Our discussion is related to a part of electrical networks and that is the Medium Voltage (MV) networks.

The transmission and sub-transmission networks may be complex considering the nature of protection schemes and construction of overhead transmission lines & high-voltage substation. However, MV network plays an important role in the whole transmission and distribution scheme as this is the level from where the consumers actually started to consume the electrical power.

The main purpose of MV network is to transform the power from sub-transmission level which is usually 66kV or 132kV to a medium voltage level which is either 6.6kV or 11kV or 17.5kV or 24kV or can go up to the level of 40kV in some countries.

In the HV sub-station which can either be indoor or outdoor type, power passes through different electrical components such as HV circuit breaker, current transformer, potential transformer, surge arrester and enters into a step-down transformer which reduces it from the level of say 132kV to 11kV. From transformer 11kV power enters to a 11kV switchboard which is located into a switchboard room inside the HV sub-station.

When the power leaves the 11kV outgoing circuit breaker of the switchboard there are two ways of power distribution, either using the over-head distribution lines or using the underground cables.

This distribution scheme can be a bit different for industrial sites having their own generation at medium voltage level. In this scheme power from MV generator goes directly to the incoming circuit breaker of the 11kV switchboard. Since no voltage transformation is required in this scheme therefore a step-down transformer discussed above is eliminated. However, it is important to mention that using diesel generator is expensive as compared to import power from national grid. At industrial sites power is commonly distributed using the underground cables.

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