Mapping matrices against vectorial frame in the instantaneous reactive power compensation

The original instantaneous reactive power or p-q theory has been used in a systematic way in the control of active power niters (APFs). When the APF is switched in parallel to a nonlinear and unbalanced load, the application of p-q theory allows a constant power compensation strategy. This means that after the APF connection, the supply instantaneous power is constant and presents the same value as the load average power. The use of other compensation strategies such as unity power factor, sinusoidal or balanced supply currents, among others, is also possible, as the development of another compensation strategy is possible within the p-q theory frame. A p-q theory reformulation without using mapping matrices is presented, which makes obtaining compensation currents easier. Finally, an exhaustive analysis of practical cases has been carried out at simulation and experimental levels through a laboratory prototype that has allowed verification of the proposed approach.     

Experimental results of tri-axial HTS cable

Recently, a tri-axial cable composed of three concentric phases has been intensively developed, because it has advantages such as reduced high-temperature superconducting (HTS) tape, small leakage field and small heat loss as compared to three single-phase cables. However, there is an inherent imbalance in the three-phase currents in tri-axial cables due to the differences in the radii of the three-phase current layers. The imbalance of the currents causes additional loss and a large leakage field in the cable, and deteriorates the electric power quality. We have already proposed that it is possible to obtain a balanced three-phase distribution by adjusting all of the twist pitches. In order to verify the theory, we designed and fabricated a 1-m-long tri-axial HTS cable and carried out the cable test. The balanced three-phase voltages of the cable were measured by supplying an AC transport current with frequency from 50 to 500 Hz at 77 K. It is found from the test results that the balanced three-phase distributions can be realized by adjusting all of the twist pitches.     

A new approach to the definition of power components in three-phasesystems under nonsinusoidal conditions

A method has been proposed for the definition of active and nonactive power components in three-phase systems under nonsinusoidal conditions. The method is more attractive than others since it is not a mere extension of methods employed in single-phase systems, but comes from the application of a quite powerful and synthetic mathematical tool specifically studied for the representation of three-wire three-phase systems in any possible condition: the Park transformation and the Park vectors. It is proven that the application of this method leads to the definition of two quantities, the real and the imaginary power, that are measurable in a quite simpler way than those proposed by other theories. The two satisfy all properties typical of the electrical power and are directly related, under sinusoidal and balanced conditions, to the active and reactive powers. It is shown how this method fits with other proposed methods that can be regarded in terms of this more general theory     

On the meaning of nonsinusoidal active currents

L.S. Czarnecki's (1985, 1987, 1988) orthogonal decomposition of nonsinusoidal currents is briefly discussed. The minimum property of Czarnecki's active currents in single-phase and three-phase nonsinusoidal circuits is discussed, using a Lagrangian multiplier approach. Czarnecki divided current, i, into three components: the active current i_a, the scattered current i _s, and the reactive current i_r. For three-phase four-line systems, several meanings of Czarnecki's active current i_a are given     

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。     

基于DSP的一种三相电路谐波电流检测算法

为能用数字信号处理器(DSP)实现三相电路的谐波电流检测,从基于瞬时无功理论的一种谐波电流检测方法出发,分别讨论了三相电流中谐波正序和谐波负序分量检测的数字方法,由此得出了一种实时的谐波电流检测算法.该算法能准确地检测出三相电流中的谐波正序和谐波负序分量.当有源电力滤波器(APF)等装置的谐波电流检测电路采用该算法时,可根据需要适时调整DSP程序来形成不同的参考电流,以便更好地满足APF的不同补偿要求.仿真和实验研究结果表明了该算法的有效性和可行性.     

Three-Phase Unbalance Prediction of Electric Power Based on Hierarchical Temporal Memory

The difference in electricity and power usage time leads to an unbalanced current among the three phases in the power grid. The three-phase unbalanced is closely related to power planning and load distribution. When the unbalance occurs, the safe operation of the electrical equipment will be seriously jeopardized. This paper proposes a Hierarchical Temporal Memory (HTM)-based three-phase unbalance prediction model consisted by the encoder for binary coding, the spatial pooler for frequency pattern learning, the temporal pooler for pattern sequence learning, and the sparse distributed representations classifier for unbalance prediction. Following the feasibility of spatialtemporal streaming data analysis, we adopted this brain-liked neural network to a real-time prediction for power load. We applied the model in five cities (Tangshan, Langfang, Qinhuangdao, Chengde, Zhangjiakou) of north China. We experimented with the proposed model and Long Short-term Memory (LSTM) model and analyzed the predict results and real currents. The results show that the predictions conform to the reality; compared to LSTM, the HTM-based prediction model shows enhanced accuracy and stability. The prediction model could serve for the overload warning and the load planning to provide high-quality power grid operation.     

Decomposition of Currents in Three- and Four-Wire Systems

Various methods of instantaneous decomposing of three-phase currents in three- and four-wire systems for the purpose of instantaneous compensation without energy storage are reviewed, analyzed, and compared. It is found that the method of Akagi et al. and other equivalent methods are optimum in the three- and four-wire systems where the neutral current is constrained to be zero after compensation, whereas the more general method of Depenbrock et al. is optimum in the four-wire systems where the neutral current after compensation is not restricted. Furthermore, both methods can be expressed in a simpler equation if the correct "null" voltage reference is used. The equation is shown to be optimum through the mathematical properties of vector space. The orthogonal instantaneous nonactive current components are accordingly identified. The required compensating currents are compared.     

