direct current transmission
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POWER SYSTEM
STABILITY
,
Volume
i Elements
of
Stability
Calculations,
1948, John
Wiley
&
Sons,
Inc.
Volume
ii Power
Circuit
Breakers an d
Protective
Relays,
1950, John
Wiley
& Sons, Inc.
Volume
n i
Synchronous
Machines,
1956,
John
Wiley
&
Sons,
Inc.,
(republished
by
Dover
Publications,
Inc.,
1967)
ELECTRICAL
TRANSMISSION
OF
POWER
AND
SIGNALS,
1949,
John
Wiley
&
Sons,
Inc. (Also
published
for the
Asiatic
market
by
Toppan
Co., Ltd.,
Tokyo, Japan,
1964)
f-
I:
f.
DIRECT
CURRENT
TRANSMISSION
I
Volume
i
EDWARD
WILSON
KIMBARK,
Sc.
D
Fellow
I.E.E.E.
Bonneville
Power
Administration
Portland,
Oregon
WILEY-INTERSCIENCE
a
Division
of
John.Wiley
&
Sons,
Inc.
New
York
London
Sydney
Toronto
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Copyright
1971
by John
Wiley
& Sons,
Inc.
All
rights
reserved. Published
simultaneously
in
Canada.
Reproduction
or
translation of
any part
of this work
beyond
that
permitted
by Sections
107
or
108
of the 1976
United
States
Copy
right
Act
without
the
permission
of
the
copyright
owner is
unlaw
ful.
Requests
for permission or further
information
should be
addressed
to the
Permissions
Department,
John
Wiley
& Sons, Inc.
Library
of
Congress
Catalog
Card Number:
72-142717
ISBN
0-471-47580-7
Printed in
the United
States of America
10
9
8
7
6
5
PREFACE
s
I
'
'
H
The
most exciting
new
technical
development
in
electric
power
systems in
the last two
decades
is
direct-current
transmission.
From
1950
to 1970, eight
direct-current
links have
gone
into
commercial
operation
in
various
parts of
the
world.
From the first
of these
links to the
last,
the
voltage
has increased
from
100
to 800 kV;
the
rated
power,
from
20
to 1440
MW; andthe
distance
from 96 to 1370
km
(60 to 850
miles).
Several other dc links
are
under
con-
*
struction
or
proposed.
Preceding
and
accompanying
this rapid growth
of
direct current
transmis
sion were
developments
in
high-voltage,
high-power valves, in
control
and
protective systems, in
dc cables,
an d
in
insulation for
overhead
dc lines.
Industrial,
governmental,
and
academic
laboratories
were
involved
in
this
development.
Dc
transmission
became
a
favored
subject
for
research
by
graduate students of electrical engineering.
The circumstances
leading
to
the
adoption of
direct-current
transmission
are
diverse:
long
water
crossings
requiring
submarine
cables,
frequency
j
changing, asynchronous
operation
of systems
having
the same
nominal
frequency, large hydroelectric resources remote from
load
centers,
long
in-
s
terregional
ties, and transmission through congested
metropolitan
areas.
The rapid
growth
of
dc
transmission,
combined with the
diversity
of reasons
for
itsuse, assures
for
it
a
brilliant
future and also
points
to
the need for a
ne w
and
better
book
on
the
subject.
The art
of
dc
transmission in the
past
two
decades has
been based on
the
use of improved mercury
arc valves.
Consequently,
this
book,
in
endeavoring
to
describe
the
present
state
of
the
art,
is necessarily
based
largely on
the
technology
employing such
valves.
There are
indications that mercury
arc
valves have
reached
almost the
peak
of their
development. At
least,
solid-
state
controllable
valves
(thyristors), though
not yet used
in
an y major
dc
1
transmission project,
are
appearing
as formidable contenders for
future
pro
jects.
Fortunately,
the
technology
of
employing
thyristors
for
dc
transmission
differs
more
indimensions than in principles
from
that
of employing
mercury
arc valves. Hence, it
is
not
primarily the transition
to
thyristors
that will in
v
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VI
PREFACE
time
render this
book
obsolete, but rather the
continuing rapid
development
of
all phases of
the art.
My interest
in
direct
current
transmission
was
awakened
in
1962
when the
Bonneville Power
Administration
(BPA) asked
me to
teach
two
courses in
this
subject
for their
engineers.
Th e lack of
an
adequate
text
book for those
courses
determined
me
to write one. S ince
1962,
I
have
taught
three more
courses
on dc
transmission,
and
have been involved
in studies of
various
aspects
of
this
subject.
I
am
indebted
to
the
BPA for
the
opportunity
to
teach
these
courses,
to
work
on
problems in
the
field,
and to confer
with my col
leagues,
as
well as for
access
to
the
BPA's excellent
library
services.
However,
I
wish
to
make
clear
that this book
is notan official publication of the
BPA
nor
one
sponsored by
it.
It has
been a
spare-time
project.I
alone am
respon
sible
for its
contents,
including any errors
which
may
inadvertently
appear in
it.
The
large
amount
of essential
information
now available
on direct-current
transmission
and the
time
required
to organize it
led
to
the
decision to divide
the work
into
two volumes of
which this is the
first.
The
proposed
contents
of
the
second volume
are indicated
on
page
xi.
Units
of
physical
quantities
used
hereinare
those
of
the International
System
(SI)
recommended
by
the
I.E.E.E.
and
I.E.C.
I
am indebted
to various
engineers
at
the
BPA
and
elsewhere
for
supplying
information,
especially
to Dr. John
J.
Vithayathil
for
many
enlightening
technical
discussions.
I
am
indebted
to my wife,
Ruth
Merrick
Kimbark,
for
typewriting much
of the
manuscript
an d pertinent
correspondence
and
for her
valued
advice
and
encouragement.
Edward
Wilson
Kimbark
Portland, Oregon
March, 1971
n
-
i
CONTENTS
1.
GENERAL ASPECTS OF
DC
TRANSMISSION
AND COMPARISON
OF IT
WITH
AC TRANSMISSION
1
1-1
Historical
Sketch
1
1-2 Constitution
of EHV
AC and
DC
Links
9
1-3
Kinds
of
DC Links
11
1-4
HV
DC Projects
from 1954
to
1970
12
1-5
Limitations
and Advantages of
AC and
DC
Transmission
19
1-6 Summary
of
Advantages
and Disadvantages
of
HV
DC
Transmission
32
1-7
PrincipalApplications
of
DC
Transmission
32
1-8
Economic Factors
33
1-9 The
Future
of
DC
Transmission
35
Bibliography
36
.
2.
CONVERTER
CIRCUITS
49
2-1
Valve Characteristic
49
2-2 Properties
of
Converter
Circuits
,
50
2-3
Assumptions
51
2-4
Single-Phase
Converters
/
51
2-5
Three-Phase
Converters
56
i
2-6 Pulse
Number
61
2-7
Additional
Six-Pulse
Converter
Circuits
62
2-8
Choice
of Best
Circuit
for
HV
DC
Converters
65
2-9 Twelve-Pulse Cascade
of
Tw o
Bridges
67
Problems
68
Bibliography
70
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Viii
CONTENTS
3. ANALYSIS
OF THE
BRIDGE
CONVERTER
71
3-1
Analysis with
Grid
Control
but no Overlap
73
3-2
Analysis
with
Grid Control
and
with
Overlap
less
than
60 80
3-3
Analysis
with
Overlap Greater than
60
92
3-4
Complete
Characteristics
of Rectifier
103
3-5 Inversion
105
3-6
Series and
Parallel
Arrangements of
Valves,
Anodes,
or
Bridges
112
3-7
Multibridge
Converters
115
Problems
12 3
Bibliography
126
4.
