Note: Descriptions are shown in the official language in which they were submitted.
- lQ~320
The present invention relates to the art of electric
power transmission and more precisely to a method of reducing
current unbalance in a three-phase power transmission line
operating with one faulty phase; the method ca~,be advantageously
used in 110 - 500 kV A.C. power transmission lines.
Damage to the insulation of overhead power trans-
mission lines caused by short circuits is quite common at the
present time. Most short circuits are caused by atmospheric
phenomena, such as thunderstorms, strong winds, snowfall, and
the like.
The time required to repair the damage caused to
the insulation is determined by the time required for the
repair proper, the time for bringing the repair crew to the
site of damage, as well as the time for detecting the ~' -
location of the fault, the greatest part of the time being
consumed by the process of detecting the point of fault
and the process of bringing the repair crew and the equipment
needed to the site of fault.
IIt should also be noted that very often weather con-
ditions present additional hazards preventing these operationsfrom being performed in a shortest possible time with the
result that repairs take from a few hours to a few days.
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If the consumer was supplyed by the damaged line only,
he will be deenergized for the duration of repairs, In case
the additionally available power sources are not sufficient
to provide power to all the region involved, partial restrictions
are imposed on the consumers until the fault is eliminated.
Any reqtriction of power consumers results in losses
which are incurred to the industrial enterprises in the region
affected for the duration of said restriction.
It is therefore common practice at present, in case
of a sustained one-phase short circuit in the line to continue
operating it with two conductors only while carrying out necessary
repairs. The process of operating with two conductors is often
termed in the literature as a two-phase operating mode or an
incomplete phase regime of the line.
The term "incomplete phase regime" will be used here-
inbelow in the sense of operating a line with two conduc~ors
only.
The incomplete phase regime, howeve~, if fraught with
the following disadvantage.
The power transmitted through the line is limited by
the current unbalance appearing in the stator windings of current
generators, synchronous condensers, synchronous and asynchronous
motors, as well as by the unbalance of illumination and household
loads. The term "current unbalance" for example in an electric
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~;94~
machine serves to designate the ratio of negative sequence
current to the positive sequence current in the machine.
The value of current unbalance is subject to ~ -
specification and is determined in turn by the design features
of electric machines and apparatuses and their op ration ~
conditions. - ;
In order to reduce the value of current unbalance when
operating under the conditions of incomplete phase regime use
is often made of a prior art method wherein the disconnected
damaged line is grounded fro~ both ends thereof.
The above method can be used provided the short circuit
is not accompanied by a line break and the wire insulation re- ~-
lative to earth can sustain the voltage induced by the two
remaining conductors which carry current. ~-
As a rule a short circuit in the lines rated at 220 kV -
and higher is not accompanied by a break of wire and after the
short circuit is deenergized the wire insulation relative to
earth is capable of sustaining a voltage which is 25 to 50
percent of the rated voltage.
The above method for reducing current unbalance permits J'
~ '
increasing the power transmitted over the line which operates
under the conditions of incomplete phase regime by 6 to 8% ~;
as compared with the conventional incomplete phase regime
operation of the line.
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However, the present method of reducing current
unbalance permits only a negligible increase of transmitted
power. Moreover, as stated earlier, the method requires pre-
conditions, such as absence of a broken wire, while the in-
sulation at the point of fault must not be damaged by the vol-
tage induced by the current in the sound wires.
These requirements tend to cause problems when using
the method for 110 kV lines, which lines are generally made
with conductors of a rather small cross-section, which often
break as a result of a short circuit.
The object of the present invention is to provide a
1.4- to 1.6 -fold increase of po~er transmitted over a line
operating in the incomplete phase regime as compared with
conventional incomplete phase regime of the power line, the
current unbalance thereof being equal.
Another object of the present invention is to reduce
current unbalance in case the faulty wire in the line is broken
or its insulation relative to earth is not capable of sustaining
the voltage appearing in it when the wire is earthed from both
ends thereof.
These objects are achieved in a method for reducing ^- `
current unbalance in a three-phase alternating current power
transmission line operating in an incomplete phase regime, wherein
use is made of the wires of this line and transformers electrically
coupled thereto, comprising the following steps: providing
an electrically closed loop made up of the line operating in an
incomplete phase regime, a wire which is insulated relative to
earth and disposed in a parallel manner along the whole length
of said line, and neutral wires of said transformers, connecting
said electrically closed loop to a source of alternating -~ -
voltage, which is operating in step with the electrical system --
into which system said line is connected, selecting a required
phase and value of voltage of said source to provide a required -
increase of current flowing through the neutral wires of said
transformers.
