Abstract: Power frequency bypass operation is an important operating mode of high-voltage frequency converters. Improper switching between the high-voltage frequency converter and the power frequency power supply can cause large current surges and severe electromagnetic interference. This paper proposes the concept of soft switching between the high-voltage frequency converter and the power frequency power supply, discusses the principle and implementation method of soft switching, and uses PSCAD/EMTDC electromagnetic transient simulation software to simulate several switching methods. The simulation results show that the soft switching method of the high-voltage variable frequency speed control system effectively avoids excessive inrush current and ensures a smooth transition of motor speed. Keywords : High-voltage frequency converter; Power frequency power supply; Soft switching; PSCAD/EMTDC Introduction When a high-voltage frequency converter malfunctions or requires maintenance, it is necessary to switch the motor from the high-voltage frequency converter power supply to the power frequency power supply. When multiple motors share one high-voltage frequency converter, the high-voltage frequency converter must perform soft starting of each motor in a certain sequence, and finally switch to power frequency operation. Therefore, power frequency bypass operation is an important operating mode of high-voltage frequency converters. In addition, when the high-voltage frequency converter is momentarily de-energized and then put back into operation, it needs to continue to supply power to the motor; when fans, water pumps, etc. switch from power frequency operation to high-voltage frequency conversion speed regulation operation, they all need to switch to high-voltage frequency converter operation. Improper switching between high-voltage frequency converter and power frequency power supply will cause a large current surge and serious electromagnetic interference [1]. Reference [2] divides the switching methods between high-voltage frequency converter and power frequency power supply into asynchronous switching and synchronous switching. Reference [3] introduces several methods to eliminate switching surge current: identifying the voltage phase when putting into the grid, connecting a three-phase demagnetizing resistor to the motor stator, etc. These methods are relatively troublesome, have poor real-time effect, the motor speed drops a lot, the motor slip increases by a factor of two, and the current at the moment of motor switching may also increase by a factor of two. As we all know, the current surge caused by asynchronous switching is the largest, and synchronous switching can reduce the current surge, but there is still a certain current surge. When switching the high-voltage frequency converter to power frequency bypass, it is not allowed to perform multiple switching in the actual system to verify the effect of various switching methods. This paper proposes the concept of soft switching between high-voltage frequency converters and power frequency power supplies, discusses the principle and implementation method of soft switching, and uses PSCAD/EMTDC electromagnetic transient simulation software to simulate and study several switching methods. Simulation results show that the soft switching method of the high-voltage variable frequency speed control system effectively avoids excessive inrush current and ensures a smooth transition of motor speed. 1 Switching Method of High-Voltage Frequency Converter The power frequency bypass operation of the high-voltage frequency converter is shown in Figure 1. When the high-voltage frequency converter is powered, QSi, QS2, KM1, and KM2 are closed; when the power frequency grid is powered, QS3 and KM3 are closed. QS2 and QS3, KM2 and KM3 must be mutually interlocked during the operation of the high-voltage frequency converter to ensure switching between the power frequency power supply and the high-voltage frequency converter power supply. QS1, QS2, and QS3 are used to completely disconnect the power supply of the system. [align=center]Figure 1 Bypass operation of inverter system[/align] Based on motor operating characteristics, power frequency bypass switching frequency, and system requirements, the commonly used switching methods between high-voltage inverters and power frequency power supplies are divided into direct switching, asynchronous switching, synchronous switching, and soft switching. a. Direct switching. This refers to directly switching the power frequency power supply and the high-voltage inverter power supply while ensuring the phase sequence of the high-voltage inverter and the grid power supply is consistent. During direct switching, the voltage amplitude, frequency, and phase are not detected. This method requires the system and protection to tolerate fluctuations and impacts during switching, and is therefore rarely used. b. Asynchronous switching. This refers to switching that detects the voltage amplitude and frequency but not the voltage phase. The most serious situation during asynchronous switching occurs when the phase difference between the high-voltage inverter output voltage and the grid voltage is 180°, causing a large inrush voltage and current, with the inrush current reaching approximately 30 times the rated current. This method requires the system to tolerate impacts and torque changes during switching and is generally only used in low-power, low-voltage inverter systems. c. Synchronous switching. This refers to the switching of the amplitude, frequency, and phase of the detected voltage. Synchronous switching technology ensures that the switching current does not exceed 2.5 times the motor's rated current. During synchronous switching, the speed changes minimally before and after the mains frequency power supply is switched on. d. Soft switching. This refers to controlling the high-voltage inverter to output a voltage with the same frequency, phase, and controllable amplitude after detecting the voltage amplitude, achieving a "disruptive" switching. Soft switching has two scenarios: switching on before switching off and switching off before switching on. During the switching from mains frequency power to high-voltage inverter power, phase detection and phase-locked loop control are first used to ensure the high-voltage inverter power supply maintains phase consistency with the residual voltage at the motor terminals. With the V/F ratio of the high-voltage inverter remaining essentially unchanged, the optimal switching operating point of the high-voltage inverter is selected, and the output voltage and frequency of the high-voltage inverter are adjusted. In this way, the motor experiences virtually no inrush current when the high-voltage inverter is switched on, and the motor torque remains essentially unchanged. During the switching from high-voltage inverter to mains frequency power, a phase-locked loop is used to lock the phase and frequency of the mains frequency power supply, controlling the high-voltage inverter to switch the motor at a slightly higher voltage and frequency than its rated value. This enables soft switching between high-voltage frequency converters and power frequency converters. 