In addition to the general protection, backup impedance protection or distance protection is also used in Generator, Transformer & Reactor to protect during external faults in the power system. Generally, impedance protection is most commonly used for the protection of transmission lines. When the Impedance value goes below the set value during the fault or overload condition, distance protection of the transmission line is operated.
Dedicated protection line differential protection, Restricted Earth Fault protection(REF), etc. are used in generators, transformers, and Reactors. However, as a backup to the main protection, Impedance or distance protection is also used to secure the equipment.
Working Principle of Backup Impedance Protection
Under the protection, voltage and current both inputs are given to the relay. By monitoring the voltage and current inputs, impedance is calculated by the relay. When the impedance value goes below the set value, it will operate and isolate the system.
Z=V/I, Here, Z= Impedance of the protection zone, V=Input voltage, I= input current
Based on the system voltage, full load current, and an impedance value of the connected element like the Transformer, Generator, or Reactor, the impedance setting is decided and set to the relay. The following key points are to know under backup impedance protection.
- During an internal fault condition, the current will be increased and the voltage also dropped. Under the condition, the calculated impedance value is found below the set impedance value in the relay and the relay will operate under the protection.
- Voltage input is necessary for backup impedance protection. During the fuse fail condition, the voltage goes zero to the relay and the relay will operate. To prevent the situation, the fuse failure block function is used in the relay.
- Impedance protection is not as sensitive as differential or REF protection in transformer and Reactor. As a backup of the differential and REF protection, it is used to protect the Generator, Transformer & Reactor in Power System.