Effects of DC bias on magnetic performance of high grades grain-oriented silicon steels

Journal of Magnetism and Magnetic Materials 426 (2017) 575–579

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Effects of DC bias on magnetic performance of high grades grain-oriented silicon steels

MARK

⁎

Guang Maa, Ling Chenga, , Licheng Lub, Fuyao Yanga, Xin Chena, Chengzhi Zhuc a b c

Global Energy Interconnection Research Institute, State Key Laboratory of Advanced Transmission Technology,Beijing 102211, China State Grid Corporation of China, Beijing 100031, China State Grid Zhejiang Electric Power Company, Hangzhou 310007, China

A R T I C L E I N F O

A BS T RAC T

Keywords: HVDC transmission Grain-oriented silicon steels DC-biased magnetization Magnetic performance

When high voltage direct current (HVDC) transmission adopting mono-polar ground return operation mode or unbalanced bipolar operation mode, the invasion of DC current into neutral point of alternating current (AC) transformer will cause core saturation, temperature increasing, and vibration acceleration. Based on the MPG200D soft magnetic measurement system, the influence of DC bias on magnetic performance of 0.23 mm and 0.27 mm series (P1.7=0.70–1.05 W/kg, B8 > 1.89 T) grain-oriented (GO) silicon steels under condition of AC / DC hybrid excitation were systematically realized in this paper. For the high magnetic induction GO steels (core losses are the same), greater thickness can lead to stronger ability of resisting DC bias, and the reasons for it were analyzed. Finally, the magnetostriction and A-weighted magnetostriction velocity level of GO steel under DC biased magnetization were researched.

1. Introduction With the construction and operation of ± 500 kV, ± 800 kV, and ± 1100 kV high voltage direct current (HVDC) transmission projects, the DC biased magnetization of the transformer is widely concerned. It is reports that the biggest DC current of neutral point of main transformer in Chuncheng substation of China is 34.5 A (noise is 93.9 dB) under mono-polar ground return operation mode of Guiguang ± 500 kV (750 MW) HVDC transmission system [1]. The DC current invasion into neutral point of 500 kV auto transformer in Wunan substation of China is 9.4 A when Sanxia-Changzhou HVDC transmission system operated by mono-polar ground return [2]. Moreover, the invaded DC current of neutral point of main transformers in Lingao and Dayawan nuclear power stations was below 4%, influenced by Sanxia-Guangzhou HVDC transmission system [3]. In order to weaken the negative effects (saturation of iron core, increasing of temperature, strong vibration, et al.) caused by DC bias, the existing researches are mainly conduct in terms of the grounding form, design of winding and core structure of transformer [4–6]. Paper [2] reported that adopting reverse current method, series capacitance method, and series resistance method can inhibit the influence of the HVDC current on the AC transformer. Paper [7,8] demonstrate that the ability of anti DC bias for transformers (from high to low) is: threephase three-climb transformer, three-phase five-climb transformer, and single-phase three-climb transformer unit. Besides, AC magnetic

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properties of a GO steel core under DC-biased magnetization were shown in paper [9]. However, there are few reports about reducing the adverse effects of DC bias by rational selection of GO steels considering the DC bias characteristics of different types of high grades GO steels. In this paper, the DC biased magnetic properties of 10 kinds of high grade (75−105) GO steels are systematically researched in terms of type selection of transformer core material, and a new method to reduce the negative effects of DC bias is proposed. 2. Test materials, methods and experiment conditions design 2.1. Test materials and methods The grades of domain refined high magnetic induction GO steels used for analysis of DC bias magnetic properties are shown in Table 1. The size of single sheet sample is 500×500 mm, and the thickness includes 0.23 mm and 0.27 mm. The magnetic properties are tested on the Bruckhaus MPG 200D soft magnetic measurement systems, which can accurately control the DC offset and the excitation signal waveform, measure the magnetostriction and A-weighted magnetostriction velocity level (AWV). The core loss and magnetic induction of 10 kinds of high grades GO steels are measured, and then the total losses are obtained under DC bias magnetic field by adding different DC current in excitation current. The eddy current loss and hysteresis loss of GO

Corresponding author.

http://dx.doi.org/10.1016/j.jmmm.2016.11.089 Received 19 July 2016; Received in revised form 18 November 2016; Accepted 19 November 2016 Available online 20 November 2016 0304-8853/ © 2016 Elsevier B.V. All rights reserved.

