Application of Unified Power Flow Controller to Improve the Performance of Wind Energy Conversion System

This research introduces the unified power flow controller (UPFC) as a means to improve the overall performance of wind energy conversion system (WECS) through the development of an appropriate control algorithm. Also, application of the proposed UPFC control algorithm has been extended in this research to overcome some problems associated with the internal faults associated with WECS- voltage source converter (VSC), such as miss-fire, fire-through and dc-link faults

The Impact of Harmonics on the Power Cable Stress Grading System

With the continuous growth of non-linear power electronic components and the increasing penetration of the distributed generation (DG), the potential for degradation in the power quality of the existing grid exists. There are concerns that the total harmonic distortion (THD) could reach unacceptable levels of 5% or higher. Moreover, there is additional potential of the presence of amplified harmonic components in the power network grid when the harmonic frequencies align with the resonant frequencies that are being injected by power electronic components of the DG. The above conditions could increase the electrical stresses on the insulation system of the power system components, and in particular, cable terminations are a concern. Standard cable terminations are designed to operate under power frequency in the power system network and their service life is considered accordingly. The research work aims to provide an understanding of the performance of the stress grading (SG) system of a commercial cable termination when the voltage waveform is distorted due to the presence of harmonics and when the high frequency and high dV/dt voltage waveforms are present from a typical power electronic drive. An aging experiment was performed for over a 600 hour time period using the pulse width modulated (PWM) high-voltage generator to quantify the impact of high frequency stress on SG system of cable termination. Furthermore, the cable termination was tested under power frequency, distorted voltage waveforms composed of fundamental and low order harmonics using an experiment setup that generate distorted voltage waveforms. Diagnostic techniques such as surface potential distribution measurements and surface temperature monitoring are used to analyze the termination performance. The surface tangential field is calculated based on the gradient of the termination surface potential as measured with an electrostatic voltmeter. The study shows that distorted voltage waveforms with high frequency and high dV/dt components, increase the electric field, resistive heating, and surface temperature rise in the terminations that use the field-dependent SG materials. The rise of electric field by as high as 27.1% and surface temperature rise of as high as 17C demonstrates the severity on the cable terminations. Such electric field enhancements for a period of time have a potential to initiate partial discharge that could lead to degradation of the termination. Moreover, surface temperature rise of 17 deg C could reduce the allowable ampacity of the cable conductor, reduce the short circuit levels, and reduce the feeder loading limits. The field-dependent electrical conductivity (σ(E,T)), permittivity (ε), and the temperature dependencies of (σ(E,T) and ε) have strong impact to degrade the electrical and thermal properties of the termination due to stress from the non-sinusoidal distorted voltage waveform. In order to minimize the surface temperature rise from the hotspot and electrical stress enhancement that eventually lead to insulation degradation and failure, the following recommendations are made for a suitable SG design for a termination to handle the severe voltage stress: Apply the capacitively graded termination in the grid where the distortion levels are low. Under the increased total harmonic distortion levels and HF components, resistively grading with higher degree of nonlinearity (achieved through the use of ZnO filler) is beneficial. The utilities could take preventive maintenance on medium voltage power cable accessories to prevent the termination failure before it actually occurs. Researchers could focus to resolve and minimize the rising power quality issues when the distribution generations are operated, improve the power electronic converters, and provide cost-effective harmonic filter solutions for harmonic mitigation

Microgrid Enabling Towards the Implementation of Smart Grids

Smart grids have emerged as dominant platforms for effectively accommodating high penetration of renewable-based distributed generation (DG) and electric vehicles (EVs). These smart paradigms play a pivotal role in the advancement of distribution systems and pave the way for active distribution networks (ADNs). However, the large number of smart meters deployed in the distribution system (e.g., 200 million smart meters will be installed in Europe by 2020) represents one of the main challenges facing the management and control of distribution networks and thus the enabling of smart grids. In addition to the data tsunami flooding central controllers, the concerns about privacy and system vulnerability are fast becoming a key restraint for the implementation of the smart grids. These concerns are prompting utilities to be more reluctant to adopt new techniques, leaving the distribution system mired in relatively old-fashioned routines. Microgrids provide an ideal paradigm to form smart grids, thanks to their limited size and ability to ‘island’ when supplying most of their loads during emergencies, which improves system reliability. However, preserving load-generation balance is comprehensively challenging, given that microgrids are dominated by renewable-based DGs, which are characterized by their probabilistic nature and intermittent power. Although microgrids are now well-established and have been extensively studied, there is still some debate over having microgrids that are solely ac or solely dc, with the consensus tending toward hybrid ac-dc microgrids. Furthermore, while some research has addressed using solely ac microgrids, the planning of hybrid ac-dc microgrids has not yet been investigated, despite the many benefits these types of microgrids offer. Additionally, developing steady-state analysis tools capable of handling grid-connected mode and islanded mode for the operation of ac microgrids and hybrid ac-dc microgrids still has uncertainties about their computational burden, complexity, and convergence. The high R/X ratio characterized distribution systems result in ill-condition that hinders the convergence of conventional Newton Raphson (NR) techniques. Moreover, calculating the inversion of the Jacobian matrix that is formed from the calculation of derivatives adds to the complexity of these techniques. Therefore, developing a simple, accurate, and fast steady-state analysis tool is crucial for enabling microgrids and hence smart grids. Driven by the aforementioned challenges, the broad goal of this thesis is to enable microgrids as building clusters to smooth and accelerate the realization of smart grids. Achieving this objective involves a number of stages, as follows: 1) The development of probabilistic models for loads and renewable DG-based output power. These models are then integrated with the load flow analysis techniques to form a probabilistic power flow (PPF) tool. 2) The proposal of a novel operational v philosophy that divides existing bulky grids into manageable clusters of self-adequate microgrids that adapt their boundaries to keep load-generation balance at different operating scenarios. 3) The proposal of planning a framework for the newly constructed grids as hybrid ac-dc microgrids with minimum levelized investment costs and consideration of the probabilistic nature of load and renewable generation. 4) The development of a branch-based power flow algorithm for steady-state analysis of ac microgrids and hybrid ac-dc microgrids