Harmonic Compensation Strategy Research Articles

A Direct Adaptive Controller With Harmonic Compensation for Grid-Connected Converters

Grid-connected converters are uncertain time-varying systems, whose controllers must deal with the relevant inductive content and high harmonic distortion of the sampled voltages. Adaptive controllers are feasible control strategies for these applications, because their gains are continuously adjusted in response to system perturbations, and they are also robust to unmodeled dynamics and exogenous disturbances. However, adaptive controllers are not necessarily capable of rejecting all harmonic components on grids with high harmonic distortion. Therefore, this work presents a Robust Model Reference Adaptive Control-based with an adaptive super-twisting sliding mode, which is improved with a harmonic compensation strategy in which additional control actions to deal with the harmonic contents of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$5^{th}$</tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$7^{th}$</tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$11^{th}$</tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$13^{th}$</tex-math></inline-formula> of the fundamental component of the electrical grid are incorporated into the control structure. Experimental results are provided of a 7.5 kW converter connected to a distorted and unbalanced grid with an LCL filter, where the developed control structure presents a total harmonic distortion (THD) of 2%, while the controller without harmonic compensation obtains a 2.5% THD.

A Decentralized Power Management and Sliding Mode Control Strategy for Hybrid AC/DC Microgrids including Renewable Energy Resources

This paper presents a new decentralized robust strategy to improve small and large-signal stability and power-sharing of hybrid AC/DC microgrids and improve its performance for nonlinear and unbalanced loads. In addition to the sliding mode controller for DC/DC converters, for the sake of improving power sharing and regulating active and reactive powers injected by distributed energy resources, and moreover, controlling harmonic and negative-sequence current in the presence of nonlinear and unbalanced loads, two separate controllers for positive sequence power control and negative sequence current control are designed based on the sliding mode control and Lyapunov function theory, respectively. The theoretical concept of the proposed robust control strategy, including mathematical modeling of microgrid components, basic theorems, controller design procedure, and robustness and closed loop stability analysis are outlined. Also, this direct power/current/voltage control scheme is governed by a new hybrid AC/DC hierarchical control scheme that exploits a harmonic virtual impedance loop and voltage compensation scheme. To show the effectiveness of the proposed robust control scheme, offline time-domain simulations are done on a hybrid AC/DC wind/photovoltaic/fuel-cell microgrid with nonlinear and unbalanced loads in MATLAB/Simulink environment, and the results are experimentally verified by OPAL-RT real-time digital simulator.

Adaptive PI + VPI Harmonic Current Compensation Strategy under Weak Grid Conditions

With the increase of the penetration rate of renewable energy power generation in the power system, the power grid gradually tends to be weak. Under the scenario of nonlinear load connecting to weak power grid, the grid-side voltage will produce unbalance, distortion and frequency deviation due to the influence of large internal impedance, insufficient inertia, three-phase current unbalance and harmonics of weak power grid. The operation of active power filters (APF) under above conditions can lead to significant degradation in harmonic compensation performance. An adaptive proportional integral (PI) + vector PI (VPI) harmonic compensation strategy for a three-phase APF is proposed for use in complex conditions of weak grid. The strategy consists of an adaptive notch filter based synchronous phase-locked loop (ANF-PLL) and an adaptive PI + VPI current control method. In this strategy, a series of adaptive lattice notch filters are introduced to improve the accuracy of grid phase and frequency estimation under the condition of voltage unbalance, distortion and a large range of frequency deviation. Then the phase and frequency information are used in the Park transformation of harmonic detection and current compensation control and the update of PI + VPI current control law parameters. A VPI current tracking controller with adaptive resonant frequency is constructed, which can improve the gain of the resonant frequency point and ensure that the APF can maintain the best compensation performance under the above weak grid conditions. Finally, the simulation and experimental results indicate that the ANF-PLL shows a better phase and frequency estimation performance under complex conditions of weak grid than the conventional and notch filter based methods, and the harmonic compensation strategy presented in this paper achieves a significant performance improvement under complex conditions of weak grid compared with Fixed VPI/SRF-PLL strategy and adaptive VPI/NF-PLL strategy.

