1

Abstract Application of EHV lines formed by series

connected overhead sections and XLPE-insulated cables sections is planned in various projects in Europe.

At first the maximum feasible length of 380kV-50Hz and of 500kV-60Hz XLPE cable lines is calculated as a function of the power carrying capacity derating due to charging current. Then shunt compensation and optimal voltage-reactive power control are analysed with use of shunt reactors with tapped windings and on-load tap changer for regulation of Mvar output.

The authors ATP-EMTP electromagnetic transient analysis of the long mixed EHV lines has revealed the risk of sustained overvoltages due to resonance on 3rd harmonics. The phenomenon is described and countermeasures are proposed.

The feasibility of the single-pole high speed reclosure of mixed EHV lines is analysed, and means for limitation of the secondary arc current are examined.

A protection scheme is proposed for fast selective detection of faults and for implementation of the single-pole reclosure only in the cases of 1--to-Gr faults in the overhead sections of mixed EHV lines.

Index TermsCable ampacity, EHV AC cables, mixed overhead-cable lines, shunt compensation, voltage and reactive power control, overvoltages, resonance, single-pole reclosure.

I. INTRODUCTION N most of the European countries the congestion of the

infrastructures and the strong demand of conservation of the environment by the population and by the local and central public administrations, make very difficult the construction of new overhead lines. Use of long stretches of EHV underground cables is therefore considered also in non-urban areas.

The growing economic integration of the member countries of the European Union requires an increase of the transfer capacity of roads, railways, EHV transmission lines, telecommunications, pipelines, etc.. Where the inter-country borders are at high altitude in mountainous regions, the construction of long new motorway or railway tunnels is considered, usually provided with a service/safety tunnel which can accommodate also EHV cable lines. Such tunnel projects and the associated 380kV cable lines (generally,

The authors are with the Department of Electrical Engineering, University

of Rome La Sapienza Via Eudossiana 18, 00184 Rome, Italy (e-mail: francesco.iliceto@uniroma1.it).

double circuits) are considered between Spain and France (35km), between France and Italy (54 or 65km) and between Austria and Italy (60km).

On the other hand, the crosslinked polyethylene (XLPE) insulation technology is considered mature and reliable for the EHV cables up to 500kV AC, as confirmed by various urban 380kV and 500kV cable lines in successful operation throughout the world. XLPE cables are simpler, less expensive and have a lower electrostatic capacitance than the paper oil filled insulated cables that for many decades had been the only available reliable technology for HV and EHV AC.

The scope of this paper is to analyse the main design and operational features of the EHV (380-500kV), 50 and 60Hz transmission lines, consisting of series solidly connected long sections of XLPE insulated cables and overhead line, as may be used in the above outlined projects. These lines will be referred to in the following as mixed lines.

Authors analyses address the main special features and phenomena which diversify the planning, analysis and operation of the mixed lines from the so far applied conventional EHV overhead lines and from urban EHV cable lines of moderate length.

As well known, the charging current and capacitive reactive power of long EHV AC cables adds vectorially to the load current and power, and reduces their active power carrying capacity at the thermal limit in a measure increasing with length of cables, operation voltage and frequency. At first the feasible lengths at the thermal limit are determined, in particular for the 380kV-50Hz cables and 500kV-60Hz cables, for various cross sections of conductors and typical cable laying conditions.

The steady state operation criteria are analysed, covering the optimal shunt compensation and controls of reactive power flows and voltages along the long mixed lines.

The ATP-EMTP analysis of the electromagnetic transients addresses particularly the risk of resonance on harmonics of low (3rd) order, a phenomenon not encountered with the EHV overhead lines. The feasibility of the single-pole high-speed reclosure (SPHSR), is also analysed with the ATP-EMTP program and ad hoc teleprotection schemes are proposed for implementing the SPHSR only in the cases of 1--to-Gr faults occurring in the overhead line section.

Design and operation of EHV transmission lines including long insulated cable and overhead

sectionsL. Colla, F. M. Gatta, F. Iliceto, Life Fellow, IEEE and S. Lauria, Member, IEEE

I

2

II. LIMIT LENGTH OF 380kV-500kV XLPE INSULATED CABLE LINES

The straightforward operating criterion of long EHVAC cable lines at their thermal limit, enabling the maximum possible active power transfer for a given cable length, consists in enforcing the equality of the reactive power flows at the two cable terminals. In other words the target is enforcing about unity p.f. operation at cable midlength. This concept could apply also to the individual stretches of a cable line compensated at intermediate locations along its route.

Given the cable characteristics and the desired receiving-end continuous active power transfer, this concept can be used to calculate the allowed maximum cable length at the thermal limit (MCLTL). To this aim the analysis of reactive power flows and of voltage profile can be made for the lossless cable line of Fig. 1.

SendingNetw.

Receiv.Netw.

EHV cable line

P =

VS VR 0

Sz P =R SzQ =S Sz 1 Q = R Sz 122

S

Fig. 1- Single-line diagram of a EHV lossless cable line, and applied constraints: Sz: apparent power at rated voltage and thermal limit of the cable

The above definition leads (see [1] and Appendix A) to the following simple but meaningful expression for the MCLTL of the lossless line:

)1(12

12

2

4

22

=

zc

S

zc

S

SZV

SZV

arctgk

MCLTL

where: - VS is the sending end voltage; - Sz is the apparent power at rated voltage and thermal limit; - is the derating, in p.u., of active power flow at cable

ends with reference to power at cable midpoint; is also the power factor at cable ends: =PR/Sz;

- Zc is the surge impedance of the cable line; - k is the transmission constant of the cable line.

