[Abstract] This article analyzes the advantages and applications of variable frequency speed control technology in various aspects, and provides a reference for the future development of the power system by looking at its future development trend. With the development of the power industry and the construction of more and more power plants, electricity consumption accounts for a considerable proportion of my country's total energy consumption each year. In order to save electricity and avoid resource waste, new methods must be adopted. After long-term development, computer technology has gradually matured and penetrated into every industry. At the same time, automatic control technology has also made corresponding progress. Against this background, AC speed control technology has emerged, solving many problems that DC speed control could not solve. Variable frequency speed control technology is a fundamental component of this technology and plays an irreplaceable role in the power system.
1. Overview of Variable Frequency Speed Control Technology
AC variable frequency speed control technology developed rapidly in the 20th century. This is related to breakthroughs in several key technologies, such as vector control technology for AC motors, direct torque control technology, and fully digital control technology based on microcomputers and large-scale integrated circuits.
(1) Vector control technology
Vector control technology, proposed by Siemens in 1971, is a novel control concept and theory. It uses rotor field orientation and vector transformation to decouple the stator current excitation component and torque component, achieving separate control of the flux linkage and current of the AC motor, thus obtaining excellent static and dynamic performance. To date, vector control technology has made significant progress and is widely used.
(2) Direct Torque Control Technology
Following the emergence of vector control technology, in 1985, M. Depenblock of Germany proposed a new high-performance variable frequency speed control technology—Direct Torque Control (DTC). Compared with vector control technology, DTC technology has higher performance, uses an electronic field orientation, does not require decoupling current, and directly controls the motor flux linkage and torque to achieve rapid torque response. Moreover, motor and rotor parameters have little impact on DTC technology, its working principle is relatively simple and easy to master, and its prospects for further development and application are quite broad.
(3) Digital control technology
With the further advancement and development of science and technology, digital control technology has gradually become the mainstream, aligning with the current trend of development. Early vector control and direct torque control technologies could not meet market needs to a certain extent, leading to the emergence of digital control technology. Digital control technology can perform rapid calculations and achieve excellent control precision, significantly reducing operating noise and shortening operating time. Furthermore, frequency converters using digital control technology are significantly smaller, improving information processing efficiency and achieving effects that were previously impossible with manual and analog technologies.
(4) PWM Control Technology In 1964, A. Schnung et al. in Germany first proposed the idea of pulse width modulation (PWM) frequency conversion, opening up a new development field for modern AC speed control systems. PWM control technology controls the amplitude of the inverter's output AC fundamental voltage by changing the width of the rectangular pulse, and controls its output frequency by changing the modulation period, thus simultaneously controlling the output voltage amplitude and frequency on the inverter. PWM technology simplifies the inverter structure, significantly improves the inverter's output waveform, reduces motor harmonic losses, and reduces torque ripple, while also improving the system's dynamic response performance. PWM technology can also be used for rectifier control, enabling the input current to be very close to a sine wave and achieving a power factor of 1 in the grid. PWM rectifiers are therefore called "green" converters. Currently, PWM technology has become the most widely used control technology in frequency converters, and the continuous improvement of AC motor speed control performance is largely due to the continuous advancement of PWM technology. Currently, the widely used methods are the quasi-optimized PWM method developed on the basis of regular sampling PWM, namely the third harmonic superposition method and the voltage space vector PWM method. In terms of frequency converter circuit topology, AC-DC-AC frequency converters and matrix frequency converters based on dual PWM technology represent the latest development trend in frequency conversion speed control technology.
2. Future Development Trends of Variable Frequency Technology
(1) Indirect High-Voltage Frequency Converter The indirect high-voltage frequency converter, also known as a high-low-high type frequency converter, consists of input and output transformers and a low-voltage frequency converter. The input transformer is a step-down transformer, which reduces the high-voltage power supply to the voltage allowed by the frequency converter. After passing through the low-voltage frequency converter, it is then stepped up by the output transformer, i.e., the step-up transformer, before being supplied to the high-voltage motor. Because the high-low-high type high-voltage frequency converter undergoes two voltage transformations, it increases energy loss, affects energy-saving performance, occupies a large area, and generates a large number of high-order harmonics, exhibiting significant drawbacks. Due to its relatively low technical difficulty and relatively low investment, it is generally suitable for high-voltage motors with a power output less than 200kW.
(2) Direct high voltage frequency converter
Unit-series multilevel high-voltage frequency converters generally employ multiplexing technology. Multiplexing technology uses several low-voltage PWM power units connected in series to achieve direct high-voltage output, as shown in Figure 1. Each power unit is powered by a multi-winding isolation transformer, and control and communication are achieved using a high-speed microprocessor and optical fiber. This technology was invented and patented by Robicon Inc. in the United States and named the "Perfect Harmonic-Free Frequency Converter." This technology fundamentally solves the harmonic problems generated by general 6-pulse and 12-pulse frequency converters, achieving perfect harmonic-free frequency conversion. It features low harmonic pollution to the power grid, high input power factor, no need for input harmonic filters and power factor compensation devices, and eliminates problems such as additional motor heating, torque pulsation, noise, and common-mode voltage caused by harmonics. Its output voltage is 2kV, 3kV, and 6kV, and its power is 800~5600kW. It is suitable for auxiliary equipment applications in power plants with power outputs above 1MW. Its disadvantages are high cost, large space occupation, and difficult installation.