CONVERTER
CHARTS
129
4-1
Chart
1with
Rectangular Co-ordinates
of
Direct
Current
and
Voltage
129
4-2
Chart
2
with
Rectangular Co-ordinates
of
Active
and Re
active
Power
138
4-3
Relations
between the
Tw o Charts
146
Problems
146
Bibliography
147
5.
CONTROL
148
5-1
Grid Control
148
5-2
Basic Means
of
Control
152
5-3 Power Reversal
153
5-4 Limitations
of Manual Control
154
5-5
Constant
Current
versus Constant
Voltage
156
5-6
Desired
Features
of
Control
157
5-7
Actual
Control
Characteristics
158
5-8
Constant-Minimum-Ignition-Angle
Control
164
5-9 Constan t-Curren t
Control
165
5-10
Constant-Extinction-Angle
Control
167
5-11
Stability of Control
174
5-12
Tap-Changer
Control
179
5-13
Power
Control
and
Current
limits
180
5-14 Frequency
Control
18 2
5-15
Multiterminal
Lines
183
CONTENTS
IX
5-16
Measuring
Devices
Problems
Bibliography
187
192
194
6.
MISOPERATION
OF
CONVERTERS
198
6-1
Malfunctions
of
Mercury-
Arc
Valves
198
6-2
Bypass
Valves
199
6-3 Arcback
206
6-4 Short
Circuit on
a Rectifier
1
220
6-5
Commutation Failure
222
6-6 Arcthrough
227
6-7
Misfire
228
6-8
Quenching
229
6-9
Generalization
of Inverter
Faults
and
Certain
Rectifier
Faults
230
6-10 Consequential
Faults in
Rectifier
,
231
Problems
233
Bibliography
234
7.
PROTECTION
235
7-1
General
7-2
DC
Reactors
7-3 Voltage
Oscillations
and
Valve
Dampers
7-4
Current
Oscillations
andAnode
Dampers
7-5
DC
Line Oscillations and
Line Dampers
7-6 Clearing Line
Faults and Reenergizing
the
Line
7-7
Circuit
Breakers
7-8
Overvoltage
Protection
Problems
Bibliography
235
235
247
260
270
272
280
282
291
292
8.
HARMONICS
AND FILTERS
295
8-1 Summary
8-2 Character is ti c
Harmonics
8-3
Uncharacteristic
Harmonics
8-4
Troubles
Caused by Harmonics
8-5
Definitions
of Wave Distortion
or
Ripple
295
296
318
323
325
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X
CONTENTS
8-6 Means of Reducing
Harmonics
332
8-7 Telephone
Interference
333
8-8 Harmonic
Filters
343
Problems
38 6
Bibliography
386
9. GROUND RETURN
39 1
9-1
Advantages
and
Problems
391
9-2
The Current
Field in
the
Earth Near an Electrode
393
9-3
The
Current Field
between
the
Electrodes
417
9-4
The Natural
Current
Field
in
the
Earth
419
9-5
Compatability
with
Other
Services
423
9-6 Design of
Electrodes
General
443
9-7
Design
of Land
Electrodes
445
9-8
Design of
Sea
and
Shore
Electrodes
465
Problems
476
Bibliography
478
APPENDICES
484
A.
Effective
Value
of
Alternating
Current
of
a
Six-pulse
C onverter 484
B.
Fundamental
Current,
Power,
and
Reactive Power of a
Six-pulse
Converter
490
C.
Inclusion
of
Direct
Voltage
Drops Du e
to
Resistance and
Arcs
in
Converter
Equations
494
INDEX
496
TENTATIVE
CONTENTS
OF
VOLUME
II
10. OVERHEAD LINES
i
11.
DC CABLES
12 .
FORCED
COMMUTATION
13. OPERATION OF
A
DC
LINK AS PART OF AN
AC
SYSTEM
14.
HIGH-POWER VALVES
15 .
CONVERTER
TRANSFORMERS
AND
REACTORS
16.
RADIO
INTERFERENCE
17.
ASYNCHRONOUS TIES
18 .
MODELS AND
SIMULATION
;
i.'
xi
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ABBREVIATIONS
A
ampere
ac
alternating-current
A.C.S.R.
aluminum
cable, steel reinforced
A.E.G.
Allgemeine Elektricitatsgesellschaft
A.G.
Aktiengesellschaft
Ah
ampere-hour
Amer.
Power
Conf.
Proc. American
Power
Conference
Proceedings (Illinois
Institute
of
Technology,
Chicago)
ASEA
Allmanna
Svenska
Elektriska
Aktiebolaget,
Sweden
Assn. Association
AWG
American
Wire
Gage
B.E.
&
A.I.R.A.
British
Electrical
an d Allied
Industries Re
search Association
(later
known
as E.R.A.)
B.I.C.C.
British Insulated Callenders Cables
B.I.L. basic
insulation level
BP A
Bonneville Power
Administration
(U.S.
Dep't.
of the
Interior,Portland, Oregon)
B.T.S.
Bell Telephone System
Bull.
Bulletin
C
coulomb,
Celsius (temperature scale,
formerly
Centigrade)
CAB consequential
arcback
cal/g
calorie
per
gram
CAT
consequential
arcthrough
C.C.
constant
current
C.C.I.F. Comite
Consultatif
International
T616phonique
(International Consultative
Committee on
Tele
phony).
Xlll
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XIV
ABBREVIATIONS
C.CXT.
Comite
Consultatif International
T61egraphique
(International
Consultative Committee
on Tele
graphy)
C.C.I.T.T.
Comite
Consultatif
International
T61ephonique
et
T61egraphique
(International
Consultative
Committee
on Telephony
and
Telegraphy),
Geneva,
Switzerland
C.E.A.
constant
extinction angle
C.E.G.B.
Central
Electricity
Generating
Board, Great
Britain
C.G.E.
Compagnie
Generale
d'Electricite,
France
C.I.G.R.E.
Conference
Internationale des
Grands
Reseaux
Electriques
a
Haute Tension (International
Conference
on
Large High-Voltage
Electrical
Systems), Paris
cm
centimetre
Conf. Conference
const.
constant
.
cos
cosine
cosh hyperbolic
cosine
cot
cotangent
coth
hyperbolic
cotangent
CP
Conference
Paper
(A.I.E.E.
or
I.E.E.E.)
csc
cosecant
dB
decibel
dc
direct-current
deg. degree (of
angle)
Disc.
discussion
e
free
electron
E.E.I.
Edison
Electric
Institute,
New
York
eh v
extra
high
voltage
Elec.
Electrical
elec.
deg.
electrical
degree
Elec.
Eng.
Electrical
Engineering,
formerly
published
by
the
A.I.E.E.
Elec.
World
Electrical
World
ABBREVIATIONS
XV
Elek.
Stantsii
EM F
Engg.
Eq.
Eqs.
E.R.A.
ETZ
E.u.M.
exp
F
Fe
Fe+
+
Fe(OH)2
ft
Gen.
Elec.
Rev.
GW
H
H
H+
H2
h
HV
Hz
ibid.
Id.
I.E.C.
I.E.E.
I.E.E.E.
J
Jour.
K
kA
kg
Elektricheskie
Stantsii
(Electric
Powerplants),
U.S.S.R.
electromotive force
Engineering
equation
equations
Electrical
Research
Association,
Great
Britain
Elektrotechnische
Zeitschrift.
