Three winding transformers with tertiary not connected (harmonics?)

[guiyermo]

Hello all,

I have few questions about three winding transformers and harmonics that I would like to get some help from you.

I have seen three winding transformers wye-wye-delta with the delta not connected to any load; the neutrals of the “wye” are grounded and these transformers are used to supply an industrial/mining facility. I’ve heard that this is to trap zero sequence currents in the delta which come from the secondary (wye) and avoid letting them flow upstream through the primary (wye).

My questions are the following:

1. If a wye-wye-delta transformer is used for “trapping” third (or other triplex) harmonic currents wouldn’t it be cheaper to install a regular two-winding transformer with harmonic filters tuned to the specific harmonic current? Moreover, wouldn’t it be cheaper to install a delta-wye transformer (delta on the primary or high-voltage side)

2. What’s the purpose of having wye grounded configuration on the primary and the secondary?

In the mean time I will search for my old books and try to analyze the positive, negative and zero sequence networks with different transformer configurations to see if I can get and answer.

Thanks

 

[pwrtran]

1) Harmonic filters need to be tuned to certain order(s) of the harmonics. To maitain then is extra cost. Delta-wye tx is generally at a higher cost as the delta windings need full BIL insulation design than wye

2) Need wye-wye tx - no phase angle shifting. Grounded wye on both side - to hold phase to neutral voltage in place on primary; provide power to single phase load on secondary.

 

[guiyermo]

Thanks for your answer PWTRAN,

If we consider that the three winding transformer is supplied by a 230 or 115 kV network and the secondary is 34.5 kV I have the following questions based on PWTRAN response:

1. In general a three winding transformer is cheaper than buying a regular two winding transformer with a separate harmonic filter plus the cost of the switchgear CB for the filter?

2. Provided that the secondary is 34.5 kV, no single phase loads are supplied. I still don't understand the need for a primary and secondary wye grounded connection. In my lmited experience, for industrial purposes I've seen main transformers with a detla-wye configuration.

Also, I would like to confirm my initial statement that a three winding transformer where the primary and secondary have wye configuration and the tertiary is delta is used to "trap" third harmonic currents in the delta.

Thanks for your help

 

[davidbeach]

Is there generation on the low side of the transformer?

In many cases it can be less expensive to build a wye-wye with a three-legged core and a delta tertiary than it would be to build a two winding wye-wye on a five-legged core.

 

[prc]

I dont think the stabilising tertiary winding in a star/star transformer is to "trap" harmonic currents.Its purpose was to (1) to stabilise the neutral point of the fundamental frequency voltages (2) to protect the tarnsformer and system from excessive third harmonic voltages (3) to prevent telephone interference due to third harmonic currents and voltages in the lines and earth.
In due course these reasons became not that important and stabilising windings were eliminated from star/star transformers with 3 phase 3 limbed core construction.(see Are stabilisings necessary? B A Cogbill ,AIEE Octo,1959)

To mitigate the problems from harmonic loads, best way is to use harmonic filters.

In today's power systems, generating end transformer connection is delta/star and distribution end delta/star with all other inter connections with star/star connection.In HV and EHv range star/star connection is cheaper to make than HV delta/star connections.In earlier days Industrial supplies were given with HV delta( up to 110 Kv) /star secondary.I believe this was for the LG faults on secondary side not affecting the primary and also to take care of unequal loadings on three phases.

Please the classic GE Paper The Whys of Y :

http://www.geindustrial.com/publibrary/checkout/White Papers|GET-3388B|generic

 

[cuky2000]

Check withe the transformer manufacturer if they can use the transformer tank in lieu of the buried delta winding. Usually this option is more cost effective than the extra winding.

 

[cmelguet]

Guillermo, by any chance are you working at the Morro Proyect in Chile? Cheers.

 

[guiyermo]

Hello,

Sorry for the delay in answering.

Davidbeach: No, there is no generation downstream the transformer. Generation is electrically close to the high voltage side (115 kV or 130 kV)

PRC: Thank you for the reference. I will read it thoroughly. Actually you recommendation of using filters was my first guess instead of a third winding.

Cucky2000: Could you please explain how can the transformer tank could repalce the usage of the third winding?

cmelguet: No, this project is not in South America but I would be very interested in knowing about your project.

Thanks,