As recent advances in power plants and energy resources expand the applications of DC-DC converters, ensuring their reliability and stability remains critical. This necessitates accurate parameter estimation for effective condition monitoring. In this context, this paper presents a novel digital twin model for DC-DC converters, combining mechanism modeling with data-driven techniques to achieve high-fidelity simulations and accurate parameter estimation. The proposed framework integrates a physics-based converter model with a long short-term memory (LSTM) neural network for error compensation, enhanced by an ant lion optimizer (ALO) algorithm for parameter estimation. It has been demonstrated on a buck converter that the proposed method outperforms in modeling accuracy and parameter estimation precision compared to conventional approaches. Extensive simulation results validate the framework’s effectiveness in condition monitoring and parameter estimation, showcasing its potential for power converter health assessment.

Grid integration of renewable energy (REN) requires efficient and reliable power conversion stages, particularly with an increasing demand for high controllability and flexibility seen from the grid side. Underpinned by advanced control and information technologies, power electronics converters play an essential role in large-scale REN generation. However, the use of power converters has also exposed several challenges in conventional power grids, e.g., reducing the system inertia. In this article, grid integration using power electronics is presented for large-scale REN generation. Technical issues and requirements are discussed with a special focus on grid-connected wind, solar photovoltaic, and energy storage systems. In addition, the core of the energy generation and conversion—control for individual power converters (e.g., general current control) and for the system level (e.g., coordinated operation of large-scale energy systems)—is briefly discussed. Future research perspectives are then presented, which further advance large-scale REN generation technologies by incorporating more power electronics systems.

Abstract Learning methods have been increasingly used in power engineering to perform various tasks. In this paper, a fault selection procedure in double-circuit transmission lines employing different learning methods is accordingly proposed. In the proposed procedure, the discrete Fourier transform (DFT) is used to pre-process raw data from the transmission line before it is fed into the learning algorithm, which will detect and classify any fault based on a training period. The performance of different machine learning algorithms is then numerically compared through simulations. The comparison indicates that an artificial neural network (ANN) achieves remarkable accuracy of 98.47%. As a drawback, the ANN method cannot provide explainable results and is also not robust against noisy measurements. Subsequently, it is demonstrated that explainable results can be obtained with high accuracy by using rule-based learners such as the recently developed quantitative association rule mining algorithm (QARMA). The QARMA algorithm outperforms other explainable schemes, while attaining an accuracy of 98%. Besides, it was shown that QARMA leads to a very high accuracy of 97% for highly noisy data. The proposed method was also validated using data from an actual transmission line fault. In summary, the proposed two-step procedure using the DFT combined with either deep learning or rule-based algorithms can accurately and successfully perform fault selection tasks but indicating remarkable advantages of the QARMA due to its explainability and robustness against noise. Those aspects are extremely important if machine learning and other data-driven methods are to be employed in critical engineering applications.

Two-three (2/3)-level dual-active-bridge (DAB) DC-DC converters have high potential to be applied in the high-voltage (HV) applications due to their higher voltage blocking capability compared to the two-level DAB converters. However, capacitor voltage balancing will be a crucial issue for the neutral-point-clamped (NPC) bridge in the HV side. In order to regulate the capacitor voltages, this paper proposes a complementary-small-vector (CSV) balancing control scheme. According to the relationships among the polarity of the neutral-point current, the small vectors, and the polarity of the transformer current, two CSV pairs are obtained. In the proposed scheme, if a small vector is diagnosed as an adverse small vector for the capacitor voltage balance, it will be replaced by its CSV. After that, the polarity of the neutral-point current can be changed to increase the required charge injected into or drawn from the neutral point. Meanwhile, the waveforms of the current and voltage during the balancing process will remain unchanged. Therefore, the power fluctuation and current overshoot can be avoided. Furthermore, the implementation of the proposed scheme under different operating modes are demonstrated. Finally, the simulation results verify the performances of the proposed balancing control scheme.

