Abstract: This paper focuses on the application of variable frequency coordinated control technology in the retrofitting of high-voltage variable frequency systems for primary air in power plant boilers. It emphasizes the design concept and system structure of variable frequency coordinated control technology, as well as the main problems and solutions addressed in the primary air system. This provides a new approach and method for the successful application of high-voltage variable frequency speed regulation technology in primary air systems. Keywords: Variable frequency coordinated control technology, primary air system, high-voltage variable frequency. I. Overview In coal-fired power plant units, primary air is the main power source for the boiler's fuel delivery system. The typical structure and principle of a direct-fired coal-fired boiler system are shown in Figure 1. The system mainly consists of four double-inlet, double-outlet ball mills, two primary air fans, and two air preheaters. The pulverized coal ground by the mills enters the furnace directly for combustion through the primary air duct. The system controls the boiler load by controlling the primary air volume. [align=center]Figure 1: Schematic diagram of a direct-fired coal-fired boiler system[/align] During normal operation, the primary air system maintains the primary air duct pressure within the range of 9.0–11.0 kPa by controlling the pressure through the fan inlet damper. The temperature of the primary air entering the coal mill is controlled by adjusting the opening of the cold and hot air dampers, ensuring the efficiency of the coal mill. The primary air volume is controlled by the mill inlet damper, thus adjusting the coal mill load according to changes in the boiler load. When the generator output power changes, the boiler's combustion system and fuel control system also change accordingly. To further reduce the plant's power consumption and achieve optimized system operation, frequency conversion modification of the primary air system has become a new research topic following the induced draft system and condensate system. Currently, the primary air system mainly faces the following problems: 1. To ensure the primary air velocity is within a certain range, it is currently controlled by the primary air fan inlet damper. The opening is between 40% and 60%, resulting in significant throttling losses. 2. In the fuel system, the primary air volume is controlled by the mill inlet damper opening, resulting in low system efficiency and poor economic indicators. 3. The opening and closing speed of the primary air fan inlet damper and outlet electric gate is slow, resulting in poor regulation quality. In the event of an emergency or a single primary air fan tripping, the RB (Rotation Controller) cannot respond effectively and promptly, potentially leading to boiler shutdown, fire extinguishing, and other accidents, causing significant economic losses. 4. Primary air fans typically exhibit "hump" characteristics and poor adjustment characteristics; improper pressure and airflow adjustments can significantly reduce fan efficiency, potentially causing direct overload protection tripping. With the increasing maturity of high-voltage variable frequency technology and the continuous practical application of new technologies and products, variable frequency energy-saving retrofitting of primary air fan systems, through variable frequency coordinated control technology, can solve the problems existing in variable frequency applications, achieving the goal of improving production processes and reducing equipment unit consumption. II. Primary Air Variable Frequency Coordinated Control Technology Through in-depth research on the primary air system and combined with the characteristics of high-voltage variable frequency speed regulation technology, this paper specifically studies the practical application and specific design implementation of high-voltage variable frequency coordinated control technology. Based on the main problems faced by variable frequency drives (VFDs) in primary air systems, the VFD coordination control unit has the following main functions: 1. Accurately and promptly and effectively locating faults during the automatic switching from VFD to power frequency operation of the primary air fan. 2. Ensuring uninterrupted operation of the primary air fan through coordination between VFD and power frequency operation modes. 3. Ensuring no pressure loss in the primary air by coordinating the VFD speed with the opening of the primary air regulating damper. 4. Ensuring that both primary air fans operate within their safe characteristic range and preventing "air grabbing" through coordinated control between the faulty primary air fan and the operating primary air fan on the other side. The control structure block diagram of this coordination control unit is shown in Figure 2. It mainly consists of more than ten modules, including: coordination control module, fault point analysis module, fault identification module, fault diagnosis and self-processing module, primary air fan system protection module, protection action connection module, damper opening function unit, analog I/O module, digital input module, and digital output module. [align=center]Figure 2: Control Structure Block Diagram[/align] Its working principle is as follows: The integrated protection device of the primary air fan automatic switching system for power frequency and variable frequency drives serves as the main detection method for both the variable frequency and power frequency circuits, receiving secondary detection signals from