An Efficient MPPT Technique-Based Single-Stage Incremental Conductance for Integrated PV Systems Considering Flyback Central-Type PV Inverter

Central-type photovoltaic (PV) inverters are used in most large-scale standalone and grid-tied PV applications due to the inverter’s high efficiency and low-cost per kW generated. The perturbation and observation (P&amp;O) and incremental conductance (IncCond) have become the most common techniques for maximum power point tracking (MPPT) strategies of PV/wind generation systems. Typically, the MPPT technique is applied in a two-stage operation; the first stage tracks the MPP and boosts the PV voltage to a certain level that complies with grid voltage, whereas the second stage represents the inversion stage that ties the PV system to the grid. Therefore, these common configurations increase the system size and cost as well as reduce its overall footprint. As a result, this paper applies two IncCond MPPT techniques on a proposed single-stage three-phase differential-flyback inverter (DFI). In addition, the three-phase DFI is analyzed for grid current negative-sequence harmonic compensation (NSHC). The proposed system efficiently provides a MPPT of the PV system and voltage boosting property of the DC-AC inverter in a single-stage operation. Moreover, the MPPT technique has been applied through the DFI using the conventional and modified IncCond tracking strategies. Furthermore, the system is validated for the grid-tied operation with the negative-sequence harmonic compensation strategy using computer-based simulation and is tested under uniform, step-change, as well as fast-changing irradiance profiles. The average efficiencies of the proposed system, considering the conventional and modified IncCond MPPT techniques, are 94.16% and 96.4% with tracking responses of 0.062 and 0.035 s and maximum overshoot of 46.15% and 15.38%, respectively.

Modular Single-Stage Three-Phase Flyback Differential Inverter for Medium/High-Power Grid Integrated Applications.

This paper proposes a single-stage three-phase modular flyback differential inverter (MFBDI) for medium/high power solar PV grid-integrated applications. The proposed inverter structure consists of parallel modules of flyback DC-DC converters based on the required power level. The MFBDI offers many features for renewable energy applications, such as reduced components, single-stage power processing, high-power density, voltage-boosting property, improved footprint, flexibility with modular extension capability, and galvanic isolation. The proposed inverter has been modelled, designed, and scaled up to the required application rating. A new mathematical model of the proposed MFBDI is presented and analyzed with a time-varying duty-cycle, wide-range of frequency variation, and power balancing in order to display its grid current harmonic orders for grid-tied applications. In addition, an LPF-based harmonic compensation strategy is used for second-order harmonic component (SOHC) compensation. With the help of the compensation technique, the grid current THD is reduced from 36% to 4.6% by diminishing the SOHC from 51% to 0.8%. Moreover, the SOHC compensation technique eliminates third-order harmonic components from the DC input current. In addition, a 15% parameters mismatch has been applied between the flyback parallel modules to confirm the modular operation of the proposed MFBDI under modules divergence. In addition, SiC MOSFETs are used for inverter switches implementation, which decrease the inverter switching losses at high-switching frequency. The proposed MFBDI is verified by using three flyback parallel modules/phase using PSIM/Simulink software, with a rating of 5 kW, 200 V, and 50 kHz switching frequency, as well as experimental environments.

Power quality improvement using three‐phase transformerless grid interfaced SECS enabling leakage current suppression

This paper presents a harmonic compensation strategy for a 3-ϕ grid-tied solar energy conversion system with leakage current attenuation feature. As parasitic capacitance exists between the solar photovoltaic panels with the ground, the deviation in common-mode voltage compels the probable safety hazards such as electromagnetic interference and harmonics injection in the grid currents. Additionally, the power quality indices of the grid, are exacerbated with the coupling of the non-linear load. Owing to these issues, a generalized integrator based approach is introduced to ensure the harmonics suppression, power factor correction, leakage current alleviation. Additionally, the grid current distortion limits are ensured within limits in order to comply with the IEEE std. 519. The presented algorithm ensures a robust operation with respect to various anomalies in the grid voltages. Simulation results show the satisfactory response of the harmonics controller even under abnormal operating scenarios including the symmetrical/unsymmetrical faults in the grid side network. In comparison with the traditional algorithms, the harmonic controller suppresses the leakage current and harmonics within restricted limits despite the abnormal conditions. Real-time simulator results demonstrate the performance of the controller under distinct operating conditions such as load unbalancing, distorted grid voltages, etc.

Switching-Table-Based Direct Torque Control of Dual Three-Phase PMSMs With Closed-Loop Current Harmonics Compensation

In this article, the new closed-loop current harmonics compensation strategy is introduced in the switching-table-based direct torque control (ST-DTC) of dual three-phase permanent magnet synchronous machines. The harmonic current distortion is the major problem of ST-DTC of multiphase machines and is typically caused by the lack of control in the auxiliary x-y subspace under the vector space decomposition model. The synthetic vectors to obtain average zero voltage in the x-y subspace can be a solution. However, this cannot compensate for current harmonics; for example, due to dead-time and back-electromotive force distortion, which is also mapped into the x-y subspace. Therefore, in this article, the x-y voltage references are obtained by the closed control loop that controls x-y currents to zero. To modulate the nonzero x-y voltage reference, the new modulation approach employing voltage vector groups in the x-y subspace together with the new switching table is developed and incorporated into ST-DTC. Meanwhile, the analysis of the two available sets of 12 voltage vector groups illustrates a tradeoff between linear range in the x-y subspace and the dc-link voltage utilization rate. Finally, the effectiveness of the proposed ST-DTC strategy is verified through experiments and simulation.

