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[email protected]
A SYSTEMATIC APPROACH TO ELECTROMAGNETIC COMPATIBILITY
ANALYSIS AND DESIGN IN UTILITY SYSTEMS
A. XMARD1 P. Y. VALENTINB. BRESSAC
J. MAHSEREJIAN*A. COUTU**
G. JOS
Electricit de FranceR&D
France
RTEGestionnaire du Rseau
de TransportdElectricit
France
*Hydro-Qubec IREQ**Hydro-Qubec
TransnergieCanada
CEA TechnologiesInc.
Canada
1 Introduction
In France, the constant evolution of the transmission network
and of the telecommunication andpipeline systems results in a
significant number of 50 Hz coupling studies being performed every
year;more than 70 studies are performed every year by RTE. Such
studies are performed when a line isinstalled or modified (for
example in case of increase of the short-circuit current), or when
apotentially disturbed system is installed or modified (pipeline,
telecommunication cable) or in somemore specific cases (live work,
accident analysis). RTE has signed an agreement with France
Telecom
(phone utility) and SNCF (railway utility) to specify the way
coupling studies are to be performed andto define rules for the
financing of constructions required to limit disturbances. The
internalspecifications of RTE propose some simple conservative
methods to calculate disturbance on nearbysystems but its role is
in most of the cases limited to detect if a detailed study is
required. Theexperience of the last years has shown that the
modeling of the disturbed system is very often a sourceof
difficulty for design engineers. HQ has similar needs regarding the
computation of fundamentalfrequency induced voltages, but in
addition requires a tool to assess interference in telecom lines
inorder to specify dc filters performance in HVDC transmission
projects. This feature involves thecomputation of the coupling
impedance at several harmonic frequencies of the audio range.The
above considerations have convinced CEATI, EDF, HQ, ELIA and others
that it was essential thatan advanced modeling tool be available
for this purpose. It must provide a structured framework forinduced
voltage calculation and include an accurate representation of
telecommunication and gas
pipeline equipment. It can be advantageously based on EMTP
(Electromagnetic Transients Program)type software [1]. This paper
presents the importance of a structured approach to the problem,
gives areview on theoretical considerations and illustrates
important issues through practical examples.
2 General presentation of the proposed new software
The proposed automatic computation technique will use the new
EMTP [2] transient simulationenvironment as a computational engine
and will use an accurate electrical representation
oftelecommunication and gas pipeline equipments. This choice of a
background software layer isparticularly appropriate because it
offers accurate models of overhead lines and underground
cablesbased on the line theory and can work in the frequency domain
(frequency scan and steady-statesolutions).
The purpose of the complete software will be to generate
automatically from the electro-geometricalconfiguration under
study, an EMTP representation, using the rules specified in [3]
[4]. The software
21, rue d'Artois, F-75008 Parishttp://www.cigre.org CIGR
Session 2004C4-205
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will include a database for the representation of the different
types of equipments (towers, conductors,underground power cables,
telecommunication cables, pipelines, fences).The user will have the
possibility to input the geometries of the power lines and of the
disturbedsystems using a digital table, the screen of a PC, or GPS
coordinates.The next paragraph gives a general review of the
theoretical principles applied in the software.
3 Computation of interference
3.1 Inductive and capacitive couplingA current circulating along
an electrical system generates in its vicinity a magnetic field
which is at theorigin of the inductive interference between the
electrical system and a disturbed system situated in thesame area.
In the case of a three-phase system the magnetic field takes into
account the magnetic fieldgenerated by each conductor, including
ground wires in the case of overhead lines and sheaths in thecase
of underground cables.Capacitive coupling originates from the
electrostatic field due to the charge along the electricalsystems.
