INOMAX VFD solution for steel plant

1 Introduction

　The frequency drive steel making process of Henan Yaxin Steelmaking Plant is complex, with many detection parameters and frequent equipment actions. The accuracy of process parameter detection and the level of automatic control are directly related to the quality and output of molten steel. For this reason, there must be a set of control systems with accurate and timely detection and a high level of automatic control in order to stabilize production and meet the needs of enterprise survival and development.

2　The load characteristics of frequency drive tilting

　　Steel making is to put molten iron and steel scrap from the blast furnace into the steel making furnace, through the process of oxidative decarburization and slagging, to reduce harmful elements, remove furnace gas and slag, and smelt molten steel that meets the requirements.

　　 There are currently three main steelmaking methods, namely, open hearth steelmaking, frequency drive steelmaking, and electric furnace steelmaking. Most of them use oxygen top-blown frequency drives for steelmaking. Its superiority is conducive to the automation of the production process.

2.1　The main equipment for oxygen top-blown frequency drive steelmaking is

(1) Raw material supply equipment: including the supply of molten iron scrap, bulk materials and ferroalloys;

(2) The main equipment of the frequency drive: it is composed of the furnace body, the furnace body supporting device and the electric drive control system for the tilting of the furnace body, etc.;

(3) Oxygen blowing device: When oxygen frequency drive steelmaking, large amount of oxygen is used, and timely oxygen supply is required, oxygen pressure is stable, and safe and reliable. Therefore, there must be a complete set of equipment to ensure oxygen supply to the frequency drive;

(4) Flue gas purification treatment equipment;

(5) Slag processing equipment, refining equipment outside the furnace, and ingot casting equipment.

According to process requirements, the tilting angle of the frequency drive is plus or minus 360°. The lower part of the furnace trunnion of the frequency drive is higher than the upper part, and the lower part is heavier than the upper part. It is designed according to a positive moment. Therefore, when the frequency drive electric control system fails or the brake force is insufficient, rely on the positive moment of the furnace body to ensure that the furnace mouth is upward, and no steel falling accident occurs.

When the frequency drive is working normally, if the molten steel needs to be dumped, the motor will output a positive torque to drive the frequency drive to tilt slowly. After the molten steel is poured, the furnace body needs to be returned to the normal position slowly. At this time, the potential energy of the frequency drive needs to be fed back to the system, and the motor is working in the feedback state.

2.2　frequency drive's requirements for the transmission system

　　Due to the characteristics of the frequency drive's technology and transmission technology, the frequency drive has high requirements on the transmission system:

(1) The mechanical tilting frequency drive can continuously rotate 360°, and can accurately stop at any position, and should also have speed control performance according to process requirements. Its tilting position can have certain interlocking requirements with related equipment such as oxygen lances, ladle carts and smoke hoods;

(2) During operation, there must be maximum safety and reliability. When a certain part of the electrical or mechanical failure occurs, the tilting machine should be able to continue to operate for a short time and maintain it until the end of the first furnace of steelmaking, even if the tilting machine occurs When the accident cannot be controlled, the furnace will not automatically tip over and cause a "steel down" accident;

(3) Tilting machinery should have good flexibility to buffer shock loads and torsional vibrations caused by starting and braking.

Another key equipment for frequency drive steelmaking is oxygen lance.

Oxygen lance is a typical potential load. As soon as the brake device is opened, the oxygen lance motor will immediately have a 100% load. When the oxygen lance is raised, the electromagnetic torque of the motor overcomes the load torque. The motor works in an electric state. When the oxygen lance descends, the load torque pulls the motor to rotate. The motor is working in the feedback braking state. Similar to the tilting control system, the oxygen lance transmission control system must also work in coordination with the brake device to prevent the phenomenon of "rolling", and it also has sufficient starting torque and overload capacity. And the speed can be adjusted.

The frequency drive is generally equipped with two sets of oxygen lances, one set is working, and the other set is spare or overhauled.

The configuration of the frequency drive control system of the 3　 frequency frequency drive in the steelmaking plant of Sangang Company

　　According to the tilting of the frequency drive and the characteristics of the oxygen lance control system, the ACS880 series inverter of INOMAX was selected.

3.1 Technical characteristics of INOMAX ACS880 series inverter

The technical characteristics of INOMAX ACS880 series inverter are as follows, it is especially suitable for frequency drive control here:

By direct torque control technology, stator flux and torque are used as the main control variables. The combination of high-speed digital signal processor and advanced motor software model enables the state of the motor to be updated at 40,000 times/s. Since the motor state and the comparison value between the actual value and the given value are constantly updated, each switching state of the inverter is determined individually. This means that its drive system can produce the best combination of switches and respond quickly to dynamic changes such as load disturbances and instantaneous power failures. In DTC, there is no need for a PWM modulator that controls voltage and frequency separately. Therefore, there is no fixed chopping frequency. In actual operation, the high-frequency noise emitted by other inverters driving the motor will not be generated, and the power consumption of the inverter itself will also be reduced.