The reactive power outgoing from the cable line is equally shared between the cable ends, both loaded at S=Sz.

In Fig. 2 the voltage, current and power profiles along the line are plotted for a cable line length equal to the MCLTL. Active power losses, although not nil as assumed, are in reality small.

As regards voltage constraints, with the same lossless approximation, MCLTL operation involves equal terminal cable voltages.

In Table I, the MCLTL values calculated with equation (1) are reported for 380kV-50 Hz XLPE cable lines equipped with 2500mm2 Cu conductors, directly buried at 1.5 m, in flat formation; soil thermal resistivity has been taken at 1.2 Cm/W; conductor and ground temperatures are 90 C and 25

C, respectively. Charging power at 400kV is 12 Mvar/km. Two different values of the transmitted active power, PR=0.9SZ (Cases 1-3) and PR=0.95SZ (cases 4-6) and three values of carrying capacity, SZ, have been considered. The MCLTL calculated with formula (1) and a realistic cable spacing 0.5m, is up to 91 km for =0.90 and up to 63 km for =0.95.

QS Q=0

P

QR

P

l=MCLTL

P

VS V mx VR

N=Sz N=SzN=P

I IRIminS

Fig. 2 Voltage profile, reactive and active power flows along a lossless MCLTL EHV cable line

If the 380kV-50Hz XLPE cables have Cu conductors of 1600 mm2 and the same laying and operation conditions, the MCLTL is found to be up to 81 km for =0.9 and up to 58 km for =0.95.

TABLE I - MCLTL OF 380kV-50Hz CABLES WITH SCu=2500mm2 CaseN

Inter-phase Dist.

m

X1

/km

SZ

MVA

Zc

SIL

MVA

PR

MW

MCLTL

km

QS and QR

MVAR1 0.35 0.177 1101 48.5 3299 0.9 991 86.6 480 2 0.50 0.200 1140 51.5 3107 0.9 1026 91.2 497 3 1.00 0.243 1225 56.8 2817 0.9 1103 99.5 533 4 0.35 0.177 1101 48.5 3299 0.95 1046 63.0 344 5 0.50 0.200 1140 51.5 3107 0.95 1083 66.7 356 6 1.00 0.243 1225 56.8 2817 0.95 1164 73.1 382

(VS=400 kV; C1=240 nF/km)

The analysis has been extended to the 500kV-60Hz XLPE cable lines. These are commercially available with the same conductor sizes of 380kV cables, i.e. up to 2500 mm2 Cu. However, on an equal conductor basis, 500kV-60Hz cables bring along tighter restrictions. The thicker XLPE insulation causes lower ampacity and lower capacitance per unit length. The latter effect is, however, largely offset by the combined effect of the higher operating frequency1 and voltage, so that the overall capacitive reactive power generated by a 500kV-60Hz cable is almost the double of that of a 380kV-50Hz cable of same length. For the considered 500kV-60Hz cable, at full load, it is about 20 Mvar/km.

Table II provides the MCLTL of 500kV-60Hz XLPE cable lines with 2500 mm2 Cu conductors in the same laying and operation conditions assumed for the 380kV-50Hz cable lines.

1 The increase of longitudinal inductive reactance has little overall impact.

3

A realistic spacing (0.5 m) of 500kV-60Hz cables allows MCLTL up to 63 km for =0.9, and up to 45 km for =0.95. For 500kV-XLPE cables with 1600mm2 Cu conductor, in the same laying and operation conditions, MCLTL is found to be up to 57 km for =0.9 and up to 41 km for =0.95.

TABLE II - MCLTL OF 500kV-60Hz CABLES WITH SCu=2500mm2 Case N

Inter-phase Dist.

m

X1

/km

SZ

MVA

Zc

SIL

MVA

PR

MW

MCLTL

km

QS and QR

MVAR1 0.35 0.207 1432 51.1 5394 0.9 1290 60.3 624 2 0.50 0.236 1482 54.6 5048 0.9 1335 63.1 646 3 1.00 0.286 1593 60.1 4586 0.9 1435 69.7 694 4 0.35 0.207 1432 51.1 5394 0.95 1360 43.6 447 5 0.50 0.236 1482 54.6 5048 0.95 1410 45.8 462 6 1.00 0.286 1593 60.1 4586 0.95 1515 50.8 497

(VS=525 kV; C1=210 nF/km)

In practice, the control of parameter at same value at the two ends, in particular in the international interconnection lines, may be not precisely achieved in operation, with a consequential erosion of the above reported MCLTL values.

III. STEADY-STATE OPERATION OF MIXED LINES. CASE STUDIES To enforce the desired reactive power flow distribution at

the cable ends, the inductive shunt compensation required at both ends of long cable lines should be provided with some regulation capability. An EHV shunt reactor with a tapped winding and an on-load tap changer [5], can vary the reactive power absorption in a wide range, from 55-60% to 100% of Qn, in discrete but rather small steps (e.g. each step equal to 2.5% of Qn). These tapped windings variable output shunt reactors (TWVO-SRs) can be an attractive and cost-effective solution for steady-state reactive power and voltage control. The local voltage regulation capability thus gained can be used to control the reactive power flow along the cables.

