What is relay and busbar
If the fault occurs on the busbars, the supply is interrupted and all healthy feeds are disconnected. The majority of faults are single phase and these faults are temporary. The error in the bus zone occurs for various reasons, e.g. B. Failure of the post insulators, failure of disconnectors, accidental dropping of foreign objects over the busbar, etc. To remove the bus fault, all circuits connected to the faulty section must be open.
The most common schemes for protecting bus zones are:
- Backup protection
- Differential overcurrent protection
- Circulating current protection
- Overvoltage protection
- Frame leak protection.
Backup protection for busbars
This is the simple way to protect the busbar from the fault the fault occurs on the busbar due to the supplying system. This provides backup protection for the supply system. The following figure shows the simple arrangement for protecting the busbar. Bus A is protected by the distance protection of bus B. When the fault occurs at A, B works. The relay operating times are 0.4 seconds.
The busbar protection system has few disadvantages such as the protection system is slow. Such a system is mainly used to protect the transmission lines. However, since the protection system is very economical, it is also used for busbar protection.
This protection scheme is not used for small switchgear. The backup protection system has many disadvantages, e.g. B. Delayed action, requiring more circuits to be disconnected for two or more transmission lines, etc.
Frame leakage or fault bus protection
This method isolates the bus carrier structure and its switchgear from the ground, which connects the entire framework, the circuit breaker tanks, etc., and provides a single floor box connection via a current transformer that feeds an overcurrent relay. The overcurrent relay controls a multiple contact auxiliary relay that triggers the breakers of all circuits connected to the bus.
With this type of protection, the only metal is the supporting structure or the fault bus is grounded through a current transformer that is secondarily connected to an overcurrent relay. Under normal operating conditions the relay will not function, however a fault involving a connection between a conductor and the ground support structure will result in a current flow through the fault bus to ground, causing the relay to operate. The operation of the relay triggers all circuit breakers that connect devices to the bus.
Differential overcurrent protection
Current differential protection
The current differential protection system works on the principle of circulating current, which states that the current enters the busbar, it is equal to the current that leaves the busbar. The sum of the incoming and outgoing nodes is zero. If the sum of the current is not equal to zero, the fault occurs in the system. The differential protection system is used for both phase-to-phase fault protection and earth fault protection.
The relay is shown in the following figure. The current transformers are located at both the incoming and outgoing end of the busbar. The secondary terminals of the current transformer are connected to one another in parallel.
The disadvantage of such schemes is that the iron core current transformer causes the failure of the relay at the time of the external failure.
Voltage differential protection relay
In this scheme, the coreless CTs are used. The linear couplers are used to increase the number of turns on the secondary sides of the current transformers. The secondary relays are connected in series with the help of the control lines. The relay coil is also connected in series with the second terminal.
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