LCL filter design for grid-tied N-level cascaded inverters used in renewable energy systems

Renewable energy resources are usually needed to be connected to the distribution grids through a power electronics converter with an appropriate filter in the output. Therefore, harmonics injected to the grid by converters must be handled to maintain power quality indices within standard limits. LCL filters with passive damping resistors are the most reliable and efficient devices to fulfill the standards. The main objectives in the design of an efficient LCL filter are to reduce the cost and weight of the filter, as well as to increase robustness and stability. Due to the outstanding ability of multilevel inverters in diminishing the output voltage harmonics, conventional 2-level inverters are being replaced with them. However, the output filter design for inverters based on multilevel Pulse Width Modulation algorithms (PWM) has not yet been considered in the literature. Hence, exact formulas should be derived to design each element in the filter considering N-levels of the output voltage. In this paper, a comprehensive and mathematical approach is proposed to calculate the maximum current ripple of a PWM-based N-level cascaded inverter, which helps to precisely calculate the size of the inverter-side inductor. Finally, an algorithm is proposed to design an LCL or LCCL filter step-by-step by modification of previous algorithms.</p

Harmonic emissions in 0-9 kHz frequency range and transient effects in grid-connected inverters utilised in solar farms

The penetration level of large-scale inverters utilized in solar farms is continuously increasing. A large part of PV power generation comes from large-scale and high-power inverters, which are normally connected to the grid through a distribution transformer. The operation of these high-power inverters with switching frequency mainly below 9 kHz due to design constrains need to be studied and their short-term incidents need to be understood. For that purpose, in this paper, output currents of three inverters have been measured using field measurements of a 3MW solar farm. The currents are then analyzed in time and frequency domains. The results show large short-term DC current and significant harmonic emissions (0-9 kHz) during short period of times, while the steady-state harmonic levels do not seem to be significant

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

Grid-tied inverters in renewable energy systems: Harmonic emission in 2 to 9 kHz frequency range

Grid distortion due to the high switching frequency of energy conversion systems has become a major concern that reduces power quality in renewable energy systems. However, the existing international standards (IEC 61000-3-2&12&16 and IEEE Std. 519) list grid harmonic limitations only up to the 40th or 50th harmonic. Due to limited knowledge of supraharmonic (Harmonics above 2 kHz) currents and voltages, an appropriate standard for this high frequency range is yet to be published. This paper presents an analysis of grid current distortion in the frequency range of 2 to 9 kHz when an active front end inverter is connected to the grid under different filter designs. New filter design approach is used to reduce or eliminate harmonics at frequencies beyond the ranges considered in conventional standards. The results show significant mitigation of high frequency harmonics in stiff grids. High frequency harmonic behavior with conventional and newly designed filters connected to a weak grid, are compared to find small amplitude current and voltage harmonics using the new filters. Furthermore, this study highlights the need for a new standard to address supraharmonics caused by grid-tied inverters

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

A novel approach in filter design for grid-connected inverters used in renewable energy systems

Renewable energy resources are utilized in distribution networks based on an Active Front End (AFE) technology as a bidirectional power flow energy conversion system. Low and high frequency harmonics generated by switching pattern of the power electronics converter should be reduced by an appropriate output filter. According to international regulations, harmonics injected to the grid by Pulse Width Modulated (PWM) power converters must be handled to maintain power quality indices within standard limits. LCL filters with passive damping resistors are the most reliable and renowned devices to fulfil the standards. The main objectives in the design of an efficient LCL filter are to reduce the cost and weight of the filter, as well as to increase the robustness and stability of the power electronics converter. In this paper, a comprehensive and mathematical approach is proposed to calculate the maximum current ripple of a PWM-based inverter, which helps to precisely calculate the size of the inverter-side inductors in the LCL filter. The mathematical approach to solve the problem of optimal damping is presented, which can be implemented analytically in different configurations of the LCL filters. The proposed method also works to solve the optimal damping problem for any type of filter