Study of transformer magnetic properties considering temperature influence of electrical steel sheet

Abstract

In this paper, the magnetic field and temperature field of transformer are analyzed, and the influence of temperature on electrical steel sheet is considered. Experiment are supplied to obtain the magnetic properties curves (B-H curves, B-P curves, and magnetic permeability curves of 30QG120), which are required for the accurate FEM (finite element method) simulation setup. Finally, the magnetizing characteristics and temperature distribution of the transformer under different temperatures and loads are studied experimentally. The simulation results of a singlephase transformer are verified by the experimental results.

Keywords

Electrical steel sheet different temperature magnetizing characteristics transformer simulation and experiment

1. Introduction

In recent years, The UHV and long-distance transmission projects have developed rapidly, so electrical equipment such as transformer has become an indispensable electrical appliance for our daily life and industrial production, as shown in Fig. 1. As a main core material of electrical equipment such as transformers and motors, electrical steel sheets are affected by the ambient temperature, and then affects the working performance of electrical equipment. Under normal circumstances, the analysis of the transformer mainly focuses on the magnetic induction curve and the loss curve at room temperature. Usually, the influence of the actual working temperature on the transformer is not considered. Therefore, it is possible to amplify the error when designing and analyzing transformers [1].

Fig. 1.

The UHV and long-distance transmission line.

At present, domestic and foreign scholars have carried out extensive research work on electrical steel sheets for electromagnetic devices. In [2], the magnetic properties of oriented silicon steel sheets under different temperature and harmonic conditions were tested. The magnetic properties of electrical steel sheet for transformer are studied in [3,4]; The magnetic properties of electrical steel sheet under different working conditions are studied in references [5,6]. In [7], the electromagnetic characteristics of electrical steel sheet under magneto-thermal coupling are studied.

The analysis methods for transformers mainly include empirical methods, analytical methods and finite element methods. At present, the finite element method is commonly used to analyze the electromagnetic-thermal characteristics of transformers. In [8], the two-dimensional finite element method is applied to study the magnetic field of a converter transformer, but the temperature rise is not considered. The three-dimensional finite element method is used to improve the magnetic field, loss and temperature of high-frequency transformer in [9]. The study did not consider the changes in the magnetic properties of electrical steel sheets at different temperatures. The leakage magnetic field and temperature field of an 800 kVA transformer were analyzed [10], but also did not consider the magnetic properties of electrical steel sheets at different temperatures.

Foreign scholars have studied the electrical steel sheets earlier. In [11], the iron loss characteristics of non-oriented electrical steel sheet under high temperature harmonic environment are measured and analyzed. In [12], the electromagnetic properties of oriented electrical steel sheet under tension and compression stress are studied. The loss of non-oriented electrical steel sheet at liquid nitrogen temperature was measured in reference [13]. In [14], the core loss of the motor under harmonic action is analyzed, but the temperature factor is not considered. In [15], the loss and noise of transformer are studied, but the influence of temperature on the loss and noise is also not considered.

This paper investigated the temperature effect on the electrical steel sheet by theory analysis and experiment verification. Experiment are conducted to obtain the magnetizing characteristics at different temperature. The FEM model considering different temperature magnetic properties curves and modified for better performance. In addition, the test setup and experimental result of temperature distribution in a single-phase transformer are also presented to verify the simulation result.

2. Measurement principle, instrument and result

The measuring coil system consists of a primary winding to create magnetic field which then creates magnetic flux in the steel. The magnetic field is defined by the current, the winding and the number of windings as well as the magnetic length of the coil. The computer converts the measured voltage into magnetic field strength. $\begin{eqnarray}\displaystyle H(t)={\displaystyle \frac{N_{1}\cdot I(t)}{l_{m}}}={\displaystyle \frac{N_{1}\cdot u_{1}(t)}{R_{n}\cdot l_{m}}}. & & \displaystyle\end{eqnarray}$ (1) The polarization is defined via measurement of the decoupled voltage induced in the secondary winding of the coil system. $\begin{eqnarray}\displaystyle {\displaystyle \frac{dJ}{dt}}=-{\displaystyle \frac{u_{2}(t)}{N_{2}\cdot A_{m}}}\Rightarrow J(t)=-{\displaystyle \frac{1}{N_{2}\cdot A_{m}}}\cdot \int _{0}^{t}u_{2}(t)dt & & \displaystyle\end{eqnarray}$ (2) where, H (t) is magnetic field strength, N₁ is number of primary windings, I (t) is primary current, l_m length of magnetic path, u₁(t) is primary voltage, R_n is equivalent resistance, u₂(t) is secondary voltage, N₂ is number of secondary windings, A_m is equivalent area, and J (t) is magnetic induction intensity.