A control algorithm for one TWVO-SR at a time, using only voltage and power measurements at the reactors locations [1] can be applied to the simplified mixed overhead-underground 500 kV-60 Hz transmission line shown in Fig. 3, where A1 and A2 are two overhead line stretches, C is the 500 kV cable line some tens of kilometers long. The latter requires shunt compensation at both ends of the cable, i.e. at the overhead/cable interfacing stations.

4CA1 A2

2 51

(E2, I2)[Send] [Rec]

QSR2 QSR4

3

(E1, I1) (E4, I4) (E5, I5)(E3, I3)

Q4Q2

P5

Q1 Q5

XSR4XSR2

Fig. 3 Single-line diagram of a shunt compensated mixed line

It is assumed that TWVO-SRs are used in regulation mode one at a time: with fixed shunt compensation at one end of the cable, say, node 2 in Fig. 3, the reactor at the other end is adjusted in order to establish equal reactive power flows out of the cable (Q2=-Q4), i.e. to have:

Q20 +Q2 = (Q40 +Q4), (2) where Q20 and Q40 are the reactive powers flowing out of the

cable prior to the regulation and Q2, Q4 are variations thereof. For small variations, an accurate linear relationship

between the changes in TWVO-SRs power, QSR2 and QSR4, and reactive power flows can be found (see [1] and Appendix B), i.e.:

Q2=k24QSR4, Q4=k44QSR4 (3) And, lastly, the desired regulation QSR4 for the reactor in

busbar 4 can be obtained:

QSR4 = K4(Q20+Q40) (4)

The above concept can be extended to any reactive power flow along the mixed line, as well as applied to several TWVO-SRs at the same time. For the reactive power flow at the generic section i of the Fig. 3 system, this yields:

Qi =ki2QSR2+ki4QSR4 (5)

Given the availability of two degrees of freedom, a couple of conditions can be imposed on the reactive power flows, and then a simple two-equation linear system in the unknowns QSR2; QSR4 can be set up.

At full load, however, one of the constraints will always be Q2= Q4. The other condition can involve the reactive power flow at either end of the mixed line, say, Q1= Q10 or Q5= Q50 in Fig. 3. An application of the method is presented in the following. Partial or no-load operation allows to eliminate condition (2), so that the control action can be focused on the reactive power exchanges between the mixed line and the connected networks, up to the regulation capability of the TWVO-SRs.

Let us consider the mixed 500kV-60Hz line of Fig. 3, where line stretches lengths are LA1=20 km, LC =40km (close to the MCLTL value found for a 2500mm2 Cu-XLPE cable buried with 35 cm spacing, SZ=1432 MVA and =0.95, see Case 4 of Table I) and LA2=140 km; the receiving-end power, P5, is 1360 MW. The interconnected systems can be reduced to two ideal voltage sources of the desired amplitudes, phase-shifted to have the desired active power flow at the receiving end of the mixed line. Line constants are reported in Table III.

TABLE III TEST SYSTEM DATA OHL A1 Cable C OHL A2

r1 [/km] 0.0174 0.0126 0.0174 x1 [/km] 0.365 0.207 0.365 c1 [nF/km] 12.0 210.0 12.0 g1 [S/km] 0.0 0.053 0.0

(cable cross-section: 2500 mm2; overhead line equipped with twin bundle ACSR Chukar conductors, 2902 mm2)

In Fig. 4-a, with U1=525 kV, U5=500 KV and P5=1350 MW, two different steady-state operations are compared: fixed-type shunt reactors, rated 2200 Mvar at 525 kV at each end of the cable, and TWVO-SRs, rated at 2225 Mvar at 525 kV, regulated in range 125-225 Mvar to have equal reactive power flows at the cable ends. With fixed compensation (curves labeled QSR2=400 Mvar, QSR4=400 Mvar in Fig. 4-a) plots show a large reactive power flow unbalance along the cable; a quantitative assessment of the current unbalance at the cable terminals is provided in Fig. 5-a.

4

500

503

506

509

512

515

518

521

524

527

530

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200d (km)

U (

kV)

-420

-315

-210

-105

0

105

210

315

420

525

630

Q (M

var)

1 2 3 4 5

P5 = 1360 MWQSR2 = 400 MvarQSR4 = 145 Mvar

P5 = 1360 MWQSR2 = 400 MvarQSR4 = 400 Mvar

a)

515

516

517

518

519

520

521

522

523

524

525

526

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

d (km)

U (k

V)

-510

-425

-340

-255

-170

-85

0

85

170

255

340

425

Q (M

var)

1 2 3 4 5

P5 = 0 MWQSR2 = 450 MvarQSR4 = 450 Mvar

P5 = 680 MWQSR2 = 450 MvarQSR4 = 405 Mvar

b)

Fig. 4 Voltage profiles (fine lines) and reactive power flows (heavy lines) for the Fig.3 system: a) with 1360 MW power flow; b) with 680 MW power flow and at no-load.

Application of the control algorithm to the TWVO-SRs installed at node 4 (curves labeled QSR2=400 Mvar, QSR4=145 Mvar in Fig. 4-a) gives the desired reactive power profile along the cable; the attendant steady-state is given in Fig. 5-b.

The plots of Fig. 4-b show that the Mvar control by TWVO-SR warrants satisfactory operation of the mixed line also at 50% of load and at no-load.

Fig. 5-a (corresponding to the lighter curves of Fig. 4) refers to the fully compensated line, with 2200 Mvar (rated power at 525 kV) of shunt reactors installed at each intermediate station; it can not be operated with the intended limit power transfer of 1360 MW, due to the unbalance of reactive power flow along the cable and the ensuing cable overload near the receiving end (i.e. I>Iz in the last 11 km of cable).

