XXVth Int. Symp. on Discharges and Electrical Insulation in Vacuum - Tomsk – 2012

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⋆This work was supported by National Nature Science Foundation of China (No. 50977004), Fok Ying Tung Education Foundation (No. 131057), New Century Excellent Talents in University (No.NCET -10-0282) and China Postdoctoral Science Foundation (No. 201003614), (No. 20110491522).

Static electric field distribution of hybrid circuit breaker Based on Ansoft

Cheng Xian, Duan Xiongying, Chen Jianhua, Liao Minfu*, Zou jiyan School of Electrical Engineering, Dalian University of Technology, Dalian, P. R.China

Abstract- Hybrid circuit breaker (HCB) based on the series of vacuum interrupter and SF6 interrupter uses strong interruption capacity of vacuum and the linear insulation property of SF6 gas to be a high-voltage and high-current breaker. The numerical calculation of 3D electric field is an important way to analyze reasonable and optimization of its structure. The 3D model of the vacuum interrupter and SF6 interrupter are established; the internal electric field distribution of vacuum interrupter and SF6 interrupter are calculated by Ansoft software, and their self-capacitances are solved. Then a 3D computational model of HCB based on models of vacuum interrupter and SF6 interrupter is established, and the distribution characteristics of its electric field are analyzed. The simulation result of electric field distribution of HCB is obtained, and the structure of HCB is optimized designed. It provides a theoretical basis for the structural design of the experimental prototype of HCB.

I. INTRODUCTION

HCB with excellent interruption capacity of vacuum and great insulation properties of SF6 gas can obtain greater breaking capacity. Vacuum interrupter first withstands the initial TRV in short-circuit current interruption process and helps SF6 interrupter’s dielectric strength recovery as soon as possible; SF6 interrupter would withstand the post-peak voltage and the static insulation voltage; HCB gets a better breaking capacity in this way [1-4]. Therefore, Modeling of HCB and 3D electric field calculation is necessary. The calculation results could point out the ideal connection mode between two interrupters in the respect of voltage distribution. At present, a study on HCB’s electric field distribution characteristics haven’t saw yet.

In this paper, computer-aided modeling software Solidworks is used to model vacuum interrupter and SF6 interrupter. Then the solid geometry model is entered into Ansoft. It is disassembled by 4-nodded tetrahedron elements. Ansoft calculates internal electric field distribution and self-capacitance of vacuum interrupter and SF6 interrupter by finite element method. The finite element model of HCB is established by vacuum circuit breaker and SF6 circuit breaker in series. The detailed

design and electric field analysis using Ansoft simulator have been carried out. Based on the finite-element technique, the electric field energy method is used to compute self-capacitance of each interrupter and stray capacitance. The results can provide a reliable basis for giving the self-capacitance of the interrupter in the dynamic simulation model, and also is a reference for selection of grading capacitance. The reference can be used to fix impact of external measuring equipment in the voltage distribution.

II. THE CALCULATION PRINCIPLE OF ELECTRIC

FIELD

The electric field numerical analysis bases on the Maxwell differential equations with the finite element discrete form. It transforms the calculation of electromagnetic field into a large matrix solution. The discretization form of finite-element changes a continuous structure into a finite number of units and sets a limited number of points in each unit. The continuum would be a collection as connected by a group of cells, which continuous domain infinite degrees of the freedom problem will be transformed into a discrete domain with the restricted degree of freedom. The extreme values of the electrostatic field energy converted into problems on extreme values of function of many variables. The finite element solution process including the establishment of the model, mesh generation, fixed excitation source and the boundary conditions, post-processing and analysis of the results and so on [5, 6].

The process of Model established in Solidworks had been ignored. This paper employed the most stable tetrahedral element in the finite element theory, used a four node. Fig. 1 is a diagram of four-node tetrahedral element.

Fig.1. Four-node tetrahedral element

978-1-4673-1266-0/12/$31.00 ©2012 IEEE

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The scalar Φ is the unknown quantity in 3D electric field solver. We can obtain the following (1) with boundary conditions.

0( )r v (1) Φ (x, y, z) is a 3D scalar potential, εr (x, y, z) is the

relative dielectric constant on the three directions, ε0 is the vacuum permittivity, and ρ (x, y, z) is the charge density. The three-dimensional scalar potential Φ is solved in the electric field model. Once the potential value of the scalar is solved, electric field intensity E and electric displacement vector D can be obtained directly by the Maxwell differential equations. The two basic quantities are calculated by (2).

