IEC TR 60071-4:2004

TECHNICAL IEC
REPORT TR 60071-4
First edition
2004-06
Insulation co-ordination –
Part 4:
Computational guide to insulation co-ordination
and modelling of electrical networks

Reference number
IEC/TR 60071-4:2004(E)
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TECHNICAL IEC
REPORT TR 60071-4
First edition
2004-06
Insulation co-ordination –
Part 4:
Computational guide to insulation co-ordination
and modelling of electrical networks
 IEC 2004  Copyright - all rights reserved
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– 2 – TR 60071-4  IEC:2004(E)
CONTENTS
FOREWORD.7

Scope and object .9
2 Normative references.9
3 Terms and definitions .9
4 List of symbols and acronyms .12
5 Types of overvoltages.12
6 Types of studies .13
6.1 Temporary overvoltages (TOV) .14
6.2 Slow-front overvoltages (SFO) .14
6.3 Fast-front overvoltages (FFO).15
6.4 Very-fast-front overvoltages (VFFO).15
7 Representation of network components and numerical considerations .15
7.1 General .15
7.2 Numerical considerations.15
7.3 Representation of overhead lines and underground cables .18
7.4 Representation of network components when computing temporary
overvoltages .19
7.5 Representation of network components when computing slow-front
overvoltages .25
7.6 Representation of network components when computing fast-front transients.30
7.7 Representation of network components when computing very-fast-front
overvoltages .42
8 Temporary overvoltages analysis .44
8.1 General .44
8.2 Fast estimate of temporary overvoltages.45
8.3 Detailed calculation of temporary overvoltages [2], [9] .45
9 Slow-front overvoltages analysis .48
9.1 General .48
9.2 Fast methodology to conduct SFO studies .48
9.3 Method to be employed.49
9.4 Guideline to conduct detailed statistical methods .49
10 Fast-front overvoltages analysis.52
10.1 General .52
10.2 Guideline to apply statistical and semi-statistical methods .53
11 Very-fast-front overvoltage analysis .58
11.1 General .58
11.2 Goal of the studies to be performed .58
11.3 Origin and typology of VFFO .58
11.4 Guideline to perform studies .60
12 Test cases.60
12.1 General .60
12.2 Case 1: TOV on a large transmission system including long lines .60
12.3 Case 2 (SFO) – Energization of a 500 kV line .68
12.4 Case 3 (FFO) – Lightning protection of a 500 kV GIS substation .73
12.5 Case 4 (VFFO) – Simulation of transients in a 765 kV GIS [51] .80

TR 60071-4  IEC:2004(E) – 3 –
Annex A (informative) Representation of overhead lines and underground cables .86
Annex B (informative) Arc modelling: the physics of the circuit-breaker .90
Annex C (informative) Probabilistic methods for computing lightning-related risk of
failure of power system apparatus .93
Annex D (informative) Test case 5 (TOV) – Resonance between a line and a reactor in
a 400/220 kV transmission system .99
Annex E (informative) Test case 6 (SFO) – Evaluation of the risk of failure of a gas-
insulated line due to SFO . 105
Annex F (informative) Test case 7 (FFO) – High-frequency arc extinction when
switching a reactor . 113

Bibliography . 116

Figure 1 – Types of overvoltages (excepted very-fast-front overvoltages).12
Figure 2 – Damping resistor applied to an inductance .17
Figure 3 – Damping resistor applied to a capacitance .17
Figure 4 – Example of assumption for the steady-state calculation of a non-linear
element.17
Figure 5 – AC-voltage equivalent circuit.19
Figure 6 – Dynamic source modelling .20
Figure 7 − Linear network equivalent .21
Figure 8 − Representation of load in [56] .24
Figure 9 – Representation of the synchronous machine .26
Figure 10 – Diagram showing double distribution used for statistical switches .29
Figure 11 – Multi-story transmission tower [16], H = l + l + l + l .31
1 2 3 4
Figure 12 − Example of a corona branch model .33
Figure 13 −Example of volt-time curve.34
Figure 14 – Double ramp shape.38
Figure 15 – CIGRE concave shape.39
Figure 16 – Simplified model of earthing electrode.41
Figure 17 – Example of a one-substation-deep network modelling .51
Figure 18 – Example of a two-substation-deep network modelling.51
Figure 19 − Application of statistical or semi-statistical methods .53
Figure 20 – Application of the electro-geometric model.56
Figure 21 – Limit function for the two random variables considered: the maximum value
of the lightning current and the disruptive voltage .57
Figure 22 – At the GIS-air interface: coupling between enclosure and earth (Z ), between
overhead line and earth (Z ) and between bus conductor and enclosure (Z ) [33] .59
2 1
Figure 23 − Single-line diagram of the test-case system .62
Figure 24 − TOV at CHM7, LVD7 and CHE7 from system transient stability simulation.63
Figure 25 – Generator frequencies at generating centres Nos. 1, 2 and 3 from system
transient stability simulation .64
Figure 26 – Block diagram of dynamic source model [55].65
Figure 27 − TOV at LVD7 – Electromagnetic transient simulation with 588 kV and
612 kV permanent surge arresters.66

