Harmonic Compensation Performance Research Articles

Clustered Voltage Control and Modulation Scheme With Switching Loss Reduction for High Power Hybrid Cascaded Converter

The hybrid cascade converters (HCC) is a potential solution to achieve harmonic current compensation in medium and high voltage applications due to the characteristics of having high frequency module. However, the high switching frequency inevitably leads to an increase of undesirable switching loss. In addition, since the reduction of power loss and the balance of clustered voltage are mutually interrelated, the existing modulation and control schemes cannot cope with them decently. To address this problem, the HCC is divided into a high-frequency module (HFM) and a low-frequency module (LFM), and a corresponding hybrid modulation scheme is proposed to reduce the power loss while maintaining a high harmonic compensation performance. Then, based on the analysis of the system voltage constraints, the dc-link voltage and threshold voltage under different parameters are redesigned. Furthermore, since the power flow between modules has been changed with the hybrid modulation scheme, the conventional clustered voltage control strategy is not capable of clustered control. Therefore, based on the re-modeling of the power flow, a negative-sequence voltage injection control strategy is proposed to achieve voltage balance. Finally, simulation and experimental results are demonstrated to verify the effectiveness of the proposed scheme.

Analysis and stabilization for full harmonic compensation oscillation in SAPF system with source current direct control

AbstractShunt active power filter (SAPF) has emerged as a promising solution for mitigating harmonic distortions. Due to the interaction between the current inner‐loop of SAPF, source impedance and non‐linear loads, harmonic oscillation issue arises and adversely affects the stability of the system, thus in turn exacerbating the harmonic distortion. However, previous studies paid less attention to the value of phase adjustment in improving the dynamic behaviour of the loop and thus stabilizing harmonic oscillation. Taking the dynamic characteristics between source impedance and non‐linear load into consideration, this paper models and analyzes the SAPF system with source current direct control to reveal the law of full harmonic compensation oscillation. Based on the analysis, an enhanced phase adjustment strategy using load current feedforward is proposed and designed to stabilize harmonic oscillation, while maintaining a favourable trade‐off between stability and harmonic compensation performance. The small‐signal model validated for stability analysis and the effectiveness of proposed stabilization strategy are verified by simulation and experiment.

Improved Harmonic Compensation Performance through Simulation-Based Transform Less HPF Using SSTL Inverter

Modern trends place a significant deal of attention on power quality and stability in distribution networks. The main cause of the PQ issue is the nonlinear loads, which result in current harmonics. It causes noise on the regulating and control circuit, which leads to incorrect operation of such equipment, and it causes considerable interaction with communications circuits along with AC Power line. This is one of the main problems with electricity quality. Using HPF in conjunction with the SSTL inverter, this power quality trouble can be reduced. HPFs can be connected straight to grid without the need for matching transformers by using smaller rating inverters and LC filters as part of their HPF design. Conversely, employing two converters results in a greater quantity of switches. The system is dependable, easy to use, and cost-effective when Transformer-less HPF with SSTL inverter is used to improve harmonic compensations in addition to harmonic compensation.

A System-Level Harmonic Control Method Based on Multibus Voltage Detected APF Without Exact Phase Synchronization

The decentralized distribution of nonlinear harmonic loads drives the trend of harmonic control from local compensation to system-level mitigation. Based on the active power filter (APF) with multi-bus harmonic detections, comprehensive control of harmonic distortions in multiple buses achieves, which is so-called system-level harmonic mitigation. However, existing system-level harmonic mitigation methods require high-speed data exchange for real-time phase synchronization among multiple buses, which reduces the feasibility of the methods. To reduce the burden of communication, a system-level harmonic mitigation method without inter-bus phase synchronization is proposed in this paper. Firstly, the phase synchronization problem is analyzed, and control system of multi-bus detected APF is proposed, in which the reference phase of each detector could be set locally. Then, a new online system-level harmonic mitigation model is introduced and solved to obtain the optimal APF current reference. Subsequently, the closed-loop control strategy of multi-bus detected APF is raised and four control stages are designed to cope with different situations. Finally, an eight-bus network with dynamic harmonic loads is designed in simulation and experiment. According to the results, multi-bus harmonic voltage distortions in the network could meet the standard. The proposed method contributes to achieving good harmonic compensation performance without multi-bus phase-synchronization.