Elektrotechnik und
Maschinenbau
(Vienna)
exponential function
farad
iron
atom
ferric
ion
ferric
hydroxide
feet
General Electric Review
gigawatt
henry
hydrogen
atom
univalent
positive
hydrogen
ion
hydrogen
molecule
hour
high-voltage
hertz
Latin for
in
the
same place
Island
International
Electrotechnical
Commission
Institutionof Electrical
Engineers
(London)
Institute of Electrical
and
Electronic
Engineers
(New
York, U.S.A.),
founded in
January,
1964,
by
merger of the A.I.E.E.
and the
I.R.E.
joule
Journal
Kelvin
(temperature
scale)
kiloampere
kilogram
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Xvi
ABBREVIATIONS
kHz
km
kV
kVA
kvar
kW
k$
lb
lbf/in2
LC
In
log
mA
MCM
mH
MHD
MHz
mi
mm
MMF
ms
mV
MVA
Mvar
MW
N
nF
N.I.I.P.T.
No.
NW
N.Z.
OH
Ont.
kilohertz
kilometre
kilovolt
kilovolt-ampere
kilovar
kilowatt
thousands
of
dollars
pound
pounds
force
per
square
inch
inductance-capacitance
natural
logarithm
common
logarithm
milliampere
thousands
of circular-
mils
millihenry
magnetohydrodynamic(s)
megahertz
mile
millimetre
magnetomotive
force
millisecond
millivolt
megavolt-ampere
megavar
megawatt
newton
nanofarad
Nauchno-Izsledovatel'skii
Institut
Postoyannovo
Toka,
Izvestiya
(Proceedings
of the Direct
Current
Research
Institute),
Leningrad.
number
northwest
New
Zealand
negative
hydroxyl
ion
Ontario
PA.
&S.
P.I.V.
Proc.
Publ.
PVC
rad
rad/s
Ref.
Rev.
RLC
rms
SC R
sec
S.E.V.
S.I.L.
sin
sinh
SW
tan
T.H.F.F.
TIF
Trans.
Trans,
and Dist.
U.S.
U.S.A.
U.S.S.R.
V
vs
W
w.r.t.
yd
yr
pP
ABBREVIATIONS
XVII
Power
Apparatus
and Systems
peak
inverse
voltage
Proceedings
publication
polyvinyl
chloride
radian
radian
per
second
reference
Review
resistance-inductance-capacitance
root-mean-square
silicon
controlled
rectifier
second (time),
secant
Schweizerischer
Elektrotechnischer Verein,
also
known as Association Suisse
des Electriciens
(Zurich)
surge-impedance
loading
sine
hyperbolic
sine
Southwest
tangent
telephone
harmonic
form factor
telephone influence
factor
Transactions
Transmission
and
Distribution
United States
United States
of
America
Union of Soviet
Socialist Republics
volt
versus
watt
with
respect
to
yard
year
microfarad
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xviii
ABBREVIATIONS
flS
pY
a
Q
-m
microsecond
microvolt
ohm
ohm-metre
(unit
of
resistivity)
DIRECT CURRENT TRANSMISSION
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I
General
Aspects
of
DC
Transmission
and
Comparison
of
it
with
AC
Transmission
1-1
HISTORICAL
SKETCH81
7181
Early
Discoveries
and Applications
Both electrical
science
and
the
practical
applications of
electricity
began
with
direct
current.
Alternating
current
came
later.
The basic
discoveries
of Galvani, Volta,
Oersted,
Ohm,
and Ampere
per
tained
to
direct
current. The first
widespread
practical application
was
dc
telegraphy powered
by electrochemical
batteries and
using ground-return
circuits.
Electric
lighting
an d
power
also began
with direct
current
powered
by
dynamos.
First
came
carbon arc iamps
operated
in
series
at constant
current
a nd fed from
series-wound
generators.
Later
came
carbon-filament
incan
descent
lamps operated
in
parallel at constant
voltage an d
supplied
with
current
from
shunt-wound
generators.
Th e
first
electric
centralstation
in
the
world,
on Pearl
Street, in Ne w
York,
was
built
by
Thomas
A.
Edison
an d
began
operation
in
1882. It
supplied
direct current at
110 V
through
underground
tubular
mains
to
an
area
roughly
1
mi(1.6km)
in
radius.
It
hadEdison
bipolar
dc
generators
driven
by
steam
engines.
Within
a few
years
similar
stations were
in
operation
in
the
central districts of
most
large
cities
throughout
the
world.
In
view of the initial supremacy of direct current
it
is interesting to see
wh y
it wa s
almost
completely
superseded by alternating
current and
wh y direct
current is
again
being
used
for
some high-voltage
transmission lines.
*
Superior numerals and,
in
some
chapters,
including
this
one,
superior
letters
alone or
followed
by numerals
refer
to
items
or.
to
groups
of items
in
the
bibliography at the
end
of the
chapter.
1
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2
GENERAL
ASPECTS
OF
DC
TRANSMISSION
Later
Ascendancy of
Alternating
Current
-
The
advent
of
the
transformer,
polyphase
circuits,
and the
induction motor
in
the 1880s
and
1890s
led
to
ac electric
power
systems.
The
transformer,
simple,
rugged,
an d
efficient,
made
possible the use of
different voltage
levels
for generation,
transmission,
distribution,
an d
use.
In
particular,
it
made long-distance,
high-voltage power transmission
possible.
Th e
exploitation of
water power,
usually
available
at
sites distant from
major
load
centers, gave
impetus
to
such
transmission.
i
The
induction
motor,
especially
the polyphase
induction
motor,
is also
simple, rugged,
and
cheap
and
serves
the
majority
of
industrial
and
resi-
f
dential
purposes. The commutators
of
dc
motors
andgenerators,
in
addition
j
to
requiring
maintenance,
impose
limitations on
the
voltage,
speed, and size
of
these machines.
Th e
voltage
per ba r of the
commutator
should not exceed
about
22
V
lest
excessive
sparking occur.
Thus a
high voltage
pe r
commutator
requires
many
bars, resulting in
a large
diameter.
A
large diameter
requires
a low
speed in
order that the
commutator and
windings
may withstand
the
centrifugal
force. And a
low-speed
machine is heavier
an d
more
expensive
f
than a high-speed
machine
of equal rating.
The
advent of
steam turbines,
|
which
are best at high
speed, gave a
great advantage
to ac
generators. j.
When
ac
systems
first
appeared,
there
were
heated
arguments
between the
proponents of
dc
an d
ac
systems.
Advocates
of
dc branded ac
dangerous
because of
the
high voltages
used.
As
a result
of
their
advantages, however,
ac
electric
power systems
became almost universal.
Power was
generated,
transmitted,
distributed,
and used
as
alternating
current.
If
direct current
was
needed for
some
particular
purpose,
such
as
adjustable-speed
motor
drives
or electrolytic
processes,
alternating
current wa s
converted to
direct
current
locally
by
motor-generator sets
or
synchronous
converters or, later,
by
{
mercury-arc
rectifiers.
The las t
vestiges of
dc
distribution were
the
low-voltage
networks
in
the
f
centers
of
large
cities and
electric
traction
(streetcar,
trolley
bus,
rapid
transit,
j
interurban and
suburban railways,
and
some
long
tunnels or mountainous
sections
of
main-line
railways).
Finally,
however,
low-voltage
ac
networks
replaced
low-voltage
dc
networks,
diesel locomotives
replaced
steam
loco
motives and
many electric
locomotives,
and gasoline
or
diesel
buses
replaced
j
most of
the
streetcars
an d
interurban
lines.