In practical terms capacitive coupling is of interest only for
overhead lines.The method used to evaluate inductive and capacitive
coupling between power lines and disturbed
systems is based on two steps.- The first step consists in
splitting the disturbing and disturbed systems into sections in
such a waythat the coupling may be represented based on the line
theory. If a section i of the disturbing linesegpower[i] is
considered as coupled with a section of the disturbed line
segdisturbed[j], then the system(segpower[i], segdisturbed[j]) is
treated as a multi-conductor propagation line, supposing that the
sectionsare parallel. The accuracy of this approach is related to
the way the sections are determined and thecorrespondence between
sections is established. IUT [4] has proposed a method which will
bepresented below.- The second step consists in evaluating the
induced voltages on the disturbed system.
3.1.1 Splitting into coupling sectionsThe coupling sections are
determined in the zone of influence, corresponding to the zone in
which thecoupling is significant. In most of the cases this area
extends [4] to less than 3 km from the line forinductive coupling
and to a few tenths of meters for capacitive coupling. The
determination of thesections and the correspondence between the
sections of the disturbing and disturbed systems leads toconsider 3
different cases (Figure 1): parallel, oblique and crossing. An
oblique case may be
approximated by a parallel case if some geographical condition
is fulfilled: 12
d13
3 d
1d2d
1d
2d
Parallel Oblique Crossing
Disturbed line
Disturbing line
1d2d
1d
2d
Parallel Oblique Crossing
Disturbed line
Disturbing line
Figure 1: Represented coupling cases between the disturbing and
disturbed lines
In the case of crossing, an equivalent parallel segment is used
as a section of the disturbed line such asd1 < 10m, d2 < 10m
[3][4].
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3.1.2 Evaluation of the induced voltagesThis step corresponds to
the application of line theory. The coupled segments segpower[i] of
the powerline and segdisturbed[j] of the disturbed line are treated
like a multi-conductor EMTP line. The multi-conductor overhead line
includes directly the electrical representation of phase conductors
and groundwires. In the case of an underground cable it includes
the representation of phase conductors and
sheaths. If the disturbed line is sheathed the sheath is
included directly in the representation.This approach leads to the
computation of impedance +Z = R sL and admittance +Y = G sC
matricesused in the solution of wave equations:
(x,s) (x,s)
x
=
VZ I and
(x,s) (x,s)
x
=
IY V
The primed variables are given per unit length, x is a position
in the line and s stands for Laplacetransformation. The above
equations are used for each segment of the disturbing and disturbed
system,coupled or not, taking into account the boundary
conditions.
This calculation is readily available in EMTP frequency domain
computations for solving togetherboth the disturbing and the
disturbed system. The EMTP PI-Exact model (which is exact at
everyselected frequency) can be used in order to avoid the
approximations related to the application of themodal theory. The
frequency domain approach is however unusable for taking into
account nonlinearcomponents, such as surge-arresters or air-gaps
which may be used to limit the level of inducedvoltage. If a study
requires the modeling of such components then the time-domain
solver must beused in conjunction with distributed parameter line
and cable models that can correctly account for thefrequency
dependency of parameters [5].
3.1.3 Presence of screening conductorA metallic conductor
present in the zone of influence and connected to ground can reduce
significantlythe inductive influence of the disturbing system on
the disturbed one. This conductor is coupled at thesame time to the
disturbing and to the disturbed systems. It is possible to
represent its effect by acoefficient which reduces the mutual
inductance between the coupled segments of the disturbedsystem and
the disturbing system.
It is also possible to represent it as a separate electric
system coupled both to the disturbing anddisturbed one. If the
screening conductor is parallel to the disturbing one (the case of
a ground wire) itcan be treated as a supplementary conductor of
this one. A similar approach can be applied if it isparallel to the
disturbed system. In other cases (non-parallel conditions, as in
Figure 2) the followingapproach may be applied:
At first the current circulating along the screening conductor
is evaluated by applying themethods described in 3.1.1 and 3.1.2,
then the effect of the screening conductor on thedisturbed system
is represented by equivalent voltage sources, which are evaluated
by:
o splitting the disturbed system, considering that it is
disturbed by the screeningconductor;
o applying for each section i of the disturbed system influenced
by section j of the
screening conductor the formula i ij je M I= , where ijM is the
mutual conductance
between segments i and j and jI is the current circulating along
the segment j .
