electricalquarter: SUBSTATION BUS SCHEME

INTRODUCTION

Before more in-depth discussion about each type of substation it is better to know few common essential features of a substation. Here we discuss about the bus schemes commonly implemented in an electrical substation. The Bus scheme is the arrangement of overhead bus bar and associated switching equipments in a substation. The operational flexibility and reliability of the substation greatly depends upon the bus scheme. Here I reiterate that the electric substation is a junction point where usually more than two transmission lines terminate. Actually in most of EHV and HV substations more than half a dozen of lines terminate. In many large transmission substations the total numbers of lines terminating exceeds one or two dozens. In this scenario obviously the first requirement is avoidance of total shutdown of the substation for the purpose of maintenance of some equipment(s) or due to fault somewhere. Total shutdown of substation means complete shutdown of all the lines connected to this particular substation. So the switching scheme is adopted depending upon the importance of the substation, reliability requirement, flexibility and future expansion etc.. Of course substation construction and operational cost is also to be considered. Clearly a EHV or UHV transmission substation where large numbers of important lines terminate is extremely important and the substation should be designed to avoid total failure and interruption of minimum numbers of circuits.

There are mainly six bus schemes. These are:

Single Bus

Main Bus and Transfer Bus

Double Bus Double Breaker

Double Bus Single Breaker

Ring Bus

Breaker and Half.

Before we proceed further I would like to discuss in brief about the Circuit Breaker and Isolator. It will be helpful for novices. See the figure below where two buses are connected by circuit breakers and isolators as shown. A circuit breaker is a device whose main purpose is to break the circuit carrying load current or fault current. As the breaker is opened then current is interrupted in the circuit. But it is not safe to work with opened breaker as one or both sides of the breaker terminals may be still energised. The breaker is then isolated from the rest of the circuit by opening the isolators on both sides of breaker. The isolators are used to isolate the breaker or circuit. It should be remembered that the isolators are never opened or closed to interrupt or make the circuit. That means when the circuit is to be made on, first the isolators on both sides of a breaker are closed then breaker is closed to allow current flow. When the circuit is to be made off or interrupted, first the breaker is opened(tripped), hence load current is interrupted. Then to isolate the breaker, isolators are opened. Isolators are designed to interrupt small current. Breakers are designed to interrupt large load current and heavy fault current. Both breaker and isolator carry load current in normal state

SINGLE BUS

As the name indicate the substation with this configuration has a single bus (Fig-B). All the circuits are connected to this bus. A fault on the bus or between the bus and a breaker results in the outage of the entire bus or substation. Failure of any breaker also results in outage of the entire bus. Maintenance of any circuit breaker requires shutdown of the corresponding circuit/line and maintenance of bus requires complete shutdown of the bus. A bypass switch across the breaker should be used for maintenance of the corresponding breaker. This case the protection system is disabled. Single Bus configuration is the simplest and least cost of all configurations. The system can be easily expanded. This configuration requires less area. The reliability of this system being low, it is not to be implemented in the substation where high reliability is expected. Large substations usually do not utilize this scheme. By sectionalising of the bus the reliability and availability of the single bus system can be improved by expanding and sectionalizing the bus.

MAIN AND TRANSFER BUS

In this arrangement one or more busses is added to the single bus substation scheme. One or more circuit breakers may be used in this arrangement to make connections between the main and transfer bus. When no Tie CB is present, for maintenance of a circuit breaker, the transfer bus is energized by closing the isolator switches to the transfer bus. Then the circuit breaker to be maintained is opened and isolated on both sides. Circuit protection will be disabled in this maintenance arrangement.

When a tie circuit breaker is present, circuit breaker maintenance is achieved by closing the tie breaker. The transfer bus is energized and the isolator nearest the transfer bus of the breaker to be maintained is closed. The circuit breaker to be maintained is now opened, isolated and removed for maintenance. The circuit under maintenance is transferred to the transfer bus.

In the main and transfer bus configuration, the protective relay scheme is quite complex due to the requirement of the tie breaker to handle each situation for maintenance of any other circuit breaker. This bus scheme is more costly than the single bus configuration but is more reliable and can be easily expanded.

The switching procedure is complicated for maintenance of any circuit breaker. Failure of a breaker or fault on the bus results in an outage of the whole substation.

Double Bus Double Breaker

This configuration utilizes two buses and two breakers per circuit. Both buses are normally energized and any circuit can be removed for maintenance without an outage on the corresponding circuit. Failure of one of the two buses will not interrupt a circuit because all of the circuits can be fed from the remaining bus and isolating the failed bus.

Substations with the double bus double breaker arrangement require twice the equipment as the single bus scheme but are highly reliable. Load balancing between buses can be achieved by shifting circuits from one bus to the other. This scheme is typically found in EHV transmission substations or generating stations.

4. Double Bus Single Breaker

Substations utilizing this configuration are supplied with two busses. Each circuit is equipped with a single breaker and is connected to both buses using isolators. A tie breaker connects both main buses and is normally closed, allowing for more flexibility in operation. A fault on one bus requires isolation of the bus while the circuits are fed from the opposite bus.

The double bus single breaker scheme is more expensive and requires more installation space than the single bus configuration. It is common to find this scheme with an additional transfer bus in EHV transmission substations.

5. Ring Bus

In the ring bus configuration, as the name implies, the circuit breakers are connected to form a ring, with isolators on both sides of each breaker. Circuits terminate between the breakers and each circuit is fed from both sides. Any of the circuit breakers can be opened and isolated for maintenance without interruption of service.

This scheme has good operational flexibility and high reliability. If a fault occurs in this configuration, it is isolated by tripping a breaker on both sides of the circuit. By tripping two breakers, only the faulted circuit is isolated while all the other circuits remain in service.

The main disadvantage of the ring bus system is that if a fault was to occur, the ring is split which could result into two isolated sections. Each of these two sections may not have the proper combination of source and load circuits. This can be somewhat avoided by connecting the source and load circuits side by side.

Ring bus schemes can be expanded to accommodate additional circuits, but its generally not suited for more than six. Careful planning should be used with this scheme to avoid difficulties with future expansion.

6. Breaker and Half

When expansion of the substation is required to accommodate more circuits, the ring bus scheme can easily be expanded to the One and Half breaker configuration. This configuration uses two main buses, both of which are normally energized with three breakers connected between the buses.

In this bus configuration, three breakers are required for every two circuits - hence the "one and half" name. Think of it as, to control one circuit requires one full and a half breaker. The middle breaker is shared by both circuits, similar to a ring bus scheme where each circuit is fed from both sides.

Any circuit breaker can be isolated and removed for maintenance purposes without interrupting supply to any of the other circuits. Additionally, one of the two main busses can be removed for maintenance without interruption of service to any of the other circuits.

If a middle circuit breaker fails, the adjacent breakers are also tripped to interrupt both circuits. If a breaker adjacent to the bus fails, tripping of the middle breaker will not interrupt service to the circuit associated with the remaining breaker in the chain. Only the circuit associated with the failed breaker is removed from service.

The breaker and half configuration is very flexible, highly reliable, and more economical in comparison to the Double Bus Double Breaker scheme. Protective relay schemes in this configuration are highly complicated as the middle breaker is associated with two circuits. It also requires more space in comparison to other schemes in order to accommodate the large number of components.