Reactive Current Research Articles

Hysteresis Controlled Based Power Quality Improvement in a Renewable Energy Source Connected to Utility grid with BESS Based STATCOM

This Paper presents a control strategy of the grid interconnected inverter Renewable Energy Sources (RES). This system can achieve the maximum benefits from these grid interconnected inverter when installed in 3-phase 4-wire distribution system. Increasing electrification of daily life causes growing electricity consumption and the rising number of sensitive or critical loads demand for high quality electricity. One of the main problems facing today is that related with the transmission and distribution of electricity. Due to the rapid increase in global energy consumption and the diminishing of fossil fuels, the customer demand for new generation capacities and efficient energy production, delivery and utilization keeps rising. Utilizing distributed generation, renewable energy and energy storage can potentially solve problems as energy shortage. With the increase in load demand, the Renewable Energy Sources (RES) are increasingly connected in the distribution systems which utilizes power electronic Converters/Inverters. The inverter can perform as a multi function device by incorporating active power filter functionality. The inverter can thus be utilized as: 1) power converter to inject power generated from RES to the grid, and 2) shunt APF to compensate current unbalance, load current harmonics, load reactive power demand and load neutral current. All of these functions may be accomplished either individually or simultaneously. This new control concept is demonstrated with extensive MATLAB/Simulink simulation studies and validated through digital signal processor-based laboratory experimental results.

Hybrid automaton-fuzzy control of single phase dual buck half bridge shunt active power filter for shoot through elimination and power quality improvement

This paper addresses the problem of controlling a single phase shunt active power filter (SAPF) in presence of nonlinear loads. The considered SAPF is based on a Dual Buck Half Bridge converter (DBHB), which has the ability to eliminate the shoot-through problem arising in the conventional inverter circuit. The aim is to design a controller that is able to achieve the following three control objectives: (i) simple and indirect estimation of harmonic components, (ii) compensating for the harmonic and reactive currents generated by the nonlinear load for assuring a satisfactory power factor correction (PFC) in the grid side, (iii) regulating the DC capacitor voltage of the DBHB converter. In order to meet these control objectives, a new controller based on multi-loop structure is proposed. In the inner loop, a hybrid automaton representation of the DBHB-SAPF is used for the purpose of designing an appropriate control law so that to ensure a unity power factor. In the outer loop, a fuzzy logic controller is developed to guarantee a tight regulation of the converter DC voltage to a desired value. The effectiveness of the proposed controller is verified and validated by numerical simulation using MATLAB/Simulink environment. From the obtained results, the designed controller shows significant performance in terms of robustness and tracking compared to the standard strategy based on PI regulator.

CPC-Based Minimizing of Balancing Compensators in Four-Wire Nonsinusoidal Asymmetrical Systems

The article presents the possibility of using the currents’ physical components (CPC) theory to generate the reference current of the active power filter (APF). The solution proposed by the authors is based on the cooperation of minimizing balancing compensators (MBC), which, due to their use in 4-wire systems, have been divided into two structures. The first compensator, which purpose is to minimize and balance the reactive current and the unbalanced current of the zero sequence, is built in the star system (STAR-MBC). The purpose of the second compensator, which operation occurs in the delta system (DELTA-MBC), is to minimize and balance the other two components, i.e., the unbalanced current of the negative sequence and the unbalanced current of the positive sequence. The two structures cooperating with each other significantly reduce the currents associated with the reactive elements, i.e., reactive current, and the unbalanced current. As mentioned, these currents are reduced but not compensated to zero or to the reference value. In order for the compensation and balancing to bring the preferable effect, an APF system should be included, which will cooperate with MBC compensators. This solution is presented in this publication. The control of the active part of the hybrid active power filter (HAPF), which was presented in the paper, consists of the reflection of the waveform of the nonsinusoidal active current. In this approach, no current shift in relation to voltage is obtained, but the waveforms of these quantities remain distorted. The reactive current is compensated and the unbalanced currents are balanced. The second definition of generating a reference current can also be used. Through this approach, the active current with a sinusoidal waveform is achieved. The second approach allows for an additional reduction of the three-phase RMS value of the load’s current. In both of these approaches, the active currents flowing through the lines will reflect the amplitude and phase asymmetry that is present in the supply voltage. The APF system will follow the changes in power or load conditions and generate the correct value for the reference current. The calculations presented in the article, as well as the current and voltage waveforms, were created as a result of the constructed mathematical models, which were used for theoretical illustrations. Calculations and waveforms were generated based on a script written in Matlab.

