A Novel Phase-locked-loop and its Application in STATCOM System

Phase-Locked-Loop (PLL) plays important role in grid connected power electronics system. a good PLL system can detect the grid phase angle and frequency fast and accurately, and additionally it can extract the positive sequence exactly. in real applications, source signal (voltage or current) usually includes harmonic distortion, unbalanced components. Conventional PLL strategy cannot solve the unbalanced and harmonic distortion problems. Different PLL solutions are proposed in literature in recent years. the general considerations for these different approaches are to design positive sequence estimator to eliminate the negative sequence components and use filters to filter out the higher order harmonic distortions. in this paper, a novel moving average filter based synchronous reference frame PLL with positive sequence estimation feature is presented. the proposed PLL has good performances in filtering harmonic, eliminating unbalanced components. to verify the proposed PLL, a STATCOM system is built in Matlab/Sim PowerSystems. Simulation results of different operation modes are shown in this paper

Adaptive Quadrant Filter Based Phase Locked Loop System

Phase-Locked-Loop (PLL) is one of the key technologies extensively used in grid connected power electronics system. A good PLL system can detect the grid phase angle and frequency fast and accurately, and additionally it can extract the positive sequence (or fundamental component for single phase system) exactly. In real applications, source signal (voltage or current) sensed for PLL usually includes harmonic distortion, unbalanced components, noises and frequency variations. Conventional PLL strategy cannot solve all the problems, especially the unbalanced and harmonic distortion. There is a trade-off between the dynamic response and phase angle tracking accuracy. Different PLL solutions are proposed in literature in recent years. The general considerations for these different approaches are to design positive sequence estimator to eliminate the negative sequence components and use filters to filter out the higher order harmonic distortions from the PLLs. In this paper, an adaptive quadrature filter based synchronous reference frame PLL (SRF-PLL) with positive sequence estimation feature is presented. The proposed PLL has good performances in filtering harmonic, eliminating unbalanced components and auto-adjusting frequency change. The simulation model is built in Matlab/simulink and the simulation results are given to verify the mathematical analysis

Fokker-planck Equation Application to Analysis of a Simplified Wind Turbine Model

This paper presents a new method to evaluate the stochastic dynamic model of the wind turbine system using stochastic differential equations (SDE). the wind speed is described by the Rayleigh distribution which is constructed as the stationary solution of a one-dimensional nonlinear SDE. the dynamic model of the wind turbine system can be combined with this SDE of wind speed to expand to multi-dimensional stochastic differential equations. the time evolution of the probability density function of the system is described by the Fokker-Planck equation (FPE) which can be derived from the corresponding stochastic differential equations. the procedure is illustrated using a constant-speed wind turbine model with squirrel cage induction generator. © 2012 IEEE

Feedback Linearization Internal Control for the Unified Power Flow Controller

This paper presents feedback linearization-based controller design methodology for a unified power flow controller (UPFC) to achieve rapid reference signal tracking in the internal level. Feedback linearization control (FBLC) is a nonlinear technique based on differential geometry theory and overcomes the drawback of traditional PI control linearizing at one specific operating condition. FBLC is applied on UPFC dynamic model via an appropriate coordinate transformation and LQR control is then applied on the transferred linear system. the proposed control is validated on the IEEE 118-bus system with full-order generator and network models. ©2010 IEEE

Improving QC Relaxations of OPF Problems via Voltage Magnitude Difference Constraints and Envelopes for Trilinear Monomials

AC optimal power flow (AC~OPF) is a challenging non-convex optimization problem that plays a crucial role in power system operation and control. Recently developed convex relaxation techniques provide new insights regarding the global optimality of AC~OPF solutions. The quadratic convex (QC) relaxation is one promising approach that constructs convex envelopes around the trigonometric and product terms in the polar representation of the power flow equations. This paper proposes two methods for tightening the QC relaxation. The first method introduces new variables that represent the voltage magnitude differences between connected buses. Using "bound tightening" techniques, the bounds on the voltage magnitude difference variables can be significantly smaller than the bounds on the voltage magnitudes themselves, so constraints based on voltage magnitude differences can tighten the relaxation. Second, rather than a potentially weaker "nested McCormick" formulation, this paper applies "Meyer and Floudas" envelopes that yield the convex hull of the trilinear monomials formed by the product of the voltage magnitudes and trignometric terms in the polar form of the power flow equations. Comparison to a state-of-the-art QC implementation demonstrates the advantages of these improvements via smaller optimality gaps.Comment: 8 pages, 1 figur

