| The rapid development of power electronics technology creates feasible and exciting opportunities to develop novel electrical equipment for better utilization of the existing power systems. During the last decade, a great number of control devices incorporating specifically the concept of "Flexible AC Transmission Systems" (FACTS) technology have been proposed and implemented all over the world. Almost all the FACTS elements are based on controlled switching of the power electronic devices to effectively realize power flow control, voltage regulation, enhancement of transient stability and mitigation of low-frequency system oscillations. However, very steep transient pulses are generated due to frequent switching of the large power electronic devices, hence resulting in considerable high-frequency conducted and radiated emissions through near-field coupling and far-field radiation, which causes electromagnetic contamination to both the power grids and the victim electronic devices positioned nearby. In the mean time, more and more electronic units have also been utilised in the secondary side of nowadays power substations to realize online monitoring and diagnosis, data acquisition and communication, automation control and protection of the power system operation. Unfortunately, these electronic units are normally quite vulnerable to electromagnetic disturbances, and sometimes can not withstand the emission levels generated from the FACTS equipment, subsequently rendering malfunctions or even being damaged. Furthermore, the case may become even worse if higher power FACTS equipments are installed. So the electromagnetic environment within substations employing FACTS equipments needs to be re-evaluated and thereafter some immune measures should be taken to guarantee all the secondary electronic units function properly.A comprehensive strategy is adopted in this dissertation to address the prominent electromagnetic compatibility (EMC) issues in FACTS-based substations. The research work mainly concentrates on the following several aspects: Equivalent modelling and digital rectifying technology for transducers and antennas, establishment of on-site measurement system, feature extraction methodology for electromagnetic interference (EMI) data, modelling and simulation scheme of high-frequency EMI mechanisms, EMI suppressing methods and electromagnetic susceptibility test measures within FACTS-based substations, etc.Due to non-flat amplitude-frequency characteristics of the antennas used in EMC measurement, it is desired to establish the equivalent transfer functions of the antennas so as to make digital correction to the measured data, from which the actual electromagnetic emissions can be traced back. Two digital rectifying algorithms based on frequency domain transfer function are proposed, one is Improved Vector Fitting Method which is achieved by introduction of the first-order derivative and also right-multiplication to the coefficient matrix by a diagonal matrix, the other one is called Poles Replacement Method which is achieved by replacing the initial poles of the target transfer function with the zeroes derived from another relative transfer function whose starting poles are manually preset. Simulation results show the proposed algorithms both render good performance in fitting accuracy.The weather conditions of on-site measurement may differ a lot from the standard calibration condition of an antenna, so it is necessary to analyze the impact of weather factors, such as temperature, humidity and atmosphere pressure, on the measurement accuracy of the antenna. Based on experimental study, digital signal processing, improved vector fitting method and artificial neural network (ANN), a comprehensive scheme is presented to quantitatively analyze and effectively predict the impact of weather factors on an antenna's frequency-domain characteristics. Analysis and experimental verification indicate that, the proposed scheme can address the concrete impact from weather factors, and thereby realize reliable prediction of the antenna's specific frequency-domain parameters with regards to variation of weather conditions.Based on circuit and field transducers, optical fibre transmission, digital oscilloscope and shielded terminal unit, an advanced data acquisition system is established to implement on-site measurements of the electromagnetic emissions generated from FACTS equipments within several different substations, which demonstrates that the measurement system can realise effective acquisition of both the conducted and radiated wide-bandwidth interference. The switching bursts of the EMI waveforms within a power cycle are analyzed to present in details the frequency and amplitude range of the electromagnetic emissions, which facilitates accumulation of large amount of first-hand EMI data.A systematic methodology for feature extraction and expression of the electromagnetic emissions is established in both time- and frequency-domains. Two novel algorithms to accurately track the instantaneous frequency of EMI signals are proposed, with one based on bi-orthogonal digital filters and the other one on Hough transform, which can respectively cope with low and fast frequency-varying EMI signals accurately. Further, with the utilization of short-time Fourier transform, Wigner-Ville distribution, smoothed pseudo Wigner-Ville distribution and rearranged smoothed pseudo Wigner-Ville distribution, a feature extraction scheme based on time-frequency analysis is presented, and more detailed and useful information of the electromagnetic emissions can be obtained through 2-D and 3-D displays, which presents fundamental reference for characterizing the interfering mechanisms and effectively evaluating the electromagnetic environment.Based on the dynamic switching characteristics of the thyristors, a modelling and simulation methodology is established to investigate on the high-frequency EMI mechanisms in substations operational with Static Var Compensator (SVC) devices. With a nonlinear time-varying resistor as the kernel of the high-frequency macro model for thyristors, a concrete simulation model of SVC including a Thyristor-switched Capacitor (TSC) element and a Thyristor-controlled Reactor (TCR) element is further proposed. The conducted emissions by simulation are correlated well with that from on-site measurements, which indicates the dynamic switching process of the thyristors is the main cause and principal mechanism of the high-frequency conducted interference. Specifically, a SVC simulation model incorporating randomly varying loads is also established to predict the EMI levels under different loadings.Referring to the topologies of FACTS equipment, and based on on-site measurement as well as theoretical analysis, pertinent measures for EMI suppression are put forward, such as earthing, shielding, filtering and installation of ferrite rings so as to reduce coupling impacts on the electronic units of the secondary side. With combination of the state-of-the-art achievement of FACTS-based EMI research, a correlation is made to the present EMC standards IEC 61000-4 series and concrete recommendations of changes for immunity tests are presented to ameliorate the IEC standards.The research work of this dissertation further develops and enriches both the theories and methodologies for power system EMC study, which possesses practical significance for engineering design and wide application of the FACTS equipment.
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