Abstract : With the implementation of national energy policies, power plants are increasingly using high-voltage frequency converters to drive motors. This paper focuses on a new frequency converter motor differential protection device developed by a technology company. It employs the sampling value differential principle to achieve differential protection for the frequency converter motor; and integrates transformer and motor protection functions into a single device, enabling backup protection through a hard-plate switch via a bypass switch.
Keywords: motor protection; transformer protection; phasor differential; sampled value differential; bypass switch switching
In accordance with national energy policy requirements, energy conservation and emission reduction efforts have been fully implemented, and reducing the plant power consumption rate is imperative for large thermal power plants. For high-voltage motors, which account for the vast majority of plant power consumption, a crucial energy-saving technology is the application of high-voltage variable frequency drive (VFD) technology. With the development of power electronics technology, VFDs have been widely used in power plants. Currently, newly built power plants generally require VFD drives for important auxiliary equipment such as fans and pumps; an increasing number of existing power plants are undergoing or have already completed the retrofitting of high-voltage motors with VFDs. After high-voltage motors are driven by VFDs, how to configure motor protection to ensure the safe and reliable operation of the unit has become a concern for power plants, design institutes, and protection device manufacturers.
1. Traditional motor protection configuration
Faults in asynchronous motors include stator winding phase-to-phase short circuit faults, winding turn-to-turn short circuit faults, and single-phase grounding faults. Abnormal operating conditions mainly include overload, locked rotor, excessively long starting time, unbalanced three-phase power supply or single-phase operation, and abnormal voltage. Therefore, for high-voltage motors, according to regulations, differential protection or instantaneous overcurrent protection is the primary protection method, with overload protection, overcurrent protection, negative sequence protection, zero sequence protection, and undervoltage protection as backup protection.
2. Current protection configuration for frequency converter motors
To ensure system reliability, power plants typically use frequency converters with power frequency bypasses for high-voltage motors. This allows the motors to continue operating normally even during frequency converter maintenance. Figure 1 shows a schematic diagram of the on-site high-voltage motor frequency converter modification. Switches K1 and K2 ensure that there are no contact points between the frequency converter and the main circuit during frequency converter maintenance. At this time, switch K3 is closed, and the motor runs through the bypass.
When the motor operates via bypass, it is directly driven by the power frequency voltage of the high-voltage bus in the plant's auxiliary power supply. The protection device at the incoming switch QF protects both the outgoing line of the switch and the motor itself. Therefore, motor protection should be configured according to the requirements of conventional motor protection. If differential protection is required, differential motor protection should be configured.
When bypass switch K3 is open and the motor is driven by the frequency converter, the protection device at the incoming switch QF protects both the outgoing line of the switch and the frequency converter. Since frequency converters used in power plants generally consist of a rectifier transformer and control cabinet, the protection device at the incoming switch QF protects only the outgoing line of the switch and the rectifier transformer. At this time, the motor becomes the load of the high-voltage frequency converter, which is isolated from the plant's auxiliary power bus. Therefore, the motor's protection should be implemented by the controller of the high-voltage frequency converter system. For 6-10kV rectifier transformers, conventional transformer backup protection is generally configured, with slight differences in setting time compared to conventional transformers. In this situation, the conventional differential protection for the motor cannot perform differential protection due to the inconsistency between the frequency of the current at the switch and the frequency of the motor's neutral side current, and must be deactivated.
Currently, typical inverter motor protection configurations include: motor protection and control devices, motor differential protection devices, and transformer protection and control devices. The motor protection devices and transformer protection devices are activated and deactivated via a bypass switch: when the bypass switch is open, the inverter drives the motor, the transformer protection device is activated, and the motor protection devices and motor differential protection devices are deactivated; when the bypass switch is closed, the power grid directly drives the motor, the motor protection devices and motor differential protection devices are activated, and the transformer protection device is deactivated.