Rough signal processing of AC power intelligent sensor under the background of smart grid

In the rough signal processing of AC intelligent sensor, the effective value and initial phase of voltage/current determine the test accuracy. To improve the harmonic detection and compensation performance of the existing APF and promote the improvement of power grid power quality. The direct positioning method is used as the comparison method, and the error LMS method is proposed to obtain and test the voltage and current signals of intelligent sensors. The simulation results of error LMS method show that the accuracy of voltage RMS and initial phase value calculated by method 1 increases with the increase of the number of sampling points, while the accuracy of voltage RMS of method 2 and method 3 does not change significantly. The results of correlation analysis method show that the test accuracy of the proposed method is 1/2–1/3 of the direct definition method when the amplitude of interference noise signal is 5%, 10% and 15%. Compared with the direct definition method, the rough signal processing technology has lower sampling amount and higher test accuracy, which helps to simplify the system and save the overhead cost.

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.

Single Phase Harmonic Mitigation Using Particle Swarm Optimization in Shunt Adaptive Power Filter

Purpose – These load sources are the main cause of generating harmonics, generating power loss in the transmission system and electrical equipment system, as well as reducing the life of electrical equipment, reducing the accuracy of electrical measuring devices, and reducing the signal transmission of communication devices. Methodology/Approach – This study applies the Particle swarm optimization (PSO) algorithm to DC-link capacitor voltage control based on the Proportional-integral (PI) control algorithm to determine the reference current. MATLAB/Simulation 2019a is used to simulate research results. Findings – The harmonic compensation performance of the SAPF circuit gives good results and meets the standard for total harmonic distortion (THD) of less than 5%. Originality/Value of paper – Bridge rectifiers with RL load and Non-linear load with active RC are applied in a single-phase power supply. The single-phase voltage source inverter is used to shunt the adaptive power filter (SAPF) to compensate for reactive power and current harmonics.

An Improved Deadbeat Control Strategy Based on Repetitive Prediction Against Grid Frequency Fluctuation for Active Power Filter

In order to improve the harmonic compensation performance of active power filter (APF) in distribution network, based on deadbeat control theory, the command current prediction algorithm and current tracking control strategy are optimized in this article. Firstly, the command current repetitive prediction in abc coordinate system is transferred to dq for improving its accuracy in lead compensation, and the equivalent for fractional delay beat is achieved by Lagrange Interpolation Polynomial to solve the problem of inaccurate prediction caused by grid frequency fluctuation. Then, considering the inherent half-sampling-period delay of sinusoidal PWM (SPWM), an improved deadbeat control strategy for current tracking is proposed by estimating the output current of next sampling period. Because the output current in next sampling period is replaced by that in current sampling period with traditional deadbeat control strategy, this estimation could make up for the defect of low control precision caused by that replacement. After that, adding error repetitive correction into the improved deadbeat control channel to reduce the periodic tracking error of output current. Finally, the stability and accuracy of the improved control system are analyzed theoretically, and its feasibility and effectiveness are verified by the simulation and hardware-in-the-loop (HIL) experiments.

A novel active damping control based on grid-side current feedback for LCL-filter active power filter

LCL filter is widely used in grid-connected converters for good harmonic mitigation performance. However, resonance poles of LCL filter challenge system stability, especially in a digital control system. This paper studies the suppression method of the active power filter output LCL filter resonance. Because of the defect that the traditional active damping based on the filter capacitor current or capacitor voltage feedback needs to add additional sensors, a novel active damping with grid-side current component feedback is proposed. And analysis and design of the feedback function, the resonance suppression characteristics, and the impact on harmonic compensation performance are analyzed in detail respectively and proposes a novel active damping control method based on a quasi-resonant controller grid-side current feedback. On this basis, a mathematical model of the SAPF system with the proposed novel active damping was established, and the resonance characteristics were analyzed. Finally, the proposed method was verified through relevant experiments.

