# Substation Bus Configuration / Scheme: The Definitive Guide

- Link: https://studyelectrical.com/2019/10/substation-bus-configurations.html
- Published: 2019-10-03T06:06:12+00:00
- Author: Tesla

In this article, you will learn different types of substation bus configuration 
and their application.

The equipment and buses installed in the substation switchyard are arranged and 
connected in specific ways to form **bus configurations**.

![Substation Bus Configurations](https://studyelectrical.com/wp-content/uploads/
2019/10/Substation-Bus-Configurations.jpg)

Substation Bus Configurations

The industry has developed several standard bus configurations that vary in complexity,
cost, and reliability.

The standard bus configurations/scheme are

 1. **Radial bus**
 2. **Sectionalized radial bus**
 3. **Main and transfer bus**
 4. **Single breaker double bus**
 5. **Ring bus**
 6. **One-half breaker**
 7. **Breaker and one-half**
 8. **Double breaker double bus.**

The layout of a substation for any particular configuration may vary to accommodate
differences in equipment type, size and arrangement, and site-specific criteria.

## 1. Radial Bus Configuration

The radial bus configuration consists of **one main bus**. The transmission lines,
[transformers](https://studyelectrical.com/2015/04/working-principle-of-transformer.html),
and shunt capacitor banks are connected to the main bus through [circuit breakers](https://studyelectrical.com/2018/12/circuit-breaker-operating-principle-and.html),
circuit switchers, or motor operated or manually operated disconnect switches.

![Radial Bus Configuration](https://studyelectrical.com/wp-content/uploads/2019/
10/Radial-Bus-Configuration.jpg)

Radial Substation Bus Configuration

Radial bus substations are the **simplest to operate**. But they have the **least
system reliability and flexibility** of operation.

 * Bus faults and failure of a breaker to operate for a fault require an outage 
   of the complete substation.
 * In radial bus substations, it is necessary to take an outage of a circuit to 
   perform periodic or emergency maintenance on its associated circuit breaker.

As shown in the figure, breaker bypass switches can be installed to allow removal
of a circuit breaker from service for maintenance without an outage of the associated
circuit, but this leaves the circuit without relay protection.

When the breaker is isolated for maintenance, the bypass switch is closed, and the
circuit breaker and its associated disconnect switches are opened.

All protective relaying and control for the circuit at the local substation are 
removed from service when the circuit breaker is isolated.

A fault on the circuit with its associated circuit breaker bypassed requires an 
outage of the complete substation.

### Advantages and Disadvantages of Radial Bus Configuration

**Advantages of the radial bus substations** over other configurations are the lowest
cost, small required land area, ease of expansion, simple operation, and simple 
protective relaying.

**Disadvantages of radial bus configuration substations** include low reliability,
low flexibility of operation for maintenance, and the removal of the substation 
from service in cases of bus faults and failure of a breaker.

**Application**

The radial bus configuration is generally **applied in substations from distribution
voltage through 161 kV and in locations where system reliability is not critical.**

## 2. Sectionalized Radial Bus Configuration

The sectionalized or split radial bus is a **modification of the radial bus**. This
configuration is **two radial buses** tied together through a sectionalizing or **
bus tie circuit breaker**.

![Sectionalized Radial Bus](https://studyelectrical.com/wp-content/uploads/2019/
10/Sectionalized-Radial-Bus.jpg)

Sectionalized Radial Bus Configuration

The sectionalizing circuit breaker can be operated normally open or normally closed,
depending on system requirements.

Bus faults or the failure of a breaker (other than the tiebreaker) to operate for
a fault **requires an outage of only the affected bus section.**

The grouping of circuits on bus sections is determined by examining system operating
criteria.

 * Circuits should be arranged to prevent outages on similar or redundant circuits.
 * Circuits fed from the same source or circuits feeding the same load should be
   on different bus sections.

Breaker bypass switches can be applied in sectionalized radial bus substations and
operate the same as in radial bus substations.

