Current Harmonic Content Research Articles

Active disturbance rejection control of three-phase LCL power conversion system under non-ideal grid conditions

Under non-ideal grid operating conditions such as unbalanced grid voltage and harmonic grid that are commonly found in microgrid conditions, the negative sequence components of the microgrid voltage interfere with the active and reactive current controls in the power conversion system, leading to an increase in the harmonic content of the grid-side current and affecting the power quality of the microgrid system. To solve these problems, firstly, the mathematical model of the LCL-type power conversion system is analyzed, and a linear active disturbance rejection control based on model compensation is designed. Secondly, the influence of non-ideal power grid conditions on the control of the LCL-type power conversion system is analyzed, and the active disturbance rejection control strategy of the LCL-type power conversion system based on frequency-locked loops with harmonic cancellation modules (HCM-FLL) is proposed, which speeds up the system, improves the system’s robustness, and reduces the harmonic content of the network measurement current under the condition of power grid voltage unbalance and harmonic power grid. Finally, by using the verification of MATLAB/Simulink simulation, the current power quality obviously under the condition of voltage unbalance and harmonic power grid is evidently improved by the proposed control strategy. When compared to the traditional control methods, the control strategy proposed in this study features a simple control structure, making it easy to implement in engineering without requiring high controller performance or additional circuits. This reduces design costs and provides a wide range of controller parameters, ensuring strong anti-interference performance without the need for frequent controller parameter adjustments.

Algorithm for Fast Detection of Stator Turn Faultsin Variable-Speed Synchronous Generators

Faults between stator winding turns of synchronous generators have led to specific changes in the harmonic content of currents. In this paper, these changes are used to detect faults in variable-speed synchronous generators connected to three-level converters during their operation. Currents typically measured for control purposes are used to avoid installation of additional sensors. The neutral point current of the three-level converter is also evaluated under faults in the generator. Encoder-tuned dynamic filters based on Park transformation and Fourier coefficients together with low-pass filters are used to detect the selected harmonics under variable speeds. The geometric loci of these components are proposed as a method to distinguish between healthy and faulty conditions. Simulation and experimental data are used to test sensitivity, selectivity and detection time of the proposed technique for different fault types. Generalization for a different generator is also presented and tested. Most fault cases were detected using the harmonics.

Model Predictive Control for Open Winding PMSM Considering Dead-Zone Effect

To improve the steady-state performance in conventional single vector model predictive control (CSV-MPC) of open winding permanent magnet synchronous motor (OW-PMSM) with common dc bus, an optimized model predictive control (MPC) considering dead-zone effect is proposed. First, the influence of dead-zone voltage vector (DZVV) in CSV-MPC is analyzed, and the process of judging DZVV is summarized. Then, the optimized MPC method considering dead-zone effect is put forward where the applied time of DZVV is redistributed. Finally, the experiment verifies that the proposed method can effectively reduce the current harmonic content without increasing the switching frequency and the suppression of zero-sequence current (ZSC) can be guaranteed.

Axial and radial electromagnetic‐vibration characteristics of converter transformer windings under current harmonics

AbstractThe winding current harmonic content of converter transformer in operation is rich. However, affected by harmonics, the radial vibration characteristics are quite different from the power transformer, structural loosening and deformation caused by winding vibration are more prominent. Therefore, in view of the influence of current harmonics on axial and radial vibrations, this study builds a 3D winding electromagnetic vibration finite element simulation model, analyses the components and contents of current harmonics, compares the leakage flux and vibration distribution under the fundamental, harmonic and superimposed currents. The correlation relationship between the harmonic components and axial and radial vibration characteristics is obtained, and the vibration measurement test of ±800 kV converter transformer in operation is carried out to verify the correctness of the analysis method. The results show that the winding radial vibration amplitude will exceed the axial vibration affected by harmonics. More importantly, under the superposition of the fundamental, 5th and 7th harmonic current, the vibration contribution of the converter transformer tank is mainly radial vibration, and the vibration dominant frequency is shifted from 100 Hz under the fundamental current to 400 Hz.

