Medium Voltage Technical Guide

Basics for MV Design According to IEC Standards

MV Networks

The term "medium voltage" is commonly used for distribution systems with voltages above 1 kV and generally applied up to and including 52 kV(1). For technical and economic reasons, the service voltage of medium voltage distribution networks rarely exceeds 36 kV. 

Medium Voltage Panel Connections
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The connection of an electrical installation to a MV utility distribution network is always realized by means of a dedicated MV substation usually designed "Main substation". Depending on its size and specific criteria, mainly related to the loads (Rated voltage, number, power, location, etc….), the installation may include additional substations designed "Secondary substations". The locations of these substations are carefully selected in order to optimize the budget dedicated to MV and LV power cables. These secondary substations are supplied from the main substation through the internal MV distribution.

Generally, most of the loads are supplied in low voltage by means of MV/LV step down transformers. Large loads such as asynchronous motors above or around 120kW are supplied in MV. 

Only LV consumers are considered in this electrical guide. MV/LV step down power transformers are indifferently located either in the main substation or in the secondary substations. Small installations may only include a single MV/LV transformer installed in the main substation in most cases. 

A main substation includes five basic functions: Function 1: Connection to the MV utility network Function 2: General protection of the installation Function 3: Supply and protection of MV/LV power transformers located in the substation Function 4: Supply and protection of the internal MV distribution Function 5: Metering

Protection of a power system depends on its architecture and the operating mode.

A main Substation Includes Basic Devices Such as: 

1. Circuit breaker: 

The circuit breaker is a device that ensures the control and protection of a network. It is capable of making, withstanding and interrupting load currents as well as fault currents, up to the short-circuit current of the network. 

2. Switches: 

The alternating current switches and switch-disconnectors for their switching function, with load making and breaking current ratings. 

3. Contactors: 

Contactors are used to switch off and to switch on loads requiring these operations during normal use, especially as used in a particular activity such as the MV public lighting and industrial motors. 

4. Current-limiting fuses: 

MV current-limiting fuses are primarily used in protection of transformers, motors and other loads. This is a device that, by the fusing of one or more of its specially designed and proportioned components, opens the circuit in which it is inserted when this exceeds a given value for a sufficient time. Current limiting fuses may have difficulties in clearing intermediate current values (exceeding service values by less than a factor of 6 to 10) and are therefore often combined with a switching device. 

5. Disconnectors and earthing switches: 

The disconnectors are used to get a separation between two circuits which could be live and independent, without impairing their insulation level. This is typically used at the open point of a loop network. They are often used to separate a part of installation from the power supply with better performances than those provided by another switching device. A disconnector is not a safety device. Earthing switches are dedicated devices to connect conductors to earth in a reliable manner so the conductors can be accessed safely. They may have a rated short-circuit making current to ensure they can withstand a mistake in operation like closing on live conductors. 

6. Current transformer: 

It is intended to provide a secondary circuit with a current proportional to the primary current (MV) current.

7. Voltage transformer: 

The voltage transformer is intended to provide its secondary circuit with a secondary voltage that is proportional to that applied to the primary circuit. For installations including a single MV/LV power transformer, the general protection and the protection of the transformer are merged. 

The metering can be performed either at MV level or at LV level. It is usually done at LV level for any installation including a single MV/LV transformer, provided that the rated power of the transformer remains below the limit fixed by the local utility supplying the installation. In addition to the functional requirements the construction of both main and secondary substations shall comply with the local standards and regulation. IEC recommendations should also be taken into consideration in all circumstances

This article was extracted from " Medium Voltage Technical Guide"  by Schneider Electric

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