Design of reactive power compensators for distribution systems with harmonic distortion

Abstract

The objective of this paper is to derive a systematic algorithm to decide the optimal location and size of shunt capacitors and filters for distribution systems with harmonic distortion. In this paper, the problem of reactive power compensation is first formulated as a nonlinear programming of minimization of real power loss and capacitor cost under voltage constraint to decide the optimal locations and sizes of shunt capacitors. The harmonic load flow is then applied to solve the total voltage harmonic distortion factor (HDF). Finally, the tuning frequency of the single‐tuned filter and the capacity and voltage ratings of the corresponding reactor and capacitor are determined so that both the harmonic distortion and the reactive power compensation can be solved simultaneously. To demonstrate the performance and effectiveness of the proposed methodology, a practical distribution feeder with nine large industrial customers is selected for computer simulation. It is concluded that proper design of harmonic filters should be undertaken to solve the harmonic resonance problem, as well as the reactive power compensation for distribution systems with nonlinear loads.     

A grid-tie PV inverter with the ability to improve power quality under unbalanced and distorted source voltage conditions

ABSTRACT

This paper proposes a grid-tie PV inverter that is able to improve power quality under conditions of both distorted and unbalanced source voltage. The presented strategy is based on the instantaneous power theory and uses a second-order generalized integrator-quadrature signal generator (SOGI-QSG). The presented control strategy is aimed at compensating reactive power, eliminating current harmonics, load balancing, and enabling the PV to inject maximum power to the grid. The advantages of the control system are the use of SOGI-QSG adaptive filter and frequency-locked loop (FLL), and removing the low-pass filter and phase-locked loop (PLL). DC–DC and DC–AC converters are utilized for connecting the PV to the grid. The DC–DC and DC–AC converters are responsible for maximum PV power tracking and achieving the control aims, respectively. Using 4-leg converter structure for grid-tie inverter enables achieving the control objectives in 3-phase 4-wire distribution network without any transformer. The presented control strategy is applied to a 3-phase 4-wire distribution network and is simulated in MATLAB/SIMULINK environment. The results of this simulation are then compared with the conventional instantaneous power method in areas including load balancing, reactive power compensation and the elimination of current harmonics, under unbalanced and distorted source voltage conditions.     

交流变频调速系统仿真建模及谐波特性分析

随着电力电子技术的发展,交流变频调速系统以其强大的优势逐渐取代传统的直流调速传动,其在给人们生产生活带来方便和效率的同时,也造成了电力系统谐波污染加重的问题。因此,设计了一种开环正弦脉宽调制（sinusoidal pulse width modulation,SPWM）控制的交流变频调速系统并搭建仿真模型,实现了对系统中变频器输出电压频率和幅值及电机转速的灵活控制。对系统网侧电压和电流进行了谐波特性分析,结果表明：网侧电压的波形比较理想、畸变很小,而电流波形畸变严重;且随着变频器输出频率的增加,电压波形畸变程度有所加剧,电流畸变程度有所减轻。     

Input filter state feed-forward stabilising controller for constant power load systems

An input filter state feed-forward stabilising controller is presented to stabilise a constant power load and is validated using a brushless DC motor drive system. The strategy is to feed-forward a stabilising signal which is a function of the DC-link filter variables, capacitor voltage and the inductor current, into the current control loop of the motor drive to modify the magnitude and phase of the system input admittance around the input filter natural frequency and thereby damp the input filter. The controller design and parameter selection are described. The impact of the stabilising controller is examined on the motor controller performance and finally the effectiveness of the controller is verified by simulation and experimentally.     

Analysis and design of a single-phase AC/DC step-down converter for universal input voltage

This work presents a single-phase AC/DC step-down converter, which is composed of two power stages, buck-boost converter and buck converter. The front stage is used for a power-factor-correction (PFC) circuit and is operated in discontinuous conduction mode (DCM) by using the pulse-width modulation (PWM) technique to achieve almost unity power factor and low total harmonic distortion of input current (THD_i). The rear stage is also operated in DCM to achieve voltage step-down and low DC-link voltage. The proposed converter can be applied for universal input voltage (85-265 V) and wide output power range. Also, the steady-state analysis of voltage gain and boundary operating condition are presented. Moreover, the selections of inductors, capacitors and input filter are depicted. Finally, a hardware circuit with simple control logic is implemented to illustrate the theoretical analysis.     

