Long-Distance DC Electrical

September/October — Vol. 29, No. 5 37

F E A T U R E A R T I C L E

Long-Distance DC Electrical Power TransmissionKey words: HVDC, power transmission technology, long DC cable, overhead line, insulation, aging model, threshold stress, space charge, resistivity, breakdown

0883-7554/12/$31/©2013/IEEE

Rongsheng LiuSenior Member IEEE, ABB AB, Corporate Research, 721 78 Västerås, Sweden

Long-distance DC transmission sys-tems are reviewed. Aging models and the design of DC cables are dis-cussed. Extruded and mass-impreg-nated cables are environmentally friendly and thus are alternatives to overhead lines.

IntroductionTransmission and/or distribution systems are needed to trans-

port electrical power from its generation site to the user loca-tions. This paper reviews major DC power transmission technol-ogies, concentrating on systems that are capable of long-distance transmission. Special attention is paid to lifetime models and the safe use of DC insulation systems. A relationship between the probability of failure and the volume of stressed insulation (the volume effect) is discussed briefly.

Major DC Power Transmission TechnologiesIn 1906 two DC cables, each 4 km in length, were manufac-

tured by Cable de Lyon, the insulation consisting of cellulose paper impregnated with resin-oil compound. They were installed between the power station at Moutiers and the city of Lyon as part of a 124-km mostly overhead line route. The system oper-ated at DC voltages of ±75 to 125 kV with transmission power of up to 30 MW [1], [2]. It is believed that this was the first truly commercial use of HVDC cables. In 1954 another commercial HVDC link was commissioned in Sweden, namely a submarine mass-impregnated (MI) insulation cable system with a transmis-sion length of 100 km, nominal DC voltage of 100 kV, and trans-mission capacity of 20 MW [2], [3]. It was used to connect the power systems on the Baltic island of Gotland with the Swedish mainland, and after 16 years of service, it was upgraded to 150 kV.

The insulation of a DC cable may also consist of polymeric material (extruded DC cable). Research and development of ex-truded DC cables have been underway for more than 40 years, and the world’s first HVDC extruded cable system was com-

The eighth of a series of invited reviews to be published during 2013 to mark

the 50th anniversary of DEIS.

mercially established in Gotland in 1999 [4]. Its nominal DC voltage was 80 kV, its capacity 50 MW (bipolar cables), and its length 72 km.

HVDC power transmission systems may consist of overhead DC line, oil-filled (OF) DC cable, MI DC cable, extruded DC cable, gas-insulated DC line and superconducting cable sys-tems. Overhead line is today the most economical system for power transmission over long distance [5]–[12]. China created a record in transmitting 7,200 MW of DC power over 2,090 km at ±800 kV. Another system, scheduled to be completed in 2014 [13], will transmit 8,000 MW of DC power over 1,670 km at ±800 kV. Using two bipolar lines on a single tower, or one bipo-lar line on each of several parallel towers, it should be possible to transmit up to 18 GW [14], [15]. An ultra-high-voltage DC system operating at ±1,100 kV is under development [16].

Oil-filled cables have long been used for power transmission over distances of 100 km or less [17], [18]. They can work up

38 IEEE Electrical Insulation Magazine

to 90°C with a design DC stress up to 40 kV/mm. Direct-cur-rent ±600-kV high-pressure, oil-filled or self-contained, fluid-filled cables, with oil-impregnated cellulose paper insulation, have been investigated in the laboratory for more than 30 years [19]–[23]. Ultra-high-voltage DC systems based on OF cables (up to ±1,400 kV and 3,000 MW per pole) have been studied [24], [25]. In principle, OF cables have the advantage of con-ductor cooling; forced cooling of the conductor can triple the transmission capacity of an OF cable. The power-carrying ca-pacity can be as much as 10 GW at DC voltage 600 kV [20], [23]. A polypropylene-laminated-paper insulation system has been used in OF cables for many years, and a polypropylene-laminated-paper submarine cable system was installed in Japan in June 2000. This system has a targeted transmission capacity of 2,800 MW, rating voltage of ±500 kV, and route length of 48.9 km [26]–[28].

