Voltage Harmonic Compensation Research Articles

Decentralized Goal-Function-Based Microgrid Primary Control with Voltage Harmonics Compensation

Decentralized Goal-Function-Based Microgrid Primary Control with Voltage Harmonics Compensation

Harmonic voltage compensation and harmonic current sharing strategy of grid-forming inverter

In a multi-inverter parallel system, the harmonic governance control strategy may worsen the system's current circulation problem due to the line impedance difference, which could further affect the stable operation of the system. To solve the problem, the strategy considering both power sharing and harmonic control is proposed in this paper. First, the microgrid system with background harmonics is modeled and analyzed. Based on the modeled system, a comprehensive control strategy for the grid inverter with harmonic voltage compensation and harmonic current sharing is established. In this paper, the output signal is controlled by frequency division. This paper adopts grid-forming control in the fundamental wave domain and employs harmonic voltage compensation control in the harmonic domain. The harmonic voltage is synthesized by collecting the specific harmonic current through the band-pass filter, compensating for the harmonic voltage drop on the line to improve the harmonic voltage quality of the point of common coupling(PCC). Aiming at the sharing problem caused by line impedance difference, virtual impedance control is introduced, harmonic impedance is redesigned according to the adjusted output impedance, and the harmonic output impedance before and after adjustment is measured and compared. The simulation results show that the proposed control strategy can not only reduce the harmonic content of the PCC voltage but also achieve the effect of equal harmonic current division.

Power quality and efficiency functionalities for a three-level Dynamic Voltage Restorer on low voltage grids

Back-to-back PWM inverter based bidirectional dynamic voltage restorers (DVR) may have worse efficiency than mechanical or SCR based solutions, but they are able to provide a continuously variable voltage in series with the grid. As the injected waveform does not need to be sinusoidal, additional features may also be implemented in software. In this paper it is shown that the compensation of voltage harmonics, flicker, and even voltage transients are all possible to some extent. The power required for the series voltage injection does not need to originate from the same phase where it is injected. Drawing power from the less problematic phases can also be used to increase the voltage symmetry on the unregulated side of the device. It is shown in the paper, that at partial load the complete deactivation of one phase in the active rectifier stage can not only help with asymmetry, but can also increase the efficiency of the system. A method is proposed for the seamless transfer in the active rectifier stage from 3-phase to 2-phase operation and back. It is also shown in the paper, that phase deactivation and electronic bypass of the injection transformer is possible in the output stage as well. The three level half bridge circuit configuration enables this without additional components.

Research on power quality improvement system based on dynamic voltage restorer

AbstractPower quality is a critical issue in power systems that can affect the performance and reliability of sensitive equipment. The dynamic voltage restorer (DVR) is a widely used technology for improving power quality, but it mainly focuses on mitigating voltage sags and often neglects the issue of harmonic distortion. This paper proposes a DVR voltage compensation strategy based on the discrete Fourier transform (DFT). This strategy utilizes the DFT to analyze the voltage waveform, separating the fundamental and desired harmonic components. With this strategy, the DVR can effectively compensate for voltage sag and achieve selective harmonic compensation, thereby improving the power quality of the power system. Additionally, a method utilizing hardware circuitry and an LED as a timestamp is proposed to dynamically estimate digital siginal processor (DSP) resource occupancy rate in real‐time, ensuring effective compensation while minimizing resource occupancy. A 3 kW DVR experimental prototype was constructed and tested to evaluate the practicality of the proposed strategy. The simulation and experimental results show that the method proposed in this paper successfully realizes the compensation of voltage sag and harmonics in the power system, and improves the stability and reliability of the power system.

Compensation of Voltage Harmonics for LCL-filtered Inverters in Islanded Microgrids

In a microgrid (MG), it is possible to have some sensitive loads that require high voltage quality. In this paper, a new technique is proposed for selective compensation of main voltage harmonics in islanded MGs. The proposed technique is capable of adjusting the compensation percentage in proportion with the existing disturbance in the sensitive load bus (SLB) and sharing the compensation workload among DGs in proportion with their nominal power. This compensation, is performed by controlling of the interface inverters of distributed generation (DG) units with LCL filters. Also, to decrease the asymmetry among phase impedances of MG and mitigate the voltage distortion after the output LCL of filters, a novel structure is proposed to generate selective virtual impedance. At fundamental frequency, the proposed structure of the virtual impedance improves the control of the fundamental component. On the ather hand, at main harmonic frequencies, it tries to adaptively improve nonlinear load sharing among DG units and mitigate the voltage distortion after the output LCL filters. Simulation results in MATLAB/SIMULINK environment show the efficiency of the proposed approach in improving load load sharing and decreasing voltage harmonics.