Optimization of Improved PID Control Strategy Based on Genetic Algorithm

Conventional PID control, although has the advantage that the robustness and adaptability, has certain limitations, but for nonlinear systems is difficult to achieve the desired effect. When the environment changes, control parameters cannot be adjusted in time. Aiming at these problems, this paper presents an improved PID control strategy based on genetic algorithm. Taking single-phase LCL as the research object, by analyzing the operation process of single-phase LCL grid-connected inverter and optimizing genetic algorithm, the genetic iteration process is optimized through adaptive adjustment, and the weight of objective function is adjusted, so that the system can respond quickly, better monitoring of grid-connected current improves the robustness of the system. So as to better ensure the stability of grid connection. Finally, the MATLAB model is built for simulation. Simulation endings indicate that the boosted control strategy has great stability and can effectively decrease the grid current harmonic content. The improved algorithm is global and has relatively high error tolerance, which can meet the needs of grid connection.

Analysis of Torsional Vibrations on the Resolver Under Eccentricity in PMSM Drive System

Since any error in the rotor angle caused by the resolver affects the entire permanent magnet synchronous motor (PMSM) control drive system, it is necessary to investigate its effect on mechanical vibrations. Torsional vibration is accordingly considered in the following study. Thereby, first, the effect of resolver errors on the harmonic content of motor current and, subsequently, torque ripple (TR) was investigated. Then, dynamical modeling is done for the generator–motor–resolver set to get the mechanical frequency response (FR) resulting from the resolver errors. Based on the above modeling, the dynamic equations (DEQs) of the system were derived, and the mechanical characteristics of the set, including natural frequency values and mode shapes, were extracted. Thereafter, TR was employed in the DEQs, and the mechanical FR caused by the resolver error was obtained. Eventually, comparing mechanical FR with electrical FR was done according to diagnosis superiority. All analyses were done using experimentally verified models.

Combined feed-forward control strategy for LCL-type grid-connected inverters

LCL-type grid-connected filter circuits are widely used for line network transmission because of their high ability to suppress current harmonics. However, due to the presence of resonance points in this third-order circuit, the system can generate severe oscillations. In order to improve power quality, a combined feed-forward factor control strategy is proposed in this paper. The control system introduces a combined feed-forward factor and incorporates the idea of dual PI control to ensure that the order of the system is reduced while improving its stability, followed by a detailed derivation of the constraint equations for each link parameter. The simulation verifies that the control scheme can regulate the output voltage of the line network and effectively reduce the harmonic content of the line network, providing a reference for reducing the harmonic content of grid-connected currents in engineering.

Input and output total currents characterization in BCM and CCM Interleaved Power Converters Under Inductance Mismatch

• Input and output currents for topologies of BCM-CCM interleaved power converters under inductance mismatch are quantified. • Proposed method is based on the analysis of the geometry of each phase and the selective sum of the phases value in certain key points. • To present a general methodology to obtain the required key points, a waveform that includes the key points of all types of phase currents is presented. • Once the selective sum is performed, it is possible to compute any of total current characteristics. • The characterization method can be used as a tool for a complete analysis, design and optimization of interleaved power converters. This paper presents a complete characterization of input and output currents in interleaved power converters with inductance mismatch, operating in Boundary Conduction Mode ( BCM ) and Continuous Conduction Mode ( CCM ). The proposal allows to compute these currents in several interleaved converter topologies for the entire range of operating points, considering any number of phases and any inductance ratio. Input and output currents are recovered from the values obtained when adding the phase currents in the instants where the slopes change; values that are thus defined as key points. This methodology is based on the coincidences that exist between the instants of the phase current key points and those of total currents. By using the computed key points, ripple amplitude, rms value and harmonic content of input and output total currents for the entire range of operating points can be easily obtained. Simulations are conducted on a 5-phase boost converter and a 5-phase buck converter under different conditions in order to validate the proposal expressions. Experimental tests on a 5-phase buck converter are presented under different operation conditions to verify that the proposed method can be applied in real situation.

Frequency-Based Active Ripple Compensation Technique to Reduce Bulk Capacitance in Integrated Offline LED Drivers

In order to increase the power density and the lifespan of LED drivers, several studies have proposed Active Ripple Compensation (ARC) techniques for minimizing the converter bulk capacitance. However, a common side-effect of the ARC method is the increase in the Total Harmonic Distortion (THD) of the converter input current. In this context, a frequency-based ARC technique is proposed as an alternative to the conventional ARC methodology, in which the duty-cycle is modulated. The analyses presented in this paper show that the frequency-based ARC approach applied to resonant converters has a better performance than the one used in hard-switching converters, since it allows for a huge capacitance reduction with a small THD increase. Furthermore, this paper presents a generalized analysis of ARC techniques applied to integrated off-line LED drivers, which reduce the number of components and the overall LED driver cost. The input current harmonic content and the output ripple reduction have been theoretically predicted for several converters. Experimental results gathered from a 96-W laboratory prototype supplied from a 127-V 60-Hz grid attested the superior performance of the frequency-based active ripple compensation, since a capacitance reduction of 66.6% has been obtained with an increase of only 0.9% in the THD.

