power transformer auxiliaries. 1

Is there any way we can avoid using station auxiliary transformer?

Click to expand...

Why?
Will your 220kV line be able to handle the inrush of the main power transformer?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

There is some confusion in the terms used for describing the transformers connected to a generator. Let us make it clear
1) Unit Transformer (USA) Generator Transformer (UK)
2) Station Service Transformer (USA) Station or station start-up Transformer (UK)
3) Unit Auxiliary Transformer (USA) unit Transformer (UK)
If you say just station auxiliary transformer, it is usually meant the distribution transformer used for station lighting and other auxiliaries used in a substation or power station. The unit auxiliary transformer is the one with input from generator output and feeding auxiliaries for power generation (boiler feed pump, air blasting fans for boilers, etc) at MV voltage.
Ref- IEEE std C57.116-2014 Transformers directly connected to Generators; J&P Transformer Book ed12.0,1998, Chapter 7


You can avoid station service or start-up transformer by using a generator circuit breaker. These are special 11-36 kV high current (10-25 kA) breakers with rupturing capacity above 100 kA. With GCB between generator and bus in the open position, the generator transformer is energized by back-charging from HV bus. The LV side of GT then feeds the unit auxiliaries through the station auxiliary transformer. After the generator picks up the voltage, synchronization to the grid is done by closing the GCB.

the generator transformer is energized by back-charging from HV bus.

Click to expand...

Hence my question. You may not be able to back energize the main transformer from the 220 kV side.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

There is a limitation in the scheme prc detailed. The unit will go without auxiliary power in case of generator transformer or generator busbar fault as both generator and generator transformer will be tripped in such a case.
The solution could be providing a tie feeder from the MV bus of the next generating unit.

1) Sure, transformers can be energized from 220 kV or 400 kV side. In fact, the inrush will be less as the energizing winding is the outermost. Two points can be of concern that can also be taken care of - mal-operation of the differential relay due to low percentage content of second harmonics in inrush current & transferred surges to LV winding of GT. Back charging shall be done only with LA and Surge capacitors of GCB or of Generator (in case GCB is not used)

2) Raghu, I am not sure. This scheme with GCB is widely used around the world, including in India. In case of a transformer fault, GCB will be opened and the auxiliaries will continue to get power from the Generator, running at no-load.

Sorry, Raghu. I went wrong. I don't know how they manage the situation you pointed out.

Thanks, prc.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

Hum
I think that the question could be re phrased as where does a power station obtain its auxiliary power supplies from?

In the UK in the 'old days' you would have say 3 x 10MVA generating sets (steam turbine) generating at 11 KV supplying a town and a 'house set' of 1 MVA generating at 400 V.

Translate for USA and other voltage systems.

All the low voltage systems, lights, boiler feed pumps whatever would be supplied by the 'house set'.

Larger more expensive systems might have two house sets and or an 11kv to 400 V step down transformer or a supply from the local utility. There are many ways to achieve this.

In the UK we decided in the 1970's to have generation from landfill gas, these were often remote sites (who wants a landfill site near their residence?)

So, you could get an 11 KV connection for the power export but no LV connection for the auxiliaries.

What to do? The sets were often 1000 kw and generated at LV, with a step up transformer (another story…) from LV to 11KV.

So, you took the auxiliaries supply from upstream of the generator breaker as the transformers were permanently energised, and usually via fuse switches.

Generally, the genset auxiliaries were supplied by each transformer and site supplies were manually switched between sets.

I can post a diagram if requested.

My experience with combined cycle plants is that the plant carries it own load off a unit aux transformer. I've seen them connected to both the CTG and STG sides of the GSU (or multiple unit aux transformers in the case of a 6+1 plant). There is then also a plant standby transformer with a source of supply entirely unrelated to the transmission connection to the GSU. Standby transformer is almost always energized but unloaded, standing by. The critical plant support systems then transfer to the standby transformer on loss of the transmission outlet of the GSU. The most critical plant support systems are those that used to cool the plant down. The condensate system may be the most important as blowing rupture disks turns a bad day at the plant into a really horrible day.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

Standby transformer is almost always energized but unloaded, standing by.

Click to expand...

Rarely the practice in my experience [ but that's just mine ]; I'm almost universally familiar with all loads common to site, like battery chargers, ventilation systems, compressed air, sump pumps, air conditioning for control room if provided, et cetera and so forth, and half of the luminaires across the plant, all being serviced [normally] from one or more grid-supplied "reserve station service" trafos. The other halves of the lighting circuits were also fed from the RSSs whenever the associated units were idle but would be transferred onto the "unit station service" transformers once the unit in question was on line and loaded to > 10- 20% of that unit's maximum continuous rating.

In the event of planned outages, contingencies or out-and-out grid black-outs, the reserve-fed loads could also be carried from the unit station service transformers of any in-service / surviving units, flexibility, redundancy and reliability being the watchwords.

Different strokes for different folks . . . or perhaps different sensibilities for different utilities . . .

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

For the units I was referencing, the generator owner is not the local distribution company. Power off the unit aux is just “losses” while power off the standby produces a power bill from the local distribution provider.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

It may or may not, depending where the metering is performed . . . if @ POCC then different [higher] price will simply be charged for incoming power than for generated power [export]. Power off the standby may not therefore necessarily be separately billed when the unit is operating.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]