Abstract: This paper briefly introduces two control implementation methods for high-voltage variable frequency speed control devices using a unit-series multilevel method. It proposes the definition of an intelligent power unit, introduces the functions implemented by the intelligent unit, and compares centralized control devices and intelligent unit devices. The paper concludes that high-voltage variable frequency speed control devices using intelligent units are more advanced and reliable. Keywords: High-voltage variable frequency, unit-series multilevel, intelligent unit 1 Introduction There are various high-voltage variable frequency methods, such as high-low-high, three-level, two-level (components directly in series), and unit-series multilevel. The unit-series multilevel method is the fastest developing and most widely used in China. Unless otherwise specified, all high-voltage variable frequency speed control devices discussed in this paper are of the unit-series multilevel method. The unit-series multilevel high-voltage variable frequency speed control device mainly consists of three parts: an input transformer, a power unit, and a main controller. The main function of the input transformer is to provide power to the power unit; the power unit mainly performs the rectification and inversion function from three-phase AC input to single-phase AC output; the main controller mainly coordinates the work of all units and provides external interfaces. From the perspective of control method implementation, there are two main approaches: centralized control and intelligent unit type. In centralized control type high-voltage variable frequency speed control devices, the SPWM wave of the power unit is generated by the main controller. The power unit receives the switching control signal from the inverter bridge (the inverter power element is usually IGBT) and the control signal from the bypass switch. The state variables formed by hardware measurement within the unit are encoded by an encoder and sent to the main controller. The main controller receives the codes from all units for logic control. Most high-voltage variable frequency speed control devices on the market currently use a multi-level series connection method for units, belonging to this centralized control type. In intelligent unit type high-voltage variable frequency speed control devices, the SPWM wave of the power unit is generated by the unit according to the frequency modulation command from the main controller, after the unit corrects itself based on its own state. The main controller receives the state signals from the units for logic control and coordinates the synchronous operation of all units. The MAXF series high-voltage variable frequency speed control devices developed and manufactured by Shanghai Keda Electromechanical Control Co., Ltd. belong to the intelligent unit type. 2. Definition of Intelligent Unit What kind of unit is an intelligent unit? Here we define an intelligent unit. An intelligent unit is a power unit that has its own independent controller, capable of independently performing real-time control functions such as measurement, protection, and SPWM waveform inversion, and can receive external commands to complete corresponding functions. The unit has its own independent controller, meaning it needs an internal CPU. However, simply having a CPU doesn't automatically make it an intelligent unit. This CPU needs to be able to perform measurement and control, meaning the unit can perform A/D conversion for analog measurement and implement software logic control and protection functions based on the measurement results. Simultaneously, the CPU needs to perform the unit's own inverter control function, meaning the unit can output a single-phase AC waveform as needed, undertaking the main function of frequency conversion control. To coordinate with other units, the unit needs interactive control capabilities, meaning it can receive commands and data, and also output its own status and data. In short, an intelligent unit is essentially a three-phase AC input, single-phase AC output low-voltage frequency converter with all major functions such as measurement, inversion, and interface. 3. The Purpose of Using Intelligent Units Why use intelligent units? (1) The intelligent unit is a decomposition of control functions, that is, the power unit undertakes the main function of inverter control, and the unit CPU directly controls the power element without intermediate links. Such real-time control is more direct and faster. (2) The power unit independently completes its own measurement, protection and inverter control. The control function is modular and independent, which reduces the mutual influence between module control and improves the reliability and independence of module control. (3) After adopting the intelligent unit, the control logic of the main controller is simplified, the CPU control resources are sufficient, the software programming is simplified, and the software reliability is improved. (4) With the intelligent unit, since the unit has its own CPU, it has rich I/O resources, can monitor more digital quantities, and can also perform analog quantity measurement, and can monitor more internal quantities of the unit. (5) With the intelligent unit, more functions can be realized, such as the unit switching function, the output waveform optimization function, the carrier frequency adjustment function, etc. (6) With the intelligent unit, it is convenient to expand and improve the product functions. 4 Functions realized by the intelligent unit Since the intelligent unit has its own CPU, it can easily realize various functions. (1) In terms of measurement, it can measure analog quantities such as DC side voltage, output voltage, output current, and power element temperature of the unit, and can monitor the input of digital quantities such as overvoltage and overcurrent of the hardware; (2) In terms of protection, it can conveniently implement software overvoltage, overcurrent, and overtemperature protection logic based on the data measured by analog quantities, and can also read the hardware protection status based on the input status of digital quantities; (3) In terms of control function, the unit can generate SPWM output waveform as needed, complete the refinement and optimization of the waveform, and automatically adjust the output voltage and output frequency according to the frequency modulation command; because the unit itself can perform inverter control, it can achieve a higher carrier frequency and effectively reduce the output current harmonics of the device; the use of intelligent units can realize arbitrary control of the unit output voltage, that is, the output voltage of each unit can be consistent or inconsistent as needed, which facilitates the automatic balance of the three-phase output of the entire device. (4) The use of intelligent units can realize the rapid bypass of units, that is, the rapid exit of units; this exit can be active exit or passive exit. Active exit refers to the active exit of individual