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Within future homes and electrical power networks, emphasis is being placed on intelligent, distributed measurement devices. In particular, the recognition of individual or aggregated loads through harmonic signature has been proposed as a useful way to enhance the value of home energy monitoring/control. Clearly, the cost of implementing such measurement devices is a major barrier to acceptance. In a recent project, a challenge was set to implement real-time software on an ARM® Cortex™ LPC1768 microcontroller platform (chip cost c. £4). The software must be capable of measuring a single-phase AC frequency, real and reactive power flows and provide a full breakdown of the voltage and current (and power) behaviour via harmonic analysis from DC to the 40th, in real-time with a new output every 20 ms. In addition, the algorithm must be capable of adapting the measurement when the frequency is not nominal (50 Hz) so that spectral leakage is minimised. It is found that the LPC1768 processor is capable of supporting such an algorithm when it is coded appropriately. This knowledge de-risks the proposed use of such cheap microcontrollers for these relatively complex tasks.

Electric vehicles (EVs) can be utilized as mobile storages in a power system. The use of battery chargers can cause current harmonics in the supplied AC system. In order to analyze the impact of different EVs with regardto their number and their emission of current harmonics, a generic harmonic current model of EV types was built and implemented in the power system simulation tool PSS®NETOMAC. Based on the measurement data for different types of EVs three standardized harmonic EV models were developed and parametrized. Further, the identified harmonic models are used by the computation of load flow in a modeled, German power distribution system. As a benchmark, a case scenario was studied regarding a high market penetration of EVs in the year 2030 for Germany. The impact of the EV charging on the power distribution system was analyzed and evaluated with valid power quality standards.

Based on the inherent characteristic of the carrier-based pulse-width modulation converters, the response of the first carrier frequency harmonic (FCFH) current in the voltage source converter (VSC)-based high-voltage direct current (HVDC) transmission system is analysed under different fault conditions in this paper. A new protection scheme for the VSC-based HVDC transmission systems is proposed based on the FCFH currents. By extracting the harmonic currents at both the endings of the DC transmission cable, the fault type can be identified. The VSC-based HVDC test system is modelled in the PSCAD/EMTDC and the proposed protection scheme is evaluated with a variety of values of the fault resistance and the fault locations. Comprehensive test studies show that the performance of the proposed protection scheme is inspiring. It can recognise the internal and the external faults correctly.

Integration of dynamic grid support is required for distributed power systems that are interconnected with medium voltage grids. This study proposes a comprehensive control solution to enhance fault ride through (FRT) capability for utility-scale photovoltaic (PV) power plants. Based on positive and negative sequence control schemes and PV characteristics, the approach alleviates dc-bus double-line-frequency ripples, reduces voltage stress on inverter power switches and DC-link capacitors, and minimises undesirable low-order voltage and current harmonics that are presented on the ac side. The study proposes a new feature to achieve superior FRT performance by using the overload capability of grid-tied inverters. A weak electric grid is used for the test case including a wind turbine induction generator, diesel engine driven synchronous generators and various loads. A comprehensive simulation verified the capability of the proposed control schemes for mitigating the voltage dip, enhancing the voltage response and further improving the stability of interconnected distributed generation in reaction to severe unbalanced voltage conditions because of asymmetrical grid faults.

Harmonic modeling of low-voltage networks with many devices requires simple but accurate models. This paper investigates the advantages and drawbacks of such models to predict the current harmonics created by single-phase full-bridge rectifiers. An overview is given of the methods, limiting the focus to harmonic analysis. The error of each method, compared to an accurate numerical simulation model, is quantified in frequency and time domain considering realistic input scenarios, including background voltage distortion and different system impedances. The results of the comparison are used to discuss the applicability of the models depending on the harmonic studies scale and the required level of detail. It is concluded that all models have their applicability, but also limitations. From the simplest and fastest model, which does not require a numerical solution, to the more accurate one that allows discontinuous conduction mode to be included, the trade-off involves accuracy and computational complexity.

Conventional current transformers (CTs) suffer from nonlinearities due to their ferromagnetic cores. On one hand, it is well-known that severe core saturation may occur because of large overcurrents or unidirectional transient components: this may substantially impact the operation of relays. On the other hand, weaker nonlinear effects are also present during regular working conditions. In particular, the spectral content of typical current waveforms is characterized by a strong fundamental term responsible for harmonic distortion affecting the frequency components at the secondary side. In turn, this has a significant impact on the accuracy that can be reached as long as current harmonics must be monitored. The target of this work is implementing a simple signal processing technique that allows compensating for this effect. The method, characterized by extremely low computational complexity, is first introduced and validated using numerical simulations. After this, it was tested experimentally to improve the harmonic measurement capability of inductive CTs. The achieved results highlight a noticeable reduction of errors at low-order harmonics over a wide range of primary current amplitudes. It is worth noting that the black-box approach makes the technique suitable also for compensating nonlinearities introduced by current transducers based on different operating principles. Thanks to this peculiarity and to the low computational complexity, the proposed method is suitable to be employed in power quality analyzers and merging units. In this way, high-accuracy monitoring of current harmonics in power systems can be achieved, opening the way to advanced power quality management and to the location of disturbing users.