Some dc
rapid-transit
systems still
remain.
The
victory
of
alternating
current over direct
current,
however, was
almost
complete.
Status
of
DC Transmission
During the Ascendancy of
AC
Transmission
Despite
the
general
acceptance
of
ac
transmission,
some
engineers
never
forgot the
obvious advantages
of
dc
transmission
(discussed
in Section
1-4).
1-1
HISTORICAL SKETCH
3
They
proposed,
however,
not
to replace ac but
to
supplement it with dc.
Specifically,
they
would
superpose
a dc transmission
link
on
an
ac system or
interconnect
two ac
systems
by a
dc transmission
tie
line. Generation, use,
and even
most
t ransmission and
distribution, would remain
by
ac.
Such
a dc
transmission
scheme
requires
that ac be converted
to
dc at the
sending end
of the dc
link
and
that dc
be converted
to ac at
the
receiving
end.
The
feasibility
and
advantageousness
of
the scheme
depended
on the develop
ment
of
suitable
converters
for
the required
high
vo ltage and
power.
The
development of suitable converters is considered
shortly.
First, however, let
us turn aside to
describe
the Thury dc
system.
The
Thury
System318
A
system
of
hv
dc transmission
designed by
a
French engineer,
Ren6
Thury,
came
into
use at
a
time
when
ac
systems were
in their infancy,
and
it
persisted
well
into
the
era
of
ac
predominance. This system is
interesting
both
as an engineering
achievement
and because of
certain similarities
to modern
hv dc
systems.
At the
sending
end of the transmission
line
a number of
series-wound dc
generators,
driven
by
prime
movers,
were connected in
series
to
generate
the required high voltage, an d
at the
receiving
end, a
com
parable
number
of
series-wound dc motors, connected in series, drove
low-
voltage dc or ac
generators.
Th e system
operated
at
constant current.
The
voltage of
each
machine
in
the hv
series circuit wa s
regulated by
shifting the
brushes.
Since
the series circuit was
normally grounded
at
only
one
point, many of
the machine
windings
had a high potential
with respect
to ground. It was not
feasible
to
provide
insulation between
windings
and
frame for
such
voltages;
instead, the frames were insulated
from
ground by setting
them
in a
floor
of
asphalt
over
asphalt
concrete, and
were
insulated from
the driving
or
driven
machines by insulated
couplings.
j
Switching an d
instrumentation
were very simple. Each
machine
was
pro
vided with a
short-circuiting
switch.
A
machine
was
taken
out of service
by
reducing
its
terminal
voltage
to
zero
and then short-circuiting it.
It
wa s
brought
into
service
by
the reverse of this
procedure.
An
ammeter
and a volt
meter were
the only instruments
required.
From
1880
to 1911
at least 19 Thury systems
were installed in Europe,
principally
for the use of water power. Th e most
important
of
these
was
that
from
Moutiers,
.in the
French Alps,
to
Lyons,33
installed in 1906 with a
route
length of
112
mi
(180
km)
of
which 2.8 mi (4.5 km) were
in
under
ground
cable,
the remainder
being
open-wire
line.
Initially, its rated
power
of
4.3 MW
was
transmitted at 57.6
kV,
75
A.
This line
was
built as a
reinforce
ment of
an
existing
ac
system and was
integrated
with
it. The
Moutiers plant
-
7/24/2019 Direct Current Transmission
12/264
4
GENERAL ASPECTS OF DC
TRANSMISSION
had
four
water
turbines,
each
drivingfour
generators
of
3.6 kV
each. At Lyons
the greater
part
of
the power
received
by
h v
direct
current was converted to
alternating
current and the remainder
to 600
V
dc for the
street railway.
Th e
over-all
efficiency was
70.5%,
which wa s considered
satisfactory for
a hydro
electric system.
In
1911 a
second
hydroelectric
plant at La
Bridoire, situated
about
halfway
along the
line
and
rated at 6 MW,
was
added (in
series). The
line
current was
then
doubled
(to
150
A).
In
1912
a
third
hydro
plant,
located
at
Bozel, 7
mi
(11
km)
beyond Moutiers,
an d
rated at 9 MW, was
added,
raising
the total
generating
capability
on the
line
to 19.3 MW.
Th e
maximum
circuit
voltage
became
125
kV and
the
route
length
140 mi (225 km). Operation
of the
line
continued
until
1937,
when
it
was dismantled.
Thury himself died in 1938.
The
Thury
system
performed
reliably
in
spite
of the
large
number of com
mutators
in
series. The
limitations
of dc machines,
already
mentioned,
how
ever,
made
it
unsuitable to the
larger amounts
of
power
that had
come to
be
required.
Further
development of
hv dc
transmission
required
better con
verters than
motor-generator
sets.
Development
of
a
Practical Converter
A
converter
is
basically nothing
more than
an
assemblage
of
controlled
switches.
Th e commutator
of
a dc
motor,
generator,
or synchronous con
verter
is
such
a
device.
The
vibrating reed
is
an
even
simpler
switching device,
used
for stepping
up
direct voltage
from a storage
battery to a value suitable
to plate
supply
in
automobile
radios or as a chopper in stabilized dc
ampli
fiers.
In these two
applications
the
input
and
output are dc , with ac
in the
intermediate
circuit,
which
is just the opposi te of
dc
transmission
inter
connecting
two ac systems.
Two
of
the more
serious
attempts
to
develop
a switching converter suitable
to
hv
dc transmission
are the transverter
and
the
Marx atmospheric-arc
converter.
The
transverter,
patented
in 1920
by
two British
enineers, W.
E. Highfield
and
J.
E.
Calverley,
consisted
essentially
of
polyphase transformers
com-
mutated
by
synchronously
rotating brush
gear.
It
performed
the
three basic
operations
of
voltage transformation, phase
multiplication,
and commuta
tion
an d
could
be
used
either
as
a
rectifier
or as
an inverter. Since the com
mutators
were
stationary
and only
the
brush
gear
rotated, the
problem
of
centrifugal force
was mitigated.
Several
experimental
transverters were
built,
the
largest of which
was
rated
at
2
MW,
20
A, 100
kV on
the
dc
side,
but none
has
been
used
commercially.
The
atm&spheric
arc
converter,
devised by
E.
Marx
of
Braunschweig
in
1932,
is a
switching
device
in
which an arc between
two
like
water-cooled
1-1
HISTORICAL
SKETCH 5
main
electrodes
is
ignited
by a high-frequency
spark
getween
auxiliary
electrodes
in
the path of the
main arc and
is
extinguished
after
a
current
zero
by
a
blast of air
or
mixed
gases that continually plays
on the
arc path. At
one
time such
converters
could
handle more power
(40 MW)
than
any
other
converter then available.
Th e life
of
the
electrodes,
however,
was
short,
and
the voltage
drop
across
the
arc
was
high (500
V).
The loss
in the arc,
together
with the
power
required for
ignition,
air blast,
and cooling,
amounted
to
2.5
to
3%
of
the
transmitted
power
at
each
terminal. This
is
considerably
greater
than
the
corresponding
loss
(about 0.3%)
in mercury-arc
converters.
Valves
Th e
synchronously
controlled
switches heretofore
described for
use as
converters can conduct in either
direction, and
the
actual
direction
of
current
depends on the
controlled
instants of
closing
and
opening an d
on the
emfs in the circuit.
Generally,
unidirectional
conduction is
desired.
Devices
having inherent
unidirectional
conduction
are
herein called
valves.
Among
such
devices
are
vacuum
and vapor or
gas-filled tubes
having
thermionic
cathodes,
the
mercury-vapor tube with
mercury-pool
cathode,
an d
various
solid-state devices.