Feedforward simple control technique for on-chip all-digital three-phase AC/DC power-MOSFET converter with least components

A novel simple control technique for on-chip all-digital three-phase alternating current to direct current (AC/DC) power-metal oxide semiconductor field-effect transistors (MOSFET) converter with least components, which is employed to obtain small current and DC output voltage ripples as well as excellent performance, and using a feedforward simple control method for DC output voltage regulation is proposed. The proposed all-digital feedforward controller has the features of low cost, simple control, fast response, independence of load parameters and the switching frequency, it has no need for compensation, and high stability characteristics; moreover, the proposed controller consists of three operation amplifiers and few digital logic gates that are directly applied to the three-phase converter. The power-MOSFETs are also known as power switches, whose control signals are derived from the proposed all-digital feedforward controller. Instead of thyristors or diodes, the application of power-MOSFETs can reduce the loss of AC/DC converter that is proper to the power supply system. The input stage of an AC/DC converter functions as a rectifier and the output stage is a low pass inductor capacitor (LC) filter. The input AC sources may originate from miniature three-phase AC generator or low-power three-phase DC/AC inverter. The maximum output loading current is 0.8 A and the maximum DC output ripple is less than 200 mV. The prototype of the proposed AC/DC converter has been fabricated with Taiwan Semiconductor Manufacturing Company (TSMC) 0.35 mum 2P4M complementary MOS (CMOS) processes. The total chip area is 2.333 1.960 mm². The proposed AC/DC converter is suitable for the following three power systems with the low power, DC/DC converter, low-dropout linear regulator and switch capacitor. Finally, the theoretical analysis is verified to be correct by simulations and experiments.     

Negative sequence compensation within fundamental positive sequence reference frame for a stiff micro-grid generation in a wind power system using slip ring induction machine

An isolated wind power generation scheme using slip ring induction machine (SRIM) is proposed. The proposed scheme maintains constant load voltage and frequency irrespective of the wind speed or load variation. The power circuit consists of two back-to-back connected inverters with a common dc link, where one inverter is directly connected to the rotor side of SRIM and the other inverter is connected to the stator side of the SRIM through LC filter. Developing a negative sequence compensation method to ensure that, even under the presence of unbalanced load, the generator experiences almost balanced three-phase current and most of the unbalanced current is directed through the stator side converter is the focus here. The SRIM controller varies the speed of the generator with variation in the wind speed to extract maximum power. The difference of the generated power and the load power is either stored in or extracted from a battery bank, which is interfaced to the common dc link through a multiphase bidirectional fly-back dc-dc converter. The SRIM control scheme, maximum power point extraction algorithm and the fly-back converter topology are incorporated from available literature. The proposed scheme is both simulated and experimentally verified.     

Improving the performance of hysteresis direct torque control of IPMSM using active filter topology

This paper describes an active filter topology to improve the performance of hysteresis direct torque control (HDTC) of interior permanent magnet synchronous motor (IPMSM). The filter topology consists of an active filter and two RLC filters, and is connected to the main power circuit through a 1:1 transformer. The active filter is characterized by detecting the harmonics in the motor phase voltages and injecting equivalent harmonic voltages to produce almost sinusoidal voltage waveform to the motor terminals. The active filter uses hysteresis voltage controller while the motor main circuit uses hysteresis direct torque control. The simulation results of this combined control structure show considerable torque ripple reduction in the steady state range and adequate dynamic torque performance as well as considerable harmonic voltage and EMI noise reduction.     

Power calibrator using switched mode voltage source

The paper is focused on a sophisticated switched mode power supply with a pure sinusoidal output voltage waveform with frequency (40-65 Hz) and voltage range (30-500 V). In order to cope with stringent voltage (10.2%) and distortion constraint (THD<0.7%) in a modern calibration system, traditional inverter design with an output LC filter is upgraded with a feedback-feedforward control principle. The paper presents the voltage source topology, analyzes its steady-state response and gives its comprehensive evaluation. Using a simple sensor that is capable to detect a small voltage asymmetry at the presence of a high AC component (500 VRMS), the parasitic DC component is limited below 5 mV. Experimental results demonstrate exceptional voltage accuracy (<0.2%) and low total harmonic distortion (THD)     

Harmonic and reactive power compensation as ancillary services in inverter-based distributed generation

