In an electrical power system, busbars are the very important element that needs to be protected against the various faults that occur on them. In case of fault on a bus bar, the fault current and power may be very large and cause serious damage to the system.
The continuity of supply is affected by faults on the busbar and causes discontinuation of power to a large portion of the system even though the power generation and feeders are normal. The stability of the system is affected by a fault in the busbar zone.
In the case of large power systems protection of busbar sections are necessary so that faulty sections are isolated before the fault spreads over the whole system. For the purpose of protection, the busbar zone includes the bus bars, circuit breakers, and isolating switches. On the occurrence of a fault in a section, all the equipment associated with that section must be also tripped out to isolate the section completely.
Frame Leakage Protection :
Frame leakage protection is one of the most simple forms of bus-bar protection available, suitably designed to withstand the earth faults occurring within the indoor installations. This type of protection is applicable to metal-clad switchgear of small size. All the metal frameworks are attached together and insulated from the ground.
It is essential to isolate the switchgear framework from lead cable sheaths, such that when a leakage to the framework occurs then the leakage current will pass through the primary of the current transformer provided in the earth connection. The current transformer secondary is connected to an earth leakage relay.
The metal-clad switchgear is grounded through a current transformer, the secondary of which is connected to an overcurrent relay. Under normal working conditions, the overcurrent relay remains inoperative but when a fault takes place, the leakage current will pass through the faulted section to the ground causing the relay to operate resulting in tripping of the circuit breaker connecting equipment to the bus.
Working Principle of Frame Leakage Protection :
The below shows the frame leakage scheme of protection applied to a single busbar substation.
Supply is provided to the station busbars having earthed star-connected secondary also to which a current transformer is connected which energizes the operating coil of the check relay connected to its secondary. When an earth fault occurs within the protected area, the contacts of both the leakage and check relay are closed energizing the tripping relay and enabling the operation of the circuit breaker.