Power system security enhancement by HVDC links using a closed-loop emergency control

In recent years, guaranteeing that large-scale interconnected systems operate safely, stably and economically has become a major and emergency issue. A number of high profile blackouts caused by cascading outages have focused attention on this issue. Embedded HVDC (High Voltage Direct Current) links within a larger AC power system are known to act as a “firewall” against cascading disturbances and therefore, can effectively contribute in preventing blackouts. A good example is the 2003 blackout in USA and Canada, where the Québec grid was not affected due to its HVDC interconnection. In the literature, many works have studied the impact of HVDC on the power system stability, but very few examples exist in the area of its impact on the system security. This paper presents a control strategy for HVDC systems to increase their contribution to system security. A real-time closed-loop control scheme is used to modulate the DC power of HVDC links to alleviate AC system overloads and improve system security. Simulations carried out on a simplified model of the Hydro-Québec network show that the proposed method works well and can greatly improve system security during emergency situations.Peer reviewedFinal Accepted Versio

Heuristic optimization of supplementary controller for VSC-HVDC/AC interconnected grids considering PLL

This paper presents a new approach to enhance the dynamic responses of AGC in power systems by means of heuristic optimization of VSC-HVDC supplementary controllers. The upcoming power electronics-based VSC-HVDC transmission systems offer new features that would be advantageous for improving the frequency control and thus for enhancing the stability of the transmission grids. In this paper, the parameters of the proposed control modulation are tuned using Genetic Algorithm and Simulated Annealing methods. The performance of the proposed intelligent based tuning approach is assessed through MATLAB simulations for an AC/DC interconnected system. For the sake of detailed analysis, the effects of PLL and frequency measurements are also included in the VSC-HVDC system modeling. Furthermore, to show merits of the proposed strategy, a comparison between AC and DC transmissions is presented.Peer ReviewedPostprint (author's final draft

A Committee Machine with Intelligent Systems for Estimation of Total Organic Carbon Content from Petrophysical Data: an Example from Kangan and Dalan Reservoirs in South Pars Gas Field, Iran

Total Organic Carbon (TOC) content present in reservoir rocks is one of the important parameters which could be used for evaluation of residual production potential and geochemical characterization of hydrocarbon bearing units. In general, organic rich rocks are characterized by higher porosity, higher sonic transit time, lower density, higher gamma-ray, and higher resistivity than other rocks. Current study suggests an improved and optimal model for TOC estimation by integration of intelligent systems and the concept of committee machine with an example from Kangan and Dalan Formations, in South Pars Gas Field, Iran. This committee machine with intelligent systems (CMIS) combines the results of TOC predicted from intelligent systems including fuzzy logic (FL), neuro-fuzzy (NF), and neural network (NN), each of them has a weight factor showing its contribution in overall prediction. The optimal combination of weights is derived by a genetic algorithm (GA). This method is illustrated using a case study. One hundred twenty-four data points including petrophysical data and measured TOC from three wells of South Pars Gas Field were divided into eighty-seven training sets to build the CMIS model and thirty-seven testing sets to evaluate the reliability of the developed model. The results show that the CMIS performs better than any one of the individual intelligent systems acting alone for predicting TOC

A committee neural network for prediction of normalized oil content from well log data: An example from South Pars Gas Field, Persian Gulf

Normalized oil content (NOC) is an important geochemical factor for identifyingpotential pay zones in hydrocarbon source rocks. The present study proposes an optimaland improved model to make a quantitative and qualitative correlation between NOC andwell log responses by integration of neural network training algorithms and thecommittee machine concept. This committee machine with training algorithms (CMTA)combines Levenberg-Marquardt (LM), Bayesian regularization (BR), gradient descent(GD), one step secant (OSS), and resilient back-propagation (RP) algorithms. Each ofthese algorithms has a weight factor showing its contribution in overall prediction. Theoptimal combination of the weights is derived by a genetic algorithm. The method isillustrated using a case study. For this purpose, 231 data composed of well log data andmeasured NOC from three wells of South Pars Gas Field were clustered into 194modeling dataset and 37 testing samples for evaluating reliability of the models. Theresults of this study show that the CMTA provides more reliable and acceptable resultsthan each of the individual neural networks differing in training algorithms. Also CMTAcan accurately identify production pay zones (PPZs) from well logs