This work aims to cover the measurement, modeling, and analysis of waveform distortions in railway power systems. It is focused on waveform distortion as a phenomenon that includes harmonic distortion, interharmonic distortion, and supraharmonics. A comprehensive view of the interactions of waveform distortions in railway systems is needed, together with a grid perspective of power quality incorporating all aspects, sources, propagation, requirements, and effects. It is understood that the communities interested or involved in the subject of railway power systems would benefit from an integrated overview of the literature on the complex problem of waveform distortion. The literature review is divided into four categories: characterization and measurements, modeling, the application of artificial intelligence, and specific issues. For each category of work, the contributions are highlighted, and a discussion on opportunities, gaps, and critical observations is provided. The work successfully builds a framework for the subject with two main characteristics; the review is informative and propositional, providing a road map of opportunities for future works. Some aspects and recommendations can be highlighted. Suggestions for future works and research practices on waveform distortion in electrical transportation are offered.

Voltage source inverters (VSIs) are usually used for all kinds of distributed generation interfaces in a microgrid. It is the microgrid's superiority to power the local loads continuously when the utility fails. When in islanded mode, the voltage and frequency of the microgrid are determined by the VSIs; therefore the power quality can be deteriorated under unbalanced and non-linear loads. A voltage unbalance and harmonics compensation strategy for the VSIs in islanded microgrid is proposed in this study. This method is implemented in a single synchronous reference frame (SRF) and is responsible for both the voltage unbalance and harmonic compensation. Furthermore, the virtual impedance loop is modified to improve the compensation effect. The impedance model of the VSI is built to explain the compensation ability of the proposed strategy. The whole control system mainly includes power droop controllers, a modified virtual impedance loop and inner SRF-based voltage unbalance and harmonics compensators. The proposed strategy is demonstrated in detail and validated with simulations and experiments.

For grid-connected inverters, switching harmonics can be effectively attenuated through an LCL-type filter. In order to suppress resonance and guarantee good performance, many strategies (e.g. active damping (AD), harmonic resonant control, repetitive control and grid feedforward) have been proposed. However, the wide variation of grid impedance value challenges system stability in practical applications. The aforementioned methods need to be investigated. This study evaluates the applicability of each part of the overall control in a weak grid case with the use of a stability criterion. It has been demonstrated that the feedback-based AD control can work well in a wide range of grid conditions. However, the resonant and repetitive control methods meet constraints. The grid feedforward method brings in an extra positive feedback path, and consequently results in high harmonics or even instability. Finally, a recommendation for system design has been presented. Simulations and experiments have been provided to verify the analysis.

Three-phase diode bridge rectifiers are widely employed in various industrial applications because of their inherent simplicity, robustness, low electromagnetic interference and good overall performance. However, their use causes harmonic distortion in the electric power network line currents due to their nonlinear nature, which, in turn, affects the electric power quality. The fundamental approach to limit the line currents’ total harmonic distortion (THD) introduced by the diode bridge rectification systems is based on increasing the number of steps in their waveform per power supply cycle and drawing them closer to the pure-sine waveforms. This can be achieved by employing the conventional twelve-pulse rectification system composed of two parallel connected six-pulse diode bridge rectifiers, in which the DC circuit is expanded on the auxiliary circuit responsible for adequately shaping the line currents’ waveforms per power supply cycle. When the auxiliary circuit is connected to the interphase reactor (IPR) additional (secondary) winding, the ability of the rectification system to reduce the line current THD depends mainly on the auxiliary circuit current waveform and its parameters. This paper provides a space vector analysis of the twelve-pulse diode bridge rectifier operation. It leads to devising a formula for the auxiliary circuit current related to the phase angle of the rectification system line currents’ space vector and the load current, which has been missing in the literature so far. The formula explicitly defines the auxiliary circuit current waveform that guarantees the optimal line currents’ THD for the twelve-pulse diode bridge rectifier which is expanded with the auxiliary circuit connected to the IPR secondary winding. The theoretical studies are validated through experimental investigations.