arXiv:1303.5452v2 [] 5 May 2014

Fast Computation of the Series Impedance of
Power Cables with Inclusion of Skin and
Proximity Effects
Utkarsh R. Patel, Bjørn Gustavsen, and Piero Triverio∗†‡
Published in IEEE Transactions on Power Delivery, vol. 28, no. 4,
Oct. 2013. DOI: 10.1109/TPWRD.2013.2267098. §
May 6, 2014

Abstract
We present an efficient numerical technique for calculating the series
impedance matrix of systems with round conductors. The method is
based on a surface admittance operator in combination with the method
of moments and it accurately predicts both skin and proximity effects.
Application to a three-phase armored cable with wire screens demonstrates a speed-up by a factor of about 100 compared to a finite elements
computation. The inclusion of proximity effect in combination with the
high efficiency makes the new method very attractive for cable modeling
within EMTP-type simulation tools. Currently, these tools can only take
skin effect into account.

1

Introduction and Motivation

Electromagnetic transients have a significant impact on the design, operation
and performance of electrical power systems [1, 2]. Transients can be caused by
several phenomena such as lightning discharges, breaker operations, faults, and
the use of power electronics converters. Since electromagnetic transients involve
a wide band of frequencies, ranging from DC up to the low MHz range, their
∗ This work was partially supported by the Norwegian Research Council (RENERGI Programme) with additional support from an industry consortium led by SINTEF Energy Research.
† U. R. Patel and P. Triverio are with the Edward S. Rogers Sr. Department of Electrical and Computer Engineering, University of Toronto, Toronto, M5S 3G4 Canada (email:
utkarsh.patel@mail.utoronto.ca, piero.triverio@utoronto.ca).
‡ B. Gustavsen is with SINTEF Energy Research, Trondheim N-7465, Norway (e-mail:
bjorn.gustavsen@sintef.no).
§ Published with the revised-title “An Equivalent Surface Current Approach for the Computation of the Series Impedance of Power Cables with Inclusion of Skin and Proximity Effects”

1

simulation requires broadband and accurate models for all network components
to predict the network response outside the nominal sinusoidal regime. For
the modeling of underground cables, it is necessary to calculate the per-unitlength (p.u.l.) cable series impedance matrix [3] over a wide band of discrete
frequencies while taking into account frequency-dependent phenomena such as
skin and proximity effects. The series impedance is next used as input data for
alternative frequency-dependent cable models [4],[5].
Traditionally, the series impedance is calculated using analytic formulas
which account for skin effect only, since they assume a symmetrical distribution
of current in all conductors [3]. These formulas are combined with systematic
procedures for computing the series impedance matrix for multi-conductor systems [6]. Although simple and highly efficient, this traditional approach neglects
proximity effects. Proximity-aware formulas are available only for two conductor systems [7, 3]. Ignoring proximity effect is acceptable for overhead lines and
for widely spaced single core cable systems. However, in the case of three-phase
cables, pipe-type cables and closely packed single core cables, the small distance
between conductors in combination with the non-coaxial arrangement leads to
significant proximity effects. This issue is relevant also in the modeling of umbilical cables for offshore oil and gas power supply and control [8]. Ignoring
proximity effect leads to an underestimate of the cable losses at the operating
frequency, and the transient waveforms are also affected, in particular in situations where waves propagate between the screens and between the screens
and ground [9, 10]. The latter situation is highly relevant in the simulation of
cross-bonded cable systems.
Because of the limitations of analytic formulas, several numerical techniques
have been proposed. The harmonic expansion method accounts for proximity effects at high frequencies under the assumption that the skin effect is
fully developed [3, 11, 12, 13]. Techniques based on the finite element method
(FEM) [14, 8, 15] fully predict proximity effect at both low and high frequencies, but they tend to be excessively time-consuming because of the fine mesh
required to properly discretize the cross section. A similar issue arises with
techniques based on conductor partitioning [16, 17, 18, 19, 20]. Moreover, with
both FEM and conductor partitioning techniques, the discretization must be
refined as frequency increases to properly capture the pronounced skin effect,
further reducing computational efficiency.
In this paper, we overcome these issues by developing an efficient numerical
technique for computing the series impedance matrix of cables with round conductors. Our approach, denoted henceforth as MoM-SO, combines the method
of moments (MoM) with a surface admittance operator (SO) introduced in [21]
for rectangular conductors. The proposed approach is not available elsewhere in
the literature, since the authors of [21], after presenting the surface admittance
operator for both rectangular and round conductors, focus their attention on
the first case. Through the surface operator, we replace each conductor with
the surrounding medium, while introducing an equivalent current density on
the surface of each conductor. The current density and the longitudinal electric field on all conductors are then related through the electric field integral
2