Integrated energy systems become more and more important to realize the energy complementary property. Micro-integrated energy system, served as the terminal integrated energy system, will have the electricity delivered directly to the local customers by energy hubs (EHs). Here, the data and information of the EHs during the operation are confidential and should be kept by each owner. Therefore, this article designs a dual-decomposition-based distributed algorithm to address this problem, where the optimal consensus problem is used for the dual problem to update the multipliers. The primary and dual problems are alternatively solved until the Karush-Kuhn-Tucher condition is satisfied. For the proposed distributed algorithm, the feasibility can be strictly guaranteed during the iteration process. Moreover, theorems and lemmas are proved for the linear convergence rate. The numerical results verify the effectiveness of the proposed algorithm.

In this period when the technology has been developing rapidly, resources are being exhausted as well conversely. Therefore, possible problems and the ways of handling them are changing and new problem-solving techniques are being tried. Due to the intermittent nature of photovoltaic (PV) systems, which have solar irradiance and temperature as a source, the problem of maximum power attaining arises. The solution to this problem aims to make optimal use of PV energy production. This study presents a metaheuristic algorithm to solve the problem of maximum power point tracking (MPPT) from PV systems which are an indispensable part of renewable energy technology. Symbiotic organisms search (SOS), a powerful and dynamic metaheuristic optimization algorithm, is adopted as a solution to this problem. The SOS algorithm has been inspired by the symbiotic interactions adopted their behavior to survive in the ecosystem, which has developed to solve optimization and engineering problems. The proposed algorithm, i.e., SOS, has been embedded in MATLAB/Simulink platform to test for accuracy and efficiency. From the obtained results, this evolutionary SOS algorithm is seen obviously to outperform in certain points more than the classical Perturb and Observe (P&O) and Incremental Conductance (INC) methods for the same system and conditions.

In this paper, a novel 5-level inverter with common ac and dc ground is proposed for solar photovoltaic (PV) applications. The newly established common-ground-T-type (CGT-type) inverter is a single-stage topology restructured from the combination of a classical T-type inverter and a front-end three-level boost converter. Its distinctive topological structure can effectively eliminate the common mode voltage issue, prevent leakage currents, and ensure a continuous input current in PV systems. More importantly, it requires only two capacitors to realize an ac voltage which can be significantly boosted from a single dc source. A pulse width modulation (PWM) scheme is also established to undertake the following tasks simultaneously: charging of the boost inductor with a constant duty-cycle, and switching of two capacitors for 5-level ac voltage generation. The operation and feasibility of the proposed CGT-type inverter is analyzed and validated through simulation results.

This paper proposes an optimal modulation scheme for two-three (2/3)-level dual-active-bridge (DAB) DC-DC converters to guarantee zero-voltage-switching (ZVS) operation and minimize the current stress. Based on the unified ZVS principle, six operating modes are defined and constraints of each ZVS mode are then obtained. According to the operating modes, the power transmission model and current stress under all voltage conversion ratios are derived for optimization. The analytical solutions for minimum current stresses are obtained by using the Lagrange multiplier method with Karush-Kuhn-Tucker conditions. With the proposed modulation scheme, both the conduction and switching losses can be reduced, and thus, the entire efficiency can be improved. Finally, simulation results verify the theoretical analysis and the performance of the optimal modulation scheme.

In order to achieve low leakage currents, a symmetrical Z-source hybrid converter is proposed to simultaneously obtain a wide range of DC and AC output voltages. The proposed converter is derived from a traditional Z-source converter by replacing the control switch to a highly efficient and reliable inverter concept (HERIC) inverter. To be applied in PV applications, the impedance network is symmetrical, which can clamp the common-mode voltage of the proposed converter to be a constant (i.e., obtaining low leakage currents). A dedicated modulation scheme is introduced so that the symmetrical Z-source hybrid converter can achieve low leakage currents, a wide range of output voltages, high efficiency and reactive power injection. Simulations are performed on a 2-kW symmetrical Z-source hybrid converter to verify the effectiveness.

