Conductor Short Ckt Cal

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400 KV SUBSTATION STRANDED CONDUCTOR BUSES- Tests and Calculations of Short-Circuit Constraints and Behaviour -

Norbert STEINFGH, Forschungsgemeinschaft fr Elektrische Anlagen

und Stromwirtschaft, D-68219 Mannheim, Germany

Amir M. MIRIInstitute of Electric Energy Systems and High-VoltageEngineering, University of Karlsruhe, P.O. Box 6980,

D-76128 Karlsruhe, Germany. Phone++49-721-608-3061,Email [email protected]

Wolfgang MEYERUniversitt Erlangen-Nrnberg,

Lehrstuhl fr Elektrische Energieversorgung,

D-91058 Erlangen, Germany

Abstract: FGH and VDE have completed an extensiveprogramme of short-circuit tests on strainedbus conductors.Planning of the test series was done by DKE WG 121.2.2'Short-Circuit Tests' and was performed under theirguidance and that of CIGRE 23-03 ESCC TF (Effects ofShort-Circuit Currents). The tests were performed onarrangements with and without droppers fitted to the busconductors. For the arrangements with droppers the di-

fferent possible current paths were studied. Evaluation andfurthergoing study of test results are in progress. ParallelFE numerical simulations have been carried out by Prof.

Miri at IEH, Karlsruhe University. Beside the numerous

variations of the short-circuit data - s.c. current and du-ration - the geometrical/mechanical parameters of the testarrangements were varied - e.g. conductors, phase distan-ces, suspension insulator chains, anchoring steel structurestiffness and eigenfrequency - to a degree to cover a widescale of practical applications. The latter parameter varia-tions were achieved by studying typical and exemplary100kV and 400kV arrangements. The present paper dealswith the recent 400kV part of these investigations (the 100kV

part to be published in ETEP [6]) with respective testresults, together with parallel numerical studies and, also,

first possible consequences to be drawn as to the standard-ized rules for assessing short-circuit performance by IECTC73 for Publ. 865. The authors give a survey of the testedarrangements, together with the parameter variations

(structure mechanical and electrical) thus achieved, thedetailed measurements of structure mechanical constraintsand displacements and respective test results in exemplary

form. They report on the numerical studies applying FEMtothe test programme, which show a remarkable accuracy incomparison with the actual test results. Finally, firstapproaches as to modification requirements for the men-tioned IEC/EN/VDE calculation procedures for assessingshort-circuit performance and strength are presented.

Key words: Short circuits, measurement of mechanicaleffects, structural analysis, Finite Element Method

1. INTRODUCTION

It was with the intent to further the development of the

short-circuit assessment methods of IEC 865-1 andVDE 0103 so as to cover the so far not accessible ar-rangements of droppers and long spans of strandedconductors with droppers that FGH and DKE 121.2conducted the former study [2] and the present one, [1]having been the basis for introducing the calculationprocedure for so-called 'Long Spans' into the standard.The second aim of the present tests is to further thestudies of the ESCC Task Force of CIGRE 23-03 on'Equivalent Static Load' for portal structures and their

foundations.The new test series comprises 100kV and 400kV

arrangements of long spans of stranded conductor bus-bars strained between portals, without and with drop-pers leading down to a lower conductor level. The basicgeometric parameters of the former [1, 2] have been ap-plied, where possible, for compatibility. In particular,conductor sag values were maintained. The availablematerial is immense, and the present paper had there-fore to be confined to the 400kV part and its results asfar as the applicability of the procedures of [3, 4, 5] for'long spans' holds valid at present: tensile forces on the

suspension points and bus conductor displacement. Therespective 100kV results will be in [6], and a com-prehensive presentation of the complete data is planned.

