In recent years, driven by the urgent need for energy conservation and the ever-increasing demands for product quality, high-voltage variable frequency speed control technology for large-capacity motors has been widely applied, basically covering major industries such as power, metallurgy, petroleum, chemical, and papermaking in China. Therefore, research on variable frequency speed control systems in China is very active. The main challenge is achieving high-voltage variable frequency speed control using power switching devices with limited voltage withstand capability. The solution is to connect low-voltage switching devices in a specific topology, using low voltage in series to form high voltage. Traditional control systems employ centralized control, which is complex to install and debug, has relatively limited functionality, and is difficult to expand, making it hard to achieve intelligent control. Distributed control systems, on the other hand, have a simple structure, flexible data processing methods, strong scalability, and their modular structure provides strong fault tolerance, making them the development trend for intelligent control of high-voltage variable frequency systems. Addressing the shortcomings of overly complex traditional control systems, this paper proposes a distributed control strategy for high-voltage variable frequency systems, employing a distributed control structure of "central control unit - bus - distributed control unit". 1 Principle of Distributed Control The high-voltage frequency converter designed requires the phase voltage to change within the range of 0 to 4320V. The system uses low-voltage power devices and adopts the voltage series superposition method to achieve high voltage [1]. The implementation principle of its high-voltage frequency converter system is shown in Figure 1. In the figure, there are 24 distributed units, each of which is composed of the same control and drive system. The control unit uses PWM control to adjust the output voltage of the drive unit to change between 0 and 540V. The 24 units are divided into three phases, and each phase is connected in series with 8 units, and the phase voltage changes within the range of 0 to 4320V. The three groups of power units are connected in star to form a distributed control topology, and high voltage output is achieved with low-voltage power devices. Each group superimposes a phase voltage waveform for motor drive, and the phase difference between the phase voltages is 120°. In this way, the line voltage can be controlled between 0 and 7500V to meet the control requirements of high-voltage motors. The distributed unit drive circuit is shown in Figure 2, which consists of a three-phase bridge rectifier circuit and a square wave inverter. The output voltages uao and ubo of the two square wave inverters are pulse-width adjustable square waves, while the voltage uab between the midpoints a and b of the two bridge arms is the superposition of the uao and ubo square wave voltages, i.e., uab = uao - ubo. Assuming the phase angle difference between uao and ubo is 180° + Φ, adjusting the Φ angle can adjust the pulse width of the output voltage, thus changing the amplitude of the fundamental and harmonic components of the output voltage. This improves the waveform of the output voltage and achieves the purpose of regulating the output voltage. 2. Design of the Distributed Control System The structure of the high-voltage frequency converter control system designed in this paper is shown in Figure 3. The distributed system consists of three parts: a central control unit, a CAN bus fiber optic communication section, and a distributed unit controller. The central control unit establishes communication with the distributed unit controller via the CAN bus, establishes task division, and coordinates the operation of the control system. Replacing the traditional centralized control system with a distributed control system solves the problems of centralized control in terms of local data acquisition, processing, and independent control, reduces the burden on the central processing unit, expands system functions, realizes remote intelligent monitoring and control of high-voltage frequency converters, improves system performance, and is more suitable for industrial applications. Since the CAN bus uses optical fiber as the communication medium, the central control unit and distributed units can be assembled by connecting two optical fibers and a HUB. Because the distributed units have identical structures, the system's replaceability and scalability can be enhanced by using hardware ID software identification. 2.1 Central Control Unit: The central control unit, based on an ARM-based embedded control system, mainly includes peripheral functional modules such as I/O modules, A/D data acquisition modules, LCD display modules, GPRS remote communication modules, and CAN bus communication modules, as shown in Figure 4. The central control unit, designed using the 32-bit computing power of ARM and employing the functional division of a distributed system, performs the following tasks: power control of the high-voltage frequency converter system, human-machine interaction operation, GPRS remote communication for remote monitoring and parameter setting of the frequency converter, remote operation control, motor speed regulation, and control of the speed regulation system's operating status. Among them, motor speed control is the core of the system. Maintaining real-time synchronization of distributed units and the reasonable allocation of real-time calculation tasks for frequency conversion control algorithms are key links in realizing speed control. This paper ensures the synchronization of distributed units through real-time correction of the central unit and the reliability of the bus protocol. The principle of task allocation is that the central control unit processes the overall information about frequency conversion speed control in the human-machine interaction information and transmits this information to each distributed unit, which then completes the PWM algorithm to achieve voltage superposition and motor control. The implementation of task allocation is based on a stable and efficient bus protocol. 