First, I'm surprised, because the ditributed Medieum Voltage that you should be used is normally defined by the ” Utility “, as the Utility defines accordingly to the consumed power from where the load should be electrified. In Syria for exp. The Utility defined as follow :
– The load up to ” 35kW ” will be connected directly to LV network ” 0.4kV “.
– The load up to ” 10MW ” should be connected to MV network ” 20kV “, that means the client will have a substation ” 20/0.4kV “.
– The load more than ” 10MW ” should be connected to HV network ” 66kV “, that means the client will have 2 substations ” 1st : 66/20kV ” & ” 2nd : 20/0.4kV “
But if you have the possibility to select one of 33kV or 11kV, please note that the transformer ” 33/0.415 kV ” is more expensive than ” 11/0.415 kV ” by around ” 15% “, and the Circuit Breakers ” 33 kV ” are also more expensive than ” 11 kV ” by around ” 3-4% “.
About the ” Step-Down ratio ” between HV and MV, please note that's approx ” 3.3 – 3.6 “, and some manufactures can produce transformers with more than this ration but the cost will be very expensive.
I always did electrical installation design in industries, commercial and residential buildings.
The problems comes when i want to choose the distribution voltage.
What i did is to select the nearby distribution system either 33kV or 11kV and stepped it down to 0.415kV.
I NEED A FORMULA ON HOW TO CALCULATE THE REQUIRED DISTRIBUTION SYSTEM TO BE USED AND THE
CRITERIALS USED TO SELECT IT.
Anyone with clear details need your help. ( i will be happy if Egineer GHALI will participate on this)
To begin with 33 & 11kv are high voltages,
you can use these voltages in secondary transmission and distribution networks in a power system depending on your supply utility voltage level(330/132/66/33kv for zambia's primary,secondarytertiary transmission networks and distribution networks).
33kv is handled via overhead lines and is economic for such while 11kv depends on application and costs of its transformation to other voltage levels.
Why and where to use any voltage level depends on the voltage level requirements of your drives,have worked with a 2.5MW Compressor at 11kv voltage.
The main worry is the cost of insulation on your equipment which makes the cost high all together .
for 0.4kv, this is used for consumer purposes mainly and few industrial machines use the system voltage maybe single phase motors and for lighting purposes.
The other worry is the switchgear to use for a particular voltage level e.g you need a VCB or ABCB or SF6 for 33kv which is expensive and VCB or OCB for 11kv with VCB and ABCB being expensive to buy and replace with additional maintenance costs for ABCB.