Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
The present invention relates to zero-current
detectors for high voltage DC power transmission lines
and in particular to such a detector employing a
saturable magnetic reactor.
The sensing of current tincluding zero current)
in an AC line is typically done with a "current transformer"
which consists of a large iron core which circles the
AC line and is dielectrically insulated from it. If
a multi-turn winding on the iron core is loaded with
a sufficiently small load resistance, then the voltage
generated across the load resistance is a faithful
replica of the current in the AC line.
For high voltage direct-current transmission,
the magnetic flux surrounding the conductor is relatively
constant so a conventional iron-core current transformer
would not work, i.e., the core would merely saturate.
If one omitted the iron core, i.e., merely used the
voltage output of a coil adjacent to the line, this also
is incapable of indicating the magnitude of the DC
current. The reason for this is that the coil output
voltage merely indicates the time-rate-of-change of the
line current, i.e., the coil output would be identical
for a current change going from 2,000 amp to 1,500 amp
(in a`given time) as for a current change going from 500
amp to zero amp in the same time. Thus, coil output
would merely indicate a change in current, but not the
magnitude of the current.
The desirability of providing zero-current
sensors for high voltage DC power transmission lines is
becoming greater because of the increased use of DC
transmission lines for long distance power transmission.
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Any inefficiencies which may result in converting the
alternating current to direct current and back again
at the terminal ends of the transmission line are
compensated for by the increased transmission efficiency
of direct-current power. To provide increased system
reliability, the DC power transmission lines are typically
provided in side-by-side pairs, so that if there is a
failure in one pair, service may be restored through the
system through the other pair. The use of such pairs is
particularly prevalent along those portions of the
transmission line where repair is difficult or would be
slow, for example, along an underwater path. If a
failure occurs on one of the transmission line pairs,
it is highly desirable that the DC power be rapidly
switched to the alternate line in order to minimize
the disruption of service. This is done by gradually
lowering the DC current on the line until the current
is low enough so that the switches employed can
interrupt it. The switches now employed cannot interrupt
a DC current of more than approximately 10 amperes. Hence,
it is necessary to locate a zero-current sensor at each
switch to detect when the line current is low enough
so that the switch can be opened without damage.
Typical high voltage DC power transmission
lines operate at power levels of approximately 400,000
volts and 2,000 amperes. For such lines, present zero-
current sensors must not only be placed near the
terminal ends of the line but also along the lines at
switch positions which may be located tens or hundred of
miles from the terminal equipment. Each of these zero-
current sensors disposed along the transmission line
now comprises a free standing device called a "transducer",
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typically costing approximately sixty thousand dollars
if rated at 400 kV. The primary reason for this high
cost is the insulation required. Accordingly, the
cost of such sensors significantly adds to the ultimate
price of the DC power system.
United States Patent No. 4,087,701 issued
May 2, 1978 to John M. Anderson and assigned to the
same assignee as the present invention describes a
transformer cascade for delivering relatively low
levels of electric power to electronic systems and
instrumentation operating on high voltage transmission
lines. These transmission lines may be either direct
current lines or alternating current transmission
lines. In the Anderson invention, power is transmitted
from a source at ground potential to instrumentation '
circuits operating at line potential through a cascade
of transformers, each having relatively low voltage
insulation and a turns ratio of approximately 1:1.
i ~ Summary of the Invention
In accordance with a preferred embodiment of
the present invention, a saturable magnetic reactor is
disposed about one of the conductors of a high voltage
DC power transmission line. This reactor is coupled to
impedance detecting means through a step-down transformer
cascade. The impedance detecting means typically
comprises a bridge circuit driven by an oscillator
operating at super audio frequencies. A filter is also
preferably provided which is turned to the oscillator
frequency. In accordance with another embodiment of th~--
of the present invention, the saturable reactor may also
include a second winding terminated in a resistive load.
At zero DC current, the magnetic core unsaturates, thus
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causing a large change in the input impedance of the
cascade transformer. This impedance change may be
detected by a simple bridge circuit at ground potential.
