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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]

Starting-up of photovoltaic (PV) inverters involves pre-charging of the input dc bus capacitance. Ideally, direct pre-charging of this capacitance from the PV modules is possible as the PV modules are current limited. Practically, the parasitic elements of the system such as the PV module capacitance, effective wire inductance and resistance determine the start-up transient. The start-up transient is also affected by the contactor connecting the PV modules to the inverter input dc bus. In this work, the start-up current and voltages are measured experimentally for different parallel and series connections of the PV modules. These measurements are used to estimate the stray elements, namely the PV module capacitance, effective inductance and resistance. The estimation is based on a linear small-signal model of the start-up conditions. The effect of different connections of the PV modules and the effect of varying irradiation on the scaling of the values of the stray elements are quantified. The System model is further refined by inclusion of connecting cable capacitance and contactor resistance. Dynamics of the resulting fifth-order model are seen to be consistent with those of the simplified third-order model. The analysis of this paper can be used to estimate the expected peak inrush current in PV inverters. It can also be used to arrive at a detailed modelling of PV modules to evaluate the transient behaviour. [ABSTRACT FROM AUTHOR]

Second-order generalized integrator (SOGI) based phase-locked loops (PLLs) are commonly used for grid voltage synchronization in single-phase grid-connected power converters. SOGI-PLLs are attractive because of their simple structure that makes them suitable for implementation even in low-end digital controllers. In this paper, an SOGI based fixed-parameter PLL structure with full dc offset rejection capability is presented. This PLL uses two cascaded SOGI structures and it is termed as cascaded generalized integrator PLL (CGI-PLL). A systematic design procedure is proposed for the CGI-PLL that minimizes the response time and unit vector harmonic distortion. This design achieves minimum settling time for any given worst-case frequency deviation in the grid voltage and ensures that the unit vector THD is less than 1%. The PLL designed using the proposed method has good harmonic attenuation capability. The steady-state and transient response of this PLL have been validated experimentally and are found to agree with the theoretical analysis. [ABSTRACT FROM AUTHOR]

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]

Inverters are required to convert the dc power from photovoltaic panels to ac. In grid connected inverters, the ac output is directly fed into the electric grid. In this paper, a high-frequency transformer based inverter topology is presented. This leads to a very compact inverter. The modulation of this inverter with a lossless snubber circuit is proposed. Proposed modulation eliminates the over-voltage spikes due to transformer leakage inductance. Commutation of the leakage inductance energy with this modulation is also discussed. The performance of the inverter with the proposed modulation is validated using simulation and experimental results. [ABSTRACT FROM AUTHOR]

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]