Feeder Protection Relays are an essential component of electrical power networks, providing reliable protection against faults and disturbances in distribution and transmission systems. They detect abnormal conditions such as overcurrent, short circuits, and earth faults, and initiate appropriate actions to isolate the faulty section from the network.
There are several types of feeder protection relays, each designed to address specific protection requirements based on the network’s characteristics. The choice of relay type depends on parameters such as fault current levels, fault types, and the desired speed and accuracy of protection.
Overcurrent Relays (OCR): These relays are the simplest and most widely used type for feeder protection. They are designed to detect overcurrent conditions caused by faults or overload situations. OCRs operate based on current magnitude exceeding set thresholds. They provide effective protection against both phase and ground faults. They can be further categorized into definite time, inverse time, and instantaneous time overcurrent relays.
Directional Overcurrent Relays (DIR OCR): These relays incorporate an additional directional element to provide selectivity in protection schemes. By considering the direction of current flow, they can determine the fault location and selectively isolate the faulted section while maintaining power supply to healthy portions of the network. DIR OCRs are commonly used in interconnected networks where fault currents can vary in magnitude and direction.
Distance Relays (DR): These relays protect against faults occurring at different distances from the relay location. DRs utilize the concept of impedance measurement, comparing the apparent impedance of the network with a predetermined characteristic impedance. They offer high-speed and accurate fault detection, making them suitable for long-distance transmission lines. Distance relay settings are typically based on percentage impedance, with coordination between relays ensuring appropriate fault clearance.
Differential Relays (DIFF): Differential protection is employed to detect internal faults within protected zones, such as transformer windings or generator stator windings. Differential relays measure the difference in current between the input and output sides of the protected equipment. If a fault occurs within the zone, the differential current exceeds a set threshold, initiating tripping operations. Differential relays are highly sensitive and provide secure protection for critical assets.
Transformer Differential Relays (TDIFF): As the name suggests, transformer differential relays are specifically designed to protect power transformers. They ensure fast and selective tripping during internal faults within the transformer, offering enhanced protection against hazards like winding short circuits or core ground faults. Transformer differential relays apply similar principles as differential relays with specialized algorithms tailored for transformers.
A comprehensive relay protection scheme combines multiple types of relays, utilizing their individual advantages to achieve reliable and selective protection across the entire electrical network. The coordination of relay settings, time grading, and selectivity ensures appropriate fault clearance and minimizes disruption to power supply.
Let’s consider a numerical example to illustrate the application of feeder protection relays. Suppose we have a 132 kV transmission line with an OCR and a DIR OCR installed for protection. The OCR is set to operate at 300 A and includes a time delay of 0.2 seconds. The DIR OCR uses a directional element with a pickup setting of 200 A. During a fault, the fault current through the line rises to 500 A.
In this scenario, the OCR will detect the overcurrent condition as the fault current exceeds its set threshold of 300 A. However, due to the time delay of 0.2 seconds, the OCR will not immediately initiate tripping. It allows for temporary overcurrent conditions caused by heavy starting currents during motor energization, for example, to prevent unnecessary tripping.
On the other hand, the DIR OCR incorporates a directional element that ensures selectivity in fault detection. By considering the direction of current flow, it avoids unnecessary tripping for faults located downstream of its position. In this example, the DIR OCR will detect the fault because the pickup setting of 200 A is exceeded, and it will initiate the tripping operation.
Thus, by combining the OCR and DIR OCR relays, we achieve both overcurrent protection and selectivity in fault detection for the feeder. This example highlights the importance of coordinating the settings of different relay types to achieve appropriate and efficient protection.
It’s worth noting that relay protection practices and standards may vary across different regions and organizations. The Institute of Electrical and Electronics Engineers (IEEE) and the International Electrotechnical Commission (IEC) provide guidelines and standards like the IEEE C37 series and IEC 61850 that help ensure consistent and reliable protection practices in power networks.