Chapter 2 gives a survey of the complete tests, thegreat number of measuring points, exemplary results asregards time dependency and maximum values oftensile loads on the suspension portals in comparisonwith results of FEM calculations by the University ofKarlsruhe as explained in Chapter 3. Maximum dis-placement bh and minimum air clearance dmin are given.Chapter 4 reports on a first approach to the extension ofthe existing IEC/VDE rules to spans with droppersdeveloped at the University of Erlangen-Nrnberg. Both

7th International Conference on Optimization of Electrical and Electronic Equipment OPTIM 2000,Brasov (Romania), 11.-12. May 2000; Proceedings pp 251-258


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The portal structures, beside their design drawings andconstruction data, are defined as regards their static and,in particular, their oscillatory behaviour by essentialstructural properties Stiffness and Eigenfrequency,measured in separate mechanical tests and listed in the

following. These mechanical tests show absolutelylinear elastic characteristics for both portals given interms of the values of stiffnes SN and SM, the resultingstiffness to be used for the IEC/VDE simplified modelis Sres. The respective values for the 400kV suspensionpoints at 11.22 m crossarm height and phase distance a= 3 m are:

SN = 1.086 kN/mm SM = 1.223 kN/mm

Sres = 0.575 kN/mm.

The relevant first eigenfrequencies excited at the

mounted mid crossarm, i.e. next to the suspensionpoints, are 9 Hz for the N-portal crossarm and 9.5 Hzfor the M-portal crossarm, while the complete portalshave basic frequencies of 3 Hz and 4.3 Hz, respectively,the M-portal having the stiffer construction (see above).

Fig. 2 Exemplary measured (above) and FEM-calculated

(below) oscillograms of short-circuit tensile forces

KP for 100 kA/40 kA - 0.5 s test on variant No. 9

(curent path B)

For each combination of test parameters as compiledin Table 1, at least two identical consecutive tests wereperformed to show the variance of behaviour andeffects. For reasons of symmetry, this gives at least four

values to be considered for every parameter of eachconstellation. The variance is, as can be seen exemplari-ly from Figs. 4 to 6, astonishingly small.

Figs. 2 a) and 3a) show exemplary oscillograms ofthe considered quantities from 100 kA / 40 kA - 0.5 s

tests on arrangement No. 9 (current B). The samearrangement and test parameters are used in Chapter 3to demonstrate the quality of FEM calculation throughthe conformity of measured (Fig. 2a and 3a) andcalculated (Fig. 2b and 3b) oscillograms.

Short-circuit tensile forces: The only loads to be con-sidered are the swing-out maxima Ftand the conductor-fall maxima Fr. Conductor pinch forces do not occur, orare negligible with single or close bundle conductors.Fig. 4 gives for current paths A, B and C (8 - 12) themeasured valus ofFt and Fr over the respective valuesof short-circuit duration. The mean values are connect-

ed by straight lines only for the purpose of betterreadability. Linear interpolation between measuringpoints could be misleading, as intermediate 0.2 s values,for instance, in Fig. 4b show.

For current paths A and B a clear tendency for areduction of values for very low short-circuit durationsis obvious, which is less significant on C. The valuesfor case C, where the short-circuit current uses only halfof the span length, are clearly reduced against bothother cases A and B.

Bus conductor displacement: Figures 5 and 6 givethemaxima of the horizontal bus conductor displacements

towards the outside bh and the minimum air clearancebetween these conductors dmin in the swing-back phaseafter short-circuit for variants 8 - 12. These are acces-sible by present IEC/VDE rules for the case A. Theextrema occur on the first swing-out (at or near theinstant of Ft for tk 0.2 s) and the first return of thespan conductor(s) after short-circuit. The oscillations ofthese conductors in most of the cases persist over ratherlong periods of time after short-circuit with onlymoderate damping of the original displacements. Al-though values are not given in this paper, it should bementioned that large and persisting oscillations occur,in a much more important degree, in the movements ofthe droppers in cases B and particu-larly in C. Short-circuit durations of 0.2 to 0.3 s produce the worst casesof span-conductor approach, while displacement to-wards the outward is roughly constant beyond a short-circuit duration of 0.2 s. It is obvious that case C valuesfor short-circuit displacement bh of the span conductorand minimum clearance between the neighboring spansdmin, with the short-circuit current using only half of thespan, must be less than both other cases.


