Purpose of Parallel Transformers - example

Hello GISdude

Parallel Transformers are used so that in case of a transformer failure, there is power still available, on a reduced level, for the load.

It is extremely important that the transformers are designed, and are connected in the same phase angle/vector angle = Dyn11, Dyn22 and so on.

It is normal for large Hospitals, which require large amounts of power in critical areas 24/7, that no cut in supply level is made.

Hospitals generally also have one or more Diesel Alternator units, which may supply power through the Essential Switchboards located next to the non-Essential Switchboards throughout the Hospital.

So in a hospital, Essential loads would include the following:

1. Surgery Theatres
2. Critical wards
3. Boiler house
4. Gas Supply = Oxygen and so on
   
5. Circuits with Computers
6. Circuits with patient monitoring equipment
   
7. XRay
8. Cat Scan
9. Emergency Lighting
10. 1 mains outlet per room, marked "Essential"
    
11. HVAC Equipment
12. And so on

Unable to give you screen shot/s, but the pair or more of identical Phase angle/vector angle transformers are just connected in parallel.

Protection Relays are very carefully set, so that in case of a fault, the transformers drop off one by one, with Circuit Breakers disconnecting the faulty Transformer and/or Circuits as required, then switching back on the OK transformers, via discrimination relays.

All that above normally is set to happen in under a second.

Meanwhile the Diesel Alternator system has cut in, at first fault indication, generally taking around 5 seconds to supply the Essential load, in parallel with the remaining transformer/s, if there are any, and that Alternator normally is set to shut down some 10 minutes after the power system stabilises.

Major Hospitals normally have an identical pair or more, of Emergency Diesel Alternator units, set to automatically start and switch onto supplying the Essential Load as required.

Major Hospitals will normally have a High Voltage Supply from 2 feeder cables, at 11kV, 33kV and once again the reason is to retain a power supply to the Hospital Main Switchboard, in case of fault/s.

The Protection Relay Systems for a large Hospital Complex, are quite sophisticated, requiring careful calculations, and normally being originally set with faults simulated via special Potential and Current Transformers, so that the Electrical Engineers know the system is as fail-safe as possible.

The J-Boxes you use will be smaller than the "Primary Pullbox", because in most cases the power supplied from a pair of identical transformers is equal.

This in practice, only very small mis-match currents actually flow through the J-Box, unless a major outage occurs in one transformer.

I hope your understanding of the situation is improved.

Advise further if I missed something you need to know.

Kind Regards....