Current unbalance in an electrical system can be
reduced as a result of increasing the currents flowing through
the neutral wires of transformers which are electrically connected
to the line operating in the incomplete phase regime.
Since the power transmitted over a line operating in
the incomplete phase regime is restricted by the value of current
unbalance, the method according to the present invention permits
said power to be increased 1.4 to 1.6 times as compared to
conventional incomplete phase regimes, the value of current
unbalance being equal.
The disconnected wire of said line, grounded from one ~-
end is advantageously used as the wire that is insulated relative ~ -
to earth and disposed along the entire length of the line
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1044~0
operating in the incomplete phase regime, whereas said alternating
voltage source is conveniently provided by a transformer arranged
on the opposite end of said disconnected wire, one winding of
aid transformer being connected between the disconnected wire
and earth and the second winding being connected to the phase
or line voltage of the electric system to which said line is
connected.
The disconnected wire can be used also when the short
circuit did not result in a broken wire and the insulation at
the point of fault can sustain the voltage induced by the
current in the live wires of the line. As a rule, this is the
case with lines rated at 220 kV and higher. Therefore, a
disconnected wire is recommended for use in case of lines rated
at 220 kV and higher which operate in the incomplete phase
regime.
The method according to the present invention can be
advantageously used by the provision of a single-phase or three-
phase transformer at one end of the line, or a single-phase -
or three-phase autotransformer.
In view of the fact that the transformer or autotrans-
former are used as a source of e.m.f. and it is basically im-
material whether the function is performed by a transformer or
a~ autotransformer, the term "transformer" will be used through-
out hereinbelow. It is, however, understood that an auto-
transformer can be used as a source of e.m.f. for the purpose.
- ^ - - ,^ -. - .-. .. . . , ~
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~432(~
The idea of using a transformer as a voltage source
can al~ays be easily reduced to practice since a line usually
; connects two sub-stations where stand-by transformers are gene-
xally available, or there is always a possibility to unload one
of the transformer of the sub-station.
On the other hand there is no difficulty in providing
a special transformer at one of the sub-stations, which can
also be used for normal operation of the electric system when it is -
used in parallel to increase the reliability of electric system
as a whole.
Voltage across one winding of said transformer must
be within 15 to 35% of the rated voltage of the line, whereas
the voltage of the second winding must correspond to one of the
rated'voltages of the sub-station where it is installed. In -
case the transformer is of a multi-winding type, the voltage
of at least one of the windings thereof must correspond to one
of the rated voltages of the sub-station where it is installed.
The present invention also envisages earthings at the
ends of the three-phase line. At present all 110-500 kV lines
are provided with such earthings at the beginning and the end
of the line. - ~-
The function of a wire which is insulated relative -
to earth and disposed along the entire length of the line ~ ~ ;
operating in the incomplete phase regime can be advantageously
performed
,
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~04~3~¢~
by the lightning protection wire rope which is insulated relative
to earth along its entire length and earthed at one of its ends,
while the function of said alternating voltage source can be
performed by a transformer located at the opposite end of said
wire rope, one of the transformer windings being connected between
the earth and the wire rope, whereas the second winding is con-
nected to the phase or lin,e voltage of the electric system to
which said power transmission line is connected.
Short circuits occurring on 110 kV lines often result
in wire breaks, therefore a disconnected wire of the line can
be used for the purpose only after checking it for breakage.
Otherwise the purpose is advantageously achieved by using the
lightning protection wire rope of the power transmission line.
Since the present method involves application of
voltage to the wire rope, the insulation thereof must naturally
withstand the voltage i~pressed. A lightning protection wire
rope is easily insulated against 10 - 25 kV current. It is
common practice at present to insulate the wire rope relative to
earth, the wire rope being grounded at one point only, at other
2 points the wire rope is connected to the ground via spark gaps.
A wire rope which insulated relative to earth achieves a number
of objects, such as the melting of ice on the rope, it can also
be used for the purpose of establishing communication, etc.
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Since the wire rope used in the method according to
the present invention will carry current whose magnitude determines
the effect of current unbalance reduction in the electric
syste~, the wire rope is preferably made of aluminum. This can
be easily provided on newly erected power transmission lines.
