Control Theory in Power Electronics

Background

Power electronics can be defined as the use of electronic devices to control and convert electric power. Therefore, every application of power electronics involves some aspect of control. Since the mathematical tools used to solve one control problem are often common to other problems, some researchers specialize in formulating control algorithms for a variety of power electronic applications. Therefore, it is useful to review the major areas of power electronics from the control perspective, in order to understand the broadbased connection between modern control theory and power electronics as a whole.

Static Power Conversion and Control

One fundamental characteristic of power electronic circuits is the discrete switching between two or more different circuit configurations. This is generally referred to as switched-mode conversion. At the heart of any power electronic converter is the switch controller. Since the ideal output of a converter is usually either pure dc or a pure sinusoid, one of the primary concerns of switch control strategies is to reduce, or at least choose, the higher harmonics which are created by the switching. Another goal of switch control is to reduce, or eliminate, harmonics which are also produced on the line (input) side of the converter. These harmonics can affect all other loads on the power grid, and therefore are a prime concern for the utilities.

Static Power Supplies

Power supplies, including uninterruptible power supplies (UPS) for computers, copiers and fax machines, as well as the huge HVDC systems and VAR compensators, depend on power electronics. Their efficiency and harmonic quality is largely a function of the controller. To reduce hardware expense, the small dc power supplies usually incorporate simple control algorithms, but as harmonic standards are tightened, manufacturers will be forced to use more complex switch control techniques (and more expensive hardware) to improve the harmonic distortion on both the supply side and the load side of the converter. Additionally, note that while new topologies can provide improvements such as greater reliability for a UPS, they also usually involve more complex control algorithms. Researchers in this area therefore must be facile in handling advanced control theory.

Motion Control

Power electronic motor drives are used in virtually all newly installed motion control applications, such as industrial machine tools, conveyers, cranes and hoists, electric vehicles, elevators, forklifts and trains. Therefore, a natural extension of power electronics in this area is the development of motion control algorithms which provide improved control response, improved efficiency and reduced hardware requirements. Additionally, the power electronic converters used in the motor drive are themselves power electronic converters with the same issues of reliability, efficiency and harmonic quality as described above.

Heating and Lighting

Electric heaters, ballasts and light dimmers in industry and homes consume a large percentage of the nation's generated electricity. Power electronics can often achieve these tasks more efficiently and more effectively than the traditional devices. Because of the switched mode nature of power electronic circuits, the efficiency and harmonic quality of these devices depends heavily on the control strategies employed.

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