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.

Voltage Detection-Based Selective Harmonic Current Compensation Strategies for Photovoltaic Inverters

Voltage and current detection-based harmonic current compensation (VDB-HCC and CDB-HCC, respectively) strategies allow the use of photovoltaic (PV) inverters to enhance the grid power quality. CDB-HCC strategies require converter hardware retrofit by inserting an extra current sensor to measure load or downstream grid currents. On the other hand, VDB-HCC strategies are straightforward solutions employing only embedded measurements used for protection, control, and synchronization purposes. This paper provides a comprehensive comparison of three VDB-HCC strategies: VDB-HCC based on a single current loop (SCL); VDB-HCC based on dual parallel current control loops (DCL); and VDB-HCC based on parallel voltage and current control loops (VCL). The comparison includes the requirement of harmonic detection algorithms, PV array maximum power point tracking (MPPT) performance, stability over grid impedance variations and inverter power rating, steady-state harmonic compensation, enabling transient response, and computational burden. Experimental results using a 1.5-kW commercial PV inverter are conducted. Based on distributed HCC applications by PV systems, VCL has shown the best performance in terms of voltage distortion reduction under weak grids.

A novel control scheme for load current components sharing improvement, and unbalanced and harmonic voltage compensation, in islanded provisional-microgrids

Microgrids (MGs) are the building blocks of future smart distribution grids, bringing several benefits to both utilities and consumers. In recent years a new class of MGs has been introduced, called provisional-microgrid (P-MG), consisting of master-MG and slave-MG. Slave-MG cannot operate isolated from master-MG. However, power quality is of the major challenges in MG investigations. In this study, a novel hierarchical and distributed control scheme is proposed for the improvement of unbalanced and harmonic load current components sharing among the distributed generation (DG) units as well as voltage quality (VQ) enhancement of sensitive load bus (SLB) in provisional-microgrid. Compared to the previous works, the proposed control strategy takes the instantaneous free capacity of DGs and multi-voltage-quality level (MVQL) indices of different buses into account for both load current sharing and voltage quality improvement functions. Furthermore, it considers the nature of variable power generation-renewable energy resources (VPG-RERs) and does not need any information about the loads or lines impedances. The efficiency of the proposed control scheme is evaluated by simulation of a typical islanded P-MG under several cases. Simulations are carried out in MATLAB/Simulink environment, and their results demonstrate the appropriate speed and accuracy of the presented method.

Wideband Series Harmonic Voltage Compensator Using Look-Ahead State Trajectory Prediction for Network Stability Enhancement and Condition Monitoring

A wideband series harmonic voltage compensator (WSHVC) with an operating bandwidth of 10 kHz for microgrids with a plurality of grid-connected inverters (GCIs) is proposed. It generates a voltage to compensate for high-frequency harmonic voltage at the point of common coupling (PCC), resulting in a virtually zero high-frequency impedance at the aggregate output of the GCIs and, thus improving system stability. The generation of the harmonic voltage is based on predicting the state trajectory of the WSHVC after a switching period per half-switching cycle to determine the moment at which the switches in the WSHVC change their states. The PCC impedance and the equivalent impedance of the GCI network are extracted by analyzing the system response after injecting the entire system with particle-swarm-optimization (PSO) optimized multisine power disturbances. Therefore, system stability is predicted. The condition of passive components in the WSHVC is also monitored by estimating their values with the sampled state variables. A 500 VA, 1 MHz, FPGA-controlled, GaN-based prototype is evaluated on a 6.5 kVA testbed that consists of three commercial GCIs, nonlinear load, and adjustable grid impedance. A comparative study of the system performance with and without the WSHVC is conducted. Experimental results are favorably compared with theoretical predictions.

Enhanced compensation strategy of voltage harmonics for Doubly-Fed induction generator

Using the idle capacity to compensate voltage harmonics at the point of common coupling (PCC) is an important reflection of participating in power grid regulation for the doubly-fed induction generator (DFIG), which is very beneficial to improve the power supply quality in the regional power grid. Due to the pouring of power electronized devices, the uncharacteristic harmonics of which frequencies are unfixed appear in the power grid. The existing voltage harmonics compensation methods mainly focus on the traditional characteristic harmonics. The frequencies of these kinds of harmonics are fixed and pre-known (6n ± 1 orders). This fact makes the existing methods only work at the specific and fixed frequency, and cannot deal with the harmonics of which frequencies are uncharacteristic or unfixed. Therefore, starting from the theoretical analysis of voltage harmonics at PCC, this paper investigates the mechanism that can simultaneously compensate for the characteristic and uncharacteristic harmonics of PCC voltage, and indicates that decreasing the impedance of DFIG in the frequency range of harmonics is the core of voltage harmonics compensation. Then, this paper proposed a wideband control structure and the matched controller, so that the impedance of DFIG can be effectively decreased in a wideband frequency range. The design procedure of the harmonics controller co-currently considers the compensation performance and system stability. Finally, this paper carries on the evaluation based on the hardware-in-the-loop platform and scale-down experimental platform to verify the correctness of the theoretical analysis and the effectiveness of the control strategy.

