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Switchgear installations

The arrangement of outdoor switchgear layouts and installations is mostly influenced by economic considerations, in particular adaptation to the space available and the operational requirements of reliability and ease of supervision.

The Most Used Outdoor Switchyard Layouts You Should Know About
The Most Used Outdoor Switchyard Layouts You Should Know About (photo credit: ABB)

Contents:

    1. Switchyard layout in general
    2. Selected layout examples:
      1. Low rise (classical layout)
      2. In-line layout
      3. Transverse layout
      4. High-rise layout
      5. Diagonal layout
        1. “Busbars above” layout
        2. “Busbars below” layout
        3. Bypass bus, single-row arrangement layout
      6. 1½-breaker layout
    3. Comparison of different switchyard layouts

1. Switchyard layout in general

To meet above conditions, various layouts (see Table 1) have evolved for the circuit configurations.

Many electric utilities have a preference for certain arrangements which they have adopted as standard. The spacing of the branches is determined by the switchyard configuration. A span length of 50 m is economical for guyed wire (strain) busbars. The number and design of portal structures is governed by the overall length of the installation.

The larger bay width T1 and T2 of the busbar step-down bays (starting bay, end bay) must be taken into account when planning the layout. For stations with busbar current ratings above about 3000 A, tubular busbars offer a more economical solution than tensioned wires.

In 123 kV stations, the tubular busbars are supported at each alternate bay, but at each bay with higher voltages.

The overhead lines leading from the transformer stations are generally also used for power-line carrier telephony. The necessary equipment (line trap, capacitor) is incorporated in the outgoing overhead lines as shown in Figure 1.

Arrangement of overhead line bays for power-line carrier telephony
Figure 1 – Arrangement of overhead line bays for power-line carrier telephony a) Line trap suspended, capacitor standing, b) Line trap mounted on capacitive voltage transformer

Where:

  1. Circuit-breaker
  2. Feeder disconnector
  3. Current transformer
  4. Inductive voltage transformer
  5. Capacitive voltage transformer
  6. Capacitor
  7. Line trap

Points in favour of rotary and vertical-break disconnectors are their mechanical simplicity and the fact that they are easier to position as feeder disconnectors.

The single-column disconnector makes for a simple station layout owing to its isolating distance between the two line levels. It saves some 20% of the ground area needed for two-column disconnectors.

Table 1 – Outdoor switchyard configurations, preferred application

Layout≤ 145 kV245 kV420 kV≥ 525 kV
Low rise (classical layout)xx
In-line layoutx
Transverse layoutxx
High-rise layoutx
Diagonal layoutxx
1½-breaker layoutxxx

Each branch (bay) consists of the circuit-breaker with its disconnectors, instrument transformers and control cubicle. The apparatus is best placed at a height such that no fencing is needed.

Here, it must be noted that according to DIN VDE 0101 (see figure), the height to the top edge of the earthed insulator base must be at least 2250 mm. The high-voltage apparatus is generally mounted directly on equipment support structures.

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2. Selected examples of switchyard layouts

2.1. Low rise (classical layout)

With the low-rise (classical) layout (Figure 2), the busbar disconnectors are arranged side by side in line with the feeder. The busbars are strung above these in a second level, and in a third plane are the branch lines, with connections to the circuit-breaker.

A great advantage of this layout is that the breaker and transformer can be bypassed by reconnecting this line to the feeder disconnector. Features of this configuration are the narrow spacing between bays, but higher costs for portal structures and for means of tensioning the wires.

The classical layout is also used for stations employing the 2-breaker method.

245 kV outdoor switchyard with double busbars, low-rise (classical) layout
Figure 2 – 245 kV outdoor switchyard with double busbars, low-rise (classical) layout

Where:

  1. Busbar system I
  2. Busbar system II
  3. Busbar disconnector
  4. Circuit-breaker
  5. Current transformer
  6. Voltage transformer
  7. Feeder disconnector
  8. Surge arrester
  9. T Bay width
  10. T1 Width initial bay
  11. T2 Width final bay at busbar dead-end

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2.2. In-line layout

An in-line layout with tubular busbars is shown in Figure 3. It is employed with busbar current ratings of more than 3000 A. The poles of the busbar disconnectors stand in line with the busbars. Portals are needed only for the outgoing overhead lines.

This arrangement incurs the lower costs for supporting steelwork and results in an extremely clear station layout.

In stations including a bypass bus, the layout chosen for the bypass bus and its disconnectors is the same as for the busbars. In stations with feeders going out on both sides, the bypass bus must be U-shaped so that all branches can be connected to it.

123 kV outdoor switchyard with double busbars, in-line layout
Figure 3 – 123 kV outdoor switchyard with double busbars, in-line layout. The busbars are tubular.