　　The standard built-in AC reactor significantly reduces the high-order harmonic content of the incoming power supply, greatly reduces the electromagnetic radiation of the inverter, and protects the rectifier diode and filter capacitor from the impact of voltage and current.

　　 Zero-speed full torque: The motor driven by ACS880 can obtain the rated torque of the motor at zero speed, and does not require the feedback of an optical encoder or a tachometer motor. The vector control inverter can only achieve full torque output when it is close to zero speed.

　　 The precise torque control provided by DTC enables ACS880 to provide a controllable and stable maximum starting torque. The maximum starting torque can reach 200% of the rated motor torque.

　　Automatic start: The automatic start characteristic of ACS880 exceeds the performance of flying start and integral start of general frequency frequency drive. Because ACS880 can measure the state of the motor within a few milliseconds, it can be started quickly within 0.48s under any condition. The vector control inverter needs to be greater than 2.2s.

　　In the flux optimization mode, the motor flux is automatically adapted to the load to improve efficiency and reduce motor noise. This is due to the optimization of the magnetic flux. Based on different loads, the total efficiency of the inverter and the motor can be increased by 1%-10%.

　　 Precise speed control: The dynamic speed error of ACS880 is 0.3%s in open-loop application and 0.1%s in closed-loop application. The vector control inverter is greater than 0.8%s in open loop and 0.3%s in closed loop. The static accuracy of the ACS880 frequency frequency drive is 0.01%.

Accurate torque control: The dynamic torque step response time can reach 1-5ms in open loop applications, while the vector control inverter needs 10-20ms in closed loop and 100-200ms in open loop.

The frequency drive capacity of the Helan Yaxin Steelmaking Plant is 100t, the tilting motors of the frequency drive are 4 sets of 90kW frequency conversion motors, and the oxygen lance motors are 2 sets of 75kW frequency conversion motors;

3.2 　 Tilting and oxygen lance control situation

(1) Tilting control

　　Usually, the tilting of the frequency drive is done by 3-4 motors, so in order for the system to work more reliably and stably, these motors must be load balanced, that is, all motors have the same output.

　　 Since these 4 motors are rigidly connected, the speeds of all motors are required to be absolutely synchronized. Therefore, the transmission of the 4 motors adopts master/slave control, and the 4 inverters are connected by optical fibers. Therefore, in the system configuration, one frequency frequency drive is used as the master, and it is used for speed adjustment and output torque setting, and other frequency frequency drives are used as slaves to follow the torque response of the master, as shown in Figure 2. Compared with the conventional control method, this use improves the performance of the system to a new level. Completely solve the phenomenon of "nodding" and "shaking the head" of the frequency drive caused by the asynchronous motor operation.

4 inverters can be switched between master and slave, but there can be only one master at the same time, and the other three are slaves. The master adopts speed control mode and the slave adopts torque control mode. The master sends control commands to the slaves. The slave accepts the start and stop commands from the host and the torque set value to act.

When a slave machine fails, it does not affect the communication and actions between the master and other slaves, so it can continue to run and wait for proper maintenance; when the master fails, the slave stops because it cannot receive the signal from the master At this time, one of the slaves is switched to the master, and the failed master is switched to the slave. After the switch is completed, the master sends a reset command to synchronize the signal, and the slave failure signal disappears. When the system is normal, continue to run. Wait until the appropriate time to overhaul the faulty equipment.

When the master is normal and 3 slaves report faults at the same time, it means that there is a problem in the communication. In order not to affect the normal production, all slaves are switched to the master at this time, that is, the 4 inverters are performing speed control at the same time. When available, check for communication problems immediately.

It should be noted that when inspecting and repairing the inverter, if other inverters are required to continue running at this time, since the optical fiber communication network has been disconnected after the inverter is powered off, other inverters cannot perform master/slave control and should be switched to The host can continue to run. After the overhaul is completed, switch to master/slave control mode.

　　 If a normal master/slave switch is performed, the slave will report a failure due to the temporary failure of the master signal. When the master/slave switch is completed, after the master sends a reset command, all alarm signals can be eliminated.

　　Because the frequency drive is a load with high starting torque, the starting method is a high torque starting method with constant excitation, and the stopping method is a ramp stop method. The braking chopper and braking resistor are used to absorb the energy returned.

In order to coordinate with the brake device accurately, when the speed of the inverter reaches absolute zero speed, the inverter outputs instructions to control the brake device, and the brake contactor is driven by the intermediate relay, and then the inverter stops the excitation, which can ensure The tilting device will not appear "rolling" phenomenon.

(2) Oxygen lance control

　　The key to oxygen lance control is that after the brake is opened, the motor must have a 100% load. For this reason, the transmission must output at least 150% torque at zero speed to prevent the oxygen lance from "slipping".

The direct torque control mode of the INOMAX inverter can provide up to 200% output torque at zero speed. Its dedicated lifting software has a torque memory function to memorize the starting torque required by the oxygen lance. Once the system is started, Just output the required torque to ensure the smooth operation of the oxygen lance.

　　 The brake interlock control function of the INOMAX lifting software ensures the safe operation of the system.

summary:

Inomax drives is the idear choice for steel plant ,with better performance and more cost effective.