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Fig. 3 Exemplary measured (above) and FEM-calculated(below) oscillograms of short-circuit constraintMAFU at the junction tower/foundation for 100kA / 40 kA - 0.5 s test on arrangement 9 (currentpath B). Same test as Fig. 2

0

10

20

30

40

50

F

40 kA

28,3 kA 20 kA

F t

F f

kN

F tF t

F fF f

a)

0

10

20

30

40

50

F

kN

Ff

40 kA

28,3 kA

20 kA

Ft

Ff

FtFf

Ft

b)

0

10

20

30

40

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0

tk

F

40 kA

28,3 kA

20 kA

Ft

Ff

s

kN

Ft

Ff

c)

Fig. 4 Short-circuit tensile force Ft and drop force Ffa) Cases A, B b) Case C

0,0

0,5

1,0

1,5

2,0

2,5

3,0

dmin

40 kA

28,3 kA

m

A

A

B

a)

20 kA

0,0

0,5

1,0

1,5

2,02,5

3,0

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0

tk

dmin

40 kA28,3 kA

20 kA

s

m

b)

Fig. 5 Minimum air clearance dmina) Cases A, B b) Case C

0,00

0,25

0,50

0,75

1,00

b h

40 kA

28,3 kA

20 kA

m A

A

A

B

B

B

a)


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0,00

0,25

0,50

0,75

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0

tk

b h

40 kA

28,3 kA

20 kA

s

m

b)

Fig. 6 Maximum horizontal displacements bha) Cases A, B b) Case C

3. CALCULATION WITH ADVANCED METHODS

The simplified methods of calculation in IEC/VDE areuseful and necessary for typical design cases by hand orcomputer-aided [7] calculation and allow parameter-sensitive in-vestigations in a very short time by use ofpersonal com-puters. Only general input data are re-quired, and the results are maximum values of tensileforces and dis-placements. The procedure is adjusted topractical requirements and contains simplifications withsafety margins.

Advanced methods use finite element or finite dif-ference modelling, and powerful software is availableon workstations and personal computers. They can beapplied to any structural configuration with single and

bundled conductors and forcing function [8]. The com-putation of the dynamic response of the completestructure including their nonlinear behaviour is pos-sible, and accurate results can be obtained, limited onlyby the degree of detail in the modelling and the avai-lability of reliable basic structural data. The calculationof eigenfrequencies, time histories of forces, momentsand deformations allows to study the system behaviour,to detect and improve weak points, or to ascertain theshort-circuit strength even for complex cases. The rangeof validity of simplified methods can be investigated.

During more than 20 years, the users of the ad-vanced methods have acquired an excellent know-how

in the modelling and computation of substation struc-tures. Test results have always been taken for compari-son and adaptation.

The possibilities and experience gained with thesedetailed methods now allow to fill out and extend therequired framework set up by singular test result data byinter- and extra-polation varying the original test para-meters in a degree that could not be done in actualtesting. Only thus can the control basis for the develop-ment of simplified calculation methods for new appli-cations be laid.

In consequence, the test structure of Fig. 1 is dis-cretized in a full detail FE model, using appropriate

beam elements for the framework of the portals and

adjusting the model to achieve first the proper stiffnessand then eigenfrequency values. As usual conductorsare done in truss elements - with particular dashpotelements for duplex conductors. The programme ap-plied was ABAQUS. The calculation was so far per-

formed for variants 1 to 10 of Table 1 and a short-cir-cuit duration of 0.3 or 0.5 s. The remarkable accordanceof calculation with measurement expresses itself inparticular in the comparison of the measured and calcu-lated exemplary oscillographs and absolute values ofFig. 2 and 3. The achieved results are evidence at oncefor the validity of the applied method, as well as of itspractical use.

