Three Winding transformer

[kssschsekhar]

Hi all

Separate Generator Step up transformer (GSUT, To step up and export power)and Station Services transformer (SST, For plant auxiliary loads)used to be there in almost all power plants.

Generally these two transformers are two winding transformers

To meet the power plant one can use three winding transformer in place of GSUT and SST.

Why most of the power plants will use two two winding transformers instead of one three winding transformer

What might be selection criteria? why design engineering still not changing with change in thoghts of the people or raliability of the equipment.

 

[Marmite]

Station services transformer provides auxiliary supplies when the unit is not running. Normally a minimum of two station transformers are provided to give diversity of supply with all units shut down.If you combine the two (GSU and SST) transformers you have no station supply when all units are off. If you meant to ask why the GSU and UNIT transformers are not combined into one transformer, then I suspect there are lots of reasons, including transport weight and height, cost, conflicts in design. ie GSU needs low impedance, UAT needs higher impedance to limit fault level on unit switchboard. Both are very specialised transformers in their own right. UAT and SST are sometimes combined on gas turbine stations because the auxiliary load is very small and the SST would be difficult to design and uneconomic for such a small load. This requires a generator switch or circuit breaker ito isolate the generator when it is not running which is costly in itself.
Regards
Marmite

 

[kssschsekhar]

Hi Marmite

I agree few reasons. As you said generally two SST will be there in the system to meet plant aux. loads.

As you are much aware during Generator unit shutdown respective Generator CB will be open and still we can supply aux. load.

Economics wise Three winding transformer will better option comparing to two winding transformer becuase it reduces space, material and equipment.

Weight and height will not a problem in this present world for transformer only.

Anyway I am expecting much better reasons
However one very important point you expressed is Higher and lower impedance each transformer.

Is there other reasons?

 

[slavag]

Good and intresting Q.

If you have big steam turbine, UAT about 10-12% of size of generator.

If you have hydrogenerator, is about 1% of size or less,( if I remember right)

If you have gas turbine, is also about 1% or less.

That means it's trafo design reasons
Im not sure if possible design trafo with 100MW primary/seconadary and 1MW teartiry windings.

Another issue steam turbine, it's more UAT design, usually installed two UAT ( UAT-A, UAT-B), but if you would like only one UAT, for my pinion, possible order trafo something like to 400MW primary/secondary and 50MW teartiry windings.

But, take in account in case of trip on the aux bus, your generator is shutdown.

Best Regards.
Slava

 

[Marmite]

kssschsekhar, I think you need to give more details of the plant you have in mind. For example you can't dismiss the height and weight implications of transporting a GSU for a 500MVA steam turbine with a 50MVA unit auxiliary load.
Transformers with double secondaries are always a compromise. The best technical solution is always to use two separate transformers. The economies from the use of double secondaries arise where is there are worthwhile savings on HV switchgear, or where there are other real reasons, for example physical space availability, for needing to reduce the quantity of HV switchgear.
There are basically two alternatives for the transformer. It can have either close-coupled or loosely-coupled secondaries. To limit fault levels you would need to use a loosely coupled arrangement, but a loosely-coupled transformer will have a high degree of magnetic imbalance, so from that point of view you would need to use a close-coupled arrangement. If the transformer is represented by a T-section network then for the close-coupled arrangement all the impedance is in the upright part of the T, i.e. the primary. With the loosely-coupled arrangement all the impedance is in the secondary, which is represented by the cross piece. One other significant disadvantage of the close-coupled arrangement is that with high load on one of the secondaries, both will experience high regulation.
Regards
Marmite

 

[odlanor]

I guess one big problem is short circuit.
A short circuit in auxiliary system using tertiary winding of stepup transformer as source, is much more than source were 2 winding transformer.

 

[dpc]

Many large generators do not have a circuit breaker on the low side. They are synchronized and isolated using the high side breaker(s). With a 3-winder, this means the generator can't be tripped off without tripping the auxiliary power.

