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The role of medium voltage switchgear in power system

The role of medium voltage switchgear in power system by SIEMENS

IntroductionAccording to international rules, there are only two voltage levels:

1. Low voltage: up to and including 1 kV AC (or 1,500 V DC)2. High voltage: above 1 kV AC (or 1,500 V DC)

Most electrical appliances used in household, commercial and industrial applications work with low voltage. Highvoltage is used not only to transmit electrical energy over very large distances, but also for regional distribution to theload centers via fine branches.

However, because different high voltage levels are used for transmission and regional distribution, and because thetasks and requirements of the switchgear and substations are also very different, the term medium voltage hascome to be used for the voltages required for regional power distribution that are part of the high voltage range from1 kV AC up to and including 52 kV AC (figure above).

Most operating voltages in medium voltage systems are in the 3 kV AC to 40.5 kV AC range .

The electrical transmission and distribution systems not only connect power stations and electricity consumers, butalso, with their meshed systems, form a supraregional backbone with reserves for reliable supply and for thecompensation of load differences.

High operating voltages (and therefore low currents) are preferred for power transmission in order to minimizelosses. The voltage is not transformed to the usual values of the low voltage system until it reaches the load centersclose to the consumer.

In public power supplies, the majority of medium voltage systems are operated in the 10 kV to 30kV range (operating voltage). The values vary greatly from country to country, depending on thehistorical development of technology and the local conditions.

Medium voltage equipmentApart from the public supply, there are still other voltages fulfilling the needs of consumers in industrial plants withmedium voltage systems; in most cases, the operating voltages of the motors installed are decisive.

Operating voltages between 3 kV and 15 kV are frequently found in industrial supply systems.

In power supply and distribution systems, medium voltage equipment is available in following forms:

Power stations

Power stations, for generators and station supply systems.

Transformer substations

Transformer substations of the primary distribution level (public supply systems or systems of large industrialcompanies), in which power supplied from the high voltage system is transformed to medium voltage

Secondary distribution level

Local supply, transformer or customer transfer substations for large consumers (secondary distribution level), inwhich the power is transformed from medium to low voltage and distributed to the consumer.

Reference: Siemens Energy Sector Power Engineering Guide Edition 7.0

The role of medium voltage switchgear in power systemIntroductionMedium voltage equipmentPower stationsTransformer substationsSecondary distribution level