This paper describes the function and working principle of the slip regulator, the hardware equipment of the slip regulator control system, and the PLC control of the slip regulator. It focuses on the solution of practical problems during accident protection and commissioning. This equipment has been applied in the No. 1 and No. 2 motor-flywheel-AC pulse generator sets of the HL-2A magnetic confinement plasma experimental device at the Southwestern Institute of Physics, China National Nuclear Corporation. During the unit startup process, it ensures that the stability of the motor stator current reaches below 1%. The entire equipment operates stably and reliably, fully meeting the design and practical requirements. Keywords: Slip regulator, PLC, Programming 1. Function and Working Principle of the Slip Regulator The HL-2A tokamak is a large-scale nuclear fusion research experimental device in China. The power supply system of this experimental device is powered by the motor-flywheel-AC pulse generator set through a transformer and thyristor converter to supply power to the load coil. For such a power supply system, directly supplying power from the grid would cause a huge impact on the grid and generate serious electromagnetic pollution. Therefore, we adopted the internationally common practice of using the device's pulse duty cycle and employing a power supply method that draws energy from the power grid and uses an AC pulse flywheel generator set for isolation, energy storage and conversion, power amplification, and energy release. Specifically, two 90MVA AC flywheel generator sets (hereinafter referred to as generator sets or units) are used to pulse-power the experimental device. The entire unit consists of a 2500kW wound-rotor asynchronous motor, a 90-ton flywheel, and a 90MVA generator. Its working principle is that the 6000V power grid supplies power to the motor. When the motor rotates, it drives the flywheel and generator to reach the motor's rated speed of 1477 rpm, after which it accelerates freely. At this point, under the action of the exciter, the generator converts the mechanical energy stored in the flywheel into electrical energy for the HL-2A device to discharge. In the experimental device, the liquid slip regulator (i.e., slip control regulator) mainly plays two roles: First, it enables the starting process of the two units. When the unit reaches a speed of 12 rpm after turning, the vacuum switch connected to the 6000V power grid is closed, and the slip control regulator is activated. Subsequently, by adjusting the height of the movable electrode in the slip regulator, the generator set with a flywheel torque of 280 t-m² is accelerated to its rated speed of 1477 r·pm under constant current (I=230A), and then allowed to accelerate freely. II. Achieving Speed ​​Regulation of the Generator Set. When the pulse generator supplies pulse power to the experimental setup, the experimental discharge of the entire setup constitutes a large pulse load for the 2500kW motor, forcing the generator set's speed to decrease. The generator set releases the mechanical energy stored in the flywheel, helping the motor overcome peak loads. To protect the motor and reduce the impact on the power grid during pulse operation, an appropriate resistor must be connected in series in the motor rotor circuit before the experimental discharge, i.e., the movable electrode must be raised to a suitable height (as shown by KB in Figure 1). After the experimental discharge is completed, the generator set current (I=230A) is kept constant by adjusting the height of the movable electrode, and then accelerated to the rated speed of 1477 r·pm, followed by free acceleration. This speed regulation process is repeated until the experiment ends. 2. Hardware Equipment for Slip Regulator Control Figure 1. Hardware Equipment of Slip Regulator Control System. The hardware structure of the slip regulator control system is shown in Figure 1. It mainly consists of an LQJ-10 type current transformer (101HL in the figure) with a current ratio of 400/5A, a current transmitter, a PID feedback control board, a regulated power supply, and a PLC. The PLC is an FX-80MR programmable controller manufactured by Mitsubishi Corporation of Japan. This PLC has 40 input points and 40 output points, and its CPU, RAM, and communication functions are integrated. It is expandable with ROM, and the program can be easily input and modified using a handheld programmer. Encryption can also be used to ensure program security. The current transmitter converts the secondary current output from the current sensor into a DC voltage output. The PID feedback control board compares the setpoint with the output signal of the current transmitter, and controls the output of the DC speed-regulating power supply through PID control, thereby controlling the electrode to increase the motor speed, causing the moving electrode of the liquid resistor to change with the stator current. The entire control system achieves the following: during the electrode descent process, when the stator current is greater than or equal to 230A, the moving electrode remains stationary; when the stator current is less than 230A, the moving electrode descends, and the descent speed of the moving electrode increases as the stator current decreases. 3. PLC Control of the Slip Regulator 3.1 Overview of PLC Control of the Slip Regulator The PLC control of the slip regulator completes the stable and reliable operation of the entire slip regulation system according to technical requirements. This includes controlling the time-sharing start-up and time-sharing re-acceleration of the two units; detecting the electrode lifting height; communicating with the outside world; protecting the alkali circulation pump from overload and power phase loss; protecting the electrode lifting motor from overload and undercurrent of the excitation current; and monitoring the temperature of the alkali solution. The PLC control of the slip regulator utilizes the PLC's input/output relays, auxiliary relays, and timers, and employs parallel connection of series circuit blocks and multiple output circuits to complete the modular program design. Each module independently completes a specific task. The entire program flowchart is shown in Figure 2. The PLC ladder diagram of the program is shown in Figure 3. Figure 2. Flowchart of the slip regulator control system. Figure 3. Ladder diagram of the slip regulator program. 