HV Transformer Design for Apartment Residential Building

[Maple2]

Hi Gurus,

For the first time, I want to design a 3ph HV utility transformer for an Apartment building with a switchboard rated for 120/208Y, 3000A, 65kA (AIC). Utility HV at proposed site is unknown.

CEC 26-254 (2) stipulates that OCP is not needed on the Primary Side if secondary side has OCP rated at not more than 125% of rated secondary current.
Since Switch Board provides OCP of 3000A , hence Secondary Side current = 3000A/1.25 = 2400A
Xfmr size = 2400A x 0.208 x 1.73 = 863.6 kVA (Hence I chose 1000kVA Transformer)
Tamper resistant Oil Liquid Filled Transformer (ONAN).

Question 1: If I select a Transformer with FCAN and FCBN that's in increment of 2.5%, meanwhile, Transformer primary HV is not yet known, is there a way to still determine how many tappings the Transformer should have?

I learnt that Impedance grounded XFRMS provide better voltage-to-ground stability, but whole residential building power could be disrupted in the event of fault current. Non impedance grounded Xfmrs I learnt provides are better where continuity of service is critical, in the event of fault current, however voltage-to-ground stability not as good.

Question 2: How do one design a transformer that could be good at both? Under what condition, could one be preferred over the other in their application to residential building transformers?

Your Opinions would be appreciated. Thanks

 

[xnuke]

Doesn't the utility provide this transformer? After all, they're the ones providing the service to the building.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[bacon4life]

In my region of the USA this would be utility owned, solidly grounded transformer. Typically impedance grounded is only possible when there are no line-to-neutral connected loads.

Given the transformer size, I assume this is a rather large building where voltage drop at 208 V can be an issue. For large buildings, sometimes the utility will provide 15 kV/480V oil filled transformer outside the building, then the building owner has a 480V/120-208V on each floor closer to individual apartments.

Transformers with either zero taps or 5 taps are common.

 

[jghrist]

Transformer taps will depend on expected source voltage and voltage drop. What is the actual load? Not necessarily equal to the switchboard rating.

 

[unclebob]

Even if it's not needed, I would put a simple fuse on the primary side. In case of a strong S-C inside the transformer or on the secondary side, the fuse will open
faster than a breaker.

 

[Maple2]

2385 Amps is the actual Load.

 

[cuky2000]

Here are a few observations to consider.

1) Appears that 2,385A was forced to be approx. 80% of the SWGR breker rating.
2) The utility SC has a small impact on the load side since the transformer and feeder impedance are the dominant factors.
3) For 208V, motor contriution usually is accepted < 50% total installed load.
4) Voltage drop at 208V need to be considered that force to overside the feeders.
5) The prospective SC at the feeder end is estimated in the graph below
6) There is an estimated connected load and the larger feeder length?

 

[FacEngrPE]

4) Voltage drop at 208V need to be considered that force to overside the feeders.

Depending on branch length, voltage drop may control the sizing of many of your branches. In the extreme case undersized branches may delay protection tripping, and increase arc flash energy.