Reference Current Calculation Research Articles

Optimal Searching-Based Reference Current Computation Algorithm for IPMSM Drives Considering Iron Loss

Optimal Searching-Based Reference Current Computation Algorithm for IPMSM Drives Considering Iron Loss

Analysis and Comparison Between Different Methods of Current Reference Generation for Active Filters Control.

In modern distribution systems the proliferation of non-linear loads results in a deterioration of the quality of voltage waveforms at the point of common coupling (PCC) of various consumers. Therefore, power-conditioning equipment is becoming more important for electric utilities and their customers. With the rapid development of semiconductor devices in power and control circuits, a new generation of equipment for power quality, the active power filters, has been developed. Their advantages over conventional means are more flexibility and very fast control response. The control of an active filter comprises two major parts: the reference current computation and the current control. There are two fundamental methods of generating the reference current: (i) frequency–domain methods, based on the Fourier analysis and (ii) time-domain analyze, based on the theory of instantaneous imaginary power in the three-phase circuits, often called p-q theory. The paper begins by presenting the principle of active filtering and the basic instantaneous imaginary power theory. In the hypothesis of a distorted and/or unsymmetrical load voltage system, the p-q theory has proven limitations; consequently the paper reviews and evaluates other two reference current calculation methods. Finally a comparative analyze of the three methods features is carried out.

New Reference Current Calculation Method of a Hybrid Power Filter Based on Customer Harmonic Emission

This paper presents a novel reference current calculation method for harmonics mitigation and reactive power compensation in power systems. This method was applied to a unique hybrid power filter topology. The motivation for this study comes from the increasing power quality issues, such as harmonic distortion and resonances, caused by the widespread integration of converter-interfaced generation (CIG) and modern nonlinear loads into the power system in recent years. The goal of this study is to propose a cost-effective and practical solution for current harmonics filtering and power factor correction by combining the advantages of passive and active filters into a hybrid solution. The developed reference current calculation method, which is based on customer current harmonic emissions, enables effective reference current calculation. Theoretical analyses along with simulation results obtained from a medium-voltage benchmark model in PSCAD verify the effectiveness of the proposed solution in harmonics filtering and show a substantial reduction in filter current ratings across various harmonic components. In addition, the simulation results were evaluated by comparison with the results obtained from a real-time simulator.

DT-CWT and type-2 fuzzy-HSAPF for harmonic compensation in distribution system

Extensive utilization of power electronics in different applications injects harmonics into degrading the power quality. Harmonics can be eliminated in distribution systems using low-cost passive filters, but, they become bulky and complex in design with the increase in harmonic order. Hence, active power filters (APFs) are proposed as a voltage source converter (VSC) for providing better compensation, but active filtering is costly. To minimize such demerits, this article represents a hybrid shunt active power filter (HSAPF). For reference current calculation, dual-tree complex wavelet transform (DT-CWT) is used here and conventional proportional–integral (CPI), type-1 and type-2 fuzzy logic controllers (T1FLC and T2FLC) are employed for parameter optimization of HSAPF, to enhance the harmonic compensation ability and power factor. The case studies are performed in both simulations as well as real-time experimental setup using dSPACE, and it is analyzed that the proposed DT-CWT and type-2 fuzzy-based HSAPF provide better compensation performance in comparison to other controllers under different operating conditions.

Advanced Torque Ripple Minimization of Synchronous Reluctance Machine for Electric Vehicle Application

The electric machine and the control system determine the performance of the electric vehicle drivetrain. Unlike rare-earth magnet machines such as permanent magnet synchronous machines (PMSMs), synchronous reluctance machines(SynRMs) are manufactured without permanent magnets. This allows them to be used as an alternative to rare-earth magnet machines. However, one of the main drawbacks of this machine is its high torque ripple, which generates significant acoustic noise. The most typical method for reducing this torque ripple is to employ an optimized structural design or a customized control technique. The objective of this paper is the use of a control approach to minimize the torque ripple effects issue in the SynRM. This work is performed in two steps: Initially, the reference current calculation bloc is modified to reduce the torque ripple of the machine. A method for calculating the optimal reference currents based on the stator joule loss is proposed. The proposed method is compared to two methods used in the literature, the FOC and MTPA methods. A comparative study between the three methods based on the torque ripple rate shows that the proposed method allows a significant reduction in the torque ripple. The second contribution to the minimization of the torque ripple is to propose a sliding mode control. This control suffers from the phenomenon of “Chattering” which affects the torque ripple. To solve this problem, a second-order sliding mode control is proposed. A comparative study between the different approaches shows that the second-order sliding mode provides the lowest torque ripple rate of the machine.

