[Abstract] This paper analyzes the electrical control system of the Angang Medium Plate Mill, focusing on the fault analysis and handling of the Cycloconverter system during abnormal shutdown. [ Keywords ] SIMADYN D; AC-AC frequency converter; fault handling The AC-AC frequency converter consists of three three-phase bridge converters with natural grid commutation and reversibility, corresponding to the three phases A, B, and C of the synchronous motor. The three-phase AC-AC frequency converter adopts a logic-free circulating current and a three-phase neutral point configuration. The motor's three phases are star-connected, and the motor's star point and the frequency converter's star point are independent. The stator circuit of the drive consists of a high-voltage circuit breaker HVS, a rectifier transformer T, a three-phase AC-AC frequency converter Cyc, an output-side contactor MCB, and an AC synchronous motor SM. The rotor (excitation) circuit consists of a circuit breaker HVR, an incoming line reactor L, and an excitation rectifier Con. A simplified system diagram is shown in Figure 1. [align=center] Figure 1: Simplified System Diagram[/align] The advantages of this scheme are: it allows the use of AC bias technology, which reduces the secondary voltage of the rectifier transformer, increases the thyristor voltage safety factor, reduces the frequency converter capacity, and eliminates third harmonics within the motor. 1.2 Control System Composition: The drive control level is based on SIMADYN D. The control system is controlled by a fully digital multiprocessor SIMADYN D, consisting of five processors, a power supply, a frame, and peripheral interfaces. It performs speed control of the two motors on the upper and lower rolls of the finishing mill, including vector control of the synchronous motor, motor start/stop control, and motor protection. To enable SIMADYN D to achieve local data acquisition, fault display, and diagnosis, a local communication network was established. A unified data communication network is used to achieve integrated control, organically connecting the two SIMADYN D devices and the host computer to achieve advanced control and centralized management. A Win CC system is configured for screen display, report generation, historical data storage, retrieval, and fault diagnosis functions. The system configuration is shown in Figure 2: [align=center] Figure 2: Simplified Diagram of Control System[/align] 1.3 Control Software Function The system adopts the STRUC G graphical editable software package. This software is a UNIX graphical editing software, a table-based editing software STRUC L, and an online testing software IBS G, service software, etc. The user assembles and edits the function blocks of the function block diagram according to the system requirements to form the engineering software. The system software runs on the processor template and includes the operating system, function blocks, monitoring program, data transmission and fault diagnosis programs. The functions implemented by the system are: speed control, vector control, AC-AC frequency conversion current control, logic control, protection control, mechanical torsional vibration prevention control, and fault diagnosis. 2. Overview of S7-400 PLC This PLC is mainly the control center for peripheral signal detection, interlocking logic control, speed setting and enable generation, and is also the automation control center of the roughing mill area. Data exchange between S7-400 PLC and SIMADYN D is achieved through DP bus network communication, using PPO4 data type, with eight communication words for sending and receiving. The meanings of the communication words are as follows: 2.1 PLC Þ SIMADYN D Information Control Word: The SD cabinet receives 8 words from the PLC: 1. Lower Roller Speed ​​Assignment: Bits 0 to 14 are the speed values, and the corresponding maximum value percentage is the maximum motor speed of 120 RPM. Bit 15 is a positive/negative bit, representing the forward and reverse rotation of the motor. 2. Upper Roller Speed ​​Assignment: Bits 0 to 14 are the speed values, and the corresponding maximum value percentage is the maximum motor speed of 120 RPM. Bit 15 is a positive/negative bit, representing the forward and reverse rotation of the motor. 3. Lower Roller Control Word: Bit 0 is the contactor MCB on/off command signal. Bit 1 is the spindle positioning start signal. Bit 2 is the reference value valid signal. Bit 3 is the sled characteristic activation signal. Bit 4 is the emergency stop signal. Bit 5 is the steel clamp reset signal. 4. Upper Roller Control Word: Bit 0 is the contactor MCB on/off command signal. Bit 1 is the spindle positioning start signal. Bit 2 is the reference value valid signal. Bit 3 is the sled characteristic activation signal. Bit 4 is the emergency stop signal. Bit 5 is the steel clamp reset signal. 