Accurate Damping Factor and Frequency Estimation for Damped Real-Valued Sinusoidal Signals

The interpolated discrete Fourier transform (IpDFT) is one of the most popular techniques to estimate the parameters of a damped real-valued sinusoidal signal (DRSS). However, its accuracy is affected by strong noise presence and short observation windows. To this end, this letter proposes a novel two-point IpDFT method, called I2pZDFT, for the parameter estimation of a DRSS. The proposed I2pZDFT uses the zero-padding technique to increase the sampling rate in the frequency domain. The conjugate symmetry and the parity of the zero-padded signal are utilized to eliminate the influence of the spectral leakage. Simulation results highlight that the proposed I2pZDFT outperforms the existing IpDFT-based methods in terms of noise immunity, especially in the case of observation windows as short as 0.5-1 cycles

Protection of multi-inverter based microgrid using phase angle trajectory

This thesis presents a simple, yet a clever way of using the current phase angle to develop low bandwidth communication-assisted line protection strategies for medium and low voltage AC microgrids, particularly those with multi-inverter interfaced distributed generators. It is now a trend in both AC transmission and distribution segments of power network that inverters interface renewable energy to the system. Unlike synchronous generators the fault feeding, and control characteristic of these generators are different and mostly influenced by the topology, switching, control deployed in the power electronics interface. The limited and controlled fault current challenges the existing conventional protection schemes. Offering higher power supply reliability and system resilience than conventional radial distribution systems, multi-inverter based microgrids, particularly those with loop and mesh typologies, are characterised by bidirectional power flow. This further constrains traditional protections such that communication-less protection schemes become ineffective for such systems. So unit protection types, such as differential protection, become more technically suitable for such microgrids despite the necessity for a communication system. In this thesis, two current direction based protection schemes for medium voltage islanded microgrids have been developed. The change in current flow direction in a line is detected using the cosine of the positive sequence current phase angle. Expressing the change and no-change of the flow directions as binary states, a low bandwidth communication based protection scheme is proposed comparing the binary states from local and remote ends of the line. To further enhance the scope and reliability of this scheme, a second protection scheme is proposed in Chapter 7 whereby the cosine function is combined with the rate of change of the slope of the phase angle (ROCOSP). This combination allows the detection and isolation of a fault even with the failure of the communication channel between relays protecting a faulted line. Furthermore, these scheme can work together and share the communication infrastructure as primary and backup protections. The performance of these schemes was assessed through simulations of microgrid models developed in Matlab/Simulink.Open Acces

Electrical Signature Analysis of Synchronous Motors Under Some Mechanical Anomalies

Electrical Signature Analysis (ESA) has been introduced for some time to investigate the electrical anomalies of electric machines, especially for induction motors. More recently hints of using such an approach to analyze mechanical anomalies have appeared in the literature. Among them, some articles cover synchronous motors usually being employed to improve the power factor, drive green vehicles and reciprocating compressors or pumps with higher efficiency. Similarly with induction motors, the common mechanical anomalies of synchronous motor being analyzed using the ESA are air-gap eccentricity and single point bearing defects. However torsional effects, which are usually induced by torsional vibration of rotors and by generalized roughness bearing defects, have seldom been investigated using the ESA. This work presents an analytical method for ESA of rotor torsional vibration and an experimentally demonstrated approach for ESA of generalized roughness bearing defects. The torsional vibration of a shaft assembly usually induces rotor speed fluctuations resulting from the excitations in the electromagnetic (EM) or load torque. Actually, there is strong coupling within the system which is dynamically dependent on the interactions between the electromagnetic air-gap torque of the synchronous machine and the rotordynamics of the rotor shaft assembly. Typically this problem is solved as a one-way coupling by the unidirectional load transfer method, which is based on predetermined or assumed EM or load profile. It ignores the two-way interactions, especially during a start-up transient. In this work, a coupled equivalent circuit method is applied to reflect this coupling, and the simulation results show the significance of the proposed method by the practical case study of Electric Submersible Pump (ESP) system. The generalized roughness bearing anomaly is linked to load torque ripples which can cause speed oscillations, while being related to current signature by phase modulation. Considering that the induced characteristic signature is usually subtle broadband changes in the current spectrum, this signature is easily affected by input power quality variations, machine manufacturing imperfections and the interaction of both. A signal segmentation technique is introduced to isolate the influence of these disturbances and improve the effectiveness of applying the ESA for this kind of bearing defects. Furthermore, an improved experimental procedure is employed to closely resemble naturally occurring degradation of bearing, while isolating the influence of shaft currents on the signature of bearing defects during the experiments. The results show that the proposed method is effective in analyzing the generalized roughness bearing anomaly in synchronous motors

