Transformer sizing for variable load and harmonics

[Hanimlat]

Hi,
I'm designing a pilot project for energy storage unit based on an induction generator coupled with a frequency drive.
there are also several large motors driven by VFDs as well so we are talking about approximately 2300kVA peak load on the transformer.
the problems are:
1. load varies throughout the day a lot of times going from full load to almost zero
2. 95% of the loads on the transformer are frequency drives so the transformer will suffer from harmonics.

these problems are very similar to what we see in wind turbine generators and we now know
that regular distribution transformers are a bad solution for wind turbines.
a possible solution would be to use a 3000kVA which seems to me sufficiently oversized but:
1. this is not a very cost effective solution.
2. the transformer losses are higher.
3. short circuit currents are bigger.

the question is what measures can the transformer vendor take to make the transformer more robust
without oversizing it?

 

[wayne440]

I cannot answer your question - but I am reasonably sure that directing your question to a competent transformer manufacturer
would be time well spent.

 

[mparenteau]

What are the VFD specs? Do they have chokes or input filters?

Mike

 

[Hanimlat]

The equipment wasn't specified yet so chokes/filters can be added if needed

 

[dpc]

Transformers can be specified with a K factor rating to help with harmonics. First step is to determine the type of drives and what harmonic mitigation will be applied at the drives. Then an expected distortion level can be calculated and an appropriate transformer specified.

 

[prc]

1) Rating in kVA = Factor 1.10-1.25 (to take care of future demand increase) x total connected load x Demand Factor x Diversity Factor
Where Demand Factor= Maximum Peak Demand/ Total connected load < 1.0
Diversity Factor= Sum of individual maximum demand of feeders/ Maximum Demand
of the whole system > 1.0
2)Estimate the harmonic loading on the transformer. Either increase MVA or select nominal rating with a k-Factor. Please refer to C57-110: 2018 Transformer capability for harmonic loading to decide the uprating.
3) Cost of the transformer will not increase linearly with MVA increase but to the power of 0.75.

 

[bacon4life]

Have you done the calcs using your anticipated loading factor to verify this assumption? In many cases a larger transformer can be more energy efficient due to lower I[sup]2[/sup]R.

Some of the issues with early wind turbine transformers may have been related to frequent/extreme thermal cycling. Slight oversizing will also reduce the extremeness of thermal cycles.