Comparative Analysis of Two Different Cooling Methods in High Voltage Variable Frequency Applications

Abstract This paper introduces application examples of two different cooling methods in the frequency conversion energy-saving retrofit of the primary air turbine of a 600MW unit. A comparative analysis of the technical solutions and economic benefits of the air-water cooling system and the closed-loop cooling system is conducted, illustrating their differences and characteristics. This provides important and referable experience for reducing cooling system energy consumption and improving system safety and reliability in high-voltage frequency conversion energy-saving retrofits.

Keywords: high-voltage frequency converter; air-water cooling system; closed-loop cooling system

0 Introduction

Currently, high-voltage variable frequency drive (VFD) energy-saving technology has been widely applied in the power industry. With 600MW and above generating units becoming the mainstay 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 a significant aspect of 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.

1. 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. That is, 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 per hour will be needed to cool the inverter itself. This not only requires a large investment in equipment but also sacrifices the electricity saved by the primary fan inverter; it is neither economical nor in line with the original intention of inverter retrofitting. Using duct cooling can save energy; however, due to dust on site, it will lead to increased equipment maintenance and a higher risk of equipment failure, affecting the safe operation of the equipment.

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.

1.1 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.

1.1.1 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 duct to the air-cooling unit for heat exchange. The cooling water pipes inside the exchanger exchange the heat with the hot air through a non-contact process, directly carrying away the heat lost by the frequency converter; thus preventing the frequency converter 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 ℃, the ambient temperature inside the frequency converter room can be controlled below 40 ℃ after the hot air passes through the exchanger. This ensures a good operating environment for the frequency converter.

Meanwhile, due to the enclosed room, the frequency converter utilizes the indoor circulating air for equipment cooling, resulting in low dust levels and minimal maintenance; this also reduces the adverse effects 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 as follows:

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)     Frequency converters have long maintenance-free cycles, require minimal maintenance, and are environmentally friendly.

1.1.2 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 pipeline is located inside the high-voltage room, potentially leading to leaks and jeopardizing 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 calculated and designed 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 ).

1.2 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.

1.2.1 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, with exhaust fans on the top.

The hot air exiting the system passes through a heat exchanger, where the refrigerant directly carries the heat outdoors; the compressor unit then dissipates the heat into the atmosphere. This system ensures that the inverter operates at a constant temperature of 30-35℃, significantly extending the filter replacement cycle and reducing on-site maintenance. Furthermore, it is unaffected by on-site installation space, location, or environment. It has stronger adaptability to temperature and other conditions. The refrigeration compressor unit is installed near the frequency converter, allowing it to operate continuously in ambient temperatures ranging from -15°C to +45°C.

The system has the following characteristics:

1)     The system features an integrated design with sealed cooling and frequency converter, resulting in a compact structure and small size.

2)     It adopts an industrial-grade 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.

1.2.2 Safety performance evaluation

The entire unit is installed on top of the inverter power cabinet. The cooler is designed with high air volume 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. One or both units can be operated depending on the inverter's operating power load, 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.

1.3 Summary

A technical comparative analysis of the application 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 closed-loop cooling is an ideal solution when space is limited.

2   Running effect

The temperature curves of the primary air fan systems A and B of Unit 3 operating under different unit load conditions using variable frequency drive (VFD) mode are shown in the figure below. 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; the temperature of the VFD using closed-loop 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 below. Data analysis reveals that, under the same load operating conditions for the same unit, the power consumption of the cooling system using… Inverters with air-water cooling systems use an average of 251 kWh less per day than inverters with closed-loop cooling systems, resulting in a 66% energy saving.

Based on 7,000 hours of operation per year, air conditioning cooling for both inverters would consume 504,000 kWh of electricity; while the annual power consumption of the air-water cooling system for primary air fan inverter A is 37,000 kWh, and the annual power consumption of the closed-loop cooling system for primary air fan inverter B is 110,000 kWh. By adopting the new cooling method, the two inverters save 357,000 kWh of electricity annually, demonstrating significant economic benefits.

3. Conclusion

Since its commissioning in April 2009, the system has operated stably for over 16,000 hours without any over-temperature faults or inverter outages. 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 these 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.

About the Author

Lin Zhimou (1964–), male, has long been engaged in the management of electrical equipment in thermal power plants and the technological transformation of high-voltage electrical energy conservation.