Vibration Measurement and Numerical Modeling Analysis of Transformer Windings and Iron Cores Based on Voltage and Current Harmonics

The operating condition and structural state of the converter transformer are closely related to vibration. Abundant harmonics aggravate the vibration of windings and iron cores, resulting in frequent mechanical structural failures, which seriously affect the stable operation of the power system. Traditional research mainly focuses on the vibration of AC transformers without harmonics and there is no in-depth discussion of the vibration mechanism and the numerical calculation model of windings and iron cores under harmonics. In addition, the influence of harmonics, winding connection method and other factors on the vibration characteristics are not clear. Therefore, this paper analyzes the voltage and current harmonic components and contents, establishes a harmonic-vibration numerical model and compares the vibration time-frequency characteristics with or without harmonics and different valve side winding connections through vibration measurement experiments. Finally, a combined simulation analysis reveals the contribution of the windings and core to the tank. The results show that the tank vibration amplitude and dominant frequency will increase under harmonica and the valve side current will affect the dominant frequency. Among these results, when there are harmonics, the amplitude increases by three times, the vibration dominant frequency changes from 100 Hz to 400 Hz and the frequency spectrum widens to 2000 Hz. In particular, the contribution of the winding vibration under the harmonic current will exceed the iron core. The research results reveal the influence of converter transformer harmonics on vibration, which can provide a theoretical basis for numerical calculation of vibration and monitoring of operating conditions and guide the design of structural vibration reduction to reduce mechanical failures caused by vibration.

Research on Harmonic Generation Mechanism of PSM High-Voltage Power Supply in Dynamic Operation

To study the harmonic generation mechanism of high-voltage power supply (HVPS) in dynamic operation, the mathematical model of the PSM HVPS is built. The mathematical model of the equivalent circuit is established by analyzing the basic operational unit of the HVPS. The initial parameters of the mathematical model are updated according to the number of the modules in series on the load side, and the key electrical parameters set in the mathematical model are obtained. After that, according to the number and the position of the series modules in the HVPS, the current superposition and phase shift calculation are carried out to obtain the harmonic current content in the power grid. This article also verified the accuracy of the mathematical model and calculating method by building a module simulation and HVPS simulation model. Furthermore, the harmonic generation characteristics in the dynamic operation of the HVPS are studied in this article.

Low–Harmonic Control Strategy of a Dual Three–Phase Synchronous Reluctance Motor Based on Three–Vector Synthesis

Dual three–phase synchronous reluctance motors (DTP–SynRM) have the advantages of simple structure, high power density, fast dynamic response, and small torque ripple, and have broad application prospects in flywheel batteries. However, the synchronous reluctance motor has no permanent magnet, and the inductance value will change with the current change in actual operation. Direct torque control (DTC) is more suitable for the control strategy of dual three–phase synchronous reluctance motors because of its low dependence on motor parameters. However, traditional direct torque control uses a large vector control motor within one control period, which can not suppress the inherent 5th and 7th current harmonics in the motor. A new harmonic suppression method is proposed in this paper: that is, using a low harmonic vector to replace a large vector in traditional direct torque control, which can be synthesized by adjusting the action time and order of three adjacent large vectors within one control period. Through this improvement, torque control can have a harmonic suppression effect. The three vector synthesis method can make full use of the existing space voltage vector, and to adapt to different working conditions, two synthesis methods of switching frequency reduction and constant torque response are proposed. An improved direct torque control strategy is obtained by optimizing the design of the switch table using a new vector synthesis method. Finally, the suppression effect of traditional DTC and improved DTC on current harmonics is compared and analyzed. The results show that the direct torque control with low harmonic vector can suppress the harmonic current in x–y subspace, and the current harmonic content is greatly reduced.

