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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. Index Terms--Low voltage ride through, power system, renewable energy, stability, variable-speed generation, weak grid, wind energy, wind farm, wind power plant, wind turbine.

Renewable-energy systems that are under development need a simulation-based analysis to ensure system stability, power quality, and reliability. Such an analysis may reveal design modifications that need to be made before the system is implemented in the field. The modular simulator RPM-Sim, available on the National Renewable Energy Laboratory Web site (http://wind.nrel.gov/designcodes/simulators/rpmsim/), facilitates a low-cost application-specific study of the dynamics of the wind-solar-diesel hybrid power systems. This paper discusses the principal modules of the simulator and provides several examples of a simulation-based analysis of the renewable-energy systems. These examples illustrate the importance of a simulation-based study of the grid-connected and stand-alone or autonomous systems. The RPM-Sim is open-ended and can be easily expanded. Index Terms--Modeling and simulation of power systems, peak-power tracker (PPT), renewable-energy systems, stiff grid, water pumping, weak grid, wind-farm aggregation.

We analyzed a power system network that consisted of two types of power generation: wind turbine generation and diesel generation. The power quality and the interaction of diesel generation, the wind turbine, and the local load were the subjects of investigation. From an energy-production standpoint, producing as much wind energy as possible is advantageous in terms of decreasing the fuel consumption of the diesel engines and reducing the level of pollution. From the customer point of view, good power quality at the receiving end is important. The purpose of this paper is to show the impact of the wind power plant on the entire system. In addition, we discuss how the startup of the wind turbine and the transient condition during load changes affect the system's voltage and frequency. Index Terms--Diesel generator, hybrid power system, power quality, renewable energy, wind turbine.

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. We built a prototype of one unit module and performed preliminary tests in our laboratory. Follow-up tests will be conducted in our laboratory to improve the design. Index Terms - Axial flux, modular generator, permanent magnet, wind energy.

To optimize the operation of a wind turbine, variable turbine speeds are desirable. To determine the advantages of a variable-speed turbine, the annualized energy production of the turbine needs to be considered. This is done by using a Rayleigh probability distribution to determine the number of hours of a particular wind speed which occur in a given year. This distribution is used with the aerodynamic power generated at a given average wind speed to determine the total energy generated during the year. The total energy for fixed- and variable-speed turbine control are compared. The variable-speed turbines show improved energy production over the constant-speed systems. Index Terms - Variable speed, wind energy, wind turbine.

The application of photovoltaics (PV's) 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. Index Terms - Peak-power tracker, photovoltaic, PV array, renewable energy, village electrification, water pumping.

This paper proposes a new series compensated induction generator/battery supply topology which provides a constant voltage and frequency at the terminals, allowing minimum current harmonic distortion while at the same time providing a source and sink of real and reactive power. With appropriate control of the reactive power, the speed of the generator is allowed to vary within a relatively wide range. This technique can be further expanded by applying ac capacitors in parallel with the load to lessen the burden of the PWM inverter. Both simulation and experimental results are in agreement with the theory which leads to the conclusion that the system could be feasible for isolated power generation systems (wind, hydro, diesel, or hybrid generation).

The most common practice for starting a single-phase induction machine (SPIM) is to install a starting capacitor in series with the auxiliary winding. In some applications, two capacitors are used. One is used during the starting period to help create the starting torque. The other one is used during the running condition to improve efficiency. This paper discusses the possibility of using an electronic switch in parallel with the running capacitor, thereby providing the equivalent of a starting capacitor. The capacitor is shorted during each cycle to vary the effective size of the ac capacitor. By using this method, only one capacitor is used for both the starting and running condition, and a similar starting performance can be obtained when compared with the conventional method using two capacitors.

An innovative power conversion system to convert energy from a variable-frequency wind-powered induction generator to a fixed frequency output. 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. Index Terms--DC-AC power conversion, induction generators, inverters, power conversion, wind energy, wind power generation.

This paper covers the operation of variable-speed wind turbines with pitch control. The system we considered is controlled to generate maximum energy while minimizing loads. The maximization of energy was only carried out on a static basis and only drive train loads were considered as a constraint. In low to medium wind speeds, the generator and the power converter control the wind turbine to capture maximum energy from the wind. In the high-wind-speed regions, the wind turbine is controlled to maintain the aerodynamic power produced by the wind turbine. Two methods to adjust the aerodynamic power were investigated: pitch control and generator load control, both of which are employed to regulate the operation of the wind turbine. Our analysis and simulation show that the wind turbine can be operated at its optimum energy capture while minimizing the load on the wind turbine for a wide range of wind speeds. Index Terms--Pitch controlled, renewable energy, variable speed, wind turbine generator.