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

21, rue d'Artois, F-75008 Parishttp://www.cigre.org © CIGRÉ

Session 2004B1-304

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 pre-moulded 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

1600mm2 XLPE trefoil formation (2cables/phase)

2500 mm2 XLPE vertical formation 0.5 m spaced in tunnel (1 cable/phase)

3250 mm2 XLPE flat formation 1m spaced (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

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].

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:

⎪⎩

⎪⎨⎧

⋅+⋅−=

⋅−⋅=

SESERE

SESERE

IAVCI

IBVAV

⎪⎪

⎩

⎪⎪

⎨

⎧

⋅ϕ=⋅ρ=

=+

=

PtgQSP

)2(SQP

VV

RERE

nRE

n2SE

2SE

nSE

)3(.1)cos(

95.0

⎩⎨⎧

=ϕ=ρ(1

)

Figure. 1: Rating of 500 kV XLPE underground cable systems

1000

1200

1400

1600

1800

2000

2200

2400

2600

0 500 1000 1500 2000 2500

Phase spacing [mm]

Pow

er ra

ting

[MV

A]

S=1600 (mm²)S=2000 (mm²)S=2500 (mm²)S=3250 (mm²)S=4000 (mm²)

In (1), the quadrupole equations from receiving to sending ends (RE and SE) have been considered, with C,B,A 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 (Sn) 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.

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 1x1600mm2and 1x2500mm2 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

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.

~

S=Sn = Cable nominal power

LAT: Length | n

RE

SP = 95%

SE : sending end

RE : receiving end

Pre=Pload Power factor = 1 Vse : Vnominal

Figure 2: Reference scheme for LAT calculation

20

25

30

35

40

45

50

55

60

0 500 1000 1500 2000

Phase sp acing [mm]

LAT

[km

]

S=1600 mm²

S=2500 mm²

Trefoil formation

Figure. 3: LAT- for the 500 kV UICS considered

Phase spacing [mm]

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 mm2 UICS when 50% of shunt compensation at the receiving end is adopted (see curve LAT_sh50%). 3.2 Summary of cable length constraints

For reference the 500 kV 2500 mm2 XLPE UICS configuration shown in Table 1 has been considered. The Steady State stability limit “LSTAB” has been calculated according to (4), considering an angle swing ∆ϑ = 15° between SE and RE; whereas Pc is the cable SIL and β is the propagation constant. The Charging Current limit evaluation “Lcrit“ has been calculated according to (5), in no-load conditions, where Vn and In are the nominal cable voltage and current rating, and Zc is the characteristic impedance. The Voltage Variation limit “LDV”, 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 “LDV“ 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

0

50

100

150

200

250

0 500 1000 1500 2000Phase spacing [mm]

LAT

[km

]

LAT_sh50%

L-DV

LAT

Lcrit

L-Stab. - 15(°)

LAT - tunnel

Figure. 4: 500 kV, 2500 mm2 UICS - Length limitations vs. phase spacing

( )

( ) ( )( ) ( )

( )⎪⎪⎪⎪

⎩

⎪⎪⎪⎪

⎨

⎧

β≅

⎟⎟⎠

⎞⎜⎜⎝

⎛

⋅+

⋅−β

=

⎟⎟⎠

⎞⎜⎜⎝

⎛ ϑ∆⋅β

=

)6(95.0arccos1L

)5(IZ3VIZ3V

arccos21

L

)4(PtgP

arctan1L

DV

2nc

2n

2nc

2n

CRIT

cSTAB

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.

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Time [ms]

V [p

.u.]

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Time [ms]

V [p

.u.]

Figure.6. (a,b) Voltage on the OHL (red), Voltage on UICS: SE (green), RE (blue)

a) L=1500 m

0.00 0.05 0.10 0.15 0.20 0.25 0.300.00 0.05 0.10 0.15 0.20 0.25 0.30

b) L=2000 m

Zc

Cable SE Cable RE10 km 1500 m ÷ 2000 m

Figure.5. UICS Connecting a substation

Zc

10 km 100 m

Zc

Figure.7. UICS in siphon configuration

Cable SE Cable RE

BIL p.u.

a) L=100 m, lightning stroke: 4/50 µs b) L=100 m, lightning stroke: 4/500 µs

0.00 0.04 0.08 0.12 0.16 0.200.00 0.04 0.08 0.12

0.20.40.60.81.0

0.20.40.60.81.0

0

Time [ms]

