A digital method for the identification of the source of distortionin electric power systems

The presence of distortion in voltages and currents in the electric power systems causes many inconveniences, ranging from an increment in the line loss to the irradiation of disturbances. The correct identification of the source of distortion is important for attaining an effective compensation of the nonactive components of power. This paper refers to a method, based on the evaluation of the sign of the harmonic active powers, originally designed for identification of the source of harmonic distortion in three-phase systems. An application of this method to the development of a new instrument is described. The instrument accuracy and the results of some experimental work are discussed     

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     

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.     

High power factor operation of resonant converters on the utility line

Operation and characteristics of resonant converters on the utility line are presented. Series-parallel (LCC-type) resonant converter operating with discontinuous current mode and continuous current mode (variable frequency control as well as fixed-frequency) are considered. Design examples are presented. SPICE simulation and experimental results obtained for the designed converters (rated at 150 W) are presented to verify the theory. It is shown that high line power factor (>0.95) and line current total harmonic distortion (THD) of <25% are obtained for the LCC-type converter for a wide load range (from full load to 10% rated load) without any active control, and the switch peak current decreases with the load current. With active line current control, low distortion and zero voltage switching for the entire cycle are realized.     

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.     

Optimal placement and sizing of harmonic filters aboard an electric propulsion ship

An optimisation technique using simulated annealing is described, in order to find the best placement of the harmonic filters to be installed in a distribution system with their minimum power size. The filtering currents are then calculated not to cancel the resulting harmonic voltages but to reduce their values with respect to the fixed limits. The objective is to minimise the power size in view of maximum savings in the equipment cost. The aboard power system of an electric propulsion ship with a great number of nonlinear loads is analysed in order to illustrate the efficiency of the optimisation method.     

电气化铁道对无功电能计量影响的研究

电气化铁道为电网注入了大量谐波电流,降低了电网电能质量.而目前谐波对无功电能表计量特性影响的研究还不完善.文章在传统无功功率定义的基础上,按照C.Budeanu频域法,给出谐波下非正弦无功功率的定义,并且比较了电气化铁道中谐波对目前无功计量两种电能表的准确度影响,给出了电气化铁道中无功电能表的选择原则.     

Analysis of the voltage harmonic distortion at buses feeding office loads

The harmonic distortion of the phase-to-neutral voltage at low-voltage buses feeding office load is investigated for various cable configurations, lengths and cross-sections. The investigation is carried out with a software model that was validated with analytical expressions and measurements taken at a real installation. It is shown that the selection of a cable should be based not only on its ampacity and allowable voltage drop but also on its configuration, in order to avoid excessive voltage harmonic distortion. Configurations where the neutral conductor is part of a four-core cable lead to relatively small harmonic distortion, whereas cable configurations where the neutral conductor runs separately cause high-voltage harmonic distortion. In the latter case, the neutral conductor develops a significant voltage with respect to the ground. Increasing the cross-section of the neutral conductor, as the existing standards dictate, may prevent its overloading from zero-sequence harmonic currents but hardly improves the voltage harmonic distortion at the load bus.     

Analysis and evaluation of harmonic suppression of lumped-element networks suitable for high-frequency class-E operation condition approximation

The harmonic output of microwave lumped-element class-E load-coupling topologies is assessed. Measurement results on fabricated C-band class-E power amplifiers with either a modified lowpass and an optimised second-harmonic impedance network show that the latter provides better performance not only in terms of output power, gain and efficiency, but also in regard to harmonic suppression. Compared with the common modified lowpass network, harmonic content in the load of the optimised second-harmonic impedance topology at the second-, third- and fourth-harmonic frequencies are lower by more than 5.8, 10 and 5.8 dB, respectively.     

Series compensation method to mitigate harmonics and voltage sags and swells

The use of a series compensator (SC) to improve power quality is investigated. The role of the compensator is not only to mitigate the effects of voltage sag and swell, but also to control the level of harmonic distortions in the network. This is accomplished through extension of the voltage injection scheme of the SC by the incorporation of load current feedback and an external inductive filter. The compensation process is accompanied by an exchange of energy between the SC and the external network. Through a phase adjustment technique, it is shown that the voltage quality across the terminals of the protected load can be controlled. Simulation studies, supported by laboratory experiments, show that the resulting series compensation scheme is effective in improving the supply quality of the power system.     

Simulation and performance evaluation of shunt hybrid power filter using fuzzy logic based non-linear control for power quality improvement

This paper deals with design and simulation of a three-phase shunt hybrid power filter consisting of a pair of 5th and 7th selective harmonic elimination passive power filters connected in series with a conventional active power filter with reduced kVA rating. The objective is to enhance the power quality in a distribution network feeding variety of non-linear, time-varying and unbalanced loads. The theory and modelling of the entire power circuit in terms of synchronously rotating reference frame and leading to a non-linear control scheme is presented. This work involves introduction of individual fuzzy logic controllers for d and q axis current control and for voltage regulation of the DC link capacitor. The simulation schematic covering the power and control circuits have been developed taking into account severe harmonic distortion caused by non-linear and unbalanced loads. The effectiveness of the fuzzy logic controller for the compensation of harmonics and reactive power has been verified by successive simulation runs and analysis of the results. The proposed controller is also able to compensate the distortion generated by the voltage- and current-fed non-linear loads, unbalanced and dynamically varying loads. Further, excellent regulation of the DC link voltage is accomplished, which significantly contributes to improvement of power quality.     

