Cours Effets

8/12/2019 Cours Effets

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A SHORT-CIRCUIT DESIGN FORCES IN POWER LINES AND

SUBSTATIONS

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1. INTRODUCTION

Short-circuit currents in power lines and substations induce electromagnetic forces acting onthe conductors. The forces generated by short-circuit forces are very important for high-voltage bundle conductor lines, medium-voltage distribution lines, and substations, wherespacer compression forces and interphase spacings are significantly affected by them.

Power Lines and Substations

Short-circuit mechanical design loads have been a subject of significant importance fortransmission line and substation design for many years, and numerous papers, technical

brochures and standards have been published (Manuio 1!"#$ %oshino 1!#&$ %avard et al.1!'"$ )*+ 1!!"$ )*+ &&$ ) 1!! and 1!!"$ /ilien and 0apailiou &&&. 2ndershort-circuit forces, there are some similarities and some differences between the behavior offle3ible bus and power lines.

4or both the power lines and substations, the electromagnetic forces are similar in their originand shapes because they come from short-circuit current () 1!''. 5evertheless, as listed

below, there are some major differences between short-circuit effects on substation bussystems and power lines6

0ower lines are subjected to short-circuit current intensity, which is only a fraction of thelevel met in substation bus systems. The short-circuit level is dependent on short-circuitlocation, because longer lengths of lines mean larger impedance and lower short-circuitlevel. The level also depends on power station location and networ7 configuration.

0ower line circuit configuration may not be a horiontal or vertical arrangement, thusinducing other spatial components of the forces than in bus systems, and the movement

may be 8uite different. 0ower lines have much longer spans and thus much larger sags than fle3ible bus and rigid

bus. This induces a very low basic swing fre8uency of the power line span (a fraction ofone %. Therefore the oscillating components of the force at the networ7 fre8uency (andits double have negligible action on power lines.

0ower line phase spacings are much larger than those in substations, and this has adramatic reduction effect on forces between phases.

9undle conductors in power lines have much larger subspans than in substations, and

bundle diameter is often larger, too. Sometimes very large bundle diameter and a largenumber of subconductors are used compared to bundled substation fle3ible bus. This hassignificant effects on the phenomenon because long subspans reduce the effect of bundlecollapse upon the tension in the subconductors during short circuit conditions. 4ig. 1demonstrates the distortion of the subconductors of a 8uad bundle around a fle3ible spacerduring a short-circuit, 7nown as the pinch effect, which causes the tension increase.

:ue to differences in structure height and stiffness, power line towers have significantlylower fundamental natural fre8uencies than substation structures. ;ne result is that thesubstation structures are more li7ely to respond dynamically to the sudden increase intension that results from the pinch effect.


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0ower line design load includes severe wind action and in some cases heavy ice loadsacting on much larger spans than in substations. Therefore design loads due to shortcircuits may be of the same order as design wind and ice loads in substations, but muchless in transmission lines.

Bundle Conductor Lines

4or bundle conductor lines, during a fault, the subconductors of the bundle move closer toeach other due to strong attraction forces because of the very short distance betweensubconductors (4igure1.

:etailed discussions of this phenomenon were given by Manuio and %oshino (Manuio1!"#$ %oshino 1!#&.

4rom their initial rest position, the subconductors move towards each other, remaining moreor less parallel in most of the subspan, e3cept close to the spacer (4igures 1 and . pinch,?while it is associated primarily with the change in angle, can be further increased by the risein tension in the subconductors due to bundle collapse. This jump results from the fact thatsubconductor length in the collapsed condition is greater than in the normal condition.

The pinch is ma3imum when the wave propagation stops towards the spacer, position e in4igure . The triangle of collapse then performs oscillations through positions d-c-d-e-d-c-e-d-c and so on as long as electromagnetic force is still on, but with decreasing amplitude. )f theshort circuit is long enough, the pinch oscillations result in a >permanent? oscillating force,

sensibly lower than pea7 value, typically @&A.:uring the fault, the spacer is strongly compressed. The compression is related to ma3imum

pinch force in the conductor and the angle between the spacer and the subconductor.

