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Home / Technical Articles / Circuit configurations (single line diagrams) for HV and MV switchgear installations

Single line diagram

The starting point for planning a switchgear installation is its single line diagram. This indicates the extent of the installation, such as the number of busbars and branches, and also their associated apparatus.

Circuit configurations for HV and MV switchgear installations
Circuit configurations for HV and MV switchgear installations (on photo: Ardnacrusha 110 kV GIS substation, Ireland; credit: ABB)

The most common circuit configurations of high and medium-voltage switchgear installations are shown in the form of single line diagrams next paragraphs.


Circuit configurations

The circuit configurations for high- and medium-voltage switchgear installations are governed by operational considerations. Whether single or multiple busbars are necessary will depend mainly on how the system is operated and on the need for sectionalizing, to avoid excessive breaking capacities.

Account is taken of the need to isolate parts of the installations for purposes of cleaning and maintenance, and also of future extensions.

When drawing up a single line-diagram, a great number of possible combinations of incoming and outgoing connections have to be considered. The most common ones are shown in the following diagrams.

  1. Most common circuit configurations
  2. Special configurations, mainly outside Europe
  3. Configurations for load-centre substations
      1. Branch connections
      2. Connections of instrument transformers
      3. Busbar coupling connections

1. The most common circuit configurations

Single busbars

Suitable for smaller installations. A sectionalizer allows the station to be split into two separate parts and the parts to be disconnected for maintenance purposes.

Single busbars
Single busbars

Double busbars

Preferred for larger installations. Advantages: cleaning and maintenance without interrupting supply. Separate operation of station sections possible from bus I and bus II. Busbar sectionalizing increases operational flexibility.

Double busbars
Double busbars

Double busbars in U connection

Low-cost, space-saving arrangement for installations with double busbars and branches to both sides.

Double busbars in U connection
Double busbars in U connection

Composite double bus/bypass bus

This arrangement can be adapted to operational requirements. The station can be operated with a double bus, or with a single bus plus bypass bus.

Composite double bus/bypass bus
Composite double bus/bypass bus

Double busbars with draw-out circuit-breaker

In medium-voltage stations, draw-out breakers reduce downtime when servicing the switchgear; also, a feeder isolator is eliminated.

Double busbars with draw-out circuit- breaker
Double busbars with draw-out circuit- breaker

Two-breaker method with draw-out circuit-breakers

Draw-out circuit-breakers result in economical medium-voltage stations. There are no busbar isolators or feeder isolators. For station operation, the draw-out breaker can be inserted in a cubicle for either bus I or bus II.

Two-breaker method with draw-out circuit-breakers
Two-breaker method with draw-out circuit-breakers

Double busbars with bypass busbar (US)

The bypass bus is an additional busbar connected via the bypass branch. Advantage: each branch of the installation can be isolated for maintenance without interrupting supply.

Double busbars with bypass busbar (US)
Double busbars with bypass busbar (US)

Triple (multiple) busbars

For vital installations feeding electrically separate networks or if rapid sectionalizing is required in the event of a fault to limit the short-circuit power. This layout is frequently provided with a bypass bus.

Triple (multiple) busbars
Triple (multiple) busbars

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2. Special configurations, mainly outside Europe

Double busbars with shunt disconnector

Shunt disconnector “U” can disconnect each branch without supply interruption. In shunt operation, the tie breaker acts as the branch circuit-breaker.

Double busbars with shunt disconnector
Double busbars with shunt disconnector

Two-breaker method with fixed switchgear

Circuit-breaker, branch disconnector and instrument transformers are duplicated in each branch. Busbar interchange and isolation of one bus is possible, one branch breaker can be taken out for maintenance at any time without interrupting operation.

Two-breaker method with fixed switchgear
Two-breaker method with fixed switchgear

1 ½ breaker method

Fewer circuit-breakers are needed for the same flexibility as above. Isolation without interruption. All breakers are normally closed. Uninterrupted supply is thus maintained even if one busbar fails.

The branches can be through-connected by means of linking breaker V.

1 ½ breaker method
1 ½ breaker method

Cross-tie method

With cross-tie disconnector “DT”, the power of line A can be switched to branch A1, bypassing the busbar. The busbars are then accessible for maintenance.

Cross-tie method
Cross-tie method

Ring busbars

Each branch requires only one circuit-breaker, and yet each breaker can be isolated without interrupting the power supply in the outgoing feeders.

The ring busbar layout is often used as the first stage of 1 ½ breaker configurations.

Ring busbars
Ring busbars

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3. Configurations for load-centre substations

Configurations for load-centre substations
Configurations for load-centre substations

Where:

  • A and B – Main transformer station,
  • C – Load-centre substation with circuit-breaker or switch disconnector.
The use of switch-disconnectors instead of circuit-breakers imposes operational restrictions.

Switch-disconnectors are frequently used in load-centre substations for the feeders to overhead lines, cables or transformers. Their use is determined by the operating conditions and economic considerations.

Switch-disconnectors used in load-centre substations
Switch-disconnectors used in load-centre substations

3a. Branch connections, variations a) to d)

a) Overhead-line and cable branches

Earthing switch 7 eliminates capacitive charges and provides protection against atmospheric charges on the overhead line.

Overhead-line and cable branches
Overhead-line and cable branches (1 – Busbar disconnector, 2 – Circuit-breaker, 3 – Switch-disconnector, 4 – Overhead-line or cable branch, – 5 Transformer branch, 6 – Branch disconnector, 7 – Earthing switch, 8 – Surge arrester)
b) Branch with unit earthing

Stationary earthing switches 7 are made necessary by the increase in short-circuit powers and (in impedance-earthed systems) earth-fault currents.

