Effects of DC Magnetic Bias on the Magnetic and Sound Fields of Transformer

doi:10.4236/epe.2013.54B209

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Energy and Power Engineering, 2013, 5, 1097-1100

doi:10.4236/epe.2013.54B209 Published Online July 2013 (http://www.scirp.org/journal/epe)

Effects of DC Magnetic Bias on the Magnetic and Sound

Fields of Transformer

Yan Li, Yanchao gao, Longnv Li, Di Zhang, Fangxu Han

Research Institute of Special Electrical Machines, Shenyang University of Technology, Shenyang, China

Email: [email protected]

Received March, 2013

ABSTRACT

DC bias current flowing into the neutral point of power transformer will seriously affect the normal operation of AC

power system. In this paper, exciting current, harmonic component of the excitation current, magnetic flux density and

noise of transformer were analyzed when the transformer is in the no-load operation state based on field-circuit coupled

method. Through the calculation and analysis, some reference bases are provided for design of transformer.

Keywords: DC Magnetic Bias; Exciting Current; Harmonic; Magnetic Flux Density; Noise

1. Introduction

As the implementation of the "transmit the electricity

from west to east, north and south to send each other,

nationwide network" strategy of China's power grid,

high-voltage direct current transmission (HVDC) tech-

nology [1-2], which is suitable for high power and long-

distance transmission, has been used more and more

widely. Under unipolar-earth way, the DC into the earth

will be large up to 3 kA [3]. It causes the uneven distri-

bution of surface potential. Thus there is potential dif-

ference between different grounds which makes some of

the DC flows from one transformer end and sides out

from the other end. Field tests show that only a small DC

flows through the transformer neutral point will seriously

harm or even affect the normal operation of alternat-

ing-current power system. Therefore, it is necessary to

pay more attention to the problem of transformer DC

magnetic bias.

Taking a single-phase transformer as an example, on

the basis of field-circuit coupled finite element method,

this paper simulates the DC magnetic bias's impact on

exciting current, exciting current harmonic, magnetic

flux density and noise of transformer when the trans-

former is switched to the zero-loaded state. It will be of a

great significance for the further study of controlling DC

magnetic bias.

2. Model of Transformer under DC

Magnetic Bias

2.1. Finite Element Modeling

A single-phase transformer with a parameter of 240

MVA was chosen as an example. In order to analyze the

DC magnetic bias's impact on transformer, field-circuit

coupled finite element method was used and the finite

element model is shown in Figure 1. The field domain

was shown on the left side of Figure 1, and the circuit

domain was on the right.

2.2. Calculation on Excitation Current

Zero-loaded is an operating state, it refers to disconnect

in the secondary side and rated voltage in the primary

side. The exciting current was consisted of core loss cur-

rent

(active current) and magnetizing current I



(reactive current), which is shown in Figure 2.

(%) 10

(%) ()

eFe ff

qG NqA















(1)

*This work was supported by NSFC, under Project 51177103 and

Pro

ram for LNIRT in Universit

(

LT2011002

)

. Figure 1. Field-circuit coupled finite element model.

Y. LI ET AL.

1098

Figure 2. Phase diagram of transformer on zero-load.

In formula (1), 0 is the zero-load loss, P

S is the

rated capacity.

e is the magnetization capacity of unit

mass, q



is the magnetization capacity of unit area at

seam crossing,

e is the total weight of the core, G

is the net area at seam crossing and

N is the Number

of joints;

0(%)(%) (%)

III





(2)

3. Magnetic Field Characteristics under DC

Magnetic Bias

3.1. Exciting Current of Different DC Magnetic

Bias Level

The relation between allowable DC current and rated

current was defined in the reference 4. The allowable DC

current was 0.3% rated current in single-phase trans-

former, 0.5% rated current in three-phase five-limb trans-

former and 0.7% rated current in three-phase three- limb

transformer [4]. The allowable DC current of single-

phase three-limb transformer in this paper is 2.27 A. The

DC current applied in this simulation is 0.6 A、1.13 A、

2.27 A、4.54 A、6.81 A、9.08 A (0.07%、0.15%、0.3%、

0.6%、0.9%、1.2% rated current res pecti vel y ). Chan gi n g

rule of exciting current is calculated when different DC

current flows through the winding. Results are shown in

Table 1.

