AC Current Controller with Error-Free Feedback Acquisition System

Abstract

Abstract-In this paper, we introduce an improved ac current controller with robustness against the noise in the feedback path. This three-phase current controller is suited for inverter supplied ac machines and grid-connected power converters. Conventional solutions make use of symmetric pulse-width modulation (PWM) techniques with feedback sampling in the middle of the voltage pulses. Single-sample-based feedback acquisition gets affected by the noise and parasitic phenomena. We perform a thorough analytical and experimental study on feedback errors with conventional sampling, and we consider the impact of the lockout time, motor cables, winding capacitance, and anti-aliasing filters. In order to suppress a significant spectral content in the area of low-order harmonics, we apply an improved acquisition technique, which uses a period-average and removes any PWM noise from the feedback signals. Both anti-aliasing filter and proposed period-average filter introduce delays, which impair the control-loop performance. The conventional current controller is then extended to suppress the effects of the delays. Parameter setting procedure is devised to achieve both the bandwidth and the robustness against the parameter changes. Analytical and experimental studies prove that the proposed feedback acquisition technique improves the robustness in the presence of variable delays and switching transients. Experimental tests show that the extended current controller with period-average feedback acquisition reaches the same bandwidth and robustness as the state-of-the-art controller.

A Three-Phase Digital Current Controller With Improved Performance Indices

“…The value of un+1 is loaded into the pulse-width registers of the PWM peripheral unit at (n+2)TS, and it affects the commutation  and the average output voltage between (n+2)TS and (n+3)TS. The interrupt (n+1) uses the feedback sample i F n+1, obtained by the averaging over one PWM period [11], representing the average output current in the interval [(n-1)TS ... (n+1)TS]. Thus, the consequential transport delay corresponds to 2.5TS, and it restrains the closed loop bandwidth of the digital current controller.…”

“…The zerocrossing instants are shifted by the lockout time, by gating delays and also by the phase shift of the anti-aliasing filters, thus introducing the sampling errors [10], [11]. The errors are further increased by the switching-excited oscillations caused by the power cable parasitic inductances and capacitances [12].…”

“…The errors are further increased by the switching-excited oscillations caused by the power cable parasitic inductances and capacitances [12]. An error-free measurement that overcomes the above listed problems can be obtained by using the oversampling technique and calculating the average of the output current over the past PWM period [11]. This averaging over one PWM period increases the transport delays from 1.5/fS up to 2.5/fS and further constrains the closed loop performance.…”

“…In this paper, we adopt the error-free feedback acquisition approach of [11], and we propose the control enhancements which reduce the impact of delays and result in the closed loop bandwidth that outperforms the similar contemporary solutions. The two essential modifications include a novel approach to scheduling of the control interrupt routines, suited to reduce the overall transport delays, and the introduction of a series connected differential multiplier.…”

“…The relevant pulse transfer functions are summarized next, both for the conventional controller and for the controller enhanced by the addition of a differential multiplier, on the basis of [11].…”