The electrical sheet steel working under different temperature is achieved by the temperature control system, and the excitation can provide the AC voltage with different DC bias and controlled harmonic. The data collection system is based on a computer. The measurement instrument system of single sheet is shown in Fig. 2. The main performances of measurement instrument are shown in Table 1. Electrical steel measuring gauge for the determination of magnetic characteristics of all types of soft magnetically sheets, especially to measure electromagnetic properties of non-crystal orientation silicon sheet and crystal orientation silicon sheet. The system meets the IEC (IEC60404 and IEC 60205) and ASTM Standards.

Fig. 2.

Magnetic properties test device of electrical sheet steel.

Table 1

Measuring instrument performance

Item	Parameters	Accuracy
Temperature range	20–300 (°C)	-
Sample size	150 ∗ 150 ∗ 0.5 (mm)	-
Magnetic field intensity	1–10000 (A/m)	0.2%
Intensity of polarization	0.001–2 (T)	0.2%
Frequency range	1–5000 (Hz)	-

Fig. 3.

Magnetic properties of 30QG120 at different temperatures.

It can be seen from Fig. 3(a) that as the temperature increases, the working point of the 30QG120 electrical steel sheet shifts to the right, and the saturation magnetic induction intensity decreases. From Fig. 3(b), shows that under the same magnetic induction intensity, the loss of 30QG120 electrical steel sheet decreases with the increase of temperature. From Fig. 3(c), the magnetic permeability of 30QG120 electrical steel sheet increases with the increase of temperature.

The results of Fig. 3 show that the increase in temperature causes the magnetic domain change of the electrical steel sheet to be more active, and the hysteresis characteristic is lowered, thereby lowering the magnetic induction strength, reducing the loss, and increasing the magnetic permeability. It indicates that the temperature has certain influence on the magnetic properties of 30QG120 electrical steel sheet. When applying 30QG120 electrical steel sheet for electromagnetic product design, the influence of working temperature should be considered.

3. Study of the temperature distribution of single-phase transformer

The simulation analysis of the electromagnetic characteristics of a single-phase transformer at different temperatures was carried out based on COMSOL finite element analysis software. Table 2 shows the basic parameters of the transformer.

Table 2
Main parameters of the single-phase transformer

Item Primary side Secondary side

Rated capacity 3 kVA 3 kVA

Rated voltage 220 V 110 V

Rated current 13.6 A 27.3 A

Electromotive force per turn 2 V/Z 2 V/Z

Turns 110 55

Silicon steel sheet type 30QG120 30QG120

Item	Primary side	Secondary side
Rated capacity	3 kVA	3 kVA
Rated voltage	220 V	110 V
Rated current	13.6 A	27.3 A
Electromotive force per turn	2 V/Z	2 V/Z
Turns	110	55
Silicon steel sheet type	30QG120	30QG120

Figure 4 shows the electromagnetic and thermal simulation flow chart of the transformer.

Fig. 4.

Calculation process of the FEM.

Calculation can be conducted by substituting the B-H curve in Fig. 3(a) and B-P curve in Fig. 3(b) obtained by experiment into step III and step IV.

3.1. Flux density analysis of transformer core

Figure 5 shows the flux density of the transformer under different temperature. As shown in Fig. 5, the flux density of the transformer decreases as temperature decreases.

Fig. 5.

Magnetic induction intensity of transformer under different temperature.

3.2. Temperature distribution analysis of transformer core

The temperature field of transformer is obtained by considering the influence of temperature on electrical steel sheet. Figure 6 shows the temperature distribution of transformer core.

As shown in Fig. 6, the maximum temperature of transformer core is 40.5 °C and the maximum temperature of winding is 60 °C.

Fig. 6.

Transformer temperature field simulation.

4. Experiment on a single-phase transformer analyzing the temperature

An experiment is conducted to verify the effectiveness of simulation, and experimental system is shown in Fig. 7.

Fig. 7.

Experimental system.

4.1. Experimental research of magnetic characteristic of transformer at different environment temperature

The Fig. 7(a) shows the experimental system used to measure the magnetic characteristic of transformers at different environment temperatures. Figure 8 shows the magnetization curves of the transformer at 20 °C, 80 °C and 150 °C, respectively. Obviously, the magnetic induction strength of the transformer decreases with the increase of temperature.

Fig. 8.

B-H curve of the transformer at different temperatures.

4.2. Experimental research of temperature of transformer

The Fig. 7(b) shows the experimental system for measuring temperature of the transformer. Figure 9 shows that the maximum temperature of the transformer core is 42.2 °C at full load operation. The comparison between simulation results and experimental results are shown in Table 3.

