Y. LI ET AL.

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Figure 9. Sound pressure distribution.

Table 2. Sound pressure level of different dc magnetic bias

level (db)

dc

I/A 100/Hz 200/Hz 300/Hz 400/Hz 500/Hz

0 52.88 76.51 69.3 46.3 60.3

0.6 62.21 86.17 79.7 58.6 69.5

1.13 65.8 85.68 76.3 67.4 69.3

2.27 65.2 86.74 82.2 58.7 67.1

4.54 70.85 94.68 85.4 70.7 78.5

6.8 76.53 99.69 98.2 75.6 67.5

9.08 77.18 100.70 90.7 78.0 82.4

Audible noise is generated due to the vibration of the

transformer. DC bias current flowing through the neu-

trals of ac power transformers with the neutral grounded,

will force the transformer to generate more leak magnetic

flux. Therefore, the core and windings would vibrate

strongly due to the magnetostriction and the electrody-

namics effects. The audible noise level of ac power

transformers neighboring the dc grounding electrode

increase with the increase of the DC current.

In this paper, the characteristics of structural acoustic

radiation are investigated by the indirect boundary ele-

ment method. In order to simulate the ground, symmetry

plane is added in the bottom of transformer core. The

plane is regarded as rigid plane and has no normal accel-

eration, then the noise is reflected entirely. This model

can simulate noise caused by core accurately.

The basic frequency of transformer noise is twice as

high as the source frequency, videlice t 100 Hz. Ther e are

also high frequency noise integer times of basic fre-

quency. Study show that, low-frequency noise is a large

percent in the noise spectrum of frequencies. Therefore,

the noise be l o w 500 Hz wa s taken into account.

As seen in Table 2, noise mainly concentrates in the

frequency of 100、200、300 Hz. The noise level increases

with the increase of the DC current. The noise level is

76.51 dB when there is no DC current and 100.71 dB

when D C current rise to 9 . 08 A.

5. Conclusions

Based on field-circuit coupled finite element method, this

paper analyzes the DC magnetic bias's impact on ex citin g

current, exciting curren t harmonic, magnetic flux density

and noise of transformer.

1) With the increasing of DC current, the exciting cur-

rent wave get distortion, and the distortion become seri-

ous with the increase of DC current. When the DC cur-

rent is 9.08 A, the amplitude in positive half-cycle is

35.1944 A, in neg ative half-cycle is -0.0025 A. After DC

current exceeds 4.54 A, the exciting current change

slowly with the DC current growth. The main reason is

that ferromagnetic material has been highly saturated.

2) It is approximately linear relationship between each

exciting current harmonics and DC Bias current. Lower-

order harmonic is influenced by DC Magnetic Bias sen-

sitively.

3) With the increase of DC magnetic bias current, the

flux density in air way changed from 3.664 to 10.84 mT,

has increased by 197 percent. It is shown that DC mag-

netic bias makes leakage magnetic field rise seriously.

4) The audible noise level of transformer increase with

the increase of the DC magnetic bias current. When the

DC magnetic bias current change from 0 to 9.08 A, the

amplitude of the sound increase from 76.51 dB to 100.70

dB.

REFERENCES

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Measures of Restraining DC Current through Transformer

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[2] M. Lahtinen and J. Elovaara. “GIC Occurrences and GIC

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doi:10.1109/61.997938

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Large Power Transformers,” Shenyang: Shenyang Uni-

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