1. Characteristics of the converter drive system
The converter is a key production equipment in a steel plant, and its external shape is shown in Figure 1. The converter resembles a "hanging bucket." The main electrically driven equipment in the converter system includes the electric drive system for tilting the furnace body and the electric control system for driving the oxygen lance. The electric drive system for tilting the furnace body is described below.
In large and medium-sized converter systems, the tilting mechanism of the furnace body generally uses four tilting motors, which are rigidly connected by a reducer and adopt a fully suspended fixing method and a torque absorption method.
In actual production operations, the converter tilting device in a steel plant requires frequent starting, braking, and acceleration/deceleration operations. Therefore, high requirements are placed on the control accuracy of the tilting device and the system's response time. The load characteristics of the converter tilting device and the operating status of the motor are analyzed below:
According to the converter control process requirements, the converter tilting angle is 360° in both directions. When the axis of the converter opening and bottom is perpendicular to the ground plane, it is in a vertical state. Therefore, the converter tilting load torque is a function of the angle, i.e., Tfz = f(θ), belonging to a reverse resistive potential energy load. Furthermore, according to the process design specifications, the converter is designed for positive torque, meaning the lower part of the converter trunnion is higher and heavier than the upper part, thus ensuring the stability of the converter control system. In the actual operation of the converter tilting device, negative torque may occur during slag removal or steel tapping. When the converter body is in a positive torque state, the motor is in electric operation; when the converter body is in a negative torque state, the motor is in generator operation, and in this case, the motor is in a regenerative braking state. The mechanical characteristics and load characteristics of the driving motor of the tilting device are shown in Figure 2.
2. Basic Requirements for Converter Tilting Control System
(1) The four motors start, brake and run synchronously. The tilting speed of the converter can be adjusted between 0.2 and 1.0 r/min as required. The converter can rotate ±360°.
(2) When the converter is tapping steel and slag, the AC power system fails. At this time, the UPS power supply is used to open the 4 brakes. The converter relies on self-repositioning to be in a safe position.
(3) When an accident such as a converter collapse occurs, the electromechanical equipment of the tilting machine can be overloaded for a short time, the converter rotates at a speed of 0.2r/min, the tilting converter pours out the contents of the furnace, and then the accident is handled.
(4) The converter is designed with positive torque, that is, the tilting in the entire working tilting angle from 0° to ±180° is positive torque.
(5) To prevent the sudden start of the motor from impacting the equipment, the motor speed should be gradually accelerated from zero when the converter starts to tilt. The acceleration time from zero to normal speed is 2 seconds.
(6) Due to the large braking torque of the brake, in order to prevent the impact on the equipment during braking, the motor should be decelerated by energy consumption braking first when the converter is braking. When the tilting speed of the converter approaches zero, the brake is de-energized and braked for 2 seconds.
(7) The requirements for the rotation angle and speed control range of the converter smelting process are shown in Table 1.
3. Key Control Challenges of a 50T Converter in a Steel Group in Shandong
3.1 Key Control Challenges
The converter control system primarily handles the furnace door operation and furnace tilting, the furnace tilting and ladle/slag removal operations, the fume hood lifting and lowering with interlocked control of cooling water flow and pressure, the oxygen lance lifting and lowering with interlocked control of oxygen lance positioning and cooling water flow and pressure, oxygen lance lateral movement and replacement, and charging and weighing operations. It also collects real-time data on temperature changes at multiple temperature measurement points in the water-cooled fume hood, water-cooled furnace wall, and water-cooled oxygen lance system, as well as real-time data acquisition and monitoring of cooling water system pressure and flow, and provides alarms for over-limit and accident events. Real-time data transmission between the PLC and the operator station is achieved via Ethernet, and the production process is monitored through a human-machine interface. Figure 3 shows a diagram of the converter operation.
Among these challenges, controlling the four converter tilting motors was the most technically difficult aspect of the project, requiring rapid response and high levels of control precision, safety, and reliability. The transmission control device used in the design was the Fengguang JD-BP32 series frequency converter, model JD-BP32-315T (315kW/380V), which effectively solved the aforementioned problems.
3.2 Selection of Converter Tilting Frequency Conversion Control Method
Based on the design and application of converter tilting control systems in recent years, the design unit carefully compared the "one-to-four" control method (i.e., one frequency converter driving four motors) and the "four-to-four" master-slave control method. They concluded that the "one-to-four" control method is simpler, easier to program, debug, and maintain. From a safety and rational perspective, each converter is equipped with an additional frequency converter as a backup to mitigate production stoppages caused by frequency converter failures during normal converter production. Ultimately, the design unit determined that the four motors would use the "one-to-four" control method. The converter tilting main circuit is shown in Figure 4.
3.3 Characteristics of Converter Tilting Frequency Conversion Control
(1) Handling of regenerative energy: For loads such as converter tilting, one of the most important principles for selecting frequency converters is that the selected frequency converter must be a high-performance frequency converter with constant output torque under full load or overload to ensure the safety of converter production. In addition, it must have regenerative braking function. In order to ensure the continuity of converter production, traditional braking units and braking resistors are used instead of feedback units to handle the regenerative power generation of the motor.
(2) Control of brake release: During the control of converter tilting, "slippage" faults should be prevented. By using the PLC control program, the brake is released when the motor torque is sufficiently large after the system starts, which greatly improves the reliability of the system application.
