With more renewable energy based distributed generation (DG) units connected to utility power grids, deterioration of power quality at the point of common coupling (PCC) becomes a major concern. There are two types of harmonics associated with DG units, and together they may cause excessive harmonic distortion at the PCC. The first type of harmonics is generated by power electronic devices in DG units such as photovoltaic systems, which contains high-frequency harmonic components at multiples of the carrier frequency of the DG interfacing inverter. Such harmonics are first reviewed in this paper, and the potential operational effect at the system level due to LCL or LC filters installed at the inverter output to mitigate such harmonics are discussed. The second type of harmonics is generated by other nonlinear local, PCC, and utility loads in the system, which are common type of harmonics at multiples of the power grid frequency, 50/60 Hz. Harmonic mitigation for such harmonics achieved through advanced control of the DG interfacing inverter operated as a power quality conditioner are reviewed and summarized. This systematic review can facilitate better understanding of harmonics associated with renewable energy based DG units and provide guidelines on advanced control schemes to realize ancillary harmonic compensation service through DG interfacing inverters.

Harmonics and Mitigation Techniques Through Advanced Control in Grid-Connected Renewable Energy Sources: A Review

High performance grid-tied inverters have stringent control requirements both under steady-state and under transient conditions. Many different control systems have been applied to grid-tied inverters. However, there are few publications reviewing the literature on these control systems and their classification, particularly with regard to recent developments in this area. In this paper, a review of solutions for the control of grid-tied inverters is carried out. These control systems are compared and classified as implementation platform, reference frame, output filter of inverter, control strategy, modulation method, and controller. The major advantages and disadvantages of these parameters are highlighted and compared. Then, the most important characteristics of these parameters have been presented in a table to show which parameters can be used in various control systems for grid-tied inverters.

Review and Classification of Control Systems in Grid-tied Inverters

This paper presents a comparison study of a typical grid-tie photovoltaic system between two types of topologies: the two stages (boost circuit adds inverter) and the single stage (inverter alone adds a step-up transformer) Some essential models built in this paper include PV array, Boost chopper and grid-tie inverter The basic characteristics and descriptions of these two topologies are described Perturb and observe and incremental conductance methods are introduced for MPPT (maximum power point tracking) As key-point, the out loop controller design details for the grid-tie inverter are investigated The PV array, two system topologies and two MPPT strategies are implemented in PSCAD/EMTDC using its user- defined facility Simulation results show that the output power of PV array can achieve a stable and accurate MPPT function under ambient conditions change At the same time, the inverter can maintain a unit power factor by controlling the transferred instantaneous reactive power to zero in the synchronous d-q frame The simulating results are compared with that of the two stages case in aspects including energy converting efficiency, power quality, and MPPT accuracy Through this study work, engineers and researchers can obtain an overall comprehension about advantages and disadvantages of these two topologies in designing a PV system

Comparative study of two stages and single stage topologies for grid-tie photovoltaic generation by PSCAD/EMTDC

This paper proposes a grid interfaced solar photovoltaic (SPV) power generating system consisting of a SPV, DC-DC boost converter, voltage source converter (VSC), interfacing inductors, ripple filter and a three phase grid feeding variety of linear and nonlinear loads. The reference grid currents are estimated by using power balance theory (PBT) to control the three-leg voltage source converter (VSC). The proposed SPV power generating system provides unity power factor, reactive power compensation for voltage regulation in a three phase distribution system. In this two stage SPV power generating system, the DC link voltage of VSC is regulated through a DC voltage PI (Proportional integral) controller and the maximum power point tracking (MPPT) is achieved using a DC-DC boost converter. The grid interfaced SPV power generating system is modeled and simulated in the MATLAB using the Simulink and the sim power system (SPS) toolboxes.

Power balance theory based control of grid interfaced solar photovoltaic power generating system with improved power quality

Despite the vast benefits of integrating renewable energy sources (RES) with the utility grid, they pose stability and power quality problems when interconnected with the existing power system. This is due to the production of high voltages and current overshoots/undershoots during their injection or disconnection into/from the power system. In addition, the high harmonic distortion in the output voltage and current waveforms may also be observed due to the excessive inverter switching frequencies used for controlling distributed generator’s (DG) power output. Hence, the development of a robust and intelligent controller for the grid-connected microgrid (MG) is the need of the hour. As such, this paper aims to develop a robust and intelligent optimal power flow controller using a grasshopper optimization algorithm (GOA) to optimize the dynamic response and power quality of the grid-connected MG while sharing the desired amount of power with the grid. To validate the effectiveness of proposed GOA-based controller, its performance in achieving the desired power sharing ratio with optimal dynamic response and power quality is compared with that of its precedent particle swarm optimization (PSO)-based controller under MG injection and abrupt load change conditions. The proposed controller provides tremendous system’s dynamic response with minimum current harmonic distortion even at higher DG penetration levels.

