Chika O. Nwankpa

Bio: Chika O. Nwankpa is an academic researcher from Drexel University. The author has contributed to research in topics: Electric power system & Emulation. The author has an hindex of 24, co-authored 154 publications receiving 2597 citations.

A new type of STATCOM based on cascading voltage source inverters with phase-shifted unipolar SPWM

Abstract: In this paper, a new type of static compensator (STATCOM) is proposed. This new STATCOM is constructed by cascading several identical full-bridge (H bridge) voltage-source inverters (VSIs). A so-called phase-shifted sinusoidal pulsewidth modulation (SPWM) unipolar voltage switching scheme is applied to control the switching devices of each VSI. The harmonics in STATCOM current caused by the DC voltage ripple is rejected by a new method developed in this paper. As a result, the size of inductor and DC capacitors can be further reduced. A very effective startup procedure is proposed to start up the STATCOM. The proposed STATCOM has the advantage of a fewer number of VSIs, the VSIs being identical and extremely fast in response to reactive power change.

An Exact Method for Computing Delay Margin for Stability of Load Frequency Control Systems With Constant Communication Delays

Abstract: The extensive usage of open communication networks in power system control causes inevitable time delays. This paper studies impacts of such delays on the stability of one-area and two-area load frequency control (LFC) systems and proposes an analytical method to determine delay margins, the upper bound on the delay for stability. The proposed method first eliminates transcendental terms in characteristic equation of LFC systems without making any approximation and transforms the transcendental characteristic equation into a regular polynomial. The key result of the elimination process is that real roots of the new polynomial correspond to imaginary roots of the transcendental characteristic equation. With the help of new polynomial, it is also possible to determine the delay-dependency of system stability and root tendency with respect to the time delay. An analytical formula is then developed to compute delay margins in terms of system parameters. For a large set of controller gains, delay margins of LFC systems are computed to investigate the qualitative effect of controller gains on the delay margin. Finally, simulations studies are carried out to verify the effectiveness of the proposed method.

Available transfer capability and first order sensitivity

Abstract: A method of calculating available transfer capability and the exploration of the first order effects of certain power system network variables are described. The Federal Energy Regulatory Commission has ordered that bulk electrical control areas must provide to market participants a "commercially viable" network transfer capability for the import, export and throughput of energy. A practical method for deriving this transfer capability utilizing both linear and nonlinear power flow analysis methods is developed that acknowledges both thermal and voltage system limitations. The available transfer capability is the incremental transfer capability derived by the method reduced by margins. A procedure for quantifying the first order effect of network uncertainties such as load forecast error and simultaneous transfers on the calculated transfer capability of a power system snapshot are explored. The quantification of these network uncertainties can provide information necessary for system operation, planning and energy market participation.

Local bifurcation in power systems: theory, computation, and application

Abstract: This paper provides an of overview of local bifurcation theory and its application to power system voltage stability analysis. The qualitative behavior of power system dynamics as modeled by differential-algebraic equations is discussed, followed by a summary of the concepts and tools for the analysis of local bifurcation from equilibria. Computational methods for locating and classifying bifurcation points as they have been applied in power system analysis are reviewed. Several examples are given.

Induction motor tests using MATLAB/Simulink and their integration into undergraduate electric machinery courses

Abstract: This work describes MATLAB/Simulink implementation of three induction motor tests, namely dc, no-load, and blocked-rotor tests performed to identify equivalent circuit parameters. These simulation models are developed to support and enhance electric machinery education at the undergraduate level. The proposed tests have been successfully integrated into electric machinery courses at Drexel University, Philadelphia, PA, and Nigde University, Nigde, Turkey.

Multilevel inverters: a survey of topologies, controls, and applications

Abstract: Multilevel inverter technology has emerged recently as a very important alternative in the area of high-power medium-voltage energy control. This paper presents the most important topologies like diode-clamped inverter (neutral-point clamped), capacitor-clamped (flying capacitor), and cascaded multicell with separate DC sources. Emerging topologies like asymmetric hybrid cells and soft-switched multilevel inverters are also discussed. This paper also presents the most relevant control and modulation methods developed for this family of converters: multilevel sinusoidal pulsewidth modulation, multilevel selective harmonic elimination, and space-vector modulation. Special attention is dedicated to the latest and more relevant applications of these converters such as laminators, conveyor belts, and unified power-flow controllers. The need of an active front end at the input side for those inverters supplying regenerative loads is also discussed, and the circuit topology options are also presented. Finally, the peripherally developing areas such as high-voltage high-power devices and optical sensors and other opportunities for future development are addressed.

Multi-level conversion : high voltage choppers and voltage-source inverters

Abstract: The authors discuss high-voltage power conversion. Conventional series connection and three-level voltage source inverter techniques are reviewed and compared. A novel versatile multilevel commutation cell is introduced: it is shown that this topology is safer and more simple to control, and delivers purer output waveforms. The authors show how this technique can be applied to either choppers or voltage-source inverters and generalized to any number of switches.<>

Medium-Voltage Multilevel Converters—State of the Art, Challenges, and Requirements in Industrial Applications

Abstract: This paper gives an overview of medium-voltage (MV) multilevel converters with a focus on achieving minimum harmonic distortion and high efficiency at low switching frequency operation. Increasing the power rating by minimizing switching frequency while still maintaining reasonable power quality is an important requirement and a persistent challenge for the industry. Existing solutions are discussed and analyzed based on their topologies, limitations, and control techniques. As a preferred option for future research and application, an inverter configuration based on three-level building blocks to generate five-level voltage waveforms is suggested. This paper shows that such an inverter may be operated at a very low switching frequency to achieve minimum on-state and dynamic device losses for highly efficient MV drive applications while maintaining low harmonic distortion.

Multicarrier PWM strategies for multilevel inverters

Abstract: Analytical solutions of pulsewidth-modulation (PWM) strategies for multilevel inverters are used to identify that alternative phase opposition disposition PWM for diode-clamped inverters produces the same harmonic performance as phase-shifted carrier PWM for cascaded inverters, and hybrid PWM for hybrid inverters, when the carrier frequencies are set to achieve the same number of inverter switch transitions over each fundamental cycle. Using this understanding, a PWM method is then developed for cascaded and hybrid inverters to achieve the same harmonic gains as phase disposition PWM achieves for diode-clamped inverters. Theoretical and experimental results are presented in the paper.

A review of wind energy technologies

Abstract: Energy is an essential ingredient of socio-economic development and economic growth. Renewable energy sources like wind energy is indigenous and can help in reducing the dependency on fossil fuels. Wind is the indirect form of solar energy and is always being replenished by the sun. Wind is caused by differential heating of the earth's surface by the sun. It has been estimated that roughly 10 million MW of energy are continuously available in the earth's wind. Wind energy provides a variable and environmental friendly option and national energy security at a time when decreasing global reserves of fossil fuels threatens the long-term sustainability of global economy. This paper reviews the wind resources assessment models, site selection models and aerodynamic models including wake effect. The different existing performance and reliability evaluation models, various problems related to wind turbine components (blade, gearbox, generator and transformer) and grid for wind energy system have been discussed. This paper also reviews different techniques and loads for design, control systems and economics of wind energy conversion system.