equation [22], and their spatial dependence is expressed in terms of Fourier components. Finally, the method of moments [23] is applied to compute the p.u.l.
parameters of the line. Numerical results will show that the proposed approach
is much more efficient than state-of-the-art FEM techniques, since few Fourier
components are sufficient to accurately model skin and proximity effect at any
frequency. Moreover, the discretization of the electric field integral equation is
entirely perfomed using analytic formulas, avoiding numerical integration used
in previous works [13, 21]. This achievement further improves the robustness
and speed of MoM-SO.
The paper is organized as follows. In Sec. 2 we state the problem from
a theoretical standpoint and review the two fundamental relations exploited in
this work, namely the electric field integral equation and the surface admittance
operator. In Sec. 3, the two relations are discretized with the method of moments. In Sec. 4, the new MoM-SO method is first validated against analytical
formulas for a simple two-conductor system, and then compared against FEM
on a three-phase armored cable.

2
2.1

Impedance Computation via a Surface Admittance Operator
Problem Statement

We consider a transmission line made by P round conductors parallel to the z
axis and surrounded by a homogeneous medium. An example of line cross section is shown in Fig. 1, where the permittivity, permeability and conductivity of
the conductors are denoted as ε, µ, and σ respectively. The surrounding medium
is assumed to be lossless, with permittivity εout and permeability µ0 . Our goal
is to compute the per-unit-length (p.u.l.) resistance R (ω) and inductance L (ω)
matrices of the line as defined by the Telegraphers’ equation [3]
∂V
R (ω) + jωL
L (ω)] I
= − [R
∂z

(1)

where V = [V1 · · · VP ]T is a P × 1 vector collecting the potential Vp of each
conductor. Similarly, the P × 1 vector I = [I1 · · · IP ]T is formed by the current
Ip in each conductor. The parameters R (ω) and L (ω) in (1) are commonly
referred as partial p.u.l. parameters [24]. From them, one can easily obtain
the p.u.l. parameters of the line with any conductor taken as reference for
the voltages and as return path for the currents [3]. In this work, the cable
parameters are computed assuming that the electric and magnetic fields are
longitudinally invariant along the cable. We neglect “end effects” that may
arise from the finite length of the cable, and which may be noticeable for shortlength cables [25]. We refer the Reader to [26, 25] for more details on this
aspect.

3

ε, µ, σ

n̂
εout , µ0

Figure 1: Sample line cross section. The conductors are depicted in white
(permittivity: ε, permeability: µ, conductivity: σ). The surrounding medium
is depicted in grey (permittivity: εout , permeability: µ0 ). The unit vector n̂ is
the normal to the conductors surface.
Js

ap

θ

cp

εout , µ0

Figure 2: Line cross section after application of the equivalence theorem. The
conductors medium is replaced by the surrounding medium, and an equivalent
current density Js is introduced on the surface of the conductors. The contour
and radius of the p-th conductor are denoted with cp and ap , respectively.

2.2

Surface Admittance Operator

In order to compute the p.u.l. impedance of the line, we follow the approach
of [21] which relies on a surface admittance operator. We replace each conductor
with the surrounding medium and, to maintain the electric field Ez outside the
conductors’ volume unchanged, we introduce a surface current density Js on
their contour, as shown in Fig. 2. The electric field inside the conductors’
ez . The current density Js , directed
volume takes instead a fictitious value E
along z, can be found with the equivalence theorem [27, 21] and reads
et ,
Js = Ht − H

(2)

where Ht is the component of the magnetic field tangential to the conductor’s
surface, evaluated before the application of the equivalence theorem (configurae t is the same quantity evaluated after the equivalence theorem
tion of Fig. 1). H
has been applied (configuration of Fig. 2).
On the boundary of each conductor, the tangential component of the magnetic field is related to the longitudinal electric field [21]
Ht =
where

∂
∂n

1 ∂Ez
,
jωµ ∂n

(3)

denotes the directional derivative [28] with respect to the unit vector

4

e t we can write
n̂ normal to the conductors surface. Similarly, for H
et =
H

ez
1 ∂E
.
jωµ0 ∂n

(4)

By substituting (3) and (4) into (2), we obtain the relation
"
#
ez
1 1 ∂Ez
1 ∂E
,
Js =
−
jω µ ∂n
µ0 ∂n

(5)

which defines a surface admittance operator that maps the electric field on the
conductor’s boundary onto the equivalent current density Js . Equation (5)
extends the formula given in [21] to the case of magnetic conductors (µ 6= µ0 ).