Journal of Magnetism and Magnetic Materials 426 (2017) 575–579

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3. Analysis of basic magnetic performance of GO steels

Table 1 Thickness and grades of commercial GO steels used in experiments. Thickness

Grade

0.23 mm 0.27 mm

23QG075 27QG085

23QG080 27QG090

23QG085 27QG095

23QG090 27QG100

Fig. 2 shows the core loss and magnetic induction of 0.23 mm and 0.27 mm series high grade GO steels under sinusoidal waveform magnetic field excitation, in order to provide reference for the analysis of the magnetic properties of the GO steels under DC bias condition. The core losses (P1.7) of 23QG075、23QG080、23QG085、 23QG090、23QG095 steels are 0.736、0.774、0.836、0.896、 0.931 W/kg respectively, and the magnetic induction (B8) are all above 1.90 T. For the samples of 27QG085、27QG090、27QG095、 27QG100、27QG105, the iron losses are better than the nominal values about 1 grade, and the magnetic induction exceed 1.89 T.

23QG095 27QG105

steels are separated at 50 Hz and 60 Hz test frequencies to explain the ability of resisting DC bias. Finally, the magnetostriction coefficient and AWV of GO steel under different DC bias magnetic field were measured.

4. Analysis of DC bias characteristics of GO steels 2.2. Design of experiment conditions for DC bias

4.1. Influence of thickness under DC-biasing

The acceptable DC bias current for transformer is related to transformer structure, core structure, working magnetic density, vibration amplitude and noise level, acceptable degree of waveform distortion, and so on. At present, the DC current allowed in the transformer windings are not conclusive, and it mainly depends on relevant experience or regulations in HVDC projects. For example, the standard of the first HVDC transmission line project in China is less than 1.5 times of the exciting current [2]. The maximum withstanding DC current in each phase of converters of ABB and SIEMENS Ltd is 5 A at Long-zheng ± 500 kV HVDC transmission project [10]. The standard of DL / T 437–2012《Technical guide of HVDC earth electrode system》provisions that the DC current must below 0.7% of the rated current for three-phase three column transformer and less than 0.3% of the rated current for single-phase transformer [11]. In order to conduct a representative result for GO steels under DC bias, the selection of DC current consider the above standard or provision (5 A) and the actual monitoring DC current (34.5 A) invaded in transformer neutral point. Taking the single-phase three-climb transformer (parameters see Table 2), which most affected by DC bias, as an example to calculate the equivalent DC bias magnetic field. The calculated formula for the DC bias magnetic field Hdc is:

Fig. 3 displays the effect of material thickness on the loss of GO steels under DC bias condition, taking 23QG085 and 27QG085, 23QG090 and 27QG090, 23QG095and 27QG095 as research objects. The loss (P1.7) gap between the same grade different thickness GO steels is below 2.5% in normal condition of pure sinusoidal waveform. As shown in Fig. 3, 6 kinds of GO steels showed a monotonically increasing trend with the increasing of DC bias magnetic field. Most importantly, the loss increase rates of 0.27 mm series GO steels are obviously lower than that of 0.23 mm series samples. That is, the thicker GO steels have a stronger ability to resist DC bias. Table 3 presents the statistical results of loss increase rate under different bias magnetic field. When superimposed DC bias magnetic field Hdc is 10 A/m, the loss of 0.23 mm series (23QG085、23QG090、 23QG095) GO steels increased by 4.7~8.0% compared with normal condition, while this value is 2.1~3.4% for 0.27 mm series (27QG085、 27QG090、27QG095) samples. When Hdc is 100 A/m, the loss of 0.23 mm series GO steels increased by 33.5~43.2%, and it only increased by 20.7~23.7% for 0.27 mm series materials. The reasons for these differences are discussed in Section 5 of this paper.

