Effect of Variable Speed Wind Turbine Generator on Stability of a Weak Grid

In this paper, we illustrate the effect of adding a hypothetical 100-MW doubly fed induction generator (DFIG) wind power plant to a weak transmission system. The effects of various wind plant load factors (100, 60 and 25% of nameplate rating) are investigated. System performance is compared to a 100-MW conventional synchronous generator interconnected at the same location. The conventional generator is installed some distance away. The simulations demonstrated that DFIG generators provide a good damping performance under these conditions. These results support the conclusion that modern wind power plants, equipped with power electronics and low-voltage ride-through capability, can be interconnected to weak power grids without reducing stability. To conduct the studies, we selected an area of the Western Electricity Coordinating Council power system that is electrically far from major generation centers and is weakly connected to the bulk transmission system. The area contains large motor loads. We observed the dynamic response of large motors in the vicinity, especially their ability to ride through fault events. The studies were conducted using positive sequence phasor time-domain analysis     

Induction Machine Phase Balancing by Unsymmetrical Thyristor Voltage Control

High inrush currents and unbalanced supply voltages are two of the major causes for failure of polyphase induction machines. While thyristor voltage controllers, sometimes called static starters, are in widespread use to limit damaging inrush currents, little attention has been paid to the use of these controllers to balance the phases voltages of the machine. The technical feasibility of unsymmetric control to limit unbalanced currents is explored. A substantial reduction in losses is shown to be possible for two specific types of unbalance.     

A cell-to-module-to-array detailed model for photovoltaic panels

This paper presents a modified current–voltage relationship for the single-diode model. The single-diode model has been derived from the well-known equivalent circuit for a single photovoltaic (PV) cell. A cell is defined as the semiconductor device that converts sunlight into electricity. A PV module refers to a number of cells connected in series and in a PV array, modules are connected in series and in parallel. The modification presented in this paper accounts for both parallel and series connections in an array. Derivation of the modified current–voltage relationships begins with a single solar cell and is expanded to a PV module and finally an array. Development of the modified current–voltage relationship was based on a five-parameter model, which requires data typically available from the manufacturer. The model accurately predicts voltage–current (V–I) curves, power–voltage (P–V) curves, maximum power point values, short-circuit current and open-circuit voltage across a range of irradiation levels and cell temperatures. The versatility of the model lies in its accurate prediction of the aforementioned criteria for panels of different types, including monocrystalline and polycrystalline silicon. The model is flexible in the sense that it can be applied to PV arrays of any size, as well as in simulation programs such as EMTDC/PSCAD and MatLab/Simulink. Accuracy of the model was validated through a series of experiments performed outdoors for different configurations of a PV array.     

Zero sequence method for energy recovery from a variable-speed windturbine generator

An innovative power conversion system to convert energy from a variable-frequency wind-powered induction generator to a fixed frequency output is presented. A standard six-switch DC link current regulated pulse width modulated (CRPWM) inverter is simultaneously modulated with two current components. A three-phase balanced current component at the induction generator's optimum operating frequency transfers energy from the generator to the converter. A single phase zero sequence current component at a fixed 60 Hz frequency transfers energy from the converter through a zero sequence filter to the load. Unity power factor output is shown both in simulation and experiment, though any arbitrary power factor output may be readily commanded. Maximum power capture for a variable-speed wind turbine is achieved using proven control techniques. This method uses only half of the active power switching devices of conventional conversion methods. Simulation and experimental verification are shown     

Review of Marine Hydrokinetic Power Generation and Power Plant

Abstract

Marine hydrokinetic power generation is a relatively new type of renewable generation. Its predecessors, such as wind power generation, hydropower plant generation, geothermal generation, photovoltaic generation, and solar thermal generation, have gained a lot of attention because of their successful implementation. The successful integration of renewable generation into the electric power grid has energized the power system global communities to take the lessons learned, innovations, and market structure to focus on the large potential of marine hydrokinetic to also contribute to the pool of renewable energy generation. This article covers the broad spectrum of marine hydrokinetic generation. The state of the art of power take-off will be discussed. Types of electrical generators will be presented, and the options for implementation will also be presented.     