Distributed Event-Triggered Control for Harmonic Voltage Compensation in Islanded AC Microgrids

Due to the proliferation of critical nonlinear loads in the islanded AC microgrids, the power quality improvement at the sensitive load bus (SLB) is a challenging task. To deal with this issue, a distributed event-triggered control strategy for the voltage harmonic compensation is proposed in this paper. By applying the proposed strategy, the distributed generator (DG) units can be employed in a distributed way to compensate for the harmonic voltage at the SLB. As a result, the power quality at the SLB is greatly improved. In addition, the suggested technique allows the SLB voltage to be measured by one or several dedicated DG units for the voltage harmonic compensation, which increases the system’s robustness. Furthermore, by applying the proposed event-triggered mechanism, the communication data is only sampled and transmitted among DG units when the predefined triggering condition is satisfied, which effectively alleviates the communication burden. Moreover, the stability and inter-event interval with the proposed control strategy are analyzed in this paper. Finally, several experimental case studies are presented to demonstrate the effectiveness of the proposed control strategy.

Analysis and Control of Current Harmonic in IPMSM Field-Oriented Control System

There is a certain amount of current harmonics in interior permanent magnet synchronous motor. In order to accurately control the current harmonic, this article focuses on three aspects: establishing current harmonic calculation model, extracting current harmonic, and designing current harmonic regulator. A current harmonic calculation model, which considers the effect of voltage harmonics induced by current controllers and digital control delay on the initial phase angle of voltage harmonics, is established for the first time. With the help of multiple synchronous rotating frame transformation, a current harmonic extraction method more suitable for motor drive field, which aims to extract the current harmonic characteristics of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> -axis and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -axis currents, is proposed. On this basis, the controlled quantity and control quantity are unified in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d–q</i> coordinate frame for analyzing the coupling relationship of current harmonic characteristics. And then a PI current harmonic regulator, including current harmonic decoupling and voltage harmonic initial phase angle compensation, is constructed, so as to realize the effective control of current harmonics. Finally, the effectiveness and superiority of the proposed method are verified by simulation and experiments.

Wideband Series Harmonic Voltage Compensator for Enhancing Stability of Microgrids

A wideband series harmonicvoltage compensator (WSHVC) for mitigating the adverse effect of unknown grid impedance and load condition on the stability of microgrids with multiple grid-connected inverters (GCIs) is presented. The concept is based on extending the concept of series active power filter. A wideband series voltage source inverter is used to compensate for the high-frequency harmonic voltage caused by the impedance at the point of common coupling, thereby creating a virtually zero high-frequency impedance at the output of the GCIs. The system stability is then assured under any operating condition. The bandwidth of the WSHVC ranges from the second harmonic of the grid frequency to 8 kHz, which is sufficiently higher than the cut-off frequency of typical GCIs. Such fast dynamic behavior is realized by a novel fixed-frequency predictive control scheme with nonlinear switching surfaces. Since the WSHVC handles harmonic power only, its volt-ampere rating is lower than that of the entire system. A 500 VA prototype for a 6.5 kVA testbed with three commercial GCIs, nonlinear load, and adjustable grid impedance has been evaluated. The power dissipation of the WSHVC is less than 1% of the VA rating of the testbed. The effectiveness of the WSHVC on improving system stability is studied.

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.

Improved Control Method for Voltage Regulation and Harmonic Mitigation Using Electric Spring

An electric spring (ES) with series connection to non-critical loads (NCLs) is mainly known as a smart load (SL) technology that can compensate undervoltages and overvoltages. In this paper, an improved control method is presented for the electric spring to regulate the effective value of the critical load (CL) voltage to the nominal value and to compensate the harmonics of the critical load voltage caused by the grid side. The electric spring’s performance for simultaneous voltage magnitude regulation and harmonic distortion compensation is investigated through simulation studies. The results are compared with those of a conventional control scheme. It has been demonstrated that the electric spring, using the improved control system, reduces the amplitude deviation and distortion of critical load voltage.

