A higher order passive power filter (LLCL filter) for the grid-tied inverter is becoming attractive for industrial applications due to the possibility to reduce the cost of the copper and the magnetic material. However, similar to the conventional LCL filter, the grid-tied inverter is facing control challenges. An active or a passive damping measure can be adopted to suppress the possible resonances between the grid and the inverter. For an application with a stiff grid, a passive damping method is often preferred for its simpleness and low cost. This paper introduces a new passive damping scheme with low power loss for the LLCL filter. Also, a simple engineering design criterion is proposed to find the optimized damping resistor value, which is both effective for the LCL filter and the LLCL filter. The control analysis and the power loss comparison for different filter cases are given. All these are verified through the experiments on a 2-kW single-phase grid-tied inverter prototype using proportional resonant controllers. It is concluded that, compared with the LCL filter, the proposed passive damped LLCL filter can not only save the total filter inductance and reduce the volume of the filter but also reduce the damping power losses for a stiff grid application.

A New Design Method for the Passive Damped LCL and LLCL Filter-Based Single-Phase Grid-Tied Inverter

Global warming is the main driving force behind worldwide interest for the generation of bulk electrical energy from renewable sources. As a consequence of advancements in solar cell fabrication and converter technology, solar PV has emerged as one of the most promising renewable sources for bulk power generation. If the current commissioning rate continues, PV power would lead to the modification of several aspects of power system and could influence the stability of the system. This paper extensively reviews the technical challenges, on particular, the stability issues associated with the integration of large-scale PV into the power system. In addition, the paper also reviews the dynamic model of large-scale PV for stability studies as well as the grid codes for large-scale PV integration into the system. Finally, this paper summarizes the research findings about the technical solutions to overcome the power system stability challenges regarding the large-scale PV integration into the transmission and sub-transmission or medium voltage distribution system.

A review of key power system stability challenges for large-scale PV integration

Transformerless photovoltaic (PV) inverters are going to be more widely adopted in order to achieve high efficiency, as the penetration level of PV systems is continuously booming. However, problems may arise in highly PV-integrated distribution systems. For example, a sudden stoppage of all PV systems due to anti-islanding protection may contribute to grid disturbances. Thus, standards featuring with ancillary services for the next-generation PV systems are under a revision in some countries. The future PV systems have to provide a full range of services as what the conventional power plants do, e.g., low-voltage ride-through (LVRT) under grid faults and grid support service. In order to map future challenges, the LVRT capability of three mainstream single-phase transformerless PV inverters under grid faults is explored in this paper. Control strategies with reactive power injection are also discussed. The selected inverters are the full-bridge (FB) inverter with bipolar modulation, the FB inverter with dc bypass, and the Highly Efficient and Reliable Inverter Concept (HERIC). A 1-kW single-phase grid-connected PV system is analyzed to verify the discussions. The tests confirmed that, although the HERIC inverter is the best candidate in terms of efficiency, it is not very particularly feasible in case of a voltage sag. The other two topologies are capable of providing reactive current during LVRT. A benchmarking of those inverters is also provided in this paper, which offers the possibility to select appropriate devices and to further optimize the transformerless system.

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Low-Voltage Ride-Through of Single-Phase Transformerless Photovoltaic Inverters

This paper presents a new methodology for optimal design of transformerless photovoltaic (PV) inverters targeting a cost-effective deployment of grid-connected PV systems. The optimal switching frequency as well as the optimal values and types of the PV inverter components is calculated such that the PV inverter LCOE generated during the PV system lifetime period is minimized. The LCOE is also calculated considering the failure rates of the components, which affect the reliability performance and lifetime maintenance cost of the PV inverter. A design example is presented, demonstrating that compared to the nonoptimized PV inverter structures, the PV inverters designed using the proposed optimization methodology exhibit lower total manufacturing and lifetime maintenance cost and inject more energy into the electric-grid and by that minimizing LCOE.

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Design optimization of transformerless grid-connected PV inverters including reliability

Current grid standards largely require that low-power (e.g., several kilowatts) single-phase photovoltaic (PV) systems operate at unity power factor (PF) with maximum power point tracking (MPPT), and disconnect from the grid under grid faults by means of islanding detection. However, in the case of wide-scale penetration of single-phase PV systems in the distributed grid, disconnection under grid faults can contribute to 1) voltage flickers, 2) power outages, and 3) system instability. This article explores grid code modifications for a wide-scale adoption of PV systems in the distribution grid. In addition, based on the fact that Italy and Japan have recently undertaken a major review of standards for PV power conversion systems connected to low-voltage networks, the importance of low voltage ride-through (LVRT) for single-phase PV power systems under grid faults is considered, along with three reactive power injection strategies. Simulations are presented for a PV power system with a LVRT capability and ancillary services. An example of a full-bridge single-phase grid connected system is tested experimentally to demonstrate the potential benefits. Additionally, grid codes for advanced PV systems with the discussed features are summarized.

