POWER TRANSFORMER DESIGN

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.

 

[peterb]

Should be no problem in getting the appropriate unit. I have come across transformers rated for dual primary voltages (138 kV & 69 kV) for similar generator spare duty, also 11.5/13.8 kV ratings for distribution system spare duty. Transformer manufacturers should have no difficulty meeting your spec for this.
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.

 

[jbartos]

Suggestion: A distinction might be made between the three-winding transformer [Secondary voltage / Primary 13.8kV / Primary 11.5kV] and two-winding transformer [Secondary voltage / Primary voltage 13.8kV tap and 11.5kV tap]. Three-winding transformer appears to be the better solution; however the two-winding transformer is simpler.

 

[peterb]

For this duty, a 3-winding transformer would be an extremely expensive approach - we talking about a 250 MVA unit here. Clearly a case for a 2-winding unit with a tapped LV winding.

 

[jbartos]

It appears that three one-phase three-winding transformers, each rated at (250MVA + 160MVA)/3=137MVA, would allow the protection of generators (two units: 160MVA and 250MVA) by off-the-shelf switchgear. To provide taps on delta connections at high currents, and to switch and protect high currents is posing atypical challenges. Also, the two generators will be galvanically separated.

 

[gmazumder]

definetely possible, a two winding transformer can be designed for your use. Why dont you give some more details of the transformer required by you. also we need some information of the generator and are the impedance levels of both the transformers same.

 

[jbartos]

Suggestion to Anita Gajjar: Please, would you clarify whether or not you would like to have a spare transformer for one generator needing it at a time or two generators needing it at a time. This makes a big difference in the transformer design. The last sentence in your posting needs this clarification.

 

[peterb]

I read the original post to say that one 250 MVA transformer is required that can be used as a spare for EITHER the 160 MVA, 11.5 Kv generator OR the 250 MVA, 15 KV unit, not both together.
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.

 

[jbartos]

It appears that the generators have a potential to generate millions per hour; therefore, it may be worthwhile to consider whether or not to have one transformer on a standby in case that the other one is undergoing service. This situation may use the common transformer for one generator spare capacity, which would leave no spare transformer available in case of a fault within the remaining transformer. Considering the sizes of generators and perhaps quality of existing transformers, and cost of the intended common transformer. Then, the two-generator option to connect to the grid over a three-winding common transformer may be considered. What it would actually count would be the cost difference between the two-winding 250MVA transformer and the three-single phase three-winding transformers of 410MVA total (means 160MVA added, or 64% to have spare transformer capacity for both generators).

 

[peterb]

Real-world design criteria dictate that the generator transformer be located as close as practicable to the associated generator. This is because of the high currents involved (8033A for the 160 MVA,11.5 KV unit, 9623A for the 250 MVA, 15 KV unit), which require that losses be minimized by transforming to the transmission voltage with as short a connection as possible. As the distances involved between two units of this size is likely to be considerable (if they are even in the same generating station), it would not appear to be practical to retrofit a common transformer for generator step-up duty.
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.

 

[jbartos]

Suggestion: Some cost analysis may be needed in this case; especially, if the existing step-up power transfomers are located outside where there is a plenty of space for additional transformers, switchgear and buses. Also, notice that the medium voltage power distribution analysis often neglects impedances of shorter buses, which are negligible in comparison to the impedances of generators or transformers. Also, the cost of transformers in the ~400MVA generation station is less important, when its high availability is required.

 

[ANITA]

The spare transformer is to be used for either of the units in case of failure of any transformer and not for BOTH.

 

[jbartos]

To Anita Dec 29, 2000 posting:
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.

 

[vector9]

It is important to note that typically this is an excellent solution when you have a number of generators of one power rating and couple more of another power/ voltage rating.
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.

 

[electricpete]

Interesting question about storage environment and testing.

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.