Unified calculation of eigen-solutions in power systems based on matrix perturbation theory

Calculation of eigen-solutions plays an important role in the small signal stability analysis of power systems.In this paper,a novel approach based on matrix perturbation theory is proposed for the calculation of eigen-solutions in a perturbed system.Rigorous theoretical analysis is conducted on the solution of distinct,multiple,and close eigen-solutions,respectively,under perturbations of parameters.The computational flowchart of the unified solution of eigen-solutions is then proposed,aimed toward obtaining eigen-solutions of a perturbed system directly with algebraic formulas without solving an eigenvalue problem repeatedly.Finally,the effectiveness of the matrix perturbation based approach for eigen-solutions’calculation in power systems is verified by numerical examples on a two-area four-machine system.     

Axial-flux modular permanent-magnet generator with a toroidalwinding for wind-turbine applications

Permanent-magnet (PM) generators have been used for wind turbines for many years. Many small wind-turbine manufacturers use direct-drive PM generators. For wind turbine-generators, the design philosophy must cover the following characteristics: low cost, light weight, low speed, high torque and variable-speed generation. The generator is easy to manufacture and the design can be scaled up for a larger size without major retooling. A modular PM generator with axial flux direction was chosen. The permanent magnet used is NdFeB or ferrite magnet with flux guide to focus flux density in the air gap. Each unit module of the generator may consist of one, two, or more phases. Each generator can be expanded to two or more unit modules. Each unit module is built from simple modular poles. The stator winding is formed like a torus. Thus, the assembly process is simplified and the winding insertion in the slot is less tedious. The authors built a prototype of one unit module and performed preliminary tests in their laboratory. Follow-up tests will be conducted in their laboratory to improve the design     

PV water pumping with a peak-power tracker using a simple six-stepsquare-wave inverter

The application of photovoltaics (PVs) has been increasingly popular, especially in remote areas, where power from a utility is not available or is too costly to install. PV-powered water pumping is frequently used for agriculture and in households. Among many available schemes, the system under study consists of a PV array, a variable-frequency inverter, an induction motor, and a water pump. The inverter feeds the induction motor, which drives the water pump. To seek the optimum power output of the PV array, the inverter is operated at variable frequency, to vary the output of the water pump. The inverter is operated to generate a six-step quasi-square wave, instead of a pulsewidth modulated (PWM) voltage output, to reduce the switching losses. The inverter acts as both a variable-frequency source and a peak-power tracker of the system, thus, having the number of switches minimized. The system is a low-cost design, with a simple control strategy. The DC bus is supported by a DC capacitor; thus, a balance-of-power flow must be maintained to avoid the collapse of the DC-bus voltage. Another advantage of the system is that the current is limited to an upper limit of the PV-array current. Thus, in case a short circuit is developed, the motor winding and the power semiconductor switches can be protected against excessive current flow     

Design and performance of a digitally based voltage controller forcorrecting phase unbalance in induction machines

The case of unbalanced supply presents a uniquely difficult problem in motor applications since its causes are varied. In many situations a static starter utilizing back-to-back thyristors in series with the motor lines is already installed for the purpose of soft starting or for power factor improvement. An attractive low-cost solution for phase unbalance is simply to modify the control strategy of the static starter by adjusting the firing angles of the three thyristors pairs independently. In this manner, the series connected thyristors serve the function of unsymmetrical, variable supply impedances which can be used to balance the voltage across the motor phases. A new digital controller utilizing a static starter for balancing the phase currents of an induction motor operating with unbalanced supply is presented. Experimental results show a marked reduction in the current unbalance from approximately 40% to a few percent and clearly demonstrate the attractiveness of the new phase balancer