(3) In terms of switching devices, IGBT frequency converters have become the mainstream of variable frequency speed control technology in the 1990s and will remain the dominant frequency converter in the field of electrical drives for a considerable period of time in the early 21st century. The integration of power conversion, driving, detection, control and protection functions has promoted the intelligence of power devices and frequency converters. At the same time, new functional frequency converters using new power electronic devices such as IGBT, IEGT (integrated emitter gate thyristor), GaAs (gallium arsenide), SiC (silicon carbide composite device), light-controlled IGBT and superconducting power devices will be further researched and developed.
Three-level high-capacity frequency converters employing high-voltage IGBTs and IGCTs commonly use IGBTs, GTRs, and GTOs as switching devices. Due to manufacturing limitations and raw material constraints, these devices are difficult to directly apply to 6kV voltages. In recent years, many countries have begun researching and developing new materials and high-voltage-resistance devices. ABB and Siemens have developed high-voltage-resistance switching devices, such as ABB's IGCT (field-controlled transistor) with a withstand voltage of 39kV, and Siemens' HV-IGBT with a withstand voltage of 56kV.
(4) Siemens, ABB, GE, and Cegelec have each developed their own high-voltage frequency converters using specially designed high-voltage switching devices and the traditional AC frequency converter structure. A typical example is Siemens' SIMOVER™ V series frequency converter. The SIMOVER™ V series frequency converter uses a traditional voltage-type frequency converter structure. By employing high-voltage HV-IGBT modules, the number of series devices is reduced to 12, resulting in higher reliability, lower costs, and smaller cabinet size. Because the inverter section of the SIMOVER™ V series frequency converter uses a traditional three-level method, as shown in Figure 2, it inevitably generates significant harmonic components, which is inherent to the three-level inverter method. Therefore, the output side of the SIMOVER™ V series frequency converter needs to be configured with an output filter to be used with general-purpose motors. Similarly, due to the influence of harmonics, the power factor and operating efficiency of the motor will be affected to some extent. This is a disadvantage of this type of frequency converter, thus limiting its application. Currently, high-voltage frequency converters are developing towards higher reliability, lower cost, higher input power factor, higher efficiency, lower input and output harmonics, lower common-mode voltage, and lower dv/dt. Furthermore, sensorless vector control technology based on DSP technology, as well as hot-swappable and hot-backup technologies for series power units, provide even broader possibilities for the development of high-voltage, high-power frequency converters.
(5) Achieve a high level of control
Control strategies based on electric motor and mechanical models include vector control, field control, direct torque control, and mechanical torsional vibration compensation; control strategies based on modern theories include sliding mode variable structure technology, model reference adaptive technology, nonlinear decoupling using differential geometry theory, robust observers, optimal control technology under certain indexes, and inverse Nyquist array design methods; control strategies based on intelligent control concepts include fuzzy control, neural networks, expert systems, and various self-optimization and self-diagnostic technologies.
Figure 1 Variable frequency control experimental setup
(6) Develop converters for clean electricity
Clean energy converters essentially refer to converters with a power factor adjusted to 1, minimizing harmonic components and preventing their occurrence on the grid or load side. This minimizes damage to the power grid and reduces motor torque ripple, thereby improving motor safety and extending its lifespan. For small to medium-capacity converters, this can be achieved by increasing the switching frequency. For large-capacity converters with a fixed switching frequency, clean energy conversion can be accomplished by modifying the circuit structure or employing new control methods.
3. Application of Variable Frequency Speed Control Technology in Power Systems
Firstly, the role of reactive power compensation: The purpose of installing reactive power compensation devices is to improve power supply efficiency and the power supply environment. It makes full use of the principle of energy exchange between two loads to compensate for the losses between power supply transformers and transmission lines. In the power supply system, reactive power compensation devices are an indispensable component. Only by rationally selecting compensation devices and applying them to the power system can the power factor of the power grid be effectively improved, network losses be minimized, and the power grid quality be effectively improved.
When selecting reactive power compensation devices, grouped switching capacitors and reactors are typically used. In some special cases, synchronous condensers and static var compensators (SVCs) are also good choices. After meeting the reactive power balance requirements, voltage regulation devices are also needed to ensure voltage quality standards are met. The principles of hierarchical and zonal balancing should be applied to reactive power compensation in the power grid. Simultaneously, the reactive power regulation capabilities of substations should be fully considered, and voltage optimization and power factor correction should be vigorously promoted. Advanced technologies, such as power grid reactive power management system software, should be actively applied to further improve power grid quality and ensure safer and more reliable grid operation.
