Momentary paralleling of different rated transformers. 1

[Universe_in_Flux]

We have been following 2 incomers and a bus coupler scheme arrangement with two buses in 0.415kV switchboard. Normally, each bus section is fed from incomer breaker and bus coupler breaker remains open. Auto change over scheme is employed on this switchboard. It enables automatic closing of bus coupler breaker on loss of normal supply to any of the two bus sections. Each incomer is currently supplied from 2 MVA distribution transformer.We are about to uprate these transformers with 3.15MVA rated ones. Since these replacements are to be carried out in phased manner, we are anticipating manual changeover on the switchboard during the replacement. During manual changeover, momentarily both incomer breakers and bus coupler breaker will be in ON condition. I Would like to know the thoughts from forum members on the possible risks/problems associated with the manual changeover operation on buses which are fed from different rated transformers.

 

[cranky108]

Can you calculate the circulating current?

 

[RRaghunath]

It doesn't matter even if the transformers of different ratings are paralleled if you have good controls to ensure the paralleling is only for a couple of seconds are less.
Depending on the MVA rating and %Z of the transformers, there could be circulating currents but unlikely to affect the transformers when the duration is short.

 

[waross]

Different sized transformer with equal %Imp will share the current in proportion to their KVAs.
In extreme case, a large transformer with a high %Imp in parallel with a smaller transformer with a lower %Imp may overload the smaller transformer even though the larger transformer is easily capable of carrying the entire load.
Circulating currents are caused by different secondary terminal voltages.

Circulating currents will be an issue in the event that the operators attempt to adjust the loading between the transformers by changing taps. Both transformers should be on the same tap with the same open circuit voltage.

You should be more concerned with the %Imp than with the transformer sizes.
To anticipate the load sharing between transformers with differing %Imp, the %Imp must be normalized or re-rated.
The transformer with the higher %Imp must be normalized to the same %Imp as the transformer with the lower %Imp.

Extreme Example:
Transformer T[sub]1[/sub] = 100 KVA, 2% Imp.
Transformer T[sub]2[/sub] = 800 KVA, 8% Imp.
Load = 500 KVA
To normalize T[sub]2[/sub] to 2% Imp, 800 KVA x 2% Imp / 8% Imp. = 100 KVA
The re-rating indicates that the transformers will share the load equally, or 250 KVA on each transformer.
This is an example of a hypothetical transformer bank with only one quarter of the capacity of the largest transformer by itself.
Moral: Check the % Imp. of both the old and the new transformers.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[cuky2000]

risks/problems[/b] associated with the manual changeover operation on buses which are fed from different rated transformers.]

Here are a few suggestions to consider:
1) Check the prospective SC paralleling transformer if there are within the rating of the protective devices.
2) Order a new 3.15MVA transformer with the appropriate impedance to minimize circulating current and SC level.
3) As an option, use a series reactor to match impedance and reduce SC
4) Plan according to prework performed to minimize the time of transformer changeover and therefore reduce the risk to a feasible shorter period.
5) Plan B backup plan in the event that issues arise during installation

 

[waross]

Note:
Circulating currents are caused by different voltages, (And in generators by different waveforms as a result of different winding pitches.)
KVA and Percent Impedance Voltage affect load sharing.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Dear Mr. Universe_in_Flux (Electrical)(OP)16 Nov 21 13:12
"... 1) We have... 2 incomers and a bus coupler scheme arrangement with two buses ... . Normally, each bus section is fed from incomer breaker and bus coupler breaker remains open. Auto change over scheme is employed on this switchboard.... 2) we are anticipating manual changeover on the switchboard during the replacement. During manual changeover, momentarily both incomer breakers and bus coupler breaker will be in ON condition....3) I Would like to know ..... on the possible risks/problems associated with the manual changeover operation on buses which are fed from different rated transformers...."
1. I make the assumption that the present set-up is equipped with (mechanical + electrical) inter-locks such that [two out of three breakers may be closed at any one time]. Strongly suggest to retain this fool-proof switching philosophy.
2. Do NOT defeat the present (see above 1) scheme . Refrain/avoid from trying to close three breakers at the same time, even for a short duration. Even on manual operation, there is no reason why you need to close three breaker at the same time. A warning sign board shall be prominently displayed the proper sequence of manual operation.
3. It is possible to parallel transformers of equal of different sizes. The are number of conditions such as vector group, % impedance, secondary voltage ... etc. Last but not least, the short-circuit capacity of the gears and busbar mechanical withstand up to 6.3MVA ... etc, even for a short-duration.
Che Kuan Yau (Singapore)