Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1056451
1 The present invention relates to a control
nppara~us for a converter of an ~C-DC parallel power
transmi~ion system in whlch an alternating current
power transmission system is connected in parallel to
a direct current power transmission system for trans-
mission and receipt of power.
Most of the existing power systems are
comprised of only an alternating current power trans-
mission system. With the progress of the technologi-
cal research into a large-power switching element
such as a thyristor, on the other hand, a converter
making use of such an element has almost reached a
point of practical application. In line with this,
transmission and receipt of power by means of a direct ~-
current power transmission system was planned and
being carried out in actual practice. In power trans-
mission and receipt by a direct current power trans-
mission, unlike the case of an alternating current
power transmission, it is not required to pay atten-
tion to the transient stability of the system involved.This leads to the advantage that the equipment of the
direct current power transmission system can be used,
for example, at the full current capacity of the trans-
mission line of the direct current power transmission
system. However, in view of the fact that a DC circuit
; breaker of large power is still under technological
development, the freedom of system operation is
lacking.
Therefore, the direct current power trans-
mission system, if introduced into the present power
,
. ~ .
~,,,i~
105~4Si
system, cannot entirely take the place of the ~ltcrnating
cu~rent po~er transmission sy~tem. Rather, the future
po~er system i9 most probably expected to be mainly
com,prised of the alternating current power transmis-
~ion system now used and a direct current powertransmission system arranged in parallel at strategic
points.
One of the greatest technological problems
encountered when operating the direct current power
transmission system in parallel to the alternating
current power transmission system is that of the
effect that the fault, which may occur in the alter-
nating current power transmission system, has on the
converter in the direct current power transmission
system. A typical one of such an effect is the
reduction in the AC voltage at the AC-DC connecting
point due to the fault. In other words, the large-
power switching element such as a thyristor presently
used for the converter is such that the timing at
which it is turned on with a firing signal can be
controlled, but the thyristor can not be turned off
with a control signal. Thus it is exclusively by
the use of the reverse voltage applied to the switch-
ing element that the switching element turned on by
the firing signal is turned off. Since the reverse
; voltage is given by the AC voltage at the AC-DC
connecting point to which the converter is connected,
the reduction in the AC voltage at the AC-DC connect-
ing point makes the operation of the converter
impossible.
105~45~
1 For this reason, in the event that the AC
voltage at the AC-DC connecting point connected with
the converter is reduced below a certain level, it
has been customary to suspend the operation of the
converter; and it is resumed at a later time when the
fault of the alternating current power transmission
system has been eliminated and the original AC voltage
has been restored.
In this way, the AC-DC parallel power trans-
mission system is operable. The control circuit forthe converter, however, incorporates a primary delay
circuit and other various time delay factors. There-
fore, it i~ some time later than the restoration of
the AC voltage that the converter comes to perform
its function to the full. This means that, apart
~rom the period during which the voltage of the power
system is reduced by the fault of the alternating
current power transmission system, transmission and
receipt of the power by the direct current power
transmission system is impossible for some time even
after the elimination of the fault. From the viewpoint
of the operation of the power system, however, it is
desirable that the direct current power transmission
system begins to display its full ability simultaneous-
ly with the elimination of the trouble, thus contri-
buting to stabilizing the power system.
Accordingly, it is an object of the pre-
sent invention to provide a control apparatus for
the converter so designed that the converter resumes
~0 its normal operation immediately after the elimination
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105~451
of any f~ult of an alternating current powcr trans-
mission system which may occur in a power system
including an AC-DC parallel power transmission system.
Another object of the invention is to pro-
vide a control apparatus for thé converter so design-
ed that an optimum transmission power of the direct
current power transmission system to enable the direct
current power transmi-ssion system to contribute to
the stable operation of the AC-DC parallel power
transmission system is determinable at the time of
elimination of a fault in the alternating current
power transmission system.
