1. Introduction Experts, scholars, entrepreneurs, and users from the frequency converter industry gathered to recount the remarkable achievements of my country's frequency converter industry over the past year. Although the Chinese frequency converter market is still dominated by the top ten brands from Europe, America, and Japan, my country's top ten frequency converter brands are rapidly emerging. High-level high-voltage frequency converters are now comparable to foreign brands, and their market share is increasing rapidly year by year. This is a cause for celebration within the frequency converter industry. However, we must also be aware that compared with products from Europe, America, and Japan, there is still a significant gap in the output, quality, variety, and scale of Chinese frequency converter manufacturers. Industry colleagues should take advantage of my country's vigorous promotion of building a resource-saving society and an innovative nation to narrow the gap with products from Europe, America, and Japan. With the in-depth application of frequency converters in various industries, remarkable results have been achieved, resulting in significant energy-saving effects, economic benefits, environmental benefits, and social benefits. Many projects focus on the specific module of frequency converters, neglecting the overall impact of the frequency conversion energy-saving system. They lack a clear understanding of the system's true characteristics and the frequency converter's role and function within the system. Consequently, in some projects, frequency converters merely mask the energy-saving effect of oversized pumps and fans, failing to fundamentally address the system's energy-saving issues theoretically. Alternatively, they may be reduced to a mere baffle, resulting in significant investment in frequency converters without maximizing their potential. This hinders the healthy development of my country's frequency conversion industry. I believe industry colleagues must prioritize the correct application of frequency converters. Achieving maximum efficiency through their application is the ultimate goal of frequency converter development. Only with correct, reliable, and smooth application can frequency converters attract a wider range of users, creating a significant market opportunity for growth and expansion. my country's annual energy-saving service market is worth approximately 120 billion yuan. Foreign giants have long coveted this market, and they have already taken significant steps. We can already hear the resounding footsteps of these giants. Colleagues, the situation is tense, and this opportunity is fleeting. Let us join hands and strive for the vigorous development of my country's frequency converter industry and for my country to achieve a moderately prosperous society by 2020! 2. The Role of Frequency Converters in Systems In practical applications, frequency converters are integrated into larger systems as subsystems for coordinated operation. For example, in the negative feedback control system shown in Figure 1, the frequency converter, along with the motor, fan, or pump, acts as the actuator of the control system, forming a complete control system together with sensors, regulators, and the controlled object. The performance of a control system depends on its structure, that is, the interrelationships, mutual influences, and mutual constraints among the four components of the control system. A system where each component is optimal locally is not necessarily optimal overall, and conversely, optimal overall system does not necessarily require optimal local components. Most frequency converter applications can be categorized into this operating mode. This means that if the four components of the control system are mismatched, even the best performance of the frequency converter will not be reflected. [align=center] Figure 1 The role of the frequency converter in the control system[/align] As can be seen from Figure 1, the control system composed of the frequency converter tracks the characteristic parameter θa, which is closely related to the characteristics of the system's changing process, in order to overcome the influence of interference f, rather than other parameters with weak correlation. I have seen such cases in engineering: in an air conditioning unit, the frequency converter does not track the characteristic parameter reflecting the load change, but tracks the resistance drop on both sides of the filter, which is unrelated to the load change. The frequency converter is useless and cannot play its role in tracking load changes and saving energy at all. The essence of the actuator link composed of the frequency converter, motor, and water pump is variable energy flow. What parameter changes is being tracked is something we should pay special attention to when designing variable frequency and variable flow energy-saving systems. 