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Repetitive controllers represent an attractive solution for systems having periodic reference or disturbance signals with high harmonic content. In order to obtain some performance improvements for three-phase plants, as grid-connected converters, complex repetitive controllers were proposed later in literature. In this article, the advantages of complex repetitive control systems over the real ones are analyzed. In addition, sufficient conditions for the input–output stability of complex repetitive control systems and modified complex repetitive control systems are obtained from the small gain theorem. These stability conditions are then used to derive the stability domains for complex repetitive control systems. Further, a relative stability analysis based on the sensitivity function is introduced. An experimental application based on an active power filter is used to exemplify the effectiveness of the proposed stability analysis. [ABSTRACT FROM AUTHOR]

The growth of renewable energy generation and its integration to the electrical grid began to affect the safety and stability of the electrical system. Therefore, as renewable energy sources, such as wind and solar photovoltaic, slowly began to replace conventional plants, grid integration requirements became one of the main concerns of the researchers. As consequence, new technical requirements have been established to ensure the stability of the electrical system. In this scenario, this paper performs a review of the current requirements for the integration of renewable power plants to the electrical grid. The review is carried out by comparing the main requirements related to voltage stability, frequency stability, power quality, and active and reactive power regulations in relation to grid stability. The present study indicates that, although current integration requirements may improve stability, further improvements are still needed. [ABSTRACT FROM AUTHOR]

Integration of distributed generation (DG) such as photovoltaics (PV) represents a challenge for the traditional operation of distribution power systems. As the installed power of DGs grows, grid codes are being modified to involve DGs in the provision of ancillary services. This includes the ability to ride-through large disturbances. In this paper, the behavior of a single-stage PV system under unbalanced voltage conditions is studied, and a fault ride-through control scheme is proposed which is able to support the grid through the injection of reactive power. Furthermore, adjustable power quality is enabled as a tradeoff between power ripple and current harmonics. The control scheme makes use of a current controller based on the space vector Fourier transform concept. No rotational transformation is required, and zero steady-state error is ensured when tracking distorted current references. The controller was tested in detailed PSCAD/EMTDC computer simulations, and implemented in a real grid-connected PV system to demonstrate its performance under unbalanced voltage conditions. [ABSTRACT FROM PUBLISHER]

In this paper, a controller based on the space vector Fourier transform (SVFT) is proposed. The vector controller is implemented in the stationary \alpha\beta reference frame and can be designed to track unbalanced and distorted three-phase reference signals containing specific positive-sequence, negative-sequence, and harmonic components. In order to evaluate the proposed controller closed-loop response, three-phase unbalanced and distorted reference currents are imposed in a grid-connected voltage source converter. The ability of the controller to follow the references with zero steady-state error is proved through frequency response plots and stability analysis. Experimental results are obtained to confirm the properties of the proposed scheme. [ABSTRACT FROM PUBLISHER]

Repetitive controllers (RCs) are linear control structures based on the internal model principle. This control strategy is known for its ability to control periodic reference signals, even if these signals have many harmonic components. Despite being a solution that results in a good performance, several parameters of the repetitive controller need to be correctly tuned to guarantee its stability. Among these parameters, one that has high impact on the system performance and stability is the finite impulse response (FIR) filter, which is usually used to increase the stability domain of RC-based controllers. In this context, this paper presents a complete tutorial for designing the zero-phase FIR filter, which is often used to stabilize control systems that use RC-based controllers. In addition, this paper presents a Matlab® application developed for performing the stability analysis of RC systems and designing its FIR filter. Simulation and experimental results of a shunt active power filter are used to validate the algorithm and the Matlab® application. [ABSTRACT FROM AUTHOR]

In this paper, two methods for determining the fundamental frequency and harmonic positive- and negative-sequence components of three-phase signals are investigated. Many aspects of the space-vector discrete Fourier transform and generalized delayed signal cancellation (GDSC) such as response time for different possible implementations, frequency adaptation schemes, stability of recursive implementation, and rounding error effects are discussed. A new design procedure for GDSC transformations is presented. New indices for characterizing three-phase unbalanced and distorted signals are proposed. Simulations and experiments are included in order to verify the performances and illustrate the theoretical conclusions. [ABSTRACT FROM PUBLISHER]

In this paper, a space-vector discrete-time Fourier transform is proposed for fast and precise detection of the fundamental-frequency and harmonic positive- and negative-sequence vector components of three-phase input signals. The discrete Fourier transform is applied to the three-phase signals represented by Clarke's αβ vector. It is shown that the complex numbers output from the Fourier transform are the instantaneous values of the positive- and negative-sequence harmonic component vectors of the input three-phase signals. The method allows the computation of any desired positive- or negative-sequence fundamental-frequency or harmonic vector component of the input signal. A recursive algorithm for low-effort online implementation is also presented. The detection performance for variable-frequency and interharmonic input signals is discussed. The proposed and other usual method performances are compared through simulations and experiments. [ABSTRACT FROM AUTHOR]