2. Simulation and Analysis This paper uses PSCAD/EMTDC simulation software to conduct a simulation study on the system shown in Figure 1. This software is an electromagnetic transient analysis package developed by the High Voltage Direct Current Transmission Research Center at the University of Manitoba, Canada. Its main function is to perform time-domain and frequency-domain calculations and simulations of power systems. A typical application is calculating the changes in electrical parameters over time when a power system is subjected to disturbances or parameter changes. It can obtain good simulation results for transient waveforms during the switching process. PSCAD has a network version (EE) and a personal computer version (PE). The PE version is suitable for simulation systems with fewer than 15 nodes; systems with more than 15 nodes require the EE version 151. [align=center]Fig. 2 Main circuit for PS-AD simulation[/align] The motor simulation parameters are as follows: rated line voltage is 6 kV, rated phase current is 0.159 kA, number of pole pairs is 8, power factor under rated load is 0.8, efficiency under rated load is 0.935, slip under rated load is 0.008, starting torque under rated voltage is 1, maximum torque is 1.8, starting current under rated voltage is 6 (per unit value), and mechanical damping is 0.008. 2.1 Asynchronous switching simulation The stator current waveform of the motor during asynchronous switching is shown in Figure 3. The motor runs stably under the power frequency power supply for 5.5 s, then the power frequency power supply is disconnected, and asynchronous switching to the high-voltage frequency converter power supply takes 5.55 s. As can be seen from Figure 3, the current surge during asynchronous switching is very obvious, about 30 times the rated current, which causes a great impact on the motor and the high-voltage frequency converter. Moreover, the time for the motor to recover stable operation is relatively long, exceeding 1.5 s, and the motor vibration is very severe. [align=center] Figure 3. Stator current curve of asynchronous-switching[/align] 2.2 Simulation of synchronous switching The stator current waveform of the motor during synchronous switching is shown in Figure 4. The motor runs stably under the power frequency power supply for 5.5s, then disconnects the power frequency power supply, and switches synchronously to the high-voltage variable frequency power supply at 5.55s. As can be seen from Figure 4, the current has a relatively small impact when using the synchronous switching method, which is about 2.5 times the rated current. After switching to the high-voltage variable frequency power supply for 0.25s, the motor can re-enter a new stable state. [align=center] Figure 4. Stator current curve of synchronous-switching[/align] 2.3 Simulation of soft switching The stator current waveform of the motor during soft switching is shown in Figure 5. The motor runs stably under the power frequency power supply for 5.5s, then disconnects the power frequency power supply, and switches softly to the high-voltage variable frequency power supply at 5.55s. As shown in Figure 5, during soft switching, the high-voltage inverter initially supplies power to the motor with a relatively low voltage and frequency. While ensuring constant motor torque, the inrush current on the motor is very small, essentially consistent with the motor's rated operating current. The output voltage and frequency of the high-voltage inverter are gradually increased, allowing the motor to smoothly transition to its rated operating state, achieving a smooth, non-disruptive soft switching. [align=center] Figure 5: Stator current curve of soft-switching[/align] 3 Implementation of Soft Switching Soft switching, based on the hardware of synchronous switching, achieves soft switching between the power frequency power supply and the high-voltage inverter through special logic design of the controller. When the motor needs to switch from operating on the power frequency grid to variable frequency speed control, phase detection and phase-locked loop control are first used to make the high-voltage inverter track the residual voltage phase and frequency at the motor end. The optimal operating point of the high-voltage inverter is selected and the inverter is engaged. Then, the output voltage and frequency of the inverter are gradually increased to reach the power frequency voltage and frequency values, allowing the motor to gradually operate to its rated state. At this time, the motor inrush current is minimal, and the motor torque remains essentially constant. During the switching process of a high-voltage frequency converter in bypass operation, the control converter increases its output voltage and frequency, tracks the residual voltage amplitude and phase at the motor end, and switches accordingly to achieve soft switching between the high-voltage frequency converter and the power frequency supply. 4. Conclusion When switching a high-voltage frequency converter in power frequency bypass operation, it is essential to minimize the impact on the power grid, the high-voltage frequency converter, and the motor during the switching process. This paper proposes the concept of soft switching between the high-voltage frequency converter and the power frequency supply, and discusses the principle and implementation method of soft switching. For high-voltage, high-capacity variable frequency drive systems that require frequent switching, the soft switching method can avoid excessive inrush current and ensure a smooth transition of speed and torque.