In
their
simplest
form,
as
diodes, they
can
be
used as
rectifiers but
not as inverters.
Th e mercury-arc
rectifier
with
pool
cathode is the most
suitable for
handling large
currents. It
was
invented
by
Peter Cooper Hewitt
about
1903
and initially was
made with a
glass
envelope.
The
steel tank
appeared
about
1908 to
1910.
In
order
for
a valve to b e u se d as
an
inverter,
it
must have a
control
electrode
that
can prevent the
valve
from conducting,
although the
anode is
positive
with
respect
to
the
cathode.
Such
an
electrode
(the
grid)
was added
to the vacuum
tube
(Fleming
valve)
by
de
Forest
in
1906. Th e grid
was added
to the mercury-vapor valve
in
1928. It was then
applied
to the
hot-cathode
glass-envelope
valve,
and
the
resulting
triode
was
called
the
thyratron.
Later
some
thyratrons were
made with
teel
jackets.
Control
grids
were
added
to
mercury-arc
valves
with
pool
cathodes
about 1930. Although
the grid in
the
vacuum
tube
can
start,
stop,
and
modulate
the
current
through
the
tube,
the
grid
of a
mercury-arc
valve can only
prevent
conduction
from starting.
After
it
has
started
the control
grid
can neither
stop
the
current nor
control its
magnitude.
Conduction does
not
cease
until
the
anode becomes negative
,
with
respect to the cathode.
Actually the
first
control
element used
in
a
mercury-arc
valve
with pool
cathode
was not a
grid
but
an
igniter,
introduced
in
1923. Th e
resulting valve
is called
an
ignitron. The igniter
is a
rod
theend of
which dips into
the mercury
pool. When
current
from an
auxiliary
source is sent
through
the igniter,
an
arc
is
started.
The igniter, like the
grid,
cannot
stop
conduction.
-
7/24/2019 Direct Current Transmission
13/264
6
GENERAL
ASPECTS
OF
DC
TRANSMISSION
Present-day mercury-arc
valves for
high-voltage
transmission, known as
d
excitrons,
have,
in addition
to
the
anode
an d
the
mercury-pool cathode,
an
ignition
electrode
for
starting the
arc,
one
or more
excitation
electrodes for
maintaining
the
arc,
an d
a
control grid
that prevents
the
arc from
reaching
the
anode until
it is
desired
that
the
valve
begin to conduct.
There are
also
j
several
grading electrodes
placed
between the
control
grid
and
the anode
for
obtaining
a more
uniform potential
gradient
than
would
otherwise
exist.
Th e
grading
electrodes
are
kept
at
the
desired
potentials
by connecting
them
to
taps
on an
external
resistance-capacitance potential
divider
the
ends
of
which
j
are
connected
to the
anode and
control grid.
This
system
of
grading
elec-
trodes,
invented
by
U.
Lamm
in
1939,
has
considerably
increased
the
peak
inverse
voltage that
the valves can withstand.
Valves
for
hv dc
transmission
are
invariably
of
single-phase
construction,
in
contrast to the
polyphase
valves
with
mercury-pool
cathode
formerly
used
extensively
in
low-voltage
rectifiers
for
industrial
and
railway
application.
Th e
development
of
valves
for hv dc transmission
has been
carried
out
since
World
War
II
principally
by
engineers
in
the
U.S.S.R.
and
by
the
Swedish
firm
of
Allmanna
Svenska
Elektriska
Aktiebolaget (ASEA), with
v
which Lamm
is connected.
A
noteworthy feature
of ASEA
valves
is the use of
several,
usually
four, anodes
in
multiple on
single-phase
valves.
Th e current
ratings are 200
to
300 A
per
anode.
Russian engineers
have
concentrated on
single-anode
valves,
which so
fa r appear
to have
been
less
successful
than
the
ASEA
valves.
About
1960,
control
electrodes were added to silicon
diodes,
giving
silicon-
controlled
rectifiers
(SCRs), also
called
thyristors.
At
present
these
are
not
capable
of
handling
the
highest
voltages
an d
powers
required for
hv
dc
transmission.
Their
ratings
have
increased,
however,
with
surprising
rapidity,
.
and it seems certain
that
such valves
will soon
replace
mercury-arc
valves
in
hv
dc
use.
Experimental
DC
Transmission
Projects
and First Commercial
Lines
Th e
initiative
in
exploring the use of
mercury-arcvalves
for dc transmission
was
taken
by
the
General
Electric Company.
After
two
smaller
experi
ments81,2
they
proceeded
in
December
1936
to use
direct current
on a
17-mi
(27-km) line
between
the
Mechanicville
hydroelectric plant
of
the
Ne w
York
Power
&
Light Corporation
and
the Genera l
Electric factory
in
Schenectady.815
The
line carried
5.25
MW
at 30
kV,
175
A.
The
converter
at
each
end
of the
line
had 12 hot-cathode
glass-envelope
thyratrons
in
6
series
pairs. The
ac input
at Mechanicville was at a frequency
of 40 Hz,
an d
the
output at
Schenectady
was
at 60 Hz.
Thus was demonstrated
a feature of
dc
transmission
that has been
important
in
several
subsequent installations:
frequency
conversion.
1-1
HISTORICAL SKETCH
7
The
line initially
operated
at constant
current,
the conversions from con
stant
alternating
voltage
to
constant current
and
vice versa
being
made by an
LC
bridge
circuit
called the
monocyclic
square.
Constant-current
operation
was
chosen
because
the
hot-cathode
tubes
then
used
could
not
withstand the
high short-circuit currents
expected
to occur on a
constant-voltage
system.
After
the
more
rugged
steel-envelope
mercury-pool
ignitron
became
available,
however,
the
line was
converted
in
1940 to constant-voltage
operation.
The
circuitry
then
used was basically the
same
as that
of
modern
dc transmission
systems,
fault
currents being limitedbycontrol
of valve ignition.Th e operation
of the line
was
discontinued
in
1945
in
the
belief that nothing more would be
learned
by
continuing it.
Perhaps
an
additional belief
was
that there was no
future
in
dc
transmission.
Meanwhile, two
25/60-Hz
frequency changers using
controlled
mercury-
arc valves
were installed
in
steel
mills in the
United States
in
1943. The
larger
of
these,
rated at 20
MW,
was installed
at the
Edgar Thompson
plant
of the
Carnegie-Illinois
Steel Company near Pittsburgh.
The United
States
wa s
inactive
in
the field of dc
transmission,
however,
for
nearly 20
years.
A
demonstration of dc transmission
using grid-control led
steel-tank
mercury-arc conversion
wa s
given
at
Zurich, Switzerland,
in
1939,
at
the
Fifth
Swiss National
Exhibition.84,5
Power
of
0.5 MW
at 50
kV, 10 A,
was
sent 19
mi
(30
km) from
Wettingen
power
plant
near
Baden
to
Zurich
over
a circuit of
one
conductor,
partly overhead
an d partly
in
underground cable,
with earth
return. In
1946,
Brown-Boveri
discontinued
their work on
hv
dc
transmission.
Two hv
dc
experiments
were conducted in
Germany
during
World
War
II
at the
instance of the German
Secretariat
for
Aviation.86,19
A
400-kV
three-phase
line
from
the Alps to
the
Ruhr
had
already
been planned,
bu t
the
Secretariat
intervened
in
favor of
a hv
dc
cable
line,
which,
it
felt,
would be
less
vulnerable to air-raid
damage.