– 4 – TR 60071-4  IEC:2004(E)
Figure 28 − TOV at CHM7 – Electromagnetic transient simulation with 588 kV and
612 kV permanent surge arresters.67
Figure 29 − TOV at LVD7 – Electromagnetic transient simulation with 484 kV switched

metal-oxide surge arresters.67
Figure 30 − TOV at CHM7 – Electromagnetic transient simulation with 484 kV switched
metal-oxide surge arresters.67
Figure 31 – Representation of the system.68
Figure 32 – Auxiliary contact and main .70
Figure 33 – An example of cumulative probability function of phase-to-earth
overvoltages and of discharge probability of insulation in a configuration with trapped
charges and insertion resistors.72
Figure 34 – Number of failure for 1 000 operations versus the withstand voltage of the
insulation .72
Figure 35 – Schematic diagram of a 500 kV GIS substation intended for lightning
studies.74
Figure 36 – Waveshape of the lightning stroke current.75
Figure 37 – Response surface approximation (failure and safe-state representation for
one GIS section (node)) .77
Figure 38 – Limit-state representation in the probability space of the physical variables
Risk evaluation .79
Figure 39 – Single-line diagram of a 765 kV GIS with a closing disconnector .81
Figure 40 – Simulation scheme of the 765 kV GIS part involved in the transient
phenomena of interest.81
Figure 41 – 4 ns ramp .84
Figure 42 – Switch operation .85
Figure A.1 – Pi-model.86
Figure A.2 – Representation of the single conductor line.87
Figure B.1 – SF circuit-breaker switching .91
Figure C.1 – Example of a failure domain .96
Figure D.1 – The line and the reactance are energized at the same time.99
Figure D.2 – Energization configuration of the line minimizing the risk of temporary
overvoltage . 100
Figure D.3 – Malfunction of a circuit-breaker pole during energization of a transformer . 102
Figure D.4 – Voltage in substation B phase A whose pole has not closed. 103
Figure D.5 – Voltage in substation B phase B whose pole closed correctly. 103
Figure D.6 – Voltage in substation B phase A where the breaker failed to close
(configuration of Figure D.2). 104
Figure E.1 – Electric circuit used to perform closing overvoltage calculations. 105
Figure E.2 – Calculated overvoltage distribution − Two estimated Gauss probability
functions resulting from two different fitting criteria (the U and U guarantees a
2% 10%
good fitting of the most dangerous overvoltages) . 107
Figure E.3 – Example of switching overvoltage between phases A and B .
and phase-to-earth (A and B) . 109
Figure E.4 – Voltage distribution along the GIL (ER-energization ED-energization under
single-phase fault ChPg-trapped charges) . 110
Figure F.1 – Test circuit (Copyright1998 IEEE [48]) . 113
Figure F.2 − Terminal voltage and current of GCB model (Copyright 1998 IEEE [48]). 113
Figure F.3 – Measured arc parameter (Copyright 1998 IEEE [48]). 114

TR 60071-4  IEC:2004(E) – 5 –
Figure F.4 – Circuit used for simulation . 114
Figure F.5 – Comparison between measured and calculated results (Copyright 1998
IEEE [48]) . 115