The last step consists in considering the direct effect of the
disturbing system on the disturbedone by applying the methods of
3.1.1 and 3.1.2.
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Screening conductor
Disturbing system
Disturbed system
Screening conductor
Disturbing system
Disturbed system
Figure 2: Splitting approach in the presence of a screening
conductor
3.1.4 Key technical issues allowing accurate calculationsAn
extensive representation of the power systemFor a fault condition
the induction on neighboring systems depends directly on the value
of the faultcurrent. A precise determination of the fault current
requires also an adequate representation of thepower system and
especially of the interaction between the substations located at
both ends of thepower line. In many practical cases the
representation of the electrical network cannot be limited to a
representation of the substations at both ends of the line by
some independent Thevenin equivalents,evaluated using the
short-circuit power at substation. It may be necessary to represent
moreextensively the system. Another possibility could be to
represent both ends by a generalized Theveninequivalent, which is
an impedance matrix with of-diagonal terms accounting for the
interactionbetween the substations through the rest of the network
when the power line under consideration hasbeen removed.
A precise representation of the return path of the fault
currentIn the case of a fault the induction on nearby pipelines or
telecommunication cables is stronglyinfluenced by the return path
of the fault current. For instance if one considers a single-phase
fault inthe cable of the configuration presented in Figure 3, the
fault current circulating on the sheath willdecrease the induction
effect of the current circulating on the phase conductor. If 2R is
high and 1R is
small, most of the fault current will return by the sheath and
the induction will be limited. But if 1R is
high and 2R is small, most of the fault current will return
through the resistance 2R . This simple
example illustrates the necessity to represent precisely not
only the fault current but also the way thefault current returns,
by making in particular an adequate representation of the
connections andgroundings of the sheaths. The use of an EMTP type
software rends possible such a modeling andallows to avoid too
conservative simplifications as it will be illustrated in the
example below.
Underground Cableline
+
+
+
R1
+
R2
Figure 3: Overhead line terminated by an underground cable with
grounded sheath at both
ends
3.1.5 Application examplesExample 1: 50 Hz voltage induced on a
telecommunication cable by an underground cable during afaultThe
225 kV system of this example is shown in Figure 4. It is made of a
700 m underground cableconnected to an 8 km single circuit overhead
line equipped with 2 ground wires. The undergroundcable is a
three-phase single-core system, bonded at substation A and equipped
with a parallel earth
continuity conductor which is connected at the earth electrode
of the substation at one terminal andconnected to the earth
electrode of the first tower at the other terminal. A
telecommunication cable
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whose sheath is earthed at one end, is parallel to the
underground cable at a distance of 50 m. Thisstrictly parallel
configuration has been chosen to allow an EMTP calculation without
determination ofcoupling segments. The same method will be applied
in the next examples.For a single-phase-to-ground fault on the
overhead part of the line, the level of induction on
thetelecommunication cable will depend not only on the fault
current but also on the part of the faultcurrent circulating back
to substation A by the earth conductor. This statement can be
illustrated byconsidering 2 different configurations. In the first
configuration phase-a is short-circuited at the firsttower
following the underground cable. Calculation results are summarized
in Table 1. All values arecrest values. It can be seen that the
magnitude of the current circulating along the earth conductor
issimilar to the current circulating on phase-a of the underground
cable. The voltage at the end of thecable is limited to 109 V.
Telecom line
Tower 2Tower 1
. . .
. . .
