In recent years, with technological advancements, the coal industry has proposed a strategy to build smart mines, leading to technological upgrades in complete sets of coal mining equipment. Mining explosion-proof and intrinsically safe frequency converters, as a type of power electronic drive automation equipment, are widely used in the upgrading and transformation of traditional coal mines into intelligent mines due to their excellent soft-start and soft-stop, variable frequency stepless speed regulation, multi-machine synchronous control, and multi-machine torque and power balance performance. This has greatly improved the automation and intelligence level of coal mine equipment. Previously, in 10 kV belt conveyors in underground coal mines, the 10 kV explosion-proof motors were typically driven by hydraulic couplers, CST (Continuous Speed Control), etc. This method had a low level of automation and intelligence, frequently resulting in internal liquid leakage, metal component wear, motor burnout, and the inability to balance power when multiple machines were running. Maintenance costs remained high, seriously hindering the improvement of coal mine intelligence and production efficiency. Therefore, finding excellent technical solutions to these problems was urgently needed. In 2018, a special project was launched to tackle this problem, resulting in the development of a high-power-density, explosion-proof, and intrinsically safe high-voltage substation frequency converter for mining, which can be used on 10 kV belt conveyors in coal mines, effectively solving the aforementioned problem.
1. Overall Design
The main technical parameters of the BPBJV-1600/10 high-voltage substation frequency converter, which is used in 10 kV belt conveyors in underground coal mines and is both explosion-proof and intrinsically safe, are as follows: rated input voltage 10 kV, rated power 1600 kW, rated output current 117 A, output voltage 0～10 kV, output frequency 0～50 Hz, overload capacity 150%/1 min, and explosion-proof type is both explosion-proof and intrinsically safe for mining.
After project approval, a survey was conducted on the current technical status of 10 kV belt conveyors and intrinsically safe explosion-proof and intrinsically safe high-voltage substations and frequency converters used in coal mines. Following analysis and summarization, a high-power-density 10 kV input/output explosion-proof and intrinsically safe high-voltage substation and frequency converter scheme with a built-in rectifier phase-shifting transformer was proposed for the first time. This scheme integrates the phase-shifting rectifier transformer and the inverter into a single explosion-proof enclosure, and improves power density through the use of water-cooling technology and composite power unit technology. Ultimately, a design requirement of 1600 kW rated power and 2400 kW overload power was achieved within a space with external dimensions not exceeding 4395 mm × 1560 mm × 1830 mm.
2. Explosion-proof and intrinsically safe design for mining applications
This product was developed specifically for use in underground coal mines. The underground coal mine environment is complex, containing flammable and explosive gases such as methane, creating an environment prone to explosion. Space is limited underground, severely restricting the size of the frequency converter. Coal dust, humidity, and even water seepage are also present. Furthermore, relevant national authorities have established stringent standards for electrical equipment used in underground coal mines. These environmental conditions and factors render surface-mount 10kV high-voltage frequency converters unsuitable for the application requirements, and these factors are the key challenges that this development project aimed to overcome. The product's cabinet utilizes planar explosion-proof and cylindrical explosion-proof technologies. The main circuit input and output terminals employ advanced 10kV high-voltage cable connector technology. The control ports feature intrinsically safe design, and the cabinet protection meets IP54 standards. Through finite element analysis and testing, this product meets and exceeds the requirements of national standards GB3836.1, GB3836.2, and GB3836.4.
3. Main circuit topology
Because the technological level of power electronic devices cannot adequately resolve the contradiction between their power and voltage withstand capabilities and switching frequency, in order to develop high-frequency, high-voltage, high-performance, and low-EMI high-power converters, it is necessary to comprehensively consider high-performance power electronic devices, main circuit topology, and the control strategy of the system in which the converter is located in order to find a solution. Multilevel structure has become one of the representative and relatively ideal solutions [1]. Common voltage-type multilevel frequency converters mainly have three basic topologies: diode-clamped, capacitor-bridging, and unit-cascaded with independent DC power supply. The mining explosion-proof and intrinsically safe high-voltage substation frequency converter BPBJV-1600/10 developed in this study adopts the unit-cascaded type, as shown in Figure 1.
