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POWER QUALITY phần 6

Hài có thể ảnh hưởng đến biến áp chủ yếu theo hai cách. Sóng hài điện áp sản xuất thiệt hại bổ sung trong lõi biến áp điện áp tần số hài hòa cao hơn thiết lập vòng trễ, chồng lên trên tuyến cơ bản | 4.8.1 Transformers Harmonics can affect transformers primarily in two ways. Voltage harmonics produce additional losses in the transformer core as the higher frequency harmonic voltages set up hysteresis loops which superimpose on the fundamental loop. Each loop represents higher magnetization power requirements and higher core losses. A second and a more serious effect of harmonics is due to harmonic frequency currents in the transformer windings. The harmonic currents increase the net RMS current flowing in the transformer windings which results in additional I2R losses. Winding eddy current losses are also increased. Winding eddy currents are circulating currents induced in the conductors by the leakage magnetic flux. Eddy current concentrations are higher at the ends of the windings due to the crowding effect of the leakage magnetic field at the coil extremities. The winding eddy current losses increase as the square of the harmonic current and the square of the frequency of the current. Thus the eddy loss EC is proportional to Ih2 X h2 where Ih is the RMS value of the harmonic current of order h and h is the harmonic frequency order or number. Eddy currents due to harmonics can significantly increase the transformer winding temperature. Transformers that are required to supply large nonlinear loads must be derated to handle the harmonics. This derating factor is based on the percentage of the harmonic currents in the load and the rated winding eddy current losses. One method by which transformers may be rated for suitability to handle harmonic loads is by k factor ratings. The k factor is equal to the sum of the square of the harmonic frequency currents expressed as a ratio of the total RMS current multiplied by the square of the harmonic frequency numbers .2 .2 2 .2 2 .2 .2 .2 .2 2 k I2 1 12 2 12 3 12 4 . 12 n 4.25 where 11 is the ratio between the fundamental current and the total RMS current. 12 is the ratio between the second harmonic current and the total