Analytical evaluation of DC capacitor RMS current and voltage ripple in neutral-point clamped inverters

The sizing of the DC-link capacitor in a three-level inverter is based on the RMS current flowing through it. This paper analyses the DC-link capacitor RMS current in a neutral-point clamped (NPC) inverter and expresses the same as a function of modulation index, line-side current amplitude and power factor. Analytical closed-form expressions are derived for the capacitor RMS current for single-phase half-bridge, single-phase full-bridge and three-phase three-leg topologies of a three-level inverter. The worst-case capacitor current stress is determined for each topology based on the analytical expressions. Further, analytical expressions are derived for the RMS values of low-frequency and high-frequency capacitor currents. These expressions are then used to estimate voltage ripple across the DC capacitor for sinusoidally modulated three-phase NPC inverter. The analytical expressions for the RMS current and voltage ripple are validated experimentally over a wide range of operating points.     

A new interpretation of the Akagi-Nabae power components fornonsinusoidal three-phase situations

A different interpretation of the power decomposition proposed by H. Akagi et al. (1984) for distorted three-phase situations is given. This makes it possible to generalize the technique to single-phase systems and polyphase systems and also to include rigorously zero sequence currents and voltages     

三相三线制有源电力滤波器谐波检测方法

针对三相三线制有源电力滤波器(Active Power Filter,APF)中的谐波检测,研究了基于瞬时无功理论的谐波电流快速检测算法,结合谐波检测过程中出现的频率混叠现象而搭建的二阶巴特沃斯低通滤波器,共同实现了有源电力滤波器高精度的谐波检测功能.实验结果验证了方法的正确性和可靠性,对于提高APF的补偿性能有很大的帮助.     

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.     

Orthogonal decomposition of the currents in a 3-phase nonlinearasymmetrical circuit with a nonsinusoidal voltage source

The concept of orthogonal decomposition of the current in a nonlinear single-phase circuit, with a sinusoidal voltage source, into active, scattered, reactive, and generated components is extended to three-phase asymmetrical circuits. It is shown that the source current in such a circuit can be decomposed into five orthogonal components that depend on distinctively different phenomena. Four of them have the same meaning as in the single-phase circuit, with only a change in their mathematical characterization. An additional orthogonal component appears in the source current, because of the load asymmetry     

A fast method for determining electrical and mechanical qualitiesof induction motors using a PC-aided detector

A unique computer-aided detector is presented that determines the mechanical and electrical qualities of single-phase and three-phase induction motors. Measurements of voltage, current, power factor, active power, and reactive power as well as evaluations of centrifugal switch and airgap eccentricity, i.e., misalignment between rotor and stator, of induction motors can be rapidly conducted, and test results are available on the screen within a few seconds. The feasibility and usefulness of this detector are impressively demonstrated through application examples     

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.     

A self-synchronizing instrument for harmonic source detection in power systems

The deregulation of energy markets holds out new prospects for contracts between customers and utilities, in which the price of energy can depend on voltage quality and load characteristics, as well as on the responsibility for disturbances caused to supply voltage in power systems. In this context, harmonic source detection is one of the main problems because of equipment sensibility and the proliferation of loads which absorb nonsinusoidal currents. In this paper, the authors present a new instrument based on a time-domain method for the detection of harmonic active powers in three-phase systems, which can be usefully applied even in the presence of unbalance or asymmetry. The amplitude and sign of harmonic powers can be measured directly, and no spectral analysis is required for the evaluation of the amplitudes and phase angles of supply currents. Moreover, the instrument is able to synchronize itself with the input signal to measure the total distortion factors of voltages and currents, supply voltage unbalance, and harmonic voltage amplitudes. Theoretical aspects are discussed, the measurement accuracy is evaluated, and the experimental results are presented. Finally, a comparison is made with a commercial instrument.     

Efficient reconfiguration of balanced and unbalanced distribution systems for loss minimisation

A simple and efficient two-stage reconfiguration algorithm for minimisation of active power loss in the balanced and unbalanced distribution systems is presented. In the first stage, the proposed method begins with all candidate switches closed. It uses current information obtained from a power flow, rather than from an optimal flow pattern used in other approaches to decide the switch to be opened, in a sequential manner. In the second stage, branch exchange operation is explored for further loss reduction. Results for four balanced systems and one unbalanced system have been obtained with the proposed method, and compared with those reported in the literature.     

Three-phase power-flow by direct ZBR method for unbalanced radial distribution systems

This paper introduces a novel three-phase power flow approach for unbalanced radial distribution systems. The proposed approach is developed based on the branch frame of reference, rather than the traditional bus frame of reference. On the basis of the branch frame of reference, a simple direct iterative method can be applied. Hence, the proposed approach may be called the `direct Z _BR method'. Basic graph theory and injection current technique are also applied in the proposed approach. The clear theoretical foundation and the simple topology of the radial distribution network make the proposed method efficient and reliable. To demonstrate the better convergence performance and the efficiency of the proposed approach, four three-phase IEEE test feeders are used for comparisons. The test results show that the proposed method has robust convergence characteristics and high performance, especially for large-scale radial distribution systems.     

A Virtual Instrument for the Measurement of IEEE Std. 1459-2000 Power Quantities

In this paper, the authors present a PC-based instrument for the measurement of electrical-power quantities defined in IEEE Std. 1459. The instrument is based on a time-domain technique for the detection of the fundamental and harmonic components of voltages and currents. The time-domain strategy was originally developed by the authors for three-phase, three-wire systems. In that paper, the strategy had been extended to both three-phase, four-wire and single-phase systems. Simulation tests were carried out to assess the uncertainty contribution of the proposed strategy in the absence of the measurement transducers. Moreover, the accuracy of the PC-based instrument with its transducers was evaluated; the experimental tests were carried out by using a power calibrator.