Problem Statement The #2 generator unit (125MW) at the Douhe Power Plant of China Datang Corporation is a peak-shaving unit, operating at approximately 70-80% load. Two induced draft fans utilize inlet dampers for regulation. To ensure safe and stable operation, the selected fan motors have a large spare capacity. During full-load operation, the inlet damper opening of the induced draft fans is approximately 60%, while during peak-shaving, it is around 40%, resulting in significant energy loss and low fan efficiency. To further adapt to the power system of separate power plant and grid operation and competitive bidding for grid connection, save energy, reduce plant power consumption, protect the environment, simplify operation, and reduce wear on rotating equipment, our company decided to adopt high-voltage frequency converter speed control devices for some fans and pumps at the Douhe Power Plant, Xiahuayuan Power Plant, and Zhangjiakou Power Plant. Our company conducted an international open tender for high-voltage frequency converters. Beijing Leadway Electric Technology Co., Ltd. was the only domestic winning bidder, securing the contract for 8 high-voltage frequency converters. The No. 2 boiler at the Douhe Power Plant has installed a 6 kV/1000 kW high-voltage frequency converter (VFD) manufactured by Beijing Lidehuafu Electric Technology Co., Ltd. on each of its two induced draft fan motors. 1. Principle of the HARSVERT-A06/105 High-Voltage Frequency Converter The VFD adopts multi-level series technology. The 6 kV system structure is shown in Figure 1, consisting of a phase-shifting transformer, power units, and a controller. The 6 kV series has 21 power units, with 7 power units connected in series to form one phase. Each power unit has the same structure and electrical performance and is interchangeable. Its circuit structure is shown in Figure 2, which is a basic AC-DC-AC single-phase inverter circuit. The rectifier side is a diode three-phase full-bridge. By applying sinusoidal PWM control to the IGBT inverter bridge, the waveform shown in Figure 3 can be obtained. [align=center] [/align] Each power unit is structurally identical and interchangeable. Its circuit structure is shown in Figure 2, a basic AC-DC-AC single-phase inverter circuit. The rectifier side is a diode three-phase full-bridge. By applying sinusoidal PWM control to the IGBT inverter bridge, the waveform shown in Figure 3 can be obtained. The input side is powered by a phase-shifting transformer, whose secondary winding is divided into three groups, forming a 42-pulse rectification method. This multi-stage phase-shifting superposition rectification method can greatly improve the grid-side current waveform, making its grid-side power factor close to 1 under load. Furthermore, due to the independence of the transformer's secondary winding, the main circuit of each power unit is relatively independent, and each power unit is equivalent to a single-phase low-voltage frequency converter. The output side is directly powered to the high-voltage motor by connecting the U and V output terminals of each unit in a star connection. By recombining the PWM waveform of each unit, a stepped sinusoidal PWM waveform as shown in Figure 4 can be obtained. This waveform has good sinusoidal properties and a small dv/dt, which reduces damage to the insulation of cables and motors. It eliminates the need for output filters, allowing for long output cables and eliminating the need for derating motors, making it suitable for direct retrofitting of older equipment. Simultaneously, it significantly reduces motor harmonic losses, eliminating mechanical vibrations and reducing mechanical stress on bearings and blades. When a unit fails, the soft-switching node K in Figure 2 can be turned on, bypassing the unit from the system without affecting the operation of other units. The inverter can continue to operate at a derating rate, reducing losses caused by downtime in many situations. 2. Introduction to the Variable Frequency Drive Retrofit Scheme : The #2 boiler induced draft fan consists of two units arranged on both sides. Currently, the output of the induced draft fan is adjusted manually by adjusting the baffles. Due to the large redundant power in the design of the induced draft fan, coupled with the resistance loss caused by the baffles used for airflow control, the plant's power consumption is high, affecting the economic operation of the unit. Motor Parameters & Exhaust Fan Parameters: Model: Y1000-8; Model: G4-73-11-28D; Rated Power: 1000kW; Rated Air Volume: 455000m³/h; Rated Voltage: 6kV; Rated Air Pressure: 6460Pa; Rated Current: 119A; Fan Speed: 742rpm; Rated Frequency: 50Hz; Rated Speed: 743r/min. To ensure system reliability, a power frequency bypass device is installed on the frequency converter. In case of frequency converter malfunction, the frequency converter stops running, and the motor can be manually switched to power frequency operation. The power frequency bypass consists of three high-voltage disconnect switches QS1, QS2, and QS3 (see figure, where QF is the client's existing high-voltage switch). QS2 and QS3 must not be closed simultaneously, achieving mechanical interlocking. During frequency converter operation, QS1 and QS2 are closed, and QS3 is open; during power frequency operation, QS3 is closed, and QS1 and QS2 are open. To achieve inverter fault protection, the inverter is interlocked with the 6KV switch QF. In the event of an inverter fault, the inverter trips QF, requiring the client to appropriately modify the QF's closing and opening circuits. During power frequency bypass, the inverter should allow QF to close, canceling the trip signal to QF, allowing the motor to start normally via power frequency after QF is closed. 3. Comparison of Inverter Debugging Data The measurement results of relevant parameters by the China Electric Power Research Institute are as follows: [align=center]Table 1 Comparison of Comprehensive Input Power of Two Fan Systems under Power Frequency Baffle Adjustment and Inverter Speed ​​Regulation[/align] The test results show that the energy saving rate is 59% at 72% load and as high as 46% at full load. Simultaneously, during motor inverter starting, the starting current rises steadily, and the motor starts very smoothly. 4. Calculation of Benefits After Frequency Conversion Retrofit 1) At full load throughout the year, with 2 frequency converters in operation, the estimated annual electricity savings are: 520kW * 5500h = 2,860,000 kWh. The minimum annual electricity cost savings are: 2,860,000 kWh * 0.326 yuan/kW.h = 932,000 yuan. 2) At 72% load throughout the year, with 2 frequency converters in operation, the estimated annual electricity savings are: 571kW * 5500h = 3,140,500 kWh. The minimum annual electricity cost savings are: 3,140,500 kWh * 0.326 yuan/kW.h = 1,024,000 yuan. It can be seen that, under full load or 72% load operation throughout the year, with the installation of 2 domestically produced high-voltage frequency converters manufactured by Beijing Leadway Electric Technology Co., Ltd., our company can save approximately 1 million yuan in electricity costs annually. Furthermore, the power factor of Beijing Leadway Electric Technology Co., Ltd.'s series of frequency converters can reach over 0.95, exceeding the motor power factor of 0.85, thus significantly reducing reactive power. It also enables soft starting of the motor, avoiding the impact on motor insulation caused by high-current starting surges, reducing motor maintenance, saving repair and maintenance costs, and significantly extending motor lifespan. 5. Conclusion High-voltage frequency converters, due to their significant energy-saving effects, achieve soft starting of the motor after adopting frequency conversion speed regulation, extending motor lifespan. Full opening of the induced draft fan dampers also reduces duct vibration and wear. In summary, the reliable operation and excellent energy-saving effects of domestically produced high-voltage frequency converters, represented by Beijing Leadway Electric Technology Co., Ltd., have created significant economic and social benefits for our company, making them worthy of strong recommendation and application.