Long length EHV underground cable systems in the transmission ...

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B1-304
Session 2004
© CIGRÉ
LONG LENGTH EHV UNDERGROUND CABLE SYSTEMS IN THE
TRANSMISSION NETWORK
M. DEL BRENNA, F. DONAZZI (*), A. MANSOLDO
PIRELLI CAVI E SISTEMI ENERGIA SPA
(Italy)
SUMMARY
The power transmission network has developed during the last decades based on the use of overhead
lines.
EHV underground insulated cable systems have been available since a long time (fluid filled
technology initially and solid dielectric technology more recently), but their development has always
been limited, mainly due to economic constraints, and they have been adopted for those applications
where overhead lines could not be pursued.
For long length connections, some technical constraints have been raised against the adoption of
underground cable systems.
On the other hand, environmental considerations, together with an increasing need for optimization of
the transmission network, push to reconsider the real impact of underground cable systems backbones.
Among the claimed technical issues, related to underground cable systems, the most sensitive topics
are those concerning length limitations, reliability and impact on the transmission grid. Indeed, while
at the High Voltage level (i.e. up to 170 kV) those problems have minor influence, some dispute is still
alive for EHV applications.
However, in light of the evolution of cable systems technology, new installation techniques and new
compensation concepts, this theme shall be reconsidered, studied in more depth and brought back to a
balanced rationale.
In this paper the following topics are analyzed:
· State of the art of AC EHV cable systems
· Determination of criteria for the definition of the maximum permissible length for EHV
underground cable systems, their rationale and their implications in the network
· Considerations on new compensation concepts and their impact on the network at different load
conditions
· Cable self-protecting effect in fast transients
· Considerations on reliability and availability of underground cable systems, with reference to
diagnostic and monitoring techniques
A study case is analysed to demonstrate the feasibility of using EHV underground cable systems in
long backbone transmission connections.
KEYWORDS
Interconnection, Cable System, Reactive Compensation, Transmission, Lightning
_____________________________________________________________________________
(*) Viale Sarca 222,20126 Milano (Italy). E-amil: Fabrizio.donazzi@pirelli.com

1. INTRODUCTION
For decades electricity transmission networks have been mainly national and almost exclusively based
on the use of overhead lines. In recent years, however, new drivers have started to play an important
role in their design.
Electricity markets are becoming increasingly liberalized and internationalised, and there is a strong
need, in particular in Europe, to optimise the utilization of power generation capacity and to increase
international competition by increasing the interconnections. Additional requirements, such as
environmental compatibility, impact on population and right-of-way utilization, are also having a
strong impact on the definition of new connections, sometimes causing significant delays in the
authorization process. These delays are not acceptable to most of the new private investors, who have
started to appear in the global scene and for whom speed is a key factor in making their investments
viable.
On the technical side, cable system technology has reached a development level and track record that
allows it to be considered as highly reliable. Furthermore, cable system technology can overcome the
limitation of traditional overhead lines in specific situations (i.e. densely populated areas, national
parks, tourist estates, etc.). Last, but definitely not least, cable systems can easily be inserted in
overhead line based networks with no negative impact on the surrounding system, affordable
technology being available to implement any reactive compensation or impedance balancing needed.
2. STATE-OF-THE ART OF AC EHV CABLE SYSTEMS
2.1 EHV AC cable systems with lapped insulation
Extra high voltage (EHV) cable systems of the self contained oil filled type (SCOF) have been in use
for many decades with excellent service records as part of bulk power transmission grids. In the mid
1960’s the first 400 kV cable systems for long distances were installed in Europe as feeders for
densely populated areas or as interconnections between huge power generation plants and remote
substations or load centres.
An improvement of this technology was introduced in the early 1980’s by replacing the conventional
Kraft paper by polypropylene laminated paper (PPL), which provides the advantage of low-loss
insulation.
Full cable system reliability over more than 40 years is proven by the extensive field experience
acquired: over 250 km of mainly double circuit 400 kV cable systems are now in operation in Europe
alone. Similar installations have also been realized all over the world, e.g. in North America and
Japan, even in the 500 kV range.
For highest transmission capacity, cable systems with forced cooling have been installed in the last 25
years.
2.2 EHV AC cable systems with extruded insulation
Environmental constraints regarding potential leaks and the desire to minimize regular maintenance
were the main drivers to replace fluid filled with dry cables. After extruded cables had already proven
their excellent service performance for several decades in the medium (MV) and high voltage (HV)
ranges, great efforts were spent since the 1980’s in the development of synthetic cables for EHV
applications, the main challenge being associated with high electrical stresses in cables and
accessories.
Cross-linked polyethylene (XLPE) has proven to be the best synthetic insulation from the technical
and economical points of view.
State of the art extruded EHV cables are characterized by super-clean insulation with well-bonded
semiconductive conductor and insulation shields, applied simultaneously in a triple extrusion and dry
curing process. Highest cleanliness, absence of voids, homogeneity of the insulation and perfect
smoothness of the interfaces with the semiconductive shields are paramount to guarantee long-term