V [p

.u.]

a) L=100 m, lightning stroke: 4/50 µs b) L=100 m, lightning stroke: 4/500 µs

0.00 0.04 0.08 0.12 0.16 0.200.00 0.04 0.08 0.12

a) L=100 m, lightning stroke: 4/50 µs b) L=100 m, lightning stroke: 4/500 µs

0.00 0.04 0.08 0.12 0.16 0.200.00 0.04 0.08 0.12

0.20.40.60.81.0

0.20.40.60.81.0

00.20.40.60.81.0

0.20.40.60.81.0

0

Time [ms]

V [p

.u.]

a) L=100 m, lightning stroke: 4/50 µs b) L=100 m, lightning stroke: 4/500 µs

0.00 0.04 0.08 0.12 0.16 0.200.00 0.04 0.08 0.12

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)

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Time [ms]

V [p

.u.]

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Time [ms]

V [p

.u.]

Figure.6. (a,b) Voltage on the OHL (red), Voltage on UICS: SE (green), RE (blue)

a) L=1500 m

0.00 0.05 0.10 0.15 0.20 0.25 0.300.00 0.05 0.10 0.15 0.20 0.25 0.30

b) L=2000 m

0.00 0.05 0.10 0.15 0.20 0.25 0.300.00 0.05 0.10 0.15 0.20 0.25 0.30

b) L=2000 m

Zc

Cable SE Cable RE10 km 1500 m ÷ 2000 m

Figure.5. UICS Connecting a substation

Zc

Cable SE Cable RE10 km 1500 m ÷ 2000 m

Zc

Cable SE Cable RE10 km 1500 m ÷ 2000 m

Figure.5. UICS Connecting a substation

Zc

10 km 100 m

Zc

Figure.7. UICS in siphon configuration

Cable SE Cable RE

Zc

10 km 100 m

ZcZc

10 km 100 m

ZcZc

Figure.7. UICS in siphon configuration

Cable SE Cable RE

BIL p.u.

a) L=100 m, lightning stroke: 4/50 µs b) L=100 m, lightning stroke: 4/500 µs

0.00 0.04 0.08 0.12 0.16 0.200.00 0.04 0.08 0.12

0.20.40.60.81.0

0.20.40.60.81.0

0

Time [ms]

V [p

.u.]

a) L=100 m, lightning stroke: 4/50 µs b) L=100 m, lightning stroke: 4/500 µs

0.00 0.04 0.08 0.12 0.16 0.200.00 0.04 0.08 0.12

a) L=100 m, lightning stroke: 4/50 µs b) L=100 m, lightning stroke: 4/500 µs

0.00 0.04 0.08 0.12 0.16 0.200.00 0.04 0.08 0.12

0.20.40.60.81.0

0.20.40.60.81.0

00.20.40.60.81.0

0.20.40.60.81.0

0

Time [ms]

V [p

.u.]

a) L=100 m, lightning stroke: 4/50 µs b) L=100 m, lightning stroke: 4/500 µs

0.00 0.04 0.08 0.12 0.16 0.200.00 0.04 0.08 0.12

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)

a) L=100 m, lightning stroke: 4/50 µs b) L=100 m, lightning stroke: 4/500 µs

0.00 0.04 0.08 0.12 0.16 0.200.00 0.04 0.08 0.12

0.20.40.60.81.0

0.20.40.60.81.0

0

Time [ms]

V [p

.u.]

a) L=100 m, lightning stroke: 4/50 µs b) L=100 m, lightning stroke: 4/500 µs

0.00 0.04 0.08 0.12 0.16 0.200.00 0.04 0.08 0.12

a) L=100 m, lightning stroke: 4/50 µs b) L=100 m, lightning stroke: 4/500 µs

0.00 0.04 0.08 0.12 0.16 0.200.00 0.04 0.08 0.12

0.20.40.60.81.0

0.20.40.60.81.0

00.20.40.60.81.0

0.20.40.60.81.0

0

Time [ms]

V [p

.u.]

a) L=100 m, lightning stroke: 4/50 µs b) L=100 m, lightning stroke: 4/500 µs

0.00 0.04 0.08 0.12 0.16 0.200.00 0.04 0.08 0.12

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.