1. Introduction Power electronics technology is one of the modern high-tech fields. It integrates power (high voltage), electronics (low voltage), and control technologies, and is hailed as the "crown jewel" and "king of energy conservation." The core technology of power electronics is frequency conversion technology. The focus of frequency conversion technology is high-voltage variable frequency speed regulation technology. The "Energy Conservation Law of the People's Republic of China," promulgated in 1997, stipulates the "development of motor speed regulation and power electronic energy-saving technologies." The author has proposed four high-low voltage variable frequency speed regulation integrated technologies (referred to as "high-low voltage schemes") for high and medium voltage motors, all of which have obtained national patents. This article introduces a newly launched "capacity-enhanced high-low voltage variable frequency speed regulation integrated device," which features high power, high efficiency, high power factor, high reliability, low cost, low harmonics, low temperature rise, low voltage safety, labor saving, investment saving, energy saving, and raw material saving. It provides an optimized high-low voltage variable frequency speed regulation scheme for 3-10kV and 200-3000kW medium and high voltage motors. 2. The Energy-Saving Potential of High-Voltage Motor Variable Frequency Speed ​​Regulation In China, the total power consumption of various electric motors accounts for over 60% of the country's total power consumption. While high-voltage (3kV to 10kV and above) motors account for 20% to 80% of low-voltage motors in terms of quantity, their capacity (kW) is 60% to 40%. In recent years, China's total power consumption has reached over 1.2 trillion kWh. Assuming that electric motors account for 60% of total power consumption, high-voltage motors account for 60% of electric motor power consumption, and the potential for retrofitting is 30%, and the micro-level (single unit) energy saving rate is only predicted at 30%, the macro-level annual energy saving in China could reach: 1.2 trillion × 60% × 60% × 30% × 30% = 38.88 billion kWh/year, equivalent to building 10 new 800,000 kW power plants operating for 5,000 hours per year. If the comprehensive electricity price is calculated at 0.5 yuan/kWh, the annual electricity saving value of China can reach: 0.5 yuan/kWh × 38.88 billion kWh/a = 19.44 billion yuan/a. Therefore, implementing variable frequency speed regulation for high-voltage motors has become the top priority of energy-saving technology measures in China and our province. "High-low voltage variable frequency speed regulation integrated device for high-voltage motors" is a patented technology with independent intellectual property rights in China. Compared with high-voltage motor variable frequency speed regulation products at home and abroad, it has the characteristics of high power, high efficiency, high power factor, high reliability, low harmonics, low cost, low temperature rise, low voltage safety, short construction period, and short investment recovery period. 3. Domestic and foreign solutions for variable frequency speed regulation of high-voltage motors At present, the variable frequency speed regulation technology solutions adopted by domestic and foreign high-voltage motors are mainly: (1) High-low-high voltage type It first converts high voltage to low voltage power frequency through a step-down transformer, and then converts the low voltage variable frequency power output of the frequency converter to high voltage through a step-up transformer to provide variable frequency speed regulation for high-voltage motors. Its disadvantages are that the step-up transformer must use amorphous magnetic materials, and under non-sinusoidal frequency conversion conditions, the efficiency is low, the power factor is low, the high-order harmonics are large, the cost is high, and the footprint is large. For example, a company in Liaoyang introduced a 6kV, 250kW high-low-high voltage type frequency converter, with a unit price of 700,000 yuan and a cost of 2,800 yuan per kW. See Comparison Table 1. [IMG=High-low voltage variable frequency speed control integrated device for medium and high voltage motors and its application]/uploadpic/THESIS/2008/1/2008012313293553845B.jpg[/IMG] (2) High-medium voltage type It achieves direct conversion of high voltage frequency through a single high voltage frequency converter. Its disadvantages are that the cost of a single high voltage frequency converter is very high, and due to the limitations of the withstand voltage conditions, its rated operating voltage is mostly 3 to 4.16kV. For example, in 2001, a water plant of