Electrical Bus System and Electrical Substation Layout

An electrical bus system connects circuits within a substation. Scheme selection depends on operating voltage, the role of the substation in the electrical power system, acceptable fault and maintenance outages, protection complexity, expansion plans and life-cycle cost.

The Main Criterias to be Considered During Selection of one Particular Bus – Bar Arrangement Scheme

1. Simplicity of system.
2. Easy maintenance of different equipments.
3. Minimizing the outage during maintenance.
4. Future provision of extension with growth of demand.
5. Optimizing the selection of bus bar arrangement scheme so that it gives maximum return from the system.

The sections below compare common busbar arrangements. A scheme’s actual performance depends on its protection zones, breaker-failure logic, equipment ratings and operating procedures.

Single Bus System

In a Single Bus System, every feeder and transformer bay connects to one bus. The low equipment count makes this scheme simple and usually less costly, but a bus fault or bus maintenance can interrupt every connected circuit.

Advantages of Single Bus System

1. This is very simple in design.
2. This is very cost effective scheme.
3. This is very convenient to operate.

Disadvantages of Single Bus System

1. A major issue with this arrangement is that maintenance on any bay requires interrupting the connected feeder or transformer.
2. The indoor 11 KV switch boards have quite often single bus bar arrangement.

Single Bus System with Bus Sectionalizer

A bus-section circuit breaker divides a single bus into protection and operating sections. When sources and outgoing circuits are distributed between them, a fault or maintenance outage can be limited to one section. The result depends on relay zones, breaker ratings and the available transfer capacity.

Advantages of Single Bus System with Bus Sectionalizer

If one source is unavailable, the other section may supply additional loads after the sectional circuit breaker closes. This is possible only when the remaining source, bus and breaker can carry the load and the protection permits that state. During maintenance, the healthy section can remain energised.

Disadvantages of Single Bus System with Bus Sectionalizer

1. As in the case of a single bus system, maintenance of equipment of any bay cannot be possible without interrupting the feeder or transformer connected to that bay.
2. The use of isolator for bus sectionalizing does not fulfill the purpose. The isolators have to be operated ‘off circuit’ and which is not possible without total interruption of bus-bar. So investment for bus-coupler breaker is required.

Double Bus System

1. In double bus bar system two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus.
2. Actually every feeder is connected to both of the buses in parallel through individual isolator as shown in the figure.
   

Each circuit can be selected onto either bus through disconnectors. The buses may split the circuits during normal service, while a bus coupler breaker supports transfers between buses. Disconnectors are not intended to make or break load current. Any transfer must follow the approved site procedure, interlocks and synchronism conditions. Operators must not treat this description as a switching instruction.

Advantages of Double Bus System

A Double Bus Bar arrangement lets operators allocate circuits between two buses and move a circuit when system conditions permit. It also allows one bus to be isolated while selected circuits remain on the other bus.

Disadvantages of Double Bus System

A single-breaker double-bus arrangement does not by itself provide a second breaker path for each circuit. Taking a circuit breaker out of service therefore interrupts that circuit unless a bypass or transfer facility is installed.

Double Breaker Bus System

A double-bus, double-breaker scheme connects each circuit to both buses through a dedicated breaker on each side. Either breaker can carry the circuit, and one breaker or one bus can normally be removed without interrupting that circuit. This service continuity requires twice as many circuit breakers per circuit as a single-breaker scheme, with corresponding protection and control equipment. Switching must follow the site’s engineered interlocks and approved procedure.

One and A Half Breaker Bus System

A breaker-and-a-half scheme uses three circuit breakers for two circuits between two buses. Each circuit has one outer breaker and shares the middle breaker with the adjacent circuit. The middle breaker is not a spare. With all three closed, each circuit has a path to both buses. A bus or breaker can usually be isolated without interrupting either circuit. Protection and breaker-failure logic are more involved because the middle breaker belongs to two circuits. Every breaker and current path must be rated for the power-system conditions found by the design studies.

Advantages of One and A Half Breaker Bus System

For a bus fault, protection trips the breakers connected to the faulted bus. The circuits can remain connected through the healthy bus when the other equipment and protection operate as designed.

Disadvantages of One and a Half Breaker Bus System

The scheme costs more than a single-breaker arrangement because it averages one and a half breakers per circuit. It also needs more protection, control and maintenance work.

Main and Transfer Bus System

A main-and-transfer-bus scheme adds a transfer bus and a shared coupler bay. Each circuit normally operates through its own main-bus bay. For breaker maintenance, the transfer path can bypass that circuit’s breaker and the coupler circuit breaker temporarily provides its switching and protection function. The protection settings and current-transformer zones must match the temporary arrangement. The ordered list below is a historical overview, not an operating procedure. Only authorised staff may switch a live substation under an approved site-specific procedure.

Switching Operation for Transferring a Feeder to Transfer Bus from Main Bus without Interruption of Power

1. First close the isolators at both side of the bus coupler breaker.
2. Then close the bypass isolator of the feeder which is to be transferred to transfer bus.
3. Now energized the transfer bus by closing the bus coupler circuit breaker from remote.
4. After bus coupler breaker is closed, now the power from the main bus flows to the feeder line through its main
5. breaker as well as bus coupler breaker via transfer bus.
6. Now if the main breaker of the feeder is switched off, total power flow will instantaneously shift to the bus coupler breaker, and hence this breaker will serve the purpose of protection for the feeder.
7. At last, the operating personnel open the isolators at both sides of the main circuit breaker to make it isolated from rest of the live system.

In a correctly designed main-and-transfer-bus system, maintenance of circuit breaker can proceed while its circuit remains supplied through the transfer path. The applicable voltage range is a project choice, not a fixed 33 kV or 13 kV rule.

Double Bus System with Bypass Isolators

This scheme combines double-bus circuit selection with a bypass path. One bus can act as the operating bus while the other supports a breaker bypass, subject to the station design. It can maintain a circuit during breaker work, but it adds a disconnector and more switching states for each bay. Its suitability depends on reliability targets, protection design, operating risk, space and cost. It is not an automatic best choice for every 220 kV station.

Ring Bus System

A ring bus connects each circuit between two circuit breaker positions. One breaker can normally be removed while every circuit remains connected through the open ring. That state has less redundancy: a second fault or breaker operation can interrupt the circuits between the two open points. Adding a circuit requires an engineered ring extension and may need an outage, so expansion becomes harder as the station grows. Larger ring buses are often converted to a breaker-and-a-half arrangement.