The Method Based on Original DBFs for Fast Estimation of Waveform Distortions in Ship Systems—Case Study

This paper deals with the problem of fast recognition of harmonic distortions by means of a wavelet transform. A method of fast detection of a frequency band affected by the high-frequency content and time of its occurrence is described. It is based on the auxiliary waveform distortion band factors (DBFs) calculated on the basis of wavelet coefficients. The merits of the presented method, as well as its limitations, are described on the basis of real voltages registered in ships' electrical power systems. This method is a part of the method based on the complementary application of Fourier and wavelet transforms, which was described in the previous papers of the author. However, this paper describes new research results and an expanded analysis exclusively focused on the aforementioned DBFs.     

电梯反馈能量对电能质量的影响

采用基于快速傅里叶变换的谐波检测方法,对目前国内在用的12台典型曳引电梯及能量反馈装置产品,分别在有能量回馈电网和无能量回馈电网2种情况下进行电压谐波和电流谐波的检测。依据检测结果,比较并分析了电梯反馈能量的谐波特点以及能量反馈装置的应用对于电能质量的影响。研究结果表明,电梯反馈能量会造成电网谐波污染,影响电能质量,因此,新型无污染的电梯节能新技术有待进一步研发。     

Reactive power transfer allocation method with the application of artificial neural network

A novel method to identify the reactive power transfer between generators and load using modified nodal equations is proposed. On the basis of the solved load flow results, the method partitions the Y-bus matrix to decompose the current of the load buses as a function of the generators' current and voltage. Then it uses the load voltages from the load flow results and decomposed load currents to determine reactive power contribution from each generator to loads. The validation of the proposed methodology is demonstrated by using a simple 3-bus system and the 25-bus equivalent system of south Malaysia. Next part here focuses on creating an appropriate artificial neural network (ANN) to solve the same problem in a simpler and faster manner. The basic idea is to use supervised learning paradigm to train the ANN. Most commonly used feedforward architecture has been chosen for the proposed ANN reactive power transfer allocation technique. Almost all system variables obtained from load flow solutions are utilised as an input to the neural network. Moreover, tan-sigmoid activation functions are incorporated in the hidden layer to realise the nonlinear nature of the reactive power transfer allocation. The targets of the ANN corresponding to the previously developed reactive power transfer allocation method. The 25-bus equivalent system of south Malaysia is utilised as a test system to illustrate the effectiveness of the ANN output compared with that of the modified nodal equations method. The ANN output provides promising results in terms of accuracy and computation time.     

Estimating harmonic distortion levels for systems with random-varying distributed harmonic-producing loads

With the proliferation of harmonic-producing loads, many power distribution systems are encountering distributed harmonic sources. These sources have comparable sizes and can vary randomly. A method that can determine key probabilistic parameters of the random harmonic distortion levels in such power distribution systems is presented. The results show that the distortion indices follow a normal distribution. Since a normal distribution can be characterised by two parameters, mean value and standard deviation, a method is proposed to estimate the two parameters using commercially available harmonic analysis programs. The proposed method has been tested on three systems and the results have confirmed its validity and usefulness.     

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 dynamics model for nonlinear electrostrictive actuators

This paper examines the nonlinear vibration of an electrostrictive ceramic rod actuator excited by a harmonic voltage source. A frequency-domain model was developed using the nonlinear constitutive law for electrostriction. The results predict harmonic distortion of the device's displacement due to the ceramic's nonlinear behavior. AC voltage signal and DC voltage bias were studied to determine the optimum power source parameters for minimizing distortion. The calculations show that the rod's resonance frequency and amplitude depend on the electromechanical coupling strength and differ greatly for large AC voltages from the equivalent linear piezoelectric results. The nonlinear analysis relates the device's electromechanical coupling coefficient to the computed resonance and antiresonance frequencies. This important result could provide the basis for future measurement of the electrostrictive coupling coefficient using resonance techniques.     

Assessment of harmonic distortion level considering the interaction between distributed three-phase harmonic sources and power grid

An iterative frequency domain-based harmonic analysis method for harmonic distortion assessment for systems with distributed harmonic sources has been proposed. The proposed method, unlike the traditional method, is able to consider harmonic attenuation and diversity due to the interaction between the harmonic sources and power grid. Time domain simulation for systems with distributed adjustable speed drives is used to validate the proposed method. Case studies are conducted to show the merits of the proposed method especially with a large number of distributed harmonic sources     

Multiharmonic generators for relative phase calibration ofnonlinear network analyzers

Multiharmonic generators are of interest for determining coupler phase dispersion of nonlinear network analyzers inherently dealing with nonsinusoidal waveforms for RF current-voltage (I-V) measurements. Supported by simple waveform analysis, two on-wafer devices are experimentally evaluated: a single diode and a nonlinear transmission line (NLTL). The investigation uses an active harmonic load-pull system with 2 GHz fundamental frequency. It turns out that the NLTL is well suited as a reference generator. We present measurement examples of phase dispersion for different directional couplers up to 18 GHz and the application to RF I-V waveforms of a 1-W power HBT with harmonic load tuning     

GPS-Based System for the Measurement of Synchronized Harmonic Phasors

A measurement system, based on high-performance Global Positioning System (GPS) receivers and general-purpose acquisition (DAQ) boards, for the evaluation of the synchronized harmonic phasors in the nodes of an electric distribution network, is presented. To meet the requirements of different fields of application, two measurement procedures have been implemented: One is based on a fixed observation window, whereas, in the other one, the observation interval is a function of the actual power system frequency. Experimental tests are presented to characterize the measurement system: The results show that the proposed procedures allow the accuracy that is usually required in power quality monitoring of distribution systems to be reached.