Ansteel introduced two sets of 3kV, 400kW high-high voltage type frequency converters, costing 1.9 million yuan, with a cost of 2,375 yuan per kW. When the operating frequency was close to 50Hz, it was not energy-efficient and frequently tripped. See Comparison Table 2. [IMG=High-Low Voltage Variable Frequency Speed ​​Control Integrated Device for Medium and High Voltage Motors and Its Application]/uploadpic/THESIS/2008/1/20080123132940801194.jpg[/IMG] (3) High-high voltage type It achieves direct conversion of high voltage frequency through the series connection of multiple high voltage frequency converters. Its disadvantage is that the frequency converter with multiple high voltage devices in series has very poor reliability and higher cost. For example, two of the four 6kV, 500kW high-high voltage IGCT series frequency converters introduced by a company in Golmud, Qinghai Province, have experienced high voltage breakdown. Repairing one unit costs 800,000 yuan, with a repair cost of 1,600 yuan per kW. See Comparison Table 3. [IMG=High-Low Voltage Variable Frequency Speed ​​Control Integrated Device for Medium and High Voltage Motors and Its Application]/uploadpic/THESIS/2008/1/2008012313294728511B.jpg[/IMG] (4) Multi-stage type It uses a special transformer to reduce high voltage to low voltage at different electrical angles, and then superimposes it into high voltage through multiple (15 for 6kV and 27 for 10kV) low voltage frequency converters. Its advantages are that the output waveform is closer to a sine wave and the higher harmonics are lower (its total voltage harmonics are below 1.4%); its disadvantages are that the transformer structure is complex, the efficiency is low, and the cost is high. In particular, the number of frequency conversion units used is more than 10 times that of this scheme, the failure probability depends on the number of components, and the cost is high. For example, a power plant in Fushun imported two sets of 6kV, 1250kW multi-stage frequency conversion devices for a total price of 5 million yuan, with a cost of 2,000 yuan per kW. See Comparison Table 4. [IMG=High-Low Voltage Variable Frequency Speed ​​Control Integrated Device and Its Application for Medium and High Voltage Motors]/uploadpic/THESIS/2008/1/2008012313295424473S.jpg[/IMG] (5) Conventional high-low voltage type uses transformer to step down the voltage and uses ordinary low voltage motors to achieve low voltage variable frequency speed control. Although the cost is lower, the frequency converter has large high-order harmonics (nearly 5%, the national standard requires that the 6-10kV power grid not exceed 4%), and the insulation of ordinary low voltage motors is fragile. Long-term operation at pulse frequency will accelerate insulation aging and even cause breakdown or burnout accidents. Moreover, the frame of a low voltage motor of the same capacity is generally one size smaller than that of a high voltage motor. It is inevitable that the foundation must be rebuilt or additional pads must be added due to the small base size and low shaft center height. It is also necessary to replace the coupling due to the thin shaft diameter. If a low-voltage variable frequency speed control motor is used instead, not only is the cost too high, but the frame of the low-voltage variable frequency speed control motor of the same capacity is also larger than that of the high-voltage motor. Due to the large base size and the problem of shaft center height, it is necessary to rebuild the foundation or add a base to the water pump. Furthermore, due to the shaft diameter problem, it is necessary to replace the coupling, and even affect the dynamic balance of the unit, which brings considerable trouble to the on-site modification! For example, in 1999, Qidaishan Iron Mine of Ansteel Xinsteel Co., Ltd. introduced a 690V, 1000hp (equivalent to 736kw) frequency converter with a variable frequency speed control motor, which cost RMB 1.6 million, with a cost of RMB 2174 per kW. See Comparison Table 5. [IMG=High-Low Voltage Variable Frequency Speed ​​Control Integrated Device and Its Application for Medium and High Voltage Motors]/uploadpic/THESIS/2008/1/2008012313300044617C.jpg[/IMG] (6) Internal Feedback Type It is an improved type of traditional cascade speed control. Both internal feedback (known internationally as the Cramer method) and external feedback (known internationally as the Scherbius method) are thyristor-based frequency conversion speed control methods based on wound-rotor motors (also known as slip-ring motors). The development of motor speed control has progressed from traditional DC speed control (due to the inconvenience of commutator, slip ring, and brush maintenance), to AC wound-rotor