Uncharacteristic Harmonics Research Articles

Enhanced compensation strategy of voltage harmonics for Doubly-Fed induction generator

Using the idle capacity to compensate voltage harmonics at the point of common coupling (PCC) is an important reflection of participating in power grid regulation for the doubly-fed induction generator (DFIG), which is very beneficial to improve the power supply quality in the regional power grid. Due to the pouring of power electronized devices, the uncharacteristic harmonics of which frequencies are unfixed appear in the power grid. The existing voltage harmonics compensation methods mainly focus on the traditional characteristic harmonics. The frequencies of these kinds of harmonics are fixed and pre-known (6n ± 1 orders). This fact makes the existing methods only work at the specific and fixed frequency, and cannot deal with the harmonics of which frequencies are uncharacteristic or unfixed. Therefore, starting from the theoretical analysis of voltage harmonics at PCC, this paper investigates the mechanism that can simultaneously compensate for the characteristic and uncharacteristic harmonics of PCC voltage, and indicates that decreasing the impedance of DFIG in the frequency range of harmonics is the core of voltage harmonics compensation. Then, this paper proposed a wideband control structure and the matched controller, so that the impedance of DFIG can be effectively decreased in a wideband frequency range. The design procedure of the harmonics controller co-currently considers the compensation performance and system stability. Finally, this paper carries on the evaluation based on the hardware-in-the-loop platform and scale-down experimental platform to verify the correctness of the theoretical analysis and the effectiveness of the control strategy.

Abnormal Harmonics Generated by Modified HVDC Converter.(Dept.E)

Unbalance in the operation of AC/DC converters gives rise to abnormal or uncharacteristic current harmonics, in addition to the characteristic harmonics in the supply current waveforms. This paper concentrates on the analysis of uncharacteristic harmonics caused by unbalanced AC supply voltages. Attention is given to modified converter operation which uses thyristors connected to tapping points on the secondary windings of the transformer. Control of by-pass thyristor for reducing abnormal harmonic generation are discusses, and some experimental results supporting the theoretical calculations are also included.

An Iterative Control Method for Voltage Source Converters to Eliminate Uncharacteristic Harmonics Under Unbalanced Grid Voltages for High-Power Applications

When voltage source converters (VSCs) are subjected to disturbances or faults, they may not be able to perform as per the desired control objective, which presents a significant challenge to system operations. One well-known situation is when the converter is subjected to an unbalanced ac voltage with a negative-sequence fundamental voltage injection. Under this condition, the VSC may generate undesired double-frequency ripples at the dc voltage and uncharacteristic harmonics at the ac current. This paper presents a new two-stage control method suitable for VSCs used in high-power applications. In the first stage, a simple proportional controller is used to suppress overshoots during the transients. Then, the iterative controller is activated to finely adjust the modulation indices and firing angles of the converter to compensate the uncharacteristic harmonics. To verify the performance of the proposed control method, the control algorithms are implemented in a 10 MVA VSC system and validated by means of PSCAD/EMTDC and hardware-in-the-loop simulations. The results show that the state variables converge within a few iterative steps, and the transient response is sufficiently fast. The proposed method is shown to perform better than conventional methods for attenuating and eliminating the uncharacteristic dc and ac harmonics caused by imbalances in the grid voltage.

Harmonic Modeling of a Diode-Clamped Multilevel Voltage Source Converter for Predicting Uncharacteristic Harmonics

In response to the growing demand for medium- and high-power trends, multilevel voltage source converters (VSCs) have been attracting growing considerations. One of the widely used VSCs are the diode-clamped multilevel VSC (DCM-VSC). As these converters proliferate, their harmonic impact may become significant. Nevertheless, a harmonic model for the DCM-VSC is currently lacking in the literature. In this paper, the ABCD matrix, mapping the input harmonics to the output harmonics of DCM-VSC, is derived. As the matrix is formulated in the time-domain, the output harmonics are exact and do not suffer from harmonic truncation errors. As the paper will demonstrate, the derived ABCD matrix can be easily applied to a microgrid system and users can easily predict all the uncharacteristic harmonics when a microgrid is subjected to various conditions of imbalance. In addition to all the results being validated with those of PSCAD/EMTDC, the computation time of the proposed method is in contrast much shorter.

Cylindrical harmonic analysis of the magnetic field in the aperture of the superconducting winding of an electromagnet

It is of practical interest to create such models of the electromagnetic field of an electromagnet that help correct the mean integral coefficients of the harmonics of magnetic induction by the geometric parameters of the magnet. The aim of the study was to obtain analytical expressions for the coefficients of the mean integral lengths of the cylindrical harmonics of magnetic induction created by the current in the superconducting winding inside the aperture of a dipole or quadrupole direct electromagnet on the basis of the geometric parameters of the winding. The analytical solution found is based on the sectorial spherical harmonics of the internal solution of the Laplace equation for the scalar potential of the magnetic field and allows associating them with a series of polar harmonics. The study shows the proportionality between the mean integral contributions to the magnetic induction, produced by uncharacteristic harmonics, and the mean integral value of the ground field. It has been determined that the contribution to the mean integral values of the harmonics coefficients from the end elements of the winding appears only in the presence of the iron framework as a result of its saturation. The received analytical representations make it possible to calculate necessary correction of geometrical parameters of a current winding when optimizing the magnetic field inside the aperture of dipole and quadrupole electromagnets with given mean integral factors.