Magnetic Field Analysis and Operating Characteristics of a Brushless Electrical Excitation Synchronous Generator With DC Excitation

To realize brushless excitation for an electrical excitation synchronous generator (EESG), a brushless-induced double electrical excitation synchronous generator (IDEESG) with dc excitation is investigated in this study. The static magnetic field is established by injecting dc into the stator excitation windings that are added in the stator slots. Rotor windings are divided into two parts: induction coils and rotor excitation winding. The induction coils cut the static magnetic field and provide dc current for the rotor excitation winding after the rectification. The direction of the currents in the induction coils is constant because of the half-wave rectification. The magnetic flux generated by the currents in the induction coils and rotor excitation winding is added in the air gap to induce the armature coils in the stator. In this study, the structure and the operating principle are examined. The frozen permeability method is adopted to study the influence of the currents in the induction coils and the rotor excitation winding on the magnetic field distribution. The harmonic content of currents in the induction coils is analyzed using the Fourier series form and the effect of the harmonics on the output voltage is also studied. A 1.5 kVA prototype is fabricated to verify the correctness of theoretical analysis and simulation.

Non-characteristic harmonics in modular multilevel converters—An analytical approach

This paper analyzes the harmonic content of the arm currents and voltages of the modular multilevel converter (MMC) by using the dynamic phasor (DP) theory. DP modeling of the MMC facilitates a study without relying on simplifying assumptions on the harmonic content of the currents and voltages. In doing so, the DP-MMC model used in this paper reveals generalized conditions under which non-characteristic harmonics can exist. These conditions offer a detailed insight into the transient behavior of harmonics in the MMC. An open-loop DP model of the MMC is developed, which has the harmonics of the arm currents and voltages as independent state variables. The linearized state-space matrices of the DP model are assessed under different grounding options at the ac side of the MMC, and the relationship between characteristic and non-characteristic harmonics of the MMC voltages and currents is discussed.

A Novel Integrated Method for Harmonic Suppression and Reactive Power Compensation in Distribution Network

Aiming at the serious harmonic pollution and the low power factor in the distribution network of industrial enterprises, this paper develops an integrated method for harmonic suppression and reactive power compensation suitable for the distribution network of industrial enterprises. The integrated method realizes the dual functions of harmonic filtering and reactive power compensation, and filters out the harmonic current to get a symmetrical current waveform while ensuring safe operation of the power compensator. In addition, it solves the problems of high harmonic content, small power factor in the distribution network, and device burnout caused by direct input of reactive power compensator. The main contributions of this paper are as follows: (1) According to the demand for the integration of harmonic suppression and reactive power compensation, the steps of integrated method for harmonic suppression and reactive power compensationare proposed, and then the methods for harmonic filtering and reactive power compensation are investigated; (2) a method for designing the capacity of a filter capacitor and the rated parameter of an electromagnetic coupling reactance converter is proposed, and an optimization simulation system is constructed to design the parameters of the filter; (3) a simulation system is developed, followed by parameter design and simulation analysis of harmonic filtering subsystem (HFSS), reactive power compensation subsystem (RPCSS) and the integrated system of harmonic suppression and reactive power compensation. Simulation results verify that the HFSS is put into operation first and then switched off later to ensure the normal operation of other equipment in the distribution network. After the treatment, the power factor, harmonic current content and total distortion rate all meet the national standards. The integrated method can dynamically track harmonics and reactive power changes, filter out harmonics, improve power factor and the symmetry level of the power source, and ensure the normal operation of other equipment in the distribution network. The research results lay a certain theoretical and technical foundation for the harmonic filtering and reactive power compensation theory, technology and its device innovation to achieve effective suppression of power harmonics and reactive power compensation.

Harmonic Analysis and Attenuation Strategy for a Two-Stage Matrix Converter Fed by Dual-Inverter Based on Pulse Barycenter Method

Since the DC-link of the dual-inverter two-stage matrix converter (TSMC) has no energy storage element, its grid-side current waveform is closely related to the load-side condition. The conventional modulation strategy of the dual-inverter TSMC is only the stack of the double inverters, without considering the cooperation between both modulation strategies. Thus, the fluctuations in the grid-side currents synthesized by both load-side currents will be obvious. To solve this problem, this paper analyzes the effect of the current pulse position on the grid-side current waveform quality, and relevant relationship between the pulse position and current harmonics is obtained according to Fourier transform. On this basis, the harmonic attenuation strategy for the dual-inverter TSMC is proposed. In the proposed strategy, the grid-side current harmonic is reduced based on the optimal duty-cycle combination, obtained from the harmonic optimization problem. The simulation and experimental results are provided to verify that the theoretical analysis of grid-side current harmonic content is consistent with the actual condition, and the proposed modulation strategy can effectively reduce the grid-side current harmonic.