5. Spare word 6. Spare word 7. Spare word 8. Spare word 2.2 SIMADYN D Þ PLC Information Status Word: SD sends 8 words to the PLC 1. Upper roller speed actual value: Bits 0 to 14 are the speed value, and the corresponding maximum value percentage is the maximum motor speed of 120RPM. Bit 15 is the positive/negative bit, representing the forward and reverse rotation of the motor. 2. Lower roller speed actual value: Bits 0 to 14 are the speed value, and the corresponding maximum value percentage is the maximum motor speed of 120RPM. Bit 15 is the positive/negative bit, representing the forward and reverse rotation of the motor. 3. Upper roller current actual value: Bits 0 to 14 are the current value, and the corresponding maximum value percentage is the motor current torque component value. Bit 15 is the positive/negative bit, representing the motor output direction when the motor is forward and reverse. 4. Lower roller current actual value: Bits 0 to 14 are the current value, and the corresponding maximum value percentage is the motor current torque component value. Bit 15 is a positive/negative bit, representing the direction of motor output when the motor rotates forward and backward. Bit 0 of the upper roller status word is the transmission system ready signal, "1" is valid. Bit 1 is the zero speed signal, "1" is valid. Bit 2 is the contactor open/closed enable signal, "1" is valid. Bit 3 is the contactor closed/closed position signal, "1" is valid. Bit 4 is the spindle positioning complete signal, "1" is valid. Bit 5 is the alarm signal, "1" is valid. Bit 6 is the fault signal, "1" is valid. Bit 8 is the oil station fault signal, "1" is valid. Bit 9 is the oil station alarm signal, "1" is valid. Bit 0 of the lower roller status word is the transmission system ready signal, "1" is valid. Bit 1 is the zero speed signal, "1" is valid. Bit 2 is the contactor open/closed enable signal, "1" is valid. Bit 3 is the contactor closed/closed position signal, "1" is valid. Bit 4 is the spindle positioning complete signal, "1" is valid. Bit 5 is the alarm signal, "1" is valid. Bit 6 is the fault signal, "1" is valid. Bit 8 is the gas station fault signal, "1" is valid. Bit 9 is the gas station alarm signal, "1" is valid. 7. Spare word, 8. Spare word 3 Common Fault Analysis 3.1 Incoming line high voltage circuit breaker (HVS/HVR) cannot be closed. Under normal circumstances, the corresponding switch connection interlock conditions SUC1, SUC2, and SUC3 in the SIMADYN-D 4#CPU function package must all be zero. If the conditions are not met, you can check the specific switch connection/disconnection interlock signals and trip signals according to them. Or find the cause of the fault according to the fault information displayed on the HMI screen. When operating the switch, all control voltages of the drive system must be normal. The sequence of operating the switch must be correct, that is, first connect the auxiliary power switches such as the inverter cabinet cooling fan, main motor ventilation system, and motor bearing lubrication pump, then connect the excitation circuit switch and high voltage circuit breaker through the HMI screen, and finally connect the inverter output side contactor MCB to connect the motor. 3.2 Overcurrent Trip in Transmission System (MCB) During normal operation, a sudden overcurrent trip in the transmission system can occur in two ways: phase current overcurrent and torque current reaching the limit. There are many causes of transmission system overcurrent, requiring specific analysis for each case. 3.2.1 Torque Current Reaching the Limit Generally, this is caused by external factors in the transmission system. There are three main causes: ① Excessive reduction causing stalling; ② Seizure of a single work roller or support roller bearing; ③ High-speed steel seizure causing overcurrent. 3.2.2 Phase Current Overcurrent Generally, this is caused by internal transmission issues. There are five main causes: ① Thyristor breakdown leading to a power supply short circuit; ② Problems with the trigger pulse causing a power supply short circuit and overcurrent; ③ Faulty current control board EP22; ④ Problems with the current feedback circuit causing overcurrent; ⑤ Seizure of a single work roller or support roller bearing. If the fault is caused by the transmission system itself, a fault message will usually be displayed. 3.3 The main unit suddenly stops rotating without any fault phenomena. There are no fault records on the HMI, and no system switches trip. Generally, this is caused by external factors affecting the transmission: 1) PLC closing control program logic; 2) PLC run enable control program logic; 3) Whether the communication between the PLC and SS52 is normal. If this occurs, it is necessary to check whether the PLC speed setting and PLC run enable have been issued from the PLC. In most cases, the cause of the fault can be found in the PLC program logic. Pay special attention to the detection points of the PLC closing control program logic: main unit fan, main unit lubrication, oil film bearing, transformer, high-voltage switch cabinet closing position, etc. 3.4 The zero-current detection device sends a transmission