Three-phase Power System Research Articles

Augmented MVDR Spectrum-Based Frequency Estimation for Unbalanced Power Systems

A robust technique for online estimation of the fundamental frequency of both balanced and unbalanced three-phase power systems is proposed. This is achieved by extending the recently introduced iterative frequency estimation method based on minimum variance distortionless response (MVDR) spectrum , in order to enhance its robustness in unbalanced system conditions. The approach is made optimal for the second-order noncircular nature of the unbalanced complex-valued system voltage, by combining the iterative MVDR (I-MVDR) frequency estimation and the complete available (augmented) second-order statistics. Such an approach makes it possible to eliminate the otherwise unavoidable estimation bias in unbalanced system conditions. It is also shown that the proposed method approaches the theoretical Cramer-Rao lower bound (CRLB), which we rigorously derive for the vector parameter in power systems. Simulations over a range of unbalanced conditions, including voltage sags, the presence of higher-order harmonics, and for real-world unbalanced power systems, support the analysis.

Multi‐FPGA digital hardware design for detailed large‐scale real‐time electromagnetic transient simulation of power systems

Large-scale electromagnetic transient simulation of power systems in real-time using detailed modelling is computationally very demanding. This study introduces a multi-field programmable gate array (FPGA) hardware design for this purpose. A functional decomposition method is proposed to map FPGA hardware resources to system modelling. This systematic method lends itself to fully pipelined and parallel hardware emulation of individual component models and numerical solvers, while preserving original system characteristics without the need for extraneous components to partition the system. Proof-of-concept is provided in terms of a 3-FPGA and 10-FPGA real-time hardware emulation of a three-phase 42-bus and 420-bus power systems using detailed modelling of various system components and iterative non-linear solution on a 100 MHz FPGA clock. Real-time results are compared with offline simulation results, and conclusions are derived on the performance and scalability of this multi-FPGA hardware design.

Widely Linear Modeling for Frequency Estimation in Unbalanced Three-Phase Power Systems

Real-time frequency estimation in three-phase power systems is revisited from the state space point of view, in order to provide a unified framework for frequency tracking in both balanced and unbalanced system conditions. This is achieved by using a novel class of widely linear complex valued Kalman filters, which provide unbiased frequency estimation and are faster converging and more robust to noise and harmonic artifacts than the existing methods. It is shown that the Clarke's transformed three-phase voltage is circular for balanced systems and noncircular for unbalanced ones, making the proposed widely linear estimation perfectly suited both to identify the fault and to provide accurate estimation in unbalanced conditions, critical issues where standard models typically fail. The analysis and simulations show that the proposed model outperforms the recently introduced widely linear stochastic gradient-based frequency estimators, based on the augmented complex least mean square. Comprehensive simulations on synthetic and real-world power system data, in both balanced and unbalanced conditions, support the approach.

Multiple-Loop Digital Control Method for a 400-Hz Inverter System Based on Phase Feedback

To keep phase balance and improve output voltage quality, a novel multiple-loop control system in 400-Hz inverter power supply based on unipolar sinusoidal pulse width modulation (SPWM) is proposed. The causes of phase imbalance in a three-phase combined inverter power system are analyzed, and a new multiple-loop control strategy based on phase control is presented to eliminate it. For reducing the total harmonic distortion (THD) of high-power inverter output, unipolar SPWM is added with this new control strategy. The designs of drive circuit and snubber circuit are discussed to eliminate dead time effects and voltage oscillation. The stability of the control system is analyzed and the power system and the control strategies are realized with digital signal processing. Contrastive simulation results indicate the advantages and the correctness of this novel controller for inverter. A three-phase combined inverter system based on a multiple-loop control strategy is constructed, and experimental results show that the inverter system can make a phase difference between each phase less than 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> , and the THD of output voltage can be controlled within 2%.