Fig. 5-b shows the effect of the adjustment of TWVO-SRs in node 4, according to the described control algorithm (QSR4=145 Mvar); reactors in node 2 are unchanged (QSR2=400 Mvar). The reactive power flows at the cable ends are symmetrical so that the cable ampacity can be fully exploited. Figure 5-c depicts the application of the general control algorithm, involving simultaneous regulation of TWVO-SRs at both intermediate stations, i.e aimed at enforcing two distinct reactive power constraints. At full load one constraint must always be Q2 = Q4; in Fig. 5-c the second constraint is Q1=0.

IV. ANALYSIS OF SWITCHING OVERVOLTAGES AND OF SINGLE-PHASE HIGH-SPEED RECLOSURE OF MIXED EHV LINES

IV.1 Case studies and modelling The following switching electromagnetic transients have

500.0-19.7 kV522.7-4.5524.0-2.1525.00.0

(145) (400)

1374 17

1372 -407

1368 406

MWMvar

4

(380) (145)

MWMvar

4

(1.576 kA) (1.576 kA)

1 2 4 5

1 2 4 5

1372 -8

1368 262

1350 58

525.00.0

1374 -1

1372 -26

524.3-2.1

1372 -405

(1.575 kA) (1.576 kA)

522.7-4.4

1368 408

1368 265

1350 61

500.0-19.7 kV

1350 -3

1368 204

1372 190 5421

(1.665 kA)(1.536 kA)

4

MWMvar

1368 592

1372 -204

1374 216

(400) (400)

525.00.0 521.2-2.1 516.8-4.4 500.0-19.9 kV

a)

b)

c)

Fig. 5 Voltages, active and reactive power flows of analysed 20-40-140 km, 500 kV-60Hz mixed line: a) with 2200 Mvar shunt reactors at each intermediate station; b) as case a), with optimum shunt compensation at node 4; c) as case b), with adjustments to yield Q1=0 Mvar.

been analysed with the ATP-EMTP- Alternative Transient Program for various mixed lines: No-load energization Load rejection at receiving end Single-phase high-speed reclosure (SPHSR) Special phenomena, in particular harmonic resonance.

The results are reported here for the following significant case studies: a. A mixed long 380kV-50Hz line consisting of a single-

circuit overhead line of 200km, a XLPE insulated cable line of 60km with about 100% shunt compensation and an overhead single-circuit line of 10km, as shown in Fig. 6-a. This is a preliminary scheme of one of the cross border 380kV cable lines under study for installation in a railway tunnel, for strengthening the transmission capacity across the Alpes. EHV shunt reactors cannot be installed inside the tunnel, for reason of safety (risk of fire).

b. The 380kV-50Hz mixed XLPE insulated cable-overhead line, shown in Fig. 6-b, consisting of two cable sections of 13.1 and 9.4 km without shunt compensation. At the time of writing, this line is under construction in Turkey between the 380kV substations of Ikitelli and Alibekoy. It will cross diametrically the metropolitan area of Istanbul.

The 380kV overhead sections of mixed lines and the adjacent lines have been simulated as uniformly distributed lines, with frequency dependent parameters to take into account skin effect in the ground and in phase conductors.

The 60 km-380kV cable section of the mixed line of Fig. 6-a has been simulated with a cascade of 555m long circuits. The shorter cable sections of the mixed line of Fig. 6-b, have been simulated with 110m long circuits, in cascade. Simulation of adjacent 380kV overhead lines with frequency-dependent uniformly distributed parameters, has been

5

performed one busbar behind where the adjacent lines are long, as in the case of Fig. 6-a, and two busbars behind where the adjacent lines are short, as in the case of Fig. 6-b.

A B C D

a)

tunnel cables60 Km

overhead line200 Km

overhead line

400 MVA

220 kVSASA 2X200

MVAR2X200 MVAR

10 Km

underground cables overhead line

13.1km 3x2000sqmm Cu

4km3x547sqmm ACSR

380kV Psc=12530MVA

DAVUTPASA YILDIZTEPE380kV Psc=12055MVA

9.4km 3x2000sqmm Cu

underground cablesIKITELLI ALIBEYKOY

154kV 154kV 154kV34.5kV4x250MVA

2x250MVA 2x125MVA

4x250MVAb)

Fig. 6 Simulated 380kV mixed lines for electromagnetic transient analysis. a) 60 km of tunnel cables between a 200 km and a 10 km overhead lines. b) 13.1 km + 9.4 km of underground cables + 4 km of overhead line in Istanbul (Turkey).

Generators have been simulated with a constant e.m.f. behind the subtransient reactance. Loads have been simulated with linear impedances. Metal oxide surge arresters (SAs) with rated voltage of 360kV, have been simulated with their v-i characteristic in the cases where SAs intervene to limit the switching overvoltages. SAs are assumed to be connected to the EHV cable terminals and to the shunt reactors and autotransformers terminals.

Saturation of magnetic iron cores has been simulated with typical values of the air-core reactance: 30% for the gapped core of 380kV shunt reactors; 60% and 50% for the core of autotransformers on their 380kV and 220kV terminals, respectively. As the frequency dependency of the iron losses has been neglected, the calculated overvoltages are slightly overestimated.