Secondly, inverter load standards: Compared to the heating time of transformers and motors, the heating time of semiconductor devices is often shorter, usually measured in minutes. Overload and overheating issues can cause significant problems. Therefore, strict load conditions are necessary. Inverter operating types need to be categorized: Level 1 allows for full current output without overload; Level 2 allows for continuous output of basic load current, with short-term overload operation reaching 50%; Levels 3 through 6 require even longer overload times. Currently, only Level 2 and Level 1 inverters are generally sold in the market. Furthermore, the appropriate inverter selection must consider the load performance and speed range requirements of the production machinery.
Variable frequency drive (VFD) technology, with its superior speed regulation performance, comprehensive protection functions, significant energy-saving effects, and ease of interface with automatic control systems for automatic adjustment, has become an effective way for enterprises to upgrade their technology and reduce energy consumption in equipment. In recent years, VFD technology has been increasingly widely used in power systems, gradually demonstrating its superior performance in energy saving, consumption reduction, and process improvement.
(1) Application of variable frequency speed control technology in power system energy saving is the main application area of frequency converters in power systems.
The United States, Japan, and Western European countries are actively promoting the application of variable frequency drive (VFD) technology in water pumps and fans used in thermal power plants. An independent study conducted in the United States and the former Soviet Union showed that replacing traditional non-speed-regulating electrical drives with VFDs can save 25% of energy in pumps and 30% in fans. Therefore, applying VFD technology to equipment such as fans and pumps will yield significant economic benefits.
Figure 2. Variable frequency control diagram of water pump
Application of variable frequency speed control technology in boiler feed pump transmission system.
Currently, the application of frequency converters in feedwater transmission systems in domestic thermal power plants is limited to a few units. For example, the Daqing Xinhua Power Plant adopted the "Perfect Harmonic-Free" series frequency converter from Robicon, an American company, for its 2300kW feedwater pump in a 100MW peak-shaving unit. According to the author's research, developed countries abroad have made boiler feedwater pump transmission systems one of the main targets for the widespread application of frequency converters. For instance, the EPRI (Electronic Power Research Institute) in the United States, during its field test program on the application of frequency converters in adjustable speed transmissions of power plant auxiliary equipment from 1984 to 1989, selected a 149kW boiler feedwater pump transmission system at the Sierra Pacific Power Company's Churchill I Power Plant as its first choice. The 3TBA series series-type high-voltage frequency converter developed by the Moscow Electric Power Research Institute in Russia was also first applied to the 5000kW boiler feedwater pump of a 200MW coal-fired unit in the Moshwa Regional Power Plant. Therefore, it is evident that boiler feedwater pump transmission systems are also one of the main targets for energy-saving retrofits using frequency converters in thermal power plants.
Application in the drive system of boiler forced and induced draft fans.
Currently, domestic thermal power plants have a total of 87 6kV frequency converters in use or under installation in their auxiliary drive systems, with a total power of approximately 96,400kW. Among these, approximately 66 frequency converters are used in boiler forced and induced draft fan systems, with a power of approximately 77,000kW, accounting for 76% of the total number of converters and 80% of the total power. This demonstrates that boiler forced and induced draft fans are currently the preferred and primary target for energy-saving retrofits in power plants using variable frequency speed control technology. Variable frequency speed control systems can control the induced draft fan to operate at a significantly lower operating point than its rated power at maximum flow, thereby achieving energy savings.
(4) AC variable frequency speed control technology also has practical applications in power systems to improve processes and enhance control accuracy.
AC electric drives possess excellent technical performance, including a wide speed range, high speed control accuracy, fast dynamic response, and reversible operation in all four quadrants. Therefore, in power systems, frequency converters are needed not only for energy conservation but also in many applications requiring precise control of flow, pressure, and liquid levels.
Figure 3 Controlled frequency converter
Many thermal power plants in China have adopted variable frequency speed control technology to upgrade their fuel control systems, using frequency converters and squirrel-cage asynchronous motors to form a variable frequency speed control system for the pulverizer.
Due to its linearity and stability, variable frequency speed control can quickly change the amount of pulverized coal entering the furnace, thus stabilizing the pressure before the turbine. It also offers stable speed control, good linearity, high reliability, and a wide speed range. Furthermore, its interface with the upper-level fuel control system is simple and easy to implement, improving the regulation quality of the fuel control system. Under steady-state conditions, the main steam pressure fluctuates within ±0.1% MPa; under dynamic conditions, such as changing the load by 10% at a 5% load increase/decrease rate, the main steam pressure fluctuates within ±0.2 MPa. Its dynamic regulation quality and steady-state operating performance are superior to slip-ring speed control.
4. Conclusion
In summary, the above analysis shows that variable frequency speed control technology has been widely used in many fields since its inception. Moreover, with continuous improvement and maturation of the technology, it has achieved good results in saving energy. This is a high-tech technology with energy-saving characteristics, and to adapt to the ever-changing power system, it is necessary to promote it and continuously improve its overall level, thereby accelerating production efficiency and improving product quality. With the development and improvement of power electronic device manufacturing technology, power conversion technology based on power electronic circuits, and various control technologies, AC variable frequency speed control technology will become increasingly mature and will become the mainstream of AC speed control in the future. The application of AC variable frequency speed control technology in power systems demonstrates its excellent application prospects in energy saving, consumption reduction, process improvement, and enhanced control accuracy.
