Abstract : The coal conveying system of the first phase of Suizhong Power Generation Co., Ltd. is equipped with two Russian-made ÜZR-1200/1500 bucket wheel stacker-reclaimers. In recent years, based on the operating conditions, the programmable control system of these two bucket wheel stacker-reclaimers has been upgraded, and the starting method of the main motors has been optimized. This paper focuses on introducing the background, upgrade plan, system composition and functions after the upgrade, and analyzes and summarizes the problems encountered during the upgrade process, and proposes solutions. Keywords : Bucket wheel stacker-reclaimer, PLC, soft start, frequency converter, wireless communication 1 Introduction The coal storage yard of Suizhong Power Generation Co., Ltd. is a strip-shaped coal yard, using bucket wheel stacker-reclaimers for coal stacking (coal from railway or port) and coal reclaiming operations. The first phase of the project is equipped with two Russian-made ÜZR-1200/1500 bucket wheel stacker-reclaimers, namely: stacking output of 1200 tons/hour and reclaiming output of 1500 tons/hour. The power supply for the bucket wheel stacker-reclaimer is achieved by guiding a 6KV cable to the tail platform of the main trolley via a cable reel. Power is then supplied to the motors of each part through a power transformer (6KV/0.4KV), and to the control system and lighting system through a control transformer (6KV/0.23KV). Multi-core flexible cables are used to achieve material stacking interlock between the bucket wheel stacker-reclaimer and the coal yard conveyor belt through the control cable reel, and to send stacking and reclaiming status signals to the central control room. The electrical control system of the bucket wheel excavator mainly consists of six aspects: control of the cantilever belt reducer drive motor (squirrel-cage asynchronous motor, 75KW), control of the bucket wheel reducer drive motor (squirrel-cage asynchronous motor, 75KW), control of the cantilever pitching oil pump motor (squirrel-cage asynchronous motor, 15KW) and hydraulic station solenoid valve, control of the slewing mechanism reducer drive motor (squirrel-cage asynchronous motor, 2×11KW) and brake, control of the traveling mechanism drive motor (squirrel-cage asynchronous motor, 4×11KW) and brake, control of the power cable reel drive motor (wound-rotor asynchronous motor, 11KW) and brake, and control of the control cable reel drive motor (wound-rotor asynchronous motor, 11KW) and brake. After a period of operation, the original electrical control system encountered many problems: (1) Due to the needs of the bucket wheel excavator process, its electrical control system consists of 6 control cabinets and more than 270 relays, resulting in many fault points, long processing time for defects, and a huge amount of maintenance work. (2) There are a large number of cables directly connecting the control room and the power distribution room. Because the bucket wheel boom needs to be frequently pitched and rotated, these cables in the cable room of the rotating part in the middle of the bucket wheel need to frequently reciprocate. The cable damage cycle is short and the number of replacements is large. (3) The components of the starting device of the cantilever belt and the bucket wheel motor are aging and have poor anti-interference ability. Some basic protections of the motor cannot work properly and are inconvenient to adjust. This results in a large current when the motor starts, which impacts the mechanical equipment, especially the bucket wheel that directly shovels coal. (4) When the power cable and control cable drum is winding the cable, the method of adjusting the speed is adopted by the rotor resistance of the wound motor. When the cable is unwinding, the rotor circuit of the wound motor is opened, and the cable directly pulls the drum. This tension is very damaging to the cable. At the same time, the rotation and travel of the cable drum are often out of sync, which can easily cause the cable to be pulled off, resulting in grounding and short circuit accidents, affecting the power supply and the interlocking of the bucket wheel stacker-reclaimer and the coal yard belt. During the three years of formal commercial operation of the unit, the power cables were replaced three times and the control cables twice. Finally, during the final stage of operation, a dedicated operator was assigned to monitor the cables, which not only endangered the personal safety of the operator but also increased labor costs, resulting in direct economic losses. The first phase of the Suining Power Plant has an installed capacity of 1600MW, consuming approximately 15,000 tons of coal per day at full load, with a minimum coal storage warning line of 130,000 tons in the coal yard. Frequent failures of the bucket wheel stacker-reclaimer resulted in prolonged periods without backup for critical equipment in the coal conveying system, leading to high maintenance costs. The current state of its electrical control system could no longer meet the needs of daily production. Therefore, to meet the high-load operation requirements of the Suining power plant and improve economic efficiency, it is necessary to modernize the bucket wheel stacker-reclaimer electrical control system and the starting methods of the main motors. 