Power Quality Improvement in Integrated System using Inductive UPQC

Harmonics pollution is very dangerous in medium power applications. Generally, harmonics are produced in the system due to the connection of nonlinear loads. This article presents a novel load voltage and current harmonic compensation, voltage sag and swells compensation approach using hybrid UPQC. The proposed synchronous reference controller uses the Carrier Pulse Width Modulation (CPWM) technique for the proper control of UPQC. The simulations are carried on Matlab/Simulink platform. The output results present the proposed hybrid UPQC compensates the load current harmonics, voltage sag and swell effectively. The load voltage harmonics are reduced from 11.65% to 1.53% and the current harmonics are reduced from 30.97% to 2.24% respectively. https://dorl.net/dor/20.1001.1.13090127.2021.11.2.6.5

Secondary‐control‐based harmonics compensation scheme for voltage‐ and current‐controlled inverters in islanded microgrids

In this study, a coordinated control scheme is proposed for sharing harmonics compensation effort among voltage and current controlled mode (VCM and CCM) inverters in islanded microgrids. In this method, the voltage harmonics compensation of sensitive bus (SB) is achieved by using secondary control as well as virtual impedance and admittance loops in primary control of VCM and CCM units. The limited capacity of the inverter is taken into account for harmonics compensation. Photovoltaic (PV) systems are considered as CCM units. The harmonics compensation is mainly performed by VCM inverters. However, in order to prevent these units from overloading, the PV interfacing inverters (CCM units) are called to collaborate in harmonics compensation whenever needed. The results of simulation study in Matlab/Simulink show the effectiveness of this method in coordination of CCM and VCM units.

Harmonic and unbalanced voltage compensation with VOC‐based three‐phase four‐leg inverters in islanded microgrids

This study proposes a novel method that combines sliding mode control (SMC) and virtual oscillator control (VOC) to eliminate voltage harmonics and unbalance caused by non-linear and unbalanced loads in a three-phase low-voltage microgrid. The conventional VOC is modified to fit the three-phase four-leg inverter systems and a novel SMC control is designed using the high-pass filter technique and linearised super twist algorithm. The proposed control method can keep the voltage harmonics and unbalance at the point of common coupling within the IEEE standard limits, under heavily non-linear and unbalanced load conditions. The validity of the proposed method is verified by both offline simulations in Matlab/Simulink and Control Hardware-in-the Loop simulations using an OPAL-RT platform.

Virtual Impedance Control Scheme to Compensate for Voltage Harmonics With Accurate Harmonic Power Sharing in Islanded Microgrids

In case of voltage harmonic compensation at the point of common coupling (PCC) in islanded microgrids, it is important to share the harmonic load power correctly among the distributed generators (DGs) to use the DGs' capacity effectively. In previous DG control schemes, it is impossible to satisfy the requirements of PCC voltage quality and accurate harmonic power sharing simultaneously with only a virtual impedance controller (VIC). In this article, we propose an enhanced DG VIC to provide accurate harmonic power sharing along with voltage harmonic compensation in islanded microgrids. The proposed VIC is developed based on simple integral controllers with two controllable parts and no information about the feeder impedances or load currents. The control performance is theoretically analyzed using a small-signal state-space model to evaluate the system dynamics and stability. The feasibility and effectiveness of the method are validated by simulated and experimental results.

A Broad Frequency Range Harmonic Reduction for Cascaded-Power-Cell-Based Islanded Microgrid With Lumped PCC Filter

Series connection of low-voltage power cells has been considered as an alternative microgrid configuration to supply rated voltage power to point of common coupling (PCC) loads. However, in the previous studies, the major focus was on the power regulation of system. Similar to the conventional shunt-distributed generation (DG)-unit-based islanded microgrid, the PCC voltage harmonic problem can be serious without any active control. In order to provide enhanced power quality in this system, a flexible PCC harmonic voltage compensation approach is developed via the hierarchical control of series-connected power cells. First, the selected low-order PCC harmonic voltage components are mitigated by using the closed-loop equivalent impedance shaping method, via cooperation among the PCC central and power cell local controllers. Second, the medium-range lumped output LC filter resonance is suppressed with the help of internal virtual impedance, which is implemented only at the power cell local controllers. Finally, the high-frequency switching ripples of PCC voltage are mitigated by phase-shifting pulsewidth modulation in distributed power cells, through exploring the output current signal of power cells as an inherent synchronizer for carrier shifting control. It has been experimentally validated that with the proposed approaches, the PCC voltage quality can be significantly improved in broad frequency range without any interferences.