motor cascade speed control (where slip rings and brushes still present maintenance challenges), and now to AC squirrel-cage motor frequency conversion speed control (robust, durable, and easy to maintain). Internal feedback once played a pivotal role in technological advancement; however, with the maturity of high- and low-voltage squirrel-cage motor frequency conversion speed control technology and a significant price reduction, and with frequency converters evolving from first-generation thyristors, second-generation GTR and GTO, to third-generation IGBT and IGCT, the internal feedback thyristor frequency conversion method based on wound-rotor motors is now outdated and has lost its price advantage. Furthermore, dedicated internal feedback speed control motors of the same capacity have complex structures, low efficiency, and long manufacturing cycles. In addition, the internal feedback method also has disadvantages such as narrow speed regulation range (mostly 50% to 100% of the rated speed), narrow energy-saving range, large high-order harmonics (about 5%, exceeding the national standard limit of 4%), low power factor (even with compensation capacitors and chopper technology, it is only 0.9), and easy inverter failure. For example, a 6kV, 300kW water pump with internal feedback speed regulation motor at a water station of Fuxin Water Supply Company has been shut down due to frequent failures, and a project has been initiated to replace it with a variable frequency speed regulation device. See Comparison Table 6. [IMG=High-low voltage variable frequency speed regulation integrated device for medium and high voltage motors and its application]/uploadpic/THESIS/2008/1/20080123133022873591A.jpg[/IMG] 4. Technical characteristics of capacity-enhanced high-low voltage variable frequency speed regulation device 4.1 Structure and principle The principle of the capacity-enhanced high-low voltage variable frequency speed regulation integrated device is shown in Figure 1 and Figure 2. [align=center][IMG=Schematic Diagram of High-Low Voltage Variable Frequency Speed ​​Control Integrated Device]/uploadpic/THESIS/2008/1/2008012313302925440G.jpg[/IMG] Figure 1 Schematic Diagram of High-Low Voltage Variable Frequency Speed ​​Control Integrated Device[/align] [align=center][IMG=High-Low Voltage Variable Frequency Speed ​​Control Integrated Device for High Voltage Motors]/uploadpic/THESIS/2008/1/2008012313303318607J.jpg[/IMG] Figure 2 High-Low Voltage Variable Frequency Speed ​​Control Integrated Device for High Voltage Motors[/align] The capacity-enhanced high-low voltage variable frequency speed control device consists of a step-down transformer t with at least one side winding having an extended delta connection structure (maximizing the inner triangle to a △ shape), a low-voltage high-capacity frequency converter lf, a modified ordinary high-voltage motor hm through changes in winding connection and insulation structure to form a low-voltage variable frequency speed control motor hm′, and frequency converter/power frequency switching switches ct, cf, and cw. This achieves optimal voltage matching, increased power capacity, and optimized functional integration. The transformer t is a high-efficiency (efficiency above 98.8%), energy-saving, and maintenance-free type with a high-order harmonic absorption circuit. It reduces the high voltage to a suitable low voltage, which is then supplied by the high-capacity frequency converter lf, which is matched to the voltage. The rated operating voltage un′ of a high-voltage motor is determined by the following formula: un′＝un/kw (1) Where: kw—winding structure coefficient, determined by formula (2): kw=pe×le+p△×l△ (2) Where: un—rated voltage (V) of the high-voltage motor; pe—proportion of the winding extension section (%); le—number of parallel branches of the winding extension section, equal to the product of the parallel branches of the inter-pole windings in the extension section and the number of parallel wires of the inner-pole coils; p△—proportion of the △-connected section of the winding (%); l△—parallel branches of the △-connected section of the winding, equal to the product of the parallel branches of the inter-pole windings in the △-connected section and the number of parallel wires of the inner-pole coils. When pe is zero, the winding structure is △-connected, kw=×l△; when p△ is zero, the winding structure is y-connected, kw=1×le＝le. 4.2 Main Technical Specifications (1) Input Voltage: 3-phase 3～15.7kV±10%; (2) Output Power: 110～3000kw; (3) Output Frequency: 0～55hz; (4) Efficiency: ≥96%; (5) Power Factor: ≥0.95; (6) Total Harmonic Distortion (THD): Less than 2% (National Standard GB/T14549—93 "Power Quality - Harmonics in Public Power Grids" limits the value to 4%); (7) Starting Mode: Variable Frequency Soft Start/Power Frequency Reduced Voltage Start, Variable Frequency Starting Current is lower than the Rated Current of the Motor; (8) Operating Mode: Variable Frequency/Power Frequency Switching; (9) Control Mode: Manual/Automatic Switching; (10) Control Interface: PLC, IPC, DCS or FCS system control with reserved analog and digital interfaces: Analog control interface: 0 (4) ~ 20mA, 0 (1) ~ 5, 0 ~ 10V; Digital control interface: RS-485 communication interface and RS-232 computer interface. 