This paper presents a transformerless single-phase inverter topology based on a modified H-bridge based multilevel converter. The topology comprises two legs, namely, a usual twolevel leg, and a T-type leg. The latter is based on a usual two-level leg, which has been modified to gain access the mid-point of the split DC-link by means of a bidirectional switch. The topology is referred as asymmetrical T-type five-level (5L-T-AHB) inverter. An ad-hoc modulation strategy based on sinusoidal pulse width modulation (PWM) is also presented to control the 5L-T-AHB inverter, where the two-level leg is commuted at fundamental frequency. Numerical and experimental results show that the proposed 5L-T-AHB inverter achieves high efficiency, exhibits reduced leakage currents, and complies with the transformerless norms and regulations, which makes it suitable for the transformerless PV inverters market¹.

This paper presents a grid synchronization scheme aimed to provide an estimation of the angular frequency and both the positive and negative sequences of the fundamental component of an unbalanced three-phase signal. These sequences are provided in fixed-reference-frame coordinates, and thus, the proposed algorithm is referred to as fixed-reference-frame phase-locked loop (PLL) (FRF-PLL). In fact, the FRF-PLL does not require transformation of variables into the synchronous frame coordinates as in most PLL schemes. Therefore, the proposed scheme is not based on the phase angle detection. Instead, the angular frequency is detected and used for synchronization purposes. The design of the FRF-PLL is based on a complete description of the source voltage involving both positive and negative sequences in stationary coordinates and considering the angular frequency as an uncertain parameter. Therefore, the FRF-PLL is intended to perform properly under severe unbalanced conditions and to be robust against angular frequency variations, sags, and swells in the three-phase utility voltage signal. Due to the selective nature of the scheme, it shows certain capability to alleviate the effect of harmonic distortion that is present in the utility voltage signal.

This paper presents the modeling and control processes of the single-phase cascade H-bridge multilevel converter used as a shunt active filter. Based on the obtained model, a controller is proposed to compensate for harmonic distortion and reactive power caused by a nonlinear load. These issues are solved by guaranteeing tracking of the line current toward an appropriately defined current reference. In the proposed approach, the current reference has been selected to be a signal proportional to either the line voltage or to its fundamental component; however, the proposed method can also be extended to other reference definitions. The proposed controller also includes voltage loops to guarantee regulation and balance of the involved capacitor voltages toward constant references. Experimental results on a 2-kVA prototype are presented to assess the performance of the proposed controller.

In this letter, a different feedback structure of the repetitive control that apparently is more appropriate for applications in power electronics is proposed. Moreover, a simple analog-circuit implementation is proposed which is suitable for high-frequency power electronics applications, where digital control is unpractical due to cost and performance of available DSPs and microcontrollers

A feedforward modification for both positive- and negative-feedback schemes of repetitive control is described. It was shown that repetitive controllers can be a useful tool for tracking of periodic reference signals and compensation of periodic disturbances, in other words, for harmonic compensation. It was shown that the feedforward modification considerably improves the frequency response and performance, providing higher gains with enhanced selectivity. Simple analog circuits are presented to implement both positive- and negative-feedback repetitive schemes. A description of the circuits and their corresponding experimental frequency responses are also given

This paper proposes some modifications aimed for the practical implementation of repetitive-based schemes used in harmonic distortion compensation schemes. Practical implementation of repetitive schemes requires both to limit the gain originally infinite and to restrict the bandwidth of the controller originally of infinite dimension. Roughly speaking, each delay line in the repetitive scheme must include an associated limiting gain and a low-pass filter (LPF). However, it has been observed that this practical modifications can produce a considerable phase shift at the frequencies of interest. As a consequence, the compensation of harmonics cannot be guaranteed anymore. One possibility to compensate such a phase shift deterioration consists in modifying the delay time associated to the delay line of the repetitive scheme. However, in the case of conventional LPFs, this phase shift is usually a nonlinear function of the frequency, which makes this task extremely difficult if not impossible. To overcome this issue, this paper proposes the use of low-pass finite impulse response filters, whose phase shift can be made linear by following appropriate design rules. The idea behind this approach is that an LPF with a linear phase produces a constant delay time, which is much easier to compensate in the delay line. Experimental results are presented to confirm the benefits of the proposed scheme.