Lecture2 Direct Indirect 2011

8/6/2019 Lecture2 Direct Indirect 2011

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DEPARTAMENT DENGINYERIA ELCTRICA

Qualitat del Subministrament Elctric enSistemes amb forta Penetraci de

Renovables


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Qualitat del Subministrament Elctric en Sistemes amb forta Penetraci de Renovables

II I. Non periodic disturbances

Lecture 2 30/3/2009

1. Overvoltages from direct lightning flashes to overhead lines2. Overvoltages from indirect lightning flashes to overhead lines3. Distribution lines insulation levels


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1. Introduction

Two cases should be considered whit lightning effects to distribution lines, lightningmight cause flashovers from :

Direct lightning flashes to a line.

Induced voltages from nearby lightning flashes.



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1.1 Direct lightning to a line


Direct flashes to unprotected power distribution lines cause insulation flashovers in thegreat majority of the cases.

For example, a stroke of as little as 10 kA would produce an overvoltage of around 2000kV, far in excess of the insulation levels of overhead distribution lines operating up to 69kV.

Lightning incidence to a line: basic approach

The flash collection rate N, in open ground (no trees, buildings, etc), is estimated byErikssons equation:

h: is the phase conductor height at the pole (m)b: is the structure width (m)Ng: ground flash density (flashes /km^2 year)N: flash collection rate (flashes /100 km / year)


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h: is the phase conductor height at the pole (m)b: is the structure width (m)Ng: ground flash density (flashes /km^2 year)

N: flash collection rate (flashes /100 km / year)

Source SMC report


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h: is the phase conductor height at the pole (m)b: is the structure width (m)Ng: ground flash density (flashes /km^2 year)

N: flash collection rate (flashes /km^2 year)

Source UNE 21186


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7


Lightning incidence to a line


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Electrogeometrical models


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Electrogeometrical models

Including the height of the structure


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Preparative models


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Lightning incidence to a line: Insulation level


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Lightning incidence to a line

For a particular current I:


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13

Lightning incidence to a line: Shielding Failure Rate (SFR)



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CFO: Critical Flashover Voltage

This is the peak voltage for a 50% probability of flashover or disruptive discharge.Air density (humidity too) should be taken in to account. Also pollution affects to the CFO.

Basic Impulse Insulation Level (BIL)

This is the reference insulation level expressed as an impulse crest (or peak) voltage with a standard wavenot longer than a 1.2/50 us impulse voltage waveform.

Withstand Voltage

This is the BIL level that can repeatedly be applied to an equipment without flashover, disruptive charge orother electrical failure under test conditions.


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Systemvoltage (kV) BIL (kV)

15.0 110

23.0 150

34.5 200

46.0 250

69.0 350

115.0 450

138.0 550

230.0 825-900

287.5 1050

245.0 1175-1300

500.0 1550

The insulators strings are such that:

1.The power frequency wet withstand voltage (CFO) must belarger than the highest voltage of the switching surge.

2.The critical impulse withstand voltage (CFO) must be nearthe maximum electrical potential rise of the tower during a

direct lighting stroke.


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Sobretensiones

generadas por

rayos

Tensin

soportable por

las lneas


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fo(U)

fo(U)P

T(U)

1

PT(U)


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Low-frequency, low current surge impedance of phase conductor

The surge impedance of an unprotected phase conductor of radius r=1 cm and height h=10 m over perfectground is about 228 , considering that the lightning current from the source splits into two directions. Thesurge impedance of a single wire over ground in one direction is calculated:

Several effects occur under lightning surge conditions with high current and fast rate of rise over imperfectsoil of finite conductivity.


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MVAV

ZcIV

C

LZc

5.1300*5000

*

==

=

=

MVAV

ZoIV

C

LZo

14.1228*5000

*

==

=

=

10 kA

Zc 228

V

CZ

BILI

20 =

Mxima corrienteMxima corrientesoportablecsoportablec

Si V > BIL hay falla


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Effects of corona at high voltage

The product of the impressed first-stroke current and the phase conductor surge impedance in twodirections:

Will be a voltage that exceeds 1 MV about 99 % of the time for flashes higher than 4.4 kA.

Whit this voltage on the conductor, its effective radius will increase from the corona effects, therebyincreasing the capacitance of the conductor and lowering its surge impedance.

If the radius of the corona envelope envelops the adjacent conductors, these will have nearly the samevoltage as the stricken phase from common-mode coupling and the differential-mode stress on phase-to-phase insulation will be further reduced.

Rc is the corona radius of the conductor at agradient of 1500 kV/m (m); Eo = 15 kV/cm is the critical corona gradient.

e is the voltage

Anderson formula:


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Effects of imperfect soil at high frequency

The impedance of the conductor will increase with decreasing soil conductivity and with decreasingfrequency.