performances. A metallic sheath and a rigid plastic oversheath protect the cable core from water and
mechanical damage.
The trend in accessories has gone towards factory tested prefabricated components. In particular premoulded
joints, characterized by single-piece rubber sleeves (EPDM or SIR), are easy and reliable to
install. Terminations are typically equipped with prefabricated stress relief cones, placed inside
synthetic or porcelain insulators.
A precondition for the acceptance of the new cable technology was the proof of its long-term
reliability [1]. Extensive test programs have been carried out and the excellent test results convinced
all parties that long-term performances of such advanced cable systems could be considered
appropriate.
First long distance EHV XLPE cable systems have been installed since the late 90’s, typical examples
of which are:
· 420kV XLPE cable systems with natural cooling for 800 and 900 MVA/cct (interconnection
feeders for the city of Copenhagen, Denmark (22 km + 10 km), in service since 1997 [2]
· 400kV XLPE cable systems with ventilated air cooling in tunnel for 1120 MVA/cct (diagonal
interconnection throughout the city of Berlin (~24 km), in service since 1998 [3]
· 500kV XLPE cable systems with tunnel and duct installation for 1200 MVA/cct (interconnection
feeders for the city of Tokio, Japan (~ 40 km), in service since 2000 [4]
· 400kV XLPE cable system with ventilated tunnel installation for 1720 MVA/cct (“siphon”
intersection of an existing OHL at Barajas Madrid Airport (~13 km), under construction)
Despite its relatively young age, extruded EHV cable systems technology is convincingly
demonstrating its appropriateness and increasingly extending its application for all kinds of
interconnections, leveraging on some key features, e.g. reduced environmental impact, ease of
installation and no need for maintenance.
Dedicated efforts to save costs associated with production, components and installation are
permanently contributing to increase the competitiveness of this technology.
3. MAXIMUM PERMISSIBLE LENGTH OF EHV CABLE SYSTEMS
3.1 Critical lengths and influence on cable system design parameters
Overhead lines are largely used in transmission networks due to their technological simplicity, low
costs and suitability to transmit bulk power for long distances (100-300 km). Their main intrinsic
feature is a high ratio between inductive and capacitive reactance, physically represented by the
characteristic impedance of the line.
The characteristic impedance of Underground Insulated Cable Systems (UICS) is much lower, due to
differences in both inductance and capacitance. In Table 1 some reference values are given for EHV
systems able to transmit 2000 MVA at 500 kV. The insulated cable capacitance is at least 15-20 times
that of overhead lines, while the cable inductance ranges between 0.25-1 times.
Table 1: Indicative reference electrical parameters for overhead lines and underground cables
OHL
1600mm 2
XLPE
trefoil
formation
(2cables/phase)
2500 mm 2 XLPE
vertical formation
0.5 m spaced in
tunnel
(1 cable/phase)
Current rating (A) 2310 2310 2310 2310
Transmissible power (MVA) 2000 2000 2000 2000
AC resistance (µΩm -1 ) 28 7.9 10.8 8.8
Inductance (nHm -1 ) 862 192 646 760
Capacitance (pFm -1 ) 14 362 205 229
Characteristic impedance (Ω) 250 23 56.2 39.2
Natural load (MW) 1000 10910 4490 6440
3250 mm 2
XLPE
flat formation
1m spaced
(1 cable/phase)

