Abstract: This paper briefly introduces the working principle of the series compensation device for electrified railways and the role of the thyristor AC electronic switch. It focuses on the hardware and software of the programmable logic controller (PLC) system in the 1600A 1000V thyristor AC electronic switch and its role in the device. Finally, a brief summary of the device's technical performance is given. Keywords: Electrified railway; Series compensation device; Thyristor AC electronic switch; PLC 1 Overview The harmonic problem of electrified railways (hereinafter referred to as electric railways) is a major issue related to the power quality of the public power grid and the smooth construction of electric railways. China has been committed to researching and solving this problem since the 1980s. The series compensation device can solve this problem to a certain extent, and due to its low technical difficulty, small investment, and ease of implementation, it has shown great advantages in the double-track conversion project of single-track electric railways. The working principle of the series compensation device is to change the circuit load type by inserting a capacitor pre-connected in series in the circuit when the electric railway power supply line is heavily loaded and the line inductive reactance loss is severe, thereby increasing the grid voltage and improving the power factor. (See Figure 1.) Since the load on an electric railway is a moving load, its magnitude is not only related to the number of locomotives, but also closely related to factors such as the locomotive's load, operating status, and track gradient. Capacitor switching operations are also quite frequent. If mechanical (contact) switches are used, due to the randomness of their switching timing relative to the mains voltage phase, the contacts will experience a large current surge when closing at non-zero-crossing points, causing a short circuit in the power capacitor. Their electrical life is typically only a few dozen cycles. Therefore, thyristor AC electronic switches must be used in series compensation devices to achieve zero-crossing switching of the power capacitor. 2. Requirements of the Thyristor AC Electronic Switch for the Control System and PLC Selection The main circuit of the thyristor AC electronic switch consists of two anti-parallel thyristor branches, each branch consisting of two thyristors connected in parallel, using a total of four thyristors (connection method shown in Figure 2). According to the requirements of electrification engineering design, the thyristor AC electronic switch is installed in a mobile house near the traction transformer. Based on the application conditions, the control system of this device has the following requirements: (1) It should have high reliability to meet the objective requirements of the long-term operation of the electric railway power grid; (2) It should have a complete fault alarm system, indicating the fault type after a fault occurs, and indicating the switching status of the power capacitors during fault-free operation; (3) It should have complete logic control and automatic protection functions; (4) It should ensure that the thyristor is triggered at the voltage zero-crossing point; (5) It should have good electromagnetic compatibility performance to adapt to the complex electromagnetic environment of the traction substation. The control function of the electronic switch can be completed through three approaches: ① using integrated circuits to develop a dedicated control circuit board; ② using a microcontroller and peripheral circuits to develop a dedicated control circuit board; ③ using a PLC and expansion modules to develop a dedicated control system. Because PLCs have the characteristics of reliable control, flexible configuration, small size, powerful functions, high speed, good expandability, and convenient maintenance, they not only reduce the hardware wiring of the system and improve reliability, but also can be adapted to new requirements by modifying the program. Therefore, the Siemens S7-200 series PLC is selected as the core component of the control system in the 1600A 1000V thyristor AC electronic switch. 3. The composition of the PLC control system and its role in the AC electronic switch (1) Hardware composition and its role In the PLC control system of this device, the Siemens S7-200 series CPU222, analog expansion module EM231 and text display TD200 are selected. The available hardware system resource allocation diagram is shown in the table below: CPU222 is the CPU unit in the PLC control system, responsible for receiving all switch quantities and analog quantity inputs after A/D conversion, and outputting control signals according to the control logic; EM231 is responsible for A/D conversion of the voltage and current signals of the main circuit, and sending the conversion results to the CPU unit; the text display TD200 is responsible for monitoring the output status of the CPU unit, and indicating the operating status and fault type of the thyristor electronic switch according to the combination of various output quantities. (2) Software composition and its functions In the PLC control system of this device, the software consists of two parts: text display software and logic control software. The text display software stores the correspondence between all the text content to be displayed by the text display and the combination of CPU unit output quantities. The logic control software contains all the control logic of the thyristor electronic switch, and in terms of software structure, it consists of 1 main program, 2 subroutines, and 2 timer interrupt programs. Its main functions are: to detect voltage and current according to control requirements, issue trigger signals in a timely manner, and detect various faults such as overvoltage, overcurrent, fan, and power supply faults, and issue alarm signals in a timely manner to achieve protection functions. The flowchart of the software implementation is shown in Figure 4. 4. Thyristor Control Strategy The thyristor AC electronic switch is the core component of the series compensation device, and the thyristor control strategy is the key technology of the thyristor AC electronic switch. To achieve zero-crossing triggering of the thyristor, it is necessary to rely on the synchronization of the main circuit voltage phase. However, in this device, once the thyristor is turned on, the synchronization signal will disappear. This is one of the issues that must be considered when designing the thyristor triggering circuit. In addition, when the thyristor is not turned on, the voltage it bears is in phase with the voltage of the power capacitor. After it is turned on, due to the change in circuit structure, a transition process will occur in the circuit. This is also one of the issues that must be considered when designing the thyristor triggering circuit. 5 Conclusion Due to the advantages of PLC, such as high integration, strong anti-interference ability, simple programming, and convenient system development, maintenance, and upgrades, coupled with the ability of the text display to clearly indicate the conduction and cutoff states of the thyristors and display various fault states, the thyristor AC electronic switch controlled by the PLC system has a simple structure, is easy to maintain, and has high reliability, maintainability, and flexibility.