Abstract: With the implementation of national energy policies, power plants are increasingly using high-voltage inverters to drive motors. This paper introduces a new inverter motor differential protection device developed by a technology company. It employs the sampling value differential principle to provide effective differential protection for inverter-driven motors. The device integrates both transformer and motor protection functions into one unit. A hard pressure plate, activated by the bypass switch, serves as a backup protection mechanism. Keywords: motor protection; transformer protection; phasor differential; sampled value differential; bypass switch In line with the national energy policy, energy conservation and emission reduction efforts have been fully implemented. In large thermal power plants, reducing the plant’s power consumption rate has become critical. High-voltage motors account for a significant portion of the plant's electricity use, and the application of high-voltage frequency conversion technology is a key technical measure for energy efficiency. With advancements in power electronics, frequency converters are now widely used in power plants. New auxiliary power plants, as well as essential equipment such as fans and pumps, are typically designed with frequency converter systems. Many existing power plants are also undergoing or have completed the retrofit of high-voltage motors with frequency converters. Once a high-voltage motor is driven by an inverter, how to configure its protection system to ensure safe and reliable operation has become a major concern for power plants, design institutes, and protection manufacturers. Traditional Motor Protection Configuration Asynchronous motor faults include stator winding phase-to-phase short circuits, inter-turn short circuits, and single-phase ground faults. Abnormal operating conditions often involve overload, stall, long starting time, three-phase power imbalance, or phase failure. According to regulations, differential protection or current quick-break protection is the main protection for high-voltage motors, while overload, overcurrent, negative-sequence, zero-sequence, and low-voltage protections serve as backup. Current Inverter Motor Protection Configuration To ensure the reliability of power generation, high-voltage motors are typically equipped with frequency converters that allow for power frequency bypass. This enables the motor to continue running even during inverter maintenance. Figure 1 illustrates the field configuration of a high-voltage motor inverter. Switches K1 and K2 ensure no contact with the main circuit when the inverter is under inspection. At this point, switch K3 is closed, allowing the motor to run through the bypass. When the motor operates via the bypass, it is directly powered from the factory’s high-voltage busbar. The protection device at the incoming switch QF covers the switch outlet and the motor itself. Therefore, at this stage, motor protection must be configured according to traditional standards. If differential protection is required, a dedicated motor differential protection system should be installed. However, when the bypass switch K3 is open and the motor is driven by the inverter, the protection device at the incoming switch QF now monitors the switch outlet and the inverter. Since most power plant inverters consist of a rectifier transformer and control cabinet, the protection device at QF primarily protects the switch outlet and the rectifier transformer. At this point, the motor becomes a load of the high-voltage inverter and is isolated from the factory bus. Thus, motor protection must be handled by the inverter system’s controller. For 6–10 kV rectifier transformers, conventional transformer backup protection is typically configured, with slight timing differences compared to standard transformers. Due to the mismatch between the current at the switch and the neutral side of the motor, conventional motor differential protection cannot be applied and is usually disabled. Currently, the general configuration for inverter motor protection includes a motor protection measurement and control device, a motor differential protection device, and a transformer protection measurement and control device. These devices switch functions based on the state of the bypass switch: when the bypass is open, the inverter drives the motor, the transformer protection is active, and the motor protection and differential protection devices are deactivated. When the bypass is closed, the motor runs directly from the power grid, and the motor protection and differential protection devices are activated, while the transformer protection is deactivated.

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