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ELEKTROENERGETIKAStar Lesn, 21. - 23. 9. 2005

SOME ASPECTS OF INVESTIGATION OF MAGNETIC

FIELDS PRODUCED BY MEDIUM VOLTAGE SWITCHGEARS

*Martin MACH, **Raimund SUMMER

*Department of Theory of Electrical Engineering, Faculty of Electrical

Engineering, University of West Bohemia, Univerzitn 26, 306 14 Plze,Czech Republic, Phone: +420-377634697, E-mail: [email protected]

**Areva Sachsenwerk GmbH,

Rathenaustrasse 2, D-93055 Regensburg, GermanyPhone: +490 941 46 20 206, E-mail: [email protected]

AbstractInvestigation of magnetic fields in medium voltage switchgears (up to 40 kV) becomes a

strongly topical business, which has several reasons. Nowadays, various switchgears and

substations of this kind are often installed in densely populated areas (for example in

buildings for commercial purposes etc.) and may represent a danger associated with their

possibly negative influencing of near low current and telecommunication devices as well as

living organisms. Because of a lot of various uncertainties in this domain practically all

developed countries accepted corresponding standards that, nevertheless, still substantially

differ from one another. The unambiguous trend is (at least within the European Union),however, their unification.

Manufacturers and operators of the mentioned appliances must certify them, accordingly,

i.e. must guarantee the maximum values of field quantities in its neighborhood. This may be

realized by either measurements (that is, however, time consuming, expensive and, under

common operation conditions, often complicated) or numerical simulations. The latter way

after validating results by experiments becomes relatively cheap and offers inclusion of

optimization techniques.

The authors continue in their work where they suggested a methodology of determining

magnetic field distribution in the neighborhood of switchgears that consists of several steps.

The basics steps are represented by preprocessing (input of 3D geometry), building of 3D

mathematical model, its solution (realized by a combination of ANSYS and a number of user

procedures) and consequent verification of results. The paper pays attention mainly to the

phase of preprocessing (selection of the definition area with boundary conditions, parameters

of discretization mesh) and solution. Discussed is accuracy of results and also problems

concerning measurements and evaluation of the field distribution. The methodology is

illustrated on an example switchgear substation of type WS, manufacturer Areva

Sachsenwerk, Germany. The most important results are compared with measured values.

This paper represents one of the results of cooperation between the University of West

Bohemia and Areva Sachsenwerk, Germany.

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1. Basic properties of the switchgears

The essential components of switchgear

installations are the bus bars (as well as somebar elements), switchgear device, space for

feeding of power cables and other equipment

(case for switchgear mechanism, low voltage

devices etc.). The basic structure is

schematically shown in Fig. 1. The entire

switchgear installation consists of those

switchgears panels where the arrangements

and types of used panels depend on the

requirements of the operating utility.

From the viewpoint of computer simulation it

is possible to divide the investigated mediumvoltage switchgear into next following five

basic functional parts:

container with vacuum circuit breakers,disconnector, bus bars etc

cable compartment low voltage cabinet housing for the operating mechanism of the switching devices cooler

All these parts can be then divided into two types: active parts (such as conductors, bus barsetc.) representing sources of magnetic field and passive parts (cases from magnetic or

nonmagnetic steel, coolers) that more or less shield the magnetic fields.

2. Steps of the solution

For the numerical simulation using FEM software it is usually necessary to simplify this

relatively complicated geometry and take into account only the relevant shielding parts

reducing the magnetic field outside the device. Resembling all above mentioned parts, the

authors used in phase of preprocessing a combination of possibilities of CAD systems

(Mechanical Desktop) with Ansys pre-processor. The model geometry created was then

discretized and solved in Ansys 6.1 environment.

Distribution of magnetic field is evaluated mainly outside the switchgear with the aim to

evaluate how far from the investigated device are isolines 1, 10and 100(maximum

values admitted by the standards).

The results obtained are then compared with measurement (in this case with respect to the

Switzerland national standard allowing 1). The measurement of the switchgear was

carried out under operation conditions in the factory of Areva in Regensburg.

Bus bar

Circuit-breaker

Feeding power

cables

Case for switchgear mechanism

Low voltage cabinet

Fig. 1: Schematic structure of a

switchgear panel

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3. Mathematical model of the task

The switchgear station produces open-boundary magnetic field that must be (for the sake of

computations) bounded by an artificial boundary sufficiently distant from the field sources.Even so, some other simplifications have to be accepted in order that the definition area is

acceptable as for its size. That is why we neglect the contribution of the conductors feeding

the switchgear (this is acceptable to some extent, because in a three-phase system whose

conductors are close to one another the magnetic field is small). Respected will be, therefore,

only the switchgear itself, whose active parts are placed more than 1.5 m over the earth

surface. The field is then supposed to be closed in the indicated space. It is generally

described by the parabolic equation for vector potential A

ext

1curl curl

+ =

AA J

t. (1)

with the Dirichlet boundary

condition, as indicated in Fig. 2.