Where:

  1. Busbar system I
  2. Busbar system ll
  3. Busbar disconnector
  4. Circuit-breaker
  5. Current transformer
  6. Voltage transformer
  7. Feeder disconnector
  8. Surge arrester
  9. T Bay width,
  10. T1 Width initial bay
  11. T2 Width final bay

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2.3. Transverse layout

With the transverse layout, the poles of the busbar disconnectors are in a row at right angles to the busbar, see Figure 4. With this arrangement too, the busbars can be of wire or tube. The outgoing lines are strung over the top and fixed to strain portals.

Though the bay width is small, this arrangement results in a large depth of installation.

123 kV outdoor switchyard with double busbars, transverse layout
Figure 4 – 123 kV outdoor switchyard with double busbars, transverse layout

Where:

  1. Busbar system I
  2. Busbar system ll
  3. Busbar disconnector
  4. Circuit-breaker
  5. Current transformer
  6. Voltage transformer
  7. Feeder disconnector
  8. Surge arrester
  9. T Bay width
  10. T1 Width initial bay
  11. T2 Width final bay

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Special layouts

Arrangements with draw-out breakers save a great deal of space, as the draw-out circuit-breaker does away with the need for disconnectors. The outgoing line simply includes an earthing switch. This configuration is used for stations with single busbars.

The costs are low. The circuit-breaker is fitted with suitable plug-in contacts and a hydraulically operated truck.

2.4. High-rise layout

Load-centre substations with one or two power transformers are usually in the form of simplified transformer stations. In Figure 5, two incoming overhead lines connect to two transformers (H-connection).

This gives rise to two busbar sections joined via two sectionalizers (two disconnectors in series).

In this way, each part of the installation can be isolated for maintenance purposes. The bus sections can be operated separately or crosswise, ensuring great reliability and security of supply.

123 kV load-centre station (H-connection)
Figure 5 – 123 kV load-centre station (H-connection)

Where:

  1. Busbars
  2. Busbar disconnector
  3. Circuitbreaker
  4. Current transformer
  5. Voltage transformer
  6. Feeder disconnector
  7. Surge arrester

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2.5. Diagonal layout

With this arrangement, the (single-column) busbar disconnectors are arranged diagonally with reference to the busbars. It is commonly used for 245 kV and 420 kV
stations. A distinction is made between two versions, depending on the position (level) of the busbars:

  1. Busbars above
  2. Busbars below

2.5.1. Diagonal “busbars above” layout

The advantage of this layout (Figure 6) is that when a feeder is disconnected, the busbar disconnectors are also disconnected and are thus accessible.

For installations with current ratings of more than 3000 A and high short-circuit stresses, the busbars and jumper connections are made of tubes.

Figure 6 shows a 420 kV station in a diagonal layout and using tubes. The tubes are in lengths of one bay and mounted on the post insulators with a fixed point in the middle and sliding supports at either end.

The busbars can be welded together over several bays up to about 120 m.

 420 kV outdoor switchyard with double busbars of tubular type, diagonal layout, busbars above
Figure 6 – 420 kV outdoor switchyard with double busbars of tubular type, diagonal layout, busbars above

Where:

  1. Busbar system I
  2. Busbar system II
  3. Busbar disconnector
  4. Circuit-breaker
  5. Current transformer
  6. Feeder disconnector
  7. Line trap
  8. Capacitive voltage transformer
  9. T Bay width
  10. T1 Width initial bay
  11. T2 Width final bay

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2.5.2. Diagonal “busbars below” layout

With this arrangement, the busbars are mounted on the disconnectors with the outgoing lines strung at right angles to them. At their points of intersection, single-column disconnectors maintain the connection with their vertical isolating distance.

This economical layout requires lightweight busbar strain portals only at the ends of the installation, and the bays are narrow. It can be of single or double-row form. The single-row arrangement (Figure 7) is more space-saving. Compared with a tworow layout it requires about 20 % less area.

The circuit-breakers for all outgoing lines are on the same side of the busbars so that only one path is needed for transport and operation. The lines to the transformers lie in a third plane.

245 kV outdoor switchyard with double busbars, diagonal layout, busbars below, single-row arrangement
Figure 7 – 245 kV outdoor switchyard with double busbars, diagonal layout, busbars below, single-row arrangement

Where:

  1. Busbar system I
  2. Busbar system II
  3. Busbar disconnector
  4. Circuit-breaker
  5. Current transformer
  6. Feeder disconnector
  7. Line trap
  8. Capacitive voltage transformer
  9. T Bay width
  10. T1 Width initial bay
  11. T2 Width final bay with busbar dead-end

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2.5.3. Diagonal layout with bypass bus, single-row arrangement

The 420 kV switchyards of the German transmission grid are of the diagonal type. To meet the stringent demands of station operation and reliability, double or triple busbars with sectionalizing and an additional bypass bus are customary.