N∙I dc H dc= l

4.2. Influence of permeability under DC-biasing Fig. 4 shows the losses of 23QG090, 23QG095, 27QG100, 27QG105 grade GO steels under bias magnetic field of 0–50 A/m in order to analysis the influence of permeability on DC bias resistance ability. In normal condition of sinusoidal waveform, loss (P1.7) of 23QG090 is lower than that of 23QG095 by 0.035 W/kg. However, with the increasing of DC bias magnetic field, loss of former sample is greater than the latter one when Hdc over 12 A/m, and the cross point see arrow 1 in Fig. 5. Similar conclusion can be obtained in 27QG100 and 27QG105 grade samples. Although loss of 27QG105 steel is higher than that of 27QG100 in normal condition, loss of 27QG105 sample is lower than the latter one instead when Hdc over 40 A/m (see arrow 3 in Fig. 4). Interestingly, loss of 27QG105 GO steel gives a lower total power loss than that of 23QG090 when Hdc is above 40 A/m (see arrow 3 in Fig. 5). Table 4 presents the magnetic induction and relative permeability of four grades GO steels. Results show that, the magnetic induction of 23QG095 and 27QG105 steels are 1.901 and 1.892, lower than that of 23QG090 and 27QG100 samples by 0.017 T and 0.011 T respectively. Meanwhile, the relative permeability of 23QG095 and 27QG105 steels is lower than that of 23QG090 and 27QG100 samples by 22.4% and 20.9% respectively. In summary, the materials with relatively lower magnetic induction or permeability show stronger resistance to DC bias, which mean lower increase rate of total loss. The above conclusion is similar to the results in paper [13]: by using higher permeability GO steel, transformer has a smaller excitation current, but the excitation current greatly increased under DC bias and the waveform may be spire shaped. Instead, using relatively low

(1)

In the formula (1), N is the turn number of coil; Idc is the DC current; l is the length of magnetic circuit. The calculated Hdc is 17.4 A/ m and 120 A/m corresponding to DC current of 5 A and 34.5 A, respectively. Therefore, the reasonable ranges of Hdc can be set up at 5–120 A/m when testing the total power loss (Bmax=1.7 T) of GO steels. In addition, Fig. 1 presents the magnetic hysteresis loop, magnetic flux density, and magnetic field intensity waveform of GO steel under DC bias condition [12]. In the first quadrant, the hysteresis loop quickly moves to the saturation region of the GO steel. The magnetic flux density curve rises in delta B overall. Magnetic field intensity waveform is serious asymmetry in positive and negative half-cycle. Table 2 Parameters of single-phase three-limb commercial transformer. Parameter

Value

Parameter

Value

Rated capacity

240 MVA

Structure

Rated voltage

550/ √3 / 22 kV 3.5 m/1.5 m

Winding height of high / low voltage

single-phase three-limb 2.05 m/2.08 m

Height / diameter of core High / wide of window inside

2.25 m/ 0.6 m

Winding turns of high / low voltage Phase current in high / low voltage side

508/32 756 A/12000 A

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Fig. 1. (a) Magnetic hysteresis loop, (b) magnetic flux density and (c) magnetic field intensity waveform of GO steel under DC bias condition.

1.95

0.90 0.85

1.90

0.80 0.75

1.85

0.70 0.65 0.60

0.95

1.95

0.90 0.85

1.90

0.80 0.75

1.85

0.70 0.65

1.80

0.60

Magnetic induction, B8 (T)

0.95

2.00 Core loss Magnetic induction

1.00

Core loss, P1.7 (W/kg)

Core loss Magnetic induction

Magnetic induction, B8 (T)

1.00

Core loss, P1.7 (W/kg)

1.05

2.00

1.05

1.80 27QG85 27QG90 27QG95 27QG10027QG105

23QG75 23QG80 23QG85 23QG90 23QG95

Fig. 2. Basic magnetic properties of 0.23 mm (a) and 0.27 mm (b) series high grades GO steels in normal condition.