Here denotes the magnetic

permeability, the electrical

conductivity and ext the density

of external currents.

But even this equation describing

the time evolution of 3D

magnetic field in time is difficult

to solve. On the other hand, from

the reasons mentioned above, theferromagnetic materials used in

structural parts of the switchgear

are not oversaturated and their

permeability (confirmed by

measurements) may be

considered approximately

constant. Now (1) can be

simplified to the Helmholtz

equation for the corresponding phasors

extcurlcurl j + =A A J (2)

(the boundary condition remaining unchanged). The part of the second term j A

expresses eddy current densities in electrically conductive parts of the system.

A=0

A=0

x

z

y

Fig. 2: Schematic structure of a switchgear panel

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4. Numerical simulation of WS switchgear

This part of the paper deals with a simulation model of a switchgear of type WS produced by

Areva Sachsenwerk GmbH (Germany). This type of switchgear with thoroughly gas-insulatedbus bar systems is primarily designed for installation in transformer substations, industrial

plants and infrastructure. The real arrangement of one switchgear unit and its comparison

with 3D simulation model is shown in the Fig. 3. The numerical model was solved in Ansys

6.1 environment and the 3D geometry was in combination of CAD systems and Ansys

preprocessing tools created. Simulated was one of typical produced arrangement with two

functional units of nominal current I = 1000, 1500 A. Several checking calculations had to be

carried out for obtaining a good idea about hardware requirements and mesh control

parameters.

Next table shows the basic material properties of used materials and the main dimensions of

the tested switchgear. Material of the conductors is copper, the circuit breaker container is

made from nonmagnetic steel, the cooler from aluminum and the others steel parts from

magnetic steel. Thickness of steel parts is 0.003m.

Tab. 1: Material properties of used materials and the basic the basic dimensions of the

complete device

Material/Properties Permeability (-) Resistivity (/m)Copper 1 1.8E-8

Aluminum 1 4.54E-8

Nonmagnetic steel 1 0.73E-6

Magnetic steel 700 0.12E-6

Fig. 3: Investigated switchgear of WS, comparison with simulated

low volta e cabinet

case for the sw. mechanism

cooler

cable compartment

system of conductors

cooler compartement

circuit breaker container

1 2

1.6 m

2.5

m

1.1m

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Solution of magnetic field distribution around switchgears is a typical task involving

geometrical incommensurabilities. Relatively small parts (steel sheet) are placed in large

volumes (air background) and hence the meshing of the model is not an easy business and a

suitable approach is needed.

In the domain of discretization we took advantage of mesh possibilities in Ansys environment

and in steel parts (with the thickness 0.003m) we used a regular hexahedral mesh (see Fig. 4).

The discretization adequate to skin-depth could be created and the number of elements in the

model was significantly limited. The number of elements across the thickness is given by the

fact that it is not necessary to know exactly the distribution of the magnetic field in the steel

parts and in the close space. It was found that for adequate distribution of magnetic field at

points more then 0.2 m far from the device three elements are quite enough. As the problem

does not require accurate evaluation of the current density in the conductors, we used the

same type of elements also in them (but only with one element).

The switchgear is then placed in an air volume and along its exterior surfaces the boundary

conditions are applied. In our case we used an upper half of a fictitious sphere of radius r >

7m (see Figs. 2, 5). The optimal radius of the background volume was determined

experimentally. For its discretization we used tetrahedral elements. In the course of

implementation of the model, the ANSYS elements SOLID 117 were employed for all parts.

For the sake of simplification, we took a linear magnetic permeability of the magnetic steel

parts. Nevertheless, this simplification is relatively correct, because the shielding material is

not fully saturated and our constant permeability presumption has no effects on the results of

solution. This assumption was validated also by measurements, where a linear dependence of

the magnetic field on source current was detected.

Nevertheless, in any case the number of elements in the model exceeded the limit 250 000.

Two types of solvers were also used during computations. The first one a direct sparse

solver is based on direct elimination of equations and is recommended especially for ill-

conditioned matrices, due to poorly shaped elements. Unfortunately, this type of solver has

extremely memory requirements and, therefore, bigger models had to be solved by an

iterative solver. Unfortunately, this type of solver converges in harmonic magnetic problems

only very slowly. The practical values are presented in Tab. 4 and Tab. 5.