. . .Substation A
Substation B
earth conductor
ground wires
Transmission line, 8kmUnderground cable, 700m
1
2
3
+1
2
3
RL
Thevenin
+
+
+
+
+ +
+
VM
+
+
+
+
+
+
+
1
2
3
+1
2
3
RL
Thevenin
+ +
a
b
c
Figure 4: Representation of a 225 kV system: underground cable
followed by a single circuit line
with 2 ground wires
Table1: single-phase fault occurring at the first tower
following the cable Total fault current 43 kAContribution of
substation A to the fault current 27 kACurrent circulating in the
earth conductor 27 kACurrent circulating in the faulty tower 825
ATotal current circulating along the ground wire between towers 1
and 2 15 kAVoltage at the end of the telecommunication cable 109
V
In the second configuration a single-phase-to-ground fault
occurred at the 7th tower following theunderground cable. Table 2
summarizes the computations. It appears that part of the
contribution fromsubstation A to the fault current circulates
through the earth electrodes of the towers and does notreturn by
the earth conductor. Therefore even with a smaller short-circuit
current circulating in phase-afrom substation A, the voltage at the
end of the disturbed system is significantly higher.
Table2: single-phase fault occurring at the 7th tower following
the cable Total fault current 39 kA
Contribution of substation A to the fault current 21.5 kACurrent
circulating in the earth conductor 18 kACurrent circulating in the
faulty tower 1.1 kATotal current circulating along the ground wire
between tower 1 and 2 15 kAVoltage at the end of the
telecommunication cable 290 V
This simple study illustrates the importance of a proper
representation of the fault current path in orderto obtain a more
realistic evaluation of the induction on the disturbed system.
Example 2: Influence of the homopolar current component on
inductive couplingWhen calculating inductive coupling in normal
operating condition it is of major importance toaccount precisely
for the effect of the homopolar component of the current as it is
illustrated in thefollowing example. A 255 kV single circuit line
(flat configuration) without ground wires is shown in
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Figure 5. At mid-span, the distance of phase conductors to
ground is equal to 20 m. The span length is400 m.
25 m
5.5 m5.5 m
Figure 5: Configuration of phase conductors at tower of a 225 kV
line
The line is a 30 km long antenna. The induction on a 400 m long
cable, parallel to the line andgrounded at one end is studied. The
height of the cable is 1 m, its distance to the line is 40 m.
Thepositive sequence current is equal to 520 A. The crest value of
the 50 Hz induced voltage at the end ofthe cable is calculated when
the line is transposed (the positions of phase conductors are
changed each10 km) and when it is not transposed, in order to
evaluate the effect of the absence of transposition ofthe line on
the level of induced voltage.It is found that the absence of
transposition of the phase conductors leads to an increase of 10%
in theinduced voltage (9.4 V/km instead of 8.5 V/km). This example
shows the importance of representingexactly the current circulating
in the phase conductors and not to limit the representation to
thepositive sequence.
Example 3: Influence of the coating of pipelinesPipelines are
tubes of steel, aluminum or cast-iron with a diameter ranging from
50 to 1100 mm. Theyare usually coated with polypropylene,
polyethylene, hydrocarbon, epoxy or plicoflex.
It is important to take into account the nature of the coating
when calculating induced voltage becauseits influence on the loss
factor may affect significantly the level of fundamental frequency
inducedvoltage as it is illustrated in the example below.The
studied configuration is similar to the one presented in Figure 5,
but with a 400 m long steelpipeline buried at a depth of 1 m,
replacing the telecommunication line.The influence of the nature of
the coating will be illustrated by calculating the 50 Hz voltage at
the endof the pipeline in the case of a single-phase-to-ground
fault for a bituminous coating and then for apolyethylene coating.
The loss factor of the coating is first determined and then the
frequency domainsimulation option [2] of EMTP is used.The
capacitance of the pipeline to the surrounding ground and the shunt
conductance can be calculatedby:
0 r
c
DC
d
= and
c c
DG
d
=
with D being the diameter (1 m) of the pipeline, cd is the width
of coating (1 mm), r is the relative
permittivity (5) and c is the resistivity of the coating. The
loss factor is given by:
Gtan( )
C =
The results presented in Table 3, indicate a significant
reduction of the induced voltage at the end ofthe pipeline,
originating from the higher losses in a bituminous coating compared
to polyethylenecoating.
