Transformers vs. Autotransformers 3

[evfj]

I am planning a new substation 220/138 kV Star/Star 300 MVA and I must choose between two options:
a)2 three phase units 150 MVA each
b)4 single phase 100 MVA each (one stanby)

Y appreciate a help to make the best decision:

1)Costs of solution a) and b)
2)Transformer vs. autotransformers
3)Size and voltage of delta tertiary winding
4)Which solution has greater kVAR losses
5)There are risks to the transformer by connecting a capacitor bank to the tertiary winding?

 

[cranky108]

Have you talked with any transformer manufactures? Or is this a class project?

 

[evfj]

Yes, I'm doing.

 

[desrod]

Solutions are not the same. What type of load are you supplying?

If you loose one of your 3 phase Tx then your capacity is down to 150 MVA for a year. If you loose a single phase, your are back at 300 MVA within 24 hrs. What is your design target? Is there a value in that?

You should compare 2 x 300 MVA 3 phase vs 4 x 100 MVA single phase if you need 300 MVA firm capacity.
Also why use cap banks on the tertiary?

Speak with the manufacturers.

 

[jkristinn]

Normally an auto-transformer would be cheaper for this rating, but design issues (such as tapping requirements, loading of tertiary) could alter that.
In general the tertiary should be rated 33% of full MVA to cope with imbalance.

 

[cranky108]

If you use single phase transformers, you won't have a tertuary.

Consider an auto transformer only has one winding per phase, where as a two winding transformer hase two. This may or may not have anything to do with the factors that affect a tertuary.
Many of the three phase auto transformers we have don't have a tertuary winding, per say. They have a phantom tertuary.

Cost is not only copper (or alumumium), but also core steel, oil, and steel case, and accessories. Also shell form and core form make a difference.

Many tertuary windings, in older transfoemers, only had about 35% capacity, which is one limit. The other limit is over exciting the transformer.
Most tertuarys I have seen are in the 15 kV class, but I have seen a few as low as 4.16 kV.

There are risks with using the tertuary winding, and a good text book should explain that.

 

[prc]

Questions raised by evfj. The same questions are discussed by power engineers for the past 100 years.There were many papers in AIEE during first half of last century on single phase or three phase and on many occasions figures were put up against your questions.Surprisingly up to 1940, US was very reluctant to go for 3 phase units while Europe went ahead for 3 phase units by early days of transformer engineering.

Now specific answers:

1)Whether auto-transformer or two winding transformer, the transformer cost will be as below:

300 MVA Transformer -100 %
2X150 MVA Transformer -115 %
4X 100 MVA Transformer -170 %

2)For this voltage ratio,normally only auto-transformers are used except in certan grids where solid earthing is not used.
A 300 MVA auto transformer of this voltage ratio is equivalent to a 2 winding transformer of rating 112 MVA ( 300 X220-138/220) with respect to size, weight ,losses and cost.But normally this advantage is not got in price due to presence of on-load tap -changer and tertiary winding.So price of a 300 MVA transformer may be same as 300 MVA 2 winding transformer, but will have losses comparable to that of a 112 MVA transformer.

3)Stabilising tertiary winding is a must when single phase transformer banks are used.Of course there are some exceptions esp in US for this.In three phase units stabilising winding is required when the core is 5 limbed ones. In two winding 300 MVA units,5 limbed core will be required due to transport restrictions.But Italy has some large auto-transformers without tertiary but with 5 limbed core.
In transformers with 3 phase 3 limbed core, stabilising tertiary can be eliminated as the tank will act as a winding for induced currents from third harmonic leakage flux.Size of stabilising winding shall be one third of main winding. ( ie one third of rated MVA in case of 2 winding transformers and one third of equivalent MVA in case of auto-transformers) But generally auto-transformers are also provided with tertiary of one third of line MVA for increased safety margins against short circuit current forces.The voltage is decided based on the nature loading coming on tertiary. In China for 3000 MVA 1100/500KV auto-transformer banks they opted for 69 kV to suit compensation load. Generally for stabilising windings, it is 11-33 kV, but actual insulation level of tertiary winding and bushing will be more than rated kV BIL, to take care of transferred surges from nearby windings.

4)Losses will be as below whether two winding or auto units:

Item 3 single phase 2 P/2 3 phase P 3 Phase
No-load losses 126 % 115% 100%
Load Losses 100% 115 % 100 %
Transport weight of
Heaviest package 40 % 58 % 100 %

5) No risk to transformer with tertiary capacitive loading. Manufacturer can design Auto-transformer for 300/300/100 MVA rating with simultaneous loading as indicated,. ie line MVA on primary will not exceed when secondary and teriary are loaded to 300( unity) +100 MVA( leading pf)