What is the advantage of a GSU transformer of 11KV/69 KV Connected to a 66KV grid.Why 69 KV is used instead of 11kv/66 KV

In some places 69 kV is a standard voltage.

The system voltage is 66 KV and not 69 KV.Then why the GSU Voltage is 69KV instead of 66KV

sudiptoghosh,
What is the RATED voltage of your generator unit? Is it 11 kV or otherwise?

11 KV ONLY

If the GSU puts out generated power at exactly the system voltage, it will not be able to deliver MVAR's. At best it might be able to achieve 1.0 power factor without exceeding the generator's voltage rating. The GSU voltage must be greater than the system voltage to enable the generator to supply reactive power.

MVAR's (reactive power) always flow from high voltage to low voltage. If voltages are equal no reactive power flows.

63-69 kV is only a 4.3% increase, within the +/-5% typical generator voltage range and GSU tap range.

I agree with rcwilson,
the use of an higher nominal voltage on HV side of GSU causes a shift of the whole capability curve of the generator along the Q axis.
In other words it will be more easy to supply reactive power to the grid and more difficult to absorb reactive power from the grid.

Si duri puer ingeni videtur,
preconem facias vel architectum.

sudiptoghosh,
In that case one of the objectives of your generator connecting to Utility is to provide voltage support in addition to active power export to the grid.That means
the GSU trafo should have a higher no-load voltage to push reactive power to achieve voltage support as pointed out by member rcwilson.

Thanks to all. But the MVAR will be supplied by the generator and we operate at Lagging Power factor to supply MVAR to the grid.So if the GSU voltage is 66KV instead of 69KV with an OLTC what is the harm

As stated before 69kV is more standard so probably cheaper to buy a "off the shelf" GSU and use the lowest tap on the tap changer

As long as the generator and on load tap changer can raise the 66 kV side voltage enough to push MVARs to the utility system, there may not be a problem. You could do the calculations or simulations to verify that the generator voltage will stay within its +/-5% voltage limits for all probable combinaitons of utility 66 kV voltage level and leading / lagging MVAR support.


There may also be some considerations about over excitation (volts per Hz) if the transformer's rated voltage is less than operating voltage. That problem may be coverd in the OLTC tap design specifications.