With the aim to enhance the system reliability and reduce the system cost of induction motor drives, the speed-sensorless control, implemented by various speed estimation schemes, is demanded. The speed estimation provided by the second-order generalized integrator-frequency-locked loop (SOGI-FLL) is emerging as a promising method for speed-sensorless control. However, most of the existing SOGI-FLL schemes may undergo performance degradation during frequency ramps. Additionally, the SOGI-FLL scheme has an insufficient capability to reject disturbances, e.g., DC offsets. To address these issues, a speed estimation scheme based on an improved SOGI-FLL scheme for speed-sensorless control of induction motor drives is proposed. In this scheme, a new FLL is utilized as the speed estimator, which achieves accurate estimation with frequency ramps. Specifically, by employing a second-order SOGI-based flux observer, acceptable robustness of the proposed estimation scheme against DC offsets is obtained. Simulations are performed to investigate the performance of the proposed speed estimation scheme and a comparison with the conventional SOGI-FLL scheme is made.

Nowadays, the entire world is facing challenges in energy and environment. To resolve these problems, the power systems are interconnected to promote the development of renewable energy sources (RESs). However, the economic dispatch (ED) problem for the global energy interconnection (GEI) should tackle two issues: 1) handle the uncertainty from RES and allocate the responsibility among the interconnected countries and 2) protect the information privacy through the dispatch. Motivated by the above, this article proposes a zonally adjustable robust decentralized ED model for the GEI. In the model, each country is only responsible for its own uncertainty, and tie-line power flows remain unchanged under uncertainties. Moreover, an alternating direction method of multipliers (ADMM)-based fully distributed algorithm is used, in which only limited information should be exchanged between neighboring countries. Finally, a case study on the Northeast Asian countries verifies the effectiveness of the proposed method. Note to Practitioners-Since the renewable energy generation has a spatial correlation among regional countries, global energy interconnection (GEI) aims to combine several power systems together to promote the renewable energy accommodation. However, two problems need to be considered: 1) Information Privacy: The information privacy of the power system in each country should be preserved, which prevents the GEI from conducting a centralized optimal dispatch framework and 2) Uncertainty: The uncertain output of renewable energy resources brings challenge to the power system secure operation. The main contribution of this article is to set up a zonally robust decentralized optimization for the GEI, where the zonally robust economic dispatch (ED) is conducted by the area control error (ACE) system to manage the difference between scheduled and actual generation under the uncertainties, and the alternating direction method of multipliers (ADMMs) algorithm is adopted for decentralizing the zonally adjustable robust ED model, which only needs limited information. In particular, this article uses a real-world example from Northeast Asian Countries to help engineers understand the advantages of the GEI and the new dispatch framework.

To increase the resilience of shipboard power systems, this paper presents an optimal defense strategy to protect critical lines against attacks. In the modeling, an attack is considered to destroy one critical bus, which may result in that all lines are connected to this bus will be out of service unless protection is enabled. Furthermore, after one true attack occurs, the network restoration is performed as soon as possible to pick up the critical loads and maintain system operation. To address the uncertain location of the attacks, a tri-level robust optimal defense strategy is set up to protect the critical lines under the worst attack. Moreover, a nested column-constraint generation method is employed to solve this model. A 60-bus shipboard power system is studied to demonstrate the effectiveness of the proposed model and the defense method.

The quasi-Z-source inverter (qZSI) can operate as a single-stage buck-boost inverter with a higher step-up gain when compared with the conventional two-stage inverter. The adoption of the shoot-through state makes the qZSI possible to operate under the short-circuit and open-circuit conditions. With these features, the qZSI is suitable for photovoltaic (PV) applications, considering the low-level and intermittent output characteristics of the PV panels. However, the control of the qZSI requires more detailed modeling analysis in terms of dynamics. Thus, this paper discusses the small-signal modeling of the qZSI and its dynamics. To validate the derived models and the analysis, both the frequency-domain and time-domain performances of the qZSI are presented and compared.

