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A.3 Fixing the maximum lightning current parameters for LPL I

A.3.1 Positive impulse

The mechanical effects of lightning are related to the peak value of the current (I), and to the specific energy (W/R). The thermal effects are related to the specific energy (W/R) when resistive coupling is involved and to the charge (Q) when arcs develop to the installation. Overvoltages and dangerous sparking caused by inductive coupling are related to the average steepness (d/7dt) of the lightning current front.

Each of the single parameters (/, Q, W/R, d/7dt) tend to dominate each failure mechanism. This shall be taken into account in establishing test procedures.

A.3.2 Positive impulse and long stroke

The values of I, Q and W/R related to mechanical and thermal effects are determined from positive flashes (because their 10 % values are much higher than the corresponding 1 % values of the negative flashes). From Figure A.5 (lines 3, 5, 8, 11 and 14) the following values with probabilities below 10 % can be taken:

I = 200 kA

Qflash “ 300 C

Qshort “ 100 C

W/R = 10 MJ/Я

d//dt = 20 kA/ps

For a first positive impulse according to Figure A.1, these values give a first approximation for the front time:

Ту - 11 (d/7dt) = 10 ps (Ту is of minor interest)

For an exponentially decaying stroke, the following formulae for approximate charge and energy values apply (Ту « T₂):

^short ⁼ (1/0,7) x I x T₂

W/R = (1/2) x (1/0,7) x I² x T₂

These formulae, together with the values given above, lead to a first approximation for the time to half value:

T₂= 350 ps

For the long stroke, its charge can be approximately calculated from:

Qlong “ $_FLASH - Qshort “ 200 C

Its duration time, according to Figure A.2, may be estimated from data in Table A.1 as:

^long “ 0,5 s

А.3.3 First negative impulse

For some inductive coupling effects, the first negative impulse leads to the highest induced voltages, e.g. for cables within cable ducts made of reinforced concrete. From Figure A.5 (lines 1 and 12) the following values with probabilities below 1 % can be taken:

/ = 100 kA

d/7df = 100 kA/ps

For a first negative impulse according to Figure A.1 these values give a first approximation for its front time of:

T = I /( df/dt) = 1,0 ps

Its time to half value may be estimated from the stroke duration of first negative impulses:

T₂= 200 ps (T₂ is of minor interest).

A.3.4 Subsequent impulse

The maximum value of average steepness d/7df related to the dangerous sparking caused by inductive coupling is determined from subsequent impulses of negative flashes (because their 1 % values are somewhat higher than the 1 % values from first negative strokes or the corresponding 10 % values of the positive flashes). From Figure A.5 (lines 2 and 15) the following values with probabilities below 1 % can be taken:

I = 50 kA

d/7df - 200 kA/ps

For a subsequent impulse according to Figure A.1 these values give a first approximation for its front time of:

T_y= I / (dildt) = 0,25 ps

Its time to half value may be estimated from the stroke duration of negative subsequent impulses:

T₂ = 100 ps (T₂ is of minor interest).

A.4 Fixing the minimum lightning current parameters

The interception efficiency of an air-termination system depends on the minimum lightning current parameters and on the related rolling sphere radius. The geometrical boundary of areas which are protected against direct lightning flashes can be determined using the rolling sphere method.

Following the electro-geometric model, the rolling sphere radius r (final jump distance) is correlated with the peak value of the first impulse current. In an IEEE working group report¹⁵¹, the relation is given as

r=10x/°'⁶⁵ (A.1)

where

r is the rolling sphere radius (m);

I is the peak current (kA).

For a given rolling sphere radius r it can be assumed that all flashes with peak values higher than the corresponding minimum peak value I will be intercepted by natural or dedicated air terminations. Therefore, the probability for the peak values of negative and positive first strokes from Figure A.5 (lines 1A and 3) is assumed to be the interception probability. Taking into account the polarity ratio of 10 % positive and 90 % negative flashes, the total interception probability can be calculated (see Table 5).Annex В
(informative)

Time functions of the lightning current for analysis purposes

The current shapes of

• the first positive impulse 10/350 ps,

• the first negative impulse 1/200 ps,

• the subsequent negative impulses 0,25/100 ps,

may be defined as: where

)¹⁰

xexp( -tIT₂)

(B.1)

I is the peak current;

к is the correction factor for the peak current;

t is the time;

is the front time constant;

T₂ is the tail time constant.

For the current shapes of the first positive impulse, the first negative impulse and the subsequent negative impulses for different LPL, the parameters given in Table B.1 apply. The analytic curves as function of time are shown in Figures B.1 to B.6.