5. Applicable fields of technology (1) Thermal power plants and cogeneration plants In high-voltage motors, power plants account for a considerable proportion. The power consumption of fans and water pumps alone accounts for more than 65% of the power consumption in the power plant, which is the first application field of this device. The main application equipment includes: pulverizer, blower (i.e., blower), induced draft fan (i.e., induced draft fan), coal mill; water supply pump, circulating pump, condensate pump, ash pump, etc. Taking the transformation of a 200,000 kW generator set as an example, it is advisable to focus on the transformation of three machines and four pumps. Specifically: Forced draft fans: mostly 1250kW dual units, operating primarily at low speeds with damper openings of only about 50%, offering energy-saving potential of over 60%, making them the first choice for retrofitting; Suction fans: mostly 1500kW dual units, also operating primarily at low speeds with damper openings below 60%, offering energy-saving potential of about 50%, making them the second priority for retrofitting; Dust collectors: mostly 440kW dual units with damper openings below 65%, offering energy-saving potential of approximately 40%; Water pumps: single pumps around 1500kW (applicable to this high-low voltage frequency conversion scheme), multiple pumps sharing a 2000kW or higher unit (this frequency conversion scheme currently has a maximum power of 3000kW), all offering energy-saving potential of over 30%; Circulating water pumps: also several hundredkW, offering energy-saving potential of approximately 40%. Condensate pumps: mostly dual units of about 355kw, with a power saving potential of about 40%; Ash pumps: mostly units of 500-630kw, with a power saving potential of more than 30%. (2) Self-owned power plants or thermal power plants in the metallurgical industry, same as above; water supply pumps in water plants; tailings pumps; other fans, water pumps, oil pumps; compressors. (3) Self-owned power plants or thermal power plants in the petroleum and chemical industry, same as above; medium and large oil pumps; water injection pumps, water supply pumps, and booster pumps. (4) Water supply pumps in the tap water industry, for example, a survey of nine water supply plants in Shenyang City showed that there was a need for variable frequency speed regulation energy-saving technology transformation. (5) Large and medium-sized cement plant exhaust fans in the building materials industry; large furnace exhaust fans. (6) The transformation of motorized ships into electric variable frequency ships in the shipbuilding industry can achieve both energy saving and environmental protection benefits. (7) High-voltage motors for large fans in other industries; high-voltage motors for large water pumps; high-voltage motors for large compressors; high-voltage motors for other large equipment. 6. Case Study Analysis The HLVF-6600/690V-710kW high-low voltage variable frequency speed control integrated device manufactured by the licensee of this patent—Anshan Quansheng Electronic Power Co., Ltd.—was tested at the series tailings pump of Qidaishan Iron Mine of Ansteel New Steel Co., Ltd. from December 2nd to 4th, 2003. It was found that the motor capacity (630kW) was insufficient. One reason was the increase in upstream production, and the other was the rise in the pump outlet pipeline, which made the original motor capacity insufficient. On December 29th of that year, the frequency conversion/power frequency switching test proved that the shaft power of the motor needed to be increased to at least 680kW to meet the current working conditions. Therefore, the original 6000V, 630kW motor was increased to 690V, 700kW, achieving the expected results. 