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In evaluating the energy-generation potential of a photovoltaic (PV) energy system, the system is usually assumed to work without interruptions over its entire life. PV energy systems are fairly reliable, but as any complex system, they may fail. In PV systems, the inverter is responsible for the majority of failures, and most inverter failures are blamed on the aluminum electrolytic capacitors typically used in the dc bus. This paper investigates the effects of common failure modes on the reliability of PV inverters and suggests a model framework for decomposing the inverter into subsystems for more detailed study. The challenges of statistical analysis based on small data sets are discussed, and simulations are performed to illustrate the proposed model using a simple decomposition into subsystems of the inverter used in the 342-kW PV system at the Georgia Tech Aquatic Center. Index Terms--Photovoltaic (PV) power systems, power-system reliability, power-system simulation, reliability modeling, simulation.

This paper introduces a concept of a generator coherency-based System Protection Scheme (SPS), addressing interarea oscillation events. The scheme uses wide-area generator speed measurements combined with discrete Fourier transform (DFT) analysis to determine coherency. A test case simulated in the NPCC 68-bus system is used to introduce the scheme's operating principle. The test case is also used to investigate the scheme suitability for different well-established digital signal processing (DSP) methods. Further aspects, such as evolving/sliding window functions, window size, and frequency resolution are analyzed. For the detection of interarea modes, parametric DSP methods turned out to be the most feasible. In general, evolving windows are faster than sliding windows. However, evolving windows require a trigger and were therefore rejected for the design proposed here. A lower frequency resolution may shorten the response time at the expense of the quality of the estimates of the instantaneous speed deviation and the generator coherency. The design proposed here is based on a high-frequency resolution at the cost of a lower response time. Index Terms--Coherency determination, digital signal processing, discrete Fourier transform analysis, interarea oscillations, system protection scheme, transient instability, wide-area protection.

This paper proposes a strategy for placing capacitors at multiple locations on a distribution feeder to allow: 1) deeper levels of substation voltage reduction for peak load reduction; 2) power factor correction; and 3) power loss reduction. By reducing peak demand, a utility can avoid paying high prices when purchasing power or it may sell excess generation at high prices. By minimizing system losses, savings are obtained through reduced demand and energy charges. Besides a positive economic response, load reduction associated with improved power factor at the substation has a beneficial effect on voltage stability by increasing the system stability limit margin. Index Terms--Capacitor placement, load reduction, nondominated sorting genetic algorithm (NSGA), voltage reduction.

The control of a nonlinear system with delayed inputs and measurements is illustrated on the example of transient excitation control of a generator. Through simulated studies, the direct feedback linearization approach is found to yield unsatisfactory results in the presence of large measurement processing delays. An effective way to compensate for the delay using prediction is demonstrated. Keywords: Phasor Measurements, Time Delay, Real-Time Generator Control, Prediction.

Three new techniques for frequency measurement are proposed in the paper. The first is a modified zero crossing method using curve fitting of voltage samples. The second method is based on polynomial fitting of the DFT quasi-stationary phasor data for calculation of the rate of change of the positive sequence phase angle. The third method operates on a complex signal obtained by the standard technique of quadrature demodulation. All three methods are characterized by immunity to reasonable amounts of noise and harmonics in power systems. The performance of the proposed techniques is illustrated on several scenarios by computer simulation.