1. Introduction AC asynchronous motors are widely used in various sectors of the national economy. Direct starting of asynchronous motors presents problems such as low starting torque, high starting current, significant impact on the power grid, difficulty in starting, significant impact on mechanical equipment, short motor lifespan, high maintenance workload, and high maintenance costs. Asynchronous motor soft starters can reduce the voltage drop across the power grid caused by hard starting of the motor, ensuring it does not affect the normal operation of other electrical equipment sharing the grid. They can reduce the inrush current of the motor, which can cause excessive local temperature rise and reduce motor lifespan; they can reduce the mechanical impact force caused by hard starting and the accelerated wear of the transmitted machinery (shafts, meshing gears, etc.); and they can reduce electromagnetic interference, as the inrush current can interfere with the normal operation of electrical instruments in the form of electromagnetic waves. Soft starting allows the motor to start and stop freely, reducing idling and improving operating efficiency, thus achieving energy savings. Soft starting of motors can be broadly divided into stepped and stepless types. Stepped soft starters include stator series reactor voltage reduction, liquid resistor voltage reduction, star-delta (y-Δ) voltage reduction, autotransformer voltage reduction, and extended delta voltage reduction. Stepless soft starters include switching transformer voltage reduction, magnetic saturation reactor voltage reduction, and thyristor series voltage reduction soft starters. Because stepped voltage reduction soft starters experience a certain degree of secondary current surge during adjustment, their soft starting effect on motors is limited. However, in stepless soft starters, with the improvement of power electronics technology, the development of power devices, and the significant increase in the prices of raw materials such as copper and iron, thyristor series high-voltage soft starters are increasingly recognized by the market. 2. Voltage Reduction Starting Principle Connecting the stator windings of a three-phase asynchronous motor to a three-phase power supply, and the rotor accelerating from rest to a steady state, is called starting. At the instant of closing the circuit, the motor's slip is 1, the starting current equals the locked-rotor current, and the starting torque equals the locked-rotor torque. As the rotational speed increases, the starting current gradually decreases from the stall current and finally stabilizes at a certain value. A higher stall torque indicates that the motor can start under a larger load and obtain a larger acceleration, but an excessive stall current will generate a large voltage drop on the power supply line, causing the grid voltage to fluctuate and directly affecting the operation of electrical equipment connected to the grid. The T-shaped equivalent circuit diagram of the asynchronous motor is shown in Figure 1. Figure 1 T-shaped equivalent circuit diagram of the motor The torque parameter expression of the motor is shown in Equation (1): (1) Where: m is the electromagnetic torque; s is the slip rate; p is the number of pole pairs; m1r1x1 are the number of phases, resistance and reactance of the stator winding, respectively; r｀2x｀2 are the rotor resistance and reactance referred to the stator side, respectively. The starting current (effective value) and starting torque when the circuit is closed can be obtained from the T-shaped equivalent circuit diagram and Equation (1), and the formula is: (2) (3) From the above analysis, it can be concluded that when the stator terminal voltage of the motor is reduced, the starting current flowing through the motor decreases proportionally. The starting current supplied to the motor by the power grid also decreases accordingly, thus reducing the impact on the power grid. 3 High-voltage soft starter system design 3.1 System overview The system block diagram of the high-voltage motor soft starter is shown in Figure 2. The power unit with thyristors connected in series is connected between the three-phase high-voltage power grid and the motor. The control unit adjusts the phase according to the pre-set starting curve based on the signal transmitted back from the sensor. The thyristor trigger signal sent by the control unit is transmitted to the thyristor trigger unit through optical fiber to adjust the conduction angle of the thyristor, thereby adjusting the voltage so that the voltage output to the motor rises slowly according to a certain curve, realizing the soft start of the motor. When the motor reaches the rated speed, the bypass contactor is engaged, and the motor is in bypass operation mode. The control unit still performs online monitoring, responsible for displaying the voltage and current of the motor and monitoring various faults. Figure 2. Block diagram of high-voltage motor soft starter system. 3.2 Design of thyristor series unit. Since most motors used in the domestic market are 6kV and 10kV motors, a single thyristor is insufficient to withstand the 6kV high voltage as a power actuator connected in series between the high-voltage grid and the motor. Although single thyristors have matured to withstand 6500V, considering reliability factors such as grid fluctuations, surges, and withstand voltage margin, the power unit uses three thyristors in series to improve the withstand voltage value when designing a 6kV high-voltage soft starter. Similarly, in the design of a 10kV high-voltage soft starter, five thyristors are connected in series to form a high-voltage valve group. A schematic diagram of a single-phase 6kV high-voltage thyristor power valve group is shown in Figure 3. Sc1 to Sc6 are high-power high-voltage thyristors, which are connected in series in groups of three and then in anti-parallel to form a single-phase power series valve group to realize the control of the AC motor by the soft starter. These six thyristors are selected from the same manufacturer, model, and production batch to minimize differences in their inherent characteristics, such as volt-ampere characteristics, reverse recovery charge, switching time, and critical voltage rise rate, caused by variations in manufacturing processes, thus reducing their impact on voltage equalization. R1, R2, and R3 are static voltage equalization resistors used to achieve static voltage equalization of the thyristors. These static voltage equalization resistors are non-inductive, with a resistance value of 1/40th of the equivalent resistance of the thyristor in the blocking state, and a sufficiently large power margin. R4, R5, R6, and C1, C2, and C3 together form a dynamic voltage equalization network to achieve dynamic voltage equalization. Through selection, the parameter errors of each resistor and capacitor