H/sub ∞/ loop-shaping controller designs for the single-phase UPS inverters

Abstract

This paper proposes the Hcs loop-shaping approach to the design of sinusoidal output voltage tracking controllers for single-phase UPS inverters. By modeling the uncertainty in load as plant output multiplicative perturbation, two lis loop-shaping control schemes that use only capacitor voltage sensor are presented. The first design adopts single-loop control scheme which has the advantage of simple control structure and easy implementation. But its transient performance and robustness are not satisfactory. The second controller with a dual-loop structure, additionally employing output of a capacitor current estimator as feedback, provides improved perfonnance over the single-loop scheme and has the feature of low distortion, good regulating performance and insensitive to the vanation in load. The results are verified through computer siniulation and hardware experiment. INTRODUCTlONThe widely use of UPS in business and industry has intrigued the development of high performance control scheme for generating sinusoidal output via PWM inverter. In designing the control law for PWM inverters in UPS, it is demanded that the output voltage waveform be of low total harmonic distortion (THD) and invariant under large variation in load. This can be interpreted as that the controlled UPS must possess certain degree of robustness against parameter perturbation. Thus, many robust control stiategies [ I]-[2]. [3]- [5] have been applied to the control of the UPS inverteis. On the other hand, the control of UPS inverters challenges the designers to make use of sophisticated control techniques since it requires the system output to follow a sinusoidal command rather than step command.The siding mode control has proved quite useful against load uncertainty [4]-[6]. However, the well-known chattering problem must be especially taken care in analog or digital realization of the control algorithm [6]. Desired robustness may be achieved by suitably increasing the control gain. Recent advance in the technology of microprocessors and digital signal processors has made the digital control become tractable in industrial applications. As a result, the deadbeat control schemes have been applied to regulation of the UPS inverters [7]-[9]. The closed-loop systems exhibit fast dynamic response and possess certain degree of robustness.Nevertheless, none of the above mentioned control strategies can explicitly specify the desired degree of robustness in the design goal and hence the robustness in face of parameter variations can not be guaranteed in advance. To ensure a prespecified degree of robustness. we propose the H _ loop-shaping control approach for the sinusoidal tracking control of single-phase UPS inverters. The design specifications such as tracking performance, bandwidth and robustness to model uncertainty, are expressed as constraints on the gain response of the closedloop transfer functions. For constructing the H _ controller, we propose the use of the LMI method [13]. This recently published method is more general and numerica...

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