A Dual-Layer Back-Stepping Control Method for Lyapunov Stability in Modular Multilevel Converter Based STATCOM

With the high penetration of the power electronic loads in the grid, the stability of static synchronous compensator (STATCOM) devices is greatly challenged. However, the conventional control methods for the modular multilevel converter (MMC) based STATCOM only consider the stability with small signal disturbances. This article proposes a novel dual-layer back-stepping control (BSC) for the MMC-based STATCOM. In the first layer, the BSC aims to regulate the sum of the capacitor energy and the reactive output current. In the second layer, the BSC aims to control the circulating current. Therefore, the proposed method possesses a fast dynamic response and accurate tracking with the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Lyapunov</i> stability of the MMC-based STATCOM. Compared with the arm-control-based BSC for MMC-based inverters, the proposed method has a simplified structure and a reduced computation burden. Moreover, the proposed method realizes the independent control between the output current and the circulating current. The simulation and experimental results verify the effectiveness of the proposed method. In addition, its robustness toward different circuit parameters and the operation ability under unbalanced grid fault is also verified.

Study on Half-Bridge Voltage Balancing Circuit with Two Driving Methods

A voltage balancing circuit not only can split a two-wire dc grid into a three-wire dc grid, but also can balance the voltages of series capacitors or battery packs. A half-bridge voltage balancing circuit is a simpler topology and is widely introduced. But it has not been deeply analysed. Thus, in this paper the balancing circuit with the complementary driving method is studied in detail, firstly. Secondly, for avoiding the reactive current occurred in the complementary driving method, the unipolar current driving method is also proposed. Finally, the prototype is implemented in the lab, and the simulation and experimental results verify the analyses.

Optimal Current Reference Calculation for MMCs Considering Converter Limitations

The paper addresses an optimization-based reference calculation method for Modular Multilevel Converters (MMC) operating in normal and constrained situations (when the converter needs to prioritize its quantities as it has reached voltage or current limitations, e.g. during system faults). The optimization problem prioritizes to satisfy the external AC active and reactive current set-points demanded by the grid operator through the corresponding grid code. If the operator demands are fulfilled, it uses the available MMC degrees of freedom to minimize the arm inductance losses. Otherwise, if the operator demanded AC set-points cannot be accomplished, the optimization attempts to minimize the error prioritizing between either AC active or reactive currents. The optimization problem constraints are imposed through a steady-state model considering simultaneously the external and internal AC and DC magnitudes of the converter. The steady-state model also includes the voltage variation in the equivalent arm capacitors (considering the ripple). Then, the imposed limitations are the maximum allowed grid and arm currents, the maximum allowed arm voltages and the sub-module capacitor maximum voltages. The paper presents a detailed formulation of the optimization problem and applies it to several case studies where it is shown that the presented approach can be potentially used to obtain the MMC references both in normal and fault conditions.

Research on Statcom Current Detection Method and Control Technology Based on Instantaneous Reactive Power Theory

The increase and decrease of non-linear load and impact load of power grid lead to lower and lower power quality, which has become a hot issue in reactive power compensation and harmonic suppression technology research. As one of the AC output equipments, the static synchronous compensator has strong compensation performance and can provide technical support for no power compensation. Therefore, based on the theory of instantaneous reactive power, the current detection method and related control technology of the static synchronous compensator are proposed, and finally the experimental simulation is carried out. The simulation results show that the improved detection algorithm can detect the reactive current of the system efficiently and accurately. Using STATCOM current detection method and control technology, it can adjust reactive power in time, compensate for actual power demand, further suppress system voltage fluctuations and improve system performance.

Comparison of Active and Reactive Power Oscillation Damping With PV Plants

This article proposes a flexible control scheme (FCS) of a photovoltaic (PV) power plant to support the voltage stability and provide fast recovery after the grid faults in a power system with high penetration renewable energy sources (RES). To achieve these goals, the proposed FCS includes two control loops: a reactive current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</sub> ) injection loop and a frequency-damping control (FDC) loop. The I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</sub> injection loop injects I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</sub> in proportion to the terminal voltage of the PV during a fault. After the fault clearance, the FDC loop provides active and/or reactive power modulation to stabilize system frequency; then, the FDC loop puts the system recovery forward by damping out the remaining synchronous machines' frequency oscillations. The FDC loop comprises active power damping function without PV power curtailment and reactive power damping function. We verified the proposed FDC's performance in a two-area system for various penetration levels of inverter-based resources using a PSCAD simulator. We investigated the control schemes' effectiveness by comparing the efficiency and the response in mitigating the post-fault oscillations and restoring the voltage regulation.