4igure 1 3ample of 8uad bundle before and during short-circuit test at @& 7


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4ig 33 6 rigid busbar response to ta given electromagnetic force similar to a two-phase faultwith asymmetrical component in the short-circuit current. The transient response is given fordifferent busbar first eigenfre8uency between 1.# % and 1@& %. (e3tract from )*+

brochure 5E 1&@, 1!!".

4ig 33 6 a tested rigid bus (all details in )*+ brochure 1&@, 1!!", Measurement points arelocated as S, ), (constrains. Short-circuit of 1" 7< during 1@ ms with automaticreclosure after ==@ ms and a second fault of &@ ms with same amplitude as the first one.

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4ig 33 test results. The first eigenfre8uency of the whole structure is about . %. There is8uasi no effect of the @& % nor of the 1&& % component of the force.


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2. FAULT CURRENTS AND INTERPHASE FORCES

< short-circuit current wave shape consists of an ? is rather low, typically & to '& ms, compared tosubstations where it is typically #& to && ms.

1

-

.

( (sin( sin(

( (sin( sin(

( (sin( sin(

t

rms

t

rms

t

rms

i t I t e

i t I t e

i t I t e

= +

= +

= + + +

(


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0is the vacuum magnetic permeability H =1&-#%Bm.

ais the interphase distance (m.

The force, being due to current flow, very much depends on phase shift between currents. )tgenerally includes6

0seudo-continuous : component, with a time-constant decay,

ontinuous dc component, sometimes, and

Two oscillating


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4igure = Two different geometric arrangements for a three-phase circuit and theelectromagnetic force reference directions on each phase corresponding to 8uation . Thenumbers 1, , and are phase numbers.

)t must be noted that the level of the pea7 force, about && 5Bm in 4igure @, is far greater thanthe conductor weight and is proportional to the s8uare of the current. 9ut the continuouscomponent is much lower, about & 5Bm in this case, as shown later. 2nder actual short-circuit levels and clearances, it is closer to the conductor weight, but acts, in most cases, in theother direction. See upper right panel in 4igure @.

4igure @ 3ample of calculated three-phase short-circuit current wave shape andcorresponding loads on a horiontal or vertical circuit arrangement.

1&


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Thus the interphase effects, for the case of horiontal or purely vertical arrangement only,may be summaried as6

1. The design force on the horiontal or vertical three-phase arrangement is the force due to athree-phase fault considering the outer phase with appropriate asymmetry. Ta7ing intoaccount the fact that only the continuous dc component has to be considered, the force on

an outer phase can be appro3imated by 8uation . This is the horiontal repulsion forcefor the horiontal arrangement, or the vertical repulsion force for the vertical arrangement6

- - B

.

&.(&.#@ 1."1" tF I e

a

= + (5Bm

Ghere

ais the interphase distance (m.

I3the rms three-phase fault at that location (7


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3. BEHAVIOR OF BUNDLE CONDUCTORS UNDER SHORT CIRCUITS

:etailed behavior of bundle conductors under short circuit is most easily illustrated throughshort-circuit tests in actual bundles. Some results from a program of tests at the Cei7isubstation in %ungary are used here for that purpose (/ilien and 0apailiou &&&.

The systematic single-phase fault tests on twin conductors were performed in the 1!!&s on apower line with a double deadended span, with a length of "& m, with the followingcharacteristics (4igure "6

Span length "& mSub conductor type


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Sagging tension 25 kN - 35/90 kA

-4000

-2000

0

2000

4000

6000

8000

0 0,2 0,4 0,6 0,8 1 1,2 1,4

time (s)

Compressivelo

ads(N)

subspan length 60 m


-4000

-2000

0

2000

4000

6000

8000

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6

time (s)

Compressive

load(N)

subspan length 30 m


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-1000

0

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2000

3000

4000

5000

6000

7000

0 0,2 0,4 0,6 0,8 1 1,2 1,4

time(s)

Compressiveloads(N)

subspan length 60 m


-3000

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-1000

0

1000

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3000

4000

5000

6000

7000

0 0,2 0,4 0,6 0,8 1 1,2 1,4

time (s)

Compressiveloads(N)

subspan length 30 m

4igure ' Typical tests results on spacer compression on "&-m and &-m subspan length, at @7< on twin-bundle line 3 ondor, with different sagging tensions. %alf of the compression isgiven. The drawings are covering short-circuit and significant after short-circuit time to bettersee the wave propagation effects after the end of the short circuit. (ourtesy 0fistererBSefag.