The introduction of distributed generation, DG, into low-voltage (LV) networks opens up the possibility of supplying ancillary services to aid network management and to maintain power quality. DG itself can cause voltage magnitude difficulties when injecting real power into networks with high R/X ratios, but control of reactive power injection can help overcome this. Continuous control and support only at the voltage limit conditions (to avoid unnecessary reactive power flows in normal conditions) are explored. Relatively high-impedance LV networks are prone to harmonic distortion from nonlinear loads. A variety of control methods that emphasise either harmonic-line flows or local-voltage distortion are examined, and a compromise method based on resistance emulation is shown to be effective. Experimental results from a single-phase laboratory network and 2 kVA inverter are used to illustrate how these additional control functions can be integrated into the existing control scheme for real-power management. Decomposition of observed voltages and currents into harmonic terms that are phasesynchronised to the grid voltage is a challenge in real-time systems. Kalman observers are used to achieve this with an additional advantage of avoiding explicit phase-locking while producing quadrature components useful in instantaneous calculation of reactive power and in providing feed-forward compensation terms     

Intelligent controlled shunt active power filter for voltage and current harmonic compensation in microgrid system

An integrated microgrid with a novel shunt active power filter (APF) using Elman neural network (ENN) is proposed in this study. The microgrid consists of a storage system, a photovoltaic (PV) system, the shunt APF, a linear load, and a nonlinear load. Moreover, the master/slave control algorithm is adopted in the microgrid. The storage system, which is considered as the master unit, is adopted to control the active and reactive power outputs (P/Q control) in grid-connected mode and the voltage and frequency of the microgrid (V/f control) in islanded mode. Furthermore, the PV system is considered as the slave unit to implement P/Q control in both grid-connected and islanded modes. In addition, the proposed shunt APF possesses dual functions of voltage and current harmonic compensation for microgrid under voltage harmonic propagation and nonlinear load to reduce the voltage and current total harmonic distortions (THD) effectively. Additionally, an ENN controller is adopted in the proposed shunt APF to improve the transient and steady-state responses of DC-link voltage during the switching between the grid-connected mode and islanded mode. Finally, some simulation results are provided to verify the feasibility and the effectiveness of the integrated microgrid with the intelligent controlled shunt APF.     

二元翼段间隙非线性颤振的模糊控制

根据模糊控制不依赖于被控对象的精确模型,对参数变化不敏感,具有很强鲁棒性的特点,提出一种模糊逻辑控制器,对包含间隙的二元机翼极限环振荡进行控制,并对其响应机理进行探讨和研究。数值仿真结果表明,模糊控制器能够有效抑制非线性极限环振动,使系统迅速达到稳定,提高系统颤振速度。     

Harmonic power flow analysis for the measurement of the electricpower quality

The number and power of loads that pollute, from an electric point of view, the electric network are constantly increasing. Their connection to the electric network causes a significant distortion in the line current and, when their overall power is no longer negligible with respect to the network power, also some distortion in the line voltage, thus deteriorating the electric supply quality. The paper proposes a method, based on the evaluation of the harmonic powers, to identify whether the source producing harmonic distortion in the line currents and voltages of a three-phase system is a polluting load connected after the metering section or is the distortion of the line voltages. Some parameters are also defined in order to quantify the effects of the polluting load on the power quality. A Virtual Instrument is also described, realized to implement the proposed method and to verify it experimentally. The instrument accuracy is discussed     

基于模糊PID控制的粉末包装计量控制系统

目的针对粉末包装计量控制系统中由于粉末的流量与螺杆的转速存在着一定非线性的特点,提出一种基于模糊PID的粉末计量包装控制系统。方法该粉末包装计量控制系统由螺杆计量机构、称重机构和信号采集处理单元等组成,根据系统特性,结合模糊逻辑构建模糊自适应调整机构对PID控制器参数进行在线调整,以适应系统特性的非线性变化,并基于Matlab对控制系统进行仿真。结果样机计量控制时偏差度最大为0.8%,仿真结果表明模糊PID较传统PID具有更好的效果。结论该控制系统实际控制工作稳定,鲁棒性较好,可以满足实际控制系统的需要。     

Vector-optimised harmonic elimination for single-phase pulse-width modulation inverters/converters

A new approach for calculating single-phase inverters/converters-optimised pulse-width modulation (PWM) switching angles for harmonic and voltage control is proposed. This approach takes into account the DC-link voltage ripple, due to the finite DC-link capacitance, in the optimised switching-angles calculation, using its harmonic content. New expressions for the evaluation of the PWM waveform harmonic spectrum are derived. Output voltage quarter-wave symmetry is thus not assumed and the pulse position is no longer fixed, resulting in two simultaneous sets of nonlinear equations in twice as many unknowns as in the case of conventional-optimised PWM strategy with smooth DC input/output voltage. The performance of this latter technique is hence retained while the size of the DC-link capacitor is reduced to a limit depending on the filter capacitance, the number of pulses per half modulation period and the modulation depth. The compensation of the reactive power (exchanged between the AC source/AC load and the DC link due to the rectifying/inverting process and the low-link capacitance) is also included; the converter then plays the additional role of an active power filter. Simulation examples and experimental results are used to validate the proposed method     