Analysis has shown that the short circuit power, Psc, of the 380kV network at the terminals busses of the mixed line, markedly affects the temporary and switching overvoltages when the cable line section is very long. For the mixed line of Fig. 6-a, with cable length of 60km, Psc has thus been

considered a variable parameter, keeping also in mind that the values of Psc may largely differ at the terminals of the various long tunnel cables under investigation and, on the other hand, of the fact that Psc may considerably vary in a specific busbars according to system operation conditions.

IV.2 No-load energization of long shunt compensated mixed lines. Harmonic resonance.

It is known that the resonance natural frequencies in transmission networks composed of overhead lines, as seen from a bus, are in general quite high. The equivalent impedance is inductive, of increasing value for frequencies usually up to at least 300-400Hz. The behavior is substantially different in presence of very long EHV shunt compensated cables, whose large concentrated capacitance causes rather low resonance frequencies of the network, in some cases in proximity of the third harmonic frequency.

Analysis has shown that the no-load energization of a mixed line as the one of Fig.6-a can initiate temporary overvoltages causing SAs to operate during many cycles. Transformers and shunt reactors subjected to high overvoltages saturate and significantly affect system overvoltages. The consequential large increase of the magnetizing current causes a large generation of harmonics, superimposed on the fundamental and DC components. High harmonic distortion and overvoltages then build-up if the network is resonant at such harmonic excitation.

A physical explanation of the phenomenon is provided by the analysis of the simplified equivalent circuits of the mixed line of Fig.6-a, assumed to be lossless, with a shunt compensation performed by 2x200Mvar shunt reactors at each cable terminal, i.e. slightly over 100%.

The line (see Fig.7-a) is assumed to be energized from busbar A and open-ended at busbar D. The positive-sequence circuit at network frequency of 50 Hz is shown in Fig. 7-b1. The equivalent impedance seen from busbar B (Fig. 7-b2) is capacitive and very high (-j2368) and does not cause large overvoltages at 50Hz.

3

8

33 161

overhead line cables in tunnel overhead line

10km - 3x585sqmm ACSR per phase 60km - 3x2500sqmm Cu 200km - 3x585sqmm ACSR per phase

2x200MVAR 2x200MVAR

400kV Network Psc=600020000

MVA

A B C D

Xsc A B C D

A' B' C' D'

A3 x Xsc A

A'

B

B'

C

C'

D

D'

B

B'

B(3 Xsc + 8)

B'

150Hz

(Xsc + 2)

a

b1

1c

b2

c2Fig.7 Equivalent circuits of the mixed line of Fig.6-a, assumed to be lossless: a) single-line diagram; b) positive sequence circuit at 50Hz; c) positive sequence

circuit at 150Hz

6

The positive-sequence circuit at the 3rd harmonic frequency of 150Hz shown in Fig. 7-c1, has an equivalent impedance, seen by the busbar B (Fig 7-c2) that is capacitive and very low (X1=-j46 ) and is resonant with the equivalent impedance of a supply network with Psc=11400MVA, by assuming that X150Hz=3Xsc; (Xsc=V2/Psc)+).

In the parallel resonance conditions at 150Hz in busbar B as per Fig. 7-c2, the large 3rd harmonic current liable to be generated by the saturation of one or two phases of the shunt reactors (and/or transformers) connected to busbar B at the moment of energization, cannot circulate in the network. Consequently, a large 3rd harmonic voltage distortion will build-up at busbar B, causing the circulation of an additional, opposite, 3rd harmonic current which cancels out the one due to saturation. This explains the possible build-up of large overvoltages contributed by the 3rd harmonic distortion at the cable terminals.

The ATP-EMTP analysis performed for the system of Fig.6-a has confirmed the possible build-up of the above described phenomenon caused by the saturation of a phase of one or more shunt reactors and/or transformers.

Low R/X networks, such as in proximity of large power generators, contribute to sustained overvoltages, especially at no-load, since losses are rather low.

The positive-sequence impedances of the system of the mixed line at no-load have been calculated at bus B. Their variation as a function of frequency is shown in Fig. 8.

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0[ H z ]0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

5 0 0 0 [ ]

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0[ H z ]- 2 0 0 - 1 5 0 - 1 0 0

- 5 0 0

5 0 1 0 0 1 5 0 2 0 0

[ ]

Fig. 8 Positive-sequence phase impedances, in magnitude and phase, calculated at bus B of the 380kV network of Fig.6-a, open circuited at bus D, versus frequency.

Because of the time dependence of the saturation phenomenon, a statistic study has been run, with 1000 no-load energizations from either line end. A statistical distribution was determined to check the influence of the closing time

+) In reality, also the zero-sequence and negative sequence networks are involved in the phenomenon and, on the other hand, X150Hz may be somewhat less than 3Xsc due to the effect of line shunt capacitances. Then the Psc value for which resonance occurs at 3rd harmonic may differ somewhat from the value calculated with the simplifications assumed in Fig.7.

span of the three poles of the line circuit breaker (CB). A uniform distribution for the closing times has been simulated, with a maximum time discrepancy between first and last pole to close of 5ms. The maximum energization overvoltage at bus D, calculated with one shunt reactor assumed to be not connected at bus B, i.e. with a cable shunt compensation of 83%, is plotted for phase T in Fig. 9. This overvoltage is of long duration (as long as the inrush currents), its time decay being inversely dependent on shunt reactor and system losses. In addition to the voltage stresses imposed on the shunt reactor, these temporary overvoltages may stress protective equipment; energy in SAs may exceed their dissipation capacity.