2.3

Electric Field Integral Equation

After applying the equivalence theorem to each conductor, the medium becomes
homogeneous, and we can easily relate the equivalent current density Js to the
electric field by means of the electric field integral equation [22]. This process
involves only the boundary cp of each conductor, that we describe with the
position vector
~rp (θ) = (xp + ap cos θ) x̂ + (yp + ap sin θ) ŷ ,

(6)

where θ is the azimuthal coordinate, (xp , yp ) are the coordinates of the conductor
center, and ap is the conductor radius, as illustrated in Fig. 2. The unit vectors
x̂ and ŷ are aligned with the x and y axis, respectively. We denote the equivalent
current density and the electric field on the boundary of the p-th conductor as
(p)
(p)
Js (θ) and Ez (θ), respectively. Using the electric field integral equation, we
(p)
can express the electric field Ez (θ) on the surface of the p-th conductor as
Ez(p) (θ)

= jωµ0

P ˆ
X
q=1

2π

Js(q) (θ0 )G (~rp (θ), ~rq (θ0 )) aq dθ0

0

−

∂V
, (7)
∂z

where the first term accounts for the field generated by the current on each
conductor, while the second term is related to the scalar potential V . The
integral kernel
1
G(~rp , ~rq ) =
ln |~rp − ~rq |
(8)
2π
is the Green’s function of an infinite space [27]. On the p-th conductor, the scalar
potential V is equal to the conductor potential Vp that appears in the Telegraphers’ equation (1). Therefore, we can replace the last term in (7) with (1),

5

obtaining
Ez(p) (θ) = jωµ0

P ˆ
X

+

Js(q) (θ0 )G (~rp (θ), ~rq (θ0 )) aq dθ0

0

q=1
P
X

2π

R pq (ω) + jωL
L pq (ω)] Iq ,
[R

(9)

q=1

where R pq (ω) and L pq (ω) are the elements in position (p, q) of the matrices
R (ω) and L (ω), respectively. Equation (9) combined with the surface admittance operator (5) will allow us to compute the p.u.l. parameters of the line.
In the next Section, we introduce a discretization of these relations suitable for
numerical computations.

3
3.1

Numerical Formulation
Discretization of the Surface Admittance Operator

Given the cylindrical geometry of the conductors, we approximate the field and
the current on the p-th conductor by means of a truncated Fourier series
Ez(p) (θ)

=

Np
X

En(p) ejnθ ,

(10)

n=−Np

Js(p) (θ)

=

1
2πap

Np
X

Jn(p) ejnθ .

(11)

n=−Np

The truncation order Np controls the accuracy and the computational cost of
the numerical technique. Examples will show that a low Np , of the order of
2 − 3, delivers very accurate results while minimizing the computation time.
1
Owing to the normalization factor 2πa
, the total current Ip flowing in the p-th
p
conductor is simply given by the constant term of the series [21]
(p)

Ip = J0
(p)

p = 1, . . . , P .

(12)

(p)

When Ez and Js are expressed in Fourier series, the surface admittance
operator (5) can be rewritten in terms of the Fourier coefficients as [21]


0
0
2π kap J|n| (kap ) kout ap J|n| (kout ap )
−
,
(13)
Jn(p) = En(p)
jω µJ|n| (kap )
µo J|n| (kout ap )
0
where J|n| (.) is the Bessel function of the first kind [29] of order |n|, and J|n|
(.)
is its derivative. The wavenumber in the conductors and in the surrounding
medium are given respectively by
p
k = ωµ(ωε − jσ) ,
(14)
√
kout = ω µ0 εout .
(15)

6

In order to simplify the oncoming equations, we introduce a compact matrix
(p)
notation. We collect all field coefficients En in the column vector
h
iT
(1)
(1)
(2)
(2)
E = E−N
,
(16)
·
·
·
E
E
·
·
·
E
·
·
·
N1
−N2
N2
1
(p)

and all current coefficients Jn in
h
(1)
(1)
J = J−N
· · · JN1
1

(2)

J−N2

···

(2)

JN2

···

iT

.

