The following points highlight the eight main types of bus-bar arrangements. The types are: 1. Single Bus-Bar Arrangement 2. Single Bus-Bar Arrangement with Bus Sectionalization 3. Main and Transfer Bus Arrangement 4. Double Bus Double Breaker Arrangement 5. Sectionalized Double Bus Arrangement 6. One-and-a-Half Breaker Arrangement and Few Others.
Type # 1. Single Bus-Bar Arrangement:
This is the simplest arrangement consisting of a single set of bus-bars for the full length of the switchboard and to this set of bus-bars are connected all the generators, transformers and feeders, as illustrated by single line diagram in Fig. 16.19.
Each generator and feeder is controlled by a circuit breaker. The isolators permit isolation of generators, feeders and circuit breakers from the bus-bars for maintenance. The chief advantages of such a bus-bar arrangement are low initial cost, less maintenance and simple operation.
The glaring drawback of this system is that in case of fault on the bus-bars, whole of the supply is affected and all the healthy feeders are disconnected. Moreover, when maintenance is to be carried out on any of the feeder sections or on a part of bus-bar the whole supply is to be disrupted. Thus such an arrangement provides least flexibility and immunity from total shutdown.
ADVERTISEMENTS:
Such bus-bar arrangement is employed for switchboards, small and medium sized substations, small power stations and dc stations.
Type # 2. Single Bus-Bar Arrangement with Bus Sectionalization:
The bus-bar may be sectionalized by a circuit breaker and isolating switches so that a fault on one part does not cause a complete shutdown. In large generating stations, where several units are installed, it is a common practice to sectionalize the bus as illustrated in Fig. 16.20.
Normally the number of sections of a bus-bar are 2 to 3 in a substation, but actually it is limited by the short-circuit current to be handled. In a sectionalized bus-bar arrangement only one additional circuit breaker is required which does not cost much in comparison to the total cost of the bus-bar system.
ADVERTISEMENTS:
Such an arrangement provides three main advantages over simple single bus-bar arrangement:
Firstly, in the event of occurrence of fault on any section of the bus-bar, the faulty section can be isolated without affecting the supply of other section or sections.
Secondly, one section can be completely shut-down for maintenance and repairs without affecting the supply of the other section (s).
ADVERTISEMENTS:
Thirdly, by adding a current limiting reactor between the sections the fault level (MVA) can be reduced thereby circuit breakers of lower capacity can be used.
At times air-break isolators were used in place of circuit breakers as bus-sectionalizer due to economy, but it must be remembered that any isolation affected by them must be affected under off-load conditions otherwise it may cause spark. It will be preferable to provide circuit breaker as a sectionalizing switch so that uncoupling of bus-bar may be carried out safely during load transfer. A double isolation is however necessary when the circuit breaker is employed as sectionalizing switch so that the maintenance work can be carried out on circuit breaker while the bus-bars are alive.
Type # 3. Main and Transfer Bus Arrangement:
This arrangement has been quite frequently adopted where the loads and continuity of supply justify additional costs. This arrangement provides additional flexibility, continuity of supply and allows periodic maintenance without total shutdown. Such an arrangement is suitable for highly interconnected power network in which flexibility is very important.
Figure 16.21 illustrates the main and transfer bus arrangement in a generating station. Such an arrangement consists of two bus-bars, known as main bus-bar and transfer bus-bar used as an auxiliary bus-bar. Each generator and feeder may be connected to either bus-bar with the help of bus coupler which consists of a circuit breaker and isolating switches. In this arrangement a bus coupler is usually used so that change-over from one bus-bar to the other can be carried out under load conditions. While transferring the load to the reserve bus, the following steps may be taken.
i. Close the bus-coupler (circuit breaker) so as to make the two buses at the same potential.
ii. Close isolators on the reserve bus.
iii. Open isolators on the main bus.
The load is now transferred to the reserve or auxiliary bus and main bus is disconnected.
