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HBFEM for a DC-Biased Problem in High-Voltage Power Transformers

DC-Biased Problem in HVDC Transformers

A typical HVDC transmission system is illustrated in Figure 3.10. It consists of a DC transmission line connecting two AC systems. A converter at one end of the line converts AC power into HVDC power, while a similar converter at the other end reconverts the DC power into AC power. Therefore, one converter acts as a rectifier, the other as an inverter. The basic purpose of the converter transformer on the rectifier side is to transform the AC network voltage to yield the AC voltage required by the converter [13]. Three-phase transformers, connected in either wye-wye or wye- delta, are used.

The magnetostrictive strain is not truly sinusoidal in character, which leads to the introduction of the harmonics. With a DC-biased transformer, magnetizing saturation will also cause some harmonics [14]. The B-H curve with hysteresis characteristics and a DC biased condition is illustrated in Figure 3.11, and the expression of H(B) can be obtained from a B-H curve data table.

The harmonics in a transformer noise may have a substantial effect on an observer, even though their level is 10 dB or more lower than the 100 Hz fundamental. In fact, the most striking point is the strength of the component at 100 Hz, or twice the normal operating frequency of the transformer. Deviation from a “square-law” magnetostric- tive characteristic would result in even harmonics (at 200,400,600 Hz, etc.), while the different values of the magnetostrictive strain for increasing and decreasing flux densities - a pseudo-hysteresis effect - lead to the introduction of odd harmonics (at 300, 500, 700 Hz, etc.). If any part of the structure has a natural frequency at or near 100, 200, 300, 400 Hz, and so on, the result will be an amplification of noise at that particular frequency.

 
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