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is particularly valid for long subspans, as used in power lines and as validated by ManuioKstest arrangements (Manuio 1!"#.

)n case of spring-type dampers, which could be compressed by the pinch, there could be alarge increase of these tensile loads acting on spacer attachment as the rela3ation of energystored in spring compression during short-circuit is released after the end of the short circuit.

:epending on the configurations of the spacer and spacer dampers, the short-circuit forcescould cause large bending moment in the conductor and the elements of the spacer.

4igure ! Typical tension oscillogram in one subconductor during and after the fault, for the"&-m span length configuration (1@ 75 initial. )rms @ 7< (pea7 !& 7


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4igure 1& Typical tension oscillogram in one subconductor during and after the fault for the 3 &-m span length configuration (1@ 75 initial. )rms @ 7< (pea7 !& 7


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4. INTERPHASE EFFECTS UNDER SHORT CIRCUITS

Maximum Tensile Loads during Moement of the Phases

4igure 11 shows a typical response of a bundle conductor two-phase fault in a horiontalarrangement ()*+ 1!!". 9oth cable tension versus time (4igure11 left and phasemovement in a vertical plane at mid-span (4igure 11 right are shown. ;n the cable tensioncurve, three ma3ima (and their corresponding time on the abscissa have been indicated,which is discussed below. ;n the phase movement curve at mid-span, the curve has beenmar7ed by dots every &.1 s to get an idea of the cable speed, and in particular to show that theshort circuit ends before there is significant movement of the phase.

Typical ma3imum loads (4igures 11 and 1 that could influence design appear when totalenergy (including a large input during short circuit has to be mainly transformed todeformation energy.

0ea7 design load could occur under the following three conditions6

1. Ma3imum swing-out 4t(at time ttin 4igure 11leftand s8uare 1 in 4igure 11 right6 verylittle 7inetic energy (cable speed close to ero and potential energy with reference togravity, so that a large part is converted in deformation energyIthat is, increase oftension. )n power lines, ttoccurs always after the end of the short circuit (the cable

position at the end of the short circuit (&.1 s is indicated in 4igure 11 right.

. Ma3imum 4fat the e3treme of downward motion (at time tfin 4igure 11 leftand s8uare in 4igure 11 right6 generally more critical because of a loss of potential energy of gravitydue to the cable position at that moment. tfalways occurs after the end of the short circuit.

. The pinch effect 4pi(at a very short time after short-circuit inception at tpi. The pinch

effect only occurs with bundle conductors, when subconductors come close to each other6tpialways occurs during short circuit.

4igure 11 /eft 4igure6 Tensile force (left time evolution of a typical twin-bundlespan during two-phase short circuit between horiontal phases. Three ma3ima6 4piattime Tpi(so-called pinch effect, due to bundle collapse, 4tat time Tt(the ma3imumof the force due to ma3imum swing of the span represented by circle point 1 on theright figure, and 4f at time Tf (the ma3imum of the force due to cable droprepresented by circle in the right figure. Typically, Tpi-=& ms, TtN1. s and TfH = s

1#


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4igure 1 alculated envelopes of phase-conductor movements for three types of loadingconditions on a >9eaubourg? tower (the figure is drawn in a vertical plane located at mid-span6 +, S, and T are their phase locations in still conditions (/ilien and :al Maso 1!!&61. two-phase short-circuit " 7< either +T, +S, or TS. three-phase short-circuit #. 7

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