Table 3: 50 Hz induced voltage at the end of a pipeline for two
types of coating
Coating c tan( ) 50 Hz voltage
bituminous 0.2*106 0.36*103 160 Vpolyethylene 1.*108 0.71 290
V
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3.2 Resistive coupling3.2.1 PrinciplesThe ground is not usually
used as an active conductor in transmission systems except in some
specialconfigurations of DC lines. However some faults (due to
pollution, lightning) may cause short-
circuit currents to flow through the grounding electrode of a
tower during a period of time. Thisgrounding electrode is
constituted of buried metallic elements and presents some
resistance related tothe shape, number and position of these
elements. The flow of current in the earth electrode leads to
anincrease of voltage in the soil, whose value depends on the
nature of the soil and which may posesafety problems (touch and
step voltages).Neighboring metallic structures buried in the area
of the earth electrode are submitted to its voltageand may become
damaged by it (puncture of sheath or insulation). They can also in
some cases absorbthe fault current and transmit it to areas far
away from the point where it has been injected into theground.
3.2.2 Splitting of the disturbed systemThe goal of the splitting
is to breakdown the disturbed system in the region of influence
into sections
along which the earth potential may be considered as of constant
value (the difference is less than 5%compared to the accurate
value). If one considers the earthing of a tower, at a distance d
greater thanthree times the equivalent radius of the electrode [3],
the potential rise may be approximated by:
V I2 d
=
where is the soil resistivity and I is the fault current
circulating in the electrode.
The method consists in defining a series of circles iC centered
on the current point injection, with
radius iR calculated by:
eqi
RR
1 0.05i=
with i 20<
The sections of the disturbed system are determined by finding
its intersection with the series ofcircles. If the disturbed system
is situated in the region of influence of several points of
injection offault current (tower where the fault is located,
neighboring towers if the line is equipped with groundwires,
groundings of cable sheaths) the sections are determined by
applying the same approach to allof them and by superposing the
splitting results.If the disturbed system is very close to an earth
electrode, the above equation is no longer valid and amore accurate
method needs to be used [4].
4 Conclusions
The sharing of right-of-way between transmission networks and
pipeline and telecommunicationsystems demands that the electric
utilities perform fundamental frequency coupling studies more
and
more frequently. As it has been shown in this paper these
studies require: A precise evaluation of the currents circulating
in the conducting parts of the disturbing
system in the zone of influence; An accurate modeling of the
influence of the disturbing system on the disturbed ones taking
into account precisely the geometry of the configuration; A
detailed modeling of the potentially disturbed systems.
A programmed method also allows determining the fault conditions
resulting into the most severestresses of induced coupling. In
addition, different grounding configurations can be analyzed
tominimize stresses.
This paper has proposed a structured framework for the
computation of induced voltages based onEMTP type software layer.
The choice of EMTP as a computational environment allows fulfilling
theabove stringent requirements.
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AcknowledgementsThe authors acknowledge contributions and
support to this work from FT (France Tlcom) and GDF(Gaz de
France)
5 Bibliography
[1] J. Mahseredjian, L. Dub, L. Grin-Lajoie, New advances in the
Simulation of Transients withEMTP: Computation and Visualization
Techniques, Electrimacs, August 19th, 2002, Plenarysession
paper.
[2] J. Mahseredjian, L. Dub and S. Dennetire: EMTP-RV
documentation on simulation options,2003, IREQ-Report
[3] CIGRE 36.02 CIGRE brochure 95 - Guide on the influence of
high voltage AC power systemson metallic pipelines 1995
[4] IUT Directives concernant la protection des lignes de
telecommunication contre les effetsprjudiciables des lignes
lectriques et des chemins de fer lectrifis Volume 2 1999
[5] Morched A., Gustavsen B. and Tartibi M.: A universal model
for accurate calculation ofelectromagnetic transients on overhead
lines and underground cables. Power Delivery, IEEETransactions,
Volume: 14, Issue: 3 , July 1999, pp. 1032 -1038