Energy Harvesting Based on Magnetic Dispersion for Three-Phase Power System

This paper presents a comparative study on Magnetic-Dispersion based Energy Harvesting Systems (MD-EHS) on electrical conductors that supply power for a three-phase AC motor. It introduces two MD-EHS which are based on magnetic cores of different material, named, nanocrystalline, ferrite and iron powder. The first one consists of harvesting energy from magnetic flux through three symmetrical magnetic cores installed on each power conductors of a three-phase AC motor. The second one consists of a single magnetic core for harvesting energy from magnetic flux of only one of these conductors. Both ones have an AC/DC converter and a variable resistor based load. Experimental results have agreed with the theoretical analysis and show that the first proposed MD-EHS is capable of supplying 14 times more energy than the second MD-EHS, considering nanocrystalline cores and phase current of 3 A, and 7.5 times more energy, considering ferrite cores and phase current of 9 A. Such energy can be applied to various low-power devices, especially in wireless sensor network.

Study on a Uniform Algorithm for Single-Phase and Three-Phase Load Flow Calculation

This paper presents a uniform algorithm for single-phase and three-phase load flow calculation. Firstly, two iterative equations with static factors are studied for single-phase power flow calculation based on Gauss power flow algorithm. One is used for all PQ nodes iteration similar to the traditional Gauss power flow algorithm, and the other one is exploited for all PV nodes and the slack node, which is a new algorithm studied in this paper. Then, the three-phase power flow iterative equations are studied. Virtual generator nodes are generated so that it is enough to built equations to solve the unknown parameters and each equation has a unique solution. Two iterative equations are studied for three-phase power flow calculation based on the single-phase formulation studied in this paper. The calculation result shows that the algorithm has the advantage of fast calculation speed, reliable convergence and powerful applicability. It has been shown that the algorithm is suitable for online calculation of single-phase and three-phase power system.

Frequency/Sequence Selective Filters for Power Quality Improvement in a Microgrid

A microgrid is capable of providing highly reliable power supply to sensitive loads like semiconductor fabrication or computer data processing equipment. This is especially required when the sensitive loads are located in distribution systems that also contain several single-phase and/or nonlinear loads. Voltage source inverter (VSI) based distributed energy resources can provide power quality conditioning support. They require filters to accurately detect unbalance and harmonics in the system. The state-of-the-art filtering techniques in three-phase power systems involve employing complex bandpass filters that pass frequencies within a band without much attenuation. Such methods however give good dynamic performance only when there is a wide separation between the desired and undesired frequencies. In power distribution systems containing imbalances and harmonic generating loads, the gap between the two frequencies may be very small, and therefore such methods don't offer good performance. This paper proposes improved methods of filtering signals in three-phase, three-wire power systems, especially when there is a narrow gap between the desired and undesired frequencies. The proposed techniques contain both complex bandpass and bandstop sections and are designed on three-phase space-vector quantities. Simulation and experimental results are presented to validate these novel filtering concepts.

Single-phase inverter-based neutral-current suppressor for attenuating neutral current of three-phase four-wire distribution power system

In this study, a neutral-current suppressor is proposed. The proposed neutral-current suppressor comprises a zero-sequence transformer, a single-phase power converter and a rectifier. The proposed zero-sequence transformer is implemented by only two single-phase transformers, which are connected to three-phase lines of the three-phase four-wire distribution power system to supply a zero-sequence current path. The single-phase power converter is connected between the zero-sequence transformer and neutral line. A rectifier with small power rating is connected to the DC bus of single-phase power converter for overcoming the power loss of single-phase power converter to sustain a constant DC voltage. The performance of neutral-current suppression for the zero-sequence transformer is advanced by controlling the single-phase power converter so as to make the neutral load current flow through the proposed neutral-current suppressor. Moreover, the proposed neutral-current suppressor can also avoid the neutral current caused by the unbalanced voltage of three-phase four-wire distribution power systems. Hence, the proposed neutral-current suppressor is an effective solution to the neutral-current problem of three-phase four-wire distribution power systems. Both computer simulation and experimental results verify that the proposed neutral-current suppressor can achieve the expected performance.