In this paper, a new active neutral point clamped (ANPC) five-level (5L) inverter is proposed. The conventional ANPC output voltage is half of the input voltage. To rectify the above problem, a floating capacitor is used to boost the output voltage equal to the input voltage, which further reduces the size of the dc-link capacitors. In addition to this, the proposed topology does not need any sensors to balance the floating capacitor, which minimize the complexity of the inverter. Further, in the proposed topology, the neutral point is directly connected to the mid-point of dc-link capacitors, which reduces the leakage current and common mode voltage due to the presence of two clamping diodes. Moreover, a new carrier signal is introduced, and the advantages are discussed. The performance of proposed 5L inverter topology is analysed in terms of dynamic loading condition. To show the superiority of the proposed topology, it is simulated in MATLAB and results are discussed.

LCL filters is an effective solution to suppress the switching harmonic in PWM converter. However, the LCL filter may introduce resonance pole to the current control loop, and challenge the current control stability. Thus, active damping control is commonly required to stabilize the system. In this paper, an all-pass filter is employed to reshaping the system open-loop phase-frequency curve to stabilized the converter side current controlled single-phase AC/DC converter. By introducing phase-lag with a second-order all-pass filter, the - 180° phase crossing at the resonant frequency is shifted to a designed point with negative gain. By doing so, there no -180° phase crossing with gain above 0 dB, and the Nyquist stability criterion tells that the current control stability promised. Design guide for the all-pass filter is given based on two frequency points phase reshaping to guarantee system stability with certain grid impedance changes and the LCL parameters drifts. Simulations and experiments performed on a 1kW single-phase PWM rectifier to verify the effectiveness of the active damping method.

State estimation is a fundamental function in modern energy management system, but its results may be vulnerable to false data injection attacks (FDIAs). FDIA is able to change the estimation results without being detected by the traditional bad data detection algorithms. In this paper, we propose an accurate and computational attractive approach for FDIA detection. We first rely on the low rank characteristic of the measurement matrix and the sparsity of the attack matrix to reformulate the FDIA detection as a matrix separation problem. Then, four algorithms that solve this problem are presented and compared, including the traditional augmented Lagrange multipliers (ALMs), double-noise-dual-problem (DNDP) ALM, the low rank matrix factorization, and the proposed new “Go Decomposition (GoDec).” Numerical simulation results show that our GoDec algorithm outperforms the other three alternatives and demonstrates a much higher computational efficiency. Furthermore, GoDec is shown to be able to handle measurement noise and applicable for large-scale attacks.

Damping the LCL-filter resonance is an essential issue for grid-connected inverters. In most of the prior-art solutions, active damping focuses on a dual-loop architecture, e.g., using the capacitor-current to damp the resonance, thus complicating the entire control loop and/or increasing the cost. Digital filters (such as notch and all-pass filters) directly cascaded into the current controller, referred to as a single-loop scheme, are thus more economical, which feature no extra sensors and a simple open-loop scheme with easy parameter tuning. This paper proposes a practical single-loop damping method for grid-current feedback control (GCF) based on a first-order digital low-pass filter, which realizes damping by lagging the phase and meanwhile reducing the magnitude response for a sufficient gain margin. Simulations verify the proposed method.

The recently proposed current-fed trans-quasi-Z-source inverter (CF-trans-qZSI) can buck or boost voltage and provide a bidirectional power flow. Thus, it has received much attention. To minimize the current ripples and output harmonics, this paper presents a novel space vector pulse width modulation (SVPWM) strategy for the CF-trans-qZSI. The switching sequence in this SVPWM strategy for the boost mode is realized by partitioning equally the short zero state time into four parts; while, for the buck mode, it is implemented by dividing the short zero state time and open zero state time into more parts, where the time interval of each part is not equal. In both modes, the divided short zero state time, open zero state time, and two-active-state time are symmetrical in one switching cycle. Simulation and experiments are performed to validate the performance of the proposed strategy. The results show that the proposed SVPWM algorithm can improve the performance of the CF-trans-qZSI in terms of reducing the DC inductor current ripples and output current harmonics in both modes.