should be very small. The capacitor value is calculated based on the difference between the maximum and minimum reverse recovery charge of the thyristor. The voltage equalization process is mainly completed by capacitor C. The switching speeds of the thyristors connected in series will not be completely identical, but will have slight differences. The voltage across capacitor c is the same under static conditions. During the switching process, since the voltage across the capacitor cannot change abruptly, the voltage drop across each thyristor will not jump. The effect caused by the inconsistent current in each thyristor during the switching process is compensated by the charging and discharging of capacitor c. Please log in to: Power Transmission and Distribution Equipment Network to browse more information Figure 3 Schematic diagram of a single phase of a 6kV series valve group 3.2.1 Static voltage equalization When a certain voltage is applied to the thyristor, there is always a certain leakage current when it is in the blocking state. When connected in series, the thyristor with the smallest leakage current, i.e. the largest leakage resistance, bears the largest voltage. An effective solution is to connect parallel resistors rj (r1, r2, r3 in Figure 3), which are called voltage equalization resistors. Since the resistance of rj is much smaller than the leakage resistance, the voltage borne by the tube is basically equal when it is blocked in both directions. The usual value is: Source: Power Transmission and Distribution Equipment Network (4) Where, ured is the rated voltage of the thyristor; idr is the static repetitive average current; πidr is approximately the peak value of the leakage current. The power of the equalizing resistor can be obtained by the following formula: (5) Where, um is the positive peak voltage acting on the element; n is the number of series elements; krj is a coefficient, which is 0.25 for single phase, 0.45 for three phase, and 1 for DC. 3.2.2 Dynamic equalizing Dynamic equalizing is the equalizing of voltage during the turn-on and turn-off process of the thyristors in the same bridge arm, that is, the equalizing of voltage during the transient process. The static equalizing resistor rj can only make the DC voltage or the slowly changing voltage evenly distributed to each series thyristor. When turned on, the thyristor turned on later will be subjected to a higher voltage instantaneously. When turned off, the thyristor turned off first will bear all the reverse voltage of the commutation, which may lead to reverse breakdown. Therefore, capacitors c (c1, c2, c3 in Figure 3) are connected in parallel with the thyristor. In order to prevent the capacitor discharge from causing an excessive (di/dt) at the moment the thyristor is turned on, resistors r (r4, r5, r6 in Figure 3) should also be connected in series with c. The RC snubber circuit should be placed as close as possible to the thyristor, with short leads, and preferably using a non-inductive resistor. The capacitor withstand voltage should be 1-1.5 times the thyristor voltage. The capacitance value can be calculated using the reverse recovery charge method. When the thyristor is turned off, its current crosses zero in the positive direction and does not immediately restore its blocking state. Due to the presence of the reverse recovery charge, a reverse recovery current itr is formed in the thyristor. The reverse recovery charge is obtained from the following formula: (6) Where tr2 is the reverse blocking recovery time. The reverse recovery charge determines the blocking voltage value borne by each thyristor in the series valve group. When the reverse leakage current itr flows through the thyristor in the reverse direction, the reverse blocking characteristic is restored, and the turn-off process ends. The capacitance value can be calculated from the following formula: (7) 3.3 Interface Unit Design The unit includes a voltage sensor interface, a current sensor interface, an optical fiber transmission interface, a fault detection interface, and a human-machine interaction interface. The voltage signal adopts a high-impedance voltage reduction method, and considering system compatibility, the circuit is designed to be universal for 3kV, 6kV, and 10kV to facilitate product manufacturing. The current sensor uses a standard x/5 current transformer combined with a high-precision current Hall effect sensor. The signal is processed and then sent to the CPU for computation. Signal transmission between high and low voltage is via fiber optic cable, ensuring both real-time and reliable transmission while also providing high-low voltage isolation. The signal is encoded by the interface circuit and transmitted via fiber optic cable to the trigger unit. The trigger unit decodes the signal and processes it accordingly before using it to trigger the thyristors. The trigger unit's power supply uses a combination of high-level and low-level power supplies, with each thyristor having its own independent trigger power supply. The human-machine interface uses a membrane-type soft key and an LCD display. The LCD display is 4 rows and 8 columns, designed with a 4-level menu management mode, and can be preset to display in Chinese and English. 3.4 Software Design Software design is the core of system control and directly affects the stability and reliability of system operation. To adapt to various load applications, the software design incorporates multiple different start-up curves, including voltage ramp start, current-limiting start, jump start, and soft stop curves. Simultaneously, comprehensive protection functions are designed, including short-circuit protection, overcurrent protection, overvoltage and undervoltage protection, and thyristor overheat protection. Motor parameters and various protection parameters can be set by the user according to the on-site application conditions. 4. System Experiment After the system design was completed, a load-bearing starting test was conducted using a 6kV/1000kW motor. The motor's rated voltage is 6kV, rated current is 112A, and rated speed is 1480r/min. The single-phase waveform of the starting current is shown in Figure 4. As can be seen from the figure, the starting current is stable and without impact, the peak starting current is approximately 2.6 times the rated current, the starting time is 22s, and the grid voltage shows no significant fluctuation, achieving a good starting effect. Figure 4: Soft-start single-phase current waveform 5. Conclusion This system uses a thyristor series valve group as the main power actuator and achieves soft starting of the motor through AC voltage regulation. The system features flexible control, simple operation, smooth starting, and reliable operation, effectively mitigating the impact on the grid and load during motor starting and protecting the motor's safe starting and operation.