The electric closed loop is preferably made by directly `-
connecting one of the ends of a disconnected wire of the line
operating in the incomplete phase regime to the neutral wire of -
at least one of the transformers which have electrical connection -
to said line, the neutral wire of said transformer being dis- ;~
connected from earth, and another transformer being used as a `
source of said alternating voltage, the other transformer being -
located on the other end of s~id disconnected wire and having
one of its windings connected between the disconnected wire and `
earth, while the other winding is connected to the phase or line
voltage of the electric system to which said power transmission -~;
line is connected. -;
The neutral wire of st transformers is generally rated
at a certain voltage, depending on the rated voltage of the
transformer. This makes it possible to disconnected the neutral -
wire from earth in order to reduce in the electric system short
circuit currents to earth. Insulated transformer
,,,
~Og43;20
neutrals permits the voltage to be regulated also by means of
booster transformers.
Insulated transformer neutrals offer a possibility
of using the present method, provided the disconnected wire of
- the line is connected directly to the neutral wire of trans-
former windings having electrical connection to the line which
operates in the incomplete phase regime.
The only limitation placed upon the method of connecting
transformer neutrals to the disconnected wire resides in the
fact that the value of voltage must not exceed the level of in-
sulation of the neutral. A spark gap must be connected to pre-
vent voltage build-up during transient processes between the
neutral and earth.
In addition to that said electric loop is pre-ferably
made by directly connecting one of the ends of said lightning
protection wire rope which is electrically insulated relative
to earth along the entire length thereof, to the neutral wire
of at least one of the transformers having electrical connection
to said line, the neutral wire of said transformer being dis-
connected from earth, while said source of alternating voltageis provided by a transformer located at the opposite end of
said lightning protection wire rope, one of the windings of said
transformer being connected between said wire rope and earth,
and the other winding being connected to the phase
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~443ZO ~:
or line voltage of the electric system to which said line
is connected.
Thus the method according to the present invention
can be widely used on 110-500 kV lines operating in the in-
complete phase regime, thereby increasing the power transmitted
over these lines when operating in the above mode. -
Taking into consideration the fact that single- -
phase short circuits are a common occurrence in these lines ~-
(from 0.3 to 0.7 times per 100 km per year), it will be
appreciated that the economic effect of the proposed method
will be quite substantial.
other objects and advantages of the proposed method
will become apparent from the following detailed description ,-
thereof taken in conjunction with the accompanying drawings,
wherein:
Figure 1 illustrates a current vector diagram of
a three-phase line operating in an incomplete phase regime ~'
(with phase "a" wire deenergized),
Figure 2 shows a transformation diagram of zero
sequence currents through a three-phase transformer with
a unity transformation ratio'
Figure 3 is the same as Figure 2 for first
sequence currents,
Figure 4 illustrates the vector diagram of phase
currents on the transformer side, the transformer windings
being delta-connected,
Figure 5 shows the schematic diagram of a three- ;~
phase line operating in the incomplete phase regime, wherein
one of the ends of the deenergized wire is earthed, according
to the invention,
A ~:
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;34~
Figure 6 same as Figure 5, when the deenergized wire is
connected to the neutral of the transformer windings electrically
connected to the line operating in the incomplete phase regime,
according to the invention,
Figure 7 illustrates the schematic diagram of a three-
phase line operating in the incomplete phase regime and a trans-
mission sub-station, for the case when a single-phase trans-
former is used as said voltage source and a deenergized wire
of the line, earthed at the receiving sub-station is used as said
wire;
Figure 8 same as Figure 7, for the case when a lightning
protection wire rope is used as said wire and a stand-by three-
phase transformer is used as said voltage source;
Figure 9 illustrates the diagram for connecting a de-
energized line wire to the transformer neutral,
Figure 10 illustrates the aiagram for connecting a
lightning protection wire rope to the transformer neutral.
Referring now to the Figures, Figure 1 illustrates a
current vector diagram in the wires of a three-phase line
operating in the incomplete phase regime. Since any one of the
three wires can be damaged and thus deenergized and the phenomena
occurring in the line will be the same irrespective of which
of the wires is deenergized, the processes occurring in the line
operating in the incomplete phase regime will be explained here-
inunder for the case when phase A is deenergized.
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The symbol ib in Figure 1 indicates the current in
phase "B" of the line, whereas 9ymbol iC indicates the current
in phase "C" of the line.