( )0

ED r

(2)

Driving source can be determined according to actual needs. The place far enough from the source can be assumed to zero.

III. VACUUM INTERRUPTER COMPUTATIONAL

MODEL

A vacuum interrupter (12kV, 20kA) is modeled, whose length is 162mm, diameter is 90mm. Diameter of contact is 55mm, and thickness is 21mm. Length of contact shield is 84mm, and thickness of contact end cap is 5mm. The model of the vacuum interrupter is performed subdivision and finite element calculation. Due to computational limitations, end caps, contacts and other places have been simplified. Such as slotting of contact is ignored, contact is simplified as a cylinder. Fig. 2 shows the profile of 3D calculation model of the vacuum interrupter. 1 is end cap of moving contact; 2 is end shield cover; 3 is moving contact; 4 is shield cover of contacts; 5 is static contact; 6 is the glass enclosure; 7 is end cap of static contact. Fig. 3 shows the 3D electric field distribution of vacuum interrupter.

Fig. 3 shows that the internal electric field of vacuum interrupter mainly focused on the area between the static contact and moving contact. Further study found that the size, shape and other factors of contacts and shield cover have a strong effect on internal electric field of interrupter, and it affects the value of the equivalent capacitance of interrupter. The capacitance equivalent circuit of the vacuum interrupter shows in Fig. 4. The static contact is equivalent to the electrodes l; the shield cover of contacts is equivalent to electrode 2; the moving contact is equivalent to electrode 3. The model is intercepted for half. The equivalent capacitance between the static contact and moving contact, equivalent capacitance between the static contact and shield cover, equivalent capacitance between the moving contact and shield cover are calculated. The results as following: C13 = 1.2473pF; C12 = 6.1213pF; C23 = 6.3513pF. Fig.4 is the capacitance equivalent circuit of vacuum interrupter.

Equation (3) gives the self-capacitance of vacuum interrupter:

12 2313

12 23

4.3644pFvC C

C CC C

(3)

Fig.2. Profile of 3D calculation model of the vacuum interrupter

Fig.3. 3D Electric field distribution of vacuum interrupter

Fig.4. The capacitance equivalent circuit of vacuum interrupter

IV. SF6 INTERRUPTER COMPUTATIONAL MODEL

A SF6 interrupter (40.5kV, 31.5kA) is modeled in the SOLIDWORKS. The interrupter’s length is 500mm, diameter is 176mm; inner diameter of static contact is 43mm, outside diameter 66mm; the moving contact diameter 67mm; static arcing contact diameter and moving arcing contact diameter all are 20mm. Fig. 5 shows the profile of 3D calculation model of the SF6 interrupter. The cylinder of SF6 interrupter is substituted

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by cylindrical conductor. The insulated rod of SF6 circuit breaker and other institutions are omitted also.

Fig.5. Profile of 3D calculation model of the SF6 interrupter

In Fig. 5, 1 is the moving contact end cap; 2 is a cylinder and the moving contact; 3 is interrupter porcelain; 4 is moving arcing contact; 5 is a PTFE nozzle; 6 is a static finger contact; 7 is a static arcing contact. The electric field distribution between static contact and moving contact is obtained by the numerical calculation of SF6 interrupter, as shown in Fig. 6. Insulation characteristic of SF6 interrupter can be further analysis according to the electric field distribution and can make a corresponding optimization design. Setting the moving contact as the electrode l and static contact as electrode 2, self-capacitance of the half interrupter model is solved. Solving is relatively simple Because of SF6 interrupter without shield cover. The calculation result is CG = 3.7123pF.

Fig.6. Electric field distribution of SF6 interrupter

V. HCB ELECTRIC FIELD DISTRIBUTION

As what mentioned above, vacuum interrupter (12kV) and SF6 interrupter (40.5kV) are connected in series to make up a model of hybrid circuit breaker. Fig.7 shows a 3D calculation model established in Solidworks. Vacuum interrupter sits on top of SF6 interrupter. The distance between vacuum interrupter and SF6 interrupter is 800 mm; the distance between SF6 interrupter and ground is 800 mm (actual situation for reference). In this model, connecting rods of vacuum circuit breaker and SF6 circuit breaker, connecting flange are simplified,

and insulation ceramics of HCB is omitted. The dielectric of external area of HCB model is set to air. The number of subdivision units is 96184 after HCB model is split in the entire calculation area.

