3 transformer supply configuration

[HamidEle]

Our project is increasing the capacity and the exisitng two main transformers ( 260/35KV) will not be able to support the additional loads. One option is to add one 260/35KV transformer. I wonder if anybody here has experience with 3 transformers running in parallel. The client requires to have the 35kv tie breakers closed, but this would exceed the s-c rating of 35KV SWGR. How can we balance the loads on these three transformers?

I would like to have discussions with the experts in this forum.
Appreciate it in advance.

 

[ScottyUK]

Three transformers in parallel isn't uncommon. Exceeding switchgear rating is reckless and dangerous.

You could consider interbus reactors or similar to reduce the fault level to an acceptable value. If you don't want to do that then you are essentially left with either using a 4-from-5 interlock scheme (assuming three incomers, two bus section switches), or replacing / uprating the switchgear.


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If we learn from our mistakes I'm getting a great education!

 

[HamidEle]

ScottyUK,
Can you elaborate a bit more about interlock scheme? There would be three incomers with two ties, but how could we keep all the ties closed without exceeding fault levels?

 

[ScottyUK]

The interlock would ensure that you always have at least one open breaker, thus ensuring the fault level remains within an acceptable limit. It doesn't meet the requirement for three transformers connected onto a solid bus.

If your client is insisting that the system be run as a solid bus with all three transformers in service then their options are reduced to either replace the switchboard, or use interbus reactors. A slight variation on the latter might be use a fourth smaller board as a tie bus with the existing gear connecting to it in a star configuration.


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If we learn from our mistakes I'm getting a great education!

 

[HamidEle]

With 2 transformer supply, we split the load half by half. How would you split the loads with 3 transformer supply? 33.3% under each transformer?

 

[waross]

If the transformers have equal PU impedance they will share in proportion to their KVA rating.
If the PU impedances are equal but the X:R ratios are not the same they will still share the load in proportion to their KVA ratings but the sum of the individual transformer currents will exceed the actual load current slightly.

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

 

[HamidEle]

We are planning to add the same size of the transformer. That way, each transformer shares the same loading. Or we can call it N+1 redundancy design. The loads would be shared eaqually.

 

[luffy2011]

HamidEle,

Simply order a transformer with the same specifications as the existing units. Once in parallel, the transformers would just divide the load almost equally among themselves.

For your concern on increased fault level, see ScottyUK's reply.

http://www.powerqualityworld.com

 

[rcw retired EE]

Tie the three busses with tie breakers equipped with current limiting explosive fuses. (G&W cLip is one supplier). Fuse assembly is shorted by a bus bar with shaped explosive charges that are triggered by overcurrent sensors. Teh charges blow the bus apart, forcing current through the fuse which then clears. Total time is about 5 milliseconds.

I just worked on a plant that used this approach. I was skeptical but it looks like it will work.

 

[Mubeenkhankashi]

@Hamid: As per theory, if the three transformers have same Percentage Impedances (% X/R) Ratios, then the three transformers will share load according to their ratings. i.e KVA1: KVA2: KVA3

As per theory, following conditions must be satisfied if transformers are to be operated in parallel:

1. The transformers must be properly connected with regards to their polarities.
2. Voltage Ratings of both primaries and secondaries must be identical i.e the Transformers MUST have the same Voltage Ratio.
3. The percentage Impedances should be equal in magnitude and have the same X/R ratios.
4. With transformers having different ratings, the equivalent impedances should be inversely proportional to the individual ratings.

Condition 1 is absolutely essential. 2 & 3 can accommodate some sort of tolerance as the parallel operation will be possible but not without compromising some circulating currents. The X/R Ratios of the three transformers must be equal in magnitude to avoid circulating currents between transformers.

For transformers having same X/R rations, the load will be shared by the transformers in the ratio of their KVA ratings.

I have been working in a Substation with three transformers operating in parallel and the station is in ruins and the transformers have been operational for the last 20 years. Everything is going on nicely.

 

[Mubeenkhankashi]

@Hamid: Luffy said the right thing... Have the simplest approach Three transformers with same ratings and Voltage Ratios and percentage impedances,,,, You kill all worries.

 

[HamidEle]

@Mubeenkhankashi,
Was it the origianl design to have three transformers in parallel? Can one transformer support the entire plant load if two transformers are down or two transformers can support the entire load if one is down? The latter option is less expensive.
I am assuming the worst case scenario is two transformers operate with one out of service.

 

[Mubeenkhankashi]

@Hamid: Brother, everything depends upon your load... If you can give the details of the load and the MVA rating of the Transformer, then I can have a better insight. I can give you an example....

Brother, parallel operation of transformers is always a reliable operation and when the conditions are fulfilled, Parallel operation of transformers is very safe and durable even though it becomes costlier. Lets assume both your scenarios:

1. If only one transformer Unit is installed---> If God-forbid, the transformer gets damaged due to some internal fault, the entire transformer will go into an outage resulting in complete shutdown of the plant. The one transformer unit is bigger in size due to high rating and so lifting and carriage charges will be more and obviously repair will be a difficult job. The company will have to face lots of losses in production.

2. If three transformer units are installed and one of them get damaged, then the plant can still operate lesser number of units i.e 2/3 or 66% of the plant load will be operational and the company will not go into substantial loss. The one transformer unit which gets damaged is lighter in weight and so easy lifting and carriage and repair is a swift process.

This is the general trend throughout the world. So all you have to do is to take care of the above conditions for parallel operation which can be superficially done easily just by noting down the ratings on the Rating plates of the transformers or practically by doing SC, OC impedance tests in the lab of the manufacturer. Best of Luck...

 

[slavag]

What ScottyUK saied on the 28 Sep 11 15:10.
Please pay attention on the rcwilson's post about special fuses ( possible Clip or ABB Is Limeter). You can get a good solution in combination interbus or infeed reactors with such fuses.
BTW, is not simple fuses, its included some logic for operation.

 

[HamidEle]

I would add another transformer with the same rating, impedance, and X/R ratio. To reduce the short circuit, we would add an interbus reactor.We probably worried too much, because we have no the other choices.Thanks anybody.

 

[HamidEle]

Mubeenkhankashi,
Another beauty of running three transformers in parallel is , suppose three equally rated transformers share the load under normal conditions, if one transformer is down, the rest of two would share the load. The senario when two transformer down should not be considered. Under normal condition, ony 1/3 load is under each transformer, the life expectancy of the transformer should be longer than 2 transformer supply condition. Trying to calculate the reliability of three transformer supply and compare with two transformer supply. Any comments on this?

 

[waross]

If the impedances are not equal, the KVA base of the transformer with the highest PU impedance may be adjusted so that the pu impedances are equal. The transformer will share the load based on the adjusted KVA base.


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

 

[HamidEle]

The impedances will be exactly the same for each transformer.

 

[HamidEle]

Our boss still has concern of runnung three transformers in parallel. His concerns are:

The bus ratings could be too high;
The control and protection of three units in parallel is more complex;
It is not a common practice in this industry, since Main-tie-Main is very common in this industry.

Are they the real concerns?