Mass-impregnated cables have also been used for many years; because of their “solid” insulation system (mass-im-pregnated, non-draining paper) they are still favored today for long-distance submarine and land DC power transmission. In principle the circuit length is unlimited. The maximum DC op-erating temperature is approximately 55°C but is expected to increase with the introduction of new lapped insulation and MI compounds. The currently commissioned capacity has reached 1,000 MW at ±500 kV [3], [17], [18], [29]–[34] and is expected to increase in the near future [33]–[41].

Extruded DC cables are relatively new developments. They are free from oil and grease, but a degassing process is needed for DC cables using cross-linked polyethylene insulation [42]. Direct-current voltages up to ±320 kV and power ratings up to 1 GW [43]–[47] are practicable. A ±500 kV, 3,000 MW (bipolar single circuit) extruded DC cable system was developed in the laboratory in Japan in 2002 [48]–[50]. Intensive studies of space charge accumulation, resistivity, aging, and reliability of DC in-sulation systems have been carried out [51]–[71].

Gas-insulated DC cable or gas-insulated line systems are insulated with compressed gas, the conductor structures being supported by spacers. In such systems the diameter of the con-ductor can be large, and thus increased current (>4,000 A) and voltage (up to ±800 kV) ratings are possible. However, for long-distance (>1,000 km) power transmission, this technology is still in the development phase [18], [72]–[84].

Superconducting cables date back to the 1960s; high-temper-ature superconducting cables are a more recent development. However, they are also still in the development phase. They may offer power capacities in the range 5 to 10 GW, but distance remains a challenge because of system reliability concerns [18], [85]–[102]. It was suggested in one study that superconducting cables may have lower power losses than any other technology for transmission over 1,500 km, with a power capacity of 5 GW and DC voltages of ±200 kV [86].

Given the above overview, it appears that long-distance, high-capacity power transmission will still be dominated by over-head-line systems in the predictable future, whereas extruded DC cables and MI DC cables will continue to be preferred for submarine applications (Table 1).

Tabl

e 1.

Fea

ture

s of

Maj

or H

VDC

Pow

er T

rans

mis

sion

Tec

hnol

ogie

s.1

Over

head

line

OF c

able

MI c

able

Extru

ded

cabl

eGI

LHT

S ca

ble

Com

mer

cial

pos

sibi

lity

±800

kV,

8 G

W±5

00 k

V, ~

3 GW

±550

kV,

~2

GW±3

20 k

V, ~

1 GW

Lim

ited

serv

ice

reco

rd

at D

CLi

mite

d se

rvic

e re

cord

at

DC

Rese

arch

and

labo

rato

ry p

roto

type

±1,1

00 k

V±6

00 k

V, 6

~10

GW

(fo

rced

coo

ling)

; ±1

,200

kV,

4~6

GW

(sel

f-con

tain

ed)

±600

~±80

0 kV

, 2~4

GW

±500

kV,

3 G

W±8

00 k

V±2

00 k

V

Bene

fit• L

ong

dist

ance

• Low

cos

t• S

mal

l ins

ulat

ion

prob

lem

s• U

HV a

nd la

rge

capa

city

• Hig

h ca

paci

ty is

pos

sibl

e• L

and

or s

hort

subm

arin

e

(less

than

60–

100

km)

• Lon

g se

rvic

e re

cord

• Lon

g di

stan

ce• S

olid

insu

latio

n• S

ubm

arin

e• L

and

• Lon

g se

rvic

e re

cord

• Lon

g di

stan

ce• S

olid

insu

latio

n• S

ubm

arin

e• L

and

• Low

er c

ost t

han

OF c

able

s

• Lar

ge c

ondu

ctor

are

a• L

ower

loss

es th

an

ca

bles

• Pot

entia

l exi

sts

for h

igh

po

wer

cap

acity

• Sup

er-c

ondu

ctiv

ity• H

igh

curr

ent c

arry

ing

ca

paci

ty• P

oten

tial e

xist

s fo

r hig

h

pow

er c

apac

ity

Wea

knes

s• C

oron

a di

scha

rges

• Env

ironm

enta

l

conc

erns

• Lig

htni

ng p

rote

ctio

n

need

ed

• Sop

hist

icat

ed o

il re

fillin

g

syst

ems

• Lim

ited

trans

mis

sion

leng

th

• Wor

king

tem

pera

ture

limite

d to

55°

C

(but

can

be

impr

oved

with

new

insu

latio

n

syst

em)

