GSU tap ratio. Either of those actions raises the plant's effective output voltage.

One possible problem is low auxiliary bus voltage when the generator is off, system voltage is low and the plant is backfeeding from the power system for start up. The higher tap on the step-up transformer makes the generator and auxiliary bus voltage lower than before when the generator is not on line. That might cause motor starting voltage drop problems.

 

[gerbii]

Changing the transformer tap will affect the generator. When trying to export more MVArs from a generator[..]

sorry, forgot to add: at constant power supply (only hv tap changes)

"If an experiment works, something has gone wrong"

 

[gerbii]

One more final word, to be more precise.

Just as mgtrp said, in case of regulating MVAr flow (increase production) there will be voltage increase on gen side (see synchronous machine V-I plot).
In that case, increasing HV tap on transformer (HV side voltage is fixed by the grid!) you are lowering LV side voltage, and you are capable to produce more VARs without exceding LV voltage limit.

Increasing reactive power production means that generator will work in different point of it's characteristic. There are limitations ofcourse that shouldn't be exceded for long period of time. As long as this is maintained all should be ok.

In addition, it's not this case, much more dangerous fo generator is running on lagging factor (generator is not a source but receiver of reactive power). And again this is limited by generator characteristics.

"If an experiment works, something has gone wrong"

 

[ScottyUK]

A lagging generator is a source of reactive power. An under-excited machine has a leading power factor.