Semi-explicit multilinear modeling of a [formula omitted] open-loop controlled PV inverter in [formula omitted]-frame

This paper introduces semi-explicit multilinear modeling to ac power systems, including voltage source converters. Firstly, the novel semi-explicit multilinear modeling class is explained. Then this class is applied to a small example of PV inverter connected to an infinite bus. Some part of the nonlinear current reference calculation is expressed as a multilinear algebraic constraint. The formulation of this multilinear algebraic constraint allowed modeling the whole nonlinear inverter as a semi-explicit multilinear model. The novel modeling approach is compared with common nonlinear time-based simulation in Simulink. The results show a good approximation of the nonlinear model by its semi-explicit multilinear model where the relative error is below 10−3%. This low error motivates the further usage of the semi-explicit multilinear modeling class for power system analysis.

Comparison of the impact of different current reference calculation methods on grid interface converter design under asymmetrical grid faults

The proportion of power electronic Voltage Source Converter (VSC) interfaced resources in electrical transmission systems is expected to dramatically over the coming decades as the power system is decarbonised. Unlike synchronous generator interfaced resources the fault current characteristic of VSC interfaced resources is fully controllable, and so the current injected by such resources during asymmetric grid fault scenarios is determined primarily by the control strategy applied to them. Three of the most prominent Current Reference Calculation Methods (CRCMs) are analysed in this paper: Injecting only positive sequence current as the conventional standard method; Injecting unbalanced current to suppress the active power oscillations which can limit the required sizing of the dc-bus capacitor; Flexible positive- and negative-sequence control provides a flexible way of balancing regulation of phase voltage against negative sequence voltage suppression. An analytical comparison is made with each precise grid voltage dip variation and with varying active and reactive power set-points based on the GB grid code and maximum current limitation of semiconductors. The performances of the three methods are discussed and validated in Matlab.

Hierarchical Adaptive Control of the Cyber–Physical Power System With MMC-MTDC for Transient Stability Improvement

Large-scale wind power integration into the power system has promoted the development of a multiterminal DC (MTDC) transmission grid with a modular multilevel converter (MMC). Basically, MTDC with MMC is a typical cyber–physical system with continuous coupling interactions between cyber assets and power systems. However, cyber events may introduce many internet-based vulnerabilities and even result in the loss of transient stability of the power system. Therefore, a voltage compensation-based two-level hierarchical adaptive control strategy is proposed in this article. At the higher level, a modified MMC output current reference calculation method is developed in the αβ framework to guarantee the transient stability of the power system, whereas a feedback adjustment method is proposed in the MMC control framework, at the bottom level, to contain the controller from deviating from its output reference while eliminating the impact of cyber communication delay on transient stability. The article shows that the proposed hierarchical control strategy can improve the transient stability of the power grid under the interference of three-phase ground faults in physical and communication delays in the cyber layer. Finally, the simulation results of the modified IEEE 9-bus test system with MMC–MTDC are used to demonstrate the effectiveness of the proposed scheme.

Improved Current Reference Calculation for MMCs Internal Energy Balancing Control

The paper addresses an improved inner current reference calculation to be employed in the control of modular multilevel converters operating during either balanced or unbalanced conditions. The suggested reference calculation is derived based on the AC and DC additive and differential voltage components applied to the upper and lower arms of the converter. In addition, the impacts caused not only by the AC network’s impedances but also by the MMC’s arm impedances are also considered during the derivation of the AC additive current reference expressions. Another issue discussed in this article regards that singular voltage conditions, where the positive-sequence component is equal to the negative one, may occur not only in the AC network but also internally (within the converter’s applied voltages). Several different inner current reference calculation methods are compared and their applicability during the former fault conditions is analyzed. The paper presents a detailed formulation of the inner current reference calculation and applies it to different unbalanced AC grid faults where it is shown that the presented approach can be potentially used to maintain the internal energy of the converter balanced during normal and fault conditions.

PLL-Free Voltage Oriented Control Strategy for Voltage Source Converters Tied to Unbalanced Utility Grids

Asymmetrical working conditions of the utility grid introduce the large-amplitude negative-sequence component to the output current of the voltage source converter (VSC), causing power semiconductor devices to suffer from thermal fatigue and thermal damage. Though the conventional phase-locked loop (PLL) based voltage oriented control (VOC) solution can suppress the steady-state negative-sequence current effectively, it has a weak suppression ability of transient overload current, and even aggravates the transient current asymmetry and causes more severe transient impact to the VSC. This paper first analyzes the transient performance of the conventional VOC strategy, especially its dynamic response time and the main factors for performance limitation. On this basis, the PLL-free VOC strategy for VSCs tied to unbalanced grids is proposed, and its critical parts, namely, the reference current calculation and the fast detection of the grid voltage sequence components, are implemented. Besides, to improve the frequency adaptability, a high-performance grid frequency detection strategy is developed based on the difference-frequency phase caused by the frequency variation. Finally, experiments are performed to verify the effectiveness and advancement of the proposed method. Specifically, the results proved the rapidity, accuracy, and frequency adaptability of the proposed method in suppressing the VSC negative-sequence current, both in transient and steady-state conditions.