Evaluation of the VLSS-LMS Method Parameters’ Influence on the Effective Estimation of Power System Current and Voltage Harmonics Content

Nowadays, power quality is considered as a major concern that is faced by electrical energy producer, consumers, industries and researchers. Whereas one among the main issues in such concern is the current and voltage harmonics detection, mitigation and compensation. Indeed, the proliferation of harmonics in power system can create serious problems within all levels and their impacts along the whole power system from the production to the end users can lead to critical technical and economic situations. Therefore, their accurate detection in real time is of great importance and interest where many techniques have been proposed for solving such major problem. The work presented in this paper deals with the evaluation of a technique that has been proposed for the estimation of harmonic content in a signal based on iterative computing technique. This technique is based on several keystone parameters, which have major influences on the accuracy of the identification of the different harmonics without the need of a costly computation, and that can be easily implemented and performed in real-time. It is worthy to clarify that the technique under investigation in this paper is the based on the idea of adaptive filtering process called the Variable length Step Size- Least mean squares method (VLSS-LMS), it is basically very sensitive to three main parameters such as the sampling frequency, the step size value and the measurement window. The evaluation of the effects of these parameters have been assessed based on simulation testes of a specific original signal containing harmonics components. The obtained simulation results clearly prove the optimal choices of the aforementioned parameters in ensuring precise estimation of the harmonics content and the reconstruction of the estimated signal with high precision and minimized average error.

A Simple 18-Pulse Star Rectifier Using Two Passive Auxiliary Circuits at DC Link

In this article, a simple 18-pulse star rectifier using two new passive auxiliary circuits at dc link is proposed for reducing the harmonics content in ac line current. The proposed rectifier combines a double-star rectifier and four-tapped interphase reactor (FTIPR) with four diodes. Passive auxiliary circuits increase the state of the output voltage and current of the double-star rectifier, thereby increasing the pulse number of the load voltage and step number of the input line current. When FTIPR is optimally designed, passive auxiliary circuits convert double-star rectifier to 18-pulse star rectifier. Tap ratios of the FTIPR are determined based on minimum total harmonic distortion (THD) of the input line current and minimum ripple of the output voltage. The proposed rectifier simultaneously reduces ripple of the output voltage and THD of the input line current. A prototype is developed to verify the theoretical analysis through experimental results. Theoretical THD of the input line current is about 10.1%, whereas experimental THD is about 5.44%.

All-SiC Traction Converter for Light Rail Transportation Systems: Design Methodology and Development of 165 kVA Prototype

The design and development of a high-performance 165 kVA, 750 V DC all-silicon carbide (SiC) traction converter for new generation light rail transportation systems (LRTSs) are described. In the design of the traction motor drive, the efficiency of the overall system is maximized and the line current harmonic content of the traction motor is minimized. A complete mathematical model of the physical system is derived to carry out real-time simulations and proper control of the LRTS on a real rail track. The electrical and thermal performances of traction-type SiC power MOSFET modules are compared with those of alternative hybrid and Si-IGBT modules for various switching frequencies. The implementation of the developed system is also described. The performance of the resulting system is verified experimentally on a full-scale physical simulator as well as for various track conditions. Very promising results for the next generation railway traction motor drives have been obtained in terms of performance criteria, such as very high efficiency, low harmonic distortion of the motor line current, low cooling requirement, relatively high switching frequency, and hence, superior controller performance. The effects of the SiC power MOSFET operation on the insulation of the available traction motors are also examined experimentally. This paper is accompanied by a video demonstrating the experimental work.

Defining and Specifying Design Considerations for Distribution Transformers in Large-Scale Solar Photovoltaic Plants

In the new era of the renewable energy market in South Africa, there is a knowledge gap in the technical standards relating to distribution transformers, which are exclusively intended to serve in large-scale solar photovoltaic (PV) applications. The problem arises from the reality that these transformers are compelled to facilitate an environment with a potentially high risk of an unacceptable level of harmonics and distortion. What manufacturers currently need is a comprehensive transformer specification for DT transformers in PV applications, as the standards currently in use for regular transformers have so far proven to be inadequate. The latter triggers the need for this work to contribute to the advancement of this knowledge gap by mapping a comparative analysis of defining and specifying the design considerations for solar PV transformers. Initially, the computation of the various transformer losses at multifaceted active-part structures under normal conditions by employing the proposed finite element analysis (FEA) is presented. Then, two design case studies are described and the harmonic load current content to which the transformer will be susceptible during operation is specified and used for the analysis. Lastly, the design losses computed using the proposed FEA are substantiated using practical measurements.