Studies of Uncertainties in Smart Grid: Wind Power Generation and Wide-Area Communication

This research work investigates the uncertainties in Smart Grid, with special focus on the uncertain wind power generation in wind energy conversion systems (WECSs) and the uncertain wide-area communication in wide-area measurement systems (WAMSs). For the uncertain wind power generation in WECSs, a new wind speed modeling method and an improved WECS control method are proposed, respectively. The modeling method considers the spatial and temporal distributions of wind speed disturbances and deploys a box uncertain set in wind speed models, which is more realistic for practicing engineers. The control method takes maximum power point tracking, wind speed forecasting, and wind turbine dynamics into account, and achieves a balance between power output maximization and operating cost minimization to further improve the overall efficiency of wind power generation. Specifically, through the proposed modeling and control methods, the wind power control problem is developed as a min-max optimal problem and efficiently solved with semi-definite programming. For the uncertain communication delay and communication loss (i.e. data loss) in WAMSs, the corresponding solutions are presented. First, the real-world communication delay is measured and analyzed, and the bounded modeling method for the communication delay is proposed for widearea applications and further applied for system-area and substation-area protection applications, respectively. The proposed bounded modeling method is expected to be an important tool in the planning, design, and operation of time-critical wide-area applications. Second, the real synchronization signal loss and synchrophasor data loss events are measured and analyzed. For the synchronization signal loss, the potential reasons and solutions are explored. For the synchrophasor data loss, a set of estimation methods are presented, including substitution, interpolation, and forecasting. The estimation methods aim to improve the accuracy and availability of WAMSs, and mitigate the effect of communication failure and data loss on wide-area applications

Development of a multilevel converter topology for transformer-less connection of renewable energy systems

The global need to reduce dependence on fossil fuels for electricity production has become an ongoing research theme in the last decade. Clean energy sources (such as wind energy and solar energy) have considerable potential to reduce reliance on fossil fuels and mitigate climate change. However, wind energy is going to become more mainstream due to technological advancement and geographical availability. Therefore, various technologies exist to maximize the inherent advantages of using wind energy conversion systems (WECSs) to generate electrical power. One important technology is the power electronics interface that enables the transfer and effective control of electrical power from the renewable energy source to the grid through the filter and isolation transformer. However, the transformer is bulky, generates losses, and is also very costly. Therefore, the term "transformer-less connection" refers to eliminating a step-up transformer from the WECS, while the power conversion stage performs the conventional functions of a transformer. Existing power converter configurations for transformer-less connection of a WECS are either based on the generator-converter configuration or three-stage power converter configuration. These configurations consist of conventional multilevel converter topologies and two-stage power conversion between the generator-side converter topology and the high-order filter connected to the collection point of the wind power plant (WPP). Thus, the complexity and cost of these existing configurations are significant at higher voltage and power ratings. Therefore, a single-stage multilevel converter topology is proposed to simplify the power conversion stage of a transformer-less WECS. Furthermore, the primary design challenges – such as multiple clamping devices, multiple dc-link capacitors, and series-connected power semiconductor devices – have been mitigated by the proposed converter topology. The proposed converter topology, known as the "tapped inductor quasi-Z-source nested neutral-point-clamped (NNPC) converter," has been analyzed, and designed, and a prototype of the topology developed for experimental verification. A field-programmable gate array (FPGA)-based modulation technique and voltage balancing control technique for maintaining the clamping capacitor voltages was developed. Hence, the proposed converter topology presents a single-stage power conversion configuration. Efficiency analysis of the proposed converter topology has been studied and compared to the intermediate and grid-side converter topology of a three-stage power converter configuration. A direct current (DC) component minimization technique to minimize the dc component generated by the proposed converter topology was investigated, developed, and verified experimentally. The proposed dc component minimization technique consists of a sensing and measurement circuitry with a digital notch filter. This thesis presents a detailed and comprehensive overview of the existing power converter configurations developed for transformer-less WECS applications. Based on the developed 2 comparative benchmark factor (CBF), the merits and demerits of each power converter configuration in terms of the component counts and grid compliance have been presented. In terms of cost comparison, the three-stage power converter configuration is more cost-effective than the generatorconverter configuration. Furthermore, the cost-benefit analysis of deploying a transformer-less WECSs in a WPP is evaluated and compared with conventional WECS in a WPP based on power converter configurations and collection system. Overall, the total cost of the collection system of WPP with transformer-less WECSs is about 23% less than the total cost of WPP with conventional WECs. The derivation and theoretical analysis of the proposed five-level tapped inductor quasi-Z-source NNPC converter topology have been presented, emphasizing its operating principles, steady-state analysis, and deriving equations to calculate its inductance and capacitance values. Furthermore, the FPGA implementation of the proposed converter topology was verified experimentally with a developed prototype of the topology. The efficiency of the proposed converter topology has been evaluated by varying the switching frequency and loads. Furthermore, the proposed converter topology is more efficient than the five-level DC-DC converter with a five-level diode-clamped converter (DCC) topology under the three-stage power converter configuration. Also, the cost analysis of the proposed converter topology and the conventional converter topology shows that it is more economical to deploy the proposed converter topology at the grid side of a transformer-less WECS

Detection of faults in a scaled down doubly-fed induction generator using advanced signal processing techniques.