A system of two currents can be represented as a sum
of currents of two systems of symmetric components. Let us -
define one system as zero sequence currents and the other - as
first sequence currents. The zero sequence currents are equal
to half the sum of the phase "B" and "C" currents each. For
phase "B" the first sequence currents are equal to half the
difference between the phase "s" and "C" currents, whereas for
phase "~" they are equal to half the difference between phase ~ -
C and B currents.
Denote the zero sequence currents by symbols iob for
phase "B" and ioC for phase "C", whereas the first sequence
currents will be denoted by symbols iib and iiC respectively.
Since the transformer windings (Figure 2) are connected
in series with the line, they carry currents iob, ioC, and
iic, which currents are transformed by the transformer. The
currents in the second winding of the transformer are denoted
as Iob, Ioc~ Iib' Iic-
The zero sequence currents as well as the first sequence
currents are transformed through a star-delta-connected trans-
former in a different way.
Figures 2 and 3 illustrate the transformation of zero
and first sequence currents through a transformer with a ;~
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~43,~
unity transformation ratio, and connected to star with an
earthed neutral and delta.
When phase "A" (Figure 2) is disconnected from the
line coupled to the star-connected windings of the transformer,
the resistance of delta-connected winding of the transformer
between phases "A" and "C" is very high, therefoxe all the
current IOb which this winding carries will flow in the reverse
direction in phase "A" of the load, whereas the current I
oc
in phase "C" flows without changing its direction in phase "C"
of the load.
The first sequence currents are transformed in a diff-
erent manner. Referring to Figure 3, consider that since for the
first sequence currents, as well as for the zero sequence currents,
the resistance of phase "A" winding connected to the delta circuit
of the transformer is by two orders higher than the resistance
of the other delta windings, the current in phase "A" winding
is practically zero. Therefore the first sequence current in
phase "B", Iib flows in the reverse direction in the phase "A"
load. The "C" phase current, IiC, flows without changing its
direction in the phase "C" load, whereas the "B" phase current
of the load is equal to the sum of phase "B" current, Iib and
phase "C" current, IiC, taken with a reverse sign, that is the
phase "B" current, Iib of the load is equal to double current
ib
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326;~
The sum of first and zero sequence currents of the load
are true currents flowing in the load phases. They are
illustrated in Figure 4.
Symbol Ia denotes phase "A" current of the load, symbol
Ib is phase "B" current of the load, and symbol Ic is phase ,~
"C" current of the load.
Current Ib of phase "B" equals to double first sequence
current. Current Ic of phase C is equal to current ic of -~
phase "C". Current Ia of phase "A" is equal to the value of
current ib and is opposite to it.
As seen from Figure 4, current unbalance can be reduced
by increasing current IOb to I'ob and current IoC to current
I'oC, without changing the first sequence currents. The line -~
currents IOb and IoC are equal to currents iob and ioC, respect-
' ively.
In other words, if we increase zero sequence currents
iob and ioC in the line, without changing the first sequence
currents iib and iiC, the current unbalance will be decreased.
Current unbalance is known to decrease until the angle of
20 currents iib and iiC in the line operating in the incomplete
phase regime, reaches 60. Further, the unbalance tends to
increase with the angle decreasing.
Thus in summary it may be said that in operating a
line in the incomplete phase regime we can reduce current
unbalance by increasing zero sequence currents io~ and ioC in
the line phases. ~-
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~344;~0
Figures 5 and 6 schematically represent power trans-
mission lines operating in the incomplete phase regime using
the ~ethod according to the present invention.
In these Figures, 1 indicates a three-phase line
operating in the incomplete phase regime, with a deenergized
wire 2 of phase "An. The line 1 is supplied by a generator
system 3, with a transformer 4 connected to the opposite end
of the line.
A voltage source 5 is connected to the deenergized wire
2 from the side of the generator system 3, said voltage source
being synchronously coupled with the generator system 3. The
wire 2 is earthed from the other end thereof with earthing 6. :~
In Figure 6 the neutral of transformer 4 is disconnected
from earth by means of a disconnector 7, whereas the wire 2 is
connected to the neutral of transformer 4.
As seen from Figure 5, the voltage source 5 connected -
to the deenergized wire 2 of line 1 operating in the incomplete
pha~e regime, offers an additional electrical closed loop for
the zero sequence currents iob and ioC to flow in the line 1.