(17)

The vectors E and J have size
P
X
N=
(2Np + 1) ,

(18)

p=1

the total number of field and current coefficients. Relation (13) can be written
in terms of E and J as
J = Ys E .
(19)
where Ys is a diagonal matrix. This matrix is the discrete version of the surface
admittance operator defined by (5) for each round conductor. Finally, (12) can
be written in terms of I and J as
I = UT J ,

(20)

where U is a constant N × P matrix made by all zeros and a single “1” in each
(p)
column. In column p, the “1” is in the same row as the coefficient J0 in (17).

3.2

Discretization of the Electric Field Integral Equation

We now cast the electric field integral equation (9) into a set of algebraic equations using the method of moments, a numerical method to solve integral and
differential equations [23]. First, we substitute (10) and (11) into (9), obtaining
Np
X

En(p) ejnθ =

n=−Np

ˆ
Nq
P
jωµ0 X X (q) 2π jnθ0
Jn
e
G (~rp (θ), ~rq (θ0 )) dθ0
2π q=1
0
n=−Nq

+

P
X

R pq (ω) + jωL
L pq (ω)] Iq .
[R

(21)

q=1
0

Then, we project (21) onto the Fourier basis functions ejn θ by applying the
operator
ˆ 2π
0
[.]e−jn θ dθ
n0 = −Np , . . . , Np
(22)
0

7

to both sides of the equation, obtaining
(p)

En0 = jωµ0

Nq
P
X
X

(p,q)

Gn0 n Jn(q) +

q=1 n=−Nq

δn0 ,0

P
X

R pq (ω) + jωL
L pq (ω)] Iq , (23)
[R

q=1

for p = 1, . . . , P , where
(
δn0 ,0 =

1
0

when n0 = 0
when n0 =
6 0,

(24)

(p,q)

and where Gn0 n denotes the (n0 , n) entry of the matrix G(p,q) . This matrix
describes the contribution of the current on the q-th conductor to the field on
the p-th conductor. The entries of G(p,q) are given by the double integral
ˆ 2π ˆ 2π
0
0
1
(p,q)
G (~rp (θ), ~rq (θ0 )) ej(nθ −n θ) dθdθ0 ,
(25)
Gn0 n =
(2π)2 0
0
which can be computed analytically as shown in the Appendix. Using the matrix
notation set in (16) and (17), the equations (23) can be written in compact form
as
R (ω) + jωL
L (ω)] I ,
E = jωµ0 GJ + U [R
(26)
where G is the block matrix
G(1,1)
 ..
G= .


(P,1)

G

···
..
.
···


G(1,P )
..  .
. 

(27)

(P,P )

G

Equation (26) is the discrete counterpart of the electric field integral equation (9).

3.3

Computation of the Per-Unit-Length Parameters

The p.u.l. parameters of the line can be obtained by combining the discretized
surface admittance operator (19) with the discretized electric field integral equation (26) as follows. First, we left multiply (26) by Ys and, using (19), we obtain
R (ω) + jωL
L (ω)] I .
J = jωµ0 Ys GJ + Ys U [R

(28)

The current density coefficients J can be expressed as
R (ω) + jωL
L (ω)] I ,
J = (1 − jωµ0 Ys G)−1 Ys U [R

8

(29)

where 1 denotes the N × N identity matrix. Left multiplication of (29) by UT
leads to the following expression for the line currents I


R (ω) + jωL
L (ω)] I .
I = UT (1 − jωµ0 Ys G)−1 Ys U · [R
(30)
Since (30) must hold for any I, the product of the two expressions inside the
square brackets must be equal to the identity matrix. Consequently, we have
that

−1
L (ω) = UT (1 − jωµ0 Ys G)−1 Ys U
R (ω) + jωL
.
(31)
By taking the real and imaginary part of (31) we finally obtain the formulas for
computing the p.u.l. resistance and inductance matrices
n
−1 o
R (ω) = Re UT (1 − jωµ0 Ys G)−1 Ys U
,
(32)
n
o
−1
L (ω) = ω −1 Im UT (1 − jωµ0 Ys G)−1 Ys U
.
(33)