A novel scheme for obtaining the fundamental-frequency positive-sequence grid voltage vector based on a generalization of the delayed signal cancellation method is proposed in this paper. The technique is implemented by sampling and storing the instantaneous αβ voltage vector. A mathematical transformation is then proposed through which the current and delayed voltage vectors are combined. It is shown that the proposed transformation has unity gain for the fundamental-frequency positive-sequence voltage vector, while its gain is equal to zero for some chosen components. Cascaded transformations can then be used for eliminating the fundamental-frequency negative-sequence vector, as well as chosen positive- and negative-sequence harmonic vector components and, thus, for accurately obtaining the fundamental-frequency positive-sequence voltage vector. The output of the last transformation block is input to a synchronous reference frame phase-locked loop for detecting frequency and position of the positive-sequence vector. A proposal for making the scheme frequency adaptive is also presented. The good performance of the proposed method is verified with simulations and experiments by using distorted and unbalanced signals, containing fundamental-frequency as well as positive- and negative-sequence harmonic components. The proposed method frequency adaptation capability is also verified. [ABSTRACT FROM AUTHOR]

A stator field oriented control scheme for induction motor drives is proposed. As a deadbeat controller, the algorithm calculates the voltage vector to be supplied by the frequency converter in order to eliminate the flux and torque errors each switching period. The strategy is a DTC scheme in the sense that there are no current controllers with the advantage of a fixed switching frequency. Simulation and experimental results are presented to show the effectiveness of the proposed method and the quality of torque and flux transient responses. [ABSTRACT FROM AUTHOR]

The fast detection of magnitude, frequency, and phase angle of the fundamental-frequency component is very useful in many three-phase power system applications. Most techniques to accomplish this task are based on different versions of three-phase phase-locked loop (PLL) schemes. Some variables of three-phase systems may experience phase angle jumps after some disturbances. In this paper, a method for the fast detection of phase angle jump occurrence, estimation of the phase angle jump, and its use for improving the performance of the generalized delayed signal cancelation (GDSC) PLL is proposed. The proposed method is based on the detailed evaluation of the GDSC filter transient behavior after a phase jump. Results considering typical three-phase grid disturbances are used to evaluate the proposed scheme, in comparison with other usual PLL techniques. The use of the proposed phase jump angle estimator allowed a reduction in the GDSC-PLL convergence time of about 50%. [ABSTRACT FROM AUTHOR]

In this paper, a generic complex repetitive controller ($nk + m$ RC, $k\in \mathbb {Z}$), based on the inverse transfer function of the generalized delayed signal cancelation transformation is proposed. The presented complex controller can be designed for regulating different positive- and negative-sequence harmonic components through a space-vector error signal, with improved response time as compared to traditional repetitive controllers. An evaluation of the proposed control scheme stability, based on phase margin, Nyquist diagram, and sensitivity function is presented. The effects of the controller parameters on the system's stability and also on its dynamic response are shown and allow the definition of some control design guidelines. Experimental results are presented to validate the theoretical assumptions through applying the proposed controller for regulating the currents in a three-phase active power filter. [ABSTRACT FROM AUTHOR]

The increasing rate of renewable energy penetration in modern power grids has prompted updates to the regulations, standards, and grid codes requiring ancillary services provided by photovoltaic-generating units similar to those applied to conventional generating units. In this work, a comprehensive survey presents a comparison of requirements related to voltage ride through reactive current injection/absorption; active power restoration; frequency stability regulation and active power control; voltage regulation and reactive power control; and the energy quality requisites included in the standards and grid codes of countries around the globe. The survey can be used to observe the differences between the requirements established in the grid codes depending on the power system operating characteristics, development of technology, and renewable energy penetration level. Many of these factors determine the parameters used to establish requisites for different grid codes, making a global standardization of the renewable energy interconnection requirements much harder. [ABSTRACT FROM AUTHOR]

This paper shows a scheme for improving the detection performance of the fundamental-frequency positive-sequence (FFPS) space vector through the generalized delayed signal cancellation phase-locked loop (GDSC-PLL). We implemented a variable-structure scheme in order to reduce the convergence time by changing the traditional datapath whenever even harmonics are not present in the input signal. If this condition is not true, the performances of the variable-structure GDSC-PLL and the traditional GDSC-PLL for detecting the FFPS space vector will be similar. Therefore, the variable-structure GDSC-PLL does not require a previous knowledge about the harmonic components of the three-phase signal. Furthermore, we improved the frequency-adaptive scheme. The performances of the original and proposed schemes were compared through transient test cases based on IEC 61000 and typical electrical events. The new approach led to a reduction of up to 62% in the convergence time. [ABSTRACT FROM PUBLISHER]