Th e
Siemens-Schuckertwerke A.G. began
experiments
in preparation
for
such
a
line.
They
transmitted
4
MW
at
1
10
kV
a
distance
of 3
mi
(5 km)
over
an
existing
line from a
station in
the
Charlottenburg
district
of
Berlin
to one
in
the Moabit district.86
A second,
larger experiment
was
to
be the
transmission
of
60
MW
by
means
of
a
70-mi
(110-km)
400-kV dc cable from the
Elbe
(near
Dessau)
to Marienfelde (near
Berlin).86,9
This
experiment
was to be
conducted
jointly by
Siemens and the
A.E.G.
The fortunes of
war
prevented completion of the project,
and
in 1945
such
plant
an d
pertinent
documents as
survived were taken to the U.S.S.R.
as
reparations.
In Sweden, where
the
principal new
hydroelectric
sites
are
in the north
and
the
principal
loads are in the
south, hv
transmission is
required
;
and,
because
of
the
development
of
valves
by
the
Swedish
firm
of
ASEA,
interest was
aroused in
the
possibility
of
a hv
dc
transmission
system
as
an alternative to
-
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14/264
8
GENERAL
ASPECTS
OF
DC
TRANSMISSION
ac. An experimental
transmission
between
Mellerud
an d
Trollhattan (36 mi)
began
operation
in
1944. It
aided further
development
of
valves by
permitting
them
to
be
tested under
service
conditions.
Th e
Swedish
State Power
Board
decided
to
use alternating
-current
for the
north-to-south transmission
already
mentioned.
The resul ts of
the
Mellerud-Trollhattan
transmission,
however,
encouraged
the
Board
to
proceed
with
hv
dc
transmission by sub
marine
cable from
the Swedish
mainland to
the island
of
Gotland,
96
km
(60
mi)
offshore;0
This
system,
built
by
ASEA,
began
service
in
1954
an d
may be
considered
the
first commercial
hv
dc
transmission
system.
The
line
transmits
20
MW
at 100
kV
through
a
single-conductor
cable,
with
return
path
through the
sea
an d
earth.
Each
converter
has
two
valve
groups rated
50 kV,
200
A, 10
MW,
the
groups
being
in
series on the
dc
side.
Each valve
has
two
anodes
working
in
parallel.
Building
the dc
link was
judged
more
economical
than
constructing
additional
thermal
power
plants
on
the
island.
The
distance
is
far
too
great
for ac
cable
transmission.
Power
flow is
normally
from
the
mainland to
Gotland
bu t
is
sometimes in
the
opposite
direction.
Much
of
the
time
when
power is
delivered to
Gotland,
there
are
no
generators in operation
there,
the only
synchronous
machine
being
a
condenser.
Power is adjusted
automatically
to
maintain rated
frequency (50
Hz)
in
Gotland.
Th e
link is
still
in operation
(1970)
and
has
a
good performance
record.
One
of the
mercury-arc
valves
was replaced
by
an
air-cooled thyristor
assem
bly,
which
also has
performed
well. Plans
have been
announced
for
doubling
the
voltage
and power
on the
existing
cable by the
addition
of
a
new
thyristor
valve
group
to each
terminal, thereby
doubling
the
voltage.
In
the
U.S.S.R.,
where
even
greater
distances
than in
Sweden
separate
the
potential
hydroelectric
sites
from the
principal
industrial
load areas,
the
use
of
hv
dc
transmission
was
considered
necessary,8
an d
an
extensive
program
of
research
an d
development
was
undertaken,begun
as
a
part
of
the
5-yr plan
of
industrial
development for
1946
to
1950.
An
experimental
line
between
Moscow
an d
Kashira
(112
km
or
60
mi,
30
MW,
+
100
kV)
began
operation
in December
i95o.B10'11,13'16
It wa s
basically
an
underground
cable line,
but
at
times
sections of
overhead
line
were
put
into the
circuit.
Both
bipolar
metallic
operation and
monopolar,
ground-return
operation
were
tried.
Practical ground
electrodes
were
developed,
and
various
kinds
of
valves
and
converter
control
were
tested.
A
Direct
Current
Institute312,14
was
established
in Leningrad,
which
since
1957
has published
approximately
one
volume
pe r
year
of
articles on
its
researches/3
A
full-scale 474-km
(294-mi)
overhead
line
between
a
hydroelectric
plant
at
Volgograd,
formerly
called
Stalingrad,
and the
Donets
Basin was energized
1-2
CONSTITUTION
OF EHV AC AND
DC
LINKS 9
at
reduced
voltage and
power
in
1962
and,
beginning in
1965,
was operated
at
its
full
rating of +400
kV,
900
A,
720 MW.1
Other dc
lines of
lengths of
2000
to 2500
km
and
voltage of +750
kV are
planned/6
1-2 CONSTITUTION
OF
EHV
AC
AND
DC LINKS
eh v transmission
links,
superposed
on
a
lower-voltage
ac
network,
or
inter
connecting
two such networks,
or
connecting
distant
generating plants
to
an
ac network,
are
compared
as
to
their
principal
components
an d
the
arrange
ments
thereof,
according
to
whether
the
line
operates
on ac
or dc.
The
phrase
transmission
link denotes
the transmission
line proper
together
with its
terminal and
auxiliary
equipment.
Figure
1a
shows a
single-circuit
three-phase
ac
line.
In
general, such
a
line
in
the
categories
already
mentioned,
one
which
might be
competitive with
a
dc
link,
requires
transformers
at both
ends
step-up
transformers
at
the
sending end
and
step-down
transformers
at the
receiving
end
although
in
some
cases
they can
be
omitted
at on e
or both ends.
If
the
transformers
are
operated as
an
integral
part
of
the
link,
only
low-voltage circuit
breakers
are
required.
Ac
system
ystem
Ac
system
c
system
nverter
Dc line
Ac
system
c
system
Ac
system
Fig.
1.
Constitution
of
ac
and
dc eh v
links shown
by single-line
diagrams.
O-J
-o
c
Ac
system
-
7/24/2019 Direct Current Transmission
15/264
10
GENERAL
ASPECTS
OF
DC
TRANSMISSION
Most
long
overhead
ac
lines require series
compensation of
part
of the
inductive
reactance. In
the
figure,
one bank
of
series
capacitors
for this
pur
pose is
shown
at the
middle
of the line.
Three-phase lines
cannot be operated, except
for
a
very
short
time
(less
than 1
sec)
with
one
or
two
conductors
open, because such
operation
causes
unbalanced
voltages in the ac
system
and
interference in
parallel
telephone
lines.
Therefore
three-pole switching
is always used
to
clear permanent
faults,
although
such
a
fault
ma y
involve
only
one
conductor. This
being
so,
two
parallel
three-phase circuits
are
required
for
reliable
transmission
(see
Fig.
16).
Long
two-circuit
ac
links are
usually
sectionalized
by means of
intermediate switching
stations
for
several
reasons.
Among
these
are
(a )
limiting
the
decrease
in
stability
power
limit attributable to
switching
out
one
circuit
to
clear
a fault
or
for line maintenance,
(b ) limiting
the
overvoltage
when
a line is
energized
from
one
end, (c) providing
a
place for the
connection
of
grounding
transformers
to
limit
the overvoltages
of
the
unfaulted phases
with
respect
to
ground
when
one
phase
is faulted
to ground,
an d
(d)
for con
nection of intermediate
loads
or generation. Intermediate
generation
raises
the
stability limit
of the
link.
