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.     

Frequency Estimation of Three-Phase Power System Using Weighted-Least-Square Algorithm and Adaptive FIR Filtering

A new technique for estimation of the instantaneous frequency based on simultaneous sampling of three-phase voltage signals is presented. The structure consists of two decoupled modules: the first is for adaptive filtering of input signals, and the second is for frequency estimation. A suitable and robust algorithm for frequency estimation is obtained. This technique provides better performance, compared with the technique based on a single-phase signal in relation to waveforms with noise. The technique is particularly important when asymmetric sags generate zero voltage in one of the three phases. In addition, it allows the measurement of the instantaneous frequency value of real signals for single- or three-phase systems. To demonstrate the performance of the developed algorithm, computer-simulated data records and calibrator-generated signals are processed. The proposed algorithm has been put to test with distorted three-phase voltage signals.      

Phase-locked-loop interferometry applied to aspheric testing with a computer-stored compensator

A recently developed technique for continuous-phase determination of interferograms with a digital phase-locked loop (PLL) is applied to the null testing of aspheres. Although this PLL demodulating scheme is also a synchronous or direct interferometric technique, the separate unwrapping process is not explicitly required. The unwrapping and the phase-detection processes are achieved simultaneously within the PLL. The proposed method uses a computer-generated holographic compensator. The holographic compensator does not need to be printed out by any means; it is calculated and used from the computer. This computer-stored compensator is used as the reference signal to phase demodulate a sample interferogram obtained from the asphere being tested. Consequently the demodulated phase contains information about the wave-front departures from the ideal computer-stored aspheric interferogram. Wave-front differences of ~ 1 λ are handled easily by the proposed PLL scheme. The maximum recorded frequency in the template's interferogram as well as in the sampled interferogram are assumed to be below the Nyquist frequency.     

一种电磁感应式角加速度传感器及误差分析

旋转角加速度测量是对旋转系统进行状态检测和故障分析的重要手段,由此提出了一种基于电磁感应原理的新结构角加速度传感器。对三相异步电机、单相异步电机和自制无槽转子的三相异步电机进行了实际的角加速度测试。实验表明,当旋转系统存在瞬间角加速度时,经过电磁耦合立刻以传感器输出绕组感应电势的形式表现出来,可以直观反映出各种旋转机械系统的角加速度变化情况。另外对传感器实际工作状态存在的误差及来源进行了分析,并对传感器的输出特性和传递函数进行了推导,为传感器的推广应用提供了有效的技术支撑。     

A new phase-locked loop used in a frequency synthesizer

A well-known contradiction in phase-locked frequency synthesizer design is between frequency resolution, on the one hand, and bandwidth of the phase-locked loop (PLL), on the other. To solve this problem, a technique that uses an algorithm that produces a group of different divide numbers to the programmable feedback frequency divider of PLL for each required output frequency is presented. The interference frequency at the output of the phase detector of PLL and the frequency resolution can then be set independent of each other. High resolution and wide bandwidth are achieved simultaneously, with a simple synthesizer design leading to savings in power consumed and device cost     

Switched Reluctance Machines as Three-Phase AC Autonomous Generator

The operation of the switched reluctance machine as an autonomous three-phase ac generator is considered. Two circuits are proposed. The generator circuit consists of only capacitors and load supplementary to the generator, but does not contain any power supply. Theoretical approaches for simulation of the three-phase autonomous switched reluctance generator are presented. The influence of the high harmonics in the phase current on the power output is discussed. Experimental and simulation results are presented     

A generalised instantaneous non-active power theory for STATCOM

A generalised instantaneous non-active power theory is presented. Comprehensive definitions of instantaneous active and non-active currents, as well as instantaneous, average and apparent powers, are proposed. These definitions have flexible forms that are applicable to different power systems, such as single-phase or multi-phase, periodic or non-periodic and balanced or unbalanced systems. By changing the averaging interval and the reference voltage, various non-active power theories can be derived from this theory. The definitions of instantaneous active and non- active currents provide an algorithm for a STATCOM to calculate the non-active current in the load current. The theory is implemented by the STATCOM, and four cases (three-phase balanced RL load, three-phase unbalanced RL load, diode rectifier load and single-phase load) are tested. The experimental results show that the STATCOM can perform instantaneous non-active power compensation, and both the fundamental non-active component and the harmonics are eliminated from the utility so that nearly unity power factor can be achieved. The STATCOM also has a fast dynamic response for transients.     

