Paralleling of autotransformer and normal power transformer? 1

[vidima]

Dear collegues,
Is it possible to operate an autotransformer 74MVA YN,a0d11, 275/66/11kV, 10%, in parallel with a normal three winding power transformer 74MVA, YNyn0d1, 275/66/11kV, 11.2%,? Can an autotransformer ever be operated in parallel with a conventional power transformer? Need help from transformer experts.

 

[prc]

It should be possible,but of course without connecting tertiary together as the vector numbers are different.You have to check the impedances at extreme tappings as impedance variation for auto and two winding trfs can be different.

 

[vidima]

Thank you PRC for that insight. The tertiary for the autotrafo will be left open and the one for the two winding transformer will be used to provide auxiliary station suplly. So you are saying the vector numbers should not be an issue for the 275kV and 66kV windings? What is the effect of this impedance mismatch at extreme tappings?

 

[waross]

Impedance differences will affect the load sharing between the transformers. I would be more concerned with X:R differences between the transformers. The effect of unequal X:R ratios is that the sum of the transformer currents is greater than the load current. I doubt that you will be able to service a 148 KVA load (74 KVA + 74 KVA) without overloading the transformers.


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

 

[vidima]

Thanks Waros. I'm wating for full details on the transformers to compare the X:R ratios and will post these for you to help with the analysis. Regarding the main winding connections(disregarding tertiary), are you saying it shouldnt be a big deal to parallel auto transformer with normal two winding transformer. What other precautions need to be taken into account, viz a viz earthing of neutral points etc?

 

[marks1080]

I would recommend against this. It just seems wrong for some reason. Besides the technicalities auto's are treated differently from power transformers from an operational point of view. You might end up with a nightmare when maintenance time comes.

The vectors, as long as the windings are connected properly would be fine. As previously mentioned though, there are much more concerning issues in terms of X/R ratio, how the system acts for +ve/-ne/zero sequance scenarios, etc...

 

[prc]

X/r ratio will not be an issue with 74 MVA transformers, can be problem with 74 kVA transformers as waross is suspecting.With large trfs r(pu)is negligible compared to x (pu)With Z variation transformer with lower X will get overloaded.The extent of overloading etc,please refer to IEC 60076-8 Trf application guide or any text book.

The Z variation with tap changing will be diffrent depending on the physical position of the regulating winding with respect tp nothers.Hence the need to check the impedance at extreme tappings.

Of course neutrals are to be earthed.

 

[jghrist]

The impedance of an autotransformer is usually much less that that of a two-winding transformer. This will cause the autotransformer to take most of the load.

 

[prc]

jghrist, theoretically yes,but in real practice no.When we compare an auto with two winding trf of same line rating, the active part or size of auto will be co-ratio times the two winding transformer.co-ratio = HV voltage-LV voltage/ HV voltage.So 100 MVA auto of 220/110 kV will be equivalent in weight and cost(theoretical again) to a 50 MVA two winding transformer.So the impedance will be half of 100 MVA two winding transformer.But in real applications, to withstand short circuit forces, impedance of auto trf is deliberately made higher.In the example, quoted by vidima also you can see that auto impedance is lower by only 10 %.Of course to that extent auto will get overloaded .But autos can be made with impedances to the same values as of two winding transformers and large transformers are usually specified like that.