The power rating of the UICS depends on the laying disposition and on the thermal characteristics of
the surrounding environment.
Figure 1 shows a sensitive study of the ratings for different conductor cross sections and laying
configurations. The 500 kV cables here have been designed with a maximum AC electric stress of 15
kV/mm and without exceeding an electric stress of 7.8 kV/mm at the surface between insulation and
insulation screen [5]. Before investigating the real impact of an UICS in a meshed transmission
system, it is necessary to define its maximum length technically feasible without compensation.
Several criteria have been used [6, 7, 8, 9] and many are the limiting factors which can be considered,
either external (i.e. steady state stability maximum angle, minimum and maximum voltages) or
internal (i.e. critical charging current, transmission efficiency, cable BIL). Additional constraints,
which are not considered in this paper, may appear for specific scenarios as for instance radial
connection of generators, to the main grid.
As regards transmission efficiency many approaches have been proposed [6,8,9].
2600
2400
Power rating [MVA]
2200
2000
1800
1600
1400
S=1600 (mm²)
S=2000 (mm²)
S=2500 (mm²)
S=3250 (mm²)
S=4000 (mm²)
1200
1000
0 500 1000 1500 2000 2500
Phase spacing [mm]
Figure. 1: Rating of 500 kV XLPE underground cable systems
According to [6] the optimum cable circuit length is the one that realizes, at the nominal cable power,
the maximum active power transfer between generation and load. The results are very sensitive to the
“optimal” cos φ. For load power factors (LPF) close to the maximum (cos φ ≅1), the Abacus in Figure
4 of [6] leads to maximum lengths close to nil, while for typical LPF (cos φ ≅0.95) lengths of the order
of 100 km are obtained.
In [8, 9] the maximum length is the one defined as the “Longueur d’Aptitude au Transport (LAT)”
which “guarantees that the (active NdR) power effectively transmitted be not less than 95% of the
total power input”, when the load power factor is equal to 1 (pure resistive load).
In order to better explain the LAT concept, some remarks are given below.
3.1.1 LAT (Longueur d’Aptitude au Transport )
For a generic load the following equations can be considered:
⎪⎧
V
⎨
⎪⎩ I
RE
RE
= A ⋅ V
SE
= −C
⋅ V
SE
− B ⋅ I
+ A ⋅ I
SE
SE
(1
)
⎧ V =
SE Vn
⎪
⎪ 2 2
⎨
PSE
+ QSE
= S
⎪PRE
= ρ ⋅ Sn
⎪
⎩QRE
= tgϕ ⋅PRE
n
(2)
⎧ρ=
0.95
⎨
⎩cos(
ϕ)
= 1.
(3)

In (1), the quadrupole equations from receiving to sending ends (RE and SE) have been considered,
with A , B, C representing the complex transmission coefficients that depend on circuit
electromagnetic parameters and V and I the complex voltage and current respectively.
In (2) general boundary conditions have been introduced, in terms of nominal circuit power (S n ) in
terms of real (P) and reactive (Q) components, load power factor relations (tg ϕ), and ‘efficiency’
requirements from the sending to the receiving ends (ρ).
In (3) further constraints are considered; they are included in the LAT definition itself.
In Figure 2 the reference circuit is shown.
~
SE : sending end
S=S n = Cable nominal power
V se : V nominal
RE : receiving end
P re =P load
Power factor = 1
LAT: Length |
P
RE = 95%
S
n
Figure 2: Reference scheme for LAT calculation
This definition is of particular interest, as it takes into account, at the same time, intrinsic electrical
parameter effects, nominal grid working conditions, near optimum transfer power requirements and
worst scenario load factor. The LAT curves relevant to the 500 kV UICS with cross section
1x1600mm 2 and 1x2500mm 2 described in Table 1, are shown in Figure 3. It appears that the results of
[8] for the trefoil formation are confirmed. However, with large phase spacing, efficient (LAT)
lengths, even greater than 50 km, in a bulk power transmission system, can be reached without any
60
55
50
Trefoil
formation
LAT [km]
45
40
35
30
25
S=1600 mm²
S=2500 mm²
20
0 500 1000 1500 2000
Phase spacing [mm]
Phase spacing [mm]
Figure. 3: LAT- for the 500 kV UICS considered
compensation device.
It is noteworthy to outline that, for UICS laid in a ventilated tunnel, an increase in LAT up to 60% can
be obtained even for reduced phase spacing, as shown in Figure 4 (LAT-tunnel). This is due to
nominal working conditions, which are closer to the Surge Impedance Load Level (SIL), where LAT,
in a loss free link, would be infinite.