the inverter's upper and lower ports, as well as the inverter's bypass switch. By detecting the operating parameters and working status at different locations of the main power system, the fault analysis module analyzes and determines the true location of the fault point based on the sequence of actions, reaction speed, and amplitude changes of secondary current and voltage, combined with the inverter's own operating parameter detection information. The fault identification module determines the safety level and severity of the fault, while simultaneously indicating the specific fault location and cause. After receiving the specific location and safety level report from the fault analysis, the coordination control module, based on the operating status and conditions of the field equipment, decides whether to initiate a switchover operation from variable frequency to power frequency operation. If the primary air fan's main power system allows for automatic switching from variable frequency to line frequency operation, the system directly and rapidly accelerates the other side's variable frequency fan to 100% and calculates the closing timing of the line frequency switch on the tripped side fan based on the actual load. The damper opening function is used to calculate in real time the position signal for the primary air fan damper to automatically close after switching from variable frequency to line frequency, thus minimizing primary air pressure disturbance during the switching process. This ensures that during the switching operation, the instantaneous value of the boiler's primary air pressure fluctuation does not exceed the minimum requirement of the boiler combustion system for primary air velocity, and the time is less than 2 seconds, ensuring stable boiler operation, safety, and no flameout or shutdown during primary air fan switching. The digital input/output interface module mainly receives external remote control signals to implement interlocking protection, blocking logic, and control functions for the primary air fan's variable frequency upper and lower ports and bypass switches. Simultaneously, it transmits high-voltage switch and external control signals to the coordination control module for comprehensive information processing and judgment. The fault diagnosis and self-processing module mainly analyzes and judges the detection signals of peripherally connected switch quantities, analog quantities, and secondary instruments to determine whether the signal interface is normal, whether the signal input and output are valid, and whether there are error states. Based on real-time status information, it identifies the fault port number and removes it from the logic processing loop, maintaining the integrity of signal processing through signal substitution. This improves the safety and reliability of the system's logic processing. [align=center] Figure 3: External view of the frequency converter coordination control unit[/align] III. Problems and countermeasures after primary air frequency conversion speed regulation 1. The "wind grabbing" problem after primary air fan frequency conversion Research on the structure and operating characteristics of the primary air fan reveals that it has a distinct saddle-shaped feature. There is a saddle-shaped region on the left half of the fan performance curve. During operation in this region, abnormal conditions such as large flow fluctuations sometimes occur, resulting in a "surging" problem. Surging is only one of the phenomena that may be encountered in the unstable operating condition region. Abnormal zero aerodynamic conditions may also occur in this region, which is the rotational "stall" phenomenon. When a fan operates in an unstable condition, it may experience significant fluctuations in flow rate, total pressure, and current. The airflow may reciprocate, generating strong vibrations, a phenomenon commonly referred to as "air snatching." After the boiler primary air fan is converted to variable frequency speed control, the parallel operation of two fans makes "air snatching" highly likely, threatening the safety of the fans and the entire system. The following analysis explains the operating conditions of two fans, as shown in Figure 4. [align=center] Figure 4: Parallel Operation Diagram of Fans[/align] If the fan parameters are appropriately selected and the operation is correct, the duct performance curve IV and the combined performance curve III of the two fans in parallel operation intersect at point 1. In this case, each fan will operate at point 1′, and the fan operation is stable, without "air snatching." If the fans operate improperly, the duct performance curve V and the combined performance curve III intersect at points 2 and 3, falling within the ∞-shaped area. In this case, the fan's operating point may be either point 2 or point 3. If the fan operates at point 2, both fans can still operate stably at point 2'. If the duct resistance of the two fans differs slightly, or the airflow in the duct system changes slightly, the result is that the fans operate in parallel at point 3, with their operating points at 3' and 3'' respectively. The fan operating at point 3' has a larger airflow and operates in the stable region, while the fan operating at point 3' has a smaller airflow and operates in the unstable region. Thus, the two fans with identical performance deliver different flow rates, resulting in "air competition." However, the operating conditions of the two fans at points 3′ and 3″ are not stable; their operating points may interchange. Under these conditions, in severe cases, one fan may have a large air volume while the other experiences backflow. Therefore, to avoid air competition when the two fans are running in parallel, measures should be taken to prevent their operating points from falling within the ∞-shaped area. After the boiler primary air fan underwent frequency conversion modification, the operating point of the fan during low-load operation was closer to the unstable region (left boundary), causing "air competition" between the two primary air fans during low-load operation, i.e., difficulty in parallel operation. By using a rapid coordination and balancing system for the two primary air fans, adjusting operating parameters, reducing system primary air pressure, and changing system ventilation volume, the "air competition" problem was solved. 