Branch with unit earthing
Branch with unit earthing (1 – Busbar disconnector, 2 – Circuit-breaker, 3 – Switch-disconnector, 4 – Overhead-line or cable branch, – 5 Transformer branch, 6 – Branch disconnector, 7 – Earthing switch, 8 – Surge arrester)
c) Transformer branches

Feeder disconnectors can usually be dispensed with in transformer branches because the transformer is disconnected on both h.v. and l.v. sides. For maintenance work, an earthing switch 7 is recommended.

Transformer branches
Transformer branches (1 – Busbar disconnector, 2 – Circuit-breaker, 3 – Switch-disconnector, 4 – Overhead-line or cable branch, – 5 Transformer branch, 6 – Branch disconnector, 7 – Earthing switch, 8 – Surge arrester)
d) Double branches

Double branches for two parallel feeders are generally fitted with branch disconnectors 6. In load-centre substations, by installing switch-disconnectors 3, it is possible to connect and disconnect, and also through-connect, branches 4 and 5.

Double branches
Double branches (1 – Busbar disconnector, 2 – Circuit-breaker, 3 – Switch-disconnector, 4 – Overhead-line or cable branch, – 5 Transformer branch, 6 – Branch disconnector, 7 – Earthing switch, 8 – Surge arrester)

3b. Connections of instrument transformers – Variations e) to g)

e) Normal branches

The instrument transformers are usually placed beyond the circuit-breaker 2, with voltage transformer 5 after current transformer 4. This is the correct arrangement for synchronizing purposes.

Some kinds of operation require the voltage transformer beyond the branch disconnectors, direct on the cable or overhead line.

Normal branches
Normal branches (1 – Busbar disconnectors, 2 – Branch circuit-breaker, 3 – Bypass circuit-breaker, 4 – Current transformers, 5 – Voltage transformers, 6 – Branch disconnector, 7 – Bypass disconnectors, 8 – Earthing switch)
f) Station with bypass busbar

Instrument transformers within branch.

The instrument transformers cease to function when the bypass is in operation. Line protection of the branch must be provided by the instrument transformers and protection relays of the bypass. This is possible only if the ratios of all transformers in all branches are approximately equal.

The protection relays of the bypass must also be set for the appropriate values. Maintenance of the branch transformers is easier and can be done during bypass operation.

If capacitive voltage transformers are used which also act as coupling capacitors for a high-frequency telephone link, this link is similarly inoperative in the bypass mode.

Station with bypass busbar
Station with bypass busbar – Instrument transformers within branch (1 – Busbar disconnectors, 2 – Branch circuit-breaker, 3 – Bypass circuit-breaker, 4 – Current transformers, 5 – Voltage transformers, 6 – Branch disconnector, 7 – Bypass disconnectors, 8 – Earthing switch)
g) Station with bypass busbar

Instrument transformers outside branch.

In bypass operation, the branch protection relays continue to function, as does the telephone link if capacitive voltage transformers are used. It is only necessary to switch the relay tripping circuit to the bypass circuit-breaker 3.

Servicing the transformers is more difficult since the branch must then be out of operation.

The decision as to whether the instrument transformers should be inside or outside the branch depends on the branch currents, the protection relays, the possibility of maintenance and, in the case of capacitive voltage transformers, on the h.f. telephone link.

Station with bypass busbar
Station with bypass busbar – Instrument transformers outside branch (1 – Busbar disconnectors, 2 – Branch circuit-breaker, 3 – Bypass circuit-breaker, 4 – Current transformers, 5 – Voltage transformers, 6 – Branch disconnector, 7 – Bypass disconnectors, 8 – Earthing switch)

3c. Busbar coupling connections

Experience shows that more complex coupling arrangements are usually needed in order to meet practical requirements concerning security of supply and the necessary flexibility when switching over or disconnecting.

This greater complexity is evident in the layouts for medium- and high-voltage installations.

Division into two bays is generally required in order to accommodate the equipment for these tie-breaker branches.

Double busbars
Double busbars
Double busbars (A and B = Busbar sections, LTr = Busbar sectioning disconnector)
Triple busbars
Triple busbars
Triple busbars

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Reference // ABB Switchgear Manual

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About Author

author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

5 Comments


  1. Karim
    May 16, 2018

    I am engineer electrical and now working in algeria


  2. Efren Marual Jr.
    Mar 13, 2018

    Dear sir Edvard,

    Congratulation! I am a candidate to take Professional Electrical Engineer in the Philippines.and practicing
    EE profession Thanks for all the technical references from EEP..your advocacy to help Electrical Engineers and Electrical Engineering Students around the world is a great treasure and as current president of Institute of Electrical Engineers of the Philippines Inc. (IIEE) Southern Laguna Chapter, we would like to recognize your goodwill by sending you a Certificate of Recognition from our chapter..Again, Congratulation sir Edvard!


  3. Maditsela
    Jan 28, 2018

    Very insightful. I am responsible for secondary substation maintenance. South Africa


  4. Sayed Tamal Hhossain
    Jan 24, 2018

    Dear sir,

    I am your EEP Friend. I am Electrical Engineer, Now working in Iraq,Bismayha New City Project, Bagad. I am Bangladeshi.

    Thanks for your useful Post .Your post is very helpful for electrical engineer.

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