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Wide-Scale Adoption of Photovoltaic Energy: Grid Code Modifications Are Explored in the Distribution Grid

In the last couple of years, the increasing penetration of renewable energy resulted in the development of grid-connected large-scale power plants. However, a high penetration harbors the risk of grid instability if the generating power plants are not able to support the grid. Therefore, grid stabilization, which depends on the system-type or grid of each country, plays an important role and has been strengthened by different grid codes. With this background, VDE-AR-N 4105 for photovoltaic (PV) systems connected to the low-voltage grid and the German Association of Energy and Water Industries (BDEW) introduced the medium-voltage grid code for connecting power plants to the grid and they are the most stringent certifications. In this paper, the control strategy of generating system is enhanced with VDE-AR-N 4105 and BDEW grid code, where both active/reactive powers are controlled. Simulation and experimental results of 100-kW PV inverter are shown to verify the effectiveness of the proposed implemental control strategy.

Implemental Control Strategy for Grid Stabilization of Grid-Connected PV System Based on German Grid Code in Symmetrical Low-to-Medium Voltage Network

This paper presents a current control technique for a single-phase grid-connected DC/AC inverter which is used in Photovoltaic power conditioning system (PV PCS). A Proportional - Resonant (PR) controller is used for replacing the conventional Proportional - Integral (PI) controller in this system. By comparison with the conventional PI control method, the PR control can introduce an infinite gain at the fundamental frequency and hence can achieve zero steady-state error. A theoretical analysis of the PR controller is presented and verified by experiment. Furthermore, a pseudo synchronous d-q transformation is employed in current control scheme and an all-pass filter based single-phase digital phase-locked loop (PLL) is introduced to detect the phase of grid voltage. Based on the theoretical analysis, the control strategy is implemented on a 32-bit fixed-point TMS320F2812 DSP and tested in a 3kW prototype PV PCS. Simulation and experimental verify the high performance of the implemented control scheme.

Design and control of Proportional-Resonant controller based Photovoltaic power conditioning system

Nowadays, the LCL-filter type becomes an attractive grid interfacing for grid-connected Voltage Source Inverter (VSI). LCL-filter can render the current harmonics attenuation around the switching frequency by using smaller inductance than L-filter. Moreover, system using LCL-filter does not depend on the grid impedance and has a better output response while comparing with LC-filter. Firstly, an analysis and design procedure of output LCL-filter for single-phase grid-connected Photovoltaic (PV) inverter system is presented in this paper. Due to the theoretical analysis, a comparison between the designed LCL-filter with L-filter and LC-filter based single-phase grid-connected PV inverter system is carried out. The comparison results are given to validate the theoretical analysis and effectiveness of filters.

Comparative analysis of low-pass output filter for single-phase grid-connected Photovoltaic inverter

In this paper, a new fast peak detector for single or three-phase unsymmetrical voltage sags is proposed. The proposed detector is modified from a single-phase digital phase-locked loop based on a d-q transformation using an all-pass filter (APF). The all pass filter generates a virtual phase with 90° phase delay but the virtual phase cannot reflect a sudden change of the grid voltage in the moment of voltage sag, which causes a resulted peak value to be significantly distorted and settle down slowly. Specially, a settling time of the peak value is too long when voltage sag occurs around zero crossing such as phase 0° and 180°. This paper describes operating principle of the APF problem and proposes a modified all-pass filter (MAPF) to mitigate the inherent APF problem. In addition, a new fast peak detector employing the MAPF is proposed and the detector can calculate peak value within 0.5 msec even when voltage sag occurs around zero crossing. The proposed fast peak detector is compared with the conventional detector using APF, and shows faster detection time in the whole range of phase. Furthermore, the proposed fast peak detector can be effectively applied to unsymmetrical three-phase voltage sags. Simulation and experimental results verify the advantages of the proposed detector and MAPF.

A new fast peak detector for single or three-phase unsymmetrical voltage sags

Nowadays, the PV systems have been focused on the grid connection between the power source and the grid. The PV inverter can be considered as the core of the whole system because of an important role in the grid-interfacing operation. An important issue in the inverter control is the load current regulation. In the literature, Proportional Integral (PI) controller, which is normally used in the current-controlled Voltage Source Inverter (VSI), cannot be a satisfactory controller for an AC system because of the steady-sate error and the poor disturbance rejection, especially in high-frequency range. Compared with conventional PI controller, Proportional Resonant (PR) controller can introduce an infinite gain at the fundamental frequency of the AC source; hence it can achieve the zero steady-state error without requiring the complex transformation and the de-coupling technique. Theoretical analyses of both PI and PR controller are presented and verified by simulation and experiment. Both controller are implemented in a 32-bit fixed-point TMS320F2812 DSP processor and evaluated on a 3kW experimental prototype PV Power Conditioning System (PCS). Simulation and experimental results are shown to verify the controller performances.요 약

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Trung-Kien Vu

Papers

Implemental Control Strategy for Grid Stabilization of Grid-Connected PV System Based on German Grid Code in Symmetrical Low-to-Medium Voltage Network

Design and control of Proportional-Resonant controller based Photovoltaic power conditioning system

Comparative analysis of low-pass output filter for single-phase grid-connected Photovoltaic inverter

A new fast peak detector for single or three-phase unsymmetrical voltage sags

Comparison of PI and PR Controller Based Current Control Schemes for Single-Phase Grid-Connected PV Inverter