Using distribution transformers vs direct connection to primary switch gear

Question

I am trying to learn more about transformers and low voltage electrical distribution.

Say you have a medium sized industrial building with a primary switchboard feeding several load centers elsewhere in the building feeding localized loads.

Is it more efficient* to use one voltage through out the system, i.e. the power feed into the building would be 120/208v 3phase which would be distributed through out the building, or distribute a larger voltage and hence smaller current through the building to the load centers and then transform the voltage to the desired voltage at that point.

*note: more efficient can either mean electrically, as in am i wasting more power by using more transformers vs larger cabling, or monetarily, as in is the smaller copper distribution wire going to outweigh the price of the transformers.

thanks in advance.

Answer 1

The decision between HV and LV distribution is usually driven by capital cost and safety considerations, with energy efficiency being a lesser objective.

The choice depends on:

• the size of the building
• the size of the loads
• the expense of electrical equipment with appropriate continuous current rating and fault current rating, including:
  • transformers
  • cables
  • copper busbars
  • circuit breakers.

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If your "medium sized industrial building" contains a number of arc furnaces, each of which requires thousands of amps, you are likely better off having high voltage distribution.

The high fault currents involved in a low-voltage solution are difficult to deal with - while it is possible to build one big transformer feeding an 8,000 amp switchboard, it is expensive to build an 8,000 amp switchboard to safely withstand a 100 kA fault level.

It is cheaper and safer to have two transformers feeding 2 × 4,000 amp switchboards rated for 50 kA fault level, or four transformers feeding 4 × 2,000 amp switchboards rated for 25 kA fault level.

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If your "medium sized industrial building" contains things like lathes, welding sets, mills, and so on, which are comparatively light loads, you are likely to be better off with a low-voltage distribution system.

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It is an electrical engineer's job to make this judgement on a case-by-case basis, to deliver a safe, lowest-cost, fit for purpose design.

This might include doing enough engineering design on both options to evaluate their relative safety and cost.