### Advantages and Disadvantages of Sectionalized Radial Bus

Sectionalized radial bus substations offer the advantages of

 * small land area,
 * increased reliability over the radial bus,
 * increased flexibility of operation over the radial bus,
 * and ease of expansion.

Disadvantages, when compared to the radial bus include

 * increased cost,
 * increased complexity of operation,
 * and increased complexity of [protective relaying](https://studyelectrical.com/2015/08/how-protective-electrical-relays-work.html).

**Application**

The sectionalized radial bus configuration is generally applied in substations from
distribution voltage through 161 kV and in locations where system reliability is
not critical.

## 3. Main and Transfer Bus

The main and transfer bus is another modification of the radial bus. This configuration
consists of **a main bus and a transfer bus**.

All circuits are connected to the **main bus through circuit breakers** and to the**
transfer bus through transfer switches**. The main and transfer buses are connected
through a **transfer bus circuit breaker**.

The transfer bus circuit breaker protects a circuit during maintenance of its associated
circuit breaker.

![Main and Transfer Bus](https://studyelectrical.com/wp-content/uploads/2019/10/
Main-and-Transfer-Bus.jpg)

Main and Transfer Bus

When a circuit breaker is removed from service for maintenance, the transfer circuit
breaker and its associated disconnect switches are closed, the transfer switch for
the circuit breaker to be serviced is closed, and the circuit breaker to be maintained
and its associated disconnect switches are opened. Reliability and protection are
not compromised during maintenance.

Considerable attention must be given to the selection of the protective relaying
for the transfer circuit
 breaker.

### Advantages and Disadvantages

The advantages of main and transfer bus substations when compared to other configurations
include the

 * small land area required,
 * ease of expansion,
 * increased flexibility of operation over the radial bus or split radial bus,
 * and the fact that any breaker can be removed from service without an outage of
   the circuit served.

Disadvantages over the radial bus are

 * the increased cost,
 * increased complexity of the operation,
 * increased complexity of protection,
 * and no improvement in reliability.

**Application:**

The main and transfer bus configuration is generally applied in [substations](https://studyelectrical.com/2019/04/electrical-substation-equipment-types-components-functions.html)
from distribution voltage through 161 kV and in locations where system reliability
is not critical.

## 4. Single Breaker Double Bus Configuration

The single breaker double bus configuration is a **modification of the sectionalized
radial bus**. This configuration consists of **two main buses connected through 
a tie circuit breaker**.

![Single Breaker Double Bus](https://studyelectrical.com/wp-content/uploads/2019/
10/Single-Breaker-Double-Bus.jpg)

Single Breaker Double Bus

Each circuit has one circuit breaker that can be connected to either the main bus
through disconnect switches.

This configuration allows circuits to be connected to either the main bus to balance
the load, separate critical circuits, or place sources on each bus and allows all
circuits to be connected to one bus in case of an outage on the other bus.

The switching of a circuit from one bus to the other is not automatic and requires
manual switching.

### Advantages, Disadvantages & Application

Single breaker, double bus substations have the same advantages and disadvantages
as the split bus radial, and additional disadvantages including increased cost over
the split radial bus increased complexity of protective relaying over the split 
radial bus because of the requirement for switching of bus relaying current transformer
secondary circuits.

The single breaker double bus configuration is generally applied in substations 
from distribution voltage through 161 kV and in locations where system reliability
is not critical.

**It is also the least common of the radial bus configurations discussed.**

## 5. Ring Bus Configuration

The ring bus configuration is, in reality, **a series of sectionalized radial buses**
connected together to form a **ring**. Each bus is called a **position**.

Sometimes a transmission line and a transformer are paired on one ring position.
In this configuration, only one position is removed from service for a circuit or
bus fault.

![Ring Bus Configuration (Four Position)](https://studyelectrical.com/wp-content/
uploads/2019/10/Ring-Bus-Configuration-Four-Position.jpg)

Ring Bus Configuration (Four Position)

The circuit breakers which serve the faulted position are opened. The failure of
a breaker to operate for a line or bus fault will cause two positions to be removed
from service.

