1. Background During the converter blowing process, a large amount of reddish-brown flue gas is discharged from the furnace mouth. This flue gas is high in temperature and contains flammable gases and metal particles. According to China's "Integrated Emission Standard for Air Pollutants" (GB 16297-1996) promulgated in 1996, the flue gas must be cooled and purified before being discharged to the chimney by an induced draft fan or transported to a gas recovery system for later use. The No. 2 Steelmaking Department of Fushun Xinfu Steel adopts a "three-blowing, three-stage" system. Each converter is equipped with a dust removal system using a two-stage Venturi dust purification method. The flue diameter is φ1.6m, the flue gas conveying pipeline is 820mm, the fan model is D700, and the motor power is 440kW/6kV. Due to the periodic intermittent oxygen blowing of the converter, to meet energy conservation and environmental protection requirements, the fans are required to operate at variable speeds throughout the entire steelmaking cycle, running at high speed during oxygen blowing and at low speed when oxygen is not being blown. The original speed control used a hydraulic coupler, with a high speed of 2700 r/min (designed 2900 r/min) and a low speed of 800 r/min. Due to limitations in hydraulic coupler technology, the following issues arose: 1) The speed range was unstable, limited to 30%–90%; moreover, the low speed of 800 r/min was still too high, resulting in energy waste; during high-speed operation, the hydraulic coupler sometimes lost rotation, causing smoke to billow from the converter mouth. 2) After more than 10 years of use, the hydraulic coupler was severely aged, requiring frequent bearing replacements, leading to converter shutdowns and failing to meet continuous production needs. 3) The motor had low efficiency and high losses, especially at low speeds. 4) The adjustment accuracy was low, linearity was poor, and response was slow. 5) The starting current was still relatively high, affecting grid stability. 6) In case of hydraulic coupler failure, it was impossible to switch to power frequency bypass operation, necessitating a shutdown for maintenance. 7) Severe oil leakage caused significant environmental pollution, with the ground severely corroded by oil. Given the numerous problems associated with the hydraulic coupler, its modification is urgently needed. To improve the operating efficiency of the fan and resolve the many issues arising from the use of the hydraulic coupler, Xinfu Steel decided to implement a frequency converter (VDC) upgrade. 2. VDC Upgrade Plan The dust collector fan is the power hub of the dust removal and purification system. If the dust collector fan fails to operate normally, it will not only affect production and cause huge economic losses, but may also endanger the safety of on-site personnel. Furthermore, the speed control system operates in a harsh environment; simultaneously, the converter periodically and intermittently blows oxygen. Therefore, the high-pressure speed control system配套 with the dust collector fan requires extremely high reliability. Based on these operating characteristics, the main requirements for the VDC speed control system are as follows: ① The VDC converter must have high reliability and operate without failure for a long period. ② The VDC converter must have a bypass function, allowing the motor to switch to mains frequency operation in case of a fault. ③ The speed range must be large and the efficiency high. ④ It must have logic control capabilities, automatically adjusting the speed according to the oxygen blowing cycle. ⑤ It must have a resonance point jumper, allowing the motor to avoid operating at the resonance point and preventing fan surge. After extensive research and comparison, Xinfu Steel and Beijing Lide Huafu Technology Co., Ltd. jointly formulated a frequency conversion retrofit plan for the No. 1 converter dust removal fan. The retrofit plan is as follows: 2.1 Equipment Configuration (see Figure 1) [ALIGN=CENTER] Figure 1 [/ALIGN] KM - High-voltage vacuum circuit breaker powered by frequency converter, KG1, KG2, KG3 - Vacuum contactors, BPO - HARSVERT-A067080 frequency converter, DJ - 440kW/6kV asynchronous motor, KM - Existing high-voltage switch, M - Existing asynchronous automatic motor. If the capacity is expanded to a 630kW/6kW three-phase asynchronous motor in the future, the frequency converter must have the driving capability. When the fan is running at high speed, if the frequency converter has a serious fault, the motor will automatically switch to the power grid after a delay of 1 second. After the current blowing cycle ends, KG3 will be automatically disconnected to inspect the frequency converter. After the frequency converter is inspected, it will automatically return to the original frequency conversion speed regulation state by pressing the reset button. When a serious fault occurs in the frequency converter, the system can automatically switch to the mains frequency grid without shutting down the load, meeting the requirement that the system cannot be shut down on-site. 2.2 Motor and Fan Parameters 2.3 Dust Collector Fan Process Requirements 1) Blowing Process Cycle (see Figure 2) [ALIGN=CENTER] Figure 2 [/ALIGN] A to B is the time for adding iron and scrap steel; B to C is the fan speed-up time, which can be adjusted; C to D is the oxygen blowing time; at point D, the fan begins to decelerate; D to E is the furnace tilting temperature measurement and sampling time; E to F is the steel tapping time; F to G is the slag splashing time. The entire blowing process cycle is about 21 minutes, of which the high-speed time (C to D) is 12 minutes. The high speed is set at 45Hz, which can be adjusted; the low speed is set at 5Hz, which can be adjusted. 