Introduction to High, Medium and Low Voltage

Voltages Divided into Classifications

High voltage, medium voltage and low voltage are the terms we hear most when talking about voltage classifications.From an international perspective, these classifications and scopes vary depending on where you live.In the United States, the National Electrical Code (NEC) and the National Electrical Manufacturers Association (NEMA) have guidelines and standards covering all voltage classifications.The American National Standards Institute (ANSI) oversees the development, promulgation and use of thousands of guidelines and standards that affect businesses.Every industry complies with applicable regulations.Both ANSI and NEC codes are purchased publications.The Electrical Engineering Portal (EEP) provides a breakdown of ANSI Standard C84.1-1989.This document divides voltage into five categories.These classifications can be combined into the following categories:

• High Voltage (HV), Extra High Voltage (EHV) and Ultra High Voltage (UHV) - 115,000 to 1,100,000 VAC

• Medium Voltage (MV) - 2,400 to 69,000 VAC

• Low Voltage (LV) - 240 to 600 VAC 

Generac has published a white paper titled Medium Voltage On-Site Power Generation.The white compares the NEC to the ANSI standard.It uses the following NEC voltage standards:

•  High power distribution - 1000 to 4160 volts

•  Medium power distribution - 50 to 1000 volts

• Low power distribution - 0 to 49 volts

The above list illustrates the classification of changes in voltage levels, depending on the governing body.Generac states that generators less than and equal to 600 volts are medium voltage generators,and generators greater than 600 volts are considered high voltage generators.Generators producing 4160 volts are common in many industries that require large electric motors at high voltage.Standby generators supply voltage to a single grid.Commonly used generator voltages are 4160 VAC,480 VAC,12,470 VAC,and 13,800 VAC,and when an industrial facility loses power,backup generators power distribution and control panels to continue operation.The higher voltage from the generator is stepped down through a transformer.The following provides information about each type of information.

High, Extra-High and Ultra-High Voltages

High and extra high voltages are associated with the transmission of electricity from power plants.The reason for transmitting electricity at HV and EHV is to improve efficiency.The lower current that accompanies high voltage transmission allows the use of thinner and lighter cables.This reduces the cost of tower and wire construction.The high voltage range is 115,000 to 230,000 VAC and the extra high voltage range is 345,000 to 765,000 VAC.T.High voltages require specialized switches and switchboards.The control room has a redundant switching function.They can be controlled remotely or placed in manuals to maintain and test individual supply systems.Substations provide step-down voltage for distribution to local areas.Extra High Voltage refers to voltages in excess of 765,000 to 1,100,000 VAC.China is using a maximum voltage transmission of 800,000 VAC.They are developing a 1,100,000 VAC system using cables currently rated at 1,200,000 VAC.

Medium Voltages and Industry

Large industrial parks and factories that require a lot of power usually use medium supply voltages.Electrical variational analysis shows that voltage is inversely proportional to amperage.This means that when the voltage increases,the amperage decreases to complete the operation.Motors and electrical equipment designed to run at higher voltages use less power and are more economical to run.Most main substations receive no more than 35,000 VAC from the utility supply. Primary substation can provide step-down power to secondary substations or individual buildings.The secondary substation distributes the power received from the primary substation.Secondary substations can have step-down transformers to further reduce power distribution to control panels for distribution throughout the facility.Substations are usually located in areas that can service one or more buildings on the property.The Aluminum Company of America (ALCOA) Warrick Operations is an example of a large industry that consumes large amounts of electricity.They are located in southern Indiana and have an independent power plant.They generate electricity using a coal-fired power plant located on the Ohio River.They process aluminum ingots into rolled aluminum sheets for plants that need aluminum can stocks.Ingots are melted in large electric furnaces and then processed through a series of operations to obtain the correct billet thickness.Any plant that uses medium voltage power to substations needs emergency or backup power.Generators delivering 13,800 VAC are not uncommon.This voltage source is very suitable for small and medium-sized substations and secondary substations.With proper generator support,the complex can continue to operate during a power outage.Available in a variety of designs,including mounted,soundproof enclosures and portable units.Portable unit is enclosed in a soundproof enclosure on a trailer pulled by a semi-tractor.