This paper deals with the improvement of the transient response and harmonic, subharmonic, and dc-offset voltage rejection capability of a grid voltage sequence detection scheme based on a second-order generalized integrator (SOGI). To perform that, the SOGI structure is first analyzed in deep, emphasizing both its tradeoff limits between settling time and harmonic attenuation and the sensitivity to grid subharmonics and dc-offset voltage. Then, a study of the effect of grid voltage harmonics and subharmonics in SOGI and in the SOGI-FLL and MSOGI-FLL structures is introduced. Hence, to overcome these problems, a new structure based on the use of the SOGI filter as prefilter for the previous structures is proposed to achieve a faster time response and higher harmonic rejection. This structure is used in a sequence detection scheme for the detection of the grid voltage components in the αβ-frame and it is applied in a three-phase PV system. Experimental and comparative results are shown to validate this proposal.

An Adaptive Prefiltering Method to Improve the Speed/Accuracy Tradeoff of Voltage Sequence Detection Methods Under Adverse Grid Conditions

A review of photovoltaic systems: Design, operation and maintenance

The fundamental voltage, current, and phase angle are required for a wide variety of power system applications. An algorithm that is capable of calculating or estimating these quantities in real time, in the presence of distorted waveforms, finds application in diverse areas of power systems. Techniques to detect voltage sag include the root mean square (rms), Fourier transform, and peak voltage detection methods. The problem with these methods is that they use a windowing technique and can therefore be too slow when applied to detect voltage sags for mitigation since they use historical data. Recent work in the field of signal processing has led to an algorithm that can extract a single non-stationary sinusoidal signal out of a given multi-component input signal. The algorithm is capable of estimating the amplitude, phase and frequency. In this paper, the algorithm is compared to existing methods of sag detection

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A New Method of Voltage Sag and Swell Detection

This paper proposes a control strategy for a VSC-HVDC for improving the quality of power supply to industrial plants The new idea of this control strategy is to give high priority to keep up the ac voltage and slightly decrease the frequency during disturbances The motivation for choosing this strategy is that the sensitive processing industries are much more sensitive to voltage drops as compared with frequency deviations In order to test this control strategy, the dynamic performance of a VSC-HVDC supplied industrial plant is investigated during sudden load disturbances, balanced and unbalanced faults from the grid Simulation results obtained using PSCAD/EMTDC show that with the use of the proposed control strategy, the system can ride through disturbances such as motor starting and faults by slightly decreasing the frequency The current limit of the VSC-HVDC converters has a significant influence on the dynamics of the system An increase of the current limit significantly improves the power quality of the system The comparison of the behavior between a pure ac supplied industrial plant and a VSC-HVDC supplied industrial plant clearly shows the capability of the VSC-HVDC to mitigate voltage dips during faults

A New Control Strategy of a VSC–HVDC System for High-Quality Supply of Industrial Plants

It is expected that, in future, utility service providers will enforce more strict power factor and harmonic standards. Therefore, efforts are being made so that power electronic system or any other load drawing reactive volt-amperes is made to appear as linear load drawing current in-phase with the utility voltage. One of the solutions towards this end is to employ active power filters (APF). This paper presents a control technique for unified power quality conditioner (UPQC), which is a combination of series APF and shunt APF. A control strategy based on unit vector template generation is discussed in this paper with the focus on the mitigation of voltage harmonics present in the utility voltage. Simulation results based on Simpowersystem (SPS) of Matlab/Simulink are presented to verify the effectiveness of the proposed control technique.

A simple new control technique for unified power quality conditioner (UPQC)

Flicker is a power quality problem that affects our daily lives. In this paper, the authors describe how voltage fluctuations may originate in the power system, but most frequently they are generated by the equipment or load connected to it, for example, arc furnaces, welders, etc.

Flicker Sources and Mitigation

Thunderstorms and lightning are large contributors to fault occurrence due to the fact that the spark stroke is repeated several times during a very short time interval. Lightning bolts are not the only origin for repetitive faults. Reclosers and recloser circuit breakers are among several types of cyclic loads that submit the system to a similar current pattern. The connected equipment may be able to withstand the first voltage sag but will drop out with the following one, if it arrives before the equipment reaches a stable situation or if the equipment self-recovers. The concept of recovering time was originally presented ten years ago without further development. The Computer Business Equipment Manufacturing Association (CBEMA) curve is the most widespread power acceptability or equipment sensitivity graph; however, its values are given in a one-event base so they cannot be applied for repetitive sag studies. The undervoltage part of the CBEMA curve follows very closely the expression /spl int/ /spl upsi//sup 2/ dt=k, which allows the extension of this criterion to repetitive sags. Results obtained are in good agreement with those previously published.

Voltage sag and recovery time in repetitive events

This letter proposes a gaming model with coalition among producers in subgroups for the market setting with multiple producers. The possible number of coalitions is obtained for a total number of N(N/spl ges/3) producers. The generic spatial gaming model with coalition has a vector as its objective function, with each vector element representing a coalition profit. Various constraints are formulated, including the spatial constraints (e.g., the DC power flow equations). The Cournot gaming strategy is adopted. A small system is used for testing the model with satisfactory results.

Induction Motor Behavior under Short Interruptions and Voltage Sags

Early power equipment was designed to withstand disturbances such as lightning, short circuits, and sudden overloads without extra expenditure. Current power electronics (PE) prices would be much higher if the equipment was designed with the same robustness. Pollution has been introduced into power systems by nonlinear loads such as transformers and saturated coils; however, perturbation rate has never reached the present levels. Due to its nonlinear characteristics and fast switching, PE create most of the pollution issues. This article shows how PE and electric power quality (PQ) are related in almost every aspect of commercial, domestic, and industrial application.

Electric power quality-the strong connection with power electronics

Voltage sags cause more than 80% of the industrial customer complaints today, which signifies economic losses averaging $10,000.00 US per event. Electric utilities and the industry blame each other for the voltage sag generation. It is very difficult to identify precisely the originating point of voltage sag generation — with minimum equipment data provided — due to investment restrictions. In many cases the conflict has been solved in court. Considering the extra expenses involved, a methodology for the precise location of voltage-sag origin would be very welcome. The difficulty is due mainly to the wide variety of voltage sag causes and characteristics. In this paper, a problem is presented in detail, showing several actual cases and identifications of industry internal and external causes. Some typical waveforms are analyzed, from which an important insight can be obtained about the origin of voltage sags. A discussion is given about other proposed identification methodologies, their accuracy and possible improvements.

J. C. Gomez

Papers

Flicker Sources and Mitigation

Voltage sag and recovery time in repetitive events

Induction Motor Behavior under Short Interruptions and Voltage Sags

Electric power quality-the strong connection with power electronics

A novel methodology to locate originating points of voltage sags in electric power systems