This configuration allows for any circuit breaker to be removed from service for
maintenance without an outage on any circuit.

Line disconnect switches are often installed to allow a line to be removed from 
service and the ring to remain intact. The two circuit breakers sourcing the line
are opened, the line disconnect switch is opened, and then the two circuit breakers
are closed.

Ring bus substations are highly reliable and flexible to operate. They are generally
limited to a maximum of eight positions to prevent the splitting of the ring. Sources
of generation or redundant circuits should not be terminated on adjacent positions
of the ring bus. This prevents a failed circuit breaker from removing two sources
of generation or two feeds to the same load from service.

### Advantages, Disadvantages & Application

**Ring bus substations have the advantages** of high reliability, flexible operation,
and low cost when compared to the breaker and one-half configuration, removal of
any breaker from service without circuit outage, and the possibility of expansion
to breaker and one-half configuration.

The **disadvantages** are the complex [protective relaying](https://studyelectrical.com/2016/07/basics-of-protective-relaying-in-power-system.html)
and control and the eight-position limitation.

**Application **

The ring bus configuration is generally applied in substations from 115 kV through
345 kV, and unlimited application at 500 kV in locations where high system reliability
is a requirement.

## 6. One-Half Breaker Configuration

The one-half breaker configuration is a v**ariation of the ring bus concept on a
multiple substation basis**. As with the ring bus, two breakers must be tripped 
to isolate a faulted line or transformer.

In the case of the one-half breaker configuration, one of the breakers is usually
at the other end of the transmission line. In the Figure below, Substations A, B,
C, D, and E form an extended ring bus.

![One half breaker Configuration](https://studyelectrical.com/wp-content/uploads/
2019/10/One-half-breaker-Configuration.jpg)

One half breaker Configuration

The **advantages of this configuration** are the same as for the ring bus, and on
an individual substation basis, the costs are even lower than for the radial bus.

The one-half breaker configuration is generally applied in **substations from 69
kV through 161 kV**, and in systems where several substations are located near each
other.

## 7. Breaker and One Half Bus Configuration

The breaker and one-half configuration consist of **two main buses**. Connected 
between the main buses are bays which consist of **three circuit breakers**. A circuit
is terminated between each two circuit breakers.

![Breaker and One half bus configuration](https://studyelectrical.com/wp-content/
uploads/2019/10/Breaker-and-One-half-bus-configuration.jpg)

Breaker and One half bus configuration

In this configuration, each circuit has a dedicated circuit breaker and shares a
circuit breaker with the adjacent circuit, resulting in one and one-half breakers
per circuit.

Frequently, a substation is designed to operate initially as a ring bus up through
expansion to six positions. Beyond six positions, the substation evolves to a breaker
and one-half configuration.

There are two types of the breaker and one-half configurations,

 1. **conventional **
 2. **folded.**

In the **conventional arrangement** transmission lines must pass over one of the
main buses, causing line termination structures to have higher pull-off points. 
Also, the installation of line traps, current transformers, and disconnect switches
in the lines is difficult.

![Folded Breaker and One-Half substation bus configuration](https://studyelectrical.
com/wp-content/uploads/2019/10/Folded-Breaker-and-One-Half-substation-bus-configuration.
jpg)

Folded Breaker and One-Half substation bus configuration

The **folded arrangement** locates line termination structures outside the main 
buses, allowing conventional pull-off heights to be used. The installation of line
traps, [current transformers](https://studyelectrical.com/2018/09/current-transformer-ct-construction-working-operating-principle-advantages-uses.html),
and [disconnect switches](https://studyelectrical.com/2015/09/air-break-switch-types-operation-working.html)
in the lines are relatively easy. Also, the folded arrangement can be fitted to 
oddly shaped sites more easily than can the conventional arrangement.

