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In this letter, the effect of deadtime in a gallium nitride (GaN) based grid-connected differential-mode Ćuk converter with high-frequency (HF) transformer isolation is analyzed. It is shown that different levels of lower order harmonics appear in the grid current depending on the power factor of an operation. Utilizing the leakage inductance of the HF transformer, the deadtime is suitably eliminated to result in an improved grid-current total harmonic distortion (THD). Experimental results support the proposed solution. [ABSTRACT FROM AUTHOR]

In this paper, a modular control approach has been proposed for a single- and three-phase differential-mode Ćuk inverter (DMCI) operating with a recently proposed discontinuous-modulation scheme that offers tangible performance benefits over conventional continuous modulation scheme. The modular control scheme uses a combination of a transformation and adaptive control law to meet tracking requirements in the presence of converter nonlinearity. Experimental results for the single- and three-phase DMCI are provided for startup, steady state, load transition, and total harmonic distortion using different types of loads, which validate the efficacy of the implemented modular tracking controller. Further, detailed stability of the closed-loop DMCI module is provided. [ABSTRACT FROM AUTHOR]

Microinverters for grid-connected photovoltaic (PV) power conversion systems often require voltage boost due to the low dc voltage of the PV panels. A high-frequency (HF) transformer-isolated differential-mode Ćuk inverter (DMCI) is an attractive topology due to its inherent voltage boost capability and high power density. One of the significant operational issue of the DMCI addressed in this paper is the presence of lower order harmonic distortion in the injected grid current. It is shown that lower order odd and even harmonic voltages are generated with this topology. This problem is accentuated due to the low damping offered by the grid to harmonic voltages. A systematic low-complexity control design is proposed to address this issue. It is shown experimentally that the DMCI operates stably with effective attenuation of the lower order harmonics using the proposed design. Practical operational issues of the DMCI with fast switching GaN field-effect transistor (FET) devices are analyzed and solutions are proposed. It is shown that there can be significant HF noise generation in the DMCI with a frequency range higher than the switching frequency. Practical solutions for resolving HF issues using a combination of circuit layout design and additional passive components are proposed and validated. All of the experimental results are demonstrated on a 500-W GaN-FET-based microinverter prototype. [ABSTRACT FROM PUBLISHER]

Safe start-up procedure is critical for the reliable operation of power converters. The challenges in start-up schemes of high-frequency transformer link inverters involve precharge of the capacitors without significant inrush currents and overvoltages. The precharge is normally followed by enabling of the closed-loop control. The overvoltage or inrush current problems in conventional start-up schemes available in the literature are identified in this paper. A new start-up scheme is proposed which achieves the objectives of limiting inrush current and start-up overvoltages. This method does not require any additional dedicated precharge circuitry. The proposed scheme is validated using simulation and experimental results which agree with the theoretical prediction. [ABSTRACT FROM AUTHOR]

Phase-locked loops (PLLs) are necessary in applications which require grid synchronization. Presence of unbalance or harmonics in the grid voltage creates errors in the estimated frequency and angle of a PLL. The error in estimated angle has the effect of distorting the unit vectors generated by the PLL. In this paper, analytical expressions are derived which determine the error in the phase angle estimated by a PLL when there is unbalance and harmonics in the grid voltage. By using the derived expressions, the total harmonic distortion (THD) and the fundamental phase error of the unit vectors can be determined for a given PLL topology and a given level of unbalance and distortion in the grid voltage. The accuracy of the results obtained from the analytical expressions is validated with the simulation and experimental results for synchronous reference frame PLL (SRF-PLL). Based on these expressions, a new tuning method for the SRF-PLL is proposed which quantifies the tradeoff between the unit vector THD and the bandwidth of the SRF-PLL. Using this method, the exact value of the bandwidth of the SRF-PLL can be obtained for a given worst case grid voltage unbalance and distortion to have an acceptable level of unit vector THD. The tuning method for SRF-PLL is also validated experimentally. [ABSTRACT FROM PUBLISHER]

In this paper, a simple single-phase grid-connected photovoltaic (PV) inverter topology consisting of a boost section, a low-voltage single-phase inverter with an inductive filter, and a step-up transformer interfacing the grid is considered. Ideally, this topology will not inject any lower order harmonics into the grid due to high-frequency pulse width modulation operation. However, the nonideal factors in the system such as core saturation-induced distorted magnetizing current of the transformer and the dead time of the inverter, etc., contribute to a significant amount of lower order harmonics in the grid current. A novel design of inverter current control that mitigates lower order harmonics is presented in this paper. An adaptive harmonic compensation technique and its design are proposed for the lower order harmonic compensation. In addition, a proportional-resonant-integral (PRI) controller and its design are also proposed. This controller eliminates the dc component in the control system, which introduces even harmonics in the grid current in the topology considered. The dynamics of the system due to the interaction between the PRI controller and the adaptive compensation scheme is also analyzed. The complete design has been validated with experimental results and good agreement with theoretical analysis of the overall system is observed. [ABSTRACT FROM AUTHOR]