A Novel Filter Design Method for Grid-Tied Inverters

Grid-tied inverters play a vital role in distribution power systems to utilize renewable energy systems. Pulsewidth modulation (PWM) techniques are used for the inverters switching, where low- and high-frequency harmonics are produced at the terminal of the inverters. Therefore, a suitable filter between the inverter and grid is needed to prevent the power quality issue in the system. LCL filters with passive damping resistor are known as well-performed solution to minimize the produced harmonics in a grid-tied topology. In this article, a unified filter design approach for a grid-tied inverter is introduced. The proposed filter design approach is based on finding the precise maximum ripple current according to the modulation method and calculating the accurate optimal passive damping resistor. Unique formulas for the calculation of active power losses are introduced, which consider all the influential designing parameters. Ultimately, a new optimization-based algorithm is presented to determine the optimal size of the inductors and capacitor considering constraints on damping losses. The experimental results prove the efficiency of the proposed optimization-based method as well as the accuracy of introduced filter parameters expressions. </p

Output filter design for grid-tied cascaded multi-level inverters based on novel mathematical expressions

Renewable energy resources, which are widely used in modern power systems, require power electronic-based converters to couple with the external grid. The main impact of utilizing power electronic converters on the grid power quality is harmonic generation produced by their switching process. Multi-level inverters are outstanding solutions to significantly reduce the voltage stress and harmonics. In this paper, novel mathematical expressions are derived to calculate the maximum ripple of the inverter output current, which are used to size the inverter-side inductor for an output filter. Moreover, new analytical and simplified formulas are proposed to calculate the damping losses, which lead to optimum selection of the damping resistor of the output filter. Ultimately, a mathematical expression for the grid-side inductor is attained considering compatibility level for harmonics within the range of 2-150 kHz to cover electrical and electronics equipment, which currently are the most important issues in the international standardization committee (IEC, TC77A). The efficiency of the proposed approach is finally validated by the experimental and simulation results

Harmonic analysis of grid-tied active front end inverters for the frequency range of 0-9 khz in distribution networks: Addressing future regulations

The existing international standards define grid harmonic limitations only up to the 40th or 50th harmonic. Increasing trend of grid-tied inverters in renewable energy systems causes grid distortions beyond the existing harmonic standardization range leading to low power quality, consumer load damage and excess power loss in the power network. Hence, this paper investigates the influencing factors towards these harmonic emissions of grid-tied inverter systems. Controllers for the inverters are designed for rated operation condition assuming the DC-link voltage and grid voltage is ideal, even though these systems mostly operate at partial powers and distorted DC-link and grid voltage conditions in practice. Hence, this paper presents an analysis of grid current distortion when the inverter operates with fluctuating DC-link voltage, different power levels and controller gains of the proportional resonant controller. The results show high gains of the controller, partial power operation and low frequency fluctuations of DC-link voltage cause increase in harmonics up to 2 kHz while there is no significant effect for 2-9 kHz harmonics. Further, the study demonstrates the significance of grid current distortion when grid voltage is distorted at the frequency where output impedance of inverter together with grid impedance is minimum. This analysis is presented for both stiff and weak grid conditions since the grid distortion varies with different grids.</p

A full-feedforward harmonic mitigation scheme in multi-parallel grid-tied inverters

The main power quality challenge of a set of parallel grid-tied inverters is the effect of grid voltage harmonics on the injecting current of the inverter set. In this paper, a novel full-feedforward strategy is proposed for suppression of the current harmonics, caused by the grid distortion, in a multi-parallel grid-tied inverters based system. The proposed strategy is based on producing a negative virtual admittance (equal to total system admittance) by applying a full-feedforward function from the grid voltage into only one of the inverters' set control loop. The produced negative admittance cancels the whole system admittance and consequently, the grid voltage harmonic effect on the system output current is disabled. The main contribution of this paper is that the proposed feedforward transfer function is applied to only one of the inverters. Hence, the conventional time and cost investments are considerably reduced. The simulation results prove the efficiency of the proposed method.</p

Harmonic analysis of multi-parallel grid- connected inverters in distribution networks: emission and immunity issues in the frequency range of 0-150 kHz