Fig. 9.

Transformer temperature photograph.

Table 3

Comparison of simulation and experiment

	Simulation result	Experimental result	Error
Flux density (T)	1.7348	1.7725	2.13%
Core temperature (°C)	40.5	42.2	4.03%

5. Conclusion

In this paper, the magnetic properties of electrical steel sheet at different temperatures were studied. The magnetic field and temperature field of the transformer made of electrical steel sheet are simulated and experimentally studied.

(1) The magnetic properties of electrical steel sheets at different temperatures were measured. The test results showed that when the magnetic induction intensity is 1.6 T, the magnetic field intensity was 37 A/m at 60 °C, 38 A/m at 80 °C, and 60 A/m at 150 °C; The permeability is 37181 at 60 °C, 36874 at 80 °C, 34290 at 150 °C; The unit iron loss is 0.74 W/kg at 60 °C, 0.73 W/kg at 80 °C and 0.69 W/kg at 150 °C. It shows that temperature has a certain influence on the magnetic properties of oriented electrical steel sheets. In the process of analysis and design of electromagnetic devices, the influence of temperature should be considered.

(2) An experimental platform for electromagnetic characteristics and temperature rise of 3 kW transformer is built and studied. The experimental results show that the error of simulation and experiment is less than 5%. The theoretical analysis can guide the design of transformer, and each index can meet the actual needs of the project.

Footnotes

Acknowledgements

This research was jointly supported by the Project of Liaoning Education Department (no. 201634090) and Project Support of Liaoning Science and Technology Department (no. 20180550037).

References

Josue

, Transformer hot-spot temperature estimation for short-time dynamic loading, IEEE International Conference on Condition Monitoring and Diagnosis (2012), 217–220.

Chen

Fang

Kwon

, Measurement research on magnetic properties of electrical sheet steel under different temperature, harmonic and DC bias, AIP Advances7(5) (2017), 056682.

Oda

Kohno

and Honda

, Recent development of non-oriented electrical steel sheet for automobile electrical devices, Journal of Magnetism & Magnetic Materials320(20) (2008), 2430–2435.

Freitag

Joost

and Leibfried

, Modified Epstein frame for measuring electrical steel under transformer like conditions, International Conference on High Voltage Engineering & Application (2014), 1–4.

Tone

, Magnetic characteristic analysis considering the crystal grain of grain-oriented electrical steel sheet, IEEE Transactions on Magnetics41(5) (2005), 1704–1707.

Bölling

Günther

and Huneus

, Magnetic properties of low loss non-oriented electrical steel sheet of thin gauge and at high frequencies, Physica Scripta39(6) (1989), 764–766.

Miyagi

, Measurement of magnetic properties of non-oriented electrical steel sheet at liquid nitrogen temperature using single sheet tester, IEEE Transactions on Magnetics46(2) (2010), 314–317.

Chen

Wood

A.R.

and Arrillaga

, HVDC converter transformer core saturation instability: A frequency domain analysis, IEE Proceedings-Generation, Transmission and Distribution143(1) (1996), 75–81.

Liu

and Lu

, Transient simulation analysis of electromagnetic field and temperature field of high-frequency transformer, High Voltage Engineering45(4) (2019), 1191–1200.

10.

Liu

, Analysis of Leakage Magnetic Field and Temperature Field of 800KVA Deep-sea Transformer, Huazhong University of Science and Technology, Wuhan China, 2018.

11.

Yao

and Iron

, Loss characteristics under pwm inverter excitation at high ambient temperature, in: International Symposium on Linear Drives for Industry Applications (LDIA), IEEE, Osaka, Japan, 2017, pp. 1–3.

12.

Boleslaw

Piotrowski

Jasnoch

and Chmielewski

, Impact of tensile and compressive stress on classical and acoustic Barkhausen effects in grain-oriented electrical steel, IEEE Transactions on Magnetics50(4) (2014), 1–4.

13.

Patrick

Veigel

and Liu

, Iron loss measurement of nonoriented silicon and cobalt iron electrical steel sheets at liquid nitrogen temperature using ring specimen, in: International Electric Machines and Drives Conference, IEEE, Miami, FL, 2017, pp. 1–7.

14.

Ruilin

and Zeng

, Studies of core loss of thin non-oriented electrical steel for electrical vehicle traction motors, in: International Conference on Electrical Machines and Systems, Jeju, 2018, pp. 2715–2718.

15.

Takajo

and Ito

, Loss and noise analysis of transformer comprising grooved grain-oriented silicon steel, IEEE Transactions on Magnetics53(9) (2017), 1–6.