23QG085 23QG090 23QG095 27QG085 27QG090 27QG095

1.2 1.1

1.3

Core loss, P1.7(W/kg)

Core loss, P1.7(W/kg)

1.3

1.0 0.9 0.8

20 40 60 80 Bias magnetic field, Hdc(A/m)

100

DC bias magnetic field (A/m) 10

20

50

100

85

0.23 0.27

7.7% 2.3%

16.7% 6.2%

34.2% 14.7%

43.2% 21.0%

90

0.23 0.27

8.0% 2.1%

16.7% 5.2%

33.5% 14.2%

42.5% 20.7%

95

0.23 0.27

4.7% 3.4%

10.7% 7.8%

24.3% 16.6%

33.5% 23.7%

1

1.1 1.0 0.9

3

0

10 20 30 40 50 Bias magnetic field, Hdc(A/m)

permeability GO steel (the excitation current is likely to be greater in normal condition) leading to less influence by DC bias. Moreover, paper [14] points out that, when the applied voltage is increased by 10% on the basis of the rated voltage, the exciting current is increased by about 1 time for the low permeability hot rolled silicon steel, while the exciting current is increased by about 3.5 times for the high permeability cold rolled GO steel. Although the excitation currents under DC bias and over excitation conditions are different, the mechanisms are all based on the corresponding relationship between the flux and the magnetization curve. When the flux peak is the same, the peak value of the excitation current of DC bias and over excitation must be the same. Therefore, the results of paper [13,14] as well as the research in this paper all show that the GO steels with relatively lower permeability (B8 > 1.89 T yet) have stronger resistance ability of DC biasing.

Table 3 Growth rates of core loss of different grade GO steels under DC bias condition. Thickness (mm)

2

Fig. 4. Core loss of four grades GO steels under DC bias condition.

Fig. 3. Effect of thickness on core loss of GO steels under DC bias condition.

Grade

1.2

0.8

0

23QG090 23QG095 27QG100 27QG105

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Core loss, P1.7(W/kg)

1.2 1.1

(a)

1.3

Core loss, P1.7(W/kg)

23QG075 23QG080 23QG085 23QG090 23QG095

1.3

1.0 0.9 0.8 0.7

0

20 40 60 80 100 Bias magnetic field, Hdc(A/m)

1.2 1.1 1.0 27QG085 27QG090 27QG095 27QG100 27QG105

0.9 0.8 0.7

120

(b)

0

20

40

60

80

100

120

Bias magnetic field, Hdc(A/m)

Fig. 5. Effect of DC bias on loss of 0.23 mm (a) and 0.27 mm (b) high grade GO steels.

800

Magnetic induction, B8 (T)

Relative permeability, μr

23QG090 23QG095 27QG100 27QG105

1.918 1.901 1.903 1.892

21325 17427 23356 19324

Magnetostriction length, x10

Grade

-6

Table 4 Magnetic induction and relative permeability of four grades GO steels.

4.3. Influence of DC bias on total power loss Fig. 5 presents the losses of 0.23 mm (70–95 grades) and 0.27 mm (80–105 grades) series GO steels under DC bias magnetic field strength of 0–120 A/m. On the whole, the losses of each grade GO steels increased with the increasing of bias magnetic field. When Hdc is 20 A/ m, the loss of 0.23 mm high magnetic induction samples increased by 8.3~16.7% compared to normal condition of pure sinusoidal waveform, and this value rise up to 27.7–44.6% when Hdc is 120 A/m. For the 0.27 mm series GO steels, the same tendency was obtained. In general, the core loss of GO steels rapidly increased with the rising of DC bias magnetic field, and the loss curve may appear cross due to the effect of permeability.

