The elimination of the output transformer from grid- connected photovoltaic (PV) systems not only reduces the cost, size, and weight of the conversion stage but also increases the system overall efficiency. However, if the transformer is removed, the galvanic isolation between the PV generator and the grid is lost. This may cause safety hazards in the event of ground faults. In addition, the circulation of leakage currents (common-mode currents) through the stray capacitance between the PV array and the ground would be enabled. Furthermore, when no transformer is used, the inverter could inject direct current (dc) to the grid, causing the saturation of the transformers along the distribution network. While safety requirements in transformerless systems can be met by means of external elements, leakage currents and the injection of dc into the grid must be guaranteed topologically or by the inverter's control system. This paper proposes a new high-efficiency topology for transformerless systems, which does not generate common-mode currents and topologically guarantees that no dc is injected into the grid. The proposed topology has been verified in a 5-kW prototype with satisfactory results.

Transformerless Single-Phase Multilevel-Based Photovoltaic Inverter

https://microgridnews.com/wp-content/uploads/2015/01/Hafez-et-al-2012.pdf

Optimal planning and design of a renewable energy based supply system for microgrids

For low-power grid-connected applications, a single-phase converter can be used. In photovoltaic (PV) applications, it is possible to remove the transformer in the inverter to reduce losses, costs, and size. Galvanic connection of the grid and the dc sources in transformerless systems can introduce additional ground currents due to the ground parasitic capacitance. These currents increase conducted and radiated electromagnetic emissions, harmonics injected in the utility grid, and losses. Amplitude and spectrum of the ground current depend on the converter topology, the switching strategy, and the resonant circuit formed by the ground capacitance, the converter, the ac filter, and the grid. In this paper, the ground current in a 1.5-kW PV installation is measured under different conditions and used to build a simulation model. The installation includes a string of 16 PV panel, a full-bridge inverter, and an LCL filter. This model allows the study of the influence of the harmonics injected by the inverter on the ground current.

Eliminating Ground Current in a Transformerless Photovoltaic Application

Grid-connected inverters are key components of distributed generation systems (DGSs) and micro-grids (MGs), because they are effective interfaces for renewable and sustainable distributed energy resources (DERs). Recently, multi-functional grid-connected inverters (MFGCIs) have attracted more and more attention for their benefits on auxiliary services on power quality enhancement in DGSs and MGs. These kinds of converters can not only achieve the power generation of DERs, but also can perform as power quality conditioners at their grid-connected points. It should be noted that these functionalities are optimally organized in the same device, which can significantly enhance the cost-effective feature of the grid-connected inverter, as well as can decrease the investment and bulk compared with multiple devices with independent functionalities. MFGCIs are especially suitable for DGSs and MGs application due to their good performances and benefits. Topologies and control strategies of MFGCIs are comprehensively reviewed in this paper. Additionally, detailed explanation, comparison, and discussion on MFGCIs are achieved. Furthermore, some future research fields on MFGCIs are well summarized.

Topologies and control strategies of multi-functional grid-connected inverters for power quality enhancement: A comprehensive review

Future ancillary services provided by photovoltaic (PV) systems could facilitate their penetration in power systems. In addition, low-power PV systems can be designed to improve the power quality. This paper presents a single-phase PV system that provides grid voltage support and compensation of harmonic distortion at the point of common coupling thanks to a repetitive controller. The power provided by the PV panels is controlled by a Maximum Power Point Tracking algorithm based on the incremental conductance method specifically modified to control the phase of the PV inverter voltage. Simulation and experimental results validate the presented solution.

A Single-Phase Voltage-Controlled Grid-Connected Photovoltaic System With Power Quality Conditioner Functionality

This paper proposes a single-phase two-wire inverter system for photovoltaic (PV) power injection and active power filtering (APF) with nonlinear inductor consideration. The proposed system can fully or partially perform APF, process PV power, eliminate harmonic currents, improve power factor, and take into account the nonlinear effect of its output filter inductor. In the system, even though only the utility current is sensed, both APF and maximum power point tracking features can be still achieved, reducing the number of current sensors and cost significantly. To prevent output current from exceeding switch ratings, inverter current is properly controlled through a current estimator and a defined limit circle. A self-learning algorithm is also proposed to determine nonlinear inductance, which can increase the accuracy of the estimated current. Simulations and experimental results have verified the feasibility of the proposed PV inverter system and the algorithm.

