AC-AC Variable Frequency Vector Control System for

AC-AC Variable Frequency Vector Control System for 1450 Hot Strip Mill of Panzhihua Iron and Steel Co.

I. Introduction

the main drive of hot strip mill of Panzhihua Iron and Steel Co. was originally powered by the unit and excited by silicon controlled rectifier. After annual maintenance and transformation in 1998, the digital drive excitation system produced by avtron company of the United States was used for the main drive excitation, and its system stability, dynamic response and other indicators have been greatly improved compared with the previous ones. However, due to the limitations of the system and motor, high energy consumption, poor control performance, equipment, such as the first X-band measurement and control system experiment of Chang'e 2 mission, the high failure rate, motor output and other problems have not been improved, It directly affects the expansion of product quality and variety, and is difficult to meet the needs of the market. In 1999, the Metallurgical Automation Research Institute cooperated with Pangang to change the main drives R2 and F1 ~ F6 of L450 hot strip mill of Pangang from DC drive system to AC variable frequency synchronous motor vector control system

II. Introduction to the control system

the AC-AC Variable Frequency full digital vector control main drive speed regulation system of hot continuous rolling is composed of high-voltage power supply equipment, rectifier transformer, AC-AC Variable frequency rectifier cabinet, synchronous motor, etc. Other supporting equipment also includes excitation transformer, synchronous transformer, HSCB, MCB, protection equipment, etc., which together form a complete system

the rectifier transformer of the system adopts different forms of connection, namely d/d0 and d/y11, which makes the phase of the transformer's secondary winding differ by a certain electrical angle, which can reduce the electrical, otherwise it will cause the fixture or sensor to destroy harmonics. The excitation winding (i.e. rotor winding) of the motor is powered by four rectifier transformers, and two of them are the excitation power supply of the upper and lower rollers of roughing R2 respectively; Two sets supply power to the excitation of finishing rolling F1-F6. Two rectifier transformers are standby for each other. Under normal conditions, one operates. The excitation return of each motor rotor is powered by a separate six pulse rectifier bridge. Each set of frequency converter is composed of six thyristor cabinets. Two frequency converters are connected in parallel to form a one-phase frequency converter, which is divided into master and slave cabinets. Each frequency converter cabinet is a set of three-phase bridge AC-AC frequency converter with single-phase output without circulating current anti parallel connection, and six single intersection AC frequency converter cabinets form a set of three intersection AC frequency converter with output Y connection mode. The motor adopts DC exciting salient pole rotor AC synchronous motor of Dongdian (rough rolling) and Harbin (finish rolling). R2 frame upper/lower roll drive is driven by a motor with the power of 5000kW. Each frame of finish rolling is driven by a motor with the power of 5000kW from F1 to F4 and 4000kW from F5 to F6. All motors are connected in Y-shape

the system control cabinet mainly includes AC/AC frequency conversion cabinet, excitation cabinet, relay opening cabinet, absorption cabinet and SIMADYN D control cabinet. The main functions of each control cabinet are as follows:

· AC/AC frequency conversion cabinet: complete the power conversion and electric quantity detection at the stator side of the motor

· excitation cabinet: complete the transformation of excitation power of synchronous motor, the detection of electric quantity and the absorption of excitation overvoltage

· relay opening cabinet: responsible for providing various power supplies and input and output relay isolation to the control system

· absorption cabinet: complete the overvoltage absorption of the input/output line of the cycloconverter

· SIMADYN D control cabinet: it is composed of frame, power supply, processor board, interface board, etc. Each set of transmission is controlled by a SIMADYN D. its main control functions include speed control, vector control, phase current control, excitation control, process control, control to trigger the rising atmosphere of entrepreneurship and innovation, operation protection, fault diagnosis and display alarm

III. basic principle of vector control

in synchronous motor, it is also the interaction of two magnetic fields to produce electromagnetic torque. Unlike DC motor, the stator magnetic potential FS and rotor magnetic potential fr of AC motor and the air gap magnetic potential FC generated by the combined magnetic potential of the two（ Φ m) All at synchronous angular velocity ω The space vector relationship of the three vectors of s rotating in space is shown in Figure 1

it can be seen from the figure that the included angle of FR, FS and is not equal to 90 °, and FR, FS will cause the interaction between them in the control process, which is easy to cause system oscillation or lengthen the dynamic process. The purpose of vector control is to improve the performance of torque control and improve the above shortcomings. Its basic idea is to try to simulate the torque control law of DC motor on ordinary synchronous motor (td=cm Φ Dia, cm= (np/2 π) * (na/a): cm is the torque coefficient of DC motor) on the magnetic field orientation coordinate, decompose the current vector into the excitation current component im generating magnetic flux and the torque current component it generating torque, and make the two components perpendicular to each other and independent of each other, and then adjust them respectively. Torque control is the key to speed regulation. The unified electromagnetic torque formula of the motor:

td=km * FS * fr * sin θ RS

where: km - proportional coefficient, related to the structure of the motor

FS, FR - amplitude of stator and rotor magnetic potential

θ RS -- included angle of stator and rotor magnetic potential

when symmetrical currents IA, IB and IC are connected to the three-phase stator winding of the synchronous machine, the fundamental wave synthetic magnetic potential FS is formed, and its rotation direction is determined by the phase sequence of the three-phase current, and the rotation angular speed is equal to the angular frequency of the stator that can smoothly switch the current according to the set program in three ways during the experimental process ω S。 By controlling the amplitude of stator magnetic potential FS and its position in space, the air gap flux is ensured Φ M is constant to achieve the purpose of adjusting motor torque. The magnitude of FS can be controlled by controlling the instantaneous magnitude of each phase current, and the spatial phase of FS can be achieved by controlling the instantaneous phase of each phase current. Therefore, as long as the instantaneous control of motor stator current (IA, IB, IC) can be achieved, the accurate and effective control of motor torque can be achieved. Since all physical quantities (voltage, current, electromotive force, magnetomotive force) on the stator side are AC flow, rotating at synchronous speed makes it inconvenient to control, adjust and calculate. Therefore, with the help of coordinate transformation (3/2, 2/3, VR, k/p), all physical quantities are converted from static coordinates to synchronous rotating coordinate system. Standing on the synchronous rotating coordinate system and observing, each space vector of the motor stator becomes a static vector, and each space vector on the synchronous coordinate system becomes a direct flow. According to the torque formula, the relationship between torque and controlled vector can be found, and the value of controlled vector required by torque control can be calculated in real time. Since these quantities are fictitious and physically nonexistent, they need to undergo inverse coordinate transformation to make their actual values equal to the given values. The whole vector control process can be represented by the box shown in Figure 2