Optimal power flow controller for grid-connected microgrids using grasshopper optimization algorithm

Effective control strategies for reliable photovoltaic (PV) grid-connected systems are needed to efficiently use solar energy, an abundant and clean renewable energy source. A space vector pulse width modulation (SVPWM) has been widely applied in the current control of three-phase voltage source inverters (VSIs). The control strategy combines a constant voltage tracking method for the variable photovoltaic power with SVPWM-based proptional-intergral (PI) current controller in a single stage three-phase PV grid-connected system. The controller mimics deadbeat control in the synchronous d-q reference frame, and is very simple and robust to implement. With the necessary grid voltage detection in Photovoltaic systems for protection, grid harmonics disturbance is effectively suppressed through feed-forward compensation. The simulation results show that the current controller makes the PV grid-connected VSI output current to be in phase with the grid voltage and has an excellent steady-state response as well as an extremely fast dynamic response.

Three-phase grid-connected photovoltaic system with SVPWM current controller

The invention discloses a three-phase four-leg inverter used for photovoltaic grid-connected power generation. The input end of the inverter is connected with a photovoltaic array, and the output end is connected with a three-phase power grid; the inverter comprises three legs (a, b and c) which are connected with each other in parallel and a neutral legs (n) which is connected with the two ends of the three legs in parallel; each leg comprises two serially-connected switches (S1 and S4/S3 and S6/S5 and S2); the connecting point of every two serially-connected switches is of the output end of each leg; the neutral leg comprises two serially-connected switches (S7 and S8); the connecting point of the two switches (S7 and S8) forms the output end of the neutral leg; and the output end of the neutral leg is connected with a neutral line of the three-phase power grid. The invention also discloses a photovoltaic grid-connected power generation system comprises the inverter. The system has the characteristics of very high adaptability to the imbalance of a power grid and extremely low grid-connected current harmonic content.

Three-phase four-leg inverter used for photovoltaic grid-connected power generation and photovoltaic grid-connected power generation system

Tree height is an essential indicator in forestry research. This indicator is difficult to measure directly, as well as wind disturbance adds to the measurement difficulty. Therefore, tree height measurement has always been an issue that experts and scholars strive to improve. We propose a tree height measurement method based on tree fisheye images to improve the accuracy of tree height measurements. Our aim is to extract tree height extreme points in fisheye images by proposing an improved lightweight target detection network YOLOX-tiny. We added CBAM attention mechanism, transfer learning, and data enhancement methods to improve the recall rate, F1 score, AP, and other indicators of YOLOX-tiny. This study improves the detection performance of YOLOX-tiny. The use of deep learning can improve measurement efficiency while ensuring measurement accuracy and stability. The results showed that the highest relative error of tree measurements was 4.06% and the average relative error was 1.62%. The analysis showed that the method performed better at all stages than in previous studies.

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Fisheye Image Detection of Trees Using Improved YOLOX for Tree Height Estimation

The utility model relates to a three-phase inverter with four bridge legs for photovoltaic interconnected generation, comprising an input terminal connected with a photovoltaic array, an output terminal connected with a three-phase electrical network, and three parallel-connected bridge legs (a, b, c) Each bridge leg comprises two series switching tubes (S1, S4/S3, S6/S5, S2), and the connectionpoint of each two series switching tubes forms the output terminals of the bridge legs The three-phase inverter with the four bridge legs for photovoltaic interconnected generation also comprises a neutral bridge leg (n) connected with both ends of the three bridge legs in parallel, the neutral bridge leg comprises two series switching tubes (S7, S8), the connection point of the switching tubes (S7, S8) forms an output terminal of the neutral bridge leg, and the output terminal of the neutral bridge leg is connected with a neutral conductor of the three-phase electrical network A photovoltaic interconnected generation system containing the inverter well adapts to the conditions of network unbalance, and has an extremely small content of interconnected current harmonic wave

Three-phase inverter with four bridge legs for photovoltaic interconnected generation and photovoltaic interconnected generation system

In high-power grid-connected inverter for new energy power generation system, low switching frequency makes the control loop, LCL-filter resonant peak, and sideband harmonics generated by modulation coupled so that the parameter design of filter and controller affects each other. This increases the difficulty of design of filter parameters and controller parameters. To solve this problem, the accurate current harmonic calculation method based on the harmonic state-space model and considering switching frequency and control delay is proposed in this article first. Second, based on the above model, the parameter optimization process considering current harmonic distortion rate, filter cost, and so on is constructed. The integrated optimization design of filter and controller parameters of low-switching-frequency LCL grid-connected inverter is completed through intelligent algorithm. Thus, the filter cost is minimized on the premise that inverter performances meet the specific requirements. In addition, considering the nonideal characteristic of grid, a scheme of integrated parameter design that can adapt to the large-scale change of grid strength is proposed. Finally, the effectiveness of the proposed method and scheme is verified by simulation and experiment.

Hongwei Zhou

Papers

Three-phase grid-connected photovoltaic system with SVPWM current controller

Three-phase four-leg inverter used for photovoltaic grid-connected power generation and photovoltaic grid-connected power generation system

Fisheye Image Detection of Trees Using Improved YOLOX for Tree Height Estimation

Three-phase inverter with four bridge legs for photovoltaic interconnected generation and photovoltaic interconnected generation system

Integrated Design of Filter and Controller Parameters for Low-Switching-Frequency Grid-Connected Inverter Based on Harmonic State-Space Model