4
4.1

Numerical results
Two Round Conductors

In order to validate the proposed technique against analytic formulas, we consider a line made by two parallel round conductors with radius a = 10 mm made
of copper (σ = 58 · 106 S/m, µ = µ0 ). Two different values for the center-tocenter distance between the wires have been used, namely D = 100 mm and
D = 25 mm. In the first case proximity effect is negligible, due to the wide
separation. In the second case it is instead significant.
The p.u.l. computed with a MATLAB implementation of MoM-SO have
been compared against two different sets of analytic formulas. The first set is
valid at high frequency [3] because it assumes a fully-developed skin effect, and
gives the p.u.l. resistance and inductance as
R=
Lext

Rs
q
πa

D
2a

D 2
2a

,
−1
 
µ0
D
−1
cosh
,
=
π
2a

(34)

(35)

where Rs = (σδ)−1 is the surface resistance and
δ=√

1
πf µ0 σ

(36)

is the skin depth. The second set of formulas accounts for the frequency dependence of the internal impedance of the wire Zint , which can be calculated
analytically under the assumption of wide separation [3]
Zint = √

1
ber(ξ) + j bei(ξ)
,
0
0
2πaσδ bei (ξ) − j ber (ξ)
9

(37)

Table 1: Characteristics of the cables in the example of Sec. 4.2.
Item
Parameters
Core
σ = 58 · 106 S/m, r = 10.0 mm
Insulation
t = 4.0 mm, r = 2.3
Wire screen 32 wires, r = 0.5 mm, σ = 58 · 106 S/m
Jacket
t = 2 mm, r = 2.3
√
where ξ = 2 aδ . The Kelvin functions ber(ξ) and bei(ξ) are the real and
3
imaginary part of J0 (ξej 4 π ), respectively [29]. The total p.u.l. impedance of
the line is thus
Z = 2Zint + jωLext .
(38)
Formulas (34)-(35) account for proximity effect, which is instead neglected
in (37).
The p.u.l. parameters have been computed from 1 Hz to 1 MHz with the
truncation order Np set to 3. No noticeable changes have been observed beyond
this value. The computation of the parameters took 15 ms per frequency sample
on a 3.4 GHz CPU. The discretization of the Green’s function which leads
to the matrix G took less than 5 ms. Figures 3 and 4 compare the p.u.l.
parameters computed with MoM-SO against the results obtained from the two
analytic formulas. In all cases, the numerical results correctly approach the
exact high frequency value given by formulas (34) and (35). In the case of wide
separation, shown in the top panels, the numerical results also correctly predict
the frequency-dependent behavior of the wire’s internal impedance (37) due to
skin effect. A little discrepancy is visible in the inductance at low frequency
(top panel of Fig. 4), due to the small but not negligible proximity effect. The
error introduced by (37) becomes more significant when the wires separation
is reduced to 25 mm, as shown in the bottom panel of Figures 3 and 4. The
non-uniform current distribution induced by the wires proximity is visible in
Fig. 5, which also shows the development of skin effect.

4.2

Three-Phase Armored Cable

We consider the three-phase armored cable in Fig. 6 which features three wire
screens and a steel armoring, for a total of 293 circular subconductors. The
key parameters are listed in Tables 1 and 2, respectively. Using MoM-SO we
computed the 3 × 3 series impedance matrix with respect to the three phase
conductors with the screens continuously grounded along the cable.
Table 3 shows the calculated positive and zero sequence resistance and reactance per km. The computation has been performed with three different
truncation orders: Np = 0, Np = 3 and Np = 7. As a validation we used a FEM
implementation [8] with a very fine mesh (177,456 triangles). It is observed that
with orders Np = 3 and Np = 7 we get a result which deviates by less than 1%
from the FEM result.
10

Table 2: Armor characteristics for the structure considered in Sec. 4.2.
Item
Parameters
Armor outer diameter
88.26 mm
Wire diameter
3 mm
Conductivity
107 S/m
µr
100
N.o. wires per layer
70