Even with a low side generator breaker, a three-winder makes it more difficult to de-energize and take clearances on the low side equipment for maintenance, and more difficult, obviously, to perform any necessary transformer maintenance.

Three-winding transformers also require more design compromises than two-winding transformers, so will generally be somewhat less efficient.

 

[cranky108]

Interesting discussion, However on older/smaller steam units they did sometimes use a single transformer for the GSU, and aux power. (from the 20MVA units I've seen)

I suspect the reason they don't now is a matter of making the design work. And if the 500MVA, like from above, was a big enough of a problem, they probally would consider three single phase units. But the ligististics apperently aren't a problem. (I'm not a transformer design person).

As said above, the auxilurys for hydro, and GT's are very small compaired to the size of the units. Where steam unit require a more sizeable aux load.

 

[rcw retired EE]

We have used both 3-windings and 2-winding transformers in our power plant designs, but never for a UAT. We have looked at it several times. The largest failure in terms of costs to the client we experienced on a power plant was the failure of a three winding transformer. (Failed three times.)

Three winding designs with windigns of different voltages MVA ratings adn impedances have to have compromises that can lead to failures. When a GSU or UAT fails at a plant, the facility is down until a repalcement or repiar can be made. A two winding is easier to repair/replace than a three winding. One or two weeks lost revenue from the plant is more than the cost savings of using to a three winding configuration. The initial savings are not worth the risk.

Also, on some plant's, the auxiliary switchgear is located some distance from the GSU. A 3-winding unit prevents locating the UAT near the switchgear.

An economically designed GSU transformer does not have much room left on the tank after all of the cooling systems, HV bushings, tap changers, control panels and isophase connections get placed. Adding another set of LV bushings and bus duct connections can make it very congested.

 

[prc]

Piggyriding a small transformer on to a large unit is a favourite exercise for planners.One utility wanted to load from tertiary of a 1500 MVA 765/400 kV auto bank to feed station and nearby village.I persuaded them out of that scenario and take a separate 33kV distribution line for a couple of kms.Any line fault on their 33 KV distribution line can take away 1500 MVA power flow!To meet that 2 MVA load they were making main transformer spec too complicated.

Only in Scandinavian countries such small windings are specified in large transformers to meet station loads.Such designs are possible.

In 50's Brown Boveri reported supplying a generator transformer with auto transformer -the unit working as a GT and interconnecting unit simultaneously.

Three winding transformers with two 50 % secondaries if specified (taking care impedance matching)and designed properly will give reasonable reliability and are extensively used for SST ( to reduce fault level in secondary) in steel mills ( to meet power at two MV voltages) and in GTs taking power from two generators.

Apart from the points mentioned in discussions,when we combine two trnsformers of X % reliability ,combined reliabilty comes down to X square %.So in a power station it is not advisable to compromise the GT reliability by adding UAT winding in to as the load ratio is very high and individual voltage regulation and control become difficult

 

[slavag]

Hi.

prc saied about it.
GSU is usually Xfr w/o OLTC, UAT is always with.
With big steam turbine and two aux buses, voltage control on the aux buses will be very dificult, if you will use three winding Xfr.

However, in industrial power plants, where generator connect directly to aux and factory bus( 6.6kV or 11.5kV) with cogeneration to grid, Three winding Xfr, for my pinion, is good solution.

Best Regards.
Slava

 

[royclh]

Hi,

Thank you for an interesting posting about Three Winding Transformer. I have an question regarding the choice of the transformers used at 150kV offshore wind farm. Hope some experts out there can enlighten me as I'm pretty new in this area.

I've encountered commonly in several wind farm designs, two 33kV WTG sections are jointly connected to one secondary side of the two winding 150/33kV offshore transformer. Sometimes, I also came across that the developer went for three winding transformers instead by connecting the 33kV WTG sections separately to the two secondaries.

What are the factors that decide whether we should go for two or three winding transformer? Some told me is the fault current contribution from the WTG and the grid. If this is the case, at what fault level we should consider using the three winding transformer instead? What is the cost implication?