Distributed simulation of power systems using real time digital simulator

The simulation of power system behavior, especially transient behavior, helps us in the analysis and planning of various power systems. However, power systems are usually highly complex and geographically distributed. Therefore system partitioning can be used to allow for sharing resources in simulation. In this work, distributed simulations of power system models have been developed using an electromagnetic transient simulator, namely Real Time Digital Simulator (RTDS). The goal is to demonstrate and assess the feasibility of both non-real-time and real-time simulations using the RTDS in a geographically distributed scenario. Different protocols and options used in the communication between power systems have been studied and analyzed. In this work, a test bed has been developed for data transfer between a power system simulated in RTDS at Mississippi State University and the power system simulated in RTDS at Texas A&M University. Different protocols, available for the interface and communication in the RTDS, have been studied and applied in this work. Finally, a locally distributed wide area control test bed was developed and simulated

Real-time Voltage Stability Monitoring and Control for Load Areas: A Hybrid Approach

This dissertation proposes a hybrid approach for real-time monitoring and controlling voltage stability of a load area fed by N tie lines. This hybrid approach integrates both simulation-based and measurement-based approaches for voltage stability assessment (VSA). First, for measurement-based VSA (MBVSA), a new method is proposed for monitoring and control of load areas, which adopts an N+1 buses equivalent system so as to model and monitor individual tie lines of a load area compared to a traditional MBVSA method adopting a Thevenin equivalent. For each tie line, the new method solves the power transfer limit against voltage instability analytically as a function of all parameters of that equivalent, which is online identified from real-time synchronized measurements on boundary buses of the load area. Thus, this new MBVSA method can directly calculate the real-time power transfer limit on each tie line. Second, in order to assess the voltage stability margins under an n-1 contingency, based on the proposed MBVSA method, two sensitivity analyses have been performed, which are respectively for the parameter sensitivity of the equivalent system and the sensitivity of the tie line flow under an n-1 contingency. Third, the proposed MBVSA method implemented for both the real-time condition and potential n-1 contingencies is integrated with the simulation-based VSA approach to form a hybrid approach. The MBVSA method helps reduce the computation burden by eliminating the unimportant contingencies while the simulation-based method provides accurate information for specific “what if” scenarios such as stability limit and margin indices under n-1 contingency conditions. In addition, simulation using the model of the system can provide recommendations for preventive control if potential voltage instability is identified. This proposed hybrid VSA approach has been validated on the NPCC (Northeast Power Coordinating Council) Large-scale Test Bed (LTB) system developed by the CURENT (Center for Ultra-Wide-Area Resilient Electric Energy Transmission Networks), and also implemented on the CURENT Hardware Test Bed (HTB) system. The effectiveness of the MBVSA in real-time monitoring and closed-loop control against voltage instability has been validated

Enhance OF SOGI-FLL and SOGI-PLL response to voltage sags and swells perturbations