Networked microgrid energy management based on supervised and unsupervised learning clustering

Networked microgrid (NMG) is a novel conceptual paradigm that can bring multiple advantages to the distributed system. Increasing renewable energy utilization, reliability and efficiency of system operation and flexibility of energy sharing amongst several microgrids (MGs) are some specific privileges of NMG. In this paper, residential MGs, commercial MGs, and industrial MGs are considered as a community of NMG. The loads’ profiles are split into multiple sections to evaluate the maximum load demand (MLD). Based on the optimal operation of each MG, the operating reserve (OR) of the MGs is calculated for each section. Then, the self-organizing map as a supervised and a k-means algorithm as an unsupervised learning clustering method is utilized to cluster the MGs and effective energy-sharing. The clustering is based on the maximum load demand of MGs and the operating reserve of dispatchable energy sources, and the goal is to provide a more efficient system with high reliability. Eventually, the performance of this energy management and its benefits to the whole system is surveyed effectively. The proposed energy management system offers a more reliable system due to the possibility of reserved energy for MGs in case of power outage variation or shortage of power.Peer ReviewedPostprint (published version

Convertidores de potencia para microrredes y sistemas de generación distribuidos

This paper presents an overview and critical discussion about the utilization of power converters in several microgrid configurations that incorporate non-conventional renewable energy sources and energy storage. The methodology is developed over 69 works published in this research topic. The papers are selected from databases in electrical engineering, e.g., IEEExplore, ScienceDirect, Springer, MDPI, etc. Then, the papers are classified depending on its focus, i.e., power converters in microgrids or power converters in distribution systems. At least, three classifications are proposed and one of them is made over more than 40 papers about power converters used in microgrids and electric distribution systems. Given the wide variety of microgrids and their configurations, the selection of appropriate power converters for every scenario is not trivial; therefore, this work also classifies the converters in their most common application, their advantages and disadvantages, and also point out the study domain, i.e., simulation or physical implementation. One of the main conclusions made from the overview is a gap identified in the study of direct current/ direct current microgrids despite being the simplest configuration among the three analyzed configurations. This is because hybrid and alternate current microgrids are more widely used since they allow taking advantage of the infrastructure of the current electrical systems.Este artículo presenta una visión general y una discusión crítica sobre la utilización de convertidores de potencia en varias configuraciones de microrredes que incorporan fuentes de energía renovable no convencionales y almacenamiento de energía. La metodología se desarrolla sobre 69 trabajos publicados en este tema de investigación. Los documentos se seleccionan de bases de datos en ingeniería eléctrica, p. ej. IEEExplore, ScienceDirect, Springer, MDPI, etc. Luego, los artículos se clasifican según su enfoque, es decir, convertidores de potencia en microrredes o convertidores de potencia en sistemas de distribución. Se proponen al menos tres clasificaciones y una de ellas se realiza sobre más de 40 artículos sobre convertidores de potencia utilizados en microrredes y sistemas de distribución eléctrica. Dada la gran variedad de microrredes y sus configuraciones, la selección de convertidores de potencia apropiados para cada escenario no es trivial; por lo tanto, este trabajo también clasifica a los convertidores en su aplicación más común, sus ventajas y desventajas, y también señala el dominio de estudio, es decir, simulación o implementación física. Una de las principales conclusiones extraídas de la visión general es una brecha identificada en el estudio de las microrredes de corriente continua / corriente continua a pesar de ser la configuración más simple entre las tres configuraciones analizadas. Esto se debe a que las microrredes híbridas y de corriente alterna son las más utilizadas ya que permiten aprovechar la infraestructura de los sistemas eléctricos actuales

Synchronous Machine Emulation of Vsc for Interconnection of Renewable Energy Sources through Hvdc Transmission