Your advice will be greatly appreciated.

 

[cranky108]

I've seen another utility that used three winding transformers. I suspect to keep fault currents down without incuring added losses from higher impedance transformer.

Here we add impedance to our transformers to keep the distribution fault currents below 10ka. But we don't use three winding transformers because of concerns of higher failure rate.

 

[kssschsekhar]

Hi all

I did not expect this much response. Now I am giving my actual Challenge

Total 10 Steam turbine needs to be installed. So automatically 10 steam Generator transformers will come.

Please dont expect 5 GUT becuase distant between the steam turbine is very lengthy.

Each GUT is expected rating will be around 130 MVA. Here Plant needs Auxilliary loads for one particular purpose.

The purpose have 4 x 26 MW load each. So Now I am thinking of useing 4 nos. two winding GUT and 4 nos three winding transformer to meet plant requirement.

Transformer desinger can do three/two winding transformers. That is not their constrint. However Before going to prepare specifications I started thought process in that process i started this thread.

Bye the bye three winding transformer specs planned is as follows

HV = 400kv, LV1: Generator voltage (15.75kv); LV2: 11.5kv

I am expecting other comments if any

 

[slavag]

Hi.
Xmm, not possible three winding GSU, but for aux load, long buses isn't problem?

Not used some RAT or two in such application?
What about voltage control on the aux. buses. You need three winding Xfr with OLTC.

Maybe better, need check of course prices, use two 3- winding transformers 400/11.5kV 63MVA with OLTC and high impedance. But it request two additional bays of 400kV ( so expensive)

IMHO.

Best Regards.
Slava

 

[7anoter4]

Usually in a power station you don't need 3 winding transformer –only if you have equipment rated of two voltage levels –let's say 7 and 11 kV. But it is simpler to
have one step-up transformer at the same power as the generator and for auxiliary power 2 transformers-if you have redundant equipment –pumps, fans and so on- supplied directly from generator bus bar. If you need two voltage levels- for BFP, IDF or FDF let's say 11 kV and 6-7 kV for the all other equipment you need a third transformer supplied from the generator bus bar.
If you have a generator breaker for start you may use the main step-up transformer for start. If you have not this breaker you need a SAT-Station Auxiliary Transformer supplied directly from the High-Voltage Grid in order to start the units.
The size of the auxiliary transformer depends upon the required power [as already said in the above posts]-for a fossil power station-may be heavy or crude oil or coal- you may have many auxiliaries as Coal Handling Facility or Tank Farm and filtering installation as Desulphurization, Ash Handling and so on. The Gas Turbine –with Combined Cycle or without has different required power.
In a Steam Generation Power Station the auxiliary transformer will be 2*[4-7%] and if no Generator Breaker is provided a SAT of 4.5-7% should be provided.
Gas Turbine with CC 2 Unit Auxiliary Transformers of 4% and a SAT of 11% should be provided.

 

[prc]

Details on theory of three winding transformers can be found in the classic transformer book "Transformer Engineering " by LF Blume,1951.( see the chapter by A Boyajin who first analysed this issue in 30's in an AIEE paper)Latest aspects from the angle of selection and specification of three winding transformers can be seen in ABB book on "Short Circuit duty of Power Transformers.

As I earlier said three winding GSUs are popular in South America where they use one GSU to collect power from two generators.In India three winding station supply transformers are widely used in Thermal power stations for the past 35 years.Earlier the size was 50 MVA 220/6.9-6.9 kV.Now sizes have gone up to 160 MVA 220 kV and 100MVA 400/11-11kV.Here we have seen three winding transformers up to 210 MVA /160/50 MVA 220/33/11 KV for steel mills.

Important point is the selection of impedance for various pairs.Reputed manufacturers will help you in this.Three winding units can be made with axially split secondaries(ie one above the other) or radially split (ie HV at middle with secondaries on either side) All these will have different characteristics ie regulation,fault current distribution etc.Some combinations will result in regulation at L2 when L1 is loaded.You can get more details from ABB book.