Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a high voltage d.c.
power transmission system that utilizes a metallic conductor
as a return, or neutral, conductor for carrying return
current between opposite ends of the system. More
particularly, the invention relates to means for protecting
the return conductor and associated equipment against
overvoltages produced by system transients.
An ~VDC system is sometimes called upon to operate
in a monopolar metallic return mode. This is a monopolar
mode in which the d.c. current returns through a separate -
metal conductor instead of through the earth or groundO In
such a systemr one end of the return conductor is grounded
while the other end floats with respect to d.c. The floating
end is often very remote from the grounded end. For
example, in one HVDC system presently in operation, i.e.,
the Square Butte system operating between Center, N.D. and
Arrowhead, Minn., the distance between the two ends is 4~5
miles.
- Under steady-state conditions, the neutral
~0 voltage at the floating end of the return conductor is
equal to the d.c. "IR" drop of the return conductor.
~ The voltage level of the neutral under
; steady-state conditions presents few insulation problems.
But large overvoltages can occur on the return conductor
; during transients, such as converter bypass, commutation
failures, starts, restarts, a.c. voltage transients,
and d.c~ line faults. All these conditions cause
overvoltages to be superimposed on the steady-state
voltage.
Such overvoltages require either that the insulation
level of the neutral, at its floating end, be very high or
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that some means be provided to hold down the level to ~hich
the overvoltage rises. In a technical paper appearing in
the IEEE Transactions ~ Power Apparatus and 5ystems, 1971,
pages 554~ , N.R. Hingorani describes an arrangement for
implementing the latter approach.
More specifically, Hingorani connects between the
neutral conductor and ground, at the floating end of the
neutral conductor,the parallel combination of a large capacitor
and a gap-type lightning arrestor. There are several
problems associated with this type of apparatus. First, the
capacitor must be large enough so that the gap-type arrestor
does not spark~over for most operating transients, and such
a capacitor is quite expensiVe. In one existing system, this
~, capacitor has a value of 50 ~ f. A second problem with this
prior apparatus is that harmonics generated by the converters
tend to pass through the large capacitor and ground in
preference to the metallic return conductor inasmuch as the
capacitor and ground have a lower harmonic impedance than the
, metallic return. The resulting harmonic current through
2Q ground is a major cause of telephone interference.
An object of my invention is to provide effective
oyeryoltage protection for the neutral ~ithout requiring the
large capacîtor described hereinabove that has been connected
- between the return conductor and ground at the floating end of
the return conductor, or neutral.
`~ Another object is to provide neutral overvoltage
protective apparatus which effectivel~ limits the harmonic
currents flo~ing through ground and producing telephone
interference.
~,nother object is to provide neutral overvoltage
protective ~eans whi,ch i5 capable of increasing the
effectiveness of the usual harmonic filters shunting the
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converters.
An additional disadvantage of the prior apparatus
re$erred to hereinabove`is that spark-over of the gap-type
arrestor activates the d.c. line fault protection control,
which sometimes brings the system current to zero to allow
the gap current to come to zero in order to permit the gap
to recover its dieIectric strength. This has meant
undesirable loss of the system for several hundred milli-
seconds. The previously-described large capacitor has been
relied upon to reduce the fequency of such system losses
since the capacitor has been used to limit the voltage on
the neutral under most transient conditions to values below
the spark oYer voltage of the arrestor.
Accordingly, another object of my invention is to
provide overvoltage protective means which, despite the
absence of the large capacitor, can dissipate the ener~y of
transient overvoltages under most transient operating
conditions without necessitating the above-described
interruptions in system current.
; 20 In carryin~ out the invention in one ~orm, I provide
a high volta~e d.c. po~er transmission system comprising:
(il a high voltage line, (ii~ a first converter at one end
of the line having first and second d.c. terminals of
~; opposite polarity, the first terminal being connected to
said line, (iii~ a second converter at the opposite end of
said line having first and second d.c. terminals of opposite
polarity, the first terminal of said second converter being
connected to said line, and (i.v.) a metallic return
conductor interconnecting said second d.c. terminals of the
3Q two conyerters. Means is proYided for connecting said
second terminal of said second converter to ground at said
second terminal, and means is provided to insulate the
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metallic return condu-tor from ground except for .this
ground connection. The meta;llic retuXn cQnductor has .no
capacitor connected from ground thereto of a size capable
of effectively limiting the` voltage thereon. CQnnected
between the second terminal of the fi,rst conYerter and
groundr I provide a valve-type surge arrestor having a
primarily zinc-oxide valve element and no gap in series,
there~ith. The valve element is characterized by (i~
substantial non-conductance of currents therethrough until
the voltage thereacross reaches a predetermined protective
leyel and (ii) an ability to return to its substantially
non-conductin~ state wh.en the voltage thereacross drops to a
seal-of~ leYel close to said protective level.