The scope of this thesis is related to the enhancement of the operation of distributed generators (DGs) based on renewable energies (REs) when connected to the conventional grid network. Synchronization of those DGs, i.e. their front-end inverters, with the grid is critical for injecting power into the grid. Internal control loops of voltage source inverters (VSIs) monitor parameters such as the utility voltage's phase, amplitude, and frequency to achieve proper synchronization and inject power into the grid. Therefore, an appropriate estimation of the grid parameters is needed for completing the mentioned goals. The different existing monitoring techniques still face technical challenges when appearing faults on the grid, particularly voltage sags and voltage swells. From those techniques, SOGI-FLL and SOGI-PLL are widely spread in power inverters. In the mentioned faulty conditions, a degradation of the estimated parameters occurs due to the impact of the grid defaults on the dynamic performance of the SOGI-FLL and SOGI-PLL estimators. In the literature, it was not found contributions to those problems. This research aimed to contribute by providing a fast and accurate detection of the faults, yielding a robust dynamical response of the VSIs control structure in front of such perturbations, thanks to a fast and precise estimation of grid parameters. This faults detection strategy was accompanied by minimization of the monitored parameters drift due to inherent harmonic pollution of the faults. Then a first contribution has been the design and implementation of a Finite State Machine on a synchronization structure known as SOGI-PLL and in SOGI-FLL, enhancing their dynamical response regarding transient time and steady-state response in front of voltage sags. A second contribution has been implementing the strategy to face the problems derived from voltage swells with satisfactory results. The research was carried out in the E3PACS laboratory facilities of EEBE-UPC. This work is organized as follows: In Section 1, the study is contextualized. Section 2 describes Power Quality indexes benchmarking for assessing the problem to solve and the obtained results. It is then followed by Section 3, where the SOGI-FLL response against voltage sags and swells is analyzed. Therefore, Section 4 summarizes the different approaches and results for mitigating the grid faults studied. Section 5 is devoted to showing SOGI-PLL amelioration by applying SOGI Error-Based algorithm. Finally, Section 6 summarizes the main contributions of this work, along with the general conclusions. Section 7 lists future work, in Section 8 are listed the published JCR indexed papers.L'abast d'aquesta tesi està relacionat amb la millora del funcionament dels generadors distribuïts (DG, distributed generation en anglès) basats en energies renovables quan es connecten a la xarxa elèctrica convencional. La sincronització d'aquests DG amb la xarxa és fonamental per injectar energia a aquesta, especialment quan es tracta d'inversors de potència. Els llaços de control interns dels inversors de font de tensió (VSI, Voltage Source Inverters en anglès) controlen paràmetres com ara la fase, l'amplitud i la freqüència de la tensió de la xarxa per aconseguir una sincronitzaciórequerida. Per tant, es necessita una estimació adequada dels paràmetres del bus de tensió per assolir els objectius esmentats. Les diferents tècniques de monitorització existents encara s'enfronten a reptes tècnics a l'hora d'aparèixer fallades a la xarxa, especialment caigudes i augments abruptes de tensió ("Voltage sags" i "Voltage swells" en anglès, respectivament). En el cas dels inversors, les tècniques basades en estructures SOGI-FLL i SOGI-PLL estan àmpliament esteses i, en les esmentades condicions de fallada, es produeix una degradació dels paràmetres estimats a causa de l'impacte dels defectes de la xarxa en el seu comportament dinàmic . A la literatura, no es van trobar contribucions rellevants per mitigar aquests problemes. Aquesta investigació pretén doncs contribuir proporcionant una detecció ràpida i precisa de les fallades, donant una resposta dinàmica robusta de l'estructura de control dels VSI davant d'aquestes pertorbacions, gràcies a una estimació ràpida i precisa dels paràmetres de la xarxa. Aquesta estratègia de detecció de fallades s'acompanya de la minimització de la deriva dels paràmetres monitoritzats a causa de la contaminació harmònica inherent de les fallades. Així doncs, una primera contribució ha estat el disseny i la implementació d'una màquina d'estats finits en una estructura de sincronització coneguda com SOGI-PLL i en SOGI-FLL, millorant la seva resposta dinàmica pel que fa al temps transitori i la resposta en estat estacionari davant les caigudes de tensió. Una segona aportació ha estat la implementació de l'estratègia per afrontar amb resultats satisfactoris els problemes derivats de les pujades de tensió (voltage swells). La investigació es va dur a terme a les instal·lacions del laboratori E3PACS de l'EEBE-UPC. Aquest treball s'organitza de la següent manera: A l'apartat 1 es contextualitza l'estudi. La secció 2 descriu el la referenciació dels índexs de qualitat de l'energia per avaluar el problema a resoldre i els resultats obtinguts. A continuació, segueix la Secció 3, on s'analitza la resposta SOGI-FLL contra les caigudes i les inflors de tensió. Per tant, la Secció 4 resumeix els diferents enfocaments i resultats per mitigar les falles de la xarxa estudiades. La secció 5 està dedicada a mostrar la millora SOGI-PLL aplicant l'algorisme SOGI basat en errors. Finalment, l'apartat 6 resumeix les principals aportacions d'aquest treball, juntament amb les conclusions generals. La secció 7 enumera els treballs futurs, a la secció 8 s'enumeren els articles indexats JCR publicats.Postprint (published version