2. Anti-surge control concept [align=center] Figure 5: Characteristic curves at different speeds[/align] Figure 5 shows the characteristic curves of the blower at different speeds. It can be seen that different speeds correspond to different hump points and hump flow rates. The lower the speed, the more the hump point shifts to the left, and the smaller the hump flow rate. Connecting the hump points at different speeds forms a curve; the right side of the curve represents the stable operating region, and the left side represents the unstable region. We call the hump flow rate the limiting flow rate, and the curve connecting the corresponding hump points the surge and wind-stealing limit line. Clearly, as long as the blower flow rate can be controlled to exceed the limiting flow rate at any speed, the blower will not experience wind-stealing problems. This is the basic idea behind anti-surge and wind-stealing control. Considering that the state of the intake gas, such as pressure, temperature, density, and changes in system airflow and pressure, will cause changes in the blower characteristic curve, a certain safety capacity should be considered to ensure that the actual operating point does not get too close to the unstable region limit, thus avoiding wind-stealing and surge accidents. The "speed regulation-proportional valve control method" is more suitable for the safety and energy-saving needs of power plants in the primary air system. The variable frequency drive (VFD) coordination control unit coordinates VFD energy saving and anti-surge control. Based on the requirements of the primary air system, it maintains the outlet pressure within a certain range when the blower flow fluctuates. Therefore, the outlet pressure P1 is taken and sent to the VFD energy saving and anti-surge controller. A closed-loop control system is formed by the pressure transmitter, coordination controller, high-voltage VFD, motor, and blower mechanism. This system continuously participates in the automatic adjustment of the blower speed to achieve stable outlet pressure. [align=center] Figure 6: Typical Safe Operation Curves[/align] Figure 6 shows two typical safe operation curves. Safe operation curve 1 is a fixed flow safe operation curve control. Safe operation curve 2 is a curve similar to the surge limit curve, but its flow rate is 5%–15% higher than the surge limit flow rate. This solves the energy consumption problem of safe operation curve 1 at lower speeds and is the most energy-efficient and safe control method. 3. When the primary air fan RB is running, the overload protection of the primary air fan inverter will be activated to prevent the primary air system from running continuously. If the primary air fan inverter on one side fails and cannot run continuously, the RB function of the unit will be triggered. If the system is not handled properly or reacts in time, it will eventually cause the unit to trip. Based on the analysis of the boiler primary air fan RB, the main reasons that will cause the primary air fan inverter to trip are as follows: 3.1 In the initial stage of the primary air fan RB operation, the system ventilation volume is too large. Under single-point pressure, the flow exceeds the standard, causing the inverter to overload. 3.2 In the initial stage of the primary air fan RB operation, the fan's operating condition is seriously deviated from the high efficiency point, and the operating efficiency is extremely low. 3.3 The primary air fan performance curve is steep, and the camel-hump characteristic is obvious, resulting in low efficiency. The measures to prevent the primary air fan inverter from tripping are as follows: (1) Provide a load limiting function in the design process of the primary air inverter to prevent the inverter from tripping due to overload protection. (2) Optimize the primary air system logic during RB. IV. Conclusion The research on the application of variable frequency coordinated control technology in the variable frequency retrofit of boiler primary air systems fully demonstrates that: in the process of energy-saving retrofit using high-voltage variable frequency technology, focusing on studying and solving the problems and solutions brought about by the application of high-voltage variable frequency technology is of greater significance for improving the safety and stability of system operation and reducing economic losses. Applying variable frequency coordinated control technology to various fields can significantly improve the safety and stability benefits of production systems brought about by variable frequency retrofits, and can further achieve the goal of optimizing the system and improving energy-saving effects. The research on this technology will inevitably play a positive role in promoting the widespread application of high-voltage variable frequency technology.