Suppression of Time-Varying Interharmonics Produced by Medium-Frequency Induction Melting Furnaces by a HAPF System

This paper describes the design and field implementation of a hybrid active power filter (HAPF) system to suppress time-varying interharmonics injected into the grid by medium-frequency coreless induction melting furnaces (IMFs). In a sample steel melt shop, variable-frequency load-resonant inverters of works coils are supplied from the medium-voltage grid via 12- and 24-pulse thyristor rectifiers. The cross-modulation phenomenon in ac–dc–ac link of the medium-frequency coreless IMF installations produces interharmonics and characteristic and uncharacteristic harmonics in the grid-side line current waveforms. Furthermore, frequencies of dominant interharmonics are migrating in time inside a frequency window as the operating frequency of the load-resonant inverter varies in wide range during a melting cycle. The HAPF system developed for this application consists of nine HAPF units operating in parallel in current control mode and is connected to the grid via a coupling transformer. Current control is achieved by extracting high-order fixed frequency characteristic harmonic components from the reference current signal and defining a fixed hysteresis band for each HAPF unit. The input LC filter of the HAPF system is optimized in the design stage by taking into account the frequency range of prominent interharmonics. Theoretical findings are verified in a sample steel melt shop by extensive field measurements. Field test results have shown that the developed HAPF system suppresses successfully the dominant time-varying interharmonics and harmonics in the frequency range from 250 to 650 Hz.

Shifting Window Average Method for Accurate Frequency Measurement in Power Systems

A shifting window average method (SWAM) is proposed to enhance the accuracy of the widely implemented discrete Fourier transform (DFT)-based algorithm in power system frequency measurements. First, the authors have comprehensively analyzed the error induced in the DFT-based algorithm. The SWAM is then introduced to improve frequency measurement accuracy by examining the analytic form of the error. Test results have validated that it achieves higher accuracy compared to the DFT-based algorithm. Due to its high accuracy and low required processing effort, the SWAM is a valuable candidate for online frequency estimation in real-life power systems, especially when harmonics, uncharacteristic harmonics, noises, and large frequency excursions exist.

Abnormal Harmonics Generated by Modified HVDC Converter

Unbalance in the operation of AC/DC converters gives rise to abnormal or uncharacteristic input current harmonics, in addition to the characteristic harmonics in the supply current waveforms. This article concentrates on the analysis of uncharacteristic harmonics caused by unbalanced AC supply voltages. Attention is given to modified converter operation which uses thyristor valves connected to tapping points on the secondary windings of the transformer. Control of by-pass valve for reducing abnormal harmonic generation are discussed, and some experimental results supporting the theoretical calculations are also included.

Direct harmonic analysis of the voltage source converter

An analytic technique is presented for the calculation of both characteristic and uncharacteristic harmonics in voltage source converter circuits. The proposed technique accounts for all harmonic interaction between the AC and DC sides of the converter, and may thus also be employed for accurate DC side voltage ripple calculations. In contrast to simulation-based approaches or iterative methods, the proposed technique offers a closed form solution for the current harmonics, thereby avoiding numerical errors and convergence problems. The technique is applied to the harmonic analysis of a STATCOM.

Novel calculation of HVDC converter harmonics by linearization in the time-domain

There is an increasing interest in the steady-state analysis of high power converter circuits for the computation of both characteristic and uncharacteristic harmonics. Uncharacteristic harmonics arise due to asymmetries in excitation/operation and are of particular interest given their overall adverse effect on the power system. Frequency domain methods for harmonic analysis are efficient though not necessarily comprehensive; that is not all the necessary details of the system are simultaneously modeled. This paper presents a novel technique that is both efficient and comprehensive as the time domain nature of the differential/algebraic equations describing the power converter operation is retained. Efficient convergence of the time domain boundary value problem is assured by an appropriate linearization. Subsequent Fourier analysis of the the time domain solution yields the harmonic spectra. Comparison is made with time domain steady-state solutions obtained by brute-force computation.

Generation of uncharacteristic harmonics in electrical plants with ac/dc converters

AbstractIn this paper an investigation is carried out on the correlation between dissymmetry imbalances and the generation of uncharacteristic harmonics in low‐voltage electrical plants supplying A C/DC six‐pulse converters. After a general introduction, some typical cases of dissymmetries and imbalances are considered, relevant to supply voltage, line impedances, thyristor injection angles and filter capacitors, and the corresponding uncharacteristic harmonic currents derived by means of the simulation program EMTP. Moreover, evaluation of harmonics is made for various values of plant and load parameters in order to derive the behaviour of amplitude and phase of uncharacteristic harmonics in a wide range of plant conditions. The results provide clear relationship between kind and degree of dissymmetry and presence of uncharacteristic harmonics with useful indications for plant design.

