Classification of Protective Schemes

A protective scheme is used to protect equipment or a section of the line. It includes one or more relays of the same or different types. The following are the most common protective schemes used for the protection of modern power system

  1. Overcurrent protection

  2. Distance protection

  3. Carrier-current protection

  4. Differential protection

  • Over-current protection:- This scheme of protection is used for the protection of distribution lines, large motors, equipment etc. It includes one or more overcurrent relays. It operates when the current exceeds its pick-up value.

  • Distance protection:- It is used for the protection of transmission or sub-transmission lines; usually 33 KV, 66 KV & 132 KV lines. It includes a number of distance relays of the same or different types. It measures a distance between the relay location and the point of fault in terms of impedance, reactance etc. The relay operates if the point of fault lies within the protected section of the line. There are few important types of distance relays as: impedance, reactance and mho type. These relays measure impedance, reactance and a component of admittance respectively between the fault point and the relay location.

  • Carrier-current protection:- This is used for the protection of EHV and UHV lines (132 & above). A carrier signal in the range of 50 – 500 kc/s is generated for the purpose. A transmitter and receiver are installed at each end of a transmission line to be protected. Information regarding the direction of the fault current is transmitted from one end of the line section to the other. Depending on the information, relays are placed at each end trip if the fault lies within their protected section. Relays do not trip in case of external faults. The relays are of distance type and their tripping operation is controlled by the carrier signal. 

  • Differential protection:- This scheme is used for the protection of generators, transformers, motors of very large size, bus zones etc. C.T.s are placed on both sides of each winding of a machine. The outputs of their secondaries are applied to the relay coils. The relay compares the current entering a machine winding and leaving the same. Under normal conditions or during any external faults, the current entering the winding is equal to current leaving the winding. But in the case of an internal fault on the winding, these are not equal. This difference in the current actuates the relay. Thus, the relay operates for internal faults and remains inoperative under normal conditions or during external faults. In case of bus zone protection, C.T.s are placed on both sides of the bus bar.