0.0 0.2 0.4 0.6 0.8 1.0[s ]-800

-600

-400

-200

0

200

400

600

800[kV]

Fig.9 Phase-to-Gr overvoltage in T phase at bus D for the no-load energization from bus A. SAs connected at cable terminals are simulated.

Fast Fourier Transform (FFT) analysis (Fig. 10) shows that the phase-to-ground overvoltages are due mainly to the 3rd harmonic components, as a result of the resonance of the supply system with the mixed line. Resonance is excited by shunt reactors saturation at the instant of switching-on. Energy dissipation in SAs is shown in Fig. 11.

0 5 10 15 20 25 30-50

100

250

400[kV]

harmonic order Fig.10 FFT analysis of the overvoltage of Fig. 9 (initial time 0.6s, final time 0.62s)

In order to eliminate the risk of resonance or to rapidly damp the oscillations on low order harmonics, one of the following measures can be applied: Warrant that the short circuit power at the energizing

busbar can not have values liable to cause resonance. Restriction of energization of the mixed line from one of the two interconnected networks can facilitate compliance with this condition.

Specify shunt reactors to remain linear (i.e. do not saturate) up to a voltage level as identified with the ATP-

7

EMTP analysis. This level is found to be 1.5 p.u. for the studied mixed line of Fig. 6-a. If TWVO-SRs are used, saturation can be avoided by a partialization of Mvar output prior to energization, i.e a reduction of operation flux density in the core.

In any case, all the transformers and autotransformers should be disconnected from the mixed line prior to energization.

P

0.0 0.2 0.4 0.6 0.8 1.0[s]0.0

0.5

1.0

1.5

2.0

2.5

3.0 [MJ]

phase S

phase T

phase R

Fig.11 Energy dissipation in SAs connected at bus C, during no-load energization of the 380kV mixed line of Fig.6-a

IV.3 1--to-Gr short circuit and SPHSR A parametric analysis has been performed for various mixed lines, including the ones of Fig. 6. The 1--to-Gr short circuits have been simulated at various locations along the mixed lines, by varying statistically over one cycle the fault ignition and SPHSR instants, with a uniforms statistical distribution. Analyses have shown that the fault ignition and SPHSR overvoltages are moderate (2.2 p.u.) and quickly damped as in conventional EHV lines, also in the case of the long mixed line of Fig. 6.a. Although cables have a high capacitance, their capacitive coupling contribution to the secondary arc current, Isa, is nil, because the -to- partial capacitances are nil. On the other hand, a large contribution to Isa can be caused by the electromagnetic coupling between the two energized phases and the open phase, owing to the charging currents flow in long line sections of the mixed line, if cable shunt compensation is low at the moment of the 1--to-Gr fault due to use of part of cable charging power in the overhead lines and interconnected networks.

The analysis performed for the mixed line of Fig. 6.a has shown that Isa can exceed 130Arms, namely in case of high power flow in the line with reduced shunt compensation of the cables. This value of Isa is much higher than the limit value of about 40Arms that is considered by experience to be consistent with the successful SPHSR of 400-500kV lines. In order to reduce Isa below 40Arms, the following measures can be applied for a long mixed EHV line as the one of Fig. 6-a: Switch-on automatically all the shunt reactors of faulty

phase during the dead time before the SPHSR. These reactors are switched off shortly after (say, 2-3) the

successful reclosure. Install a line CB in the interfacing station between the long

overhead line and cable sections, to limit the open phase only to the overhead line section.

Use a fast closing grounding switch. Equip the EHV shunt reactors with neutral reactors. Mixed lines including short cable sections do not have problems for the SPHSR, except the protection problem dealt with in par. V.

No special means for secondary arc reduction

0.22 0.24 0.26 0.28 0.30 0.32 0.34 0.36[s]-500-400

-300

-200

-100

0

100

200

300[A]

Temporary reinsertion of the shunt reactors

Fig.12 Secondary arc current, Isa, in case of 1--to-Gr near bus D (see Fig. 6-a), with shunt compensation at cable ends of 0Mvar-200Mvar and power flow of 1100MW. Dotted line: no special means for secondary arc current reduction; Continuous line: temporary insertion of all the shunt reactors on the faulty phase.

V. PROTECTION SYSTEMS AND HIGH-SPEED RECLOSURE OF EHV MIXED LINES

The single-phase high-speed reclosure (SPHSR) of EHV overhead lines is common practice in Europe and in many countries of the other continents, because the vast majority of line faults (90-95%) are 1--to-Gr and these are mostly (order of 90%) transient faults, i.e. liable to self-clearing with the SPHSR.

Faults rates of the HV and EHV cables are by an order of magnitude lower than in overhead lines, however are permanent faults. The high-speed reclosure must therefore be prevented, because it is useless and in order to avoid overheating of cable metallic sheaths and overstressing of equipment, due to repeated short circuits. The protection relays installed at the terminals of mixed EHV lines must therefore fastly and selectively detect the type and location of fault and initiate the SPHSR only in case of 1--to-Gr fault in the overhead line sections.

Figure 13 shows typical single-line diagrams of the interfacing stations between an overhead and a cable line section.

The scheme of Fig. 13-a is applicable for moderate cable length, not requiring shunt compensation at the cable terminals. The scheme of Fig. 13-c, with one or two shunt reactors at the interfacing station and with shunt reactor and line CBs, is appropriate when both the cable and overhead line sections are very long. When the line CB is present in the interfacing station, the cable and overhead line are separately

8

protected; then the SPHSR is easily applied to the overhead line only.