Table 1 – Classes and shapes of overvoltages – Standard voltage shapes and standard
withstand tests .13
Table 2 – Correspondence between events and most critical types of overvoltages
generated .14
Table 3 – Application and limitation of current overhead line and underground cable
models .18
Table 4 – Values of U , k, DE for different configurations proposed by [59] .35
Table 5 − Minimum transformer capacitance to earth taken from [44].37
Table 6 − Typical transformer capacitance to earth taken from [28].37
Table 7 – Circuit-breaker capacitance to earth taken from [28].37
Table 8 – Representation of the first negative downward strokes .40
Table 9 – Time to half-value of the first negative downward strokes .40
Table 10 – Representation of the negative downward subsequent strokes .40
Table 11 – Time to half-value of negative downward subsequent strokes .40
Table 12 – Representation of components in VFFO studies .43
Table 13 − Types of approach to perform FFO studies.52
Table 14 – Source side parameters .69
Table 15 – Characteristics of the surge arresters.69
Table 16 – Characteristics of the shunt reactor.69
Table 17 – Capacitance of circuit-breaker.70
Table 18 – Trapped charges.70
Table 19 – System configurations.71
Table 20 – Recorded overvoltages .71
Table 21 – Number of failures for 1 000 operations.72
Table 22 – Modelling of the system .76
Table 23 – Data used for the application of the EGM .76
Table 24 – Crest-current distribution.77
Table 25 – Number of strikes terminating on the different sections of the two incoming
overhead transmission lines .77
Table 26 – Parameters of GIS disruptive voltage distribution and lightning crest-current
distribution .78
Table 27 – FORM risk estimations (tower footing resistance = 10 Ω).79
Table 28 – Failure rate estimation for the GIS11.80
Table 29 – Representation of GIS components − Data of the 765 kV GIS.82
Table D.1 – Line parameters . 100
Table D.2 – 400 /220/33 kV transformer . 101
Table D.3 – 220 /13,8 kV transformer . 101
Table D.4 – Points of current and flux of 400 /220/33 kV transformer . 101
Table D.5 – Points of current and flux of 220 /13,8 kV transformer. 101
Table D.6 – Points of current and flux of 400 kV /150 MVAr . 102
Table E.1 – Parameters of the power supply. 105

– 6 – TR 60071-4  IEC:2004(E)
Table E.2 – Standard deviation and U for different lengths (SIWV = 1 050 kV). 108
50M
Table E.3 – Standard deviation and U for different lengths (SIWV = 950 kV). 108
50M
Table E.4 – Standard deviation and U for different lengths (SIWV = 850 kV). 108
50M
Table E.5 – Statistical overvoltages U and U for every considered configuration . 110
2 % 10 %
Table E.6 – Risks for every considered configuration. 111
Table E.7 – Number of dielectric breakdowns over 20 000 operations for every
configuration . 112

TR 60071-4  IEC:2004(E) – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSULATION CO-ORDINATION –
Part 4: Computational guide to insulation co-ordination
and modelling of electrical networks

FOREWORD
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The main task of IEC technical committees is to prepare International Standards. However, a
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example "state of the art".
IEC 60071-4, which is a technical report, has been prepared by IEC technical committee 28:
Insulation co-ordination.
– 8 – TR 60071-4  IEC:2004(E)
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
28/156/DTR 28/158/RVC
Full information on the voting for the approval of this technical report can be found in the report
on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until the
maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• transformed into an International standard
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
A bilingual version of this technical report may be issued at a later date.

TR 60071-4  IEC:2004(E) – 9 –
INSULATION CO-ORDINATION –
Part 4: Computational guide to insulation co-ordination
and modelling of electrical networks

