Electrical transient interaction between transformers and the power system: case study of an onshore wind farm

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfilment of the requirements of the degree of Master of Science in Electrical Engineering June 2016Through the Renewable Energy Independent Power Producer Procurement Program (REIPPPP) the South African government has awarded opportunities for growth of renewable energy through bidding rounds. Round 1 saw a total capacity of 397 MW being awarded to independent power producers (IPP). Subsequently Rounds 2, 3 and 4 each had a total capacity of 333 MW auctioned. The advent of renewables on the market has brought upon its own associated problems with regards to power quality issues and failure of HV equipment. This thesis will address transformer failures that occurred at an onshore wind farm. The nature of the transformer failures suggest transient overvoltages are mainly to blame. A comparison between transformer failures in South African and Brazil suggest a common failure mechanism. The failure starts with an inter-turn insulation failure which propagates to an inter-layer insulation. In worst cases the failure mode results in a puncture through the LV-HV barrier and punctures through the LV winding. An extensive literature review was performed to find appropriate methods to predict and explain the failure mode in wind turbine LV-MV step-up transformers. Of the different models which were reviewed the most notable was the Multi-conductor Transmission Line (MTL) model which was chosen as the preferred model due to its ability to predict the inter-turn/inter-layer voltage stresses. Verification of the developed MTL model by the author was then compared to published results of an MTL model of a disc winding transformer. The results of the comparison revealed a relatively good agreement between the developed model and the published model. The application of the MTL model to represent the voltage stresses in transformer windings was then extended to two specially constructed wind turbine step-up transformer prototypes. The prototypes differed in the winding arrangement of the MV coil. The other used two separate MV coils separated by an oil gap whereas the other had a single MV coil. To validate the model accuracy, a comparison of measured results versus those obtained analytically was done for the two prototypes. The analytical and measured results also had a relatively good agreement for the two prototypes considered. Measurement of switching surges was done on-site at the wind farm to understand the nature of the transients. Using analysis tools such as FFT and frequency domain severity factor it was possible to understand the impact the nature of these transients would pose on the transformer insulation. Different mitigation techniques which can be used to alleviate the transient overvoltages to within safe levels were investigated. The most notable protection device considered was the RL choke device which offered a significant reduction of the pre-strikes and is virtually transparent under power frequency operation.MT201

Identification and development of microgrids emergency control procedures

Tese de doutoramento. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 200

Modeling for harmonic analysis of ac offshore wind power plants

This Ph.D. dissertation presents the work carried out on the modeling, for harmonic analysis, of AC offshore wind power plants (OWPP). The studies presented in this Ph.D. thesis are oriented to two main aspects regarding the harmonic analysis of this type of power system. The first aspect is the modeling and validation of the main power components of an AC offshore wind power plant. Special emphasis is focused on the modeling of wind turbines, power transformers, submarine cables, and the interaction between them. A proposal of a wind turbine harmonic model is presented in this dissertation to represent the behavior of a wind turbine and its harmonics, up to 5 kHz. The distinctive structure of this model consists of implementing a voltage source containing both the fundamental component and the harmonics emitted by the converter. For the case of transformer and submarine cables, the frequency-dependent behavior of certain parameters is modeled for frequencies up to 5 kHz as well. The modeling of the frequency-dependent characteristics, due to skin and proximity effect, is achieved by means of Foster equivalent networks for time-domain simulations. Regarding the interaction between these power components, two complementary modeling approaches are presented. These are the Simulink®-based model and an analytical sequence network model of the passive components of the OWPP. A description of model development and parameterization is carried out for both modeling approaches considering a scenario that is defined according to a real offshore wind power plant. On the other hand, the second aspect of this Ph.D. thesis is oriented to the analysis of the issues that appear in offshore wind power plants in relation to harmonic amplification risk, compliance of grid codes in terms of harmonics and power factor, and the design of effective solutions to improve the harmonic emission of the facility. The technical solutions presented in this Ph.D. thesis cover aspects regarding modulation strategies, design of the connection filter of the grid side converter and management of the operation point of the grid side converter of wind turbines. This last by means of changing the setpoint of certain variables. As inferred, these are solutions from the perspective of the wind turbine manufacturer

Communications and control for electric power systems: Final report

This report is a summary of some of the work done on the Communications and Control project, with particular emphasis on the achievements during the years 1986--1996. During those years, the project moved away from concern with dispersed storage and generation and its impact on power system operation (the team was responsible for studies in this area, and for making a power system simulator that included DSG), and became involved in more concrete work aimed at applying high-tech solutions to problems of power system communications and control. This report covers work done at JPL on the following topics: (1) the measurement of electric and magnetic fields, both ac and dc; (2) the use of optical power to supply low-power electronics; (3) the design of a fault-tolerant communication system designed for distribution automation; and (4) a digital phase locked loop that allows the use of low-power transmitting electronics to recreate a good-quality signal at the receiver. In a report of this kind, only the results and highlights of the work are described

Underground distribution cable incipient fault diagnosis system

This dissertation presents a methodology for an efficient, non-destructive, and online incipient fault diagnosis system (IFDS) to detect underground cable incipient faults before they become catastrophic. The system provides vital information to help the operator with the decision-making process regarding the condition assessment of the underground cable. It incorporates advanced digital signal processing and pattern recognition methods to classify recorded data into designated classes. Additionally, the IFDS utilizes novel detection methodologies to detect when the cable is near failure. The classification functionality is achieved through employing an ensemble of rule-based and supervised classifiers. The Support Vector Machines, designed and used as a supervised classifier, was found to perform superior. In addition to the normalized energy features computed from wavelet packet analysis, two new features, namely Horizontal Severity Index, and Vertical Severity Index are defined and used in the classification problem. The detection functionality of the IFDS is achieved through incorporating a temporal severity measure and a detection method. The novel severity measure is based on the temporal analysis of arrival times of incipient abnormalities, which gives rise to a numeric index called the Global Severity Index (GSI). This index portrays the progressive degradation path of underground cable as catastrophic failure time approaches. The detection approach utilizes the numerical modeling capabilities of SOM as well as statistical change detection techniques. The natural logarithm of the chronologically ordered minimum modeling errors, computed from exposing feature vectors to a trained SOM, is used as the detection index. Three modified change detection algorithms, namely Cumulative Sum, Exponentially Weighted Moving Averages, and Generalized Likelihood Ratio, are introduced and applied to this application. These algorithms determine the change point or near failure time of cable from the instantaneous values of the detection index. Performance studies using field recorded data were conducted at three warning levels to assess the capability of the IFDS in predicting the faults that actually occurred in the monitored underground cable. The IFDS presents a high classification rate and satisfactory detection capability at each warning level. Specifically, it demonstrates that at least one detection technique successfully provides an early warning that a fault is imminent

Loss allocation in a distribution system with distributed generation units

In Denmark, a large part of the electricity is produced by wind turbines and combined heat and power plants (CHPs). Most of them are connected to the network through distribution systems. This paper presents a new algorithm for allocation of the losses in a distribution system with distributed generation. The algorithm is based on a reduced impedance matrix of the network and current injections from loads and production units. With the algorithm, the effect of the covariance between production and consumption can be evaluated. To verify the theoretical results, a model of the distribution system in Brønderslev in Northern Jutland, including measurement data, has been studied