Strategies to Increase the Transient Active Power of Photovoltaic Units during Low Voltage Ride Through

Due to a limitation in the magnitude of the three-phase output inverter currents, the output active power of the photovoltaic (PV) unit has been de-rated during low voltage ride through, which brings great instability risk to the power system. With the increase in the penetration rate of new energy, the impact of the power shortage on the system transient stability increases. It is of great significance to analyze the impact of this transient power shortage on system stability. This article explores methods to improve the active power output capability of photovoltaic units during low-breakthrough periods. A transient simulation model of a grid-connected PV generator with low-voltage ride-through (LVRT) capability is presented, under the condition of meeting the overcurrent capacity of the PV inverter and the requirement of dynamic reactive power support supplied by the PV generator specified in the China grid codes (GB/T 19964-2012) during grid fault. An example system with high PV penetration is built. The change principle and influencing factors of PV transient active power output are analyzed. The simulation model is designed in PowerFactory/DIgSILENT, and several types of three-phase voltage sags are performed in simulation to assess the impact of the active current reference calculation method and the maximum inverter output current (Imax) limit value on the PV active power output. According to the three indexes, namely the maximum active power of PV unit during the fault, the power improvement gradient and the power surge after the fault is cleared. Simulation results showed that using the orthogonal decomposition method to calculate the active current reference can make full use of the current capacity of the converter. Setting Imax to 1.1 rated current of photovoltaic inverter (IN) can reduce the cost-effectiveness ratio of the transient active power output of the PV unit. Therefore, we aim to improve the unit’s transient active power output capacity and realize the optimal effect of improving the transient active power shortage of the system.

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.

Active Filter Reference Calculations Based on Customers’ Current Harmonic Emissions

This paper deals with harmonics compensation in industrial and distribution networks using an active filter (AcF). When defining the AcF’s reference current, it is important to properly consider the network background harmonic distortion. Within this paper, we propose an AcF reference current calculation method, based on customers’ current harmonic emissions. The main novelty of the paper is the AcF reference current calculation method that considers only the customer’s contributions to the harmonic distortion at the point of common coupling (PCC). By separating the harmonic current at the PCC into components that can be attributed to the customer and to the network, it is possible to limit the required AcF power. To determine the customer’s emission, the customer’s harmonic impedance must be known. As the actual harmonic impedance cannot be determined in a real environment, a reference harmonic impedance can be used instead. To test the proposed AcF reference current calculation method, we developed a control algorithm of an AcF in the PSCAD software and tested this on a medium-voltage benchmark simulation model.

Simulation Models For Three-Phase Grid Connected PV Inverters Enabling Current Limitation Under Unbalanced Faults

Normally unbalanced grid voltage dips may lead to unbalanced non-sinusoidal current injections, DC-link voltage oscillations, and active and/or reactive power oscillations with twice the grid fundamental frequency in three-phase grid-connected Photovoltaic (PV) systems. Double grid frequency oscillations at the DC-link of the conventional two-stage PV inverters can further deteriorate the DC-link capacitor, which is one of the most important limiting components in the system. Proper control of these converters may efficiently address this problem. In such solutions, Current Reference Calculation (CRC) is one of the most important issues that should be coped with the reliable operation of grid-connected converters under unbalanced grid faults. Therefore, this paper proposes and simulates CRC methods and presents the results in order to improve the quality of the grid-connected PV system under unbalanced grid voltage fault.

Model Predictive Current Control for Fault-Tolerant Bidirectional Voltage Source Converter With Open Circuit Fault and Unbalanced Grid Voltage

To improve the fault-tolerant operation capability of bidirectional voltage source converter (BVSC), an improved model predictive current control (IMPCC) for fault-tolerant BVSC, is proposed with balanced DC-link capacitor voltage under unbalanced grid voltage. The proposed method can maintain continuous operation even if power device faults and unbalanced grid voltage faults occur together. A current predictive model of the fault-tolerant BVSC is established. By using grid voltages and 90° lagging signals in the $\alpha \beta $ stationary coordinate system, the reference current calculation method is designed for BVSC to eliminate power ripple under unbalanced grid voltage, which can avoid the complex positive and negative sequence extraction. DC-link split capacitor voltage balancing is achieved by the improved cost function. Based on the current predictive model and improved cost function, the optimal space voltage vectors are selected for fault-tolerant BVSC. Compared to the existing predictive methods, the proposed IMPCC can balance DC-link capacitor voltage and eliminate the power ripples for fault-tolerant BVSC under unbalanced grid with simple implementation. The experimental results validate the effectiveness of the proposed control scheme.