Tube-type busbars are preferred. These can handle high current ratings and high short-circuit stresses.

The space-saving single-row layout with the circuit-breakers of all outgoing lines in one row is very effective here, too. Using two-column isolators on the feeders simplifies the layout.

Single-column isolators are used for the busbars and the bypass bus (see Figure 8).

420 kV outdoor switchyard with tubular conductors, triple busbars and bypass bus, diagonal layout, single-row arrangement
Figure 8 – 420 kV outdoor switchyard with tubular conductors, triple busbars and bypass bus, diagonal layout, single-row arrangement

Where:

  1. Busbar system I
  2. Busbar system II
  3. Busbar system III
  4. Bypass bus
  5. Busbar disconnector
  6. Circuit-breaker
  7. Feeder disconnector
  8. Bypass disconnector
  9. Current transformer
  10. Voltage transformer
  11. a and b Ties for busbars 1, 2 and 3 and bypass bus 4
  12. c Outgoing line

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2.6. 1½-breaker layout

The 1½-breaker configuration is used mainly in countries outside Europe. It is employed for all voltages above 110 kV, but predominantly in the very high voltage
range.

The double busbars of these stations are arranged above, both outside or inside, and can be of tube or wire. The more economical solution of stranded conductors is often used for the links to the apparatus, because with the relatively short distances between supports, even the highest short-circuit currents can exert only limited stresses on the equipment terminals.

The branches are always arranged in two rows. The disconnectors used are of the pantograph and two-column vertical-break types. Vertical-break disconnectors are employed in the outgoing line.

Figure 9 shows a section through one bay of a 525 kV station, the busbars are of wire.

This arrangement allows the station to be operated on the ring bus principle while construction is still in progress, and before all the switchgear apparatus has been installed.

525 kV outdoor switchyard, 1½-breaker layout
Figure 9 – 525 kV outdoor switchyard, 1½-breaker layout

Where:

    1. Busbar system I
    2. Busbar system II
    3. Busbar disconnector
    4. Circuit-breaker
    5. Current transformer
    6. Voltage transformer
    7. Feeder disconnector
    8. Branch disconnector
    9. Surge arrester
    10. Line trap
    11. Transformer

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3. Comparison of different layouts

Table 2 compares different layouts of 123-kV outdoor switchyards as regards area, foundations (volume) and steelwork (weight) for one line branch and one transformer branch with double busbar, assuming a total size of the substation of 5 bays.

Table 2 – Comparison of different layouts for 123 kV

Type of branch
(bay)
Overhead lineTransformer
Type of layoutAreaFoundations
(volume)
Steel-workAreaFoundations
(volume)
Steel-work
except cable
gantry on LV
side
In-line
(tubular
busbars)
225 m2
100 %
23.3 m3
100 %
6.6 t
100 %
193 m2
100 %
52.3 m3
100 %
4.3 t
100 %
Transverse
(tubular
busbars)
282 m2
125 %
27.2 m3
117 %
7.8 t
118 %
302 m2
156 %
78.4 m3
150 %
9.6 t
223 %
Low-rise
(classical,
wire busbars)
192 m2
86 %
33.9 m3
145 %
8.4 t
127 %
201 m2
104 %
81.3 m3
155 %
8.8 t
205 %

Table 3 compares different layouts of 245-kV outdoor switchyards as regards area, foundations (volume) and steelwork (weight) for one line branch and one transformer branch with double busbar and bypass bus or 1½-breaker layout.

Table 3 – Comparison of different layouts for 245 kV

Type of branch
(bay)
Overhead lineTransformer
Type of layoutAreaFoundations
(volume)
Steel-workAreaFoundations
(volume)
Steel-work
except cable
gantry on LV
side
In-line
(tubular
busbars)
323 m2
100 %
28 m3
100 %
7.9 t
100 %
344 m2
100 %
63.2 m3
100 %
7.0 t
100 %
Transverse
(tubular
busbars)
413 m2
128 %
31.9 m3
114 %
9.1 t
115 %
433 m2
126 %
69.2 m3
110 %
9.4 t
134 %
Low-rise
(classical,
wire busbars)
324 m2
100 %
38.6 m3
138 %
10.4 t
132 %
369 m2
107 %
83.1 m3
131 %
12.5 t
179 %
1½-breaker
(tubular
busbars)
267 m2
83 %
27.4 m3
98 %
8.1 t
103 %
301 m2
88 %
47.7 m3
76 %
8.5 t
121 %

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Reference // ABB Switchgear Manual

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About Author

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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

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