1 Introduction
Currently, high-voltage variable frequency drive (VFD) energy-saving technology has been widely applied in the power industry. With 600MW and above units becoming the main type of thermal power generation, the capacity of high-voltage VFDs in VFD energy-saving retrofit applications such as induced draft fans, primary air fans, and condensate pumps is gradually increasing to over 2000kW. Equipment heat dissipation and operating environment issues in high-voltage VFD applications have become crucial factors directly affecting the reliability of the VFDs themselves and the safety and stability of the generating units. Furthermore, with the increase in installed capacity and power of VFDs, the investment and operating costs of their auxiliary cooling systems have gradually become an important aspect that cannot be ignored in project implementation. Adopting a suitable dedicated cooling system for high-voltage VFDs is of positive significance for improving equipment safety and stability, reducing the operating costs of auxiliary cooling systems, and maximizing the benefits of high-voltage VFD energy-saving projects.
2. Comparison of the two cooling schemes
In accordance with energy conservation and consumption reduction measures, a power plant undertook a frequency converter energy-saving retrofit of the 2240kW primary air fan system配套的2240kW for Units 3 and 4. Based on the rated power of the frequency converters (2240kW) and an efficiency of 96%, the rated heat generation of each high-voltage frequency converter is calculated as: qb = p × (1 - η) = 2240 × (1 - 96%) = 89.6kW. This means that to maintain the indoor environment of the frequency converters within the allowable operating temperature range, all the heat generated by the frequency converters must be carried outdoors to avoid heat accumulation inside the frequency converter room. Therefore, each frequency converter requires approximately 90kW of cooling capacity from the refrigeration or heat exchange system.
If air conditioning is used for cooling, 36 kWh of electricity will be consumed per hour for heat dissipation of the frequency converter itself. This not only requires a large investment in equipment but also sacrifices the electricity saved by the primary fan-driven frequency converter, making it neither economical nor in line with the original intention of frequency converter retrofitting. While duct cooling can save energy, it can lead to increased equipment maintenance and susceptibility to equipment failure due to dust on-site, affecting equipment operational safety.
After extensive research and evaluation, and considering the installation location and available space, air-water cooling system and closed-loop cooling system were adopted for the two Harvert series primary air fan high-voltage frequency converters, respectively. This not only ensures equipment operational safety but also solves the problem of high operating costs for cooling systems, achieving the goal of energy conservation and consumption reduction.
2.1 Air-Water Cooling Solution
The primary air fan of Unit 3a has a large site space, making it suitable for an air-water cooling system with a large space requirement.
2.1.1 System Introduction
The BLH-CK series air-water cooling system fundamentally solves the problems of high heat dissipation density and high power consumption, effectively improving system safety and reliability while reducing operating costs. This system is a highly efficient, environmentally friendly, and energy-saving closed-loop cooling system that can achieve 100% heat exchange cooling of the hot air exhausted from the top of the inverter cabinet. Due to its fully mechanical design, this system is simpler than other power and electronic equipment such as air conditioners, and offers significantly higher safety and reliability.
Its working principle is as follows: hot air from the frequency converter is directly passed through the air-cooling unit via a duct for heat exchange. The cooling water pipes inside the exchanger exchange the hot air through a non-contact process, directly carrying away the heat lost by the frequency converter and preventing it from heating the indoor environment. When the water supply pressure of the air-cooling unit is between 0.25 and 0.35 MPa and the cold water temperature does not exceed 33°C, the ambient temperature inside the frequency converter room can be controlled below 40°C after the hot air passes through the heat exchanger. This ensures a good operating environment for the frequency converter.
Meanwhile, due to the enclosed nature of the room, the frequency converter utilizes the indoor circulating air for equipment cooling, resulting in low dust concentration and minimal maintenance; this reduces the adverse impact of the environment on the operational stability of the frequency converter's power cabinet and control cabinet. The structural principle diagram of the air-water cooling system is shown in Figure 1.
The system has the following characteristics:
(1) The equipment is simple and quick to install;
(2) The equipment has a long service life, low failure rate, and reliable performance;
(3) Low operating costs. The operating cost of an air-water cooling system is only 1/5 to 1/8 of that of an air conditioning cooling system with the same heat exchange capacity;
(4) The frequency converter has a long maintenance-free cycle, low maintenance requirements, and good environmental hygiene.