Figs. 7 and 8 show the positive and zero sequence resistance and inductance
as a function of frequency, from 1 Hz to 1 MHz. It is observed that with Np = 0,
significant errors result as the proximity effects are ignored. Indeed, by setting
Np = 0 in (10) and (11), one assumes a circularly-symmetric current distribution
on the conductors. With Np = 3 and Np = 7, a virtually identical result is
achieved which agrees very well with the FEM result. At very high frequencies,
however, the FEM result deviates somewhat from that of the proposed approach
since the mesh division is not sufficiently fine to properly account for the very
small skin depth. In the proposed technique, instead, skin effect is implicitly
and fully described by the surface admittance operator, and does not affect
the discretization of the problem, which depends only on the proximity of the
conductors. As a result, the level of discretization, controlled by Np , does not
have to be increased as frequency grows, making MoM-SO much more efficient
than FEM.
Timing results, reported in Table 4, demonstrate the excellent performance
of the developed algorithm. With MoM-SO, there is first a computation time
for the Green’s matrix G of 11.6 s (Np = 3) or 16.5 s (Np = 7). Matrix G
has to be evaluated only once, since it does not depend on frequency. Then,
for computing each frequency sample one needs 2.01 s for Np = 3 or 15.5 s
for Np = 7. Since Np = 3 was found sufficient for obtaining accurate results,
the total computational cost for computing the 31 samples in this example is
T = 11.6 + 31 × 2.01 = 73.9 s. The computation time using FEM is much
higher, requiring 440 s per frequency sample. This is 220 times slower than the
per-sample computation time of 2.01 s using the new method with Np = 3. We
can therefore safely state that MoM-SO is at least 100 times faster than the
FEM approach when several frequency samples are needed.

5
5.1

Discussion
Computational Cost

A few remarks on the computational cost of MoM-SO are in order. The most
expensive step in evaluating (32) and (33) is the LU factorization of the matrix
M = 1 − jωµ0 Ys G ,
11

(39)

Table 3: Positive- and zero-sequence impedance of the three-phase cable of
Sec. 4.2 at 50 Hz. MoM-SO is compared against a finite element approach [8].
MoM-SO (proposed)
Np = 0 Np = 3 Np = 7
FEM
R+ [Ω/km] 0.06905 0.07259 0.07261 0.07218
X+ [Ω/km] 0.08703 0.09042 0.09048 0.09041
R0 [Ω/km]
0.2386
0.2437
0.2438
0.2459
X0 [Ω/km] 0.08033 0.08943 0.08958 0.08975
Table 4: Three-phase cable example of Sec. 4.2: computation time for MoM-SO
and FEM.
MoM-SO (proposed)
Np = 0 Np = 3 Np = 7 FEM
Green’s
11.6 s
13.7 s
16.5 s
function discretization
Impedance
0.085 s
2.01 s
15.5 s 440 s*
computation
(per
frequency
sample)
All computations were performed on a system
with a 2.5 GHz CPU and 16 GB of memory.
*Mesh size: 177,456 triangles.
which is used to compute the term (1 − jωµ0 Ys G)−1 Ys U. The matrix M
has size N × N , where N is the total number of unknowns used to discretize
the problem (18). If we let Np = 3 for all conductors, we obtain that N = 7P .
Therefore, the number of unknowns N scales well with the number of conductors
P , and it remains moderate even in presence of hundreds of conductors. In
the example of Sec. 4.2, which has 293 conductors, MoM-SO uses N = 2051
unknowns, as opposed to the 177,464 unknowns required by FEM. Even if the
MoM-SO matrix (39) is full while the FEM matrix is very sparse, the huge
difference in size makes MoM-SO faster than FEM, as shown by the numerical
results. The remarkable saving of unknowns stems from the use of a surface
formulation instead of a volume formulation, where one must mesh the entire
volume of the conductors and, possibly, also of the surrounding medium. The
FEM code [8] used in this paper is an in-house program which adapts stateof-the-art routines in Matlab’s PDE Toolbox to the Weiss-Scendes one-step
FEM method [14] for series impedance computation. Although the usage of a
different FEM implementation or a different meshing strategy may improve the

12

computational efficiency, the need for a very large number of triangles cannot
be overcome. When computing frequency samples over a wide frequency band,
the mesh must have a fine resolution over the entire solution domain to capture
the low frequency behavior, and at the same time have a very fine resolution at
the conductor surfaces to capture the pronounced skin effect at high frequency.
The MoM-SO approach fundamentally overcomes this issues, since it does not
require any meshing of the cross section.

5.2

Relation with Existing EMTP Tools

In the numerical example of Sec. 4.2, we considered a commonly applied threephase cable design which features wire screens and a stranded steel armoring.
This cable was modeled with MoM-SO with an explicit representation of each
strand. In available EMTP-tools, one would have to model the screens and
the armor by equivalent tubular conductors. Such approach leads to very fast
computations but errors are inevitably introduced, in particular for the steel armoring where the air gaps between magnetic strands cannot be easily accounted
for by an equivalent tubular conductor. We have shown that with MoM-SO such
cables can be modeled in full detail with an acceptable CPU time, while taking
into account both skin and proximity effects in every phase conductor, and in
every wire and armor strand.