On many long
eh v
lines,
shunt
reactors are
required
for
limiting
the voltage,
especially
at
light
loads, but
they
may be
required
even
at full load. These reactors are
usually
placed
at intermediate
switching
stations
an d are so indicated in Figure
16.
A
representative
single-circuit
dc
link
is shown
in
Figure
lc. Th e line
itself
usually
has two
conductors,
although some lines
have
only
one,
the return
path
being
in the
earth
or
seawater
or
both. At
both
ends
of the l ines
are
converters,
the
components
of
which are
transformers
an d
groups
of mercury-
arc
valves.
The converter
at
the sending end
is called
a
rectifier
,
an d
that
at
the rece iv ing end
an inverter. Either
converter,
however, can
function as
rectifier
or
inverter, permitting
power
to
be
transmitted in
either
direction.
The
ac line,
of
course, also
has this
reversibility.
Circuit
breakers
are
installed
only
on the ac
sides
of the converters. These
breakers are not used
for clearing
faults
on
the
dc line
or
most
misoperations
of
the
valves, for these
faults
can be cleared
more
rapidly by grid
control of
the valves. Th e breakers
are
required,
however,
for
clearing
faults
in the
transformers or for taking
the whole
dc
link
out
of
service.
Harmonic
f i lters and
shunt
capacitors for
supplying
reactive
power
to
the
converters
are
connected
to
the
ac
sides of the
converters.
Large
inductances
called
dc
smoothing reactors
are
connected
in
series
with
each
pole
of the
dc
line.
Some
writers
claim
that a
two-conductor
dc line
provides
the same
re
liability as
a
two-circuit three-phase line
having six
line
conductors, for
either
conductor
of the dc
line
can be used with
ground
return
continuously or
for
limited
periods,
say,
a few
days
per
year.
1-3
KINDS OF DC
LINKS
11
If
higher
reliability
is
required
of
a dc
line than
that
provided
by
two con
ductors, three or
four
conductors may
be
provided.
On e
pole
of
a
four-
conductor
line
is shown
in Figure Id, with
two converters per
terminal.
The
bus-tie
switches
1
are normally
open.
If a permanent
fault
occurred on
the
lower
conductor,
the converters
connected to it would be controlled
so
as
to
bring
the voltage
an d
current
on
it
to
zero. Then switches
3
would
be opened,
isolating
the faulted
line.
Next the
converter voltages
would
be raised
to
equality
with
those of
the
respective adjacent
converters
,
after which
switches
1
would be
closed.
The capability of
all
converters
would
then be
usable,
and
the
power
normally
carried by two conductors
would
then
be carried
by
one.
The line loss
would
be
four
times
its normal
value,
somewhat
diminishing
the
delivered power.
Th e
whole
switching
operation would
t ake abou t
0.3
sec,
a
time as short
as that
required
for rapid
reclosure on
an ac line. Each pole
would
be switched
independently
of the other.
Comparison
of
the ac
and
dc links
shows that
(a)
the
dc
line
proper is
simpler,
having one
or
two
conductors
instead
of
three, but that
(b),
on
the
other
hand,
the
terminal
equipment
is more
complex,
having the groups
of
valves
and
some
auxiliary
equipment
that the ac line does not
need.
1-3
KINDS
OF
DC
LINKS
Direct-current
links are
classified
as
shown
in
Figure
2.
)
The monopolar
link has
one
conductor,
usually
of
negative polarity,
and
ground
or sea
return.
The
bipolar
link
has
two
conductors
one positive, the o ther
negative.
Each
ternfig||||has
two
converters
of
equal
rated
voltages
in series
on the
dc
side.
Thioints
(junctions between converters) are
grounded at
one or
both
ends,
fillllh
neutrals
are
grounded, the two poles can
operate
inde
pendently. Normally
they
operate
at
equal
current
;
then
there is
no
ground
current.
In
the
event
of a
fault on
one
conductor,
the
other
conductor
with
ground return
can
carry
up
to
half
of the rated
load.
Th e rated
voltage
of
a
bipolar
link
is
expressed
as
100
kV,
for
example,
pronounced
plus
an d
minus
100
kV.
Th e homopolar link
has
two or more
conductors
all having
the
same polar
ity,usually negative,
and always
operates
with
ground
return.
In
the event
of
a
fault on
one
conductor,
the entire converter
is
available
for
connection
to
the
remaining
conductor
or
conductors,
which,
having
some
overload
capability,
can
carry
more
than half
of the
rated power,
an d
perhaps
the
whole
rated
power,
at the expense
of
increased
line
loss.
In a
bipolar
scheme
reconnection
of the whole converter to one
pole
of the
line
is more
complicated
and is
usually
not
feasible
because of
graded insulation.
In
this
respect
a
homopolar.
line
is
preferable
to a bipolar
line
in
cases
where continual
ground current
is
-
7/24/2019 Direct Current Transmission
16/264
12
GENERAL
ASPECTS OF
DC
TRANSMISSION
Rectifier
Inverter
(c)
Fig. 2.
Kinds of dc
links.
not
deemed
objectionable
(see Chapter 9).
An additional
minor advantage
is
i
the
lower power
loss
due to corona.
Negative
polarity is
preferred
on
over-
j
head
lines
because
of
its smaller radio interference.
>
Cascaded
Groups
In
each
of
these
kinds of
links
there
are
usually several
converters
connected
i
in
parallel
on the ac side
but
in
series on the dc side for
obtaining
the
desired
level
of
direct
voltage from
pole
to ground. Each such converter consists of
a
transformer
bank
and
a
group
of
valves.
1-4
HV
DC PROJECTS
FROM 1954 TO 1970
The successful
operation
of the
Gotland
link
awakened interest in
dc
transmission
in other
countries.
A
l ist of the dc
links in
operation
or
under
construction
in
1970 is
given in
Table 1. These links
are
situated
in nine
j
I
.S
c
a
e
O
I
os
=
X
a
60
c
U
Q
I
1
I
3
o
H
8
u
o
cS
S
-S
kj
o
60
II
>
>
I
CO
p
o
o.
3
O
a
M
O
cO
(U
VI
e
-
w
O
cO
cO
w
-
7/24/2019 Direct Current Transmission
17/264
-
7/24/2019 Direct Current Transmission
18/264
16
GENERAL
ASPECTS
OF
DC
TRANSMISSION
The
valves, manufactured
by
ASEA,
are
rated at 1.2 kA, 125
kY,
an d
have
four
anodes.
Konti-Skan
LinkK
This is an
interconnection
between
Sweden
and
Denmark
an d
thus,
through
previously
existing
ac
connections,between
Germany and
the rest
of
Western
Europe
and
the Scandinavian countries.
It crosses
the
Kattegat by
way of
the island
of
Laeso
an d
has
two
cable
sections
an d
overhead
sections
on the
island
and
at
each
end.
Th e dc
scheme
was
compared
with
an ac scheme
having a
shorter
cable.
The
cost
of
the
two schemes
was approximately equal, bu t
the dc scheme
presented two
advantages
over the
ac:
1
.
T he dc line
provides
an
asynchronous
tie. The
stability
limit
of the
ac
scheme was
estimated as 350
MW;
the
ultimate power
capability
of the dc
link was
500 MW. The need
for expensive
load-frequency
regulation
is
avoided.
2. The
dc
scheme
can be built
in
two
stages,
and
thus almost
half of
the
investment
can be
postponed.
The first
stage
operates
monopolarly
with
one
submarine
cable
and
sea return
at
a
power
capability
of
250
MW.
In
the
second
stage
the line
will
be a bipolar,
metallic
circuit
for
500 MW,
with
sea
return used
only
in
emergencies.