Compared with a conventional two-stage inverter, the quasi-Z-source inverter (qZSI) has several advantages for renewable applications because of its buck-boost capability and continuous input current. This paper addresses modeling and control issues resulted from the impedance-source network resonance. Considering shoot-through states, null states, and active states, the modulation ratio is included in the modeling process. Unlike prior-art modeling methods to separate the DC and AC parts, the proposed modeling method considers the impedance source network as an open-loop impedance at the input of the AC side. The resonant characteristic of the converter is investigated and the AC controller is designed in detail. Simulations are presented to validate the modeling and control methods.

Quasi-Z-source cascaded H-bridge (qZS-CHB) inverters are one promising solution for high power photovoltaic (PV) systems. This type of topologies inherits the advantages of cascaded converters (i.e., multilevel outputs) and impedance-source inverters (i.e., high conversion ratios). In addition, it allows increasing the inverter reliability (with high redundancy). However, the modulation and control of qZS-CHB inverters are challenging to a certain extent. Thus, this paper proposes modified modulation techniques to increase the performances of qZS-CHB converters in terms of voltage gains and stresses. The novelty lies in the use of the switching frequency optimal as reference signals in the modulation techniques. A comparison in terms of harmonics with selected modulation techniques is performed.

This paper presents a new multilevel breed of XY converter family, called LY Multilevel Boost Converter (LY-MBC) topologies for renewable energy applications. Four LY-MBC topologies are designed to obtain a high voltage conversion ratio and which also offer an operative solution for renewable energy systems with minimal number of components. Existing LY converter topologies includes L-L, L-2L, L-2LC and L-2LCm converter topologies and LY converter is one of the breed of the XY converter family. The proposed LY-MBC topologies are derived by attaching the Cockcroft Walton (CW) multiplier to the Y converter of LY converter topologies. The proposed LY-MBC converter topologies are compared with existing LY converter topologies in terms of voltage conversion ratio and number of components. The noticeable features of the proposed LY-MBC topologies are also discussed in details. The working operation, feasibility, and design of the proposed LY-MBC topologies are verified by simulations.

The fuel supply system plays an important role in combustion engines, and thus a higher reliability is required. A novel fault diagnosis and prevention strategy are presented in this paper. The diagnosis model is built up based on deviation calculation by introducing the flow sensor principle, which take reference flow rate and measured flow rate as criteria. The estimation function between the fuel flow rate, outlet pressure and pump rotating speed is obtained, which could be used to estimate the fuel flow rate. The estimation method can provide the flow rate to the entire system and then keep the fuel supply system working reliably even when the sensors are in failure. Experimental results show the effectiveness of the proposed strategy in terms of accurate fault detection and robust prevention in the fuel supply system.

This paper explores the fault tolerant capabilities of an Embedded Enhanced-Boost Z-Source Inverter (EEB-ZSI) for PV applications. Compared with the prior-art Embedded Source Inverters (E-ZSI) and Enhanced-Boost Z-Source Inverter (EB-ZSI), the proposed topology features that when one dc source (e.g., PV panel) is short-circuited (SC) or open-circuited (OC), the inverter can tolerate the faults and still operate with a compromised conversion ratio. However, the conversion ratio is still larger than the traditional E-ZSI. This topology can be further applied to the cascaded H-bridge inverter systems for multi-level applications with fault-handling capabilities. A detailed fault-tolerant analysis is conducted on the EEB-ZSI and simulations are provided to validate the analysis.