Generalized Design of High Performance Shunt Active Power Filter With Output LCL Filter

This paper concentrates on the design, control, and implementation of an LCL-filter-based shunt active power filter (SAPF), which can effectively compensate for harmonic currents produced by nonlinear loads in a three-phase three-wire power system. With an LCL filter added at its output, the proposed SAPF offers superior switching harmonic suppression using much reduced passive filtering elements. Its output currents thus have high slew rate for tracking the targeted reference closely. Smaller inductance of the LCL filter also means smaller harmonic voltage drop across the passive output filter, which in turn minimizes the possibility of overmodulation, particularly for cases where high modulation index is desired. These advantages, together with overall system stability, are guaranteed only through proper consideration of critical design and control issues, like the selection of LCL parameters, interactions between resonance damping and harmonic compensation, bandwidth design of the closed-loop system, and active damping implementation with fewer current sensors. These described design concerns, together with their generalized design procedure, are applied to an analytical example, and eventually verified by both simulation and experimental results.

3상 전동기 결상에 의한 전기화재 보호를 위한 장치 개발 연구

In the three-phase power system using the three-phase motor, when any one-phase is open-phase, the unbalanced current flows and the single-phase power supplied by power supply produces overcurrent to motor coil. As a result, the enormous damage and electrical fire can be given to the power system. Recently, the thermal over-current relay (THR) or electronic motor protection relay (EMPR) is mostly used as the open-phase detection device of the three-phase motor. When the over-current or overheat of electric line is generated, it detects and operates circuit breaker, but there is the defect that the sensing speed is slow, the operation can be sometimes failed, and the precision is decreased. In order to improve these problems, this paper is proposed a new control circuit topology for openphase protection using semiconductor devices. Therefore, the proposed open-phase protection device (OPPD) enhances the sensing speed and precision, and has the advantage of simple fitting in the three-phase motor control panel in the field, as it manufactures into small size and light weight. As a result, the proposed OPPD protects the three-phase motor, minimizes the electrical fire from openphase, and contributes for the stable driving of the power system. The performance and confidence of the proposed OPPD is confirmed by a great variety of the experiments of operation characteristic.

Impact of Unbalance Single-phase Traction Load to Three-phase Power System

The two most common types of supply for mass transit systems are 1.5 kV dc and 25 kV ac. This paper will firstly brief the relative merits of dc and ac traction systems. As the ac traction system is of single phase, imbalance in three-phase supply is inevitable and is an universal problem. With reference to the local 25 kV traction system, the paper will assess the degree of imbalances and the corresponding impacts at different phase connection scenarios. Cost-effective and pragmatic measures to reduce degree of imbalance by minor but appropriate modification of existing phase connections of traction transformers 132 kV will be proposed in this paper. Consequently, generation performance and efficiency are improved, unnecessary fuel consumption is reduced as much as possible, and the extra cost imposed to consumers at large would also be reduced.

Widely Linear Adaptive Frequency Estimation of Unbalanced Three-Phase Power Systems

A novel technique for online estimation of the fundamental frequency of unbalanced three-phase power systems is proposed. Based on Clarke's transformation and widely linear complex domain modeling, the proposed method makes use of the full second-order information within three-phase signals, thus promising enhanced and robust frequency estimation. The structure, mathematical formulation, and theoretical stability and statistical performance analysis of the proposed technique illustrate that, in contrast to conventional linear adaptive estimators, the proposed method is well matched to unbalanced system conditions and also provides unbiased frequency estimation. The proposed method is also less sensitive to the variations of the three-phase voltage amplitudes over time and in the presence of higher order harmonics. Simulations on both synthetic and real-world unbalanced power systems support the analysis.

