The protection scheme for a power system should cover the whole system against all probable types of faults. Unrestricted forms of line protection such as over current and distance systems, meet this requirement, although faults in the Bus bar zone are cleared only after some time delay. If unit protection is applied to feeder and plant the bus bars are not inherently protected. Bus bars have been left without specific protection. Different bus bar faults are as follows. BUSBAR FAULTS
· Majority of bus faults involve one phase and earth, but faults arise from many causes and a significant number are inter-phase clear of earth.
· With fully phase-segregated metal clad gear, only earth faults are possible ,and a protective scheme need have earth fault sensitivity only.
· For outdoor busbars , protection schemes ability to respond to inter-phase faults clear of earth is an advantage
TYPES OF PROTECTION SCHEMES
· System protection used to cover bus bars
· Frame –earth protection
· Differential protection
SYSTEM PROTECTION
· A system protection that includes over current or distance systems will inherently give protection cover to the bus bars.
· Over current protection will only be applied to relatively simple distribution systems, or as a back-up protection set to give considerable time delay. Distance protection will provide cover with its second zone.
· In both cases, therefore ,the bus bar protection so obtained is slow
Frame-earth Protection
· This is purely an earth fault system, and in principle involves simply measuring the fault current flowing from the switchgear frame to earth. To this end a current transformer is mounted on the earthing conductor and is used to energize a simple instantaneous relay.
This protection is nothing but the method of providing earth fault protection to the bus bar assembly housed in a frame. This protection can be provided to the metal clad switchgear. The arrangement is shown in the figure below. The metal clad switchgear is lightly insulated from the earth. The enclosure of the frame housing different switchgears and bus bars is grounded through a primary of current transformer in between. The concrete foundation of switchgear and the other equipments are lightly insulated from the ground. The resistance of these equipments with earth is about 12 ohms. When there is an earth fault, then fault current leaks from the frame and passes through the earth connection provided. Thus the primary of C.T. senses the current due to which current passes through the sensitive earth fault relay, thereby operating the relay.