4. CALCULATION ACCORDING TO IEC 60865-1In IEC 60865-1 [3], identical to EN 60865-1 [4] andDIN EN 60865-1/VDE 0103-[5], a method is standard-ized for the mechanical effects on substation buses with

flexible conductors due to short-circuit currents. Thetensile forces Ft, Ff and Fpi and the maximum horizontaldisplacement bh can be determined analy-tically. IEC60865-2 [9] gives an example for the calculations. Thephysical background, the assumptions made and thederivation of the method are described in detail in [10,11]. The confrontation with many test results gives agood agreement. This standard is so far only applicableto arrangements without droppers, and the aim is toshow how to take into account droppers at midspan.The basis for the investigations are the test resultsdescribed above.

At first, a calculation of the static sag is carried out

using the change-of-state equation. For variant 8 (with-out droppers) of Table 1, it corresponds to the tests. Forthe variants with droppers, an additional mass has to beadded in midspan which consists of the mass of theclamp and about half the mass of the dropper due to itsstiffness and its fixation in the clamps.

For the design of busbars, the maximum short-circuitduration Tk is stated by the protection concept. Theactual short-circuit duration tk is unknown, can be lowerand can lead to higher tensile forces than Tk. Thereforethe maximum values are determined by [3], whichoccur within 0 < tk Tk [10]. In contrast, the short-circuit duration tk is known when calculating tests andyields the swing-out angle k at the end of the currentflow, see [10, equ. (4.8)]. If tk Tres, the con-ductorswings out to its highest position at m = 21 and thenback. Tres is the resulting oscillation period of the spanduring current flow and 1 = Arctan r the direction ofthe maximum radial force Ft. r means the ratio ofelectromagnetic force and gravitational force on theconductor. Iftk Tres, m = Arccos (1 -r sin k) holds; Ftis maximum at 1 if k 1, otherwise at k. Ft acts atthe end of the drop down from m.

For the variants 2 - 7 and 9 - 12, many investigationshave been done. The best agreement with the tests could

be achieved with the following assumptions:


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- Disregard of the dropper mass: Fig. 4a, b shows thatdroppers without current have no significant influenceon the tensile forces and the displacements.- Disregard of the influence on the main-conductormovement by the droppers: In contrast to the main-

conductor movement without droppers, there is achange in the trajectory when the dropper is stretched.Kinetic energy of the main conductor is converted intoelastic energy in the dropper and back during the falldown. Due to the energy conservation law the tensileforces Ft will not differ very much compared with thecase without droppers; Fig. 4a, b confirms this.- Influence of the current path on the electromagnetic

force: Cases A and B induce in the main conductors theelectromagnetic force per unit length [10, eq. (*19)]

l

l

a

IF ck

2"2 )(

2

' 0

= (1)

With Case C, the current flows only through half themain conductor and then through the dropper. Thetransverse force on the main conductor due to theelectromagnetic force between the droppers is con-sidered by substituting Ic in (1):

l

ll

a

IF dck

2/2/)(

2'

2"20 +

=

(2)

lc is the main-conductor length, ld the dropper length

and l the span length.The comparison with the test results is given in Fig.7; calculated values on vertical axis (index c) versusmeasured values on horizontal axis (index m). Eachsign marks a compared result different for cases B andC and additional case A. Fig. 7a shows the greater oneof each pair of tensile forces Ft, Ff, which is relevant fordesign purposes. For Ik = 40 kA, Ft is always higherthan Ff; for 28.3 and 20 kA they are similar. Fig. 7bcompares the maximum horizontal displacements bhh. Inaddition, the location of the relative error 0 % and thetechnical limit 25 % are depicted as broken lines;above the 0-% line the calculated values are on the safe

side.

Fig. 7 points out:- Most of the calculated values are on the safe side,

the error is less than 25 %.- When they are not on the safe side, they are

sufficiently small and occur only in the case of tk =0.1 s.

- Due to the greater static tensile force, the tensile for-ces of case B are a little bit higher than those of A.