3.2 PLC control method of the slip regulator. To meet the requirements of both commissioning and experimentation, the PLC control system is designed with two schemes: local control and remote control. The local control scheme is used during commissioning. The remote control scheme is used during fusion reaction experiments. According to the technical requirements for generator unit startup, Unit 2 can only start or re-accelerate after Unit 1 has finished starting or re-accelerating. The electrode lifting processes of Units 1 and 2 need to be performed simultaneously. The program design of this PLC control system fully meets the above technical requirements. This PLC control system also achieves the relative independence of the slip regulator section and its coordinated cooperation with other parts of the device. During the internal self-test of the slip regulator, the PLC program disconnects the slip regulator section from the outside world, avoiding mutual interference with external equipment. During the fusion reaction experiment, the PLC program reconnects the slip regulator section to the outside world. At this time, for the input signals from the central control room, the interlocking of corresponding buttons and indicator lights is controlled by PLC programming to ensure the accuracy and error-free transmission of electrode lifting and re-acceleration signals; for the output signals, signals are given through independent relay contacts to avoid interference from external equipment. All of the above is achieved through PLC programming with a selector switch. 3.3 Accident Protection of the Slip Regulator When the current in the rotor circuit of the 2500kW motor is too high, the temperature of the circulating alkali solution is too high, so that the cooling circulating water cannot remove the excess heat released by the rotor circuit. At this time, an alarm is issued through the temperature sensor, PLC, and electric bell; when the alkali solution temperature reaches 70℃, the thermal relay of the control system activates, and the entire unit stops starting. When the electrode lifting motor or alkali solution circulating pump overheats, the corresponding thermal relay activates, and the entire unit also stops starting. 3.4 Solution to Practical Problems Because the control of the slip regulator was initially designed with only one scheme in mind—when the vacuum switch is closed and the button controlling the moving electrode to descend is pressed once, the electrode automatically descends—this scheme suddenly failed during debugging. To handle such emergencies and further achieve automatic control, the control system incorporates two additional schemes alongside existing ones. The first scheme automatically initiates the active electrode descent procedure when the vacuum switch closes. The second scheme considers unforeseen circumstances and employs a completely manual descent operation for the motor. At the instant the vacuum switch closes, the current in the primary circuit of the unit rises from 0. If the current transmitter is activated at this time, the current signal received by the PID module will also rise from 0. After processing by the PID algorithm, the measured value will be much lower than the setpoint, causing the active electrode of the slip regulator to descend rapidly—a serious malfunction. To avoid this transient process of current rise in the primary circuit, the slip regulator control system utilizes a 5-second timer delay from the PLC to activate the current transmitter. 4. Conclusion Our designed PLC-based slip regulator control system has been tested in the HL-2A unit operation experiment. The experimental records show that during the entire start-up and re-acceleration process of the unit, the stability of the stator current of the wound-rotor induction motor reaches below 1%, meeting the experimental requirements. Figure 5 shows the relationship between the stator current and time of the 2500kW motor, plotted based on experimental data. As can be seen from the figure, during the initial startup of the generator set, the starting current initially rises below the set current value (230A) and exceeds the set value, before slowly decreasing back to the set value. The explanation for this phenomenon is as follows: At the start of startup, the windings of the wound-rotor induction motor impede the increase of the starting current. Therefore, in the initial period of startup, the starting current experiences a transient rise. To avoid this transient process, the previously mentioned 5-second delay of the PLC timer is too long, causing the starting current to exceed the set value before slowly decreasing back to the set value under the action of PID feedback control. Therefore, further debugging or calculation is needed to obtain the accurate delay value of the PLC timer. 5. Conclusion The slip regulation control system of the HL-2A device was completed under conditions of limited funding; therefore, its real-time computer display and touchscreen interface were not considered. It is hoped that when conditions are suitable, the control system we designed can be improved in these two aspects. References: Zhang Jihe, Zhang Runmin, and Liang Haifeng (eds.), *Fundamentals of Motor Control and Power Supply*, Sichuan Province: Southwest Jiaotong University Press.