3.1.2 LAT and reactive compensation
When generalizing LAT definition, adding inductance in parallel to the resistive load, i.e. when shunt
compensation devices are installed at the RE of the UICS, a further LAT increasing effect is obtained.
As an example, Figure 4 shows the LAT increase for the 2500 mm 2 UICS when 50% of shunt
compensation at the receiving end is adopted (see curve LAT_sh50%).
3.2 Summary of cable length constraints
LAT [km]
250
200
150
100
50
0
( 0.95)
( ∆ϑ)
2
( Vn
) − 3 ⋅ ( Z
cIn
)
2
( V ) + 3 ⋅ ( Z I )
LAT_sh50%
L-DV
Lcrit
0 500 1000 1500 2000
L-Stab. - 15(°)
Phase spacing [mm]
LAT - tunnel
Figure. 4: 500 kV, 2500 mm 2 UICS - Length limitations vs. phase spacing
⎧
⎪L
⎪
⎪
⎨L
⎪
⎪
⎪L
⎪
⎩
STAB
CRIT
DV
1 ⎛ Pc
= arctan⎜
β ⎝
1 ⎛
= arccos⎜
2β
⎝
1
arccos
≅
β
⋅ tg
n
P
⎞
⎟
⎠
LAT
c n
2
2
⎞
⎟
⎠
(4)
(5)
(6)
For reference the 500 kV 2500 mm 2 XLPE UICS configuration shown in Table 1 has been considered.
The Steady State stability limit “L STAB ” has been calculated according to (4), considering an angle
swing ∆ϑ = 15° between SE and RE; whereas P c is the cable SIL and β is the propagation constant.
The Charging Current limit evaluation “L crit “ has been calculated according to (5), in no-load
conditions, where V n and I n are the nominal cable voltage and current rating, and Z c is the
characteristic impedance. The Voltage Variation limit “L DV ”, has been calculated according to (6), in
no-load conditions with a 5% voltage difference between SE and RE. As Figure 4 shows, the LAT is
the most limiting constraint for any traditional underground configuration studied. However, in case of
forced cooled circuits with large phase spacing, the voltage variation “L DV “ constraint can become the
limiting criteria, as shown by the curve “LAT-tunnel” of Figure 4 in correspondence of 1600 mm
spacing.
3.3 Cable self-protecting length
Although not directly influencing the maximum feasible length, cable self-protecting length is
somehow important in insulation coordination studies, in scenarios including OHL and UICS. Due to
discontinuities on surge impedance, the transition point is often protected by surge arresters against
overvoltages driven into the cable by lightning strokes on OHL. Wave reflections cause the rising of
the voltage on the cable itself that sometimes can exceed the Cable BIL. The factors influencing the
voltage increase, mainly depend on:
· Lightning stroke current shape
· Lightning strike point distance from the cable.
· OHL Vs. UICS surge impedance ratio.
· Scenarios at the far end of the cable like a substation (see Figure 5) or an OHL/siphon (Figure 7)
· Cable BIL

In Figure 6 results are shown for the worst case scenario, i.e. a substation at the far end, with a
lightning current of 200 kA 4/250 µs, striking the OHL 10 km away from the entrance of the cable.
The UICS length has been varied from 1500 m to 2000 m.
Zc
Cable SE
Cable RE
10 km 1500 m ÷ 2000 m
Figure.5. UICS Connecting a substation
V [p.u.]
1.4
1.2
1.0
0.8
a) L=1500 m
b) L=2000 m
BIL p.u.
0.6
0.4
0.2
0.0
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Time [ms]
Figure.6. (a,b) Voltage on the OHL (red), Voltage on UICS: SE (green), RE (blue)
Zc
Cable SE
Cable RE
10 km 100 m
Figure.7. UICS in siphon configuration
Zc
a) L=100 m, lightning stroke: 4/50 µs b) L=100 m, lightning stroke: 4/500 µs
V [p.u.]
1.0
0.8
0.6
0.4
0.2
0
0.00 0.04 0.08 0.12
0.00 Time [ms]
0.04 0.08 0.12 0.16 0.20
Figure 8 a) Voltage on the OHL (red), Voltage on UICS: SE (green) RE (blue)
b) Voltage on the OHL (green), Voltage on UICS: SE (blue) RE (violet)