2) Inverter and Field Interface At point B, a pair of closed nodes (nitrogen-oxygen conversion point) are provided by the field. When the field nitrogen-oxygen conversion switch switches to oxygen blowing mode, the inverter starts from low speed to high speed. At point C, after the inverter reaches high speed, it provides a pair of closed nodes (high-speed status nodes) to the field, indicating that the field inverter is ready for the field operator to proceed with the lowering of the oxygen lance. At point D, the field provides another pair of closed nodes to the inverter (fan deceleration point). When the oxygen lance is raised, the inverter begins to decelerate. The deceleration time is not specifically required, but if speed increase is needed during deceleration, the inverter should be able to meet the speed increase requirements. The relationship between the fan speed, nitrogen-oxygen switching node, and oxygen lance position node is shown in Figure 3. [ALIGN=CENTER] Figure 3 [/ALIGN] The two nodes provided to the inverter on-site (nitrogen-oxygen conversion point, fan deceleration point) and the high-speed state node provided by the inverter on-site are all passive nodes with a capacity of 2A/30VDC or 0.8A/230VAC. 3. Equipment Operation Status The high-voltage inverter model is HARSVERT-A06/080. The production, installation, and commissioning cycle of this inverter is very short, totaling only a little over three months, providing a strong guarantee for the timely commissioning of converter No. 1. Compared with the original hydraulic coupling, the high-voltage inverter produced by Beijing Leadway Technology Co., Ltd. has the following advantages: 1) Stable operation and reliable safety. The original hydraulic coupling required bearing replacement approximately every 40 days, requiring a half-day shutdown each time, resulting in huge economic losses. The HARSVERT-A inverter is maintenance-free, requiring only periodic replacement of the ventilation filter on the cabinet door, without shutdown, ensuring continuous production. 2) Significant energy-saving effect, greatly reducing power consumption per ton. 3) The motor achieves true soft start and soft stop. The frequency converter provides the motor with a harmonic-free sinusoidal current, reducing the number of motor failures. Simultaneously, the frequency converter sets a resonant point jump frequency, preventing the fan from operating at the resonant point for extended periods, ensuring stable fan operation, reducing fan bearing wear, extending the service life and maintenance cycle of the motor and fan, and improving fan utilization. 4) The frequency converter has comprehensive self-protection functions. Compared with the original relay protection, it offers more protection functions and is more sensitive, greatly enhancing the protection of the motor. During frequency converter commissioning, the frequency converter automatically protected and reported motor overcurrent. Inspection revealed that a phase conductor lug of the motor had broken, causing the motor to operate with a single phase, resulting in overcurrent. We promptly carried out repairs, preventing the accident from escalating. 5) The frequency converter has a seamless interface with field signals, meeting production needs. The frequency converter has a built-in PLC, allowing for flexible field signal input. The converter provides a pair of high-speed and low-speed nodes for the frequency converter, which automatically operates at high and low speeds according to the node status. The flue gas flow and temperature signals from the converter, previously stored in the instrument cabinet, are now connected to the frequency converter, eliminating the need for the instrument cabinet and significantly saving space. The frequency converter has built-in speed measurement, eliminating the need for the original tachometer connected to the motor; the frequency converter directly provides motor speed indications to the site. 6) It has strong adaptability to grid voltage fluctuations; even when the grid voltage reaches 6.9kV, the frequency converter can still operate normally. 7) Compared with hydraulic couplings, it significantly reduces noise during acceleration, thus reducing noise pollution. Because it eliminates the need for periodic bearing replacements, it reduces oil pollution to the environment, greatly improving the on-site environment of the fan room. 4. Energy Saving Analysis Before the modification, the conditions of converters #1 to #3 were similar. After the modification, the conditions of converter #1 can be compared with converters #2 and #3. The energy saving data is ultimately analyzed based on the dust removal power consumption per ton of steel (see attached table). Compared with the hydraulic coupling, the average power consumption for dust removal per ton of steel is reduced by 2.76 kW•h/t: The annual steel production of converter #1 is expected to be 350,000 tons this year, with an electricity price of 0.44 yuan/kW•h: The total annual power saving is: 350,000 tons × 2.76 × 0.44 = 425,040 yuan, representing a power saving rate of 39.2%. [ALIGN=CENTER] Appendix [/ALIGN] Furthermore, due to the reliability of the HARSVERT-A frequency converter, the situation of blast furnace blasting caused by converter shutdown due to the failure of the original hydraulic coupling is avoided, resulting in significant economic and social benefits. 5. Conclusion Based on several months of operation... The HARSVET-A06/080 high-voltage, high-power frequency converter manufactured by Beijing Leadway Power Equipment Co., Ltd. boasts excellent performance, high reliability, and significant energy-saving effects, meeting the requirements of continuous production speed control systems. Our company has decided to continue using it in the renovation of converters #2 and #3. The frequency converters配套的 for converters #2 and #3 have arrived on site and are currently being installed and commissioned. (Article excerpted from "Energy Saving Innovation 2006 – Proceedings of the First National Electrical Energy Saving Competition")