3. Analysis of energy-saving case of frequency converter application in thermal power plants Figure 2 is a diagram of the application scheme of frequency converter in thermal power plants. This scheme only highlights most of the components in the control loop consisting of the high-voltage frequency converter (HVIN), transformer (PT), and high-voltage motor (M), without explaining the position and role of the high-voltage frequency converter in the control system. Therefore, it is possible that the function of the high-voltage frequency converter will not be fully utilized to achieve the best energy-saving effect, or the frequency converter will be reduced to merely a baffle, without fulfilling its proper role. [align=center] Figure 2 Variable Frequency Drive Application Scheme in Thermal Power Plant[/align] In reality, the energy-saving effect achieved by frequency conversion energy-saving retrofit varies depending on the location of the fans and pumps in the power plant. Figure 3 is a schematic diagram of the production process of a thermal power plant. The motors configured for fans and pumps in thermal power plants are basically high-voltage motors exceeding 250kW, so high-voltage frequency converters are required for variable frequency and variable flow energy-saving applications. However, the target tracked by the control system composed of fans or pumps in different positions in the production process and the high-voltage frequency converter is different, resulting in different energy-saving effects. [align=center] Figure 3 Schematic Diagram of Production Process in Thermal Power Plant[/align] As shown in Figure 3, circulating water pump 8. Because the cooling water temperature fluctuates due to the influence of power generation load and climate, it affects the condensing pressure of condenser 4 and consequently the output and efficiency of turbine 3, which in turn affects the output and efficiency of generator 18. Therefore, the control system consisting of circulating water pump 8 and high-voltage frequency converter should track the condensing pressure of condenser 4 to overcome the impact of power generation load and climate change on power generation efficiency, maintaining the high-efficiency operation of the generator set under varying operating conditions. Simultaneously, since the control system can automatically adjust the cooling water volume according to power generation output and climate change, cooling pump 8 achieves high-efficiency operation of the power generation system with minimal energy consumption, thus maximizing its own energy savings. Similarly, the induced draft fan 17 in Figure 3, when analyzed, should be monitored by the control system consisting of it and the frequency converter to maintain a constant negative pressure in the boiler furnace 21, ensuring efficient combustion of the boiler under varying power generation load and atmospheric pressure changes, thereby maintaining high-efficiency operation of the power generation system under varying operating conditions. For example, consider the forced draft fan 15 in Figure 3. Analysis shows that its composite control system, consisting of the frequency converter, steam consumption sensor, and coal consumption sensor, tracks the air-to-coal ratio of boiler combustion to ensure efficient boiler combustion during power generation load changes and atmospheric pressure variations, thus maintaining the efficient operation of the power generation system. The variable frequency and variable flow control systems of the induced draft fan 17 and forced draft fan 15 are essentially constant pressure and variable flow systems, the most commonly used mode in fan and pump energy-saving projects. Although theoretically it can be proven that the constant pressure and variable flow energy-saving mode is not energy-efficient, if it conforms to the overall goal of efficient operation of the entire power generation system, achieving efficient operation of the entire power generation system at the lowest cost, the power generation system is still energy-efficient overall. It contributes to the overall energy-saving goal of the system, thus maximizing the role of the frequency converter and achieving excellent energy-saving results. Simultaneously, since the configured capacity of fans and pumps is generally larger than the actual demand capacity (at least 30%), the use of frequency converters can also mask this shortcoming, achieving a "cover-up" energy-saving effect. 4. Analysis of Energy-Saving Cases of Variable Frequency Drives in Water Injection Oil Production in Oilfields In the later stages of oilfield production, when oil reserves are nearly depleted, water injection oil production technology is widely adopted to improve oilfield productivity. Water injection oil production involves injecting a mixed aqueous solution treated with chemical raw materials such as scale inhibitors, clay stabilizers, corrosion inhibitors, and chemical displacement agents into the oil-bearing layer. The oil-bearing layer is flooded with water, and under the drive of the water, oil and water are carried out together. Figure 4 shows the comprehensive energy consumption flow diagram of water injection oil production in oilfields. P2 in Figure 4 is a high-pressure water injection pump, which uses variable frequency speed control for energy saving. [align=center] Figure 4 Comprehensive Energy Consumption Flow Diagram of Water Injection Oil Production in Oilfields[/align] Water Injection Oil Production Rate: Daqing (4.0 m³ water/ton crude oil), Jingbian Local Oilfield (17.4 m³ water/ton crude oil). Each water injection well controls approximately 0.3–1.2 km² of oil-bearing area, approximately 2 production wells, and an injection volume