Accordingly, it is an object of the present
invention to provide an inexpensive zero-current detector
for a high voltage direct current power transmission
line.
Description of the Drawings
FIG. 1 is a partial cross-sectional side
elevation and schematic diagram illustrating the zero-
current detector of the present invention.
FIG. 2 is a schematic diagram detailing the
bridge and filter circuits of FIG. 1.
FIG. 3 is a schematic diagram illustrating
an alternate embodiment for the saturable magnetic
reactor circuit.
FIG. 4 is a graph of the output voltage as a
function of the line current.
Detailed Description of the Invention
In FIG. 1, a single conductor 10 of a multi- ,
conductor high voltage DC transmission line carrying current
I passes through saturable reactor core 16. This core
preferably is toroidal and comprises material such as
iron or ferrite. For convenience, a metal corona shield
and weather housing 12 is provided atop porcelain insulator
14 which is approximately 15 feet in length. The porcelain
insulating house 14 is generally cylindrical in shape
with exterior fluting to increase the arc-over path
length. Along the central axis of the insulator cylinder
there are disposed a series of cascaded transformers
which typically comprise toroidal ferrite cores, designated,
for example, in FIG. 1 by 18a-18g. Each of these cores
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possesses two windings for coupling to its immediately
adjacent core as shown. However, the uppermost core
in the cascade is coupled to saturable reactor 16 and
the lowermost core in the cascade 18g is connected to a
circuit which essentially acts as an impedance sensor.
A preferable embodiment for this impedance
sensor includes an oscillator which drives a conventional
bridge circuit. One arm of the bridge circuit comprises
the transformer cascade terminated in the saturable
magnetic core 16. The output voltage measured across
a selected pair of bridge terminals is then preferably
connected to a bandpass filter which is tuned to accept
signals having the frequency of the oscillator and a
reasonable bandwidth surrounding this central oscillator
frequency. Accordingly, there is shown in FIG. 1
oscillator 20 connected to bridge circuit 21 having as
one arm thereof the transformer cascade, said filter
22 being connected'to an appropriate pair of bridge
circuit nodes. The output of the filter, VO is fed
out of the external housing through feed-through bushings
26. The lower portion of,the housing 13 is at ground
potential and, accordingly, none of the electronics of
the present invention are at the line potential. Also
shown in FIG. 1 is feed-through bushing 26 on the corona
housing 12. This bushing insures that the conduction of
the current occurs through the line 10 passing through
the core 16 rather than through a path which circumvents
core 16. Additionally, if desired, a metal oxide varistor
device 24 for overvoltage protection may be provided in
parallel across the windings coupling toroid 18a to
saturable reactor 16. The varistor device is selected
to be usually nonconducting and functions to absorb
voltage surges induced by sudden changes in the high voltage
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DC current. However, this is not a serious problem
since surge voltages are usually limited by the rapid
sat:uration of the sensing core. Furthermore, although
not shown in FIG. 1, capacitive and/or resistive potential
grading circuits may be employed to maintain a constant
voltage gradient between the sequential cores in the
transformer cascade. Such grading circuits are shwon
and described in the aforementioned Anderson patent.
When the high voltage DC line is carryin~ its
normal current (approximately 2,000 amperes), the
sensing core 16 is saturated. When the line current
approaches zero, the core unsaturates and the impedance
looking into the saturable reactor 16 changes from a
very small inductance to a much larger inductance. A
similar impedance change also appears at the bottom
most (input) winding on core 18g of the transformer
cascade. That is, the impedance looking into the
transformer cascade changes from being close to a short
circuit to being close to an open circuit. This impe-
dance change is maximum at zero line current and fallsoff rapidly as the line current departs from zero and
would be considerably reduced, for example, at a DC
line current of 5 amperes.
The impedance change near the zero current
condition is detected by a simple bridge circuit operating
at super audio frequencies. For example, at a frequency
of approximately 30 KHz, a transformer cascade comprising
toroidal ferrite cores exhibits an overall transmission
efficiency of 80 percent even though the core windings
are not coaxial (see FIG. 1). The cascade transformer
forms one arm of the bridge circuit. The bridge is
adjusted so that it is balanced (zero output) when the
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sensing toroid 16 is saturated. Thus, at zero line
current the bridge exhibits a large unbalance. The
bridge output voltage is thus a measure of the DC line
current near the zero-current level.