For great accuracy, the height h should be a complex number based on the soil conductivity and frequency.

The impedance over wide range of frequency should be performed to obtain time-domain result by theinverse Fourier transform.

The effect of finite soil conductivity on surge impedance can be modeled with acceptable accuracy inlightning calculations by replacing the real height of the line (h) by the effective height:

Where is the soil conductivity (mS/m).

To be realistic soil conductivity would be calculated at 124 kHz (1st RS).

The effects tend to limit the conductivity at 124 kHz to a minimum of about 1 mS/m.


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Flashover rate from direct flashes

Both corona and imperfect soil effects could introduce changes within a 30 %.

Unless the distribution line is protected by overhead ground wires (OHGW) or arresters, more than 99 % ofall direct lightning flashes will cause flashovers regardless of insulation level, conductor spacing orgrounding.

Then, to estimate the number of flashovers an approach could be obtained by previous equation:


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1.1 Indirect lightning to a line


Experience shows that many of the lightning-related outages of low-insulation lines are due to lightning thathits the nearby ground or structures in the proximity.

Most voltages induced on a distribution line by flashes that terminate near the line are less than 300 kV.Maximum peak voltages are around 400 kV.

The induced voltages tend to be unipolar and have short pulse witdhs.


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Overvoltage level of 300 kV BIL is considered sufficient for lines in areas of high soil conductivity.

An insulation lever of 400 kV BIL may be more appropriate fro areas of low 1 mS/m conductivity.

The calculation of the induced voltages requires electromagnetic coupling.

Some analytical formulas for simple configuration exists. One of these formulas is the Rusk formula.

The Rusk model serves to predict the maximum overvoltage Um (kV) for a peak stroke current Ip (kA) at adistance d (m) from a line of height h (m):

Where v is the speed of propagation return stroke (m/s) and c is the speed of light (c=3e8m/s).

Return stroke speed for natural lightning varies between 0.29x108 m/s and 2.4x108 m/s


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The limitation of Rusk model for imperfect soil could be resolved by adopting the effective height (heff).

Lightning may be collected by tall objects as wind turbines. For tall structures Rusk formula fails becausev=c (at the tower).

By means of numerical methods, Baba suggested that lightning flashes to tall objects (100 m) such aswind turbines located within 100 m of distribution lines may induce 50 to 80 % higher overvoltage thanflashes to ground at the same location.

Example of the flashover as a function of the CFO of a 10 m high single conductor infinite line:

0.001

0.010

0.100

1.000

10.000

100.000

50 100 150 200 250 300

CFO (kV)

Flashovers/100km/yr

ideal ground

ground conductivity = 10 mS/m

ground conductivity = 1 mS/m


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As a reference, a 10 m tall distribution line in open ground with GFD = 1 flash/km2/yr will haveapproximately 11 flashes/100 km/yr due to direct strokes.

In open ground, induced voltages will be a problem for lines characterized by low insulation levels and/orabove a poor conducting ground.

A grounded neutral w ire or overhead OHGW will reduce the voltage across the insulation by a

factor that depends on the spacing between adjacent groundings, on the grounding impedance and on theproximity of the grounded conductor to the phase conductors. This factor is typically between 0.6 and 0.9.

Grounded ci rcuits , i .e. ci rcu its w i th a grounded neutral w i re or overhead OHGW, are generallyexpected to have fewer number of flashovers for a g iven CFO because the grounded conductorreduces via i ts electromagnet ic shielding effect the voltage stress across the insulation.


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Estimation of failures due to direct and indirect effects

0 - ymin : Direct strikes

ymin ymax: Induced voltages that

produce failures

> ymax: Induced voltages that do

not produce failures

Number of failures associated to

indirect lightning


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Proposed activities


1.-Calculate the SFR for a three-phase overhead line (h = 10 m) with a ground wire 50 cm above of the centerphase. Three phases are distributed horizontally. Line lenght is 50 km and Ng is 3 flash/km^2 year. First calculatefor a particular current of 20 kA and then for all the current range.

2.- Estimate the (undamped) voltages on the previous overhead line when a 15 kA lightning strikes one phase.

3.- Calculate a list of induced overvoltages in a phase for the previous line for lightning of 5 kA, 10 kA, 20 kA , 50 kA

and 100 kV at distances: the minimum (to determinate), the minimum plus 25 m, the minimum plus 100 m and theminimum plus 500 m.

4.- Simulate the previous line behavior when it is exposed to 500 lightning flashes.

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Lecture2 Direct Indirect 2011