Since OHL and UICS have the same BIL, overvoltage exceeds the BIL for UICS lengths around 1500
m (Figure 6 a) whereas for longer lengths, i.e. 2000 m, it does not. In fact (see Figure 6 b), both SE
and RE Voltages stay within the BIL and therefore no extra protection devices are necessary.
Shorter distances of the striking point from the cable entrance could increase the self-protecting
distances. On the contrary, in the siphon scenario, cable BIL is never exceeded whatever be the cable
length and the lightning current shape (Figure 8 a, b).
4. RELIABILITY AND AVAILABILITY: DIAGNOSTIC AND MONITORING
The introduction of XLPE insulated cables has raised some concerns regarding long-term life since, in
modern high voltage cable systems, temperature, overloads and water ingress may become time
limitation parameters for the system lifetime. Utilities expect highest reliability from UICS and an
obvious demand is the need for little or no maintenance in spite of higher utilisation. Third party
damage, fault location and maintenance have been among the most penalizing factor for UICS
availability so far.
Two systems have already been introduced into commercial plants and have shown their excellent
performances. The main capability of the first system, called Real Time Thermal Rating (RTTR) is the
dynamical evaluation of the permissible load of a given cable circuit and its environmental variable
conditions. RTTR is based on continuous temperature and load monitoring [10].
Concerning to the second monitoring system, it is noteworthy to outline that XLPE cables do not need
any maintenance, provided the cable sheath is impervious to possible water penetration into the cable
insulation. The water monitoring system has been developed to recognise the ingress of water
immediately when entering in an accidentally damaged cable sheath [11].
5. CONCLUSIONS
Technological developments allow nowadays a much broader use of cable systems for AC power
transmission applications. Availability of two families of cable systems, fluid filled and extruded, the
combination of the long standing experience of the first and the environmental friendliness of the
latter, allow the definition of optimized solutions for all kinds of applications.
In particular accurate and unprejudiced network analysis and system design, including installation, can
significantly increase cable systems circuit lengths well above 50 km without reactive compensation.
If reactive compensation is adopted, and today’s technology allows its use at limited costs, the limits
in maximum cable lengths virtually disappear.
Overhead lines are and will continue to be an important means of power transmission, especially for
very long backbone links in areas where no environmental concerns may be raised.
The smart combination of the two technologies, based on the specific drivers of each project is the key
for the realization of efficient and reliable transmission networks.
6. REFERENCES
[1] CIGRE WG 21-03, “Recommendations for electrical tests…”, Electra No. 151, Dec. 1993.
[2] P. Andersen et al., “Development of a 420 kV XLPE cable system for the metropolitan power
project in Copenhagen”, CIGRE paper 21-201, 1996
[3] C.H. Henningsen et al., “New 400 kV long distance cable systems, their first application for the
power supply of Berlin”, CIGRE paper 21-109, 1998
[4] H. Ohno et al., “Construction of the World’s first long-distance 500 kV XLPE cable line”, CIGRE
paper 21-106, 2000
[5] A. Bolza, B. Parmigiani, F. Donazzi, C. Bisleri, “Prequalification Test Experience On EHV XLPE
Cable System”, CIGRÉ paper 21-104, 2002.
[6] R. Arrighi, “Operating Characteristics of Long Links of AC High Voltage Insulated Cables”,
CIGRÉ paper 21-13, 1986
[7] P. Argaut, J. Becker, P.M. Dejean, S. Sin, E. Dorison, “Studies and Development in France of
400kV Cross-Linked Polyethylene Cable Systems”, CIGRÉ paper 21-203, 1996.

[8] P. Couneson, J. Lamsoul, X. Delre, X. Van Merris, “Bulk Power Transmission By OHL or Cables.
Comparative Assessment and Principles Adopted in Belgium for the Future Development of the HV
Network”, CIGRÉ paper 21/22-09, 1996.
[9] EDF, “Réseaux électriques et environnement”, Épure N° 48, Octobre 1995, (pag.39).
[10] F. Donazzi, R. Gaspari, “Method and system for the Management of power cable links”, CIGRÉ
paper 21-203, 1998.
[11] L. Goehlich, F. Donazzi, R. Gaspari, “Monitoring of HV cables offers improved reliability and
economy by means of power sensors” Power Engineering Journal, June 2002.