1 Scope and object
This technical report gives guidance on conducting insulation co-ordination studies which
propose internationally recognized recommendations
– for the numerical modelling of electrical systems, and
– for the implementation of deterministic and probabilistic methods adapted to the use of
numerical programmes.
Its object is to give information in terms of methods, modelling and examples, allowing for the
application of the approaches presented in IEC 60071-2, and for the selection of insulation
levels of equipment or installations, as defined in IEC 60071-1.
2 Normative references
The following referenced documents are indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments) applies.
IEC 60060-1:1989, High-voltage test techniques – Part 1: General definitions and test
requirements
IEC 60071-1:1993, Insulation co-ordination – Part 1: Definitions, principles and rules
IEC 60071-2:1996, Insulation co-ordination – Part 2: Application guide
IEC 60076-8:1997, Power transformers – Part 8: Application guide
IEC 60099-4:1991, Surge arresters – Part 4: Metal-oxide surge arresters without gaps for a.c.
systems
IEC 61233:1994, High-voltage alternating current circuit-breakers – Inductive load switching
3 Terms and definitions
For the purposes of this document, the following terms and definitions, in addition to those
contained in IEC 60071-1, apply.
NOTE Certain references are taken from the IEC Multilingual Dictionary[1] .
___________
A consolidated edition exists, published in 2001, which incorporates the current edition, plus its amendment 1
(1998) and amendment 2 (2001).
References in square brackets refer to the bibliography.

– 10 – TR 60071-4  IEC:2004(E)
3.1
backfeeding
refers to the conditions of supplying a high-voltage overhead line or cable through a
transformer from the low-voltage side
3.2
back flashover
flashover of phase-to-earth insulation resulting from a lightning strike to towers and shielding
wires [1]
3.3
back flashover rate
number of back flashovers of a line per 100 km per year
3.4
closing of capacitive load
essentially closing of capacitor banks but also closing of any other capacitive load
3.5
critical current
minimum lightning current that induces a flashover on a line
NOTE The critical current of the line is the smallest critical current among all injection points.
3.6
direct lightning strike
lightning striking a component of the network, for example, conductor, tower, or substation
equipment [1]
3.7
energization
connecting or reconnecting to a source an element of a power system which has no stored
energy
3.8
fault clearing
interruption of the short-circuit condition on a system
3.9
limit distance
distance from the substation after which no overvoltage resulting from a lightning stroke gives
rise to an impinging surge dangerous for the substation's equipment
3.10
line dropping
disconnection of the line by opening the last circuit-breaker
3.11
line fault application
application of a line short-circuit on a system
3.12
load rejection
opening of a line breaker during normal power flow causing a certain amount of load to be
unsupplied
NOTE From a temporary overvoltage point of view, the worst case occurs when the remote circuit-breaker of a
long line transmitting a significant part of the supply of a power station is opened.

TR 60071-4  IEC:2004(E) – 11 –
3.13
line re-energization
opening and fast closing of the line circuit-breaker as the consequence of a fault or a relay
maloperation
NOTE With respect to line energization, trapped charges should be taken into account.
3.14
maximum shielding current
maximum lightning current that can hit a phase conductor on a line protected by shielding wires
3.15
parallel line resonance
overvoltage appearing on an unenergized shunt reactor compensated circuit due to capacitive
coupling with a parallel energized circuit
3.16
point-on-cycle controlled switching
energization of capacitive load at the instant that the voltage is zero across the circuit-breaker
contacts thus eliminating the switching transient
NOTE De-energization of inductive load ensures a long and weak power arc at zero-current crossing thus
eliminating the risk of re-strike and re-ignition.
3.17
representative lightning stroke current
minimum value of lightning current at a specific point of impact which produces overvoltages
that the equipment has to withstand; it is deduced from experience
3.18
slow-front overvoltage flashover rate
number of flashovers of a line per 100 km per year due to slow-front overvoltages
3.19
switching resistor
resistance inserted to match the surge impedance of the line in order to limit the switching
surge magnitude launched from the source
3.20
switching of inductive and capacitive current
includes interruption of starting current of motors, interruption of inductive current when
interrupting the magnetizing current of a transformer or when switching off a shunt reactor,
switching and operation of arc furnaces and their transformer, switching of unloaded cables
and of capacitor banks, interruption of current by high-voltage fuses
(See 2.3.3.4 in IEC 60071-2)
3.21
uneven breaker pole operations
operation caused by one or two breaker poles stuck during opening or closing of the circuit-
breaker
– 12 – TR 60071-4  IEC:2004(E)
4 List of symbols and acronyms
AIS Air-insulated substation
BFO Back flashover
BFR Back flashover rate
EGM Electro-geometric model
FACTS Flexible alternating current transmission systems
FFO Fast-front overvoltages
GIS Gas-insulated system
HVDC High-voltage d.c.
LIWV Lightning impulse withstand voltage
MOA Metal oxide surge arrester
SFO Slow-front overvoltages
SIWV Switching impulse withstand voltage
SFOFR Slow-front overvoltage flashover rate
TOV Temporary overvoltages
TRV Transient recovery voltage
VFFO Very-fast-front overvoltages
Z (or Z ) Surge (or characteristic) impedance
s c
I Critical current
c
I Maximum shielding current
m
In addition, refer to 1.3 of IEC 60071-2 as well as the list of symbols in [4].
5 Types of overvoltages
Table 1, extracted from IEC 60071-1, and Figure 1, detail the characteristics of all types of
overvoltages.
Lightning overvoltages (FFO)
Switching overvoltages (SFO)
2 Temporary overvoltages (TOV)
System voltage
µs ms s Duration
IEC  763/04
Figure 1 – Types of overvoltages (excepted very-fast-front overvoltages)