Calculation of the Capacity for the Operation of a Mini-Energy Complex Based on an Inde-pendently Operating Asynchronous Generator

The article present the analysis of the operation of the mini-energy complex (MEC) based on alternative energy sources. An asynchronous generator (AG) was taken as energy source. The IEC operates independently with standard power parameters without the use of a frequency converter. To operate independently, AG needs a source of reactive excitation current. Based on the calculations carried out and the results obtained with the use of the experimental facility,the operating conditions of the MEC with standard parameters of electricity under varying load have been analyzed. A characteristic feature of the autonomous MEC is the commensurate capacity of the generating device and consumers. Therefore, any power consumer on/off leads both to significant changes of local electric system parameters and affects the operation of the generator itself. In this article, the main attention is paid to the influence of three-phase motor load on stable AG operation. When the MEC operates independently, reliable self-excitation of the asynchronous generator and the start-up of consumers whose power is commensurate with the generating unit must be ensured. It is also necessary to ensure the maintenance of voltage stability, the possibility of automatic operation of the generating unit, and the preservation of its integrity in emergency modes. Thus, for stable MEC-based AG operation the nature of the load should be taken take into account, the parameters of the of the local grid should be known as well as the exact availability of consumers and their characteristics, and also equivalent circuit parameters of asynchronous motors. In addition, it is necessary to accurately calculate the capacity when changing the parameters of the electrical system, so as not to lose the self-excitation of the asynchronous generator, which is equivalent to disconnecting the entire load of the generator and causes a sharp increase in speed. To solve these problems, it is necessary to create a high-speed MEC control system.

Optimized Exploitation of Ancillary Services: Compensation of Reactive, Unbalance and Harmonic Currents Based on Particle Swarm Optimization

Taking into account the growth of distributed generation systems in low-voltage networks, it is important to fully exploit the energy resources and their embedded power converter. Thus, this paper proposes an optimization strategy for controlling the multifunctional inverter endowed in the distributed generator. Such approach allows maximum exploitation of its power capabilities under condition of high active power generation, and limited capability for performing ancillary services. Consequently, grid-support functions can be efficiently provided by the inverter, enhancing the power quality in low-voltage grids. The proposed approach is based on Particle Swarm Optimization and takes advantage of definitions from the Conservative Power Theory to synthesize the control references for the compensation of reactive power, as well as unbalance and harmonic distortion currents. Simulation results performed using Matlab/Simulink are presented to evaluate the optimization model for a three-phase system.

Closed-Form Asymmetrical Duty-Cycle Control to Extend the Soft-Switching Range of Three-Phase Dual-Active-Bridge Converters

The three-phase dual-active-bridge (DAB) converter is a promising topology for high-power dc–dc conversion in dc grids and industrial applications. However, the conventional single-phase-shift modulation shows a limited soft-switching range and large reactive currents under light-load operation especially with large voltage variations. This article proposes an asymmetrical duty-cycle control method for the three-phase DAB to extend the soft-switching range and reduce reactive currents by realizing three-phase triangular- and trapezoidal-current-mode operation. Closed-form solutions of the proposed modulation schemes are derived that enable a simple online calculation of the control parameters. The proposed modulation schemes can substantially reduce losses of both power semiconductor devices and the medium-frequency transformer. Thus, the three-phase DAB converter can realize high efficiency over a wide operation range, especially under light-load conditions. Considering the practical implementation, a compensation method is presented to correct the distorted current waveforms caused by the dead time and ensure the desired soft-switching operation. Besides, the power loss imbalance between top and bottom switches induced by the asymmetrical duty cycle is addressed by an active thermal balancing strategy. The effectiveness of the proposed method is validated by simulation results and experiments on a downscaled converter prototype.