Current i2 in the wire 2, impressed by the voltage f ~ ~ ~
source 5 will partially flow through the neutral of transformer ~. :
4, the line 1 and the neutral of the generator system 3. -:
In this case it is necessary for the direction of `
voltage from the said source to be such that the zero sequence
currents in the energized wires of line 1, determined by the
voltage
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source 5, have the same direction as the zero sequence currents
iob and ioC in this line when operating in the conventional
incomplete phase regime.
Such connection of the voltage source 5 enables us
to increase the zero sequence currents in the line 1, since the
zero sequence currents, determined by the generator system 3
and the voltage source 5 are summed up with one and the same
direction. Accordingly, an increase of zero sequence currents
in the line 1 results in reduced current unbalance in the
generating system 3 and in the load p~ases.
Zero sequence currents in the line can be increased
by connecting the deenergized wire 2 (Figure 6) to the neutral
of transformer 4, which is disconnected from earth by means of
the disconnector 7, and connecting to the other end of the wire
2 a voltage source 5.
Here all the zero sequence current flows along the
path made up by the generator system 3, the wire of the i.n-
complete phase regime line 1, the transformer 4 winding, the wire
2, the voltage source 5 and earth. In this case the current
in the deenergized wire 2 will be equal to the sum of zero
sequence currents iob and ioC in the line 1. -~ :
Such connection of the deenergized wire 2 to the
neutral of the transformer 4 disconnected from earth is
feasible, provided the insulation of the transformer neutral
is rated at
~4~Z~)
a voltage which is bound to appear in it after the voltage source
5 is connected to the deenergized wire 2.
Consider some examples of embodying the present
invention. Referring to the circuit illustrated in Figure 7
showing-a 220 kV line operating in the incomplete phase regi~e
with a deenergized wire 2. One end of the line 1 is connected
to buses 8 t220 kV) of the transmission sub-station, the other
end thereof being connected to buses 9 (220 kV) of the receiving
sub-station.
It is understood that in addition to said line, other
lines not shown in Figure 7 can be easily connected to the
buses 8, 9 (220 kV) of the both sub-stations as well.
The transmitting sub-station houses two three-winding
three-phase transformers 10 and 10` with windings rated at 220,
110 and 35 kV, which windings are connected via three-phase : :
circuit breakers 11, 12, 13 and 11', 12' and 13' respectively, -
the buses 8 (220kV), buses 14 (llOkV) and buses 15(35kV).
The windings (35kV) of the transformer 10 are delta connected. :
, In order to simplify the drawing, the three-phase con-
nections of transformers 10 and 10' with circuit breakers 11, 12, ; : ~:~
13 and 11', 12', 13' respectively, are shown as single lines.
A single-phase three-winding transformer 16 is also provided at ~
the transmitting sub-station to be used as a stand-by trans- ~::
former to back up any phase of the three-phase transformers 10
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or 10' for repairs. The single-phase transformer 16 can be
connected via single-phase switches 17 or 18 to any bus system
8 or 14, respectively.
In case of a fault in the wire 2 of the line 1, which
didl not result in its breakage, while the insulation at the point
of fault is able to sustain a voltage of 35 kV, the present
invention can be embodied as follows.
The deenergized wire 2 is earthed at the receiving
sub-station 9 with the earthing 6. At the transmitting sub-
station,, one of the lead-outs of the 35 kV windin3 of the single-
phase transformer 16 is earthed, whereas the other leadout of said
winding is connected via a circuit breaker 19 to the disconnected
wire 2. The single-phase transformer 16 is connected to the
respective phase of bus 8 (220 kV) or 14 (110 kV), and voltage
is fed to the disconnected wire 2 by means of the switch 17 or ~-
18, respectively.
If for some reason a single-phase transformer cannot ~ -
be used while the power supplied by one of the three-phase trans-
formers 10 or 10' is sufficient to supply the consumers connected
to the buses 15 (35 kV), one of the transformers 10 or 10' must
be disconnected from the buses 15 (35 kV) in order to supply
- voltage to the disconnected wire. In order to ~upply voltage to
the disconnected wire 2, after th~ transformer 10 or 10~ is
disconnected from the buses 15 (35 kV) one of its
-19-
4~3Z~
35 kV winding leadouts must be earthed and the other leadout of
the winding (35 kV) must be connected to the disconnected wire
2 of the line 1.
