Substation Transformer Windings

[TOMCAT]

What benefits\liabilities would be involved in constructing a 3Ph 10MVA Delta\GndY 69KV\12.47KV Transformer with Rectangualr vs Circular wound cores? Secondly what industry standards exist for Gnd protection on the high side of such a unit?

 

[GYakle]

There are huge cost differences between rectangular and circular core transformers. Additionally rectangular wound transformer will not meet ANSI/IEEE standards in regards to the percent allowable change in transformer impedance after a fault. ABB claims that they have a transformer design that will meet the allowable change in impedance but I have yet to review it. Lastly the rectangular transformers are a throw away transformer you do not rewind these units typically, manufacturers will not even warranty a rewind for a unit the size and voltage clase you are looking at. With the current lead time for transformers being out as much as a year you may want to consider circular unit that will stand-up better over time. As far as protection for the high side you will have to purchase nuetral CT's and incorporate them in to your bus differential scheme and you may want to use a nuetral over current relay in addition.

 

[peterb]

As the high side is delta connected and there is no HV neutral, HV ground fault protection is best provided by a low set ground overcurrent relay connected in the residual circuit of a 3-phase set of bushing CTs on the transformer feeder circuit breaker, which will normally also be used for transformer phase overcurrent protection.
With the transformer full load current of 83.7A, a CT ratio of 100:5A would be selected. The ground fault relay can be set down to around 15% of the CT rating, which allows for unequal CT saturation during inrush conditions (lower setting possible with detailed calculation). An instantaneous ground overcurrent relay setting can also be applied, as there is no zero sequence current source on the transformer side of the feeder breaker - any zero sequence ground current flowing in the circuit can only be as the result of a ground fault in the transformer 69 kV feeder or the HV winding.

 

[jbartos]

Suggestions:
1. It appears that the transformer efficiencies are normally taken into consideration since the transformers usually run 24hr per day and 7days per week. Lower resistance of the transformer windings will lead to smaller RI**2 watt losses. This works in favor of square windings if one does not have to be committed to industry standards.
2. HV side delta is normally not required to have the ground protection.

 

[GYakle]

Sorry I miss read the original post and thought the HV winding was Wye connected. In regards to the winding losses being favorable for rectangular windings than circular I don't understand. This will depend a lot on the winding ability to dissipate heat, and with rectangular windings there is a lot of blocking necessary for the winding to maintain its rectangular shape during short circuit events. This additional blocking and the compact design of rectangular windings decreases the amount the oil circulations about the winding causing greater heating.

 

[jbartos]

Suggestion to the previous posting: Please, notice that the heat is proportional to R x I**2. If I is constant and the same for circular winding and rectangular winding, and if R is smaller for the rectangular winding, then the rectangular winding will develop less heat; therefore, the cooling may be somewhat less intense or powerful.

 

[gmazumder]

rectangular coil transformers have been in use for quiet some time. the rectangular coil is actually the Lv winding and the same is made out of foils either copper or aluninium.
generally the HV will be conducter type and the core is rectangular in nature. the core is held in position by s frame which is welded in construction.
the foil type rectangular winding have good short circuit withstand capacity due to the even distribution of the fault current in the foil and can have lower resistance with lower losses.
i would be intrested to know about the ABB design but the rectangular construction can be benefitial but the repairs of the same are a costly affair.

 

[jbartos]

Small encore: Please, notice that the small transformers, e.g. control transformers, electronic device transformers, etc., have rectangular windings and rectangular cores.

 

[gmazumder]

i am talking of distribution transformers up to 3-4 MVA range, for 10 MVA range the size and design complexity may increase and the cooling will also have to be carefully looked into.

 

[jbartos]

To the previous posting: The MVA range increases in complexity and adds cooling to minimize the transformer size, cost, losses, etc., however, the rectangular coils and cores seem to have their legitimate place and advantages, which are dependent on the manufacturing means (various jigs, etc.) to certain extent. They are the reason for this discussion, else there would not be any. When it comes to transformer repairs, there are many factors involved; however, by the time the transformer needs repairs, it may have produced noticeable savings. After the transformer is repaired, it is projected there for some time. This time is just there to get the most out of the transformer iron and some of its accessories.