ADVERTISEMENTS:
The advantages and disadvantages of the arrangement are gives below:
Advantages:
i. It ensures continuity of supply in case of bus fault. In the event of occurrence of fault on one of the bus, the entire load can be transferred to the other bus.
ii. Repair and maintenance can be carried out on the main bus without interrupting the supply as the entire load can be transferred to the auxiliary bus.
iii. Each load can be supplied from either bus.
iv. The in-feed and load circuit may be divided into two separate groups if required from operational considerations.
v. The testing and maintenance of feeder circuit breakers can be done by putting them on spare bus, thus keeping the main bus undisturbed.
vi. The maintenance cost of substation is lowered.
vii. The bus potential can be used for relays.
Disadvantages:
i. Additional costs.
ii. The bus is maintained or expanded by transferring all of the circuits to the auxiliary bus depending upon the remote backup relays and breakers for removing faults of the circuits. During this condition a line fault on any of the circuits of the bus would shut-down the entire station.
Type # 4. Double Bus Double Breaker Arrangement:
In very important power stations two circuit breakers are employed for each circuit, as illustrated in Fig. 16.22. Such a bus-bar arrangement does not require any bus-coupler and permits switch-over from one bus to the other whenever desired, without interruption. This bus arrangement is very costly and its maintenance cost is also high.
This arrangement provides maximum flexibility and reliability as the faults and maintenance interrupt the supply to the minimum. A circuit breaker can be opened for repairs and usual checks and the load can be shifted on the other circuit breaker easily. But because of its higher cost, this arrangement is seldom used at the substations.
For 400 kV switchyards two main buses plus one transfer bus scheme is preferred. The transfer bus is used for transferring power from main bus 1 to main bus 2 and vice versa.
Type # 5. Sectionalized Double Bus Arrangement:
In this arrangement duplicate bus-bars are used with the main bus-bar in sections connected through a bus-coupler, as illustrated in Fig. 16.23. In this arrangement, any section of bus-bar can be isolated for maintenance, while any section may be synchronised with any other through the auxiliary bus-bar. Sectionalization of auxiliary bus-bar is not required and would increase the cost if done.
Type # 6. One-and-a-Half Breaker Arrangement:
This is an improvement over double bus double breaker arrangement and it affects saving in the number of circuit breakers. This arrangement needs three circuit breakers for two circuits. The number of circuit breakers per circuit comes out to be 1½ hence the name. This arrangement is preferred in important large stations where power handled per circuit is large.
This arrangement is shown in Fig. 16.24. This arrangement provides high security against loss of supply as a fault in a bus or in a breaker will not interrupt the supply. Possibility of addition of circuits to the system is another advantage. The bus potential can be used as supply to relays, however, at the time of bus fault such potential to the relay should be thrown off.
The main drawback of this arrangement is complications in relaying system because at the time of fault two breakers are to be opened. The other drawback is that for maintenance of circuit breakers if the load shedding is not used, two breakers are to be opened in which case the other circuit in the line-up will be operating with one breaker from one bus only. At the time of fault in that bus supply to the other circuit is also interrupted. The maintenance cost is higher.
The above arrangement has been used in important 400 kV and 750 kV substations.
Type # 7. Ring Main Arrangement:
This is an extension of the sectionalized bus-bar arrangement where the ends of the busbars are returned upon themselves to form a ring, as illustrated in Fig, 16.25. This arrangement provides greater flexibility as each feeder is supplied by two paths, so that the failure of a section does not cause any interruption of the supply. The effect of fault in one section is localised to that section alone.
The rest of the sections continue to operate. Circuit breakers can be maintained without interrupting the supply. The cost is also not much as the numbers of breakers used are nearly the same as that of a single bus-bar system.
The drawbacks of the system are:
(i) Difficulties in addition of any new circuit in the ring,
(ii) Possibility of overloading of the circuits on opening of any section of the breaker, and
(iii) Necessity of supplying potential to relays separately to each of the circuit.
Type # 8. Mesh Arrangement:
This is another arrangement making economical use of circuit breakers in a substation. In this bus-bar arrangement, the circuit breakers are installed in the mesh formed by the buses, as illustrated in Fig. 16.26. The circuits are tapped from the node points of the mesh. In Fig. 16.26 eight circuits are controlled by four circuit breakers.
When fault occurs on any section, two circuit breakers have to open, resulting in opening of the mesh. This arrangement provides security against bus-bar faults but lacks switching facility. It needs fewer circuit breakers than that required by one-and-a-half breaker arrangement. It is preferred for substations having large number of circuits.