In Breaker and One Half Configuration configuration, only one circuit, the faulted
circuit, is removed from service for a fault. The main bus fault does not require
that circuits be removed from service.

The failure of a circuit breaker between the main bus and a circuit to operate for
the main bus fault requires that only the circuit adjacent to the circuit breaker
be removed from service. The failure of a circuit breaker between two circuits to
operate for a fault requires the two adjacent circuits to be removed from the service.

This configuration allows any circuit breaker to be removed from service for maintenance
without an outage on any circuit.

Line disconnect switches are sometimes installed to allow a circuit to be removed
from service and all circuit breakers to remain closed.

Breaker and one-half substations are very reliable and flexible in operation. Sources
of generation or redundant circuits should not be connected in the same bay. This
prevents a failed breaker from removing two sources of generation or two feeds to
the same load from service.

### Advantages, Disadvantages & Application

**Advantages of breaker and one-half substations over other configurations are**

 * very high reliability,
 * very flexible operation,
 * removal of any breaker from service without a circuit outage,
 * and ease of expansion, with no limit on the number of bays.

**Disadvantages are**

 * the large land area required,
 * the high cost,
 * and the complex protective relaying and control.

**Application**

The breaker and one-half configuration is generally applied in substations from 
230 kV through ultrahigh voltages but can be applied at 69 kV, 115 kV,138 kV, and
161 kV.

Because of its high reliability, it is often applied at major generation facilities
and at locations where system reliability is critical.

## 8. Double Breaker Double Bus Configuration

The double breaker double bus configuration consists of **two main buses**. Connected
between the main buses are bays consisting of **two circuit breakers**, and between
the circuit breakers, a circuit. In this configuration, each circuit has two dedicated
circuit breakers.

![Double Breaker Double Bus configuration](https://studyelectrical.com/wp-content/
uploads/2019/10/Double-Breaker-Double-Bus-configuration.jpg)

Double Breaker Double Bus configuration

Only the faulted circuit is removed from the service for a fault. A bus fault requires
that no circuits be removed from service. The failure of a circuit breaker to operate
for a bus fault requires only that the circuit terminated in that bay be removed
from service.

This configuration allows any circuit breaker to be removed from service for maintenance
without an outage on any circuit. Line disconnect switches are usually not required.

Double breaker double bus substations are the **most reliable and are very flexible
to operate**. They require no separation of sources of generation or redundant circuits.

### Advantages, Disadvantages & Application

**Double breaker double bus substations have the advantages of**

 * very high reliability,
 * very flexible operation,
 * the fact that breaker removal for maintenance will not cause an outage,
 * and ease of expansion.

**Disadvantages include**

 * high cost,
 * the large land area required,
 * and the complex protective relaying and control.

The double breaker double bus configuration is generally applied in substations 
from 230 kV through ultra-high voltages, nuclear-generating facilities, major generation
facilities, and locations where system reliability is very critical.

## Comparison of Bus Configuration

The following tabulation compares the relative constructed costs and levels of reliability
of each configuration for a substation serving six transmission lines.

![Comparison of Bus Configuration](https://studyelectrical.com/wp-content/uploads/
2019/10/Comparison-of-Bus-configurations.jpg)

Comparison of Bus Configuration

Usually, some bus configurations can be eliminated from consideration for a particular
substation on the basis of its function.

A radial bus configuration would not be considered for a nuclear generating station,
nor would a double breaker, double bus configuration be considered for a distribution
substation.

![Typical Bus Configuration Voltage Levels](https://studyelectrical.com/wp-content/
uploads/2019/10/Typical-Bus-Configuration-Voltage-Levels-.jpg)

Typical Bus Configuration Voltage Levels

The selection of a bus configuration for a particular substation should always take
into account the ultimate anticipated development and function of that installation.
The figure above shows at what voltage levels each configuration is typically applied.

Reference: [_Fundamental Concepts in Substation Design_](https://ieeexplore.ieee.org/document/239554)

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