APPLICATION OF VIRTUAL IMPEDANCE TO SUPPRESS DEAD-TIME EFFECT IN CURRENT-CONTROLLED PWM INVERTERS

• Main Author: Ardriani, Tri
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/50605
• Institution: Institut Teknologi Bandung

Dead-time is necessary in a PWM inverter to avoid short-corcuiting the DC source through the semiconductor switches. Due to the momentary loss of voltage control during dead-time, the output voltage becomes distorted, particularly with low-order harmonics. In motor applications, current harmonics may increase losses, heating, and cause torque pulsation. Low-order harmonics are difficult to suppress using conventional filters. Therefore, a special mitigation strategy is required. Dead-time mitigation methods are generally grouped into two categories: feedforward and feedback methods. In feedforward method, the reference is changed according to a preconceived model of the disturbance. This method requires accurate modeling, hence difficult to implement as dead-time effect always varies with load conditions. Feedback method is simpler. However, to suppress low-frequency disturbance, a high feedback gain is needed, which may jeopardize the system’s stability. A resonant controller has infinite gain at the resonant frequency and may work well to suppress certain harmonics. Its drawback is that it rapidly turns complex as more frequencies are being targeted. Moreover, it is not suitable for variable frequency applications. In this research, a control method based on virtual impedance concept is proposed to suppress dead-time effect. Virtual impedance is a controller block that mimics the behavior of a real impedance. Because it is virtual, it can be designed only to affect the disturbance signals. It may have unique characteristics that cannot be realized with real impedance. It can also be added as a plug-in to an already existing controller. The validity of the proposed method is verified through simulation and experiment. Several combinations of virtual impedance, as well as its extended design as a virtual filter, are compared with respect to their abilities in reducing THD levels. Lastly, experimental results are provided, showcasing the effectiveness of a virtual impedance in improving a current controllers’s performance.