Protection schemes are proposed here for the interfacing single-line diagrams of Figs. 13-a and b, where line CBs are installed only at the two terminal substations of the whole mixed EHV line.

a) b)

c)

GroundingdisconnectorDisconnector

SurgeArrester

CurrentTransformer

Airgap

VoltageTransformer Cable terminal( with capacitiveVT)

Circuit breaker

Shunt reactor w/bushing CTs

Fig. 13 Typical single-line diagrams of interfacing stations between overhead and cable lines

The monitoring of the short circuit current flowing at the terminals of the various sections of a mixed EHV line does not permit the fault location as usual in MV radial lines, because the EHV mixed lines are usually part of a meshed grid. On the other hand, the selective location of faulty section by means of the reactance or impedance measurement is not sufficiently precise and reliable, owing also to the low series impedance of cable lines and to the large diversity of the electrical constants of EHV overhead and cable lines.

The protection of mixed EHV (330-380-500kV) lines consisting of a cable section interposed between two overhead line sections, as analysed in par. II, can be implemented with the scheme herebelow proposed (Fig. 14):

A B C D

OHL 1 CABLE OHL 2 f 1

E

Fig. 14 Single-line diagram of a mixed EHV line (A-B-C-D) to be protected

The mixed line, between substations A and D, where line CBs are installed, is protected with the usual distance relays applied with the directional comparison teleprotection, preferably the permissive overreaching transfer tripping scheme set to about 150% of line impedance. This protection must initiate the line tripping in 3-phase when it detects a multiphase fault. In the cases of the most frequent 1--to-Gr faults, the protection must trip only the faulty phase at both the ends of mixed line and initiate the SPHSR with a delay of 2-3.

The mixed line should be provided with a 2nd main protection, as usual in EHV lines, which can be a 2nd directional comparison teleprotection scheme using distance relays. However, if a wide-band reliable telecommunication channel is available between the

terminals A and D, consisting of fibre optics or microwaves, it is appropriate to use as 2nd main protection, a differential protection scheme, which ensures a more reliable selective detection of the faulty phase for the SPHSR and a very fast fault clearing.

The EHV cable section should be provided with an additional differential protection scheme supplied by current transformers at the cable terminals (B and C in Fig. 14), to be implemented with digital relays intercoupled via a dedicated fibre optic cable laid along the EHV cables. If fault (of whatever type) is internal to the cable section (f1 in Fig. 14), the differential protection must initiate the fast transmission of signals from station B to A and from station C to D, respectively, which will block the SPHSR and trip in 3-phase the CBs at both the ends of the mixed line (i.e. open also the non-faulty phases if one phase has already been tripped out by the local teleprotections). These transfer tripping signals should be transmitted with the normal telecommunication channels serving the overhead lines (fibre optics, power line carrier, microwaves, etc.). The transfer signals from B and C (Fig. 14) will reach A

and D, respectively, with a delay of the order of one cycle from the initiation instant by the cable differential protection. The latter will therefore perform the fast clearing of cable faults even in case of delayed operation of the distance relays at the terminal of mixed line in the 2nd or 3rd zone. Overheating of cable metallic sheaths is then avoided.

VI. CONCLUSIONS The maximum cable length at the thermal limit (MCLTL)

of EHV XLPE insulated cable lines is achieved by operation with the same charging power outflow at the two cable terminals, i.e. with about unit p.f. at mid point. With a derating of 10% due to charging power, the MCLTL of 380kV-50Hz buried XLPE cables with Cu conductors of 1600 mm2 and 2500 mm2, is about 81 km and 91 km, respectively. These MCLTL values are reduced to about 57 km and to 63km for 500kV-60Hz buried XLPE cables with Cu conductors of 1600 mm2 and 2500 mm2, respectively.

A method is proposed for the optimal planning of shunt compensation, and of voltage and reactive power control of EHV mixed lines including long cable sections. A 500kV-60Hz long mixed line is analysed as a case study and results are presented.

A physical explanation and the ATP-EMTP analysis are presented of a resonance phenomenon on 3rd harmonic liable to cause sustained no-load energization overvoltages up to 2.4 p.u. in mixed EHV lines including cable sections with length of some tens of km. Countermeasures to avoid the risk of such resonances are presented in the paper.

Apart this resonance phenomenon, the switching and temporary overvoltages of mixed EHV lines are found consistent with the conventional insulation levels, if an adequate compensation of cable charging reactive power is applied.

9

Analysis of SPHSR of 380kV-500kV mixed lines following the 1--to-Gr faults in overhead sections shows that the secondary arc current, Isa, may exceed 100-150Arms. Methods are recommended for lowering Isa to less than 40Arms and making successful the SPHSR.

At last, a protection scheme of mixed EHV lines is presented for the fast, selective detection of faults, for implementing the high-speed reclosure only following the faults occurring in the overhead line sections and, on the other hand, to contain within about 100ms the duration of the short circuit currents flow in the metallic sheath of the XLPE cables in case of faults in the cable sections.

The performed studies confirm the feasibility of EHV 50Hz and 60Hz mixed lines, including long sections of XLPE cables to be laid in motorway and railway tunnels. However some efforts should be made by cable manufacturers, to adapt the design, transport and laying techniques to cable route lengths much exceeding the ones so far applied, in particular for increasing the distance between cable joints.

