High-Voltage Inverter Retrofits in Power Plants

1 Basic Structure and Operating Mechanism of High-Voltage Inverters

1.1 Module Composition

• Rectifier Module: This module converts the input high-voltage AC power into DC power. The rectification section mainly consists of thyristors, diodes, or other power semiconductor devices to achieve the conversion from AC to DC. Additionally, through a control unit, voltage regulation and power compensation within a certain range can be realized.

• DC Filter Module: The rectified DC power is processed by a filtering circuit to smooth out voltage fluctuations, forming a stable DC bus voltage. This voltage not only provides energy support for the subsequent inverter stage but also plays a crucial role in ensuring output voltage stability and dynamic response capability.

• Inverter Module: The filtered DC power is converted back into AC power in the inverter module using power semiconductor devices such as IGBTs and pulse width modulation (PWM) technology. By adjusting the duty cycle and switching frequency of the PWM signal, the inverter can precisely control the amplitude and frequency of the output AC power, meeting the requirements of various loads such as motors, fans, and pumps. This technology enables the inverter to provide functions such as soft start, stepless speed control, optimized operating conditions, and energy savings.

1.2 Operating Mechanism

High-voltage inverters employ a cascaded multilevel topology, producing an output waveform that closely approximates a sine wave. They can directly output high-voltage AC power to drive motors. This configuration eliminates the need for additional filters or step-up transformers and offers the advantage of low harmonic content. The motor speed $n$ satisfies the following equation:

Where: $P$ is the number of pole pairs of the motor; $f$ is the operating frequency of the motor; $s$ is the slip ratio. Since the slip ratio is typically small (usually in the range of 0–0.05), adjusting the motor's supply frequency $f$ enables corresponding regulation of its actual speed $n$. The motor slip ratio $s$ is positively correlated with load intensity—the higher the load, the greater the slip ratio, resulting in a decrease in the motor's actual speed.

1.3 Key Factors in Technical Selection

• Voltage Matching: Select appropriate matching schemes such as "High-High" or "High-Low-High" based on the motor's rated voltage. For motors with power exceeding 1,000 kW, the "High-High" scheme is recommended. For motors below 500 kW, the "High-Low-High" scheme may be prioritized.

• Harmonic Mitigation: Harmonics are easily generated at the input and output terminals of high-voltage inverters. To reduce their impact, multiplexing techniques or additional filters can be employed. By properly configuring filters, harmonic distortion can be controlled within 5%, achieving effective harmonic suppression.

• Environmental Adaptability: High-voltage inverters require air-cooling or water-cooling systems to ensure the internal temperature of the control cabinet remains below 40°C. Dehumidifiers and air conditioning units are typically installed at inverter sites. In special areas without air conditioning, component temperature ratings must be considered during design, and ventilation capacity of cooling systems should be increased to ensure stable operation.

2 Application Example of High-Voltage Inverters in Power Plants

A power plant's power system typically includes equipment from turbine generators, boilers, water treatment, coal conveying, and desulfurization systems. The turbine section supplies power to feedwater pumps and circulating water pumps, the boiler section provides forced draft fans (primary fans), secondary fans, and induced draft fans, while the coal conveying section operates belt conveyors. By using high-voltage inverters for variable-speed control of these devices based on load variations, energy consumption can be reduced, auxiliary power consumption lowered, and operational economy improved.

A nickel-iron production project in Morowali, Indonesia, located on Sumatra Island, commissioned eight 135 MW generator units between 2019 and 2023. To further optimize internal operations and reduce production costs, technical retrofits involving the installation of high-voltage inverters were implemented between 2023 and 2024 for the condensate pumps of Units 1, 2, 3, 4, and 7, as well as the feedwater pumps of Units 2 and 5.

2.1 Equipment Status

The project employs a pyrometallurgical nickel-iron process with 25 production lines, equipped with eight Dongfang Electric DG440/13.8-II1 circulating fluidized bed boilers and eight 135 MW intermediate reheat condensing steam turbine generator sets. Each unit is configured with two fixed-frequency condensate pumps, two hydraulic coupler-regulated pumps, and six hydraulic coupler-regulated fans.

Feedwater pumps and fans are designed with redundancy, providing 10%–20% backup capacity. Units 5 and 6 operate in island mode with a load rate of approximately 70%. By optimizing motor speed to match actual load demands and incorporating regenerative braking energy feedback to the grid, unnecessary energy consumption from fans, pumps, and other equipment is reduced, further minimizing system energy losses.

2.2 Retrofit Scheme

Based on actual equipment operating conditions, high-voltage inverter retrofits were implemented for the feedwater and condensate pumps of the 135 MW generator sets.

• Feedwater Pump Retrofit: An "Automatic One-to-One" configuration was adopted, where each feedwater pump is equipped with a dedicated high-voltage inverter, including a bypass cabinet to ensure system reliability.

• Condensate Pump Retrofit: A "One-to-Two" configuration was implemented, where two condensate pumps share one high-voltage inverter, balancing efficiency and cost-effectiveness.

Considering the local historical maximum temperature range of 23–32°C, components were selected to operate at a 40°C ambient temperature. Additionally, the forced exhaust design of the inverter cabinet was adjusted based on a 40°C room temperature to ensure effective heat dissipation, eliminating the need for a dedicated inverter room or air conditioning systems.

2.3 Economic Benefit Evaluation

The total investment for this retrofit project was approximately 6 million RMB, including 5 million RMB for equipment, 400,000 RMB for construction, and 600,000 RMB for auxiliary materials provided by the client. Calculations show an annual energy-saving benefit of 6.58 million RMB, allowing the investment to be recovered in less than one year, successfully achieving the expected economic goals.

3 Conclusion

With the rapid development of high-voltage inverter technology, its applications have expanded rapidly across various industries. In power plant production systems, high-voltage inverter technology should be actively promoted. Priority should be given to retrofitting units with long operating hours or those urgently in need of upgrades, as such measures offer significant economic value and strategic importance.