600 400 200

Sinusoidal excitation Hdc=10 A/m Hdc=20 A/m Hdc=40 A/m Hdc=60 A/m Hdc=80 A/m Hdc=100 A/m Hdc=120 A/m

0 -200 -400 -600 -2.0 -1.5 -1.0 -0.5 0.0

0.5

1.0

1.5

2.0

Magnetic polarisation, T Fig. 7. Butterfly loops of 27QG85 grade steel under different DC bias magnetic field.

from 0 to 120 A/m, the zero-to-peak value λz-p, 1.7T (×10−6) increased from −0.515 to 1.83, and the corresponding AWV value enlarged from 52.9 to 67.5 dB. The asymmetrical butterfly loops shown in Fig. 7 illustrate that the DC bias has serious influence on magnetostriction, especially when Hdc exceeding 20 A/m. In addition to the influence of DC bias, the material conditions such as grain orientation, residual stress and coating tension are all have direct effects on magnetostriciton of GO steel [15,16], and the magnetostriciton is increased by compressive stresses in the magnetizing direction rather than tensile stresses [17]. In summary, the increased magnetostriction, vibration, as well as the noise of GO steel caused by DC bias should be highly valued in HVDC transmission system.

4.4. Influence of DC bias on magnetostriction Fig. 6 displays the effects of DC bias on magnetostriction coefficient (λz-p, 1.7T) and A-weighted magnetostriction velocity level (AWV) of 27QG85 grade GO steel. With the DC bias magnetic field increased

5. Discussion With the increasing of DC current immersed in AC excitation current, the increase rate of core loss of 0.27 mm series GO steels were obviously lower than that of 0.23 mm samples (see Table 3). In order to interpret the reason for its strong ability to resist DC bias, the total loss have been separated into eddy current loss Pe and hysteresis loss Ph at different test frequency (f1=50 Hz and f2=60 Hz) taking the 27QG085 and 23QG085 grade steels as an example. As shown in Fig. 8, under normal working condition, the eddy current loss of 27QG085 sample (0.410 W/kg) is higher than that of 23QG085 sample (0.351 W/kg). Compared with Ph, the increase rate of Pe is not obvious with the DC biased magnetic field increased from 10 A/m to 50 A/m (see the red column in Fig. 8(a) and (b)). Thus the greater the proportion of Pe in total loss under normal condition, the lower the increase rate of total loss under DC bias. For example, When Hdc is 50 A/m, the total loss of 27QG085 steel is 0.938 W/kg, only increased by 14.7% compared to the loss under sinusoidal excitation; whereas the

Fig. 6. Effects of DC bias on magnetostriction coefficient and AWV of 27QG85 grade GO steel.

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1.2 Hysteresis loss, Ph Eddy current loss, Pe

1.0

(a)

Core loss, P1.7(W/kg)

Core loss, P1.7(W/kg)

1.2

0.8 0.6 0.4 0.2 0.0

0

1.0

(b)

0.8 0.6 0.4 0.2 0.0

10 20 30 40 50 Bias magnetic field, Hdc(A/m)

0

10 20 30 40 50 Bias magnetic field, Hdc(A/m)

Fig. 8. Eddy current loss and hysteresis loss of (a) 0.27 mm (27QG085) and (b) 0.23 mm (23QG085) GO steels under DC bias condition.

by DC bias should be valued in HVDC transmission system.

loss of 23QG085 sample increased by 34.2%, up to 1.122 W/kg. The above analysis seems not enough to explain the season for thicker GO steels having obviously stronger ability of anti DC biasing, as the anormal loss Pa component was ignored. However, it is confirmed in paper [9], that more magnetic domain walls are annihilated and nucleated under DC-biased magnetization with higher delta B even at the same AC induction and effective wall number changes. It is thought that the loss increase is mainly caused by energy loss concerned with harder processes towards higher induction and partial increase of Pe due to decrease of wall numbers. In addition, the value of Pe is proportional to thickness when content of Si not change. Therefore, the increase rates of core loss of 0.27 mm series GO steels were obviously lower than that of 0.23 mm samples under DC bias condition.

