Types of Busbar Arrangements in Grid Stations and Substations

The arrangement and connection of incoming and outgoing feeders in grid stations and substations and the number of busbars have a significant influence on the supply reliability of the power system. Grid stations and substations, and the topology of the power systems must be designed in a similar way and must therefore be included in the context of planning as a single task. We have several busbar arrangements employed in grid stations and substations; they include:

• Single busbar without separation
• Single busbar with sectionalizer
• Special H-Arrangement
• Mesh arrangement
• Ring
• Double busbar arrangement
• Double busbar with reserve busbar

Single Busbar without separation

This is the simplest arrangement of a substation as illustrated in figure 1(a). The outgoing feeders are connected to a single busbar and a single transformer is installed.

Independently of the number of feeders supplied according to the topology of the system, no supply reserve exists for the outage of the transformer or of the busbar. The transformer can be loaded up to 100 % of its permissible (rated) load.

This arrangement is found in MV and LV systems but also in 110/10 kV systems where a three-winding transformer can be installed to feed two MV systems as illustrated below:

The arrangement with two transformers as illustrated in figure 1(c) offers a supply reserve for the outage of one transformer. If both transformers are loaded under normal operating conditions only to the extent that each one can take over the total load of the substation in case of outage of the other transformer, which is normally not substantially more than 50 % of the rated load.

If circuit breakers are installed in the outgoing feeders, short-circuits of the lines affect only the consumers attached to the faulted line, since the network protection disconnects the faulted line selectively. If load-break switches are installed in the outgoing feeders then one circuit-breaker is needed either on the MV or on the LV side of the transformer. In case of short-circuits on any feeder, the total load is switched off and supplied again only after isolation of the faulted line by the associated load break switch.

This busbar arrangement is characterized by the following features:

• Supply reserve in the case of busbar faults not provided by the substation itself.
• Supply reserve against transformer outage only given with second transformer.
• Deenergizing the busbar requires the interruption of supply.
• It is usually installed only in areas with small load density in the LV and MV voltage range.
• The flexibility for operation is comparatively low.
• Feeder arrangement in radial systems possible without circuit-breakers.
• Supply of ring-main systems advisable only if a remote station is available.

Single Busbar with Sectionalizer

The disadvantages presented by the single busbar without separation can be prevented by the arrangement of a busbar with a sectionalizer. In the first case the sectionalizing function is realized by a load break switch, in the second case by a circuit breaker.

Generally it is not meaningful to construct substations having two transformers with single busbar without sectionalizer. In principle with use of two transformers further arrangements of the substations are possible.

Another arrangement of a single busbar with two transformers is illustrated below:

This arrangement is characterized by the following features:

• Supply reserve in the case of busbar faults available 50 % of the load in the case of two busbar sections and 66 % in the case of three busbar sections.
• Supply reserve in the case of transformer outages available, depending on the loading of the transformers.
• Deenergizing of a busbar section requires supply interruption for 50 % or 33 % of the load depending on the number of busbar sections.
• Flexibility in operation in the medium range.
• It is usually installed in areas with medium load density in LV and MV systems.
• Used as an intermediate installation if three customers are to be installed finally.
• Arrangement of feeders in radial systems possible without circuit breakers.
• The supply of ring-main systems with remote station enables higher loading of the transformers.

Special H-Arrangement

Substations with single busbar, longitudinal bus coupler and two transformers are also installed in the 110 kV systems in urban areas. The 110 kV cables are looped in and out of the substation as shown in figure 3(a)

Supposing the parameters of the line protection are set in such a way that the longitudinal bus coupler is opened in case of short-circuits on the lines, the circuit breakers in the outgoing feeders can be avoided and only load break switches are required. A similar arrangement is applied for the transformer circuit breakers where in case of faults the feeding line is switched off.

A substation arrangement without any circuit breakers is called load disconnecting substation, but this needs two load break switches in the busbar in order to be able to deenergize each section of the busbar.

The characteristic features of this arrangement are:

• Supply reserve in the case of busbar faults available for about 50 % of the load.
• In the case of busbar faults, no power supply through the connected cables or overhead lines.
• Deenergizing of a busbar section possible without supply interruption with an appropriate arrangement on the lower voltage side.
• Supply reserve in case of outage of transformer due to faults.
• This arrangement is used in areas with medium load density, and sometimes employed in urban areas with high load density systems.
• Reduced investment cost for 110 kV possible.
• Limited flexibility in operation.