Average Periodic Delay-Based Frequency Adaptable Repetitive Control With a Fixed Sampling Rate and Memory of Single-Phase PFC Converters

This article proposes an average periodic delay-based repetitive controller for power factor correction (PFC) converters undergoing grid frequency variations. When the grid frequency is changed, a conventional repetitive controller requires an interpolation method or additional memory to achieve frequency adaptability. Furthermore, frequency adaptability can also be realized using a variable sampling frequency; however, this requires additional computation effort and a multirate digital control system. In this article, with a fixed sampling frequency and memory length, the proposed repetitive controller, which generates the average periodic delay unit, maintains its harmonic compensation performance under grid frequency variations. In the proposed method, the control variables are updated depending on the phase of the grid voltage to reflect the grid frequency deviations in the average period delay unit. Because no complicated software modification is necessary, the proposed scheme can be easily plugged in the existing control algorithm. The theoretical analysis and stability issues of the proposed scheme have been discussed in detail to ensure stable operation and disturbance rejection. Both the simulations and the experiments on a 1.5-kW PFC converter have been carried out to verify the effectiveness of the proposed repetitive control.

Performance enhancement of shunt active power filter with the application of an adaptive controller

In this study, an adaptive controller is proposed for shunt active power filter (SAPF) to improve the harmonic compensation performance. The proposed controller forces the source current to become sinusoidal, with less total harmonic distortion and also maintained in phase with the supply voltage. The merits of modified adaptive controller over existing controller are verified in terms of harmonic compensation ratio factor for various perturbations of load and filter parameters. The eficacy of the proposed indirect current control with model reference adaptive controller (ICC-MRAC) control approach is tested by comparing the existing indirect current control with propertional–integral controller (ICC-PI) with the ICC-MRAC algorithm and then justified through laboratory prototype studies followed by using dSPACE 1104. It is observed from the result of implementation that the proposed controller-based SAPF performs superior than the existing ICC-PI in terms of robustness, adaptability, reliability and flexibility to various grid perturbations, such as harmonics, noise etc. Thus, the improvement occurs in power quality is due to the perfect harmonic rejection from current.

Dynamic Terminal Sliding-Mode Control for Single-Phase Active Power Filter Using New Feedback Recurrent Neural Network

In this article, an adaptive dynamic terminal sliding-mode controller using a double hidden layer recurrent neural network (DHL-RNN) structure for a single-phase active power filter (APF) is proposed to improve harmonic compensation performance. First, a method combining dynamic sliding mode and terminal sliding mode is proposed to solve the chattering phenomenon in traditional sliding-mode control. Then, since the nonlinear dynamics of APF is difficult to obtain accurately, the DHL-RNN is used to approximate the proposed dynamic terminal sliding-mode controller. Meanwhile, an integral robust switching term is added to eliminate the approximation error of the neural network. Simulation and experimental results proved that the proposed controller has better compensation performance and tracking effect compared with a simple terminal sliding-mode controller.

Analysis and Design of PI Plus Repetitive Control for Grid-Side Converters of Direct-Drive Wind Power Systems Considering the Effect of Hardware Sampling Circuits

In inverters based on a single proportional-integral (PI) or deadbeat (DB) controller, an inherent resonance peak may emerge near their current loop cut-off frequency, which results in harmonic amplification or even resonance. Additionally, inappropriate filter circuits implemented in sampling circuits may result in the expansion of the resonance peak. Thus, this paper further investigates the influence of the sampling circuits on a PI- or DB-based control loop. Then, the RC filter in the sampling circuit is designed to reduce the inherent resonance peak. Moreover, a compound control strategy based on an improved repetitive controller (IRC) plus a PI controller is adopted for the grid-side converter of a direct-drive wind system. This strategy enhances the harmonic and reactive compensation performance by reconstructing the internal model of the classic repetitive controller (CRC) and limiting the bandwidth of the PI-based loop to a low level. The parameters of the presented IRC-plus-PI control are designed for the purpose of resonance peak elimination and system stability. Furthermore, the non-integer delay problem is solved with an inserted fraction compensator (FC), which plays the role of a low-pass filter in the IRC. Finally, the feasibility and effectiveness of the presented control method is verified by the experimental results.