p.u. voltage
(VFFO)
TR 60071-4  IEC:2004(E) – 13 –
Table 1 – Classes and shapes of overvoltages – Standard voltage shapes
and standard withstand tests
Class Low frequency Transient
Continuous Temporary Slow-front Fast-front Very-fast-front
1/f
T
f
1/f
Voltage or
over-
T
p
voltage
T
T
T
t
1/f
shapes 1/f
T 1 2
T
t
3 ns < T ≤ 100 ns
f
Range of 10 Hz < f
0,3 MHz < f
f = 50 Hz or 20 μs < T 0,1 μs < T
p  1
voltage or < 500 Hz
< 100 MHz
60 Hz ≤ 5 000 μs ≤ 20 μs
over-
30 kHz < f
0,03 s ≤ T 2
voltage t
T ≥3 600 s T ≤ 20 ms T ≤ 300 μs
t 2 2
< 300 kHz
shapes ≤ 3 600 s
1/f
1/f
Standard
1)
T
t
p
voltage T
T 1 Time (μsec)
T T
T
t 2
t
shapes
f = 50 Hz 48 Hz ≤ f
T = 250 μs T = 1,2 μs
p 1
or 60 Hz
≤ 62 Hz
1) T = 2 500 μs T = 50 μs
2 2
T T = 60 s
t t
Short-
Standard
duration
1) Switching Lightning 1)
withstand power
impulse test mpulse test
frequency
test
test
1)
To be specified by the relevant apparatus committees.

6 Types of studies
For range I voltage level (U up to 245 kV), SFO are generally not critical while the FFO due to
m
lightning have to be carefully considered. However, for higher voltage levels, SFO become of
major importance, specifically in the UHV range, while FFO become in many cases less critical.
TOV have to be studied for all system voltage levels.Table 2 provides a list of events and the
most critical types of overvoltages generated.

– 14 – TR 60071-4  IEC:2004(E)
Table 2 – Correspondence between events and most critical types
of overvoltages generated
Transient overvoltages
Temporary
overvoltages
Slow-front Fast-front Very-fast-front
overvoltages overvoltages overvoltages
TOV
SFO FFO VFFO
Load rejection X
(see 2.3.2.2 in IEC 60071-2)
Transformer energization X X
Parallel line resonance X
Uneven breaker poles X
Backfeeding X
Line fault application X X
(see 2.3.3.2 in in IEC 60071-2)
Fault clearing X X
(see 2.3.3.2 in in IEC 60071-2)
Line energization X X
(see 2.3.3.1 in in IEC 60071-2)
Line re-energization X X
Line dropping X X
1)
AIS busbar switching  X
Switching of inductive and X X X
capacitive current
(see 2.3.3.4 in IEC 60071-2)
Back flashover  X
Direct lightning stroke  X
(see 2.3.3.5 in IEC 60071-2
Switching inside GIS substation  X
1)
SF circuit-breaker inductive and X X X
capacitive current switching
Flashover in GIS substation  X

Vacuum circuit-breaker switching  X X
1)
In the case of short distance busbars and low damping, very-fast-front overoltages can also occur.