Table B.1 - Parameters for Equation (B.1)

Figure В.1 - Shape of the current rise of the first positive impulse

Figure В.2 - Shape of the current tail of the first positive impulse

Figure B.3 - Shape of the current rise of the first negative impulse

IEC 2624/10

Figure В.4 - Shape of the current tail of the first negative impulse

Figure В.5 - Shape of the current rise of the subsequent negative impulses

Figure B.6 - Shape of the current tail of the subsequent negative impulses

The long stroke can be described by a rectangular waveshape with an average current I and a duration 7"_L0NG according to Table 3.

From the analytic curves as function of time, the amplitude density of the lightning current (Figure B.7) can be derived.

Relevant frequency range for LEMP effects

7 - Amplitude density of the lightning current according to LPL IAnnex С
(informative)

Simulation of the lightning current for test purposes

C.1 General

If a structure is struck by lightning, the lightning current is distributed within the structure. When testing individual protection measure components, this must be taken into account by choosing appropriate test parameters for each component. To this end, a system analysis has to be performed.

C.2 Simulation of the specific energy of the first positive impulse and the charge of the long stroke

Test parameters are defined in Tables C.1 and C.2 and an example test generator is shown in Figure C.1. This generator may be used to simulate the specific energy of the first positive impulse combined with the charge of the long stroke.

The tests may be used to assess mechanical integrity, freedom from adverse heating and melting effects.

The test parameters relevant for simulation of the first positive impulse (peak current /, the specific energy W/R, and the charge Qshort) ^are Qi^ven in Table C.1. These parameters should be obtained in the same impulse. This can be achieved by an approximately exponentially decaying current with T₂ in the range of 350 y.s.

The test parameters relevant for the simulation of the long stroke (charge Q_L0NG and duration T_LONG) are given in Table C.2.

Depending on the test item and the expected damage mechanisms, the tests for the first positive impulse or the long stroke can be applied singly or as a combined test, where the long stroke follows the first impulse immediately. Tests for arc melting should be performed using both polarities.

Current generator for

the long stroke

Current generator for the first short stroke

NOTE The first negative impulse is not to be used for test purposes.

IEC 2847/10

NOTE The values apply to LPL I.

2. 1 - Example test generator for the simulation of the specific energy
   of the first positive impulse and the charge of the long stroke

Table C.1 - Test parameters of the first positive impulse

Table C.2 - Test parameters of the long stroke

C.3 Simulation of the front current steepness of the impulses

The steepness of the current determines the magnetically induced voltages in loops installed near conductors carrying lightning currents.

The current steepness of an impulse is defined as the rise of the current Д/ during rise time At (Figure C.2). The test parameters relevant for the simulation of this current steepness are given in Table C.3. Example test generators are shown in Figures C.3 and C.4, (these may be used to simulate the front steepness of a lightning current associated with a direct lightning strike). The simulation can be carried out for a first positive impulse and a subsequent negative impulse.

NOTE This simulation covers the front current steepness of impulses. The tail of the current has no influence on this kind of simulation.

The simulation according to Clause C.3 may be applied independently or in combination with the simulation according to Clause C.2.

For further information on test parameters simulating the effects of lightning on LPS components, see Annex D.

Table C.3 - Test parameters of the impulses

300 kV charging voltage Ui_

Figure C.2 - Definition of the current steepness in accordance with Table C.3

NOTE These values apply to LPL I.

Figure C.3 - Example test generator for the simulation of the front steepness
of the first positive impulse for large test items

3,5 MV charging voltage

NOTE These values apply to LPL I.

Figure C.4 - Example test generator for the simulation of the front steepness
of the subsequent negative impulses for large test itemsAnnex D
(informative)

Test parameters simulating the effects of lightning on LPS components

D.1 General

Annex D gives the basic parameters that may be used in a laboratory to simulate the effects of lightning. This annex covers all the components of an LPS subjected to all or a major part of the lightning current and may be used in conjunction with the standards specifying the requirements and the tests for each specific component.

NOTE Parameters relevant to system aspects (e.g. for the coordination of surge protective devices) are not considered in this annex.

D.2 Current parameters relevant to the point of strike

The lightning current parameters playing a role in the physical integrity of an LPS are in general the peak current /, the charge Q, the specific energy W/R, the duration T and the average steepness of the current d/7dt. Each parameter tends to dominate a different failure mechanism, as analysed in detail below. The current parameters to be considered for tests are combinations of these values, selected to represent in laboratory the actual failure mechanism of the part of the LPS being tested. The criteria for the selection of the outstanding quantities are given in Clause D.5.

Table D.1 records the maximum values of I, Q, W/R, T and d/7dt to be considered for tests, as a function of the protection level required.

Table D.1 - Summary of the lightning threat parameters to be considered in the
calculation of the test values for the different LPS components and for the different
LPL