Fig. 4: Discretization mesh in conductors

andsteelFig. 5: An example of discretized

background

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Most of computations were realized on a PC WIN XP machine 2*P4 3.4 GHz, 4GB RAM and

the time of computation was approximately 1 hour. Practical experience shows, that the

effective and operative computations (optimization) of real devices have really high hardware

requirements.

Number of volumes 80

Number of nodes 260 000

Number of elements 150 000

Number of equations 200 000

Tab. 2: Typical parameters of the 3D model and its solution

(the ICCG type of iterative solver implemented in Ansys 6.1)

Fig. 6 shows an example of the magnetic field distribution around the investigated switchgear

station. 1 isosurfaces of magnetic field outside the switchgear are depicted. In order toobtain correct results, a shielding wall was also considered at the left side, just as it was

during the verification measurement. Using these results, it is then possible to make a cut in

all controlled (measured) levels and compare the simulated isolines with measurements.

Matrix solver Time of solution

Direct (Sparse) 50 min

Iterative (ICCG) 80 hours

Fig. 6: 1 T isosurface, cut in the front plane of the device

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5. Measurement

In order to check the usability of the proposed method as well as its validity and accuracy,

corresponding measurements have been performed at real switchgear installation in January

2005. The device under test consists of two functional units. It was necessary to find the

distribution of magnetic field and the distance of 1T lines from the switchgear. The

measurement was performed for purpose of validation of the simulation. Measured was the

magnetic field from the switchgear as well as field from the transformer. For validation of

presumption of linear permeability we performed measurements for two values of the feeding

current. The measurement configuration is shown in Fig. 6.

Fig.7. The measurement configuration - top view, real arrangement

The switchgear station was measured in free area. The current source was a 3 phasetransformer. The 3 phase current leads were insulated and put into close contact to reduce

the magnetic emission. Steel plates were placed on top of the leads for additional shielding.

The current input was in functional unit 1 (left in Fig. 1). Both units were connected by bus

bars and the conductors in second unit were short-circuited.

The magnetic field measurement on the WS switchgear was performed using magnetic field

measurement device EFA-3 that measured the effective (RMS) value of field. Because the

measured data represent the sum of transformer magnetic field and magnetic field from the

switchgear, we measured only magnetic field of the transformer (switchgear device is

disconnected) in order to be able to secure a correct comparison between simulation and

measurement. The relevant distribution of magnetic flux density between the measuring points

was interpolated by continuous curves.. Magnetic field of the switchgear station wasdetermined from measurement data as is depicted in Fig. 8.

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0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5

distance (m)

B

mod(T)

Switchgear + transformer

Only transformer

Only switchgear

1T

Fig. 8: Determination of magnetic flux density of switchgear from measurement data

6. Comparison between simulation and measurement

The comparison between numerical simulation and measurement is shown in Fig. 9.

As can be seen, the agreement at the selected level h = 2m is very good. Along the lower

levels (see, for example, Fig. 10) the results still well correspond with measurement, but

nevertheless, difference between measurement and simulation is more obvious. This is caused

by deformation of magnetic field by shielding plates, not included in the simulation. A better

way how to shield the transformer field is to use a channel from magnetic steel for the lead-in

cable.

Fig. 9: Comparison with measurement, cut h = 2m,I = 1000A

measuringpoints

interpolated curves

resultant field

measurement

calculated 1T lines

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Fig. 10: Comparison with measurement, cut h = 1m,I = 1000A

Conclusion

The aim of this paper was to present the possibilities of numerical simulation as a powerful

tool in the field of certification of industry devices and summarize the practical experiences of

the authors in this field. A numerical model in Ansys 6.1 environment was developed forselected type of medium voltage switchgear and corresponding measurements were

performed. A very good agreement with measurements was achieved without modifying

material properties. The main part of the modeling is realized by Ansys scripting language

that is very easy to modify and extend.

References

[1] Dvok P., Mach M.: Possibilities of passive and active shielding of medium-frequency magneticfields, Proceedings EMD2004, 22.-24.9. 2004, Vilnius, Lithuania, ISBN 9986-05-776-3, pp. 79-82

[2] Chari M., Salon S.: Numerical methods in electromagnetism, Academic Press, 2000.[3] Mach M., Summer R.: Magnetic field emission of gas-insulated switchgears, Proceedings

EPE2004, 31.5.-2.6. 1005, Dlouh Strn, Czech Republic, ISBN 80-248-0842-0, pp. 79-82

[4] Jkel W. B.: Investigation of magnetic fields of single pole encapsulated switchgear installations,Proceedings EMC 2002, Wroclaw, pp. 369-372

[5] Ida N:, Engineering electromagnetic, Springer, 2000.[6]Ansys reference manual, Ansys Inc.[7] www.areva-td.com

[8] www.ansys.com

measurement

calculated 1

T lines

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