An Automated Forward Operating Base Electrical Distribution System Simulator

In this paper, an automated forward operating base electrical distribution system simulator (ODCC) is developed. the ODCC has been developed based on the open platform distribution system simulator (DSS) that is able to create variable models of electrical systems and performing many types of analysis. © 2012 IEEE

Retention of Female Faculty Members

The recruitment and the retention of female undergraduate and graduate students into engineering courses is discussed. A similar challenge lies in recruiting female faculty member from the limited pool of candidates in several fields at most universities. It is found that about half the females who were hired did not stay at the university. It is suggested that programs should be introduced to encourage mentoring and career development as such improvements would benefit all faculty members both female and male

Distributed Grid Intelligence for Future Microgrid with Renewable Sources and Storage

The FREEDM microgrid is a smart grid solution with Distributed Grid Intelligence (DGI) to efficiently manage the power distribution and storage of renewable energy. within the FREEDM system, DGI applies distributed control method in a unique way to achieve feasible load balancing in microgrid by migrating power between renewable energy generation and storage at each node. This paper presents the key aspects in implementing such a scheme and outlines the preliminary results obtained by integrating the proposed methodology. the results demonstrate the potential benefits of adopting DGI control on a microgrid

Tightening QC Relaxations of AC Optimal Power Flow Problems via Complex Per Unit Normalization

Optimal power flow (OPF) is a key problem in power system operations. OPF problems that use the nonlinear AC power flow equations to accurately model the network physics have inherent challenges associated with non-convexity. To address these challenges, recent research has applied various convex relaxation approaches to OPF problems. The QC relaxation is a promising approach that convexifies the trigonometric and product terms in the OPF problem by enclosing these terms in convex envelopes. The accuracy of the QC relaxation strongly depends on the tightness of these envelopes. This paper presents two improvements to these envelopes. The first improvement leverages a polar representation of the branch admittances in addition to the rectangular representation used previously. The second improvement is based on a coordinate transformation via a complex per unit base power normalization that rotates the power flow equations. The trigonometric envelopes resulting from this rotation can be tighter than the corresponding envelopes in previous QC relaxation formulations. Using an empirical analysis with a variety of test cases, this paper suggests an appropriate value for the angle of the complex base power. Comparing the results with a state-of-the-art QC formulation reveals the advantages of the proposed improvements

Tightening QC Relaxations of AC Optimal Power Flow through Improved Linear Convex Envelopes

AC optimal power flow (AC OPF) is a fundamental problem in power system operations. Accurately modeling the network physics via the AC power flow equations makes AC OPF a challenging nonconvex problem. To search for global optima, recent research has developed a variety of convex relaxations that bound the optimal objective values of AC OPF problems. The well-known QC relaxation convexifies the AC OPF problem by enclosing the non-convex terms (trigonometric functions and products) within convex envelopes. The accuracy of this method strongly depends on the tightness of these envelopes. This paper proposes two improvements for tightening QC relaxations of OPF problems. We first consider a particular nonlinear function whose projections are the nonlinear expressions appearing in the polar representation of the power flow equations. We construct a convex envelope around this nonlinear function that takes the form of a polytope and then use projections of this envelope to obtain convex expressions for the nonlinear terms. Second, we use certain characteristics of the sine and cosine expressions along with the changes in their curvature to tighten this convex envelope. We also propose a coordinate transformation that rotates the power flow equations by an angle specific to each bus in order to obtain a tighter envelope. We demonstrate these improvements relative to a state-of-the-art QC relaxation implementation using the PGLib-OPF test cases. The results show improved optimality gaps in 68% of these cases