2.1.2 Safety Performance Evaluation
The entire system is installed outside the high-voltage inverter room wall, with complete separation of cooling water and circulating air. Water pipelines are clearly separated from the high-voltage equipment outside the inverter room, ensuring the high-voltage equipment room is not subject to safety threats or accidents such as waterproofing or insulation damage. This avoids the serious accidents that could occur if the cooling water pipelines are laid out inside the high-voltage room, leading to leaks and endangering the safe operation of the high-voltage equipment. In the design of the air-water cooling system, to prevent condensation from the air-cooling unit's outlet from entering the room, the air outlet and air velocity of the air-water cooling unit are designed and calculated to ensure safe and stable operation under good pressure relief. Simultaneously, the cooling system provides fault alarm detection points for the fan and air-water cooling unit, which can be remotely transmitted to the DCS (Distributed Control System).
2.2 Closed-loop cooling scheme
Due to limited space available at the site for the primary air fan of Unit 3b, a closed cooling system with a smaller footprint is adopted.
2.2.1 System Introduction
The closed-loop cooling system uses R134a refrigerant as the heat exchange medium and utilizes a high-efficiency refrigeration compressor unit for heat transfer. The indoor heat exchanger is directly installed on the top of the inverter power cabinet. Hot air exhausted from the top of the cabinet passes through the heat exchanger, where the refrigerant directly carries the heat to the outside, where the compressor unit dissipates the heat into the atmosphere. This system ensures the inverter operates at a constant temperature of 30-35℃, significantly extending filter replacement cycles and reducing on-site maintenance. It is also unrestricted by installation space, location, or ambient temperature, exhibiting greater adaptability. The refrigeration compressor unit is installed near the inverter and can operate continuously in ambient temperatures ranging from -15℃ to +45℃. A schematic diagram of the closed-loop cooling system is shown in Figure 2.
Figure 2 Schematic diagram of a closed-loop cooling system
The system has the following characteristics:
(1) The sealed cooling and frequency converter are integrated into the design, resulting in a compact system structure and small size;
(2) It adopts an industrial high-efficiency scroll compressor unit, which is efficient, energy-saving and environmentally friendly;
(3) Low operating costs. The operating cost of a closed-loop cooling system is only 1/3 to 1/2 that of an air conditioning cooling system with the same heat exchange capacity;
(4) The equipment operates in a closed environment and the frequency converter is not affected by the outdoor environment.
2.2.2 Safety Performance Evaluation
The entire unit is installed on top of the inverter power cabinet. The cooler is designed with high airflow and low enthalpy difference, resulting in no condensation at the air outlet. The system is equipped with a refrigeration compressor unit and two independent power supplies. Depending on the inverter's operating power load, one or both compressors can be operated simultaneously, improving the safety and economic performance of the cooling device. In the event of a single power failure, the other power supply powers both compressors at 100% load, enhancing system safety and reliability. It also provides standard DCS signal interfaces for temperature, operating status, and fault alarms.
2.3 Comparison of two different cooling methods
A comparative analysis of the technical performance of two different cooling methods in the retrofitting of the primary air turbine frequency converter of a 600MW unit shows that, given sufficient space on site, an air-water cooling system is the best option, while a closed cooling system is an ideal cooling solution when space is limited.
3. Running effect
Figure 3 shows the temperature curves of the primary air fan systems of Unit 3 (a and b) operating under different unit load conditions using variable frequency drive (VFD) mode. As can be seen from the figure, the temperature of the VFD using the air-water cooling system varies between 22 and 38.5℃, significantly affected by the outdoor temperature difference and unit load; while the temperature of the VFD using closed-circuit cooling is relatively stable, fluctuating between 31 and 35℃, and less affected by the outdoor environment and unit load.
The actual power consumption of the cooling system for the two primary air inverters after the frequency converter retrofit of Unit 3 is shown in the attached table. Data analysis reveals that, under the same load conditions for the same unit, the inverter using the air-water cooling system consumes an average of 251 kWh less power per day than the inverter using a closed-loop cooling system, resulting in a 66% power saving.
Appendix: Statistics on the Actual Power Consumption of the Inverter Cooling System
Based on an annual operating time of 7000 hours, air conditioning cooling for the two frequency converters would consume 504,000 kWh of electricity; while the annual power consumption of the air-water cooling system for primary air fan frequency converter A is 37,000 kWh, and the annual power consumption of the closed-loop cooling system for primary air fan frequency converter B is 110,000 kWh. By adopting the new cooling method, the two frequency converters save 357,000 kWh of electricity annually, demonstrating significant economic benefits.
4. Conclusion
Since its commissioning in April 2009, the system has operated stably for over 16,000 hours without any over-temperature faults or abnormal inverter shutdowns. It has played a crucial role in ensuring the safe operation of the unit and minimizing the negative impact of the inverter on power generation. Practice has proven that the application of the two new cooling methods has significantly improved the safety and stability of high-voltage frequency converters and maximized energy-saving benefits. Furthermore, it provides valuable experience for future high-voltage frequency converter energy-saving retrofits.
Figure 3 Temperature curves of different unit loads during variable frequency operation.