• New

tech

nolo

gy• S

yste

m a

nd in

sula

tion

re

liabi

lity

• Agi

ng• R

esis

tivity

• Spa

ce c

harg

e

• Sys

tem

relia

bilit

y fo

r

long

dis

tanc

e• L

imite

d se

rvic

e re

cord

• Sop

hist

icat

ed

cr

yoge

nic

syst

em• S

yste

m re

liabi

lity

• Lim

ited

serv

ice

reco

rd

fo

r lon

g di

stan

ce

1 OF

= oi

l fill

ed; M

I = m

ass

impr

egna

ted;

GIL

= g

as-in

sula

ted

line;

HTS

= h

igh-

tem

pera

ture

sup

erco

nduc

ting.


Environmentally Friendly Power Transmission Over Long Distances

In relation to long-distance HVDC power transmission, the likely alternatives to overhead lines are MI cables (Figure 1) and extruded cables (Figure 2). These two cable systems are preferred for submarine power transmission over distances greater than 100 km (Figure 3). For overland applications, they are usually installed underground (contrasting with overhead lines). Both systems are designed to be corona-discharge free, in principle, and thus are environmentally friendly (no interfer-ence with adjacent communication systems and no additional greenhouse gas generation). Potential exists for both systems to reach the operating voltages and power ratings of overhead lines in the future (Figure 4).

With the introduction of HVDC stations using voltage source converters or line-commutated converters, as shown in Figures 5 and 6, respectively, electrical power can be converted from AC to DC (rectification) and then converted back to AC (inversion). In this way it can be transmitted over long distances at different power levels. Voltage source converters are relatively new, based on insulated gate bipolar transistors. The system uses high-fre-quency (up to 2,000 Hz) pulse-width modulation, and thus the use of small filters and independent control of active and reac-tive power are possible. The line-commutated converter is based on thyristors and operates at 50/60 Hz. A single voltage-source-converter station today can handle power levels in the range 50 to 1,100 MW; the corresponding level for a conventional HVDC

station is 8,000 MW.

Insulation Systems for Long-Distance Power Transmission

The main insulation systems used for MI cables are based on oil, or compound gel-impregnated Kraft-paper, or polymer laminated Kraft-paper (Figure 7), whereas for extruded cables they are mainly extruded polymers, e.g., cross-linked HVDC polymer, cross-linked polyethylene, or high-density polyethyl-ene (Figure 8).

Figure 1. An MI (mass-impregnated) DC cable for submarine application.

Figure 2. Extruded DC cables for submarine and land applica-tions.

Figure 3. Power transmission capacity over long distance using DC overhead line, mass-impregnated (MI) cable and extruded cable (bipolar, present capacity with proven technology).


The steady-state electrical field distribution in AC cable insu-lation is mainly governed by the relative permittivity of the insu-lation material. In DC cable insulation the insulation resistivity and accumulated space charge dominate the field distribution. However, during transients, e.g., load-on or load-off, lightning impulses and switching impulses, the admittance of the insu-lation, the wave impedance of the cable, i.e., the ratio of the transverse components of the electrical and magnetic fields, and more generally Maxwell’s equations must be taken into consid-eration.

