Nine topologies of medium voltage ring-main systems you should know about

MV ring-main systems

Power systems are constructed can be operated as radial systems, ring-main systems or meshed systems. This technical article deals with nine most common topologies of ring-main systems, their characteristics and applications. Ring-main systems are common in the medium voltage range.

A large variety of ring-main systems are in operation with respect to permissible loading of the lines, reserve capability against outages, different arrangement of the feeding station and supply reliability.

Let’s discuss about characteristics of nine main topologies ring-main systems:

1. Ring-Main System – Simple Topology
2. Ring-Main System with Remote Station (Without Supply)
3. Ring-Main System with Reserve Line
4. Ring-Main System with Feeding Remote Station
5. Ring-Main System as Tuple System
6. Ring-Main System with Cross-Link
7. Ring-Main System with Base Station
8. Special-Spare Cable System
9. Double-T Connection

1. Ring-Main System – Simple Topology

The simplest kind of ring-main system is obtained by connecting the line ends (starting from the radial system topology) back to the feeding station as outlined in Figure 1.

Usually ring-main systems are operated with open disconnection points (load disconnecting switch) at defned locations on each line, which provides for simple operation including a switchable reserve capability, depending on the loading of the lines.

The loading of the lines must be selected in such a way that in case of failure of a line the total load of this line concerned can be supplied after closing the load-switch at the open disconnection point.

The system is characterized by:

• Clear and simple structure.
• Moderate planning expenditure.
• Simple operation (similar to radial system) under normal operating conditions.
• Loading of lines during normal operation 50% of the permissible loading, higher loading possible depending on load duration.
• Reserve for outage of each line section given by the line itself.
• Investment cost not very high; reduction possible, if circuit-breakers are omitted. In this case one circuit-breaker has to be installed on the secondary side of the feeding transformer.
• Maintenance cost rather low.
• System losses can be minimized by changing the location of the open disconnection point.
• Voltage profile can be optimized, differences between feeding and receiving end of the lines depend on the location of the open disconnection point.
• Flexibility to respond to changing load conditions.
• Reserve for outage of feeding transformer or bus section usually available if an arrangement is selected as in Figure 1b.
• Standardization of cross-sections of the lines is given (usually only one cross-section shall be used).
• Feeder protection can be realized with overcurrent protection.
• Application in medium-voltage systems up to 35 kV, in case of high load density, in principle also in low-voltage systems.

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2. Ring-Main System with Remote Station (Without Supply)

Connecting the individual lines of the system at the receiving end to a station without infeed from a higher voltage supply level forms a ring-main system as outlined in Figure 2.

The permissible loading of individual lines can be above 50% of the thermally permissible load of a line, depending on the loading of all lines.

In case of outage of any feeder, the load will be supplied by separation of the faulted line section through the remote station by one or more lines connected to it.

This type of ring-main system with remote station can be seen as an intermediate step of system development between a pure radial system and a ring-main system with a feeding remote station (see below).

The system is characterized by:

• Clear and simple structure.
• Moderate expenditure for planning.
• Simple operation under normal operating conditions.
• Loading of lines under normal operating conditions more than 50% of permissible thermal loading.
• Reserve for outages available depending on the preloading of the remaining lines.
• Investment cost in the medium range; reduction possible if a topology without circuit-breakers for the feeders is selected. In this case one circuit-breaker has to be installed on the secondary side of the feeding transformer.
• Maintenance cost rather low.
• Power system losses cannot be minimized.
• Voltage profile in the system not optimal, differences between feeding station and remote station are significant.
• Flexibility to respond to changed load conditions.
• Reserve for outage of the feeding MV transformer usually available.
• Standardization of cross-sections of all lines given (usually one cross-section used).
• Feeder protection can be realized with overcurrent protection.
• Application in medium-voltage systems up to 35kV, also in case of medium load density, in principle also in low-voltage systems.

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3. Ring-Main System with Reserve Line

If a separate line between the feeding station and the remote station is constructed, without supplying load through this line under normal operating conditions, the system performance is significantly improved by this reserve line.

The outage of one line is covered under the system topology. The second line outage is no longer covered, however; to achieve this, the reserve line must have a larger cross-section.