of 30–50 m³/d. Because oil-bearing reservoirs vary greatly in geological structure, their water permeability coefficients differ significantly. Furthermore, the water permeability coefficient changes with the duration of oil extraction. Therefore, from the perspective of the water injection pump loop analysis, the load resistance characteristics of the water injection pump change with the different geological structures of the oil-bearing reservoir and the duration of oil extraction. For a given oil extraction area, the amount of water injected per unit weight of crude oil remains relatively constant over a certain period. Therefore, the system's target should be to maintain a constant water injection flow rate to ensure the oil recovery rate during water injection oil extraction. However, as the water permeability coefficient of the oil-bearing reservoir changes, the operating point of the water injection pump also changes. To ensure the water injection pump operates at its highest efficiency under any conditions and at any frequency, from a systems perspective, a valve should be added at the pump outlet as a compensation link to adjust the pump's operating point. It's important to understand that a smaller pump loop resistance does not necessarily mean less power consumption; rather, the loop resistance must match the pump's characteristics to ensure the pump operates at its highest efficiency point. Combined with the variable frequency drive (VFD) control system's feedback control that tracks the constant water injection flow rate, the energy-saving performance of the VFD system can be maximized. As shown in Figure 4, implementing variable frequency energy-saving control for the water injection pump is only one aspect of the water injection oil production energy-saving system. Other environmental protection and energy-saving projects should also be considered. For example, the oily water after oil-water separation should be treated by a wastewater treatment plant and then recycled. Otherwise, the indiscriminate discharge of oily wastewater could leave behind only a barren, polluted wasteland after the oilfield's lifespan ends. Water recycling can also reduce the extraction of groundwater and alleviate the burden on underground aquifers. Furthermore, the treated wastewater is estimated to be 30-40°C and contains 23.3-34.9 kWh of recoverable heat per ton. This heat can be used for heating hot water or heating systems. 100 m³/h of return water can supply the heating needs of approximately 200-300 households in a 20,000-30,000 m² area. If this untreated hot water is injected back into the ground, this heat is wasted. Adding a heat pump heat recovery unit CH1 can achieve heat recovery. The above analysis shows that the control system composed of frequency converters appears as an organic whole. Only when this whole operates in a coordinated manner and constantly conforms to the overall goal of the larger system can the function of the frequency converter be maximized and significant energy-saving effects achieved. "A system is the basic form and fundamental attribute of the material world." A system is a living, unified, organic whole, not a cold, impersonal collection of machines. Systems possess biological attributes—order, harmony, nature, and balance—and have life processes. We should follow the objective law that the world moves according to a systemic pattern and apply the concept of "systems thinking" to address the energy-saving issues of variable frequency and variable flow systems. Only by firmly establishing an awareness of "systems thinking" that conforms to objective laws can we clearly define the correct role and function of the frequency converter within the system, integrating it into the system and forming an organic whole with its "neighbors." Only then can the function of the frequency converter be maximized, and the variable frequency and variable flow system achieve the greatest energy-saving effect. The variable frequency and variable flow energy-saving systems we encounter are generally complex large systems. The concept of "systems thinking" is the only correct and feasible approach to handling today's complex large-scale system engineering. I believe that as time goes by and more practical cases of analyzing frequency converters using the concept of "systems thinking" increase, the concept of "systems thinking" will soon be accepted and familiar to people, and will play a significant role in my country's energy-saving service market. 5. Conclusion (1) The application of frequency converters is integrated into the system. Frequency converters, motors, and water pumps together form the actuator link of the control system. Considering the application of frequency converters solely from the perspective of the frequency converter professional module is not conducive to the healthy development of the frequency converter industry. (2) The energy-saving effect of frequency converters depends on the structure and performance of the overall system composed of frequency converters. Considering the problem solely within the scope of the frequency converter professional module, frequency converters cannot exert their ultimate function. We should follow objective laws and promote the application of frequency converters using the concept of "systems thinking".