5.3

Effect of Lossy Ground

The current version of MoM-SO does not permit to include the effect of a lossy
ground. However, in the case of transients involving armored and pipe-type
cables, the effect of the ground return is often small and can be ignored, at
least when the transient effect does not include current injection to ground. The
authors are currently extending the method to include ground return effects and
tubular conductors (sheaths). As for the example of the three-phase cable in
Sec. 4.2, the modeling in this paper is fully applicable since the sheath conductor
consists of wires and because the thick armor permits to ignore the external
medium.

6

Conclusion

We presented an efficient algorithm for computing the series impedance of systems of round conductors. The method combines a surface operator with the
method of moments which permits to compute the complete series impedance
matrix while taking into account both skin and proximity effects. This capability is of major importance in cable system modeling as the short lateral distance
between cables often leads to significant proximity effects. Due to its efficient
discretization of the underlying electromagnetic problem, the algorithm outperforms state-of-the-art techniques based on finite elements by a factor of about

13

100. The computed resistance and inductance can be used for an accurate prediction of electromagnetic transients in EMTP-type programs when combined
with an appropriate frequency-dependent cable model.

Analytical evaluation of the Green’s matrix G
The discretization of the Green’s function requires the computation of the double
integral (25). After substitution of (8), the integral reads
ˆ 2π
0
1
(p,q)
fn (θ)e−jn θ dθ .
(40)
Gn0 n =
(2π)2 0
where
fn (θ) =

1
2π

ˆ

2π

0

ln |~rp (θ) − ~rq (θ0 )| ejnθ dθ0 .

(41)

0

We first calculate the integral in (41), which can be expanded using (6) to obtain
ˆ 2π
0
1
ln r~00 (θ) − aq (cos θ0 x̂ + sin θ0 ŷ) ejnθ dθ0 =
fn (θ) =
2π 0
ˆ 2π


0
1
ln (r00 )2 + a2q − 2aq r00 cos(θ00 − θ0 ) ejnθ dθ0 ,
(42)
4π 0
where r~00 (θ) is the auxiliary vector
r~00 (θ) = ~rp (θ) − (xq x̂ + yq ŷ) = r00 (cos θ00 x̂ + sin θ00 ŷ) ,

(43)

which is constant with respect to the integration variable θ0 . We denote its
modulus and its angle with r00 and θ00 respectively1 . The solution to the last
integral in (42) is presented in [3] and reads

ln(r00 )
for n = 0 ,
jnθ 00
fn (θ) =
(44)
a|n|
e
− q 00 |n| for n 6= 0 .
2|n|(r )
Next, we solve the integral in (40). The solution involves several cases, which
are itemize for better readability:
• p 6= q , n = 0: in this case, the integral in (40) is analogous to (41) and
can be solved with the formulas given in [3]. When n0 = 0, we have
(p,q)

G0,0 =

1
ln(dp,q ) ,
2π

(45)

and when n0 6= 0 we have
(p,q)
Gn0 ,0
1 For

1
=−
4π |n0 |



 n0 
 n0
ap | |
xp,q − jyp,q
−
,
dp,q
dp,q

(46)

the sake of clarity of the notation, we omit from r00 and θ00 the dependence on θ.

14

where xp,q = (xp − xq ), yp,q = (yp − yq ), and
q
2 .
dp,q = x2p,q + yp,q

(47)

• p 6= q , n > 0, n0 ≥ 1: if we manipulate the integrand function as follows
"
#n
00
00
(aq )n ejnθ
(aq )n r00 ejθ
fn (θ) = −
=−
=
2n(r00 )n
2n
(r00 )2
n

(aq )n
xp,q + ap cos θ + j(yp,q + ap sin θ)
−
=
2n
(xp,q + ap cos θ)2 + (yp,q + ap sin θ)2
(aq )n
1
−
,
2n (xp,q − jyp,q + ap e−jθ )n
we can rewrite (40) as a complex integral

0
j(aq )n
z n −1
(p,q)
Gn0 ,n = − 2
dz ,
8π n(ap )n0
(xp,q − jyp,q + z)n

(48)

performed over the closed path z = ap e−jθ with θ = [0, 2π]. Since we
assume n0 ≥ 1, the integrand function in (48) will have no poles inside the
integration path and, using the residue theorem [30], we have that
(p,q)

Gn0 ,n = 0 .

