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Electrical Power Systems Quality, Second Edition phần 6

Ferroresonance Ferroresonance quản lý trong một hệ thống phân phối xảy ra chủ yếu khi tải nhẹ biến áp ba pha trở nên bị cô lập trên một cáp với một hoặc hai giai đoạn mở. Điều này có thể xảy ra cả vô tình và cố ý. Chiến lược để đối phó với | Applied Harmonics Applied Harmonics 263 Figure 6.23 An example of a C filter where the maximum harmonic current allowed to flow in the system is 10 30 and 50 percent at the tuned harmonic order of 5.5. Figure 6.24 A C filter with and without a notch filter. Downloaded from Digital Engineering Library @ McGraw-Hill www.digitalengineeringlibrary.com Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any Use is subject to the Terms of Use as given at the website. Applied Harmonics 264 Chapter Six ters can work independently of the system impedance characteristics. Thus they can be used in very difficult circumstances where passive filters cannot operate successfully because of parallel resonance problems. They can also address more than one harmonic at a time and combat other power quality problems such as flicker. They are particularly useful for large distorting loads fed from relatively weak points on the power system. The basic idea is to replace the portion of the sine wave that is missing in the current in a nonlinear load. Figure 6.25 illustrates the concept. An electronic control monitors the line voltage and or current switching the power electronics very precisely to track the load current or voltage and force it to be sinusoidal. As shown there are two fundamental approaches one that uses an inductor to store current to be injected into the system at the appropriate instant and one that uses a capacitor. Therefore while the load current is distorted to the extent demanded by the nonlinear load the current seen by the system is much more sinusoidal. Active filters can typically be programmed to correct for the power factor as well as harmonics. 6.6 Harmonic Filter Design A Case Study This section illustrates a procedure for designing harmonic filters for industrial applications. This procedure can also be used to convert an existing power factor correction capacitor into a harmonic filter. As described in Sec. 4.1.2 power factor correction .