Mesh

This arrangement is known as a three switch mesh substation as shown in figure 3(b). It utilizes only three circuit breakers to control four circuits. This scheme offers better features and facilities than a single busbar without a bus section switch.

The three switch mesh has the following features:

• Any circuit breaker may be maintained at any time without disconnecting that circuit. To permit for all operating and maintenance conditions, all busbars, circuit breakers and disconnectors must be capable of carrying the combined loads of both transformers and line current power transfers.
• Normal operation is with the bypass disconnectors or optional circuit breaker open so that both transformers are not disconnected for a single transformer fault.
• A fault on one transformer circuit disconnects that transformer circuit without affecting the fine transformer circuit.
• A fault on the bus section circuit breaker causes complete substation shutdown until isolated and power restored.

A development of the three switch arrangement for multiple circuit substations is the full mesh layout shown in figure 3(c). Each section of the mesh is included in a line or transformer protection zone so no specific separate busbar protection is needed. Operation of the two circuit breakers is needed to connect or disconnect a circuit; disconnection involves opening the mesh line or transformer circuit, disconnectors may then be used to isolate the specific circuit and the mesh reclosed.

This arrangement has the following features:

• Circuit breakers may be maintained without loss of supply or protection and no additional bypass facilities are needed. That particular circuit may be fed from an alternative route around the mesh.
• Busbar faults will only cause the loss of one circuit. Circuit breaker faults will involve the loss of a maximum of two circuits.
• In general, not more than twice as many outgoing circuits as infeed are used in order to rationalize circuit equipment load capabilities and ratings. Maximum security is achieved with equal numbers of alternatively arranged infeed circuits and load circuits.

Related: Surge Suppression in Power Systems

Ring Busbar Arrangement

The ring busbar offers increased security compared to the single busbar arrangement since the alternative power flow routes around the ring busbar are available. An example of a typical scheme that would occupy more space than a single busbar arrangement is shown below:

The ring is not secure as the mesh arrangement discussed earlier since a busbar fault causes all circuits to be lost until the fault has been isolated using the ring busbar isolators. Unless busbar disconnectors are duplicated, maintenance on a disconnector requires an outage of adjacent circuits. The inability of disconnectors to break load current is also an operation disadvantage.

Related: Switchgear

Double Busbar Arrangement

Switchgear with double busbar is a typical arrangement for grid stations in MV, HV and EHV systems. All the incoming and outgoing lines and transformers are connected with circuit breakers and disconnecting switches to the busbars as illustrated in figure 4(a)

A bus coupler consisting of a circuit breaker and disconnecting switches is required to separate the two busbars in case of busbar faults.

This arrangement offers a high degree of supply reliability and operation flexibility because each outgoing line and transformer can be switched without supply interruption from one busbar to the other if the busbars are operated in coupled mode. For separate operation of the busbars, separate network groups can be operated.

Features of Double Busbar arrangement

• Supply reserve in the case of busbar faults available for the entire load.
• Supply reserve for outage of transformers available, depending on the loading of the transformers.
• Deenergizing of a busbar section possible without supply interruption.
• Employed in HV and EV transmission systems.
• Used in important substations in MV systems.
• Very high operation flexibility.
• This arrangement is used for supply of industrial systems.

Related: Key Factors to Consider In Substation Design

Double Busbar with Reserve Busbar

This arrangement is very costly and therefore only advisable for very important grid stations in the HV and EHV systems. During the operation, all the three busbars are energized; the outgoing transformers and lines are connected to two busbars only whilst the third one is separated with no load and is available as a reserve busbar and for switching purposes.

This arrangement has an advantage that a busbar can be completely deenergized without reducing the operation flexibility (i.e. two busbars remain in operation). In addition, in case of a loss of one busbar due to faults, the two other busbars remain available.

Key characteristics of this arrangement are:

• Supply reserve in case of busbar faults available for the entire load.
• Supply reserve for outage of transformers available depending on the loading of the transformers.
• Deenergizing of one busbar possible without supply interruption and without reduction of the operation flexibility.
• This arrangement is seldom used, only employed in HV and EHV grid stations of very great importance.
• Used in very important power stations.
• Employed in industrial power systems and sometimes for urban supply in the 110 kV systems.
• The operation flexibility is very high.
• It has a very high investment cost.

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