Control of Distributed Generators and Direct Harmonic Voltage Controlled Active Power Filters for Accurate Current Sharing and Power Quality Improvement in Islanded Microgrids

Harmonics are regarded as one of the main challenges in a microgrid. This issue may even get worse when different distributed generators (DGs) work together to solve the load sharing problems due to mismatched feeder impedances and diversified DG ratings. Even though load sharing can be achieved, the microgrid suffers from voltage unbalance and total harmonic distortion (THD) issues at the output of DG terminals as well as at the point of common coupling (PCC). Thus, in this paper, the power quality improving method is discussed, with a target of load sharing under the hierarchical control of different DG units and an active power filter (APF) in microgrids. To achieve this objective, we propose integrating a direct harmonic voltage controlled APF with DGs to improve their harmonic compensation performance. This proposed control scheme has many advantages over conventional control using a shunt resistive active power filter (R-APF) with voltage controlled DGs. First, based on the existing THD level of the PCC voltage, the proposed scheme provides improved voltage compensation and reduction in THD in the islanded microgrid. Secondly, equal load sharing can be achieved simultaneously. Thus, the proposed scheme provides better performance and a seamless interface as the proposed study mainly contains both the voltage controlled DGs and the local based voltage detection APF.

Cooperation of Voltage Controlled Active Power Filter with Grid-Connected DGs in Microgrid

Due to the excessive use of nonlinear loads and inverter interfaced distributed generators, harmonic issues have been regarded as a major concern in power distribution systems. Therefore, harmonic compensation in microgrids is a subject of current interest. Consequently, a novel direct harmonic voltage-controlled mode (VCM) active power filter (APF) is proposed to mitigate the harmonics in a cooperative manner and provide a better harmonic compensation performance of less than 5%. Due to the dispersive characteristics of renewable energy resources, voltage feedback based on a harmonic compensation control loop is implemented for the first time. This system can be smoothly combined with the current control loop. Our method proposes a better performance while mitigating the harmonics in comparison with conventional resistive active power filters (R-APF). Based on direct voltage detection at the point of common coupling (PCC), the proposed VCM-APF can therefore be seamlessly incorporated with multiple grid-connected generators (DGs) to enhance their harmonic compensation capabilities. The advantage of this scheme is that it avoids the need for designing and tuning the resistance, which was required in earlier conventional control schemes of R-APF for voltage unbalance compensation. Additionally, our scheme does not require the grid and load current measurements since these can be carried out at the PCC voltage, which further reduces the implementation cost of the system. Furthermore, the simulation results show the significance of proposed method.

An Enhanced Control Strategy for Multiparalleled Grid-Connected Single-Phase Converters With Load Harmonic Current Compensation Capability

To increase power rating, parallel-connected converters are widely used in power systems. However, it brings challenge to output harmonic current control. For active power filter, the harmonic current compensation performance worsens. A virtual resistor has significant effect on the amplitude and phase of the output current. Based on the analysis of current control loop with a virtual resistor, an enhanced current control method is proposed. The optimal value of a virtual resistor is used to stabilize the amplitude of the output harmonic current. The phase control of a filter capacitor harmonic voltage is used to regulate the phase of the output harmonic current. Finally, simulation and experimental results verify the correctness of theoretical analysis and effectiveness of the method. Load harmonic current compensation capability is guaranteed.

Improvement in Harmonic Compensation of a Smart Charger with a Constant DC-Capacitor Voltage-Control-Based Strategy for Electric Vehicles in Single-Phase Three-Wire Distribution Feeders

This paper presents an improvement in harmonic compensation performance of a previously proposed smart charger (SC) with a constant dc-capacitor voltage-control (CDCVC) strategy for electric vehicles (EVs) in single-phase three-wire distribution feeders (SPTWDFs). A controller for 3rd harmonic currents in d-q coordinates is added to the previously proposed SC. This addition improves harmonic compensation performance of the source currents. We briefly introduce harmonic current compensation using the previously proposed CDCVC-based algorithm for the SC. Then, the basic principles of the proposed controller for the 3rd harmonic currents in d-q coordinates are discussed in detail. It is shown that synchronization of the current controllers for both the fundamental and 3rd harmonic components is required. The switching frequency of a three-leg pulse-width modulated rectifier with a bidirectional dc–dc converter, which performs the SC, is determined considering the synchronization of the current controllers. Simulation and experimental results demonstrate that balanced and sinusoidal source currents with a unity power factor are achieved during both battery charging and discharging operations in EVs, improving the harmonic compensation performance of the previously proposed SC. Experimental results also demonstrate that the total harmonic distortion values of source currents are improved by 8.4% and 3.6% with the proposed controller for 3rd harmonic currents, when the SC is discharging, for example.

