Voltage Level Selection

[Adam1980]

Dear all,

I searched in the forum concerning this question but I did not find a comprehensive answer since some implications were scattered in other queries.

When having a certain load\customer to be connected to an electrical network with different voltage levels available (transmission, sub-transmission, distribution,...).

What would be the main criteria for voltage level selection.

Obviously calculations showing losses and voltage drop will be supportive however grid codes usually specify this criteria directly independent of the distance of power transfer and based on the rating of the client.

This of course is related to size of breaker, possible bus bar ratings and so on. However I couldn't find any norm or comprehensive equipment list which generalize this criteria. Ex. maximum bus bar/breaker rating at 115 kV is 1000 A that means if load is more than 200 MVA it should be connected to higher voltage levels.

Anybody knows if such limitations are officially published? Otherwise how can we be sure that these limits are really true since each manufacturer would have their one rating of equipment.

Thank you.

 

[davidbeach]

It depends.

First, check the tariffs of the serving utility. That will provide information as to what size loads they'll serve at what voltages. There will be some overlap, but generally the size of the load will dictate the voltage level, particularly between secondary and primary. Anything at the subtransmission level or higher will also require lots of discussion with the serving utility, and at that level they'll generally tell you what voltage you can have. With your own transformer between that voltage and the remainder of your load you can then have what ever facility voltage you want.

 

[dpc]

If you have an actual example with real data you'll get better answers. There is no standard and there is no comprehensive answer. In many cases, as David says, you'll be constrained by what the serving utility is willing to do. Continuous and short circuit ratings of standard equipment are only two of the many factors involved.

You must also consider the expected ACTUAL energy usage and demand, not just a calculated maximum value, as well as the future growth potential.

 

[davidbeach]

Oh, and by the way, 3000A at 115kV is not difficult.

 

[Adam1980]

Thank you for your reply.

The basis of my question is a planning guideline of some utility which includes the following:
connection of 200 MVA to 380 kV
connection of 50 to 200 MVA to 115 kV
connection of 5 to 50 MVA to 34.5 kV
connection of up to 5 MVA to 13.8 kV

In all cases the user will have an own transformer to serve the plant's load.

I am aware that how connecting these loads would impact the network takes part of voltage level selection.

But I believe from a basic planning perspective the selection that a certain load rating should be connected to a certain voltage level is directly related to equipment limits and rating. That is either the equipment of the new consumer or the existing equipment of the utility.

Other factors such as stability issues would follow.

 

[davidbeach]

Right, but the answer will vary from utility to utility. The available voltage levels will also vary.

For example we would offer 13.2kV (customer voltage should be 12.47kV) not 13.8. 34.5kV is available in a very limited area. Some portions if the service territory would offer 115kV, but in other areas there would only be 57.5kV. And no where would 380kV be available. Other utilities will give other answers, you have to start with the utility.