High-Efficiency Bidirectional Three-Level Series-Resonant Converter With Buck-Boost Capacity for High-Output Voltage Applications

To improve the efficiency and power density of electronic vehicles, battery packs with higher voltage levels have been gradually adopted. This change poses new challenges for conventional battery charger designs based on two-level topologies and low-voltage rating switches. Therefore, in this article, a bidirectional buck-boost three-level series-resonant converter (BBTL-SRC) for high-output voltage applications is proposed. The voltage stress of switches is reduced to only half of the output voltage, so the low-voltage rating switches with lower losses and costs can be used. In addition, the phase shift and pulsewidth modulation strategies are designed to realize buck and boost conversion. All switches can achieve zero-voltage switching, and the reactive current in resonant tank decreases significantly. Hence, the switching and conduction losses are reduced and efficiency is optimized. The operation principles and characteristics are analyzed in detail, and design considerations are provided. A 3.3-kW, 600-840-V output prototype is built and tested. The experimental results verify the effectiveness of the proposed BBTL-SRC.

Phasor-domain Dynamic Model of Asymmetric Current Injection Controller for Converter-interfaced Generator

This paper presents a controller model of asymmetric current injection for converter-interfaced generators suitable for root-mean-square (RMS) phasor-domain, fundamental-frequency, three-phase, and dynamic simulation tools. The effectiveness of the proposed controller is assessed with simulations in test systems with high percentage of converter-interfaced generation. The simulations focus on the operation of protection relays that use negative-sequence quantities in their directional elements. This paper also presents and compares two strategies to limit reactive negative-sequence currents, and active and reactive positive-sequence currents. A tutorial test system and a regional system part of the actual Brazilian Interconnected Power System are used to assess the correctness of the proposed controller in three-phase fundamental-frequency RMS dynamic simulations.

CPT-Based Multi-Objective Strategy for Power Quality Enhancement in Three-Phase Three-Wire Systems Under Distorted and Unbalanced Voltage Conditions

This paper explores the application of multifunctional inverters connected to the electrical network, to propose an adaptable system to individually control reactive power, load imbalance, and harmonic currents. Based on Conservative Power Theory (CPT), the dynamic compensation of Power Quality (PQ) indices is carried out under different conditions of demand and energy generation. First, a set of global factors for reactive power, imbalances, and current harmonics is introduced, which are effective even for bidirectional power flows in systems with harmonic voltage distortion and/or voltage unbalance. This set of factors gives rise to a multiobjective decision-making algorithm to estimate coefficients of proportionality, which instantly considers the reference signals linked to the disturbances, enabling to individually keep track of the target values of each of the PQ indices, as customarily established through standards or electricity utility criteria. As a result, this proposed approach allows a multiobjective operation of the inverter connected to the network, thereby allowing maximum use of energy generated, as well as limiting energy losses of distribution networks, and enhancement of the PQ of a prosumer. Finally, to illustrate the effectiveness of the proposed approach, different case studies were simulated with this set of case studies, including one representative real case containing a PV system connected to the network and a non-linear unbalanced load with variable demand.

Analysis on Circulating Current and Split Capacitor Voltage Balance for Modular Multilevel Converter Based Three-phase Four-wire Split Capacitor DSTATCOM

We propose a modular multilevel converter (MMC) based three-phase four-wire (3P4W) split capacitor distribution static synchronous compensator (DSTATCOM), aiming at compensating unbalanced and reactive load currents. Due to the zero-sequence current compensation, the circulating current characteristics of 3P4W MMC-DSTATCOM are different from conventional MMCs. Moreover, the distinct working principle of MMC would affect the features of split capacitor voltage. The decoupled positive-, negative- and zero-sequence second-order and DC components of the circulating current are deduced explicitly. Two proportional-integral controllers with dual dq transformation are employed to suppress the positive- and negative-sequence components of second-order circulating current, while quasi proportional-resonance controller is designed to eliminate the zero-sequence component. Besides, the phenomenon of the unbalanced split capacitor voltages is revealed, and fast-tracking balancing method by controlling zero-sequence current flowing through the split capacitors is provided. Digital simulation results verify the accuracy of the analysis and the feasibility of the suppression methods.

Research on the control strategy of phase balance in H-bridge cascade SVG

Based on the H-bridge cascaded SVG as one of the best solutions to effectively compensate reactive currents, this article focuses on the cascaded high-voltage H-bridge SVG phase balance problem. While analyzing the influence factors of the SVG DC side voltage, it summarizes and introduces 5 SVG phase balance control strategy ， Analyzed its real-time performance, stability, applicable occasions, algorithm and hardware circuit complexity, etc.Some control strategies have been simulated and verified, and the simulation results have reached the expected value, which provides a reference for the selection of balance control strategies for high-voltage SVG equipment.