2.2 Distributed Control Unit The distributed control unit is designed based on the TI DSP chip TMS320F2407, making full use of the chip's rich peripheral modules. The CAN bus module realizes communication with the central control unit, receives and feeds back control information; the A/D conversion and digital input/output ports are used in conjunction to realize the monitoring and processing of protection signals such as bridge arm voltage and module overcurrent; the event module is the core of distributed unit control. Based on the voltage frequency, real-time synchronization information and error correction information sent by the central control unit, it calculates the period and duty cycle of the PWM voltage output of the current control unit using the PWM voltage series processing method. Figure 5 shows the superimposed waveform of the PWM voltage outputs from the eight distributed units in each phase. By adjusting the PWM output waveform of the distributed units, the superimposed waveform can be made to approximate a perfect sine wave, enabling the frequency converter system to achieve harmonic-free control. 2.3 Bus Protocol Design The central control unit of the system communicates with each distributed control unit via a CAN bus, which is the hub of the distributed system. Due to the complex environment of the industrial site and the presence of strong electromagnetic interference, this system designs an optical fiber hub. The differential signal of the CAN bus is transmitted via optical fiber, further enhancing the anti-interference capability of the CAN bus and ensuring the stability and reliability of system communication. To meet the strong real-time requirements of the distributed system, this paper designs a dynamic priority allocation mechanism and a time-division transmission mechanism. The priority of each node on the CAN bus is determined by its identifier; the smaller the identifier value, the higher the priority. Utilizing this characteristic of the CAN bus, a dynamic priority allocation mechanism is designed. The high 5 bits of the identifier are used as priority allocation bits and are masked during data transmission, not used as a receiving flag; the low 6 bits are used as receiving flag bits, set according to the unit ID. Dynamic priority allocation method: After system startup, the priority of all nodes is set to the intermediate value of 11100. Dynamic allocation begins after CAN bus transmission. When a unit successfully transmits data, its priority is lowered; when a unit's data transmission is arbitrated by the bus and becomes pending, its priority is raised to continue transmission. Although dynamic priority allocation solves the problem of equal data transmission among units, the system has many nodes, and using CAN's own arbitration mechanism still results in a relatively long transmission wait time. A time-division multiplexing mechanism is used between distributed units, spreading data transmission along the time axis within the control range, thus reducing bus conflicts between nodes and improving communication stability. The protocol format defined by the 8-byte data field of CAN transmission is shown in Table 1. In the table, the instruction code indicates the content of the instruction, the source address is the identifier of the transmitting unit, the length indicates the valid data, and the receiving unit processes the data according to the length. Instructions are divided into several categories, including network control, operating status setting, synchronization monitoring and calibration, alarm monitoring, and fault handling. The optimized control mechanism and application layer protocol of the CAN bus transmit control information through a stable bus network, and the stability and real-time performance of communication have been experimentally verified. 3. Experimental Verification Due to the special nature of its application environment, high-voltage frequency conversion systems cannot be directly tested for stability and reliability on-site. Based on the control principle of high-voltage frequency conversion systems, this paper designs a test system simulating on-site conditions. The test system replaces the 0-540V voltage of the power unit with the control unit voltage of 0-24V under load. The test system drives a 380V/120W three-phase AC motor, verifying the system's control accuracy, stability, and reliability. To acquire the superimposed waveform of the units, a 50kΩ/150kΩ resistor was used to divide the voltage at the voltage output terminal of the test system. The measured superimposed waveform of the phase voltage is shown in Figure 6(a). The output voltage of the distributed unit is superimposed according to the theoretical design requirements, fully conforming to the control voltage waveform required by the control model. Moreover, the synchronous operation of the eight control units in the distributed embedded control system is normal. The superimposed waveform of the line voltage shown in Figure 6(b) is the vector sum of the two-phase phase voltages. During system operation, the frequency and peak value of the changing output voltage and the line voltage waveform remain normal, indicating that the phase difference control algorithm between the phases of the distributed control is correct. The system operated stably and normally for an extended period (one week) in an experimental environment, with all functions, including frequency conversion speed regulation, remaining stable. Both the system and the high-voltage drive section designed in this paper use low-voltage signal interfaces. The test results of the test platform can basically reflect the system's industrial operation. The test experiments demonstrate the feasibility of the embedded distributed control strategy designed in this paper for practical application. This paper proposes a new control strategy for high-voltage frequency conversion and successfully designs a stable high-voltage frequency conversion distributed control system test platform. This platform realizes the control operation of the motor using a voltage series topology. The system was tested in a simulated experimental environment, verifying the correctness of the distributed strategy, communication protocol, and control algorithm, and it operated well. Compared with traditional control systems, distributed control systems have advantages such as simple control structure, strong scalability, and reconfigurability. Furthermore, they possess strong computing power, leaving considerable room for optimization of control algorithms and improvement of control performance. The high-voltage frequency conversion distributed control system designed in this paper is suitable for the control of high-voltage motors in industries such as power, oil fields, and papermaking. References [1] Wu Zhongzhi, Wu Jialin. Application Manual of Medium (High) Voltage High Power Frequency Converter. Beijing: Machinery Industry Press, 2003. [2] Wu Kuanming. CAN Bus Principle and Application System Design. Beijing: Aeronautics and Astronautics University Press, 1996. [3] TMS320LF2407, TMS320LF2406, TMS320LF2402 DSP Controllers Datasheet, Texas Instruments Incorporated. 2004. [4] S3C44B0X RISC MICROPROCESSOR, Preliminary Product Overview. SAMSUNG'S3C44B0X Datasheet. 2003.