How do you calculate the electrical supply of a 11KV site to equate to 5 - 7.5MVA capacity. thanks

Go to the profile of Wesley Tapalu
Wesley Tapalu on Nov 10, 2017 • 4 answers
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I assume you are asking what is the current rating of the supply. 11kV is the voltage and 5-7.5MVA is the electrical load of the site. Therefore the answer will be between 5000000VA divided by 11000V and 7500000VA divided by 11000V. i.e. the current required to supply a demand of between 5-7.5MVA will be between 454.54A-681.81A @11kV.

Hope that helps.

Go to the profile of Paul Davis-Moull
Paul Davis-Moull on Nov 10, 2017

One slight clarification. There's a root(3) missing from the calculations. So:
5,000kVA / 11kV / Root(3) = 262A and
7,500kVA / 11kV / Root(3) = 394A
Hope taht helps/

Go to the profile of Paul Rae
Paul Rae on Nov 10, 2017

The question is not that clear - I'm assuming that you are not actually asking about current?
11kV is just the voltage at which the plant generates. The peak capacity is lets say 5 MVA.
Simplifying lets use just 5 MW. The energy produced by the plant at full capacity is 5000 kWh each hour. Or if you prefer 5000 *8760 kWh/year = 43.8GWh/year.

Bear in mind however that a wind farm does not run at fiull capacity so the energy available is less; typically you may get 20% of the full rated capacity. So 43.8*.20 = 8.76 GWh. At 7.5 MVA it would be 50% higher.

Go to the profile of John Williams
John Williams on Nov 10, 2017

If it is a generation site the installed electrical plant would normally be rated at 0.8 pf, while the total may be be 7.5 MVA/6 MW the output would depend on both plant availability (maintenance etc outages) and prime mover capacity and availability. For thermal plant, how does it support the 6MW. For solar or wind what is the after diversity maximum output over the appropriate time period? Calculations are fairly simple and financial implications of the answer less critical than if a consumer demand site.

If it is a consumer site the energy purchase tariff is very likely to incentivise a pf around the user may be drawing as much as 6.8MW. The underlying calculations will need to determine the after diversity maximum demand of all the elements of the site load considering the nature of the load and the opportunities for Maximum Demand management. In this case the Max-Min range needs to be refined as it affects major design decisions re 400A or 630A busbars,, fault level rating of switchgear and harmonic content with the need for filtration.

Further detail is needed to assist Wesley with his inquiry.

Go to the profile of Ian Earle
Ian Earle on Nov 10, 2017