6.1 Comparison of Test Data Before and After Motor Capacity Increase Based on the inspection certificate issued by Shenyang Electric Machinery Co., Ltd. on November 12, 2003, for the conversion of the original 6000V, 630kW motors of Units 5 and 6 to 750V, 630kW motors, and comparing it with the inspection certificate issued on March 11, 2004, for the conversion of the original 750V, 630kW motors of Units 5 and 6 to 690V, 700kW motors, the rated power increased by 11.11%, the no-load loss of the same power decreased by about 24%, and the test temperature decreased by 25℃. See Table 7. [IMG=High-Low Voltage Variable Frequency Speed ​​Control Integrated Device and Its Application for Medium and High Voltage Motors]/uploadpic/THESIS/2008/1/20080123133724226083.jpg[/IMG] 6.2 A comparison of operating data of the upgraded high-low voltage variable frequency speed control integrated device with that before the capacity expansion: The operating data of the upgraded high-low voltage variable frequency speed control integrated device during the transportation of tailings slurry from three systems and waste liquid from the emergency well on March 16-17, 2004, is compared with the data of the high-low voltage variable frequency speed control integrated device before the capacity expansion during trial operation from December 2-4, 2003, under similar operating conditions. Although the high voltage of the former was only 5800V, 3.3% lower than the latter, and the low voltage of the former was only 672-674V, 1.1%-2.3% lower than the latter, the maximum operating current of the motor increased by 9.7%-15%, the maximum speed increased by 0.6%-1.4%, and the maximum outlet pressure of the pump increased by 6.7% under the condition of a 4% increase in sand concentration. The transportation capacity was significantly improved and met the requirements of normal operating conditions. (See Table 8). [IMG=High-Low Voltage Variable Frequency Speed ​​Control Integrated Device for Medium and High Voltage Motors and Its Application]/uploadpic/THESIS/2008/1/2008012313373223740V.jpg[/IMG] The operating data of the upgraded high-low voltage variable frequency speed control integrated device on March 19, 2004, while transporting tailings slurry from three systems and wastewater from two pipeline maintenance lines, is compared with the data of the unupgraded high-low voltage variable frequency speed control integrated device operating under similar conditions at the end of December 2003. The upgraded device showed a 15% increase in the maximum operating current of the motor, a 4.4% increase in the maximum speed, and a 9% increase in the maximum pump outlet pressure under conditions with a 2% increase in sand concentration. The transport capacity is significantly improved, and it basically meets the needs of abnormal operating conditions. See Table 9 for details. [IMG=High-Low Voltage Variable Frequency Speed ​​Control Integrated Device and Its Application for Medium and High Voltage Motors]/uploadpic/THESIS/2008/1/2008012313373952216R.jpg[/IMG] 6.3 Comparison of Operating Data between the Expanded High-Low Voltage Variable Frequency Speed ​​Control Integrated Device and the Hydraulic Coupler The measured data of the expanded high-low voltage variable frequency speed control integrated device under heavy load conditions of tail slurry from three systems plus maintenance wastewater from two pipelines on March 19, 2004, is compared with the measured data of the hydraulic coupler under similar operating conditions on January 29, 2004. The expanded device showed that the maximum motor speed increased by 3.6%–6%, the maximum temperature rise decreased by 3℃–5℃, and the maximum pump outlet pressure increased by 6% under conditions with a 2% increase in sand concentration. See Table 10. [IMG=High-Low Voltage Variable Frequency Speed ​​Control Integrated Device and Its Application for Medium and High Voltage Motors]/uploadpic/THESIS/2008/1/2008012313375449706I.jpg[/IMG] Subsequently, on March 3, 2004, measured data were obtained under heavier load conditions with tail slurry from three systems plus maintenance wastewater from two pipelines and waste liquid from an emergency well. Compared with measured data obtained on January 29, 2004, with only tail slurry from three systems plus maintenance wastewater from two pipelines under heavy load conditions, the maximum speed of the motor increased by 1.6% to 5.2%, the maximum temperature rise decreased by 2℃ to 4℃, and the maximum outlet pressure of the pump increased by 6% under conditions with a 6% increase in sand concentration. As listed in Table 11. The maximum conveying capacity increased by 10%. [IMG=High-Low Voltage Variable Frequency Speed ​​Control Integrated Device for Medium and High Voltage Motors and Its Application]/uploadpic/THESIS/2008/1/200801231338016985023.jpg[/IMG] After a long period of normal production (referring to the simultaneous operation of 2-3 upstream production systems plus the emergency well waste liquid pump) and abnormal production (referring to the simultaneous discharge of waste liquid from two upstream maintenance pipelines in addition to normal production equipment), it can operate normally and has obvious capacity expansion and energy saving benefits. 