Because ripple and other transient signals on
the high voltage DC line may also be coupled into the
bridge circuit near the cascade transformer, particularly
near zero-current conditions, a filter is preferably
provided at the bridge output so as to pass only signals
in a relatively narrow bandwidth surrounding the
oscillator frequency. This prevents the bridge output
from being affected by these other signals.
FIG. 2 is a schematic diagram illustrating
a bridge circuit and filter which may be employed in
the present invention. The bridge is a conventional
bridge circuit with four arms, or sides. Two of these
arms preferably comprise pure resistances Rl and R2 which
are ioined at a common node. The other two sides of
the bridge comprise circuit elements as shown. In
particular, one of these arms comprises the transformer
cascade which is coupled to the saturable reactor. The
last remaining arm comprises a series combination of an
inductance Lo and a resistance Ro. The values Lo and Ro
are chosen to match the~impedance seen looking into the
cascade transformer when the reactor 16 is saturated.
These four bridge,arms are coupled to an oscillator 20
and preferably to a filter 22. The connections to
these devices are best described if the four arms of
the bridge are considered to be, for purposes of
illustration, the four sides of a square. The output
of the oscillator is connected to two diagonally opposite
nodes and the input to the filter is connected to the
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other two diagonally opposite nodes. The filter 22'
shown in the dotted lines in FIG. 2 may be any convenient
conventionally employable bandpass filter. A simple
filter is shown in FIG. 2 comprising a transformer, the
primary and secondary of which are connected in parallel
with separate capacitors such that the inductive-capacitive
combination acts as a bandpass filter tuned to the fre-
quency of the oscillator 20. Resistor R3 is effective
to insure selective frequency filtering regardless of
the bridge impedance and is chosen to be approximately
equal to the filter impedance at resonance. The rms
output of the filter VO may be measured as shown and
is plotted in FIG. 4 as a function of the DC line current.
As seen in FIG. 4, the voltase output is maximally
sensitive at zero-current DC line conditions.
FIG. 3 illustrates an alternate embodiment of
the saturable reactor 16 disposed about transmission
line 10. In particular, a second winding 26 is provided
about the reactor core and said winding is connected in
parallel with a resistance R and a varistor 27, as shown.
The varistors function as protective devices to absorb
voltages induced because of transients or ripple currents
on line lO. Using this alternate sensing core, the
impedance seen looking into the primary (from varistor
24) changes from a small inductance when the core is
saturated to esse~tially pure resistance under zero-
current conditions in the line.
There are several significant advantages that
this invention provides over conventional transducer
methods of measuring zero DC line conditions. In
particular, because it is much easier and less
expensive to grade the high voltage down the length of
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the insulator, there is no need for expensive high
voltage bushings. Such bushings are required in the
conventional transducto~ because the sensing core in
such tranductors is not kept at line potential and
accordingly, the voltage grading must be accomplished in
a 10 or 12 inch radius by employing such dielectric
materials as SF6 or a high dielectric strength oil.
In the present invention, the sensing core is at line
potential as is shown in FIG. l. Hence, the sensing
core 16 may be much smaller in size and does not
require the high voltage insulation between it and
the line, which is re~uired in conventional transductors.
Another advantage of the present invention is that there
are no active components at line potential and only a
minimum number of passive components. Hence, system
reliability is enhanced and the need for shut down of a
DC transmission line to effect repairs is minimized.
From the above, it may be appreciated that
the present invention provides an inexpensive and highly
reliable zero-current detector for a high voltage DC
transmission line.
While this invention has been described with
reference to particular embodiments and examples, other
modifications and variations will occur to those skilled
in the art in view of the above teachings. Accordingly,
it should be understood that within the scope of the
appended claims, the invention may be practiced otherwise
than is specifically described.