2. Determination of the Modification Scheme Due to the complexity of the controlled object, the special nature of the operating environment, and the long-term continuous operation, adhering to the principle of the highest performance-price ratio, and after investigation and demonstration, we selected the SIEMENS S7-300 programmable logic controller (PLC) with a field-distributed I/O structure as the core equipment of the bucket wheel excavator's electrical control system; the original HITACHI J300 frequency converter for the slewing and traveling mechanisms was retained, and the traveling control method was optimized; the cantilever belt and bucket wheel motor were modified to use the AB 150-A180NBDB-8L4 soft starter from the United States, and a bypass system was added to improve reliability; the control cable reel was eliminated and replaced with a point-to-point wireless digital communication system; in order to reduce mechanical costs, the original power reel mechanical structure was retained, but a torque control method using a general-purpose frequency converter was adopted to ensure the smooth and reliable operation of the power cable reel. 3. Composition of the Control System (1) Hardware Configuration of PLC Control System Considering process requirements, number of I/O points, scanning speed, and self-diagnostic functions, SIEMENS S7-300 PLC was selected, with CPU model 315-2DP. A main control station was designed in the power distribution room, mainly to collect input signals from equipment under the vehicle (such as relay auxiliary contacts and sensors) and issue commands to control the actuators (intermediate relays and indicator lights) in the power distribution room; a remote cabinet was designed in the control room, containing two remote I/O substations of the PLC system, mainly to collect input signals from equipment on the vehicle (such as buttons and sensors) and issue commands, etc. A shielded twisted pair cable was used between the master station and the slave station, and communication was carried out using the PROFIBUS bus structure (see Figure 1). In this way, apart from some necessary wiring, the number of cables used for the connection between the vehicle and the under vehicle was less than 2/3 of the original, which greatly reduced the maintenance workload and improved the reliability of the equipment. The S7300-PLC program was compiled using the STEP 7 configuration software provided by SIEMENS, and LD ladder diagram programming was adopted, which is convenient for electrical maintenance personnel to understand and learn. Furthermore, a programmer communication interface is reserved on the CPU. By connecting the programmer via a programming cable, fault diagnosis can be performed quickly, greatly accelerating the defect handling speed. At the same time, when the process changes, the program can be changed quickly without having to spend a lot of effort to modify the electrical control circuit. (2) Wireless Digital Radio Wireless digital radio is a telemetry wireless device used for applications that require real-time data processing. It uses microprocessor control and digital signal processing (DSP) technology, and can provide reliable service even under very harsh conditions. Considering the environmental conditions of the coal conveying system and the actual situation that coal conveying operators use a lot of walkie-talkies, we selected the reliable and maintenance-free American MDS digital radio. Each bucket wheel excavator requires a point-to-point wireless communication system. Each system consists of two radios: one connected to the PLC installed in the bucket wheel excavator's power distribution room as a wireless communication slave station, and the other connected to a remote station PLC as a wireless communication master station (see Figure 2). This slave station also exchanges data with the substation PLC of the coal conveying control system, completing the interlocking between the bucket wheel excavator and the coal yard conveyor belt, and displaying the bucket wheel excavator's working status on the operator station screen in the central control room. The MDS digital radio is reliable, ensuring the stability of the interlocking signals and enhancing the central control room's monitoring of the coal yard. Simultaneously, the eliminated motors and brakes can be reused as dedicated spare parts for the drive section of the power cable reel. The use of the wireless digital communication system ensures the stability of the interlocking signals, reduces the workload of operators and maintenance personnel, and saves significant costs associated with replacing control cables, resulting in substantial economic benefits. 4 Optimization of motor starting methods (1) Soft starting of cantilever belt and bucket wheel motor The cantilever belt and bucket wheel motor are installed on the metal frame of the bucket wheel machine. When the cantilever belt and bucket wheel motor start, a large starting current will appear, resulting in a strong mechanical shock. However, by using a soft starter, the starting time, starting voltage and other parameters can be adjusted, so that the output voltage can rise in a certain way, so that the voltage of the controlled motor gradually and smoothly rises to the full voltage, and the starting current rises smoothly to the set value, thereby meeting the starting torque requirements and ensuring successful starting. According to the nature of the load of the cantilever belt and bucket wheel: the static load is relatively large, and once started, the resistance torque of the motor will decrease. For this load nature, a short-term high voltage (UK, the value and time of which can be set) is added in actual starting. This soft start method is usually called voltage jump start method (see Figure 3). Using a soft starter, the starting characteristic curve is good, so that the motor and its auxiliary mechanism are not impacted, accelerate smoothly, and the power supply line and the entire transmission system bear the minimum possible impact. In order to improve the reliability of the equipment, a bypass system consisting of AC contactors was added to the soft starter of the two devices. After the motor starts normally, the operation of the soft starter is cut off and the bypass contactor continues to keep the motor running. The bypass device can also be used to directly start the two devices for a certain period of time when the soft starter needs maintenance, which improves the reliability of the equipment. (2) Optimization of frequency conversion control for walking and turning The original design of the frequency converter for walking and turning was that the speed could be adjusted in the range