Single Bus System with Bus Sectionalizer
Because of cheapness and simplicity, single bus-bar is adopted with sectionalizing arrangements, as shown in the figure below. Full advantage of the scheme is realized if the incoming and outgoing circuits are distributed evenly on the sections. Where double feed is provided for any single load it is preferable to have one circuit from each section. Suppose, a single load needs to have a main supply and a standby supply. Here, the main supply will be from one section and standby will be from the other section of the single bus system with bus sectionalizer. In this arrangement, each section behaves as a separate bus. Any outage of one section does not effect the other section of the bus-bar. The sectionalizer circuit breaker will enable bus differential protection to trip only the faulty section and keep the healthy section running.
This arrangement has another interesting and useful application where two transformers are used. Suppose two 33/11kV 6.3MVA transformers are used in the scheme. The 11 kV of one transformer is connected as incoming of one side bus and 11 kV of another transformer is brought to other section of the bus. The bus sectionalizer and the two incoming breakers are so interlocked that only two breakers can be closed at a time. Normally, each section continues to cater to its load. If the incoming circuit on any section has to be taken out, the sectionalizer breaker can be closed and supply restored to the outgoing circuits of this section.
Double Bus System with Bus Coupler
The Double Bus System is a popular configuration in electrical power substations. It enhances reliability and flexibility of operations and is ideal for medium to large substations. Normally, in 220KV AIS and GIS, the double bus system is utilized. The double bus system consists of two busbars running parallel to each other, connected through a bus coupler. This system allows the switching of loads between buses without any interruption in service, ensuring high reliability and flexibility in operations. Normally, the incoming and outgoing feeders are evenly distributed among the two buses. But in extreme conditions, all the feeders may need to be connected to a single bus, especially during an entire bus shutdown. Therefore, the selection of bus rating must be such that it can carry all the feeders together simultaneously.
Key Components of a Double Bus System
- Two Bus – The system has two separate bus that run in parallel. The buses are labeled as Bus 1 and Bus 2 in most diagrams.
- Bus Coupler: A bus coupler switch is used to interconnect the two busbars, allowing the transfer of load from one bus to another.
- Circuit Breakers (CBs): Each feeder line is connected through a circuit breaker to each busbar, providing protection and control.
- Isolators: Isolators are used for isolating different sections of the system during maintenance.
- Current Transformers (CTs) and Potential Transformers (PTs): Installed for measuring current and voltage, and for relay protection.
When we need to shut down an entire bus, say Bus-1, all the feeders connected to Bus-1 must be diverted to Bus-2. For that, we first need to close the bus coupler isolators and then switch on the bus coupler breaker. Suppose a feeder is connected to Bus-1, and we need to divert it to Bus-2 without interruption. To achieve this, we shall close the bus-side isolator of the feeder associated with Bus-2. Now, both Bus-1 and Bus-2 are connected to the feeder as both bus-side isolators are closed. Next, the bus-side isolator of Bus-1 is to be opened. In this way, one by one, all the feeders are diverted to Bus-2. Finally, switch off the bus coupler breaker and then open the bus coupler isolators. Now, Bus-1 is completely isolated from the live system and can be earthed for maintenance.
Main and Transfer Bus System
The main disadvantage of the double bus system is that there is no provision for circuit breaker maintenance without interruption of the feeder. In the double bus system, bus maintenance is quite flexible and interruption-free, but individual circuit breaker maintenance is not interruption-free. This disadvantage can be overcome by the main and transfer bus system. This system provides flexibility for carrying out breaker maintenance but does not permit interruption-free bus maintenance. When a circuit breaker needs maintenance, the feeder is changed over to the transfer bus and is diverted through the transfer bus coupler breaker.
Another disadvantage of the main and transfer bus system is that more than one feeder cannot be diverted through the transfer bus at the same time.
One and Half Breaker Scheme of an EHV Electrical Substation
The One and a Half Breaker Scheme (also known as the 1.5 Breaker Scheme) is a widely used configuration in Ultra High Voltage (UHV) substations normally for 400KV and above substations. It is a compromise between the Double Bus Double Breaker (DBDB) scheme and the Ring Bus scheme, offering high reliability, flexibility, and operational efficiency.
Key Features of the One and a Half Breaker Scheme
For every two circuits (feeders), there are three circuit breakers. This results in 1.5 breakers per circuit, hence the name. The scheme typically uses two main buses (Bus A and Bus B). Each circuit is connected to both buses through the breakers. Any circuit can be powered from either Bus A or Bus B. Maintenance can be performed on any breaker or bus without interrupting the power supply to the connected circuits. The failure of one breaker does not affect the operation of the other circuits. On other hand, if one bus fails or goes under shutdown, the other bus can take over the load. Inspite of, so many advantages one and half bus system is more expensive than simpler schemes like the Single Bus or Double Bus Single Breaker (DBSB), although it is cheaper than the Double Bus Double Breaker (DBDB) scheme.
Here, the middle breaker (shared breaker) is connected to both Bus A and Bus B. The other two breakers are connected to either Bus A or Bus B. Under normal conditions, both buses are energized, and the circuits are fed through the breakers. If one bus fails, the other bus can supply power to all circuits. If a breaker fails, the adjacent breaker can take over its function.