A harmonic compensation approach for interlinking voltage source converters in hybrid AC-DC microgrids with low switching frequency

In recent years, the hybrid AC-DC microgrid has been well accepted as it combines the advantages of both AC and DC systems. As the microgrid contains both DC sub-grids and AC sub-grids, interlinking DC-AC converters are essential. Meanwhile, considering the nonlinear AC loads may deteriorate the voltage quality of the AC bus, embedding an ancillary harmonic compensation function to the interlinking converters is promising. However, the conventional harmonic control methods used for active power filters (APFs) may not be suitable for the interlinking converters due to the main purpose of it is to exchange real and reactive power between the DC and AC sub-grids. The switching frequency is preferred to be lower than the APFs when the capacity of the microgrid is large. At low switching frequency, harmonic compensation performance or even the system stability may be affected. In this paper, a harmonic compensation approach suitable for hybrid AC-DC interlinking converters at low switching frequency is proposed. Through feeding the PWM reference signal with the harmonic compensation component directly to avoid the multi-loop control path of the fundamental component, the proposed method can achieve the effective harmonics compensation without being limited by the closed-loop control bandwidth. The proposed method, modeling approaches, stability analysis, as well as detailed virtual impedance design are presented. Experimental verification is also provided.

Hybrid AC/DC System Harmonics Control Through Grid Interfacing Converters With Low Switching Frequency

The nonlinear loads in distribution grid may generate harmful low-order harmonics, polluting the grid and deteriorating the voltage quality particularly when the grid is weak. The grid interfacing converters in the distribution system, such as the distributed generation (DG) interfacing converters or hybrid ac/dc grid interlinking converters, can participate in distribution grid harmonic control. In this paper, two virtual-impedance-based harmonics control methods are developed for grid interfacing voltage-source inverters (VSIs) to improve the power quality of the distribution grid. As the control parameters are designed based on the virtual impedance theory, clear physical meanings are given to explain their impacts on the system. Also, the proposed methods do not rely on the closed loop feedback control and, therefore, are very suitable for VSIs with low switching frequency (such as those for high-power DGs or interlinking converters for hybrid ac/dc systems), whose closed-loop control feedback bandwidth may be limited for harmonic regulation. The influence of system delay and feedback control loop is considered and modeled in the design procedure; thus more accurate virtual impedance control is achieved for low-switching-frequency VSIs. Moreover, a comprehensive comparison of the two methods, including stability and harmonic compensation performance, are given. Their effectiveness is verified by experiment results.

An Adaptive Digital-Control Scheme for Improved Active Power Filtering Under Distorted Grid Conditions

The operation of active power filters (APFs) under nonideal grid conditions, such as grid-frequency fluctuation and voltage harmonics, can lead to significant degradation in harmonic compensation performance. This paper proposes an adaptive digital-control scheme for a three-phase APF for use in harmonically distorted and variable-frequency grid conditions. This scheme is comprised of a grid-frequency adaptive resonant current controller and an enhanced synchronous-reference-frame phase-locked loop (SRF-PLL). The PLL uses an inherently stable adaptive-filtering stage to improve grid phase and frequency estimates in the presence of voltage harmonics. The improved PLL frequency estimate is used to update the resonant gains of a PI + vector-proportional-integral current-control scheme, implemented in the SRF. This enables the APF to maintain optimal performance in distorted grid conditions. The performance of the proposed APF control scheme is evaluated in a test microgrid, with a 15-kVA three-phase voltage-source converter configured as the APF, a 90-kVA grid emulator utilized to replicate distorted grid conditions, and a load emulator implemented to draw harmonic currents. The control scheme presented here is shown to demonstrate significant performance improvements under nonideal grid conditions compared with equivalent adaptive and nonadaptive methods.