Feeder Protection Schemes: An Overview
Feeder protection is a vital aspect of electrical power network transmission and distribution systems. It involves the implementation of protective devices and schemes to detect and isolate faults occurring within feeders, which are the conducting paths that carry electrical power from the primary source to various utilization points. Feeder protection schemes are designed to ensure reliable power supply, prevent severe equipment damage, and minimize downtime.
There are different types of feeder protection schemes that can be employed based on the specific requirements of the power system and the nature of the faults. Some commonly used schemes include overcurrent protection, distance protection, and differential protection.
Overcurrent Protection:
Overcurrent protection is the most basic and widely used scheme for feeder protection. It relies on the detection of excessive current flow in a feeder, beyond the pre-defined limits, to indicate a fault condition. The protective relays associated with overcurrent protection schemes are set to trip circuit breakers and disconnect the faulty section from the healthy system. The time-current characteristic of the protective relays should be coordinated to ensure discrimination between relays located at different points along the feeder.Distance Protection:
Distance protection schemes are based on the principle of measuring the impedance between the relay location and the fault point. These schemes are commonly used for longer transmission lines and can be more selective and faster compared to overcurrent protection schemes. Distance relays are sensitive to both current and voltage signals, and their settings are determined by the impedance characteristics of the protected feeder.Differential Protection:
Differential protection schemes rely on the comparison of currents at two or more points along the feeder. The difference in the measured currents indicates the presence of a fault within the zone protected by the differential relay. Differential protection is highly sensitive and can quickly detect internal faults, such as transformer winding faults or cable insulation failures. However, coordination challenges may arise due to complexities introduced by load currents, shunt devices, and circulating currents.
To illustrate the concept of feeder protection schemes, consider a practical example involving a transmission line. Let’s assume a 220 kV transmission line with a length of 100 km and a fault impedance of 10 ohms. We will focus on applying distance protection schemes.
Given:
Transmission line length (L): 100 km (or 100,000 m)
Fault impedance (Z): 10 ohms
The reach of a distance relay (R) can be calculated using the equation:
where V is the system voltage.
For a 220 kV transmission line, assuming a system voltage of 220 kV (or 220,000 V), the reach (R) of the distance relay can be calculated as:
In this example, the distance protection scheme will be set to operate within a reach of 22 km. If a fault occurs within this reach, the distance relay will detect the fault and initiate a trip signal to isolate the faulty section. The actual reach setting may vary based on system requirements, fault impedance, and coordination considerations.
Feeder protection schemes, including overcurrent, distance, and differential protection, play a crucial role in ensuring the reliability and stability of electrical power network transmission and distribution systems. These schemes provide effective fault detection and isolation mechanisms, minimizing the impact of faults and improving system performance. Compliance with industry standards such as IEEE C37.112 and IEC 61850 helps ensure consistent application and interoperability of feeder protection schemes.