Still another object of the invention is
to provide a control apparatus for the converter so
designed that the transmission po~lér of the direct
current power transmission system is reduced to main-
tain the voltage at the AC-DC connecting point of
the AC-DC parallel power transmission system when
the voltage at such a connecting point drops due to
a fault in the alternating current power transmission
system.
In order to achieve the above-mentioned
objects, there is provided, according to the present
invention, a control apparatus for the converter of
the direct current power transmission system, com-
prising first means for producing a first voltage signal
corresponding to a required controlled delay angle for
giving at least a minimum margin angle determined as a function
~f the voltate at the connecting point between the AC and DC
transmission lines and the current in the direct current power
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1 transmission line, second means for producing a second
voltage signal as a function of the difference between
a current reference value of the direct current power
transmission system and an actual current value thereof,
third means for selecting one of the first and second
voltage signals, and f~21rth means for applying to
the converter, in response to a phase signal in phase
with the voltage at the AC-DC connecting point, a fir-
ing signal adapted to fire the converter at a con-
trolled delay angle corresponding to the selectedvoltage signal wherein the improvement comprises
fifth means for maintaining the phase of the phase
signal in phase with the voltage at the AC-DC connect-
ing point even after the voltage at said AC-DC con-
necting point drops transiently, and means for furthercontrolling the controlled delay angle given by the
firing signal in response to the voltage drop at the
AC-DC connection point.
~he above and other objects, features and
advantages will be made apparent by the detailed des-
cription taken in conjunction with the accompanying
drawings, in which:
Figs. la, lb and lc show block diagrams
for explaining the background of the present invention,
in which Fig. la is a diagram schematically showing
an example of the construction of the AC-DC parallel
power transmission system, Fig. lb a schematic diagram
showing an example of the fundamental construction
of the control circuit for the converter of the direct
current power transmission system, and Fig. lc a
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1056451
1 diagram schematically showing an example of the phase
locked oscillator used in the control circuit shown
in Fig. lb;
Figs. 2a, 2b and 2c are diagrams for gene-
rally explaining the effect of the present invention,
in which Fig. 2a is a diagram showing the transmission
power for the AC-DC parallel power transmission system,
~ig. 2b a diagram showing the transmission power at
the time of occurrence of a fault in the alternating
current power transmission system incorporating no
means of the invention, and Fig. 2c a diagram showing
the transmission power at the time of occurrence of
the same fault as in Fig. 2b above in the presence of
the means of the invention;
... ..
15 Pig. ~ is a diagram for explaining the fun-
damental operating principle of the invention;
Fig. 4 is a block diagram showing an embodi-
ment of the present invention;
Fig. 5 is a waveform diagram for explaining
the operation of the embodiment of Fig. 4;
Fig. 6 is a diagram for explaining another
effect of the embodiment shown in Fig. 4;
Fig. 7 is a block diagram showing an example
of a circuit used in the embodiment of the invention;
25Figs. 8a to 8c show waveforms for explain-
ing the operation of the circuit of Fig. 7;
Figs.-9 and 10 show a couple of partial
modifications of the embodiment of Fig. 4; and
Fig. 11 is a block diagram showing still
another embodiment of the invention.
,
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1056451
1 Prior to describing in detail the embodiments
of the invention, explanation will be made of the
background of the invention with reference to the
drawing3 for facilitating the understanding of the
invention.
; It i9 already explained that both the alter-
nating current power transmission system and the
direct current power transmission system have their
own advantages respectively. Therefore, in order
10 to positively introduce the direct current power -~
transmission system into the power system, an effec-
tive way will be to make use of the AC-DC parallel
power transmission system as shown in Fig. la to lc.
An overall configuration of such a system
15 is schematically illustrated in Fig. la. Reference
characters A and B show power stations, which have
power supplies Gl and G2 respectively behind the
reactances AC~ of the alternating current power
' transmission system. A couple of AC power trans-
20 mission systems A~l and A~2 and a DC power trans-
mission system D~ are interposed between the power
stations A and B. The AC power transmission systems
; A~l and A~2 are connected to the bus bars AB and BB
through circuit breakers CBl; CB2 and CBl'; CB2' res-
25 pectively. The DC power transmission system D~ is
~; connected to the bus bars AB and BB through converters
1 and 2 and transformers TRl and TR2 respectively.