(49)

• p 6= q , n > 0, n0 < 1: when n0 < 1, the integrand function in (48) has a
pole in z = −xp,q + jyp,q . Applying again the residue theorem we obtain


0
−π(aq )n n − n0 − 1 (dx − jdy )−n+n
(p,q)
Gn0 ,n =
,
(50)
(−ap )n0
−n0
(2π)2 n

n
with m
being the binomial coefficient.
• p 6= q , n < 0: the Green’s matrix entries for n < 0 can be obtained
from (49) and (50) by symmetry
(p,q)

∗
Gn0 ,n = (Gp,q
−n0 ,−n ) ,

(51)

where ∗ denotes complex conjugation. This identity follows from the symmetry relation f−n (θ) = fn (θ)∗ .
• p = q: in this case we have that xp,q = yp,q = 0, and using (48) one can
easily show that
(
1
ln ap if n0 = 0 ,
(p,p)
Gn0 ,0 = 2π
(52)
0
if n0 6= 0 ,
(
1
− 4π|n|
if n 6= 0, n0 = n ,
(p,p)
Gn0 ,n =
(53)
0
if n 6= 0, n0 6= n .

15

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Research

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16

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17

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18

−2

10

Resistance p.u.l. (Ω/m)

Analytical (no proximity)
Analytical (high−freq)
Computed
−3

10

−4

10

0

10

2

10

4

10

4

10

10

6

Frequency [Hz]
−2

10

Resistance p.u.l. (Ω/m)

Analytical (no proximity)
Analytical (high−freq)
Computed
−3

10

−4

10

0

10

2

10

10

6

Frequency [Hz]

Figure 3: P.u.l. resistance of the two wires line of Sec. 4.1 computed with
MoM-SO (crosses), the analytic formula (34) valid at high frequency (dash-dot
line), and formula (38) (solid line). Wires separation is 100 mm (top panel) and
25 mm (bottom panel).
19

−7

10.4

x 10

Analytical (no proximity)
Analytical (high−freq)
Computed

Inductance p.u.l. [H/m]

10.2
10
9.8
9.6
9.4
9.2
9
0
10

2

4

10

10

6

10

Frequency [Hz]
−7

5

x 10

Analytical (no proximity)
Analytical (high−freq)
Computed

Inductance p.u.l. [H/m]

4.5

4

3.5

3

2.5
0
10

2

4

10

10

6

10

Frequency [Hz]

Figure 4: P.u.l. inductance of the two wires line of Sec. 4.1 computed with
MoM-SO (crosses), the analytic formula (35) valid at high frequency (dash-dot
line), and formula (38) (solid line). Wires separation is 100 mm (top panel) and
25 mm (bottom panel).
20

(a) 515 Hz

(b) 3.63 kHz

Figure 5: Two wires example of Sec. 4.1 in the case of a separation of 25 mm:
current density plot for two different frequencies, obtained with MoM-SO.

40

30

20

y [mm]

10

0

−10

−20

−30

−40
−40

−30

−20

−10

0
x [mm]

10

20

30

40

Figure 6: Cross section of the three-phase cable with steel armoring and wire
sheath considered in Sec. 4.2

21

1

10

FEM
Proposed, Np=0.

Resistance p.u.l. [Ohm/km]

Proposed, Np=3.
Proposed, Np=7.
0

10

Zero sequence

−1

10

Positive sequence

−2

10

0

10

1

2

10

10

3

10
Frequency [Hz]

4

10

5

10

6

10

Figure 7: P.u.l. resistance of the three-phase cable of Sec. 4.2, obtained with
MoM-SO and FEM. For MoM-SO, three different truncation orders Np are
considered.
0.4

Inductance p.u.l. [mH/km]

0.35

FEM
Proposed, Np=0.

Positive sequence

Proposed, Np=3.
Proposed, Np=7.

0.3

0.25
Zero sequence
0.2

0.15

0.1

0.05
0
10

1

10

2

10

3

10
Frequency [Hz]

4

10

5

10

6

10

Figure 8: P.u.l. inductance of the three-phase cable of Sec. 4.2, obtained with
MoM-SO and FEM. For MoM-SO, three different truncation orders Np are
considered.
22