The majority of the energy demand over the past years has been fulfilled by centralized generating stations. However, with a continuously increasing energy demand, the integration of decentralized renewable energy sources (RES) into the power system network becomes inevitable even though these sources affect the stability of the grid due to their intermittency and use of various power converters. The transmission of power over long distances from RES is usually accomplished either by AC or DC transmission. High voltage DC transmission (HVDC) is preferred over high voltage AC transmission (HVAC) due to numerous and complex reasons, such as its lower investment cost for long transmission cables, lower losses, controllability, and limited short circuit currents. Several control methods for grid-connected voltage source converters (VSCs), such as power-angle and vector-current controls, are being adopted in RES interconnections. However, these methods face several issues when used for a weak grid interconnection. This thesis develops a control strategy for a VSC–HVDC transmission system by referring to the synchronverter concept. In the proposed method, the sending-end rectifier controls emulate a synchronous motor (SM), whereas the receiving end inverter emulates a synchronous generator (SG) to transmit power from one grid to another. The two converters connected by a DC line provide a synchronverter HVDC (SHVDC) link. Given the high demand for sustainable energy, integrating RES—which can be extended to wind-based resources—into the long-haul HVDC link becomes essential. Therefore, in this thesis, a windfarm with a type 4 permanent magnet SG is integrated into the HVDC link through a rectifier. Depending on the wind speed, the proposed control strategy automatically shares and manages the wind generator power on the DC side by using a battery energy storage system (BESS) connected to the HVDC link to stabilize the power fluctuations generated by the intermittency of the wind farm. The performance of the synchronverter-based HVDC transmission was verified by using a MATLAB Simulink model. Results show that the controller can effectively control the power flow from one grid to another and that the effect of wind fluctuation on the grid can be mitigated by introducing a BESS at the DC link. Therefore, by properly controlling the SHVDC, BESS, and RES connected to the HVDC system, the power from remote RES can be connected to a weak AC grid in a stable manner

Application of Holomorphic Embedding to the Power-Flow Problem

abstract: With the power system being increasingly operated near its limits, there is an increasing need for a power-flow (PF) solution devoid of convergence issues. Traditional iterative methods are extremely initial-estimate dependent and not guaranteed to converge to the required solution. Holomorphic Embedding (HE) is a novel non-iterative procedure for solving the PF problem. While the theory behind a restricted version of the method is well rooted in complex analysis, holomorphic functions and algebraic curves, the practical implementation of the method requires going beyond the published details and involves numerical issues related to Taylor's series expansion, Padé approximants, convolution and solving linear matrix equations. The HE power flow was developed by a non-electrical engineer with language that is foreign to most engineers. One purpose of this document to describe the approach using electric-power engineering parlance and provide an understanding rooted in electric power concepts. This understanding of the methodology is gained by applying the approach to a two-bus dc PF problem and then gradually from moving from this simple two-bus dc PF problem to the general ac PF case. Software to implement the HE method was developed using MATLAB and numerical tests were carried out on small and medium sized systems to validate the approach. Implementation of different analytic continuation techniques is included and their relevance in applications such as evaluating the voltage solution and estimating the bifurcation point (BP) is discussed. The ability of the HE method to trace the PV curve of the system is identified.Dissertation/ThesisMasters Thesis Electrical Engineering 201

Harmonic Characteristics and Voltage Support for Inverters With a Weak AC Supply

In recent years, the world-wide use of high voltage direct current (HVDC) power transmission has increased sharply. Most of the recent projects involve transmitting large amounts of power from remote generations to load centers, where the short-circuit capacity at the receiving bus is often only a few times that of the rated DC power transfer — indicative of a weak AC support that is more susceptible to various operational problems, such as harmonic amplification, poor voltage regulation and adverse effects on the firing control. This report is on a two-part investigation. The first part deals with the harmonics characteristics of an inverter with weak AC support under balanced and unbalanced network conditions using the two most common firing schemes; the individual pulse control and the equidistant pulse control. The second part examines the operational behavior of the inverter with weak AC support when supplementary VAR support in the form of static VAR compensator is used. Aside from dynamic and transient behaviors, the study also looked into possible interaction between the inverter and the thyristor-controlled reactor at the harmonic level