6.1 Temporary overvoltages (TOV)
Temporary overvoltages are of importance when determining stresses on equipment related to
power-frequency withstand voltage in particular for the energy capability of MOA. TOV can
stress transformers and shunt reactors as a consequence of over fluxing. Ferro-resonance is a
particular type of TOV which is not studied in this report.
6.2 Slow-front overvoltages (SFO)
Slow-front overvoltages play a role in determining the energy duty of surge arresters and in the
selection of required withstand voltages of equipment as well as the air gap insulation for
transmission line towers.
SFO studies require the investigation of possible network configurations and switching
conditions that result in overvoltages exceeding the withstand values mentioned above. In
decreasing order of importance, events which have to be considered typically are line re-
energization, line energization, line fault application, fault clearing, capacitive load closing and
inductive load opening (from the reactor's point of view). In reactive current switching, the

TR 60071-4  IEC:2004(E) – 15 –
circuit-breaker may break down after the final clearance due to excessive dv/dt. A dielectric
breakdown across the circuit-breaker before a quarter cycle of the power frequency after the
final clearance is known as re-ignition, but a dielectric breakdown across the CB after the
quarter cycle of the power frequency following the final clearance is known as restrike. Circuit-
breaker restrike generates high SFO.
NOTE There is no withstand value specified for range I equipment.
6.3 Fast-front overvoltages (FFO)
They are essentially produced by lightning strokes. Their magnitude is much larger than other
kinds of overvoltages.
FFO are therefore critical for all voltage levels, and it is essential to mitigate them with
protective devices, i.e. mainly surge arresters. Fast-front overvoltages are studied to determine
the risk of equipment failure and therefore to select their required withstand level in relation to
protective device configuration and tower earthing, and to evaluate line and station
performance.
NOTE Vacuum breakers can cause overvoltages in the fast-front range because of current chopping and restrike.
6.4 Very-fast-front overvoltages (VFFO)
Very-fast-front overvoltages are important for protection against high-touch voltages and
internal flashover in GIS enclosures. VFFOs appear under switching conditions in GIS (see [3])
or when operating vacuum circuit-breakers in medium-voltage systems.
Pre-striking GIS disconnector and SF circuit-breaker re-ignition would produce VFFO.
Normally, these VFFOs can be avoided by point-on-cycle (POC) switching but analysis is
required to cater for the control relay malfunction.
7 Representation of network components and numerical considerations
7.1 General
Simplified methods for the evaluation of each type of overvoltage are presented briefly in 8.1,
9.1 and 10.1, respectively. They do not require a precise modelling of each component.
However, when it is necessary to determine accurately overvoltages or overvoltages for which
simplified methods cannot deal, detailed analysis with detailed models are required. These
models representing the components of the system to be used in studies depend on the type of
overvoltage being considered. After numerical considerations, this clause presents, for each
type of overvoltage, the models which are adequate for representing each component.
7.2 Numerical considerations
7.2.1 Initialization before calculation of transients
The solution of a transient phenomenon is dependent on the initial conditions with which the
transient is started. Some simulations may be performed with zero initial conditions, i.e. with
some particular cases of lightning surge studies, but there are many cases for which the
simulation must be started from power-frequency steady-state conditions. In most cases, this
issue is solved internally by simulation tools.
However, there is presently no digital tool which can calculate the initial solution for the most
general case, although some programmes can perform an initialization with harmonics for
some simple cases. The initial solution with harmonics can be obtained using simple
approaches. The simplest one is known as the “brute-force” approach: the simulation is started
without performing any initial calculation and carried out long enough to let the transients settle
down to steady-state conditions. This approach can have a reasonable accuracy, but
its convergence will be very slow if the network has components with light damping.

– 16 – TR 60071-4  IEC:2004(E)
A more efficient method is to perform an approximate linear a.c. steady-state solution with non-
linear branches disconnected or represented by linearized models.
7.2.2 Time step
The time step has to be coherent with the highest frequency phenomenon appearing in the
system during the transient under consideration. A value of one-tenth of the period
corresponding to the highest frequency is advised.
The time step has to be lower than the travel time of any of the propagation elements of the
network. A value of half this travel time is advised.
The correctness of the time step may be verified by the method presented in [5] which involves
comparing the result given with the time step and half of the time step. If the two results are
equivalent, the first value of the time step is considered small enough.
7.2.3 Duration of the simulation
The duration time must be sufficiently long to ensure that the maxi
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