An Improved Lyapunov Function Based Control Strategy for Multitasking Distributed PV Inverter

The ELF (Extended Lyapunov Function) based control approach for single phase two-stage multitasking PV (photovoltaic) inverter is demonstrated in this paper. The introduced PV inverter can operate under the uncertainty of non-linear loads and PV characteristic beyond giving the closed feedback system stability analysis. The two stage PV inverter is introduced, in initial step a boost circuit is used to harnessing of crest power at PV panel side. Here time ratio of the boost circuit is planed with adaptive INC (Incremental Conductance Method). In the secondary step, the control of grid connected VSI (Voltage Source Inverter) is managed by ELF (Extended Lyapunov Function) strategy. To get best dynamic response, difference of PV feed forward to the loss component of DC bus capacitor. Other side, control of VSI and maintain stability of PV inverter, the ELF technique is reduces error in-between input and output, with there is no disturbance of equilibrium point in PV inverter. The PV inverter performance observed at changing of insolation and load variations using MATLAB /simulation. Various simulation results are presented to validate the harmonics compensation strategy.

Residential Distribution System Harmonic Compensation Using Priority Driven Droop Controller

As renewable energy based distributed generation (DG) units are being increasingly connected throughout today's distribution system, they can be used to mitigate harmonics caused by the wide adoption of nonlinear residential loads. To make the best use of all these DGs' ratings, it is important to develop a method to coordinate DGs' participation efforts in harmonic compensation according to their ratings and locations. Due to the low droop slope for the harmonic controller in DGs, traditional harmonic droop control methods can lead to significant harmonic sharing errors. Also, very limited work has been carried out in literature so far to identify the DGs' compensation priorities according to their locations and power rating. To address this issue, a novel priority-driven, droop-based, selective harmonic compensation scheme is developed in this work. The proposed control scheme improves the harmonic sharing accuracy. The compensation priority design and the way to integrate with droop control is studied. To ensure stability, a virtual impedance modelbased stability analysis is also discussed. Analysis, comparisons, and simulation results are used to verify the improvement of compensation performance.

Leakage Current Suppression in Double Stage SECS Enabling Harmonics Suppression Capabilities

This work deals with a harmonic control strategy for a transformerless double stage solar energy conversion system (SECS) to alleviate the leakage current in the presence of nonlinear loads enabling harmonics suppression capability. In this SECS, the PV module stray capacitance can elicit leakage current, which causes electromagnetic interferences and potential safety problems. To address these issues, the model predictive controller based adaptive harmonic compensation strategy is presented herein, which not only provides leakage current suppression but it also serves manifold ancillary power quality improvement features. The revised IEEE-519-2014 standard is incorporated in adaptive harmonic compensation strategy to achieve harmonics within recommended limits. The comparative response with a conventional control is exhibited to verify the capabilities of harmonic control strategy. Furthermore, a comparative harmonic suppression capability is analyzed herein to validate the competence of voltage controller. Various real-time simulated results are discussed to manifest capabilities of this control. The leakage current is restricted below 300 mA as per desirable safety requirements against leakage current of VDE-00126-01 and IEC-62109-2 standards. The total harmonic distortion (THD) of grid current and voltage, are achieved within permissible limits in compliance with the revised IEEE-519-2014 code.

Coordinated Operation of Energy Storage Systems for Distributed Harmonic Compensation in Microgrids

Energy storage systems (ESSs) bring various opportunities for a more reliable and flexible operation of microgrids (MGs). Among them, energy arbitrage and ancillary services are the most investigated application of ESSs. Furthermore, it has been shown that some other services could also be provided by ESSs such as power quality (PQ) improvements. This issue could be more challenging in MGs with widespread nonlinear loads injecting harmonic currents to the MG. In this paper, the feasibility of ESSs to act as coordinated active harmonic filters (AHF) for distributed compensation was investigated. An optimization model was proposed for the coordination of the harmonic compensation activities of ESSs. The model takes into account the various technical and systematic constraints to economically determine the required reference currents of various AHFs. Simulation cases showed the performance of the proposed model for enhancing the harmonic filtering capability of the MG, reduction in the compensation cost, and more flexibility of the distributed harmonic compensation schemes. It was also shown that ESS activities in harmonic compensation do not have much of an effect on the ESSs revenue from energy arbitrage. Hence, it could make ESSs more justifiable for use in MGs.

