Comments on: Parallel transformers benefits

By: Sinisa Spremic

Sinisa Spremic — Fri, 03 Feb 2012 11:08:56 +0000

Disadvantage of circulating currents is increasing of losses. Load sharing is connected with different short circuit impedance (mainly reactance because R< Parallel operation methods for parallel operation of transformers imply circulating currents minimization so circulating currents (which could not be fully removed due to different factors) are at low values and thus circulating currents increase losses at very low level.
Of course increasing overal inductance (reactance) using inductors (transformer reactance+inductor reactance) will effect in circulating current reduction. Load sharing could be equalized if added inductors equalize overall impedance (reactance) among transformers.
Inductors built in secondary side of transformers could help to decrease short circuit currents in places where it is necessary.
My opinion is that there is no need to add inductors on secondary side of transformers regarding to parallel operation of similar transformers (circulating currents and load sharing) because of inductors costs, increasing maintenance cost, decreasing reliability (more devices), need for new parametrisation of protection devices and automatic voltage regulators, etc).

Best regards, Sinisa Spremic

By: Thomas Rooth

Thomas Rooth — Thu, 02 Feb 2012 10:23:31 +0000

Question regarding possible elimination/reduction of the disadvantiges of circulatig currents, load sharing etc.:
Will the adding of inductors on the secondary side of each transformer help. It’s obvious this cause higher cost but in spite this disadvantage, will it help?

Kind regards,
Thomas Rooth

By: Sinisa Spremic

Sinisa Spremic — Wed, 01 Feb 2012 12:56:08 +0000

Mathan!

Your question is ambiguous.

Best regards!

By: mathan

mathan — Thu, 08 Dec 2011 01:40:35 +0000

hi sir .. as am a final year student. i need clear abstrat of partial breakdown of power transformer

By: trishla goyal

trishla goyal — Wed, 07 Dec 2011 14:38:22 +0000

as m an final yr. B.E. student i have done my recent training from a 220kv g.s.s. where i practically saw this
benefit of using two transformers in parallel it really increases the efficiency of the system but obviously this system is expensive. we can connect a 80% loaded unit in parallel with a 0% loaded unit. but the conditions of polarities ,voltage ratings i.e. turns ratio,inductance to resistance ratios phase displacement between primary and secondary line voltages and phase sequence in case of three phase transformers should be met.
and for your second question according to my knowledge a split second bus with a tie breaker interlocked to prevent parallel operation is necessary in this case.

By: gerard ford

gerard ford — Sun, 04 Dec 2011 20:49:03 +0000

would you know if it is ok for circuits from one transformer share a neutral from another if both neutrals are connected, will it hurt the transformers, to bond the neutrals?

By: Sinisa Spremic

Sinisa Spremic — Sat, 03 Dec 2011 21:03:43 +0000

Yes, it is possible to put in parallel transformers in case that before paralleling one have 80 % of rated load and other one 0 %. Before paralleling transformers the best is if transformers are at the same positions, but there will be no problem to paralleling them if there is a litlle difference in positions among transformers (one, two and possible morethan that), but in this case you will have circulating currents.
I did not know what did you mean with your second question about interlocking.

Best regards,
Sinisa Spremic

By: Amang

Amang — Thu, 24 Nov 2011 12:40:38 +0000

Dear Mr. Artem Kropachev,

could you explain operation procedure for this system..? Is it possible to parallel of those transformer where one of those transformer already loading 80% with the other one with load =0 (Zero), or transformers must be parallel in condition load =0 for both of transformer..? are we need an interlock between MV breaker and LV breaker of those Transformer..?
Thanks for your respond.

Best Regards,

Amangkurat

By: Sinisa Spremic

Sinisa Spremic — Tue, 20 Sep 2011 18:04:14 +0000

You can run two desribed transformers of different nominal power rating S1 and S2 (MVA or kVA) in parallel with same voltage ratio, frequency and phase sequence. Loads of transformers with different nominal power ratings will be equal only if their short circuit voltages are in this relation Uk1/Uk2=S1/S2. Transformer with higher nominal power should have higher value of short circuit voltage. This means that impedances Z1 and Z2 are equal. Impedances can be calculated from Uk (short circuit voltage) as Zk=(Uk/100)*(Un**2/Sn) where Uk (%), Un (kV) and Sn (MVA).
Transformers used in distribution ordinarily have approximate equal Uk (short circuit voltages) and according to that transformers with higher nominal power has lower value of impedance. It means that in parallel transfomer with higer nominal power will have higer load, and transformer with lower nominal power will have lower load. For example two transformers with equal short circuit voltages with load of 15 MVA – transformer of S1=20 MVA will be loaded with 10 MVA and transformer of S2=10 MVA will have load of 5 MVA.

By: Sinisa Spremic

Sinisa Spremic — Tue, 20 Sep 2011 17:35:03 +0000

If two transformers are on the same high voltage busbar and on the same low voltage busbar, difference in transformers ratios will produce circulating currents (there will be no arcing). Transformer with lower value of ratio (which means higher voltage on low voltage side of transformer) will increase reactive current (to obtain higher voltage drop in transformer imedance) due to circulating current and transformer with higher value of ratio (which means lower voltage on low voltage side of transformer) will decrease reactive current (to obtain lower voltage drop in transformer impedance) due to circulating current – this is physical explanation of circulating currents and why both transformers have the same voltage at low voltage busbar.
Any changing in position of transformer will cause arcing, and it is allowed only in transformers with On Load Tap Changers (OLTC), where arcing happens in special chamber.