Joule–Thomson Inversion in Vapor–Liquid–Solid Solution Systems

Solid phase precipitation can greatly affect thermal effects in isenthalpic expansions; wax precipitation may occur in natural hydrocarbon systems in the range of operating conditions, the wax appearance temperature being significantly higher (as high as 350 K) for hyperbaric fluids. Recently, methods for calculating the Joule–Thomson inversion curve (JTIC) for two-phase mixtures, and for three-phase vapor–liquid–multisolid systems have been proposed. In this study, an approach for calculating the JTIC for the vapor–liquid–solid solution systems is presented. The JTIC is located by tracking extrema and angular points of enthalpy departure variations versus pressure at isothermal conditions. The proposed method is applied to several complex synthetic and naturally occurring hydrocarbon systems. The JTIC can exhibit several distinct branches (which may lie within two- or three-phase regions or follow phase boundaries), multiple inversion temperatures at fixed pressure, as well as multiple inversion pressures at given temperature.     

Built-in self-test for phase-locked loops

An effective built-in self-test (BIST) structure of a phase-locked loop (PLL) in digital applications is presented in this paper. The proposed BIST structure can identify possible faults in any block such as the phase detector, charge pump, loop filter, voltage-controlled oscillator and divide-by-N of the PLL. The key advantage of this approach is that it uses all existing blocks in PLL for measuring and testing, reducing the chip area overhead. Restated, the proposed approach does not alter any existing analog circuits. Rather, the proposed approach only adds some small circuits to the PLL and requires a slight modification of the digital part. The final test outputs are digital values which can increase the reliability of the proposed BIST structure. Physical chip design and fault simulation results indicate the characteristics of the proposed BIST structure, namely, high fault coverage (97.2%) and low area overhead (2.78%).     

被动定向浮标数字解复用技术

介绍了被动定向浮标信号传输系统中复合信号解复用的工作原理,推导了解复用的过程,解释了罗盘补偿矢量传感器自转对浮标测向的影响。提出了一种解复用的数字信号处理算法．在信号处理板上软件实现解复用。设计了一种软件导频锁相环,解决了偶极子信号解调过程中频率相位精确跟踪的问题。该技术降低了浮标接收机的复杂性,保证了通道一致性,提高了可靠性和定向精度,同时降低了浮标的成本。通过仿真对导频锁相环的性能作了评价,并通过湖上试验对其进行了验证。     

Single-phase induction motor drive system using z-source inverter

Employing conventional three-phase inverter in variable speed single-phase drive suffers from limited maximum output voltage applied to motor or voltage utility factor (VUF). In this study, a z-source inverter (ZSI) has been used to drive a single-phase induction motor (SPIM). By the proposed topology, VUF and consequently torque speed characteristic of the single-phase motor is improved. The equations for employing ZSI in SPIM drive and modulation method are described. Results of two simulations using conventional inverter and ZSI are presented. The results show that using ZSI leads to an increase in motor electromagnetic torque compared to conventional three-phase inverter due to improved VUF. The experimental results confirm the theory and simulation.     

Windows and interpolation algorithms to improve electricalmeasurement accuracy

An FFT-based measurement technique, which can be easily used to determine accurately the frequency, amplitude, and phase of all the harmonic and interharmonic components of a distorted signal, is examined. Suitable windows and interpolation algorithms are examined in order to reduce undesirable effects due to spectral leakage caused by a sampling process that is not synchronized. Several results concerning the application of different windows on a set of simulated signals are compared to verify the capability of the proposed procedure. The influence of the noise is examined to study the filtering properties of the weighting functions     

A Filtering Technique for Three-Phase Power Systems

A novel filter for use in three-phase power systems is introduced. When the input to the filter is a balanced three-phase set of signals, the filter suppresses noise and distortions and extracts a smooth three-phase fundamental component. When the input signal to the filter is unbalanced, it extracts the fundamental positive-sequence component of the input signal. The filter also estimates the magnitude, phase angle, and frequency of the signal and adaptively follows the variations in all these three variables. The characteristics of the filter, including its mathematical equations, stability analysis, steady state, and dynamic responses, are discussed in this paper. The filter highly attenuates the harmonics, unknown interharmonics, and distortions. However, an extension of the filter for full removal of harmonics and unknown interharmonics is also presented, and its operating principles are discussed. The structural simplicity and robustness of the filter make it desirable for power system applications. In addition to the wide applications in power systems, it can specifically be used as an adaptive antialiasing filter for three-phase applications.     