3.4 Operation of the converter tilting control system
(1) Three-location operation: According to the requirements of the steelmaking process, the tilting of the converter needs to be controlled from three locations: the main control room main control console, the converter front control console, and the converter rear control console. Main control room main control console: The converter can be tilted, fed, discharged, and all other operations can be performed on the main control room main control console through the buttons on the main control console and the main control controller. Converter front and rear control consoles: These are mainly used by on-site workers during discharge and slag removal and converter maintenance. Since only one front of the converter can be seen from the main control room, the amount of material discharged cannot be seen during discharge and slag removal. Therefore, on-site operation by workers at the front and rear control consoles is necessary.
(2) Start-up and shutdown and speed setting - tilting furnace master controller
The start/stop and speed control are provided by the tilting furnace master controller, currently the XDA-V11122 model manufactured by TE. The XDA-V11122 master controller communicates with the PLC via analog voltage input, with a voltage range of -10V to +10V. When the input voltage is -10V to 0V, the motor reverses; when the input voltage is 0V to +10V, the motor rotates forward. In the PLC program, the -10V to +10V range is correspondingly divided into a digital range of -27648 to +27648. When the input is between -27648 and -300, the motor reverses; between 300 and 27648, the motor rotates forward; and between -300 and +300, the motor remains stationary to prevent controller drift and malfunctions.
4. Process parameters of a 50T converter in a steel group in Shandong
The 50T converter project of this steel group was a newly constructed project, adopting advanced domestic and international control processes and technologies at the time. The project was successfully put into operation in July 2003. The main process parameters of the converter's tilting mechanism are as follows:
Nominal capacity of converter: 50T
Rated tilting torque: 1000 kN.m
Maximum accident torque: 2300 kN.m
Tilting speed: 0.2～1.0 rpm
Overall transmission ratio: 563.08
Tilting motor: YGP315M-8, 45kW, the motor is a variable frequency speed regulation three-phase asynchronous motor for roller conveyors, stator Y connection, H-class insulation, duty cycle S5 (40%), cosΦ=0.76, rated current 85.9A, rated torque 659.1Nm.
Features of 5JD-BP32 Series Frequency Converters
The JD-BP32 series frequency converters are a series of high-performance, simplified, and low-noise frequency converters independently developed by Shandong Xinfengguang Electronic Technology Development Co., Ltd. using a brand-new concept. While improving stability, they incorporate a simplified PLC, practical PI control, flexible input/output terminals, online parameter modification, self-identification of signal transmission faults, power outage and shutdown parameter storage, fixed-length control, frequency swing control, RS485 control, fieldbus control, and a series of other practical and advanced operation and control functions. They provide equipment manufacturers and end customers with a highly integrated solution, greatly helping to reduce system procurement and operating costs and improve system reliability.
The technical parameters of the JD-BP32 series frequency converters are shown in Table 2.
6. Specific debugging process
Due to the frequent and complex process requiring furnace shaking, and with the combined weight of the furnace itself and the material inside reaching approximately 160 tons, the load inertia is significant. Therefore, ensuring the furnace doesn't slip at any position and determining when to open and close the mechanical brakes on each motor is crucial during commissioning. In actual commissioning, after repeated experiments, the no-load value (30-35%) of the sum of the rated currents of the four motors was selected. At this point, the motors have established a sufficiently large electromagnetic torque, and by coordinating with the PLC, the brakes can be opened. This prevents the "slippage" phenomenon.
Since the converter frequency converter is a "one-to-four" control, the frequency conversion control can only use "V/F" control. In order to improve the torque of the frequency converter at low speed, a suitable boost voltage should be applied during startup.
Voltage boost parameters: P0.09 = 6% voltage boost value during startup. P0.10 = 10Hz torque boost cutoff frequency. The main inverter commissioning parameters are shown in Table 3.
Then, according to the process requirements, the main connection signals between the frequency converter and the PLC control system are: forward operation, reverse operation, emergency stop, reset, frequency converter operation signal (brake release signal), and frequency converter fault signal. See the on-site wiring diagram in Figure 5.
7. On-site usage
The specific control process of the converter tilting motor is as follows:
(1) Start-up process
When the system is ready to run, all switches and contactors in the main circuit of the frequency converter are turned on. The master controller inputs analog values into the PLC. Based on the positive and negative values input by the master controller, the PLC determines whether the frequency converter is in forward or reverse direction. The absolute value of the input value of the master controller is used as the speed control output of the frequency converter. If the frequency converter is fault-free, it starts to run. The opening signal (based on whether the output current of the frequency converter reaches the required motor current amplitude) is given by the PLC, and the converter tilting motor starts to rotate and the speed rises to the given value.
(2) Parking
When the master controller is switched to the stop position, the PLC issues a stop command, the frequency converter blocks the output, the brake is closed, and the converter tilting stops rotating.
(3) Fault
When the frequency converter malfunctions, its fault signal is input into the PLC, the brake is closed, and the PLC issues an alarm.
8. Conclusion
On July 1, 2003, the entire converter project was put into operation. Compared with the DC speed control of other converters in the steel plant, the frequency converter speed control has the advantages of low maintenance costs, fast response during operation, and stable operation, avoiding accidents such as runaway and molten steel splashing. The stable operation of the converter tilting system for many years has proven that the solution of debugging the Fengguang JD-BP32-T series frequency converter is feasible.