전원전압의 불평형 및 왜곡 보상기능을 갖는 3상 PWM 정류기의 전류제어기

본 논문은 상용의 전원 전압이 불평형 및 왜곡을 갖는 경우, 전원전압 불평형 및 왜곡 보상기능을 갖는 전류제어기를 제안한다. 일반적으로 3상 전원 시스템은 공통 입력 단자 (Point of Common Coupling)에 단상 부하 및 비선형 부하가 3상 부하와 같이 연결될 수 있어, 종종 불평형과 왜곡이 발생한다. 이런 조건하에서 3상 PWM 정류기를 일반적인 전류 제어기로 제어 할 경우, 3상 PWM 정류기 입력 전류가 불평형 및 왜곡을 갖는 문제가 발생한다. 본 논문에서는 간단한 모델 기반 적응 제어 시스템(Model Reference Adaptive System)기법을 이용하여 3상 입력 전압의 불평형 및 왜곡을 관측하고 전향 보상하는 전류 제어기를 제안하며, 모의해석과 실험을 통하여 그 효용성을 입증한다. In this paper, a new current controller with the compensation of an unbalance and distorted grid voltages has been proposed. Generally, in the three-phase power system, single phase or nonlinear loads can be connected with the 3 phase linear load simultaneously on the same point of common coupling. Therefore, The source voltage unbalance and distortion problem can be occurred. Under these unbalance and distorted grid voltage conditions, the input current of 3 phase PWM rectifiers also have unbalance and distortion. In this paper, a current controller with the simple Model Reference Adaptive System based unbalance and distorted voltages observer is proposed to get a sinusoidal input current. The performance of the proposed algorithm is verified through the simulation and the experiment.

Harmonic Sequence Impedances: A Tool for the Three-Phase Frequency Scan Technique

In this paper, the harmonic response of an electric system under imbalance conditions is established. Symmetrical harmonic components are proposed to give a new way of interpreting resonance occurrence in three-phase power systems. A three-phase harmonic bus impedance matrix (THBIM) for a simple distribution system is derived using analytical procedures. The development of frequency scans for the THBIM elements in both phase and sequence reference frames is carried out. Simulation results show the benefits of the harmonic sequence components for characterization of the harmonic resonance conditions in unbalanced distribution systems, and for characterizing the effectiveness of the mitigating techniques. This method offers a new way to conduct harmonic analysis and diagnosis in unbalanced power systems.

Current Limitation and Power Burden of a Flux-Coupling Type SFCL in the Three-Phase Power System According to Turn's Ratio and Fault Type

Most of the transmission system has a network structure to improve the reliability and stability of a power system. Fault current is continuously expected to increase by the increase of the power demand. If fault current exceeds the cutoff capacity of a circuit breaker, the circuit breaker is broken and the damage by fault current is expanded throughout the power system. Superconducting fault current limiter (SFCL) was designed to solve this problem in a power system. In this paper, we investigated the current limiting characteristics and power burden of superconducting elements of a flux-coupling type SFCL in three-phase power system. A Flux-coupling type SFCL is one of the resistive type SFCLs. The flux-coupling type SFCL was made by using a transformer. Reactors connected in each phase shared an iron core. When the superconducting elements were quenched in fault phase, the fault current flowed into the primary and secondary coils simultaneously. Thus, the current flowed into primary and secondary coils of sound phase by the magnetic coupling flux. Meanwhile, when the current of sound phase exceeded the critical current of the SFCL, superconducting elements connected in the sound phase were quenched. The value of the fault current tended to decrease as the first reactor's ratio increased. Furthermore, the power burden of the superconducting element was reduced. The reduced power burden of the superconducting elements shortens the recovery time of the superconducting element, which is advantageous for cooperation with a reclosing system when the SFCL is applied to the system. As a result, we confirmed that the flux-coupling type SFCL operated effectively in the three-phase power system.