Fundamental-frequency and harmonic positive- and negative-sequence components detection is an important task for implementing power converters for renewable energy systems, uninterruptible power supplies, active power filters, dynamic voltage restorers, and also for power systems protection relays. Detection techniques of this kind are generally implemented in digital signal processor (DSP) with the execution time limited by the sampling period. The computational effort of the control algorithm can considerably increase the execution time, due to the sequential nature of processing in DSP. A promising technique for sequence components separation of three-phase signals is the so called the generalized delayed signal cancelation—phase locked loop (GDSC-PLL). Field programmable gate array's (FPGA's) capacity of exploring the parallelism of operations present in the GDSC-PLL is demonstrated in this paper through the mapping of this technique directly in hardware, allowing for a much shorter execution time than in DSP. The proposed architecture is presented, and the efficient detection of the fundamental-frequency positive-sequence with FPGA is demonstrated, with the obtained results compared with a traditional DSP implementation. In particular, the advantages and possibilities of the use of FPGA are demonstrated in comparison with the DSP. For this comparison, a metric for evaluating the capacity of complexity increase in application algorithms is proposed. [ABSTRACT FROM PUBLISHER]

In this paper, a novel scheme for obtaining the fundamental-frequency positive-sequence grid voltage is proposed. The method is based on four simple mathematical transformations; two of them are in the stationary reference frame, which are able to eliminate odd harmonics from the original signals. The other two transformations are implemented in a synchronously rotating reference frame in order to eliminate even harmonics. The output of the last transformation block is the input to a synchronous reference-frame phase-locked loop for detecting the frequency and position of the positive-sequence voltage vector. The proposed algorithm was verified through simulations and experiments by applying distorted and unbalanced signals, containing positive and negative-sequence components. The results are in agreement with those theoretically predicted and indicate that the proposed scheme has a great potential for use in grid-connected converter synchronization algorithms. [ABSTRACT FROM AUTHOR]

Abstract-This paper presents a modulation technique based on the generalized pulsewidth-modulation strategy for matrix converters. The proposed technique uses a discontinuous modulation to clamp each output leg of the converter during 1200 of the output voltage period, achieving a reduced number of switchings compared with the traditional modulation techniques. Aside from that, the major attraction of the proposed technique is an additional algorithm that lags the clamping of each output leg of the converter to synchronize it with the peak of the corresponding output current (load current), avoiding high switching losses (switching at high currents). Therefore, this technique reduces the number of switchings as well as guarantees only medium and low current switchings. Simulation and experimental results show the efficiency of the proposed technique. [ABSTRACT FROM AUTHOR]

Voltage profile is one of many aspects that affect power quality in low-voltage distribution feeders. Weak grids have a typically high line impedance which results in remarkable voltage drops. Distribution grids generally have a high R/X ratio, which makes voltage regulation with reactive power compensation less effective than in high-voltage grids. Moreover, these networks are more susceptible to unbalance and harmonic voltage disturbances. This paper proposes an enhanced coordinated control of active and reactive power injected in a distribution grid for voltage regulation. Voltage drop mitigation was evaluated with power injection based on local features, such loads and disturbances of each connection. In order to ensure disturbances rejection like harmonic components in the grid voltages, a repetitive vector-control scheme was used. The injection of coordinated active and reactive power with the proposed control algorithm was verified through simulations and experiments, demonstrating that it is a promising alternative for voltage regulation in weak and low-voltage networks subject to inherent harmonic distortion. [ABSTRACT FROM AUTHOR]

The main contribution of this paper is the proposal of new modulation techniques for three-phase transformerless neutral point clamped inverters to eliminate leakage currents in photovoltaic systems without requiring any modification on the multilevel inverter or any additional hardware. The modulation techniques are capable of reducing the leakage currents in photovoltaic systems by applying three medium vectors or using only two medium vectors and one specific zero vector to compose the reference vector. In addition, to increase the system utilization, the three-phase neutral point clamped inverter can be designed to also provide functions of active filter using the p–q theory. The proposed system provides maximum power point tracking and compensation of current harmonics and reactive power. To validate the simulation models, an experimental three-phase inverter is used to evaluate leakage currents and the dc link voltage control. [ABSTRACT FROM AUTHOR]

In this paper, a modified Z-source inverter (ZSI) with specific modulation techniques is proposed to reduce leakage currents in three-phase transformerless photovoltaic (PV) systems. The new topology only requires an additional fast-recovery diode when compared with the original structure. On the other hand, the pulsewidth modulation technique is entirely modified in order to reduce the leakage currents through the conduction path. Simulation results for the three-phase transformerless PV system operating in two cases, i.e., connected to a grid and connected to a grounded RL load, are presented. Experimental results of leakage currents in three-phase ZSIs connected to a RL load are obtained to validate the theoretical and simulation models. [ABSTRACT FROM AUTHOR]

In some photovoltaic (PV) applications, it is possible to remove the transformer of a system in order to reduce losses, cost, and size. In transformerless systems, the PV module parasitic capacitance can introduce leakage currents in which the amplitude depends on the converter topology, on the pulsewidth modulation, and on the resonant circuit comprised by the system components. Based on the common-mode voltage model, modulation techniques are proposed to eliminate the leakage current in transformerless PV systems without requiring any modification on the converter and any additional hardware. The main drawback is that the proposed modulation technique for two-level inverters can only be used with 650-V dc link in the case of a 110-V (rms) grid phase voltage. Comparisons among the modulation techniques are discussed, and it is proven that the proposed modulation for two- and three-level inverters presents the best results. To validate the models used in the simulations, an experimental three-phase inverter is used. [ABSTRACT FROM AUTHOR]