The study ventures into the development of a micro-based doubly fed induction generator (DFIG) test rig for fault studies. The 5kW wound rotor induction machine (WRIM) that was used in the test rig was based on a scaled-down version of a 2.5MW doubly fed induction generator (DFIG). The micromachine has been customized to make provision for implementing stator inter-turn short-circuit faults (ITSCF), rotor ITSCF and static eccentricity (SE) faults in the laboratory environment. The micromachine has been assessed under the healthy and faulty states, both before and after incorporating a converter into the rotor circuit of the machine. In each scenario, the fault signatures have been characterised by analyzing the stator current, rotor current, and the DFIG controller signals using the motor current signature analysis (MCSA) and discrete wavelet transform (DWT) analysis techniques to detect the dominant frequency components which are indicative of these faults. The purpose of the study is to evaluate and identify the most suitable combination of signals and techniques for the detection of each fault under steady-state and transient operating conditions. The analyses of the results presented in this study have indicated that characterizing the fault indicators independent of the converter system ensured clarity in the fault diagnosis process and enabled the development of a systematic fault diagnosis approach that can be applied to a controlled DFIG. It has been demonstrated that the occurrence of the ITSCFs and the SE fault in the micro-WRIM intensifies specific frequency components in the spectral plots of the stator current, rotor current, and the DFIG controller signals, which may then serve as the dominant fault indicators. These dominant components may be used as fault markers for classification and have been used for pattern recognition under the transient condition. In this case, the DWT and spectrogram plots effectively illustrated characteristic patterns of the dominant fault indicators, which were observed to evolve uniquely and more distinguishable in the rotor current signal compared to the stator current signal, before incorporating the converter in the rotor circuit. Therefore, by observing the trends portrayed in the decomposition bands and the spectrogram plots, it is deemed a reliable method of diagnosing and possibly quantifying the intensity of the faults in the machine. Once the power electronic converter was incorporated into the rotor circuit, the DFIG controller signals have been observed to be best suited for diagnosing faults in the micro-DFIG under the steady-state operating condition, as opposed to using the terminal stator or rotor current signals. The study also assessed the impact of undervoltage conditions at the point of common coupling (PCC) on the behaviour of the micro-DFIG. In this investigation, a significant rise in the faulted currents was observed for the undervoltage condition in comparison to the faulty cases under the rated grid voltage conditions. In this regard, it could be detrimental to the operation of the micro-DFIG, particularly the faulted phase windings, and the power electronic converter, should the currents exceed the rated values for extended periods

Maximum-power-point tracking with reduced mechanical stress applied to wind-energy-conversion-systems

[EN] This paper presents an improved maximum-power-point tracking algorithm for wind-energy-conversion-systems. The proposed method significantly reduces the turbine mechanical stress with regard to conventional techniques, so that both the maintenance needs and the medium time between failures are expected to be improved. To achieve these objectives, a sensorless speed control loop receives its reference signal from a modified Perturb&Observe algorithm, in which the typical steps on the reference speed have been substituted by a fixed and well-defined slope ramp signal. As a result, it is achieved a soft dynamic response of both the torque and the speed of the wind turbine, so that the whole system suffers from a lower mechanical stress than with conventional P&O techniques. The proposed method has been applied to a wind turbine based on a permanent magnet synchronous generator operating at variable speed, which is connected to the distribution grid by means of a back to back converter.González, L.; Figueres Amorós, E.; Garcerá, G.; Carranza, O. (2010). Maximum-power-point tracking with reduced mechanical stress applied to wind-energy-conversion-systems. Applied Energy. 87(7):2304-2312. doi:10.1016/j.apenergy.2009.11.030S2304231287

Reliability Improvement of Autonomous Microgrids through Interconnection and Storage

This thesis deals with reliability and power quality improvement in autonomous microgrids. The reliability is improved through the interconnection of storage, intertying two neighbouring microgrids and interlinking of microgrids cluster through a common power exchange highway. The power quality is improved by interconnecting distributed static compensator (DSTATCOM) in the microgrid. All the proposed methods are verified through extensive digital computer simulation using PSCAD