As an important part of multiple-function and intelligent single-phase grid-connected Photovoltaic (PV) system, fault ride-through (FRT) has attracted extensive attention in the recent years. According to the continuously updated grid-connected requirements, the inverters should remain connected with grid under grid faults even the grid voltage dips to zero, and provide reactive current injection to support the grid recovery. In this paper, a contrastive evaluation of two intuitive and simple flexible power control methods, which are implemented in two different reference frame, are presented in the case of zero-voltage ride-through (ZVRT) operation. Only the dq-rotating reference frame power control method provides an acceptable ability of ZVRT. In addition, the performance is verified in both the simulation and experiment results.

In this paper, magnetically-coupled impedance source (MCIS) three-phase four-switch inverters are introduced for renewable energy applications. This inverter utilizes two single-phase MCIS inverters to obtain a three-phase output, where less component-count is achieved in order to reduce the inverter volume. On the other hand, as a result of the reduced number of switches, the proposed inverter suffers from higher voltage stresses. The MCIS inverters are first reviewed. Then, the modulation and operation principle of the MCIS three-phase inverter topology are introduced before sizing the passive components. A comparison among the similar candidates is done. Finally, the analysis is verified by simulations.

This paper presents a family of cost-effective magnetically-coupled impedance source inverters for renewable energy systems. The inverters are derived from magnetically-coupled impedance source networks, featuring low cost and no ground leakage current when used in PV system. The numbers of required semi-conductor switches is reduced. More important, the elimination of leakage currents makes it particularly suitable for high-efficiency photovoltaic (PV) applications. A comparison among prior-art PV inverters is then performed. The Performances of the inverters are evaluated analytically as well as by simulations. The analysis and simulation show the superiority of the proposed inverter in terms of leakage current suppression and low cost.

The reliability aspect study of Modular Multilevel Converter (MMC) is of great interest in industry applications, such as offshore wind. Lifetime prediction of key components is an important tool to design MMC with fulfilled reliability specifications. While many efforts have been made to the lifetime prediction of IGBT modules in renewable energy applications by considering long-term varying operation conditions (i.e., mission profile), the justifications of using the associated mission profiles are still missed. This paper investigates the impact of mission profile data resolutions and electrical power modeling methods on the estimated lifetime of IGBT modules in an MMC for offshore wind power application. In a 30 MW MMC case study, an annual wind speed profile with a resolution of 1 s/data, 10 minute/data, and 1 hour/data are considered, respectively. A method to re-generate higher resolution wind speed data from lower resolution data is introduced as well. Based on the wind speed data, IEC 61400-12-1 power curve model and a wind speed-power stochastic model are compared as well. Five mission profile modeling scenarios are compared in terms of the predicted lifetime of the IGBT modules used in the MMC, resulting in significant differences. The study serves as a first step to quantify the impact of mission profile modeling on lifetime prediction, and to provide a guideline on mission profile collection for the presented application.

In order to reduce the Common-Mode Voltage (CMV) in three-to-five phase indirect matrix converters, three improved Space Vector Pulse Width Modulation (SVPWM) methods are proposed and discussed. The improved modulation schemes are achieved by reorganizing zero vectors from the inversion stage to the rectification stage. The proposed methods can not only theoretically reduce the CMV peak value by up to 47.1%, but also decrease the switching losses of the power devices. Moreover, comparisons show that a superior CMV reduction performance is achieved when only two large active voltage vectors (AVVs) in the inversion stage, which results in a large amount of third-order harmonics in output currents. In addition, the method that utilizes two adjacent active current vectors (ACVs) and the method that uses two non-adjacent ACVs in the rectification stage have the same CMV peak value. By contrast, the latter achieves a lower Total Harmonic Distortion (THD) level of the output currents. Simulation results verify the effectiveness of the proposed methods.

