Graham asks this question about transformer loadings in double-ended scheme for a hospital substation project, and what the best approach would be. It has been answered by ABB (Theme – Power):
This Q & A is one of thousands posted in our Technical Expertise area, and answered on a daily basis by our Voltimum Experts.
Question: I need your advice and technical input on the following concern from one of our clients:
We are proposing a double-ended substation scheme for the hospital project we are involved in.
In such a scheme, the two transformers have to be loaded to not more than 50% (with exception of forced air cooling, where 75% loading is allowed).
During under off peak conditions (or winter), the transformers will be loaded to not more than 30% of its overall capacity. This will lead to a low power factor (PF) and will need more capacitor banks to improve the PF.
The client has a suggestion that under this double ended-scheme, one transformer should be fully loaded (70% to 90%) and the other transformer should be standby (i.e. no-load).
Following is an excerpt from a client’s email:
“As shown in your electrical power system to the new hospital…
“It seems that all transformers will be operated on 50% from its nominal load maximum, also actually the load will not exceed 40% or 30% during the cool season.
“Such operation will cause a very low PF on the main transformers most of the time (for transformers there are big induction loads).
“My suggestion is to modify this system to one operating system (the transformer’s load will be from 70 to 90 % all the time), then to keep another system as stand-by (automatically). This will improve PF too much, so then we will save a lot of money through the monthly electricity bill - as you know, the utility company will start overcharging the electricity bill for all customers that have a low PF within a few months.
“I think that the best solution is to go for large capacitor racks to improve the PF, especially as we are talking about the high tension side”.
Do you agree?
Answer: We would normally associate I2R losses resulting in kWh unit charges to depend upon the loading of the transformer. The transformer manufacturer should be able to give accurate loss figures. You would expect the losses of two transformers to be greater than one.
For reactive compensation of the no load transformer itself, we would normally expect it to be approximately 1.5% of the kVA TX rating (calculated for standard transformers). The transformer manufacturer should be able to verify this.
As an example, for 1000 kVA TX - reactive compensation would be 1000 x 1.5 = 15 kVAr
Reactive or power factor charges are normally seen based upon the operation and type of load connected, such that any power factor correction (PFC) requirement is calculated on the connected load, i.e. 500 kW - 625 kVA, 375 kVAr, PF 0.8. From this we would calculate the power factor correction that would be required, e.g. to improve to 0.95 lag, 210 kVAr would be required.
On a monitored high voltage system, the reactive element would be from the connected load and also the reactive requirement of the transformer therefore to maintain a PF at (say) 0.95 on the HV system, we would look to improve the low voltage PF higher than 0.95 to (say) 0.98 lag.
It would be expected to be more economical to run one transformer fully loaded transformer and put the other one out of circuit, compared with two at 50% load. However, the reactive element required for two transformers only would not be very great and, therefore, most systems such as this would use both transformers and could adopt a PF compensation system, so that it operates for two transformer operation modes - and also a single transformer mode.
The power factor correction requirement would, as stated previously, be in the main dependent upon the connected load and the operating PF of that connected load.
To see many more Q & A in Voltimum UK's Experts Area, please click on the link. Experts from leading organisations provide online answers to your technical questions on a broad range of subjects. Our searchable database of existing Q & As now contains over 3,500 entries; you can browse through them here.