POWER TRANSFORMER DESIGN
POWER TRANSFORMER DESIGN
(OP)
IS IT POSSIBLE TO DESIGN A SINGLE GENERATOR TRANSFORMER OF FOLLOWING RATING:
250MVA,50HZ,12.5% IMPEDENCE WITH TWO LOW VOLTAGE LEVELS:
11.5KV AND 15KV.
We WANT TO BUY A COMMON SPARE TRANSFORMER FOR OUR TWO UNITS:
160MVA,11.5KV AND 250MVA,15KV.
250MVA,50HZ,12.5% IMPEDENCE WITH TWO LOW VOLTAGE LEVELS:
11.5KV AND 15KV.
We WANT TO BUY A COMMON SPARE TRANSFORMER FOR OUR TWO UNITS:
160MVA,11.5KV AND 250MVA,15KV.






RE: POWER TRANSFORMER DESIGN
Assuming that you have bus duct connections for the LV, you will need to spec the physical arrangement carefully to be sure that the transformer will line up with the bus duct for either voltage. Also, you may need to consider the takoff for the unit service transformer, if the bus duct connection for this comes from the main transformer.
RE: POWER TRANSFORMER DESIGN
RE: POWER TRANSFORMER DESIGN
RE: POWER TRANSFORMER DESIGN
RE: POWER TRANSFORMER DESIGN
RE: POWER TRANSFORMER DESIGN
RE: POWER TRANSFORMER DESIGN
This is a legitimate cost-saving approach, useful in cases where the transformer design can be coordinated to allow for the different physical arrangements of the two units. I have seen it successfully applied where the generators were essentially the same rating, but applied at differing primary voltages - the spare transformer has actually been used to replace a failed unit. The lesson learnt from that experience was that all physical factors must be taken into account and any adaptor hardware needs to be prepared ahead of time - this is considerably easier than having to fabricate busbar sections when under heavy pressure to get a generator back on line.
RE: POWER TRANSFORMER DESIGN
RE: POWER TRANSFORMER DESIGN
Another point that would have to be addressed with a common transformer is that of generator switchgear. With the unit connected scheme, there is no LV generator circuit breaker. In order to convert to a common transformer, LV CBs would have to be added to each generator output. While such breakers are available, they require space which will likely not be available in a retrofit situation - they are also extremely expensive. All in all, re-designing a major generating station to accommodate a common 3-winding transformer would not be a practical undertaking.
As regards transformer maintenance, this is usually carried out when the associated generator is itself down for maintenance.
RE: POWER TRANSFORMER DESIGN
RE: POWER TRANSFORMER DESIGN
RE: POWER TRANSFORMER DESIGN
Thank you for the clarification. The 2-winding 3-phase 250MVA power transformer with 11.5kV and 15kV LV taps will suffice with all advantages and disadvantages stated and unstated above. It is by your choice.
RE: POWER TRANSFORMER DESIGN
So normally we assume that this transformer will be placed in a storage yard, sometimes unused for years.
Considering this what is the best method of storage ? Should we power it up at a lower voltage ? or maybe use some heating devices ? will it ensure the dielectric strength of the insulating oil ? We will assume the enviromental to be humid and tropical.
RE: POWER TRANSFORMER DESIGN
From what I have seen of manufacturer's recommendation for long-term storage of a generator stepup transformer:
store it oil-filled with bushings installed. Run the pumps/fans every few months. Sample for DGA at least yearly and Doble test every 3 years. Visual inspection monthly.
Of course, they're not paying the bill.