284 MVA Auto Transformer

[MYAMYAMYA]

Hi All

During the design of a project by our company, we came a cross a requirement of two Autotransformers as below from the client:

170/226/284 MVA, 230/69 kV, ONAN/ONAF/ONAF, YNa0d1 , Z=(HVLV)=12.9 / (HV-TV)=27 % / (LV-TV)=27 % (170 MVA BASE), Step-Down Autotransformer

Positive Sequence impedance:

High Voltage to Low Voltage: 12.9%
High Voltage to Tertiar: 27%
Low Voltage to Tertiar: 27%

Zero Sequence impedance:
High Voltage to Low Voltage: 12.9%
High Voltage to Tertiar: 200%
Low Voltage to Tertiar: 200%

Are these requirements achievable ? especially the zero sequence.

Thanks in advance

 

[prc]

It looks like it is an auto transformer with neutral end taps for LV variation (variable induction type) with a neutral reactor (external or internal). Why more zero sequence reactance is required? A bit strange requirement.

 

[prc]

I got some more details from an AIEE paper PAS 86-8 1967 "EHV Application of Auto Transformers" by G W Alexander. (page 997) This type of neutral reactors are used in lower voltage systems where X0/X1 is less than 1.0 and hence LG fault MVA may become more than 3L fault MVA, exceeding the breaker rating. Reactors will require LA protection.

 

[FPelec]

It is an autotransformer with an additional reactor connected between neutral and ground. IMHO there are very few reasons to include a tertiary winding on such an autotransformer: being a variable flux transformer the tertiary winding could not be used for feeding a local load because of voltage fluctuations (you could still use it by using an additional booster transformer). You could instead achieve the same zero sequence impedance with a 3-phase 3-limb unit, without delta connected tertiary.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[Mbrooke]

FPele- while you are correct, I disagree. I have seen many, many tertiary wingdings feeding load. Although I do not know the specifics specd in their construction- I have seen it down and it does indeed work within acceptable limits.


As for the OP- assuming the client really, really needs this I'd call several manufactures. Someone somewhere should have the capability to make a unit like that.

 

[prc]

Mbrooke, There is not much special about it. All reputed manufacturers will be able to make it. The spec calls for a tertiary, may be stabilizing one because of 5 limbed core construction. Neutral reactor added to get more zero sequence impedance and to improve X0/X1 ratio.