Acknowledgment The authors acknowledge the financial support provided by the Science and Technology Foundation of State Grid Corporation of China under grant SGRI-WD-71-14-002. References [1] S. Chun, Measure to decrease the neutral current of the AC transformer in HVDC ground-return system, High Volt. Eng. 11 (2004) 52–54. [2] D.Z. Kuai, D. Wan, Y. Zou, et al., Impacts of long-time DC biasing magnetism on transformers, Electr. Power 8 (2004) 41–43. [3] Z. Rong, Z. Jie, S. Chun, et al., Measures to restrain the neutral current of the AC transformer in HVDC ground return system, High Volt. Eng. 11 (2004) 52–54. [4] E. Barbisio, O. Bottauscio, M. Chiampi, et al., Analysis of AC magnetic properties in SiFe laminations under DC-biased magnetization, Physica B 343 (2004) 127–131. [5] M. Philip, J.M. Anthony, P.H. Jeremy, Effect of DC voltage on AC magnetization of transformer core steel, J. Electr. Eng. 7 (2010) 123–125. [6] L. Cheng, F.Y. Yang, G. Ma, et al., Development and application of high magnetic induction grain-oriented silicon steel for power transformer, Mater. Rev. 6 (2014) 115–118. [7] H.Z. Li, X. Cui, D.S. Liu, et al., Influence on three-phase power transformer by DC bias excitation, Trans. China Electrotech. Soc. 5 (2010) 88–96. [8] Z. Lin, W. Chen, Y. Yang, Analysis of DC bias exciting current of the single-phase transformer and its effect on protection, Power Syst. Prot. Control 24 (2010) 158–162. [9] S. Yanase, Y. Okazaki, T. Asano, AC magnetic properties of electrical steel core under DC-biased magnetization, J. Magn. Magn. Mater. 215–216 (2000) 156–158. [10] L.S. Zeng, Influence of DC transmission ground electrode current on power transformers, Electr. Power Constr. 12 (2004) 22–24. [11] DL/T 605-2012, Technical Guide of HVDC Earth Electrode System. [12] D. Miyagi, T. Yoshida, M. Nakano, et al., Development of measuring equipment of DC-biased magnetic properties using open-type single-sheet tester, IEEE Trans. Magn. 10 (2006) 2846–2848. [13] Q.F. Lei, Transformer and higher harmonic, Transformer 4 (1989) 2–5. [14] Mitsubishi Electric LTD. Magnetization curve former of 500 kV autotransformer at Zeng cheng s/s. Mitsubishi Electric LTD, 1994, Accessed 3.17.1994 [15] A. Pulnikov, R. Decocker, V. Permiakov, et al., The relation between the magnetostriction and the hysteresis losses in non-oriented electrical steel, J. Magn. Magn. Mater. 290- 291 (2005) 1454–1456. [16] J.M. Shilling, G.L. Houze, Magnetic properties and domain structure in grainoriented 3% Si-Fe, IEEE Trans. Magn. 10 (1974) 195–223. [17] G.C. Eadie, The effects of stress and temperature on the magnetostriction of commercial 3 1/4% silicon-iron grain-oriented electrical steel strip, J. Magn. Magn. Mater. 26 (1982) 43–46.

6. Conclusions Combine with the special working condition of AC transformer caused by HVDC transmission, magnetic properties of 0.23 mm (23QG75~23QG95 grades) and 0.27 mm (27QG85~27QG105 grades) high permeability GO steels under condition of AC / DC hybrid excitation were researched. Results are briefly summarized as follows: 1) For the GO steels which show the same core loss under sinusoidal excitation, the greater the thickness of material, the stronger the ability to resist DC bias (lower increase rate of loss). The reason is that, loss increase is mainly caused by Ph and partly contributed by Pe due to decrease of magnetic domain wall numbers under DC biased magnetization, and the proportion of Pe in 0.27 mm series GO steels is higher than that of the same grades 0.23 mm samples. 2) Due to the relatively lower magnetic induction (B8 > 1.89 T yet), the loss of 23QG095 and 27QG105 grade GO steels were lower than that of 23QG090 and 27QG100 grade GO steels respectively, when the DC bias magnetic field exceeds 12 A/m and 14 A/m respectively. 3) DC bias has serious effect on the magnetostriction and AWV value especially when Hdc exceeding 20 A/m. With Hdc increased from 0 to 120 A/m, λz-p, 1.7T (×10−6) rose from −0.515 to 1.83, and the corresponding AWV enlarged from 52.9 to 67.5 dB. The increased magnetostriction, vibration, as well as the noise of GO steel caused

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