Relay phase-loss protection technology: principle, application and development trend

The basic principle and importance of phase-loss protection

Hazards of phase-loss faults

Phase-loss refers to the phenomenon that any one phase of the power supply in a three-phase power system is missing, which is one of the main reasons that lead to the burning of three-phase asynchronous motors. When the three-phase motor in operation when the missing phase, will produce negative sequence current component, resulting in three-phase current imbalance or too large, which in turn causes the motor winding instantaneous burnt. According to statistics, about 40% of motor failures are related to power supply phase loss.

The basic principle of relay phase loss protection

Relay phase loss protection determines whether a phase loss fault has occurred by monitoring the current or voltage abnormality of the three-phase circuit. For motors with star (Y) connection, since the line current is equal to the phase current, ordinary thermal relays can provide effective protection; however, for motors with delta (Δ) connection, since the phase current is not equal to the line current, ordinary thermal relays may not be able to act in time, and special phase failure protection relays must be used.

Differential phase-break protection mechanism

Thermal relays with phase-break protection use a differential mechanism, consisting of an upper guide plate, a lower guide plate and a lever. When a phase is disconnected, the bimetal of that phase cools and resets, pushing the upper guide plate to move to the right, while the bimetal of the other two phases continues to be bent by heat, pushing the lower guide plate to move to the left, triggering the protection action through differential action.

Technical realisation scheme of phase loss protection

Thermal relay phase loss protection

Ordinary thermal relay pushes the actuator through bimetal bending by heat, but it has limited effect on the protection of Δ-connection motor.

Differential thermal relay solves this problem by special structural design. When phase loss occurs, the bimetal on the phase loss side cools down and resets, while the other two phases continue to be bent by heat, triggering the protection by differential action.

Voltage monitoring type phase loss protection

uses three low voltage relays with adjustable release voltage, the coils are star connected and the common node is connected to the zero line.

The disadvantage is that the sensitivity is insufficient and the motor may still maintain a high voltage during phase loss, resulting in delayed protection.

Electronic Comprehensive Protection Relay

Integrating microprocessor and advanced sensing technology, it can monitor multiple parameters such as current and voltage simultaneously, realising all-round protection.

Solid State Relay (SSR) Technology

Suitable for high voltage automotive battery systems with output withstand voltages up to 900V, it features fast response and long life.

Challenges and solutions in practice

Protection challenges for Δ-connected motors

Solutions include the use of specialised phase failure protection relays, integrated motor protectors or the design of auxiliary protection circuits.

Contactor Release Voltage Mismatch Problems

Common contactor release voltages range from 0.2Us to 0.7Us, which can lead to phase loss protection failures.

A more feasible solution is to adopt a protection strategy based on current protection.

Risk of relays failing on their own

Preventive measures include the use of reliable quality products, regular testing and maintenance, redundant design and the use of intelligent relays with self-diagnostic functions.

Out-of-phase faults in PV systems

Relays can be used to disconnect the PV system from the grid in the event of a grid fault for fast protection.

Technological innovation of relay phase loss protection

CPLD-based intelligent phase loss protection system

Contains signal acquisition module, CPLD logic processing module and thyristor control protection module.

Through the algorithm of real-time judgement of the circuit state, once the detection of phase loss, immediately close the single SCR to protect the relay.

Integrated and miniaturised design

such as small solid state relays, reducing the solution size by up to 90% while reducing BOM costs.

New Material Applications

The use of silver alloy contacts, ceramic insulating materials, etc. improves the relay's resistance to high temperatures and arcing.

Wide-band semiconductor technology

The introduction of Silicon Carbide (SiC) and Gallium Nitride (GaN) devices has raised the operating temperature of solid state relays to over 200°C and brought the switching frequency into the MHz era.

Selection guide for relay phase loss protection

Selection according to the type of load

Resistive, inductive or capacitive loads require different relay characteristics.

Matching Voltage and Current Ratings

Ensure that the relay rating is higher than the maximum operating parameters of the system. Environmental Adaptability Consideration

Environmental factors such as high temperature, high humidity, vibration, etc. affect relay selection.

Functional requirements analysis

Different requirements such as basic protection, intelligent monitoring or remote control determine the product type.

Typical application scenarios of relay phase loss protection

Industrial motor protection

Phase loss protection for key equipment such as water pumps, fans and compressors.

New Energy Vehicle High Voltage System

Circuit protection for battery management, motor control, charging system and other links.

Photovoltaic power generation system

Inverter protection, grid switching, energy storage management and other applications.

Intelligent building power distribution

Power monitoring of important loads such as lifts and air conditioners

Future Development Trends

Intelligent and Digital

New generation of protection devices integrating microprocessors, digital signal processing algorithms and communication interfaces to achieve software definition and remote management of protection functions.

Solid State Relay Technology

As semiconductor technology advances, the share of solid state relays in high voltage and high current applications will gradually increase.

Predictive Maintenance and IoT Integration

Intelligent relays can detect potential failure hazards in advance through continuous monitoring of equipment status parameters and deep integration withindustrial IoT systems.

Standardised and modular design

Modern protective relays commonly use embedded mounting, standard terminals and HD LCD interfaces to simplify installation and configuration processes