Fig.7. 3D computational model of HCB

The computational results of HCB electric field distribution is showed in Fig. 8. The voltage distribution of vacuum interrupter and SF6 interrupter can be seen from Fig.9 and Fig.10. The calculation shows that the voltage of the vacuum interrupter is 66.6% of the total voltage, the voltage of SF6 interrupter is about 33.4% of the full voltage.

Fig.8. 3D potential distribution map of HCB

Fig.9. Electric field distribution of vacuum interrupter

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Fig.10. Electric field distribution of SF6 interrupter

VI. CAPACITANCE COMPENSATION

The capacitances between different parts of HCB are carried out matrix calculation. Set static contact of vacuum interrupter as electrode 1, contact shield cover as electrode 2, moving contact of the vacuum interrupter (including actuator) and static contact of SF6 interrupter as electrodes 3, moving contact of SF6 interrupter (including actuator) as the electrode 4, ground as electrode 5, as shown in Fig. 11. The results of capacitance matrix calculation are as follows:

C13=1.2888pF, C12=4.934pF, C23=5.7496pF, C34=4.3676pF, C35=3.1705pF, C25=0.2571pF, C15=0.0900pF.

Fig.11. the equivalent diagram of the self-capacitance of HCB

According to the capacitance value of matrix computation, equivalent capacitance of vacuum interrupter is CV = 3.9655pF; the equivalent capacitance of SF6 interrupter is CG = 7.8052pF.

Voltage percentage of each interrupter is as follows:

UV%= G

V G

CC C

=66.31%

UG%= V

V G

CC C

=33.69%

This result is roughly equal with the result of electric

field distribution calculated by Ansoft. It verifies the result of self-capacitance calculations. In order to reducing the recovery voltage of vacuum interrupter in later period of TRV, a compensation capacitor needs to be parallel connected on vacuum interrupter. According to Fig. 11, the recovery voltage of vacuum interrupter can be reduced to 30% of the total voltage if a capacitor (15pF) is parallel connected on it.

VII. CONCLUSION

1. The 3D model of the vacuum interrupter and SF6 interrupter are established. It is disassembled by 4-nodded tetrahedron elements. Use Ansoft software, their internal 3D electric field distribution and self-capacitances are calculated.

2. 3D finite element model of HCB with vacuum circuit breaker vertical in the above and SF6 circuit breaker vertical in the below is designed. The electric field distribution and the equivalent capacitances of vacuum interrupter and SF6 interrupter of HCB have been solved. The voltage distribution results between the calculation of electric field and calculation of equivalent capacitances are contrastive verified.

3. The voltage distribution ratio of vacuum interrupter in the later period of TRV is optimized. A paralleled capacitance, which value is 15pF, can reduce vacuum interrupter voltage to 30% of the total voltage.

REFERENCES [1] R.P.P.Smeets, V.Kertesz, and D.Dufournet, "Interaction of

a vacuum arc with an SF6 arc in a hybrid circuit breaker during high-current interruption," IEEE Trans. on Plasma Sci., vol. 35, pp. 933-938, 2007.

[2] K. Natsui, Y. Kurosawa, Y. Hakamata, H. Hirasawa, and Y. Yoshioka, "Voltage distribution characteristics of series connected SF6 gas and vacuum interrupters immediately after a large AC current interruption," IEEE Trans. Power Del., vol.3, no. 1, pp.241–247, 1988.

[3] Cheng, Xian, Duan, Xiongying, and Liao, Minfu. "Breaking capacity gain characteristics of vacuum interrupter combined with SF6 interrupter," Chinese Journal of Vacuum Science and Technology, vol.32, no. 3, pp. 201-207, 2012.

[4] Cheng, Xian, Liao, Minfu, and Duan, Xiongying, "Study on Breaking Characteristics of High Voltage Hybrid Circuit Breaker," 24th International Symposium on Discharges and Electrical Insulation in Vacuum, pp. 449-452, 2010.

[5] Tennant. A, and Ide. J. P, "Modelling a planar phase switched structure in Ansoft HFSS (high frequency structure simulator)", ICAP 2003, pp. 257-261, 2003.

[6] Sun Jing , Cao Yundong , and Liu Xiaoming, "Design and Analyses on Permanent Magnet Actuator for Mining Vacuum Circuit Breaker," 22th International Symposium on Discharges and Electrical Insulation in Vacuum, pp. 512-515, 2006.

Email of the author(s): chengxian@mail.dlut.edu.cn