Continuously expanding deployments of distributed power-generation systems (DPGSs) are transforming the conventional centralized power grid into a mixed distributed electrical network. The modern power grid requires flexible energy utilization but presents challenges in the case of a high penetration degree of renewable energy, among which wind and solar photovoltaics are typical sources. The integration level of the DPGS into the grid plays a critical role in developing sustainable and resilient power systems, especially with highly intermittent renewable energy resources. To address the challenging issues and, more importantly, to leverage the energy generation, stringent demands from both utility operators and consumers have been imposed on the DPGS. Furthermore, as the core of energy conversion, numerous power electronic converters employing advanced control techniques have been developed for the DPGS to consolidate the integration. In light of the above, this paper reviews the power-conversion and control technologies used for DPGSs. The impacts of the DPGS on the distributed grid are also examined, and more importantly, strategies for enhancing the connection and protection of the DPGS are discussed.

In many important energy conversion systems, the power electronic converters are proven to have high failure rates. At the same time, the failures of the power electronics systems are becoming more and more unacceptable because of the high cost of failures and booming power capacity. As a consequence, an appropriate assessment of reliability performance for the power electronics is a crucial and emerging need, because it is the essential information for the design improvements as well as for the extension of converter lifetime, and reduction of the cost-of-energy. Unfortunately, there is still lack of suitable and cost-effective tools for the reliability assessment in power electronics. In this paper, an advanced design tool structure, which can acquire various reliability metrics of the power converters, is proposed. The proposed reliability design tool is based on the failure mechanisms in the critical components of the power electronics system, and the mission profiles in the converter applications are also taken into account. Finally, the potential methodologies, challenges and technology trends involved in this tool structure are also discussed.

Grid-connected converters employ LCL-filters, instead of simple inductors, because they allow lower inductances while reducing cost and size. Active damping, without dissipative elements, is preferred to passive damping for solving the associated stability problems. However, large variations in the grid inductance may compromise system stability, and this problem is more severe for parallel converters. This situation, typical of rural areas with solar and wind resources, calls for robust LCL-filter design. This paper proposes a design procedure with remarkable results under severe grid inductance variation. The procedure considers active damping using lead-lag network and capacitor current feedback. Passive damping is also discussed. The design flow, with little iteration and no complex algorithms, selects the proper ratios between the switching and resonance frequency, the grid and converter inductance, and the filter capacitance and total inductance. An estimation for the grid current total harmonic distortion (THD) is also proposed. Simulation and experiments validate the proposals.

Grid converters require a simple inductor or an LCL-filter to limit the current ripples. The LCL-filter is nowadays the preferred solution as it allows lower inductance values. In order to solve the stability concerns, active damping is preferred to passive damping since it does not use dissipative elements. However, large variations in the grid inductance and resonances arising from parallel converters may still compromise the system stability. This calls for a robust design of LCL-filters with active damping. This paper proposes a design flow with little iteration for two well-known methods, namely lead-lag network and current capacitor feedback. The proposed formulas for the resonance frequency, grid and converter inductance ratio, and capacitance of the LCL-filter allow calculating all the LCL-filter parameters. An estimation for the achieved Total Harmonic Distortion (THD) of the grid current is also provided. Experimental results show very robust designs to the parameter variations.

The increased penetration of renewable energy systems, like photovoltaic and wind power systems, rises the concern about the power quality and stability of the utility grid. Some regulations for Low Voltage Ride-Through (LVRT) for medium voltage or high voltage applications, are coming into force to guide these grid-connected distributed power generation systems. In order to verify the response of such systems for voltage disturbance, mainly for evaluation of voltage sags/dips, a Voltage Sag Generator (VSG) is needed. This paper evaluates such sag test devices according to IEC 61000 in order to provide cheaper solutions to test against voltage sags. Simulation and experimental results demonstrate that the shunt impedance based VSG solution is the easiest and cheapest one for laboratory test applications. The back-to-back fully controlled converter based VSG is the most flexible solution for the system test under grid faults but also the most expensive one.