What is medium voltage? It depends on application, industry

medium voltage is far from uniformly defined, with specifications varying widely by industry, application, standards organizations, and other sources. At the broad system level, IEEE (Institution of Electrical and Electronic Engineers) Std. 100 defines nominal medium voltage (MV) as ...

07/26/2010


Dear Control Engineering: What is medium voltage?

Answer a question with a question, what could be simpler than defining one agreed-upon range for “medium voltage”? Actually, medium voltage is far from uniformly defined, with specifications varying widely by industry, application, standards organizations, and other sources.

At the broad system level, IEEE (Institution of Electrical and Electronic Engineers) Std. 100 defines nominal medium voltage (MV) as “greater than 1 kV and less than 100 kV.” Moreover, this is followed by two still higher class designations—high voltage (HV) ranging to 230 kV and extra high voltage (EHV) above that point. For power cables, the same IEEE standard specifies the top end of MV as 72.5 kV.

Closer to our area of interest, a NEMA (National Electrical Manufacturers Association) designation for MV cables has ratings between 600 V and 69 kV. The same top figure applies to some electric wire and cable manufacturers’ specifications. Other IEEE references designate MV electrical devices as “designed to operate between 1 kV and 35 kV.” Some manufacturers rate MV switchgear up to 38 kV.

As mentioned in the main article, MV variable-frequency drives have a narrower practical voltage range of around 2.3 to 15 kV, because few MV drives are built outside that range. In addition, only a few common input voltages have been “standardized” worldwide—such as 2.3 and 4.16 kV in the 60-Hz world and 3.3 and 6.6 kV in the 50-Hz world.

An anecdotal way to illustrate the blurring of MV definitions is to recall that certain servo drive vendors proudly advertise their 480-V offering as a “high voltage” product.

Frank J. Bartos, P.E., is Control Engineering consulting editor. Reach him at braunbart(at)sbcglobal.net.



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