units through instructions during the debugging process to verify the exit function of the unit, the automatic output balancing function of the device, the anti-disturbance function of the device, etc.; passive exit refers to the exit when the device malfunctions; the rapid exit of units can reduce the disturbance to the device and improve the reliability of the device. (5) The use of intelligent units can realize the active input of units in the exit state, that is, the unit that has exited operation can be added to the operation sequence when other units are running normally. With the structural design of the unit, such as the insulation design of the unit shell and the plug-in structure design of the input and output, the power unit can be maintained online, that is, the power unit that has malfunctioned can be replaced online without stopping the device, which greatly improves the continuous operation capability of the device and has strong applicability to applications with high reliability requirements. (6) In terms of communication, intelligent units can easily realize serial communication, can perform communication verification, and reduce communication errors; since the communication data does not participate in real-time control, the real-time requirement of the data is low, the data transmission volume can be relatively large, and the interface function between the unit and the outside world is realized in a more complete way. (7) The adoption of intelligent units can provide more measurement data and more real-time control methods, thus enabling more comprehensive status monitoring and more accurate and comprehensive fault location. 5 Comparison of centralized control type and intelligent unit type high voltage frequency converters Both centralized control type high voltage frequency converters and intelligent unit type high voltage frequency converters are high voltage frequency converters with unit series multi-level mode. The main circuits of the two are the same, both being voltage source type high voltage frequency converters with input transformers in the direct high-high mode. Since the control parts of the two are implemented in different ways, they have their own different characteristics. The following is a comparison of the two. (1) Control structure: The centralized control type is single processor control, that is, the main controller processor; the intelligent unit type is multi-processor control, each power unit has at least one processor, and the main controller has at least one processor. For a 6kV system, if 5 units are connected in series per phase, then at least 16 processors work simultaneously. (2) Hardware structure: The centralized control type has fewer unit functions and the hardware structure is relatively simple; the intelligent unit itself has a CPU control part and can implement more functions, so the hardware structure is relatively complex. (3) Communication rate requirements: In centralized control type devices, the main controller participates in real-time control, so the communication rate between the main controller and the unit must be very fast, otherwise the real-time control requirements cannot be met; in intelligent unit type devices, the main controller does not directly perform real-time control, so the communication rate requirement is lower. (4) Fault tolerance: For centralized control type devices, the communication data is used for direct real-time control, and communication data errors are not allowed, otherwise unpredictable results will occur. Therefore, the fault tolerance of its communication data is poor, and it is easy for the unit to exit or even the device to exit operation due to optical communication errors; for intelligent unit type devices, the communication data are instructions and unit status quantities, neither of which directly participate in real-time control. At the same time, the two parties in the communication (intelligent unit and main controller) can perform data verification and judge erroneous instructions and statuses, so it has strong fault tolerance. Because the communication rate of intelligent unit type devices is low, they can perform communication verification and error correction, and have strong fault tolerance. Therefore, the communication reliability between its main controller and the unit is very high, which is an important link to ensure the normal operation of the device. (5) Carrier frequency of the unit: Due to the limitation of communication rate, the carrier frequency of the unit in centralized control type devices is relatively low, generally 600Hz to 1000Hz; intelligent units, considering the comprehensive switching losses and harmonics, have a higher carrier frequency, usually around 2000Hz. The carrier frequency is an important factor affecting the harmonics of the output current. A higher carrier frequency results in smaller harmonics in the output current. (6) Online maintenance: For centralized control type devices, when individual power units are out of operation during device operation, it is impossible to put them back into operation. Even with structural measures, it is impossible to use backup units for online maintenance of the device; intelligent units can actively put units out of and into operation during device operation. Therefore, in conjunction with structural measures, online replacement of units can be performed, realizing online maintenance of the device. Online maintenance of the device can greatly improve the reliability of the device. (7) Functional Expansion: The control of centralized control devices is entirely completed by the main controller, and all units need to be controlled in real time. When expanding functions, the impact on the entire control structure is significant, and functional expansion is relatively difficult. For intelligent unit-type devices, only the units can perform direct real-time control, while the main controller performs logical function control. There is little coupling between them, and their mutual influence is minimal. Therefore, each unit can independently expand its functions, and independently debug and verify the expanded functions, making functional expansion relatively easy. 6 Conclusion The high-voltage variable frequency speed control device using intelligent units is a technologically advanced high-voltage variable frequency speed control device. The design and implementation of intelligent units completes comprehensive monitoring of the units, fundamentally solving the problem of communication reliability and enhancing the real-time performance of unit control. Intelligent units enable online replacement of units, improve the self-location function of unit faults, facilitate device maintenance, and improve the overall reliability and maintainability of the device. High-voltage variable frequency devices using intelligent units, with their advanced technology, reliable performance, and convenient maintenance, will be widely promoted and applied, making an important contribution to my country's energy conservation and emission reduction efforts.