Four-anode, 1.1-kA,
125-kV
valves are
used.
Sakuma
Frequency
Changer
This station
was
put
into
operation
in
1965,
interconnecting
the
50-
an d
60-Hz
systems
of Japan. It can
transmit
300
MW
in either
direction. There
is
no dc
transmission
line, t he dc
circuits
being
confined
to the station. With
minor
exceptions,
the
equipment
and
circuits
are
like thotee
of a transmission
scheme.
The valves
are similar
to those
of the
New
Zealand
an d
Konti-Skan
links.
Sardinian
SchemeL
In
order
to
use
large deposits
of
low-grade
coal on
the
Italian
island
of
Sardinia,
a
thermal
power plant
was built there,
and
a
dc
link
was
built con
necting
it,
by way of
the
French
island
of Corsica, to the Italian
mainland
near
La Spezia.
This
link
consists
mainly of
submarine
cable, with
some
overhead
line
on
Corsica
and at the
ends.
A peculiarity
of this
scheme
is
that
the
line
has two conductors
of the same
polarity,
with
sea
return.
The
polarity
is
negative when
power is
transmitted
from Sardinia to the
mainland,
which
1-4
HV
DC PROJECTS FROM
1954
TO
1970
17
is
the
usual
direction,
although
the opposite
direction
holds
when
the Sardin
ian
plant
is
shut down.
Power flow
is regulated so
as
to
keep
constant
fre
quency
on
the Sardinian ac
system. Th e valves
are
similar to those of
several
other
schemes,
are
rated at
1.0
kA,
100 kV,
an d
have
four
anodes.
Vancouver
Island
Scheme1*
This provides
a
dc connection between the
mainland
of
the Canadian
province of British
Columbia
at Arnott, south
of the
mouth
of
the Fraser
River,
and
Vancouver
Island.
It is
being built in
stages of
78
MW each,
with
an
expected
final
power
of
312
MW.
It
crosses
the
Strait
of
Georgia by
submarine
cable
anpring
Island
by
overhead
line.
The four-anode
valves are rated at
lWO
kV.
This
is the
first
schlBgP
which
a
dc
link
operates
in
parallel
with an
ac
link.
Pacific Northwest-Pacific
Southwest
IntertiesN
Th e
purpose
of this scheme
is to
take
advantage of
seasonal
diversity
in
load
and generation between the
northwest
area,
comprising the
states
of
Washington
an d
Oregon,
and
the
southwest
area,
comprising
southern
California
and
Arizona.
The entire
scheme
includes
two 500-kV ac circuits
with a total
length
of
905 mi
(1450
km)
from the
Columbia River to the
vicinity
of Los
Angeles
and
two
400-kV
bipolar
dc
circuits. The
first dc
circuit
is
from
Celilo
substation near
The
Dalles,
Oregon,
to
Sylmar
sub
station,
near Los
Angeles.
The
second
dc
circuit
is
planned
to be
built from
Celilo
to Mead
substation
near
Hoover
Dam
at Boulder City,
Nevada. The
power
ratings of the
ac lines
are 1000 MW each
an d
those
of
the dc
lines
1440
MW
each.
A third dc
line,
the so-called dc
cross
tie,
from
Sylmar
to
Mead,
about
270 mi
(430
km),
has
been
discussed,but
there
is
no
definite
plan
for building
it.
Each
of
the
two
dc lines exceeds
any
previous
dc
line
in
length
and
in
power rating, although the
rated
voltage
is
equal
to
that
of the Volgograd-
Donbass
line.
The valve
ratings
are
also
greater, being
1.8
kA,
133
kV,
240
MW
pe r
group,
with
six
anodes per valve.
Th e
dc
lines
operate
in
parallel with a
60-Hz ac
system. Because
of
the
great length
of
the
ac lines,
the
stability
of the ac
system
poses
a
considerable
problem,
an d
it
was
necessary to use a high
degree
(average 65%)
of series
compensation. A
permanent
bipolar fault
on
a fully-loaded
dc line is one
of
the
severest disturbances that
the
ac system
must
withstand,
although
the
occurrence of such
a
fault is
believed to be very
infrequent.
-
7/24/2019 Direct Current Transmission
19/264
18
GENERAL ASPECTS
OF DC
TRANSMISSION
Kingsnorth0
The Central
Electricity
Generating
Board
of Great Britain is interested
in
the
use
of
dc
links
for
reinforcing an ac
system
in
areas
of
high
load
density
without
increasing
the
interrupting
duty of
ac circuit
breakers.
A trial installa
tion
of this
kind
is the transmission
of
power
by underground
dc
cable
from
the
Kingsnorth thermal
power
plant,
situated on the sou th
shore
of the
Thames
River
estuary,
to
two
substations
in
London.This
is
a
bipolar
scheme
having three
cables:
one for each
pole
and
a
neutral cable. Each
pole goes
to
a
different
substation,
with
the result
that,
although
the whole scheme has
three
terminals, each
pole has
only
two terminals.
The Beddington substation
is 37 mi
(59 km)
from
Kingsnorth,
and
the
Willesden
substation
is
14
mi
(23
km)
beyond
Beddington.
Whenever
the
loads
of
the two substations
are
unequal,
there
will
be
neutral
current. This
current
is
not
allowed to flow in
the
ground for
fear of
damage by
electrolytic
corrosion
to some of the
many
buried
metallic
structures
found
in a metropolitan area.
The
rating of this
scheme
is
2 6
6 kV, 1.2 kA, 640 MW.
There
are four
groups
of
valves
at
Kingsnorth
and two groups
at
each
substation, each
group
being
rated at 133
kV,
1.2
kA,
160
MW.
Nelson
River,
Manitoba,
Schemep
Th e
Nelson
River
has a
potential
hydroelectric
power
development
of
about
6500
MW,
includingsome diversion
of
water
from
other streams. It
has
been decided
to
develop
this
power
and to
transmit
it
to Winnipeg by direct
current. Bipolar
+
450-kV
overhead dc lines
were judged more economical
than
500-kV
ac
lines.
Ultimately there
will
be
several
such
bipolar
circuits
as
the
development
proceeds
by stages.
With two
such lines,
the transmission
capacity will
be
3240 MW.
In
response
to
the invitation
for bids on terminal
equipment
for the
first
stage,
three
proposals
were
received for
thyristor
converters
and
two
for
mercury-arc-valve
converters.
Th e
proposal
for
mercury-arc
equipment
by
the
English
Electric
Company
was accepted.
Each
valve
group
will
operate
at
1.8 kA, 150
kV, 270
MW. There
will
be three groups
in
series per
pole.
Eel River
(New
Brunswick)
This station
provides
an
asynchronous
tie
between
the 60-Hz
ac systems
of
Hydro
Quebec
and
of
New
Brunswick. As at
Sakuma,
the dc circuits
are
confined
to the station.
In
contrast
to
Sakuma,
the nominal frequencies of
the
two
ac systems are
equal,
although one
can
drift with relation to the
1-5
LIMITATIONS
AND
ADVANTAGES OF
AC AND DC
TRANSMISSION 19
other.
Th e
distinctive
feature
of the
Eel
River
station is
that it
is
the first
large
converter station
designed
to
use
thyristor
valves
initially an d
exclusively.
The
rating of the
station
is
320
MW,
80
kV dc, 230 kV
ac.
1-5
LIMITATIONS
AND
ADggpGES
OF AC
AND
DC TRANSMISSION
Noting
the
universal
use of
alternating
current
for
electric
power
trans
mission,
as well
as
for generation,
distribution,
and