A single-phase inverter system for PV power injection and active power filtering with nonlinear inductor consideration

This paper presents a half-bridge single-phase two-wire (1phi2W) photovoltaic (PV) inverter system that can perform both active power filtering and real power injection. In the proposed system, it needs only two active switches, reducing cost significantly. In addition, output current of the inverter can be controlled to prevent switches from exceeding their current ratings. Thus, power rating of the inverter can be effectively used and power quality can be improved. For controlling inverter output current, an amplitude-clamping algorithm and an amplitude-scaling algorithm are proposed, which can determine inverter current command without needs of complicated calculation. Simulations and experimental results have verified the feasibility of the proposed PV inverter system and the two current control algorithms

PV Power Injection and Active Power Filtering With Amplitude-Clamping and Amplitude-Scaling Algorithms

This paper presents a coupled-buck converter with active-clamp circuits The active switches in the converter can sustain a long enough duty cycle while it operates with a high step-down voltage ratio, reducing current stress significantly In the converter, the active-clamp circuits provided can have benefits of recycling the leakage energy, while minimum ringing voltage is across the active switches Moreover, since both main and auxiliary switches can be turned on with zero-voltage switching (ZVS), switching loss can be reduced and conversion efficiency therefore can be improved significantly A 240 W prototype of the proposed buck converter was built and experimental results have shown that efficiency can reach as high as 89% It is relatively attractive for applications to hybrid electric vehicle on-board chargers

Interleaved soft-switching coupled-buck converter with active-clamp circuits

This study presents a buck-boost-flyback integrated converter (BBFIC), which can step up the voltage of photovoltaic (PV) module or fuel cell to a much higher level for grid-tied applications. The converter combines two buck-boost converters and one flyback converter into a single-stage structure with single power switch. In the power stage, an inductor and a coupled inductor are adopted to feature buck-boost and flyback behaviours, and three capacitors are connected in series to stack up output voltage. Even though it possesses the characteristics of buck-boost and flyback converters, the problem of reversed voltage polarity is avoided and the energy stored in leakage inductor can be recycled without additional active clamp circuits. The structure of BBFIC can be easily expanded to obtain an extra-high voltage gain by stacking buck-boost cells or flyback cells. Voltage gain derivation and theoretical analysis are detailed in this study. Simulations and practical results measured from a prototype have validated the proposed BBFIC.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-pel.2015.0482

Buck-boost-flyback integrated converter with single switch to achieve high voltage gain for PV or fuel-cell applications

A multi-input converter (MIC) to process wind-PV power is proposed, designed, analyzed, simulated, and implemented. The MIC cannot only process solar energy but deal with wind power, of which structure is derived from forward-type DC/DC converter to step-down/up voltage for charger systems, DC distribution applications, or grid connection. The MIC comprises an upper modified double-ended forward, a lower modified double-ended forward, a common output inductor, and a DSP-based system controller. The two modified double-ended forwards can operate individually or simultaneously so as to accommodate the variation of the hybrid renewable energy under different atmospheric conditions. While the MIC operates at interleaving mode, better performance can be achieved and volume also is reduced. The proposed MIC is capable of recycling the energy stored in the leakage inductance and obtaining high step-up output voltage. In order to draw maximum power from wind turbine and PV panel, perturb-and-observe method is adopted to achieve maximum power point tracking (MPPT) feature. The MIC is constructed, analyzed, simulated, and tested. Simulations and hardware measurements have demonstrated the feasibility and functionality of the proposed multi-input converter.

/pdf/multi-input-converter-with-mppt-feature-for-wind-pv-power-r2do4erru9.pdf

Chih-Lung Shen

Papers

A single-phase inverter system for PV power injection and active power filtering with nonlinear inductor consideration

PV Power Injection and Active Power Filtering With Amplitude-Clamping and Amplitude-Scaling Algorithms

Interleaved soft-switching coupled-buck converter with active-clamp circuits

Buck-boost-flyback integrated converter with single switch to achieve high voltage gain for PV or fuel-cell applications

Multi-Input Converter with MPPT Feature for Wind-PV Power Generation System