Improved Voltage Harmonics Compensation Strategy for a Stand-Alone DFIG Operating under Unbalanced and Nonlinear Load Conditions

The study presents an improved control and operation for a stand-alone Doubly-Fed Induction Generator (DFIG) energy system under unbalanced and nonlinear load conditions. This type of loads results in unbalance and distorted stator voltage and current at the Point of Common Coupling (PCC), with subsequent power quality degradation and electromagnetic torque pulsations of DFIG. A new and simple control strategy based Proportional-Resonant Controllers (PRCs) in the stationary reference frame is applied in the Load-Side Converter (LSC) for fully compensate the unbalance and harmonic distortion in the PCC and avoid the torque pulsations of DFIG. As a result, symmetrical and sinusoidal AC voltage is applied to the PCC, which indicates that the Rotor-Side Converter (RSC) can be still controlled with the traditional vector control strategy, to adjust the generator torque and to balance active power. This strategy is conceptually simple, it does not require for rotating coordinate transformation and the sequence decomposition process, in addition no current sensors need to be installed at load or stator side, so that the number of current sensors employed is reduced. Simulation results using MatLab/Simulink are given and show the effectiveness and validity of the proposed control scheme to eliminate the harmonics in the PCC and torque pulsations of DFIG.

Harmonic Compensation Strategy for Single-Phase Cascaded H-Bridge PV Inverter Under Unbalanced Power Conditions

Cascaded H-bridge (CHB) inverter has the advantages of easy modularization, the low harmonic content of output current, and that all power units are capable of achieving independent maximum power point tracking (MPPT). However, aging or partial shielding of photovoltaic (PV) panels will lead to the unbalanced output power of each PV module, which will result in the overmodulation of H-bridges and increased harmonic components of the grid current. In this article, a novel harmonic compensation strategy for single-phase CHB inverter is proposed, it can extend the linear modulation range of single-phase CHB inverters to about 1.27. Moreover, this strategy will not reduce energy harvesting, and also can maintain the system to operate in the unit power factor. Simulation and experimental results all verify the effectiveness of the proposed strategy.

Comparison between PI, PR+HC, and modified PR+HC current controller in inverter system

&lt;span&gt;This paper presents the comparison between proportional integral (PI) current controller, proportional resonance and harmonic compensator (PR+HC) current controller and modified PR+HC current controller in the inverter system. Power electronic components like inverter and current controller uses in the system produce unwanted harmonics that affect the quality of distribution power network. In this study, development and simulation of current controller using conventional proportional integral (PI), the selective harmonic compensation scheme (PR+HC), and modified version of the latter are considered so to overcome these harmonics injection. Modification is by adding control parameter randomisation technique to the PR+HC scheme. Results compare the three controllers and proved that with modification to the selective harmonic compensation scheme, the overall current THD can be reduced&lt;/span&gt;

Harmonic Compensation Strategy for Extending the Operating Range of Cascaded H-Bridge PV Inverter

For single-phase cascaded H-bridge (CHB) photovoltaic (PV) inverters, all the PV modules are able to operate at the maximum power point, which is beneficial to energy harvesting. However, due to the stochastically variable irradiance level and ambient temperature, the unbalanced output powers among PV modules may cause the overmodulation of the H-bridges with higher power, ultimately resulting in a distorted grid current. In this paper, a novel harmonic compensation strategy is proposed. Compared to the existing methods, it can not only ensure that CHB inverter operates at unity power factor with small dc-bus voltage ripple but also keep the total harmonic distortion of grid current in acceptable range under some heavy power unbalance conditions, so it has a good overall performance. Simulation and experimental results validate the effectiveness and feasibility of the proposed method.