Control of a converter system for an asymmetrical parameter type two-phase induction motor drive operating in motoring and generating modes

The control of a converter system is presented and discussed for an asymmetrical parameter type two-phase induction machine drive that is operating in motoring and generating modes. The proposed system consists of back-to-back voltage source converters. For a machine side, a three-leg voltage source converter provides both unbalanced and balanced two-phase output voltages with a scalar V/F control based on a carrier space vector pulse width modulation (SVPWM) technique. For a front end, a single-phase AC/DC doubled voltage converter with hysteresis current control is used to keep DC-link voltage constant, thus resulting in a bi-directional power flow operation for the motoring and generating modes. A closed-loop design for the DC-link voltage is fully given and also included is a review of carrier-based SVPWM for two-phase three-leg VSI. The proposed drive system was both simulated using MATLAB/SIMULINK and implemented on digital microcontrollers. The comparative performance evaluation of the whole system between balanced and unbalanced two-phase voltages for the machine is given. The simulation and experimental results show that the unbalanced phase voltage offers better performance for the whole system.     

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.     

基于自抗扰控制的微电网三相不平衡问题研究

当微电网中三相负载呈现不平衡状态时,会导致三相输出电压不平衡,若采用三相四桥臂逆变器,则可解决此问题。但在dq0旋转坐标系下,三相四桥臂逆变器输出电压和输出电流会相互耦合。同时若系统中出现负载不平衡工况,直接采用开环控制,因不平衡负载导致输出电压中含有二倍频扰动,所以负载端输出电压仍然处于不平衡状态。针对上述2个问题,建立了三相四桥臂逆变器对应的数学模型,推导被控对象开环传递函数,对三相不平衡问题进行了深入分析。同时根据系统模型信息,设计二阶自抗扰控制器,目的是对dq轴电压、电流进行解耦,同时可以减少测量原件的个数。并针对由不平衡负载产生的二倍频扰动引入比例谐振控制器,最后通过Matlab/Simulink仿真,验证了该解决方案的有效性。     

Calculation of the efficiency of polarization-insensitive surface-stabilized ferroelectric liquid-crystal diffraction gratings

A rigorous analysis is presented of the diffraction efficiency of a polarization-insensitive surface-stabilized ferroelectric liquid-crystal (SSFLC) phase grating, taking full account of the internal structure of the ferroelectric liquid-crystal layer. When no field is applied, the twisted director profile in the relaxed state gives an optimum diffraction efficiency for a device thickness between the half-wave-plate and the full-wave-plate conditions. The influence of the magnitude of the spontaneous polarization and applied ac fields are investigated, and it is shown that the diffraction efficiency of a binary SSFLC phase grating can be strongly enhanced with the technique of ac stabilization.     

Ultra wideband inphase power divider for multilayer technology

A multilayer inphase power divider with an ultra wideband behaviour is presented. The proposed divider exploits broadside coupling via a multilayer microstrip/slot configuration. The design method utilised for the device is based on the conformal mapping techniques. The developed device has a compact size with an overall dimension of 20 mm x 30 mm. The simulated and measured results show that the proposed device has equal power division between the two output ports with <0.2 dB amplitude imbalance between them, better than 10 dB return loss and isolation and < 2degrees phase difference between the two output signals across the frequency band 3.1-10.6 GHz.     

集成锁相环在激光脉冲调制系统中的应用

锁相环由鉴相器、环路滤波器及压控震荡晶体组成,是一个能跟踪输入信号频率和相位的闭环自动控制系统。研制的激光脉冲调制系统采用锁相技术,以单片、集成锁相环代替分立元件,实现了片内鉴频和鉴相的功能。研制的腔倒空驱动器能够输出 4MHz, 800kHz, 400kHz, 80kHz,40kHz, 8kHz, 4kHz, 800Hz, 400Hz 等不同重复频率的脉冲信号,输出功率达到瓦级,满足了声光布拉格池的要求。该激光脉冲调制系统已经应用在皮秒时间相关单光子计数光谱仪系统中,取得了比较理想的效果。     

Synchronization Techniques for Power Quality Instruments

The measurement of voltage characteristics in power systems requires the accurate estimation of the power supply frequency and signal synchronization, even in the presence of disturbances. The authors developed and tested two innovative techniques for instrument synchronization. The first is based on signal spectral analysis techniques performed by means of the Chirp- transform analysis. The second is a phase-locked loop (PLL) software based on a time-domain coordinate transformation and an innovative phase-detection technique. To evaluate how synchronization techniques are adversely affected by the application of a disturbing influence, experimental tests were carried out, taking into account the requirements of the standards. The proposed techniques were compared with a standard hardware solution. In this paper, the proposed techniques are described, the experimental results are presented, and the accuracy specifications are discussed.