trip signal. The zero-current detection device is used to monitor whether the thyristors in the frequency converter are conducting. It is a very critical detection device. Each phase frequency converter cabinet is equipped with one thyristor zero-current detection device. This device measures the voltage across the thyristor and determines whether the thyristor is conducting and whether it has reached zero current by checking the voltage and the presence or absence of trigger pulses. The device issued a trip signal, mainly due to the following reasons: 1) Fast fuse blown. Check if the indicator spring of the fast fuse in the corresponding inverter cabinet has popped out. If it has, replace the fast fuse. Also check the fast fuses on other bridge arms; 2) Thyristor module short-circuited. Replace the damaged thyristor module; 3) The zero-current detection device itself is faulty. Replace the spare part; 3.5 Power cabinet performance degradation After the drive system has been running for a period of time, a phase current overcurrent trip fault occurs. After resetting and running again, the fault persists. The motor, cables, etc. are all normal, and the pulse is also normal. Therefore, it is suspected that the fault may be caused by the thyristor. The thyristor modules are removed and the insulation and leakage current of the thyristor modules are checked. It is found that the leakage current of two thyristor modules is very large. After applying a voltage of 1000 V DC, the leakage current is measured to be 200 mA, while under normal conditions, the leakage current of the thyristor modules is less than 100 mA when a voltage of 1300 V DC is applied. After replacing the thyristor modules, the drive system runs normally. Therefore, insulation and leakage tests of the thyristor modules should be performed regularly, especially the DC voltage detection thyristors in the excitation cabinet, which must also be tested to prevent burnout of the demagnetizing resistor. 3.6 Photoelectric Encoder Faults The photoelectric encoder is a key detection component of the transmission system, used to detect the actual speed of the motor and the rotor position. Its quality directly affects the performance and quality of the transmission system. Common photoelectric encoder faults include no pulse, missing pulses, and uneven vibration pulses. These manifest as overcurrent and oscillation in the transmission system. AC-AC frequency converter transmission systems have an open-loop function, which is very useful for checking speed feedback loop faults. When a speed feedback loop fault occurs, rotate the motor locally in open-loop mode and check the photoelectric encoder pulses with an oscilloscope. If no pulses are detected or the detected waveform is abnormal (the phases of the A and B pulses are incorrect), the pulse generator and its wiring should be checked. Therefore, high-quality, high-performance photoelectric encoders should be selected, and the connection and installation with the rotor should be on the same axis as much as possible; the encoder should be regularly checked for coaxiality. 3.7 SIMADYN D - ITDC Control Board Fault: The main motor suddenly oscillated during operation, producing abnormal noise. Inspection revealed that the main excitation current was zero, and the CPUs P1, P2, EP22, and P4 in the SIMADYN D rack were all in normal condition. After tripping the circuit breaker, the H01 indicator light on the P4 ITDC control board remained constantly lit. This indicated a fault in the P4 ITDC control board. Replacing the ITDC control board restored normal system operation. In summary, it is essential to utilize the indicator lights on the SIMADYN D module to determine the system's operating status. 4. Conclusion There is relatively little discussion on the maintenance of AC-AC frequency converter systems. Based on my practical maintenance experience, I have provided a brief discussion of common faults in AC-AC frequency converter systems, hoping it will be helpful to engineering technicians. References: 1. Yu Mengchang, ed., *Fundamentals of Electronic Technology*, Xi'an Jiaotong University Press, 1982. 2. Chen Boshi, ed., *Automatic Control Systems for Electric Drives*, 3rd Edition, Machinery Industry Press, 2003. 3. *Instruction Manual for 6RA70 All-Digital DC Speed ​​Control System*, SIIEMENS ELECTRICAL DRIVES LTD., 1998. 4. *SIMADYND System Manual*, SIIEMENS ELWCTRICAL DRIVES LTD., 1998. 5. Niu Xiuyan, ed., *Electrical Machines*, Machinery Industry Press, 2003. 6. Wei Binggui, ed., *Fundamentals of Electric Drives*, Machinery Industry Press, 2001. Author Biography: Liu Xiaofei, male, member of the Communist Party of China, born in May 1977, graduated from Zhengzhou University of Aeronautics and Astronautics with a major in Electrical Technology, is an electrical engineer; currently working in the Electrical Workshop of the Second Rolling Mill of Anyang Iron and Steel Group Corporation in Henan Province. He has been engaged in electrical automation research, development, and maintenance.