of 0-50Hz. Through actual observation, it was found that frequent adjustment of walking speed is not very meaningful in the coal conveying operation mode of our company. Moreover, frequent and large-range adjustment of walking speed makes the synchronization between the cable drum and walking unpredictable and easy to break the cable. If the brake of the walking motor is adjusted too tightly, it will cause the vibration of the entire machine body at the moment of walking stop. Therefore, according to the operating conditions and actual needs, the speed range of the frequency converter is narrowed to 20-30Hz. For the same reason, to prevent excessive rotation speed from causing the cantilever to collide with the mechanical stop, and to prevent excessive impact between the bucket wheel and the coal pile during material handling, thus overloading the bucket wheel motor, the rotation speed is smoothly adjusted within the range of 0-20Hz according to actual operating conditions. After the actual motor speed is significantly reduced, the coaxial cooling fan's heat dissipation capacity is not as good as before. However, since the equipment is installed outdoors with good natural ventilation, and it does not require continuous long-term operation, the motor's temperature rise remains within regulations. Therefore, the original motor can still be used under low-speed conditions without needing to be replaced with a dedicated variable frequency motor, saving some costs. (3) Torque control of the power cable reel motor by frequency converter The power cable reel motor is a wound-rotor motor. In this renovation, in order to save money, the rotor part of the motor was short-circuited, making it equivalent to a squirrel-cage motor. An LG IV5 frequency converter with a vector encoder was used for closed-loop control to ensure that the power cable reel maintains a constant torque during speed adjustment, thereby improving the transmission quality of the motor. Vector control means that the magnetic field and torque do not interfere with each other and the torque is controlled according to the command. Using an encoder, the magnetic field current and torque current can be controlled independently, thus achieving smooth movement at extremely low speeds and high torque and high precision speed and torque control. At the same time, in order to protect the 6KV power cable, a motor shaft rotation detection sensor was installed at the coupling of the reel motor. If the signal of the cable reel drive motor shaft rotation is not detected after the trolley has been traveling for 3 seconds, the trolley will stop traveling and a warning signal will be issued in the control room. With the addition of cable tension protection at the cable counterweight, a comprehensive protection system for the power cable is formed to ensure the normal winding of the power cable. 5. Summary of Modification (1) The #2 bucket wheel excavator was modified before the #1 bucket wheel excavator. After the control cable was modified to wireless communication, it worked normally. However, when the wireless communication system of the #1 bucket wheel excavator was modified and put into trial operation, the #2 bucket wheel excavator experienced abnormal communication and unstable interlocking signals. Through analysis and professional software diagnosis, it was found that the communication parameters of the radios of the two bucket wheel excavators were set in the same way, causing mutual interference. After resetting the communication parameters of the #1 bucket wheel excavator, the problem was solved. (2) When the power cable reel frequency converter was modified, in order to save costs, the old motor was used. It was necessary to add a bracket and install an encoder at the end of the motor rotor shaft. The on-site construction method was to use a pistol drill to drill a blind hole at the end of the rotor shaft. As a result, due to the processing technology, the coaxiality between the encoder and the motor shaft was higher than the standard value, which caused the pulse signal emitted by the encoder to be abnormal, causing the power cable reel frequency converter to lose rotation and stop, and displaying the "encoder fault" prompt message. The solution was to replace it with a high-quality encoder-specific coupling to improve the coaxiality as much as possible. Such problems caused by the construction process should be paid more attention to in similar modifications in the future. (3) Shortcomings: In the long run, a touch screen monitoring system should be installed on the remote cabinet door of the control room. It is mainly used for fault diagnosis, guiding operators in abnormal operation; at the same time, it manages the life of the bucket wheel stacker-reclaimer equipment, records the cumulative operating time of the main motors, the number of actuations of the main contactors, and even recalls the historical current of the main motors. These processes are easy to implement in terms of programming. With these designs, fault diagnosis of the main equipment can be strengthened, the speed of defect handling can be improved, and it has great guiding significance for the point inspection and maintenance of important equipment in the coal conveying system. 6 Conclusion Since the modification and commissioning of the two bucket wheel stacker-reclaimers of Suidian Company, the electrical control system has performed relatively stably, meeting the coal receiving and unloading tasks of the coal conveying system, adapting to the coal combustion requirements of Suidian's dual-machine high-load operation, ensuring normal production, providing convenience for operators, and reducing the workload of maintenance personnel. It is an effective application of modern control technology in the coal conveying system of a power plant. References : 1. "Application and Maintenance of Frequency Converters" by Feng Duosheng, South China University of Technology Press 2. S7-300 PLC User Manual, Siemens 3. AB Soft Starter User Manual, AB Company