7. Conclusion Through successful practice, the capacity-enhancing high-low voltage variable frequency speed control integrated device has the following advantages: (1) After modification by this patented technology, the power of high-power medium and high voltage motors can be increased by 1-3 power levels. (2) The efficiency of the high-efficiency system is greater than 96%. Among them, the efficiency of the frequency converter is 98%, and the efficiency of the transformer is 98.8%. (3) The power factor of the high-power system can reach more than 0.95. This scheme adopts PWM modulation, which can maintain a high power factor in any frequency band (speed band). (4) High reliability: First, the oil-free fully sealed transformer or dry-type transformer is maintenance-free; second, the low-voltage large-capacity frequency converter with mature technology is safe and reliable, and can also be maintenance-free with a service life of up to 70,000 hours; third, the high-voltage motor with high-level insulation is operated under low-voltage frequency conversion conditions, and the insulation life is infinitely long. During the motor voltage conversion process, a thorough maintenance and repair is carried out at the same time, such as dust removal, cleaning the bearings and applying lubricating oil, dynamic balance test and parameter test, and finally painting before leaving the factory. Fourth, it is equipped with a frequency conversion/power frequency switching switch so that it can switch to the original power frequency mode in case of frequency converter failure. This switching device does not increase the cost or the floor space. (5) Low harmonic voltage: The total harmonics can be lower than 1.6%. Because the open Y-shaped structure on both sides of the high/low voltage winding of this transformer is changed to an extended triangle with a closed loop (i.e., a partial △ structure, up to a fully enclosed △ structure), it is used to absorb the high-order harmonics from the frequency converter. With the addition of LC filtering and the multi-closed loop effect of the motor, the high-order harmonics on the high-voltage side can be significantly reduced. This practical case has been tested on-site multiple times by the Northeast Power Harmonic Testing Station. The total voltage harmonics on the high-voltage side are only 0.4% to 1.3%, which is far lower than the national standard limit of 4%. (6) Low cost The cost of the transformer, frequency converter and high-voltage motor conversion is very low, lower than the current price of all high-voltage frequency converters at home and abroad. The price per kW is less than 1,000 yuan; while the price per kW of high-voltage frequency converters at home and abroad is as high as 1,400 yuan to 3,000 yuan. (7) Low temperature rise When the high-voltage motor is running at the power frequency, the grid voltage is often too high, which causes the motor stator to over-excite and heat up; there are also cases where the grid voltage is too low, which causes the motor rotor slip to be too large and heat up. This frequency conversion scheme adopts a voltage system that matches the transformer, frequency converter, and motor, and takes measures to stabilize the voltage and frequency, thereby effectively reducing the operating temperature rise of the motor and further extending the overhaul period and service life of the motor. (8) Low-voltage safety This frequency conversion device implements a transformer and frequency converter isolation method, so the operating voltage of the frequency converter and motor is only a few hundred V, making maintenance and repair safe and convenient. (9) Short construction period The transformer manufacturing only takes 15 to 20 days; the frequency converter order only takes about 1 month; the motor voltage modification does not exceed 1 week; the control cabinet processing only takes 1 week; the installation (no need to rebuild the foundation, add pads, or replace the coupling) and commissioning only takes 3 days. The total construction period is about two months, so it is effective quickly. The construction period of other schemes is at least half a year to 1 year. (10) Short investment recovery period Because this scheme saves investment and is effective quickly, the investment recovery period is generally 1 to 2 years. It is shorter than the investment recovery period of other schemes at home and abroad. In summary, this high-low voltage scheme achieves the purpose of being superior to the high-voltage frequency conversion scheme at the cost of the low-voltage frequency conversion scheme.