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Electrical Power Systems Quality, Second Edition phần 5

Do có độ dài dẫn, hãm không phải lúc nào cũng hiệu quả chống lại xung dốc. Đề án trinh sát thực tế loại bỏ những từ cáp. Ăn phân phối tại các khu vực đông dân cư sẽ có các chương trình do thám theo mặc định. Có một số lượng đầy đủ của máy biến áp đã có hãm ở hai bên của cực riser. 4,6 | Fundamentals of Harmonics 210 Chapter Five range of power system equipment most notably capacitors transformers and motors causing additional losses overheating and overloading. These harmonic currents can also cause interference with telecommunication lines and errors in power metering. Sections 5.10.1 through 5.10.5 discuss impacts of harmonic distortion on various power system components. 5.10.1 Impact on capacitors Problems involving harmonics often show up at capacitor banks first. As discussed in Secs. 5.9.3 and 5.9.4 a capacitor bank experiences high voltage distortion during resonance. The current flowing in the capacitor bank is also significantly large and rich in a monotonic harmonic. Figure 5.32 shows a current waveform of a capacitor bank in resonance with the system at the 11th harmonic. The harmonic current shows up distinctly resulting in a waveform that is essentially the 11th harmonic riding on top of the fundamental frequency. This current waveform typically indicates that the system is in resonance and a capacitor bank is involved. In such a resonance condition the rms current is typically higher than the capacitor rms current rating. IEEE Standard for Shunt Power Capacitors IEEE Standard 181992 specifies the following continuous capacitor ratings 135 percent of nameplate kvar 110 percent of rated rms voltage including harmonics but excluding transients 180 percent of rated rms current including fundamental and harmonic current 120 percent of peak voltage including harmonics Table 5.1 summarizes an example capacitor evaluation using a computer spreadsheet that is designed to help evaluate the various capacitor duties against the standards. The fundamental full-load current for the 1200-kvar capacitor bank is determined from Ic . var t -12 . 50.2 A v3 X kV-LL 3 X 13.8 The capacitor is subjected principally to two harmonics the fifth and the seventh. The voltage distortion consists of 4 percent fifth and 3 percent seventh. This results in 20 .