: The surge arrestor h.as a protective leveI that is
re~ched by the overvoltages produced on said return conductor
by normal operating transients of the sy~tem, such as those
-', overvoltages produced hy system staxt-up and by comm,utation
: failures.
For a ~etter unders.tanding of the invention,
reference may be had to the following description taken in
conjunction ~ith the accompanying drawings, wherein:
Pig. 1 is a schematic sho~ing of a prior art HVDC
~ system such, as xeferred to h.ereinabove.
~ ~Pig. la is a graph show~ng certa~n Yoltage relation-
ships present in the system o~ Fig. 1 at the floating end of
'`~ its return conductor.
Fig~ 2 is a dia~xa~m,ati:c sho~ing illustxating the
e~fect o.f ~round i~pedance on the operation of a conventional
harmonic filker shunting the inYerter.
Fig. 3 is a schematic showing of an HVDC system
embodying one ~orm o~ my invention~
Re~erring now to Fig. 1, the prior art HVDC power
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transm~ssion system schematically shown thexein comprises
a high voltage transmission line 10, ~hlch may be either an
overhead or underground line, and two converters 12 and 14 at
opposite ends of the line. Converter 12 usually functions as
a rect~fier and converter 14 as an inverter, but the role of
either converter can be reversed in a conventional manner as
occasion demands. The two converters may be separated by
great d~stances, e.g., the 465 miles present in the above-
referred to Square Butte system.
The converters can be of any conventional form, such
as, for e~ample, the form depicted in U.S. Patent No.
3,832,62Q - dated August 27, 1974 - Pollard, where the
converter compxises a plurality of controllable valves
connected in a three-phase, double-way, 6 pulse bridge
configuration. The conyerters 12 and 14 are provided with the
usual controls, an example of wh~ch is shown and claimed in
aforesaid Pollard patent, for controlling the firing angles
of t~e valves. In Fig. 1/ I have schematically shown at 35
such a control for converter 12. It is conventional to provide
2Q such a control w~th pole protection means, schematically shown
at 37, ~or detectin~ the pxesence of a d c. line fault and
for developing an output signal which is supplied via circuit
36 to control 35 upon the occurrence of a d.c. line fault.
The control 35 upon receiving this signal via clrcuit 36 in
response to a d.c. line fault, drives the rectifier into
inversion, thus forcing the system current to zero. After a
brief per~od, such as 200 to 5QQ msecs, which is normally
sufficient to permit deionization of an arcing fault, control
35 xestores the system to normal.
In certain systems, instead of driving the system
current immedi~ately to zero in response to a d.c. line fault,
as described aboye~ the s~stem current is ramped down to a
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minimum level in such a manner as to cause reversal of the
current in the fault. If the fault current thereafter
persists, the system current is reduced to zero through
appropriate valve control.
Each converter 12 and 14 has two spaced-apart d.c.
terminals of opposite polarity. The upper terminal 16 of
converter 12, the rectifier, is connected to one end of
transmission line 10 through a smoothing reactor 13; and
the upper terminal 18 of converter 14, the inverter, is
connected to the opposite end of the power line through a
smoothing reactor 15. The lower terminal 19 of the inverter
is electrically connected to the lower terminal 17 of the
rectifier through a metallic return conductor 20, sometimes
re~erred to as the neutral, or neutral conductor.
The end of the return conductor 20 at the inverter
end of the system i~ connected to ground through a ground
connection 21. The opposite end of the return conductor 20
may be thought of as floating ~ith respect to d.c.. Under
steady state conditions, the voltage between the return
conductor 20 and ground at the floating end is equal to the
"IR" drop resultin~ from return current through conductor 20.
In the 465 mile Square Butte System, referred to above, this
steady-state neutral~t~-ground voltage is about - 16 kV, as
indicated at A in the ~raph of Fig. la.