PID-type terminal sliding mode control for permanent magnet synchronous generator based enhanced wind energy conversion systems

The provision of wind farm (WF) grid-codes (GC) in sustaining grid operations has become imperative, especially for WFs with permanent-magnet synchronous generator (PMSG) wind energy conversion system. Numerous techniques are developed for executing GC requirements in the event of grid faults. Among the methods, an intriguing strategy is to enhance the performance of back-to-back (BTB) converter controllers. In this research, the PID-type terminal sliding mode control (PID-TSMC) scheme is implemented for both machine-side and grid-side converter modified controllers of BTB-converter, to reinforce the nonlinear relationship among the state-variable and control input. The application of this control scheme decreases the response time and improves the robustness of the BTB-converter controllers to parameters uncertainty and external disturbances. The grid-side converter tracks the maximum power point, contributing to the generator active power output rapid decrease during faults. This frees up converter capacity for injecting GC-compliant reactive current into grid. Besides, the machine-side converter regulates dc-link voltage, in which its variations during external disturbances decrease substantially with the PID-TSMC. The discussions over the simulations contemplate on the robustness and efficiency of the implemented PID-TSMC strategy in comparison to other BTB-converter control strategies.

Comprehensive Compensation Strategy on the Power Quality of Micro-grid Integration Based on the Multifunctional Energy Storage Converter

In order to improve the utilization rate of the energy storage system as well as the power quality of micro-grid and grid under parallel operations, this paper proposed an integrated compensation strategy of power quality based on the composite controlling principle of frequency division and hysteresis loops. On the premise of not changing the operation state of the power conversion system (PCS), combined with the current generation principle of multi-objective compensation, use the residual capacity of PCS to compensate harmonic, reactive and unbalanced currents quickly, accurately and selectively, so as to realize the current control on the compensation component of the grid-connected current under the disturbance of the distorted voltage. At the same time, aiming at the dwarf-signal dynamic models of the converter, use the designed output current control loop based on H ∞ resonance control theories to provide the tuning basis of the controller parameters, and thus the anti-disturbance ability of the system is improved, and the sudden change of the frequency is reduced. Finally, construct a micro-grid system with energy storage devices, and carry out experiments on the proposed control strategies. The results show that it has well governance abilities on the power quality.

A Fault-Tolerant Control Framework for DFIG-Based Wind Energy Conversion Systems in a Hybrid Wind/PV Microgrid

This article proposes a fault-tolerant control framework for a doubly fed induction generator (DFIG)-based wind energy conversion system (WECS) in a hybrid wind/photovoltaic system (PV) microgrid structure. It implements a fractional-order sliding mode control (SMC) for the DFIG converters to mitigate the grid faults and ensure robustness against mismatched uncertainties. It also includes a shared reactive power support strategy where both the WECS and PV system participate in providing the necessary reactive current by utilizing their converters as STATCOM. The proposed approach is validated using a wind/PV system installed in a feeder of a test microgrid system subject to short-term unbalanced grid voltage faults and mismatched disturbances. Its performance is further compared to that of a standard SMC-based approach. The obtained results show that the proposed framework improved the dynamic stability of the DC voltage and enabled grid support during both symmetrical and asymmetrical grid faults. Providing fast and robust control of the converters, ensuring compliance with the new grid codes and avoiding the activations of the crowbar system are among the positive features of the proposed framework.

An Improved ip − iq Harmonic Detection Method Based on Time-Varying Integral Duration

By analyzing harmonic related to instantaneous active current and instantaneous reactive current, we propose an improved harmonic detection method to remedy long response delay and low detection accuracy corresponding to harmonic of power system. This method introduces a series connection scheme composed of Low-Pass Filter (LPF) and a current average module firstly. Then, detection accuracy of harmonic current is enhanced by adaptive tuning cut-off frequency of LPF and integration time which was obtained by proposed algorithm of the current average module. Moreover, feedback loop is introduced to compensated delay caused by LPF. Simulations including uncontrollable rectifier and three-phase voltage type inverter show proposed method has many advantages such as high detection accuracy, low response delay and good generality. Our method provides reliable harmonic current detection for later harmonic suppression and harmonic compensation.