Grid-connected inverters based on active front end technology are of the most important components in renewable energy systems. In large scale solar farms, a set of parallel grid-connected inverters are used to scale up the amount of injecting current to the network. Contrary to a single grid-connected inverter, each inverter based on its impedance consumes other inverters output current. Therefore, the main challenge in multi-parallel grid-connected inverters is to analyze the interaction between the inverters. The harmonic rejection ability of an inverter in a set of parallel grid-connected inverters differs from that of a single grid-connected inverter. In this paper, the harmonic behavior of a set of parallel grid-tied inverters equipped with LCL filter against the background voltage of the grid is investigated. The main aim of this paper is to find the harsh circumstances of the multi-parallel grid-connected inverters from the viewpoint of power quality. For this purpose, the electrical model of an inverter is initially derived. Then, the grid current is calculated based on the share of each inverter. The investigations are performed for two main case studies including identical and different types of inverters. The mentioned case studies are examined by using three different typical inverters to justify the achievements in this paper. Besides, for each case the effect of grid condition, i.e., the value of grid impedance is studied. Finally, using the grid current frequency response, the harmonic and power quality assessment of the inverters are analyzed

Harmonic analysis of grid-connected inverters considering external distortions: addressing harmonic emissions up to 9 kHz

Grid-tied inverters, used in renewable energy sources, are exposed to distortions emitted by various sources including the reference signal, external power grid, and DC-link along with harmonics created by the pulse width modulation unit. However, the effect of these sources on grid-tied inverter output, especially near the resonant frequency of the inverter's filter, is unknown. In this study, a comprehensive harmonic model of the grid-tied inverter is presented by considering all three types of external sources. The proposed model can be utilised for low and high-frequency harmonic emission of grid-connected inverters. A new analytical expression is introduced as an indicator of the maximum possible individual grid current harmonic in the case of harmonic injection of multiple external sources. The impact of series damping resistor on harmonic rejection ability of the inverter is analysed at the range of frequencies around resonance. The simulation and experimental results fulfil the proposed harmonic model of the inverter

An enhanced full-feedforward strategy to mitigate output current harmonics in grid-tied inverters

The grid-tied inverters are the most vital components in renewable energy-based power systems. Hence, maintaining the power quality of a grid-tied inverter output within the standard range is an ongoing challenge in the power system. The appeared individual harmonics at inverter output current caused by grid voltage harmonics depend on the inverter output equivalent admittance. This consists of a combination of admittance seen from the point of common coupling and the phase-locked loop path. Therefore, the precise calculation of the aforementioned admittance is an inevitable requirement to design a harmonic mitigation strategy. By adding a virtual admittance to the system through modifications in the inverter control loop, the output equivalent admittance can be removed. For this purpose, in this paper, a novel full-feedforward harmonic suppression scheme is proposed which can effectively eliminate individual harmonics injected from the grid. Consequently, the individual output current harmonics of the inverter will be effectively suppressed. Simulation and experimental results have been carried out for a typical single-phase grid-tied inverter to verify the efficiency of the proposed scheme against emitted harmonics from the grid.</p

A Full-Feedforward Technique to Mitigate the Grid Distortion Effect on Parallel Grid-Tied Inverters

In this article, a novel full-feedforward (FF) technique is introduced to lessen the effect of grid voltage distortion on injected current from the multiparallel grid-tied inverters. In this technique, the feedforward transfer function is applied to only one of the system inverters known as the target inverter. Hence, the investment cost for designing and implementing the FF technique is remarkably reduced. The proposed technique is based on introducing a virtual negative admittance at the target inverter to cancel the total parallel admittance of the system. Moreover, the reasons and conditions of instability, caused by the conventional FF techniques, are fully explored in this article. Then, by modifying the conventional FF techniques, a feedforward transfer function is proposed, which guarantees the strong dynamic performance of the system. Ultimately, an additional solution is devised to ensure a desirable grid voltage harmonic rejection ability of the proposed FF technique. Simulation and experimental results verify the validity of the proposed FF technique for a system consisting of two identical parallel grid-tied inverters. </p