As pointed out in the introduction, this neutral d.c.
level presents ~ew insulation problems. But large over-
voltages can occur on the neutral during transients, such as
those mentioned in the introduction. Such transient conditions
cause o~ervoltages that are superimposed on the steady state
voltage as ~ndicated at B in Fig. la.
Th~s can be better appreciated if one assumes that
the system o~ ~ig. 1 is operating with -16 kV present at
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the floating end of the neutral 20, and then suddenly the
inyerter 14 is b~passed. The normal voltage at the upper
terminal 18 of the inverter, which will be assumed to be +218
~V, suddenly collapses to zero. This produces a voltage
transient that propogates down the return conductor 20
toward its floating end, driving this end to a much higher
negative voltage.
Such an overvoltage would require either that the
~; insulation level of the neutral and its associate equipment
be as high as on the high side of the converters or that
some means be provided for holdiny down the overvoltage.
n
In prior systems, such as describedJthe Hingorani paper
referred to hereinabove, there is provided a voltage-limit-
ing arrangement that comprises a large capacitor and a
gap-type li~htning arrestor connected in parallel with each
other between the floating end of the neutral conductor and
gxound. Fig. 1 shows such a capacitor at 26 and such a
gap-type arrestor at 28. The gap-type arrestor is a
conventional device comprising a gap device 30 and, in at
least one prior system, also comprising a valve element 32 of
silicon carbide connected in series with the gap device 30.
This type of arxangement has several disadvantages.
First, the capacitor 26 must be large enough so that the
gap-type arrestor 28 does not spark-over for most operating
transients, and such a capacitor is quite expensive. It is
important to pre~ent such spark-overs of the gap-type
arrestor because when such arrestor sparks over, it appears
as a d.c. line-to-ground fault and activates the pole
protection means 37 if the arxestor current persists. This
3Q may cause a loss of the d.c. system for several hundred
milliseconds, which is undesirable, following ~hich a restart
is required.
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A second disadvantage of the arrangement of Fig. 1
is that harmonics generated by the converters tend to pass
throuyh the large capacitor and ground in preference to the
parallel path through the metallic return conductor inasmuch
as the capacitor and ground have a lower harmonic impedance
than the metallic return. The resultant current through
ground is a major cause of telephone interference.
Another disadvantage of the arrangement of Fig. 1
is that the presence of the large capacitor detracts from the
effectiveness of the usual harmonic ilters 40 and 42
shunting the respective converters. For a better understanding
o~ this latter point~ reference may be had to Fig. 2 where
` the harmonic filter 42 shunting the inverter 14 is shown.
The impedance of the path through ground and the large
capacitor is schematically depicted as Zg The harmonic
current generated by the inverter 14 is designated Ih. If
this current can be forced through the harmonic filter 42
instead o~ out onto the line, noise will be significantly
reduced. Most of the current Ih ~ill flow through the filter
at re~uencies ~here the filter impedance z~ is less than the
-~ effective total impedance of the return line 20 and the
ground impedance Zg. In the case of a large grounding
capacitor 26, zg is less than Zf at all frequencies except
where the filter is tuned, thus allowing a major portion of
the current Ih at such ~requencies to bypass the filter and
flow out onto the line. This, of course, detracts from the
effectiveness of the filter.
I am able to largely overcome these disadvantages
by usin~ at the ~loatin~ end of the metallic return conductor
20 a different type of overvoltage protective means from
that shown in Fig. l Referring to Fig. 3, this overvoltage
pxotective means comprises an arrestor 49 comprising a
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valve element 50 and no gap in series with the valve element.
The valve element 50 is not of the usual silicon carbide
material typically used in arrestors but rather is o~ a
ceramic material, sintered at high temperature, and consist-
ing essentially of zinc-oxide and a small amount of other
metal-oxide additives that produce the desired non-linear
characteristics of the valve element. The basic structure
of the sintered material is a matrix o~ highly conductive
zinc oxide grains, joined by highly resistive intergranular
layers consisting primarily of the metal oxide additives.
Under sufficient electrical stress, the intergranular layers
start to conduct in a highly non-linear mode. Examples of
this type of ceramic material are disclosed and claimed in
U.S. Patent 3,928,2~5 - dated December 23, 1975 - Fishman et
al assigned to the assignee o~ the present invention. This
type of arrestor ls discussed in a technical paper by
Sakshaug et al entitled "A Mew Concept in Station Arrester
Design" appearing in the IEEE Transactions on Power Apparatus
and Systems, Vol. PAS-96, No.2 pages 6~7-656, March/April,
1977.