In Figure 1, T represents the rectifier-phase-shifting transformer; A1–A4, B1–B4, and C1–C4 are composite power units; KM1 and KM2 are output contactors. The rectifier-phase-shifting transformer has a capacity of 2000 kVA, including one primary winding, one auxiliary power supply winding of 1140 V, and 24 windings with a rated voltage of 690 V. The composite power unit has a 6-phase input and single-phase output, with an input line voltage of 690 V and an output line voltage of 1450 V. The outputs of power units A1–A4 are connected in series to form phase A, with a phase voltage of 5800 V; the outputs of power units B1–B4 are connected in series to form phase B, with a phase voltage of 5800 V; and the outputs of power units C1–C4 are connected in series to form phase C, with a phase voltage of 5800 V. Then, connect L1 terminals of A4, B4, and C4 together to form the midpoint of the inverter. L2 terminal of unit A1 serves as the A-phase output of the inverter, L2 terminal of unit B1 serves as the B-phase output of the inverter, and L2 terminal of unit C1 serves as the C-phase output of the inverter. The output line voltage is 10 kV. By controlling KM1 and KM2, the inverter output can have two drive modes: one inverter driving one motor and one inverter driving two motors. This structure employs a high-low-high power conversion method. The rectifier phase-shifting transformer converts the input three-phase 10 kV voltage into multiple sets of three-phase 690 V voltages, which serve as the input to the power units. The single-phase 1450 V voltage output from the power units is then cascaded across the outputs of each phase power unit, ultimately resulting in an inverter output line voltage of 10 kV. The power units are designed as a 6-phase inverter module with an input line voltage of 690 V and a single-phase output of 1450 V. Devices with a rated voltage of 1700 V are sufficient to meet the design requirements. If a multi-level structure is not used, a series connection of devices must be employed to address the voltage withstand capability issue. Currently, the manufacturing level of single power switching devices, such as IGBT modules, is only 6500 V withstand voltage, which is clearly insufficient for direct 10 kV voltage conversion.
7. Experiment
In November 2018, the prototype was completed. The Shanghai National Mining Equipment Testing and Inspection Center for Production Safety, in accordance with the "Implementation Rules for the Review and Issuance of Safety Marks for Mining Products – High-Voltage Frequency Converters" and enterprise standards, conducted temperature rise tests, constant torque tests, 150% overload tests, electromagnetic compatibility tests, power quality tests, and overall protection function tests at the Shenzhen Kumak Motor Loading Laboratory. All test indicators fully met the requirements. Temperature rise test data is shown in Table 1, constant torque test data is shown in Table 2, and the frequency converter output waveform during rated load testing is shown in Figure 4. Other tests (such as alternating damp heat tests, explosion-proof shell related tests, and lightning strike tests) were completed at the Shanghai National Mining Equipment Testing and Inspection Center for Production Safety. On February 27, 2019, an explosion-proof certificate was obtained; on March 20, 2019, a mining product safety mark certificate was obtained. In April 2019, this frequency converter was applied to a coal mine for industrial operation. It operated well, exhibited excellent speed regulation performance, and improved the automation and intelligence levels of the belt conveyor, receiving unanimous praise from the customer.
8. Conclusion
The newly developed explosion-proof and intrinsically safe high-voltage substation frequency converter, suitable for 10 kV underground coal mines, has passed type testing and demonstrated excellent speed regulation performance, stability, and reliability. Since its application in a coal mine in April 2019, this frequency converter has achieved excellent soft start and stop, stepless speed regulation, multi-machine synchronous control, and multi-machine torque and power balance, greatly improving the automation and intelligence level of coal mine equipment. It plays a positive role in ensuring safe coal mine production, reducing the labor intensity of coal miners, saving energy and reducing emissions, and realizing the construction of intelligent mines with less manned or even unmanned fully mechanized mining, and has broad application prospects.
Disclaimer: This article is a reprint. If it involves copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