- In tendency, the horizontal displacement is calculat-ed too high especially for 40 kA caused by physicaleffects which cannot be taken into account [10].

0

10

20

30

40

50

0 10 20 30 40 50

Fm

Fc

A B C

kN

kN

a)

+25 %

-25 %

0 %

current path:

0,0

0,2

0,4

0,6

0,8

1,0

1,2

0,0 0,2 0,4 0,6 0,8 1,0 1,2b hm

b hc

A B C

+25 %

-25 %

m

m

b)

0 %

current path:

Fig. 7 Comparison between calculation and testa) Maximum ofFt and Ffb) Horizontal displacement bh

- When comparing the horizontal displacements, it

should be kept in mind that they are measured withan increment of 5 cm.

The outcome of the calculation in comparison withthe tests permits to extend the method stated in IEC60865-1 [3] to arrangements with droppers in midspan.The procedure should be as follows:- the static tensile force Fst and the sag are estimated

with an additional mass in the span equal to theclamp mass plus half the mass of the dropper.

- Using this value ofFst, the short-circuit tensile forcesFt, Ff, Fpi and the maximum horizontal displacementbh are calculated according to subclause 2.3 of IEC

60685-1.


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- If the current is flowing over half the span and thedroppers, eq. (19) of IEC 60685-1 for the electro-magnetic force on the main conductor should bereplaced by eq. (2) above.

ACKNOWLEDGEMENTThe investigations were sponsored by the"Bundesminister fr Wirtschaft" of Germany throughthe"Arbeitsgemeinschaft.Industrieller.Forschungsver-einigungen " Otto von Guericke e.V. (AiF) under theproject number 9784. The authors wish to express theirthanks to AiF for this substantial support,and also toRWE Energie AG and Bayernwerk AG for theirpractical help to make the tests possible.

REFERENCES

1. Herrmann, B., Stein, N., Kiessling, G.: Short-circuiteffects in high-voltage substations with strandedconductors. Systematic full-scale tests and a simplecalculation method, IEEE Transactions on Power

Delivery 4 (1989), pp. 1021-1028

2. Hosemann, G., Miri, A.M., Stein, N., Zeitler, E.: Thebehaviour of droppers in high-voltage substationsunder short-circuit, in Proceedings of the 5thInternational Symposium on Short-Circuit Currents inPower Systems, Warszawa, 1992

3. IEC 60865-1, Short-circuit currents - Calculation ofeffects. Part 1: Definitions and calculation method,Geneva; IEC, 1993

4. EN 60865-1, Short-circuit currents - Calculation ofeffects. Part 1: Definitions and calculation method,Brussels: CENELEC, 1993

5. DIN EN 60865-1/VDE 0103, Kurzschlussstroeme - Berechnung der Wirkungen. Teil 1: Begriffe undBerechnungsverfahren, Berlin: VDE, 1994

6. Stein,, N., Meyer, W., Miri. A.M.: Tests and Calcu-lations of Short-Circuit Forces and Displacements in

High-Voltage Substations with Strained Conductorsand Droppers, ETEP 2000

7. PC Programme IEC865, University of Erlangen, 1999

8. Miri, A.M., Schwab, A.J., Kopatz, M.: Kurzschluss-stroeme und Leiterbewegungen in Hochspannungs-schaltanlagen in Seilbauweise, Elektrizittswirtschaft87 (1988), pp. 429-436

9. IEC 60865-2, Short-circuit currents - Calculation ofeffects. Part 2: Examples of calculation, Geneva: IEC,1994

10. IEC TC 73/CIGR SC 23 WG 11 , The mechanicaleffects of short-circuit currents in open air substations(Rigid and flexible bus-bars), Vol. 105, Geneva; IEC,Paris: CIGRE, 1996

11. Meyer, W., Herold, G., Zeitler, E-: Short-circuitcurrents - Calculation of effects. The second edition of

IEC Publication 865, in Proceedings of the 6thInternational Symposium on Short-Circuit Currents inPower Systems, Lige (Belgium), 1994.

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