As pointed out in the paper by Sakshaug et al, this
arrestor draws very little current until a voltage approaching
its protective level is reached, and then only that current
is drawn which is necessary to limit the overvoltage to the
protective level. Furthermore, the arrestor returns to its
original state of substantially no-conduction when the voltage
applied thereto drops to a voltage level very near the same
voltage level at which conduction started. Stated another way,
this arrestor has an exceptionally high degree of non-
linearity in its voltage-current characteristic. Moreover,
the protectiye characterlstics of this valve material remain
essentlally unchanged despite exposure of the valve material
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to the repetitive passage therethrough of discharge currents,
even reIativeIy high'discharge currents.
Another important point to note with respect to
the arrangement of Fig. 3 is that there is no capacitor
corresponding to the capacitor 26 of Fig. 1 connected between
the return conductor and ground. Stated another way, in the
' arrangement of Fig. 3, there is no capacitor connected
between the return conductor 20 and ground of a size capable
of limiting the voltage appearing on the return conductor.
Since no such capacitor is present, the voltage on the return
conductor 20 resulting from normal operating transients (such
as start-up, commutation failure, inverter bypass and blocking)
is perm~tted to rise withbut attenuation toward the protective
level of the arrestor 49, 50. If this voltage reaches the
protective level of the arrestor the valve 50 of the arrestor
becomes conducting, allowing suff~cient current therethrough
to clip the voltage. When the discharge voltage across the
arrestor, i.e., the voltage produced by the transient current
therethrough drops to a seal-off value slightly below the
arrestorls protective level, the valve returns to its normally
non~conducting state. There is no large follo~ current through
the arrestvr necess-itatin~ converter operations that could
bring the system current to zero. Hence, for such normal
operating transients, even though the arrestor may operate
to pass substantial currents, it is unnecessary to lose the
system for brief periods, as has often been the case when
the gap device of prior systems has sparked-over~
Of course, ~ith the capacitor no longer present,
the hereto~ore~re~uired substantial expense of providing such
~ ~ o l ~ e ~J
~0 a capacitor is o~r~e.
In addition, with the capacitor no longer present,
the impedance of the path throu'~h'earth shunting the return
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conductor 20 is much higher than when the capacitor is
present, thus greatly reducing the current through the earth,
thus reducing the telephone interference that results from
current through the earth.
Still further, referring to Fig. 2, with the
capacitor no longex present, zg is much higher than when the
capacitor is present. As a result, the absence of the
capacitor in the system of Fig. 3 results in forcing most of
the harmonic current Ih through the harmonic filter 42, thereby
increasing the effectiveness of the harmonic filter.
The valve member 50 is selected to have sufficient
thermal capacity so that it can not only handle the normal
transient conditions refexred to above, but also can handle
abnormal transient conditions such as d.c. line ~aults. A
d.c. line ~ault from the high voltage line 10 to ground at
the rectifier would impose the most severe duty on the
arrestor 49, 50. In the aforesaid Square Butte system, this
produces a peak current for several microseconds of many
thousand amperes followed by a persistent current averaging
2a about 300 to 400 amperes for 30-35 msecs. The arrestors's
energy-handling capability is much greater even than needed
~or this duty.
~ possible, but very unlikely type o~ system
A failure~ is a break in the return conductor 20 while this
system is operating at high current. This would force all the
system current through the arrestor. To prevent ~he arrestor
from being destroyed by such duty, sensing means 33a, 34a
senses excessiVe current through the arrestor and if this
current exceeds a threshold value for more than a predetermined
3~ minimum period, it develops a signal which is supplied to
the control means 35. Control means 35 responds by driving
the rectifier into inversion, thereby interrupting system
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current within a time short enough to prevent the arrestor
from being damaged by this condition.
More details of an arrestor suitable for use in
practicing this invention (as shown at 49, 50 of Fig. 3) can
be found in U.S. Patent 3,959,543 - dated May 25, 1976 -
Ellis, assigned to the assignee of the present invention.
While I have shown and described a particular
embodiment o~ my invention, it will be obvious to those skilled
in the art that various changes and modifications may be
made without departing from my invention in its broader
aspects; and I, therefore, intend herein to cover all such
changes and modifications as fall within the true spirit
and scope o~ my invention.
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