Design of a Cooperative Voltage Harmonic Compensation Strategy for Islanded Microgrids Combining Virtual Admittance and Repetitive Controller

Nonlinear loads (NLLs) in three-phase systems are known to produce current harmonics at −5, 7, −11, 13,… times the fundamental frequency. However, harmonics of the same frequencies are induced in microgrid voltage thereby reducing the power quality. Dedicated equipment, such as active power filters, can be used to compensate the microgrid harmonics. Alternatively, each distributed generation (DG) unit present in the microgrid can also be used for the same. The use of the virtual admittance concept combined with a proportional integral with resonant controller (PI-RES) control structure has previously been proposed as a harmonic compensation sharing strategy when multiple DGs operate in parallel. The drawback of this methodology is that a large number of RES controllers might be required to compensate for all the harmonic components induced by NLLs, increasing the tuning complexity as well as the execution time. This paper proposes the combined use of virtual admittance control loop and repetitive controller (RC) for harmonic compensation. The main advantage of the proposed method is that only one RC is required to compensate for all the harmonic components, significantly reducing the computational burden and the design complexity. The dynamic performance of the whole system is tested under variable NLL.

Simple Harmonics Compensation Method for Smart Charger with Constant DC-Capacitor Voltage-Control for Electric Vehicles in Single-Phase Three-Wire Distribution Feeders

This paper discusses harmonic current compensation of the constant dc-capacitor voltage-control (CDCVC)-based strategy for smart charger for electric vehicles (EVs) in single-phase three-wire distribution feeders (SPTWDFs) under distorted load current conditions. The basic principle of the CDCVC-based harmonics compensation strategy under distorted load current conditions is discussed in detail. The instantaneous power flowing into the three-leg pulse-width modulated (PWM) rectifier, which acts as a smart charger, shows that the CDCVC-based strategy achieves balanced and sinusoidal source currents with a unity power factor (PF). The CDCVC-based harmonics compensation strategy does not require any calculation blocks of fundamental reactive, unbalanced active, and harmonic currents. Thus, the authors propose a simplified algorithm to compensate for reactive, unbalanced active, and harmonic currents. Simulation and experimental results demonstrate that balanced and sinusoidal source currents with a unity PF in SPTWDFs are obtained on the secondary side of the pole-mounted distribution transformer during both the battery-charging and battery-discharging operations in EVs, compensating the reactive, unbalanced active, and harmonic currents.

An Optimized Third Harmonic Compensation Strategy for Single-Phase Cascaded H-Bridge Photovoltaic Inverter

Due to the unequal solar radiations or dust accumulation of photovoltaic modules in a single-phase cascaded H-bridge photovoltaic inverter, the unbalanced output power among photovoltaic modules will make the H-bridges with higher power overmodulation, resulting in deteriorated grid current. Concerning this issue, this paper proposes an optimized third harmonic compensation strategy that injects moderate amount of third harmonic into the overmodulation H-bridges to keep the amplitudes of their modulation waveforms just being unity, then compensates the same amount of opposite harmonic sequence and properly distributes it to rest of the H-bridges. The proposed method can ensure that all the power units will be free from overmodulation under some heavy power imbalance conditions and will not increase the third harmonic component of the grid current. The validity and effectiveness of the proposed method is verified by simulation and experimental results.

A Harmonic Compensation Strategy in a Grid-Connected Photovoltaic System Using Zero-Sequence Control

Mitigation of harmonics for a grid-connected inverter is an important element to stabilize the control and the quality of current injected into the grid. This paper deals with the control method of a three-phase Grid-Connected Inverter (GCI) Photovoltaic (PV) system, which is based on the zero-sequence current adjuster. The proposed method is capable of removing the harmonic current and voltage without using any active and passive filters and without the knowledge of the microgrid topology and also impedances of distribution bands and loading conditions. This concept is adopted for the control of a Distributed Generator (DG) in the form of grid-connected inverter. The proposed control can be applied to the grid connected inverter of the PV. The fast dynamic response, simple design, stability, and fast transient response are the new main features of the proposed design. This paper also analyzes the circuit configuration effects on the grid connected inverter capability. The proposed control is used to demonstrate the improved stability and performance.

Flexible harmonic current compensation strategy applied in single and three-phase photovoltaic inverters

Non-linear loads connected to power system have increased considerably in the recent decades. Traditional generation based on hydro and thermal power plants cannot mitigate harmonic currents. However, due to the increased connection of photovoltaic into the power system, its power electronic converter can be used to perform ancillary services such as harmonic current compensation. This work presents a dynamic method based on the Second Order Generalized Integrator coupled with a phase locked loop structure to detect and compensate the most predominant harmonic current components from the power system. It is also presented an extension to detect multiple harmonics, according to the amplitude. Technical issues related with the harmonic current compensation strategy, and its implementation for both single and three-phase PV inverters are explored to demonstrate the functionality and efficiency of the method. The results show the harmonic current compensation being compensated by a PV inverter. Finally, a losses analysis were performed in the LCL filter. The magnetic and damping resistor losses are more impacted due to high harmonic current compensation.