Pragmatic Electrical Engineering: Fundamentals

Pragmatic Electrical Engineering: Fundamentals introduces the fundamentals of the energy-delivery part of electrical systems. It begins with a study of basic electrical circuits and then focuses on electrical power. Three-phase power systems, transformers, induction motors, and magnetics are the major topics. All of the material in the text is illustrated with completely-worked examples to guide the student to a better understanding of the topics. This short lecture book will be of use at any level of engineering, not just electrical. Its goal is to provide the practicing engineer with a practical, applied look at the energy side of electrical systems. The author's pragmatic and applied style gives a unique and helpful non-idealistic, practical, opinionated introduction to the topic. Table of Contents: Basic Stuff / Power of the Sine / Three-Phase Power Systems / Transformers / Machines / Electromagnetics

Running DFT-Based PLL Algorithm for Frequency, Phase, and Amplitude Tracking in Aircraft Electrical Systems

Phase-locked loop (PLL) algorithms are commonly used to track sinusoidal components in currents and voltage signals in three-phase power systems. Despite the simplicity of those algorithms, problems arise when signals have variable frequency or amplitude, or are polluted with harmonic content and measurement noise, as can be found in aircraft ac power systems where the fundamental frequency can vary in the range 360-900 Hz. To improve the quality of phase and frequency estimates in such power systems, a novel PLL scheme based on a real-time implementation of the discrete Fourier transform (DFT) is presented in this paper. The DFT algorithm calculates the amplitudes of three consecutive components in the frequency domain. These components are used to determine an error signal which is minimized by a proportional-integral loop filter in order to estimate the fundamental frequency. The integral of the estimated frequency is the estimated phase of the fundamental component, and this is fed back to the DFT algorithm. The proposed algorithm can therefore be considered to be a PLL in which phase detection is performed via a DFT-based algorithm. A comparison has been made of the performances of a standard PLL and the proposed DFT-PLL using computer simulations and through experiments.

Improvement of Three-phase Unbalance due to Connection of Dispersed Generator by Damper Windings of Synchronous Generator

SUMMARY It is well known in the numerical simulations of a synchronous generator that the damper winding contributes to suppressing three-phase circuit unbalance in power systems. However, an experimental study has not been performed yet. In this paper, we verify experimentally the suppression of three-phase unbalance by the damper windings. In order to simulate the three-phase unbalance, a 470-W dispersed generator of single-phase two-line type was connected to a three-phase laboratory-scale power system that included a 6-kVA synchronous generator. We measured and analyzed line voltages and currents as well as damper bar currents both with and without the dispersed generator. The influence of damper windings on the unbalance of the three-phase circuit was also investigated. The results show that the damper winding contributes to improving the three-phase unbalance. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 186(1): 43–50, 2014; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22305

Shifted-Frequency Analysis for EMTP Simulation of Power-System Dynamics

Traditional power-system dynamic simulation tools make a number of simplifying assumptions to speed up computations; thus, they can only capture snapshots of the system operation, ignoring the dynamics between states. This paper applies the shifted-frequency-analysis (SFA) theory to the Electromagnetic Transients Program (EMTP) simulator in order to efficiently obtain the time-domain simulation results in the neighborhood of the fundamental frequency without making quasi-steady-state or other simplifying assumptions. The SFA-based EMTP simulator can provide dynamic-phasor results that accurately trace the evolution of the system state in three-phase power systems. Dynamic phasors provide envelopes of the time-domain waveforms and can be accurately transformed back to instantaneous time values. Test cases have verified the usefulness of the SFA-based dynamic simulation method.

Multiphase transmission line modeling for voltage sag estimation

This paper presents a novel approach for voltage sag indices calculation based on instantaneous voltage estimation. The estimation uses traditional state estimation where redundant measurements are available. The estimation is based on time domain state estimation which uses time domain modeling of the power network. The time domain current monitoring is used to have linear mapping and to achieve high performance of voltage sag estimation. The fault estimation procedure is prior of the voltage sag estimation. This paper shows a possible for fault instance detection, fault location identification and fault type estimation method that are required to estimate voltage sag for different line models utilizing residual analysis and topology error processing. Lumped parameter and distributed parameter transmission line modeling are developed to estimate instantaneous voltage at a three-phase power system in time domain. Magnitude and duration of voltage sag as main indices are calculated from the estimated instantaneous bus voltage. The performance of the novel approach is tested on IEEE 14 bus system and the results are shown.