Electromechanical relays are devices that utilize an electromagnetic mechanism to achieve switching operations in electrical circuits. They have been widely used in power network transmission and distribution systems for many decades due to their reliability and robustness. In this explanation, we will explore the different types of electromechanical relays, their applications, and how they function.
There are several types of electromechanical relays, each designed for specific purposes within the power network. The most common types include overcurrent relays, distance relays, differential relays, and directional relays.
Overcurrent relays: These relays are primarily used for protecting power transmission and distribution lines against excessive current flow. They are designed to detect and trip the circuit breaker if the current exceeds a predetermined threshold. Overcurrent relays are crucial for preventing damage to equipment and minimizing hazards caused by short circuits or overloads.
Distance relays: Distance relays are used for the protection of power transmission lines against faults such as short circuits. They use measurements of voltage and current at different points in the line to calculate the impedance of the line. By comparing this impedance with preset threshold values, distance relays can accurately determine the location of the fault and activate the tripping mechanism.
Differential relays: Differential relays are employed to protect transformers, generators, and motors against internal faults. These relays monitor the current flowing into and out of the protected equipment and compare them. If the differential current exceeds a predetermined value, the relay trips the circuit breaker to isolate the faulty section.
Directional relays: Directional relays are used to protect transmission lines against faults that are located closer to one end of the line. These relays determine the direction of current flow and trip the circuit breaker if the fault occurs in the opposite direction to the line’s intended flow. Directional relays prevent the fault from affecting other healthy sections of the power network.
The functioning of electromechanical relays is based on the interaction between an electromagnet and a mechanical switch. When a fault is detected, an electrical signal is sent to the electromagnet, which generates an electromagnetic force. This force attracts or releases a contact, thereby opening or closing the circuit. The mechanical design of the relay ensures that once the fault is cleared or the fault condition is no longer present, the relay returns to its original state.
To illustrate the application of electromechanical relays, let’s consider an example of overcurrent protection in a transmission line. Suppose we have a transmission line with a rated current of 1000 A and we want to set the overcurrent relay to operate at 120% of the rated current, i.e., 1200 A.
To calculate the relay setting, we can use the following formula:
The PSM is a device-specific parameter that accounts for any desired time delay or margin of operation. Let’s assume the PSM in this case is 0.8.
The relay setting would be:
Therefore, the overcurrent relay should be set to operate when the current exceeds 800 A to allow for a margin and avoid unnecessary tripping during momentary load fluctuations.
In summary, electromechanical relays play a vital role in the protection and control of power transmission and distribution systems. They offer reliable and robust solutions for various applications such as overcurrent protection, fault detection, impedance measurement, and directionality. By understanding the different types of electromechanical relays and their applications, engineers can ensure the safe and efficient operation of power networks.