Distribution networks are typically of two types, radial or interconnected. A radial network leaves the station and passes through the network area with no normal connection to any other supply. This is typical of long rural lines with isolated load areas. An interconnected network is generally found in more urban areas and will have multiple connections to other points of supply. These points of connection are normally open but allow various configurations by the operating utility by closing and opening switches. Operation of these switches may be by remote control from a control center or by a lineman. The benefit of the interconnected model is that in the event of a fault or required maintenance a small area of network can be isolated and the remainder kept on supply.

The MV/LV substations are a node of a network, which includes a set of equipment designed to protect and facilitate the operation of the electrical energy.

The MV/LV substations provide the interface between the distribution MV and LV. The MV / LV adapts to all modes of operation and why should fulfill the following functions:

• • Distribute the power and protect the LV departures;
  • Isolate the MV/LV substation in case of default;
  • Manage the MV network in case of default and the position by remote control.

MV/LV substation transformer has two purposes:

• • The interconnection between the lines of the same voltage level, it helps to distribute the power of the different lines from substations.
  • The transformation of electrical power, processors can switch between voltage levels to another. So, we can classify the MV/LV substations over the networks on which they are used as well as the functions they perform. The realization of MV/LV substations needs prior knowledge:
  • Standards of reference and statutory instruments (voltage, quality of supply, short-circuit power, etc.…);
  • Needs to be performing (power plant availability, operation);
  • Specific needs-related uses (voltage variations tolerated, compensation of reactive power, immunity to interference receivers, regulations related to facility security);
  • Constraints, installation and environment

The consumer must provide certain data at the earliest stage of the project. (Maximum anticipated power demand (kVA), layout plans and elevations showing location of proposed substation, degree of supply continuity required from the information provided by the consumer), the power-supplier must indicate:

• • The type of power supply proposed, and define the kind of power-supply system: over headline or underground-cable network, the service connection details: single-line service, ring-main installation, or parallel feeders, and the power (kVA) limit and fault current.
  • The nominal voltage and rated voltage (Highest volt- age for equipment)
  • Metering details which define: The cost of connection to the power network and tariff details (consumption and standing charges).