Calculation of Uncharacteristic Harmonics Generated by Three-Phase Diode-Bridge Rectifier with DC Filter Capacitor.

Three-phase diode-bridge rectifiers are used to convert the AC input into DC voltage. A large capacitor is connected as a filter on the DC side, since in most applications, a low ripple in the DC output voltage is desirable. Unbalance in the supply conditions of a three-phase diode-bridge rectifier with a DC filter capacitor gives rise to uncharacteristic harmonics on both the AC and DC sides.This paper proposes a simplified method for calculating the uncharacteristic harmonic currents generated by the three-phase diode-bridge rectifier under unbalance, taking into account the effects of both the AC and DC side impedances. Analytical expressions for the harmonic currents are derived using the switching functions with overlap period the AC circuit theory. Therefore, the proposed method can be executed only with algebraic computation and its accuracy is quite high. The validity of the proposed method is demonstrated by comparison with the results of time simulation.

Probabilistic modelling of current harmonics produced by an AC/DC converter under voltage unbalance

Harmonic currents generated by an AC/DC power converter are subject to random variations due partly to fluctuation of the converter's DC load and partly to power source disturbances. The latter are also responsible for the generation of uncharacteristic harmonics. Based on an analytical power converter model and probabilistic models of voltage unbalance and converter load variations, the random behaviour of both characteristic and uncharacteristic power system harmonics produced by a six-pulse AC/DC power converter undervoltage unbalance is analyzed. The probability density functions of the magnitude and the phase angle of harmonic currents are derived and compared with the results obtained from Monte Carlo simulations. It is found that the developed probabilistic harmonic model agrees very well with the results of Monte Carlo simulations and gives significant insight into the effects of voltage unbalance and converter DC load variations upon the random properties of the power converter current spectrum.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Minimisation of uncharacteristic harmonics in HVDC convertors through firing angle modulation

Minimisation of uncharacteristic harmonics in HVDC convertors through firing angle modulation

Reply: Minimisation of uncharacteristic harmonics in HVDC convertors through firing angle modulation

Reply: Minimisation of uncharacteristic harmonics in HVDC convertors through firing angle modulation

Three phase AC-DC load and harmonic flows

The validity of assuming that the fundamental frequency load flow conditions of an AC-DC conversion plant are unaffected by the presence of harmonics is questioned, and an algorithm is derived which is capable of determining the true load flow conditions in the presence of harmonics. The algorithm is unique in its three phase approach to the problem, which is essential if normally unfiltered uncharacteristic harmonics are to be represented. Results of a realistic and relevant test system are used to demonstrate the interaction which exists between the fundamental and harmonic frequencies in the power converter. >

Calculation of uncharacteristic harmonics of a three-phase bridge rectifier with DC current ripple

Calculation of uncharacteristic harmonics of a three-phase bridge rectifier with DC current ripple

Minimisation of uncharacteristic harmonics in HVDC convertors through firing angle modulation

A modulating signal (MS) applied to the control voltage has a profound effect on the DC and AC-side uncharacteristic harmonic currents. The presence of harmonic distortion in the AC-busbar voltage causes distortion on the DC-line current ripple. One reason for such distortion is the valve-asymmetrical firing pattern caused by the closed loop nature of constant current control. A second reason is that the DC-current ripple is related to AC-side harmonics even if no closed-loop current control is prescribed. A method of harmonic minimisation based on approximate linear relationships between uncharacteristic harmonics and the MS was developed and implemented on a convertor model. A circuit to select, adjust and inject the MS into the control voltage connected to the output of the injection circuit inhibits the MS injection during transients. This control feature is important because it allows fast transient response of the control and does not contribute to oscillatory behaviour. A full set of analytical predictions and model tests using the injection circuit described above for an MS of 50 Hz, 100 Hz and 150 Hz is presented.

Calculation of uncharacteristic harmonics of a three-phase bridge rectifier with DC current ripple.

Calculation of uncharacteristic harmonics of a three-phase bridge rectifier with DC current ripple.

Describing function applied to HVDC systems harmonic instability

The adoption of voltage-controlled oscillator based firing systems has almost completely eliminated the harmonic instabilities experienced in early HVDC systems. However, the basic feature of such systems (equidistant firing) is adversely affected by the feedback action of the current control loop, since the direct current may contain uncharacteristic harmonics which are not filtered out by the controller. Under certain circumstances (such as an imbalance and/or a distortion in the alternating voltage, DC-or AC-side resonances near DC or AC harmonic frequencies, respectively) instabilities may occur which are characterised by oscillations synchronised with the alternating voltage, i.e. harmonic instabilities. Nonlinear control-system theory, namely the describing-function method, was used to show that these oscillations are limit cycles whose conditions of occurrence can be predicted. A detailed computer model for an hvdc link was developed which allows the calculation of both the DC and the AC harmonics in the steady state and the describing function for subharmonics of the firing frequency of the convertor. Limit cycles can be predicted by the Nichols chart technique.