Figure 1: Function of monitoring relays (Source: OMRON)

Types of monitoring relays

There are different types of monitoring relays, each designed to monitor specific parameters. Some common types of monitoring relays include:

• Current monitoring relays: These relays safeguard crucial industrial components by preventing issues caused by overcurrent or undercurrent conditions. They measure the current flowing through a system using a direct connection or a current transformer. When an overcurrent is detected, such as in the case of an overloaded or stalled motor, the relay actuates and indicates the problem or shuts off the power flow, thus preventing potential damage. Similarly, in undercurrent situations, such as the failure of a heating element in a kiln, the relay ensures the problem is identified, maintaining proper temperature distribution and processing conditions.
• Voltage monitoring relays: These relays protect equipment from voltage abnormalities such as under-voltage, over-voltage, phase imbalances, phase loss, and phase sequence issues. They de-energize equipment when voltage levels deviate from safe operating parameters. A typical voltage monitoring relay comprises an electromagnetic coil for the tripping mechanism, a control circuit with adjustable features, and a contact mechanism. The relay's contacts remain closed when the voltage is within the normal range and open when abnormal voltage is detected. These relays are essential in maintaining safe voltage levels in motor control centers, generators, HVAC systems, pump controls, and other critical applications.
• Phase monitoring relays: These relays ensure the stability and correct operation of three-phase motors and other rotating equipment. They protect against voltage faults that can lead to equipment failure or unintended operation. In the event of a phase loss due to a blown fuse, tripped breaker, or open circuit, the connected motors can draw excessive current on the remaining phases, leading to overheating and potential damage to internal circuitry. Phase monitoring relays are designed to safeguard against these conditions by monitoring the voltage phases in Wye or Delta systems. They are capable of detecting and responding to single phasing, even when regenerative voltages are present.
• Ground fault relays: These relays aim to safeguard electrical equipment from ground faults caused by unintentional current flow to a grounded surface. These faults can arise from dust, water, or worn insulation, leading to short circuits and posing safety risks to workers. Ground fault relays provide continuous protection by monitoring these faults using a separately connected current transformer. They detect faults through zero sequence and direct measurement methods. Zero sequence involves monitoring phase conductors to ensure that all currents return on the same conductors, detecting any current that returns via a different path, usually the ground. Direct measurement involves reading current in the transformer neutral and ground connection.
• Insulation monitoring relays: These relays protect electrical systems by monitoring the insulation resistance between conductors and the ground. They detect and prevent insulation faults that could lead to electrical hazards or equipment damage. They are used for loads disconnected from the voltage system to ensure safety before reconnection. When an insulation fault is detected, these relays prevent the network's connection by anticipating the trip of other protective devices, such as circuit breakers with control coils. This proactive approach ensures that faults are addressed before they can cause significant issues.
• Temperature monitoring relays: These relays accurately monitor and control the operating temperature of equipment and machinery. They ensure continuous temperature monitoring, maintaining operations within optimal temperatures. When the temperature exceeds or falls below the designated range, these relays can automatically shut down the machinery, trigger alarms, or activate cooling or heating systems, thus preventing potential damage or hazards. Some temperature relays can be programmed to react to custom temperature settings, providing flexibility for various applications. They primarily focus on safety, reducing the risks of overheating due to overcurrent conditions. These relays protect equipment from damage and diminish fire risk by disconnecting power when unsafe temperatures are detected.

Selecting a right protection relay

Selecting the right protection relay is essential for ensuring the optimal performance and reliability of electrical systems. Here are the key factors to consider during the selection process:

• Application requirements: Thoroughly understand the system's specific needs for protection, including necessary voltage levels, current ratings, and load characteristics.
• Response time: In critical situations, choose relays with swift response times to detect and isolate faulty sections quickly, minimizing damage and reducing downtime.
• Standards and certifications: Ensure that the selected relay complies with relevant standards and certifications, guaranteeing safety and reliability in operation.
• Voltage and current considerations: Select a relay that matches the electrical system's voltage and current ratings to handle anticipated levels without exceeding their capacities.
• Communication and monitoring capabilities: Look for advanced features such as easy controller connection, remote monitoring, IO-Link interface, and SCADA integration for proactive system management and quick issue response.
• Ease of installation and maintenance: Choose a user-friendly relay for easy installation and maintenance to minimize errors and reduce downtime during maintenance.
• Compatibility and interoperability: Ensure the relay is compatible with existing control and monitoring systems and communication protocols such as Modbus for seamless integration and interoperability

Application of protection relays

Protective relays play a crucial role in safeguarding essential equipment in various industries. They have key applications in the following areas:

• Generator protection: Generators are designed to operate under high load factors for extended periods, making them vulnerable to various faults and abnormal operating conditions. These include phase faults, ground faults, loss of excitation, over-excitation, overvoltage, unbalanced currents, abnormal frequencies, motoring, dead machine energization, breaker failures, and system faults. Protective relays act swiftly to disconnect a faulty generator from the system, preventing damage and, if necessary, initiating a complete shutdown to protect the machine and its auxiliaries.
• Transformer protection: Protective relays provide over-excitation and temperature-based protection for power transformers, identifying adverse operating conditions and shielding them from failures. These relays address failure modes such as winding failures from short circuits, core faults due to insulation breakdown, terminal failures from loose connections, and on-load tap changers.
• Busbar protection: Busbar relays are designed specifically for high-impedance applications, protecting busbars from faults and failures, including short circuits, earth faults, overcurrents, and other abnormal electrical conditions. Upon detecting a fault, the relay rapidly triggers protective devices, such as circuit breakers, to isolate the faulty section from the rest of the system.
• Motor protection: Motors are at risk of overloads, overtemperature, phase imbalances, and ground faults. Motor protection relays continuously monitor parameters such as current and voltage to quickly identify abnormalities and take corrective actions. This prevents motor damage and reduces the risk of disturbances affecting the power grid, thereby improving overall system stability.
• Feeder protection: In power distribution feeders, faults such as arc flash, impending cable splice, and high impedance faults pose hazards to the general public and personnel. Protective relays use advanced algorithms and sensing technologies to swiftly identify faults like short circuits, overcurrents, and ground faults. These relays work in coordination with other network protection devices, such as circuit breakers and transformer relays, to offer a comprehensive and synchronized response to faults.

Application example of phase monitoring relay

It's important to note that overloading and phase power unbalance are the main reasons for motor damage. Overloading causes an increase in current consumption, leading to insufficient torque or overheating, which can permanently damage motors. In the case of 3-phase motors, phase imbalance causes excessive current flow in one or more phases, leading to increased operating temperature and damage to motor insulation. This can result in motor immobilization and unplanned machine downtime.

The Omron K8AK-PH1 monitoring relay is specifically designed to monitor 3-phase, 3-wire supplies. It tracks phase sequence and phase loss during start-up and operation while accommodating a wide range of AC supply voltages from 200 to 480 VAC. The K8AK-PH1 monitoring relay has a maximum operation reaction time of 0.1 seconds, ensuring quick detection and response to phase anomalies, thus minimizing potential equipment damage. Additionally, the relay has two independent DPDT relay outputs that can trigger alarms, initiate equipment shutdowns, or activate corrective actions based on detected phase issues. The LED indicator of the K8AK-PH1 provides real-time feedback on the relay's operational status, making troubleshooting and system monitoring simpler.