DSTATCOM integrated with Y-y connection transformer for reactive power compensation

This paper proposes a distribution static synchronous compensator (DSTATCOM) integrated with a Y-y connection transformer for reactive power compensation. A cascaded DSTATCOM is connected to a group of taps on the primary windings of the transformer to form an integrated structure. The proposed structure can achieve a flexible connection point voltage selection to achieve a compromise between the voltage rating and current rating. Therefore, the initial investment, voltage stress and size of the compensation system can be reduced. The winding currents distribution and compensation capacity constraint are analyzed in detail to determine the attainable compensation range. To improve the dynamic response performance of the system, a feedforward compensation control strategy is introduced in this paper. To prevent the transformer windings from overcurrent, a reactive reference current calculation method is also presented. Finally, both simulation and experimental results are provided to verify the effectiveness of the proposed system.

A four-switch shunt active filter based on one cycle control for compensating non-linear loads

ABSTRACT This paper introduces a Four-Switch Three-Phase Shunt Active Power Filter (FSTP-APF) based on One Cycle Control (OCC) technique for compensating the currents of three phase non-linear loads. The power stage of FSTP-APF uses only four semiconducting switches which leads to reduce the overall cost of APF. The proposed technique retains the advantages of the conventional OCC, such as no reference current calculation, no need for using Phase-Locked Loop (PLL), very simple control circuit and constant switching frequency. In addition, the modification reduces the number of semiconductor switches, reduces the filter output inductances, reducing the number of sensors to only four sensors, two of them for measuring supply currents and the others for measuring DC-link voltages, and the control algorithm becomes simpler. The feasibility and performance of the proposed technique are demonstrated through the simulation and experimental verifications.

A Novel Orthogonal Current Decomposition Control for Grid-Connected Voltage Source Converter

Grid-connected voltage source converter is used to control the flow of active power from renewable energy sources with several functionalities, such as harmonic mitigation, reactive power compensation, and power factor improvement. The reference current calculation and its accuracy decide the performance of the control algorithms during harmonic disturbances. Fundamental signal extraction techniques are utilized to overcome the disturbances present in the system voltage and current. This paper presents a novel orthogonal current decomposition control for load current decomposition into active and reactive components. These current components are obtained through character of orthogonal vector in the inner product space. The block diagram of the proposed technique is implemented with the help of adder, multiplier, and windowed integrator. Phase voltage and load current for each phase are used as inputs to this functional diagram for getting desired fundamental active current signal. Subsequently, adjustable dc-link voltage control is also presented to diminish the switching losses in the converter and to avoid the false tripping of the grid-connected system during susceptible grid conditions. The proposed control technique is simulated in MATLAB/Simulink and verified through developed hardware prototype using ARM Cortex M4-based microcontroller (STM32F407VGT6) using Waijung environment.

Enhanced Current Reference Calculation to Avoid Harmonic Active Power Oscillations

Energy storage systems play a key role in the rise of distributed power generation systems, hence there is great interest in extending their lifetimes, which are directly related to DC current ripple. One of the ripple sources is the low-frequency active power fluctuations under unbalanced and distorted grid voltage conditions. Therefore, this paper addresses a multifrequency control strategy where the harmonic reference currents are calculated to reduce harmonic active power oscillations. The stationary reference frame (StRF) approach taken here improves the precision and computational time of the current reference calculation method. Additionally, in order to ensure safe converter operation when a multifrequency reference current is provided, a computational efficient peak current saturator is applied while avoiding signal distortion every time step. If the injected current harmonic distortion is to be minimized, which is a feature included in this work, the peak current saturator is a necessary requirement. Active power ripple is reduced even with frequency variations in the grid voltage using a well-known frequency-adaptive scheme. The simulation and experimental results prove the optimized performance for the control objective: power ripple reduction with minimum current harmonic distortion.

Online MTPA Control of IPMSM Based on Robust Numerical Optimization Technique

This paper presents an online maximum torque per ampere (MTPA) control that considers both magnetic saturation and cross-magnetization effects of interior permanent-magnet synchronous machines (IPMSMs). For IPMSM drives, especially in torque-controlled applications, torque accuracy and high-efficiency operations are important issues. They can be dealt as a constrained optimization problem to satisfy both torque reference tracking and loss-minimizing operation. In this paper, nonlinear simultaneous equations are derived from Lagrange multiplier method, which could be solved by numerical algorithms. Among them, Levenberg-Marquardt algorithm (LMA) is employed to guarantee a robust calculation of optimal current references. It is optimized to alleviate calculation burden while maintaining the stability of the proposed algorithm. In addition, a torque reference limiter is implemented to satisfy a current limit in real time. The feasibility of the proposed method is verified under various operating conditions by simulation and experimental results. Through the proposed algorithms, accurate MTPA control is achieved under not only unsaturated but also highly saturated operating conditions.