Aging ModelsThe reliability of an insulation system is affected by aging

under multiple stresses, e.g., electrical, thermal and mechanical, and space charge accumulation [103]–[110]. The fundamental relationship between the aging of an insulation system and the aging parameters is given by the Arrhenius equation [111], and variations such as Montsinger and Dakin’s thermal aging model

[111]–[113], the inverse power law (electrical aging) [114]–[116], Eyring’s multiple stress aging model [104], [107], [117], and the Weibull distribution [104], [118], [119]. Specifically, we have

Figure 4. Expected increase of voltage and power ratings for mass-impregnated (MI) cable and extruded cable compared with overhead line (bipolar, present voltage level with proven technology).

Figure 5. A simplified diagram of a voltage source converter (VSC).

Figure 6. A simplified diagram for conventional HVDC station (monopolar transmission with earth return).

Figure 7. A lapped Kraft-paper insulation system for mass-im-pregnated (MI) cables.


(a) the Arrhenius equation

K A G

RT = exp −

,

(1)

where K is the aging rate, A is a pre-exponential or rate factor, G is the activation energy of the aging process, R is the gas constant, and T is the absolute temperature;

(b) Dakin’s model (thermal aging)

log τ = A* + 0.434G/RT, (2)

where τ is the service lifetime of the insulation and A* is a constant;

(c) inverse power law (electrical aging)

Ent = constant, (3)

where E is the applied electrical stress, n is an aging exponent, and t is the time to breakdown;

(d) Eyring’s model (multiple stress aging, combining ther-mal stress with either mechanical or electrical stress)

L kT

h LGkT

cEkT

= 0 exp exp ,−

∆

(4)

where L is the aging rate, h is Planck’s constant, k is Boltzmann’s constant, T is the absolute temperature, ΔG is the Gibbs free energy, E is the electrical stress, and L0 and c are constants; and

(e) Occhini [118] and Densley et al. [104] considered the parameters of the Weibull distribution, i.e., the scale, shape, and location (threshold stress value) of the elec-trical field, and the variation of failure probability with time.

Occhini [118] and Densley et al. [104] took into account the so-called breakdown volume effect, namely that the larger the stressed insulation volume, the lower the applied field strength at which breakdown occurs. The breakdown volume effect is con-sistent with the weakest link theory, i.e., the larger the stressed volume, the greater the probability that it will include the weak-est link in the total insulation volume [120]–[122]. We have

p E t

E EE

tt

l D D

l

( , )

exp

=

− −−

−( )1

2 2γα β

s s

c

00 02

02D D−( )

c

,

(5)

where p is the probability of failure under an electrical field E at time t; Eγ is a threshold stress value below which breakdown will probably not occur; Es and ts are the scale parameters for the electrical field and the time; α and β are the shape parameters for the electrical field and the time; l, Dc, and D are the length, the conductor screen outer diameter, and the insulation screen inner diameter of the cable; and l0, Dc0, and D0 are the corresponding initial values, respectively.

The inverse power law is often used to predict the service lifetime of a cable following accelerated aging. It follows from (3) that if a cable is to have a service lifetime of at least 50 years at a working stress of 20 kV/mm, then, assuming n = 9, it should not breakdown under an accelerated aging field of 40 kV/mm for at least 36 days. Similarly, if the same cable is to have a service lifetime of at least 50 years at a working stress of 25 kV/mm, it should not breakdown under an accelerated aging field of 40 kV/mm for at least 266 days. In practice the situation is more com-plicated. Usually the volume effect will dominate the service lifetime of a long cable installation (over 100 km for instance). The volume effect is incorporated in (5), where the probability

Figure 8. Extruded cables with cross-linked HVDC polymer as insulation: (a) an example of sub-marine cable with copper conductor area of 1,000 mm2 and (b) an example of submarine cable with copper conductor area of 1,650 mm2.


of failure will increase with increasing l D Dc( ),2 2− which is di-rectly proportional to the stressed volume of the insulation. On the other hand, it is believed that the threshold stress value Eγ is dependent on the material feature of the insulation. For a given insulation system, a typical Eγ value might exist, below which electrical breakdown will probably not take place.

