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I have a installation designed with a duty standby transformer .in case of failure of duty transformer the standby one will take the full load. normaly standby will have no load connected.
I wonder will it be better to load both transformer 50% and incase of failure of one transformer,the other transformer will be loaded 100%.

RE: transformer

Load losses are proportional to the square of the load, so having each transformer loaded at half load will result in one half of load losses.

One transformer: Loss = 1 x Full Load Loss (FLL)
Two transformers: Loss = 2 x FLL/4 = FLL/2

RE: transformer

The other advantage with both transformers connected to busbar, sharing 50% load each is that even in case of failure of bus coupler (failure to close) still half the load is live and continues to be running.
The difficulty could arise when the loads cannot be divided as required.
It is not a good idea in any case to parallel both the transformers at the busbar.

RE: transformer

Arc flash and Fault currents may be an issue. Sometimes it WAS done right the first time.

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

RE: transformer

The downside is the no-load loss will be double with two trafos. Do a loss analysis.


RE: transformer

Calculate the capital and OM & A costs of the added equipment needed to SAFELY operate with the two trafos normally in parallel; then complete a full cost/benefit analysis.

In other words, it CAN be done; the question to answer is, "Is it worth it?"


"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: transformer

1) The point raised by edison is also relevant. Normally no-load losses will be less than 1/4 of load losses. Hence even in the worst case, total loss with two units in parallel will be load loss of one unit, while the total loss in one unit at full load will be 1.25 times load loss.
2) There is one more advantage with two units in parallel. It is always better to keep transformers in warm conditions to reduce moisture absorption.

RE: transformer

I had assumed that standby meant the second transformer was installed and energized, so that load could quickly be transferred to the standby transformer. The the no-load losses would be the same whether the load was split 0/100 or 50/50. The energization losses should be sufficient to avoid moisture issues common with de-energized spares.

If the second transformer not actually installed and energized, the OP would also need plans in place to periodically test the spare transformer as well as a plans in place to have an electrician on call.

For any loss evaluation, the OP would need to consider the annual duty cycle rather than just the peak loading. Not many transformers are loaded to 100% 24x7. As an anecdote, the DOE efficiency standards evaluate transformer efficiency at 35-50% of nameplate.

RE: transformer


Good point about keeping the transformers warm.


RE: transformer

bacon4life, in case you keep the spare transformer energized, but without loading, total losses will be 1.5 times load loss of one transformer; ie losses will be higher than half loading both units. No-load losses will not be sufficient to warm up the unit with the risk of energization added.

RE: transformer

prc-How much additional heat would you suggest to avoid moisture issues? The typical oil-filled pole-mounted 25 kVA transformer around me has roughly 50 watts of energization losses. I had assumed this was about the same as a typical strip heater in a control cabinet. Is that not enough heat? The OP did not mention the type of transformer, so the OP could have also been referring to dry-type or substation transformers.

I have seen the ratio of load-to-energization losses vary quite significantly between bids from different manufactures, with typical values ranging from 4 to 8. The table below shows possible savings for 4 different transformers that have equal efficiency per the DOE guidelines. The OP would need to evaluate losses based on their actual transformers.

Regarding heating to prevent moisture damage, Column O shows the loading percent where load losses and energization losses are equal. For lots of distribution transformers in my area, the transformer load never gets high enough to provide more than 50% of the heating. If the energization losses do not provide enough heat, any idea how big of a lifetime hit would a lightly loaded transformer take? A while back I had tried to pencil a business case for swapping out transformers loaded to less than 10% of nameplate, but could not justify the labor cost to do so based solely on energization loss reduction. If I could show that lightly loaded transformers tend to fail early due to moisture, it could help justify changing out the transformers.

RE: transformer

No, we cannot say lightly loaded transformers are more prone to failures. The only point is when we keep a transformer live without loading, there is the risk from dielectric stress. Then why not put both units under half loads or part loads, so that total losses will be less than 100 % + no-load combination?

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