Consider the diagram illustrated in Figure 8. As a
result of a short circuit in the wire 2, the power transmission
line 1 i5 operated in the incomplete phase regime mode. The
failure resulted in a broken wire 2. A lightning protection
wire rope 20 is provided on the line 1, being earthed at the
receiving ~ub-station 21, its insulation along the entire length ~ .
thereof being capable of sustaining 10 kV voltage.
The power transmission line 1 connects buses 21
(110 kV) of the receiving sub-statïon and buses 22 (110 kV) of ~ ~ .
: the transmitting sub-station. Connected to the buses ;
; 22 (110 kV) via circuit breakers 23 and 24 two three-phase
two-winding transformers 25 and 26 respectively, the rated vol-
tage of their windings being 110 kV and 10 kV, one of the trans-
formers being used for supplying power consumers connected across ~.
buses 2!7 (10 kV). Generally, taking into consideration the value
of permissible overload, the power delivered by one of the trans-
formers 25 or 26 is sufficient to supply all consumers connected .
across buses 27 (10 kV). These buses are coupled to the trans~
former 25 via circuit breaker 28. Figure 8 illustrates the . :~
coupling of only one transformer (25) since the other transformer :
.(26).. is disconnected from the buses 28 (10 kV), ~
`
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-20- `
o
while one of the leadouts of the 10 kV winding of the latter
transformer is earthed and the other leadout of the 10 kV
winding is connected to the lightning protection wire rope 20.
Figure 9 illustrates a 110 kV line 1 with a disconnected
wire 2. The sub-station diagram is the same as in the preceding
example. The transformer 4 is connected to the line at the
receiving sub-station. Figure 9 shows only 110 kV winding of
the transformer, which is disconnected from earth by the dis-
connector 7. At the transmitting sub-station the disconnected
wire is connected to one of the leadouts of the 10 kV winding
of the transformer 26. Other operations at the transmitting `
sub-station are the same as in the previous example.
In case the disconnected wire 2 is broken as a result
of the fault, use can be made of the lightning protection wire
rope.
Referring to Figure 10, illustrated is the line 1 with
the disconnected wire 2. The diagram of receiving and trans-
mitting sub-stations is the same as in the previous example.
The line 1 is provided with a lightning protection
wire rope 20 which is insulated relative to earth. On the
receiving sub-station side the wire rope is earthed by a dis-
connector 29. In order to switch the line to operate in the in-
complete phase regime, the disconnector 29 is switched off, the
wlre rope
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32~
20 is connected to the neutral of the transformer 4 and the
neutral is disconnected from earth by means of the disconnector
7. At the transmitting sub-station, the wire rope 20 is con-
nected to one of the leadouts (10 kV) of the transformer 26.
Other operations at the transmitting sub-station are the same
as in the previous example.
Thus, a method is proposed for reducing current un-
balance when operating the electrical system of a three-phase
A.C. power transmission line in an incomplete phase regime by
using the wires of the line involved and transformers electric-
; ally coupled thereto, the method permitting the power trans-
mitted over a line operating in the incomplete phase regime to
be increased 1.4 to 1.6 times with only negligible expenses
involved, with a simultaneous reduction of zero sequence currents.
Consider two calculated examples of increasing the
transmitted power in the incomplete phase regime of the line.
Example 1
A 150 km long 220 kV line is operating in the incomplete
phase regime. From the transmitting sub-station side the line
is connected to a 1400 MWt power station. Operating in the
receiving system are generators having a total output of 400
MWt.
.' ,. ' ' .
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3Z'O
Owing to current unbalance in the generator windings the
power transmitted over the line operating in the conventional
incomplete phase regime does not exceed 62 MWt.
If the deenergized wire of the line earthed at the
receiving sub-station is connected from the transmitting sub- -
station side to a voltage source of 35 kV, the power trans-
mitted over the line can be increased to 108 MWt, the current ~ -
unbalance in the generator windings being equal.
Example 2
A 40 km long 110 kV line is supplying through a trans-
former located at a 1~ MWt receiving sub-station a region which
consumes 10 MWt. Current unbalance of electric receivers is
limited to 20%. In the conventional incomplete phase regime
the power transmitted is limited by current unbalance to 3.2
MWt.
If the disconnected wire of the line is connected to
the disconnected from the earth neutral of the transformer of
the receiving sub-station, while a voltage of 10 kV is impressed
at the other end of said disconnected wire, the power transmitted
20 in the incomplete phase regime of the line and with the above-
mentioned current unbalance can be increased to 7.4 MWt.