VII. APPENDIXES

A. MCLTL calculation With the symbols defined in par. II, the following

equations can be written at the cable ends:

)1.(

cos1

cos1

1sin

22

22 A

BAV

BVVSQ

BAV

BVVSQ

withBVVSPP

SRS

zS

RRS

zR

RSzSR

==

==

10

( )( ) ( )[ ] (B.9)4404024244120241442

24

nSRQEEXcXcEXcXc

QQ

++

+

From (B.9) we have:

( ) ( )( ) ( )B.10

1

402044

44

402024

QQKQ

QK

QQQQ

nSR

nSR

+

+=+

The K4 coefficient is finally given by:

( ) ( )[ ] (B.11)1

4040242441202414424 EEXcXcEXcXc

K ++=

The desired operation condition, can be also obtained by acting on the TWVO-SR at node 2, XSR2. The unknown is now the change of rated reactive power QSR2n. Former results still hold, but formulas (B.9) to (B.11) are replaced by:

(B.12)3

;3 2

202442

20222 SR

nSR

n

QE

EXEQ

EEX

E

The expression sought for becomes:

( )( ) ( )[ ] (B.13)2204022224120221242

24

nSRQEEXcXcEXcXc

QQ

++

+

In conclusion, we have:

( ) ( )B.14402022 QQKQ nSR + The K2 coefficient is:

( ) ( )[ ] (B.15)1

2040222241202212422 EEXcXcEXcXc

K ++=

VIII. REFERENCES [1] F. M. Gatta, S. Lauria, " Very long EHV cables and mixed overhead-

cable lines. Steady-state operation," accepted for presentation at 2005 IEEE St. Petersburg Power Tech Conference. June 26th-July 1st, 2005

[2] UCTE. (2003, Dec.). First UCTE Comments on the Background Paper Undergrounding of electricity lines in Europe. [Online]. Available: http://www.ucte.org/pdf/Publications/Library/e-default_2004.asp

[3] Commission of the European Communities. (2003, Dec.). Background Paper Undergrounding of electricity lines in Europe. [Online]. Available: http://europa.eu.int/comm/energy/electricity/publications/index_en.htm

[4] R. Arrighi, Operating characteristics of long links of AC insulated cables, in Proc. CIGRE Genera1 Session 1986, Paper 21-13.

[5] A. Babare, G. Bertagnolli, F. M. Gatta,, F. Iliceto, Design and application of variable Mvar output shunt reactors with on load tap-changer. Operation experience in Africa, in Proc. CIGRE Genera1 Session 1998, Paper 12-308.

IX. BIOGRAPHIES

Francesco Iliceto (SM '71; F '85) was born in Padua (Italy) in 1932. He received a doctor degree (Hons) in Electrical Engineering in 1956 from Padua University. From 1956 to 1965 he worked with two power utilities, on the design and construction of steam thermal power plants (Milan, Italy), of the HVDC link between Sardinia and the Italian mainland and HVAC transmission (Rome, Italy). In 1965 he received the Professor degree of Power System Analysis from the Ministry of Education (Rome, Italy) and joined the Faculty of Engineering of Rome University, where he has served as Ordinarius Professor and Head of the Electrical Engineering Department until 2004. In 1966 and 1971 he was visiting professor in Sweden and in the USA.

Since 1968 he has been consultant to the Turkish Electricity Authority. In this capacity, he contributed substantially to the planning, design and operation of the EHV and HV systems of Turkey and interconnections with neighbouring countries, up to the present stage, and to research.

Since 1977 he has been consultant to the Volta River Authority for planning, design and operations of Ghana's national power system and extensive rural electrification.

At the request of the World Bank, of the European Investment Bank, the Inter American Development Bank, the Asian Development Bank and of National Electricity Corporations, he has served as technical consultant in several other countries (Zaire, Portugal, Togo, Benin, Canada, Egypt, Burkina Faso, Northern Cyprus, Sierra Leone, Pakistan, India, Costa Rica, Panama, Honduras, Nicaragua, Brazil, Laos, Senegal, Mali, Mauritania, Ivory Coast, Germany, Ethiopia, Finland, China, Mozambique, Russia, Italy).

His main fields of interest are EHVAC and EHVDC transmission, power system analysis, power system planning and design, rural electrification with a new low cost technology. He is author or co-author of more than 100 technical papers and tutorial books (5 volumes). He has served as chairman or member of various national and international Technical Committees.

Fabio Massimo Gatta was born in Alatri (Italy) in 1956. In 1981 he received a doctor degree in Electrical Engineering from Rome University (Hons). He then joined the Rome University's Department of Electrical Engineering where he was appointed Researcher and in 1998 appointed Associate Professor in Applied Electrical Engineering.

His main research interests are in the field of power system analysis, long distance transmission, transient stability, temporary and transient overvoltages, and series, shunt compensation, SSR, distributed generation.

Stefano Lauria (M' 99) was born in Rome, Italy, in 1969. He received the master degree and the Ph.D. in electrical engineering from the University of Rome "La Sapienza" in 1996 and in 2001, respectively. In 2000 he joined the Department of Electrical Engineering of University of Rome "La Sapienza" as a Researcher. His main research interests are in power systems analysis, distributed generation, power quality and electromagnetic transients.

Luigi Colla was born in Marino, Italy, in 1980. He received the master degree in electrical engineering from the University of Rome "La Sapienza" in 2004. He is currently working towards his Ph.D. in electrical engineering. His main research interests are in power systems analysis, electromagnetic transients and long distance transmission.