Space Charge Effects and Space Charge Measurement

Accumulation of space charge in the insulation of a DC cable can cause degradation of the insulation and thus aging of the cable [110], [120], [123]. The measurement of space charge distribution in the insulation of a coaxial cable, using the pres-sure wave propagation method, was first reported in 1989 [124] and again in 1991 [125]; corresponding measurements using the pulsed electro-acoustic (PEA) method were first reported in the early 1990s [126]–[135]. The measurement frequency bandwidth of the PEA method was significantly increased fol-lowing the replacement of lead zirconate titanate transducers by polyvinylidene fluoride transducers [136], [137], and it became possible to measure space charge distributions sufficiently accu-rately without resorting to deconvolution procedures. Deconvo-lution was introduced for signal calibration in cylindrical cable geometry [133], and a flat electrode system was used (in the PEA method) for the measurement of space charge in the insu-lation of coaxial cables with insulation thickness up to 20 mm [132], [134], [135], [138]. Figure 9 shows the use of the PEA method for space charge measurements in a cable with cylindri-cal geometry.

Cable aging due to space charge accumulation increases with increasing applied electrical field strength. It was found [66] that, at low DC field (18 kV/mm), the space charge profile dis-tribution and density within the insulation (extruded polymer) of a model cable aged in tap water varied little over a three-year pe-riod with the field applied (Figure 10). Initially, limited hetero-charge build-up was observed in the vicinity of the insulation screen/insulation interface, which caused a 40% increase in the electrical field strength at the interface. The increase was stable throughout the three-year period, although slight redistribution of space charge near the interface was observed. The cable did not break down during the three-year period.

ConclusionsAt present, overhead lines, MI cables, and extruded polymer-

ic cables are commonly used for long-distance HVDC power transmission. They are more suitable than OF cables, supercon-ducting cables, and gas-insulated DC lines. For submarine ap-plications, MI and extruded polymeric cables are favored. For bipolar applications, MI cables can handle up to 2 GW at operat-ing voltages of at least ±500 kV. The corresponding figures for extruded cables are 1 GW and ±320 kV.

For overland applications, overhead lines are preferred for high-capacity power transmission over long distances. However, environmental considerations could reverse this preference in

favor of MI and extruded polymeric cables. A hybrid DC trans-mission system could contain all three.

Cellulose-based (paper) DC insulation systems have per-formed satisfactorily for more than 100 years. Extruded DC insulation systems are more recent and offer oil-free and thus “easy” manufacture. However, attention must be paid to prob-lems associated with resistivity, space charge, and long-term reliability, especially at higher operating voltages. For a given insulation system, a threshold value of DC electrical stress may exist below which DC electrical stress would probably have lit-tle influence on the aging process.

AcknowledgmentThe author thanks Claire Pitois, Marc Jeroense, Hossein

Ghorbani, Per Skytt, Dong Wu, Christer Törnkvist, and Carl-Olof Olsson for their valuable discussions.

Figure 9. Principle of the pulsed electro-acoustic (PEA) method for the measurement of space charge in cylindrical geometry. a and b are respectively the inner and outer radii of the insula-tion; ρ(r) is the volume space charge density at radius r; and s, l, and c are respectively the thickness of the outer semicon-ductive layer, the distance between the outer semiconductive layer and the piezo device (a polyvinylidene fluoride film), and the position of the piezo device. R and C are respectively the resistance and capacitance of the PEA measurement system, V is the voltage of the DC source, and vp(t) is the time-dependent voltage output of the pulse generator. vs(t) is the output voltage from the piezo device, from which the space charge distribution is deduced [129].


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Rongsheng Liu received the PhD degree in 1988 from the Department of Electrical Engineering at Xi’an Jiaotong University in China and did postdoctoral work at the Electronic Measurement Laboratory of the Musashi Institute of Technology in Tokyo between July 1990 and June 1992. He then joined ABB Corporate Research in Swe-den and has since been working at ABB

in the field of power technology. His research interests include dielectrics and electrical insulation, especially related to power transformers and power cables. He holds more than 20 patents and has been an author or coauthor of 123 technical publications and reports. He is a winner of 3 technical awards and is a Senior Member of the IEEE.

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