Table 1. Exciting current peak of different dc magnetic bias

level.

DC Current/A Peak of Positive

Half-cycle/A Peak of Negative

Half-cycle/A

0 1.51 -1.51

0.6 7.93 -0.30

1.13 13.1 -0.25

2.27 22.1 -0.16

4.54 27.1 0.11

6.8 32.5 -0.05

9.08 35.2 -0.002

With the increasing of DC current, the exciting curren t

wave get distortion, and the distortion become serious

with the increase of DC current. The peaked wave of the

positive half cycle is more obvious, and the amplitude of

the wave increase. The negative half-cycle is approxi-

mately flat-topped wave, and the amplitude tends to zero.

When the DC current is 9.08 A, the amplitude in positive

half-cycle is 35.1944 A, in negative 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. Therefore, changing the magnetic characteris-

tics of ferromagnetic material is a method of inhibiting

DC bias.

3.2. Analysis of Exciting Current

Based on formula 3, exciting current waveform's Fourier

transform is analyzed. Results are shown in Figures 3-4.

()sin( )

itIIn tn









 

 (3)

Figure 3. Change curves of exciting currents' harmonic

ratios along with DC current.

Figure 4. Change curves of exciting currents' harmonic

components along with DC current.

Y. LI ET AL. 1099

As shown in Figure 3, with the increasing of DC cur-

rent, the ratio of each exciting current harmonic to fun-

damental exciting current increase rapidly, and the sec-

ond exciting current harmonic is the most obvious. When

the DC current is 2.27A, the increment speed of current

harmonic ratios begin stable.

The corresponding relation between each exciting

current harmonics and DC Bias current is shown in Fig-

ure 4. The DC magnetic bias's impact on each exciting

current harmonics are basically identical, it is approxi-

mately linear relationship. Lower-order harmonic is in-

fluenced by DC Magnetic Bias sensitively, higher-order

harmonic is less affected.

3.3. DC Magnetic Bias's Impact on Flux Density

The calculated position of flux density is shown in Fig-

ure 5. Position A is in the halfway of core window

height. Position B is the centre line of iron beam. Posi-

tion C is in the middle of the two windings.

Transformer operates in inflection point of B-H curv e.

Core model in this paper is 30 RGH120, th e linear region

is 1.7-1.75 T. As shown in figures 6-8, when a curren t in

DC nature in the transformer exists, with the growth of

DC current, the magnetic flux density increases signifi-

cantly. The Maximum flux density on the iron beam is

changed from 1.8347 T to 1.94994 T, on ferrite yoke is

changed from 1.707 T to 1.866T. With the increasing of

DC current, the magnetic flux density increases, but the

amplitude is narrow. Because of the influence of DC

current, ferromagnetic material is easy to get saturated.

The flux density in air way increases significantly. It

changed from 3.664 to 10.84 mT. It is shown that DC

magnetic bias makes leakage magnetic field rise seri-

ously.

4. Acoustical Characteristic of Core under

DC Magnetic Bias

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.

Figure 5. Calculated position of flux density.

Figure 6. Flux density of position A.

Figure 7. Flux density of position B.

Figure 8. Flux density of position C.

Y. LI ET AL.

1100

Figure 9. Sound pressure distribution.

Table 2. Sound pressure level of different dc magnetic bias

level (db)

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

[1] Y.Y. Zhu, W. P. Jiang and Z. H. Zeng, “Studying on

Measures of Restraining DC Current through Transformer

Neutrals,” Proceedings of the CSEE, Vol. 5, No. 13, 2005,

pp. 300-305.

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

test 400kV System Transformer,” IEEE Transaction on

Power Delivery, Vol. 17, No. 2, 2002, pp. 555-561.

doi:10.1109/61.997938

[3] Y. Y. Yao, “Research on the DC Bias Phenomena of

Large Power Transformers,” Shenyang: Shenyang Uni-

versity of Technology, 2000. pp. 415-420.

[4] S. Yuan and T. S. Wang. “Summary of the Research on

Transformer DC Magnetic Bias,” High Voltage Appara-

tus, Vol. 46, No. 3, 2010, pp. 83-87.