Large value AC capacitor for harmonic filtering and reactive power compensation

To overcome the limitations of conventional shunt passive filters, which are invariably used for harmonic filtering, a quasi-passive filter (QPF) has been proposed. It comprises a parallel and series tuned LC tank circuit. Unlike the conventional shunt passive filter, the QPF utilises a large value AC capacitor. Unipolar DC capacitors and power semiconductor devices have been used to realise the large value AC capacitor. The operation of the QPF is simple and it does not require the complex control methods of active power filters. With certain modifications in the QPF, a modified quasi-passive filter (MQPF) has been proposed, which can be used for reactive power compensation in addition to harmonic filtering. The proposed QPF and MQPF have been verified through analysis and simulation. Experiments are carried out to verify the validity of the QPF.     

电力系统谐波水平的主要控制方法

为了控制电力系统谐波电压水平,对于大、中型非线性用户负荷接入电网,IEC、IEEE和英国G5/4等标准中有不同的规定。本文在扼要介绍上列标准中相关规定的基础上,总结其特点,为谐波国标(GB/T14549-1993)的修订提出一些建议。     

A passive auxiliary circuit with interphase transformer applied in 12-pulse converters to provide clean power utility interface

This paper proposes a passive auxiliary circuit which can be added to an interphase transformer (PAC + IPT) configuration to reduce the total harmonic distortion (THD) existing in 12-pulse diode rectifier converter systems at AC mains. The proposed PAC + IPT compensation method is a simple structure, with low power consumption and requires no extra DC power supply. We present the theoretical analysis of the proposed topology that lessens the total harmonic distortion (THD) and evaluate the dynamic simulation results on a 12-pulse converter system and a 3-kW laboratory prototype. Both the simulation and the experimental results show that the proposed PAC + IPT compensation method can improve the power quality and provide a clean power utility interface of AC line input currents for a conventional 12-pulse diode rectifier converter.     

Design and implementation of PV-based three-phase four-wire series hybrid active power filter for power quality improvement

This paper proposes a Photovoltaic (PV)-based three-phase four-wire Series Hybrid Active Power Filter (SHAPF), it comprises of a Series Active Power Filter (SAPF) and an LC shunt passive filter. The proposed system eliminates both the current and voltage harmonics and compensates reactive power, neutral current and voltage interruption. A SAPF demands a source of energy for compensating the voltage sag/swell. This system found a new topology for SHAPF which utilizes the PV with DC–DC boost converter as a source of DC power for the series active filter. The compensation current reference evaluation is based on the twin formulation of the vectorial theory of electrical power theorem with Fuzzy Logic Controller (FLC). The PV array/battery managed DC–DC boost converter is employed to step up the voltage to meet the DC bus voltage requirement of the three-leg Voltage Source Inverter (VSI). The foremost benefit of the proposed system is that, it will provide uninterrupted compensation for the whole day. This system utilizes the renewable energy; accordingly saves the energy and provides the uninterruptable power supply to critical/sensitive load, through the PV array/battery bank during both day time and night time. An experimental model was established and results were obtained, which indicated the capability of the proposed control scheme.     

敏感用户高品质交-直流级联供电方案研究

工业过程中大量使用的变频器(variable frequency drive,VFD)在采用传统交流供电时易受到电压暂降影响,传统治理方式存在检测延时、控制复杂等问题。为此,该文从敏感负荷变频器在电网扰动工况下的供电需求量化出发,围绕供电拓扑设计、DC/DC变换器的控制策略及储能单元容量配置等方面,构建一种实用化工业用交-直流级联供电方案。该文利用电压暂降过程中直流供电回路与敏感负荷间的压差自感应控制,实现交-直流供电回路无扰动切换,提高电压暂降治理响应的快速性;采用两相交错并联DC/DC变换器接入集中式储能,减少储能系统纹波系数,并给出两相均流控制方法。最后,通过Matlab/Simulink中的建模仿真与样机的实验测试,验证所提方案能够在交流系统发生不同严重程度电压暂降过程中为各类敏感负荷提供稳定的连续供电,为解决工业敏感用户高品质供电问题提供可实用的新思路。