By the way, the DC power transmission system D~ in-
cludes a DC reactor DC~. High harmonics filters
30 Fl and F2 are inserted between the bus bar AB and
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A .. . . . .
. . . .
lOS6~51
the ~round and between the bus bar BB and the ground
respectively.
The fundamental con~truction of the control
apparatus 9 for the converter 1 of the above-mentioned
system is shown in Fig. lb. The control apparatus 9'
for the converter 2 is the same in construction a~ the
control apparatus 9 and hence Fig. lb omits the detail
of the apparatus 9'. Also the explanation will be
made, hereinafter, of only the control apparatus 9.
The control apparatus 9 is impressed with voltage
signal from the bus bar AB and current signal from
the direct current power transmission line DL, res-
pectively, through AC potential transformer 15 and
DC current transformer 25 on the one hand, and cur-
rent reference signal Idp for the converter 1 and a
command for determining whether or not a current
margin signal ~I should be given depending on the ope-
ration mode, invertor- or rectifier-operated mode of
the converter on the other. Reference numeral 4 shows
a con3tant ext.i.nctJ.on angle control circuit which produces, in
response to signals from the AC potential transformer 15 and
the DC current transformer 25, an output voltage signal for
giving a controlled delay angle corresponding to the constant
margin angle whereby to assure the operation of the converter
1 without any commutation failure. Numeral 11 shows
an adder which is impressed with a current refer-
ence Idp, an actual current Idr of the direct current
power transmission system obtained from the DC current
transformer and the current margin signal ~I from the
switch SWl turned on in response to a command to give
~OS~.451
1 ~he cu~rent margin si~nal ~ 11 of these signals
are applied to the adder 11 a~ polarities shown in
the drawing under consideration. Numeral 13 shows
an amplifier for amplifying the output from the adder
11. Numeral 10 shows a voltage selector circuit
for producing a control voltage Ec corresponding
to a delay angle determined by the output of the
constant extinction angle control circuit 4 or a
delay angle determined by the output of the amplifier
13, whichever is smaller. Numeral 6 shows a phase
locked oscillator which produces a phase signal in
phase with the line voltage of the power station
given by the AC potenti~l transformer, i.e., the AC . :
- voltage at the AC-DC connecting points tl and t2.
:: 15 Numeral 8 shows an automatic pulse pAase shifter for
applying a firing signal to the converter 1 at the
controlled delay angle a corresponding to the control
~ignal Ec. 'rhe automatic pulse phase shifter 8 has
a controlled delay angle from amin to amaX as its
characteristics are briefly illustrated in the draw-
ing, even though it is not limited to the one shown
in the drawing. As is well known, the various con-
stants of each circuit are determined under the normal
control conditions in such a manner that a takes an
; 25 appropriate value smaller than 90 (for example, 15)
in the off state of the switch SW1, nameiy, in the
rectifier-operated mode and larger than 90 (for
instance, 140) in the on state of the switch SW1,
namely, in the inverter-operated mode. In many cases,
amin and amaX assume the values of about 5 and 160
_ g _
1056451
r~spectively.
When the converter 1 is operating as an inverter,
the constant extinction angle control circuit 4 determines the
controlled delay angle ~ corresponding to the minimum margin
angl~e y for the converter to operate without commutation failure
as a function of the voltage E2 at the connecting point t
and the dc current Id by using the following equation:
~ = r + U = ~
cos y - cos B = XId
where X = the commutating reactance
and ~ = the control angle
The constant extinction angle control circuit also
provides a control voltage (first voltage signal) Ec which is
determined from the controlled delay angle a by taking into
consideration the phase-shift characteristics of the automatic
pulse phase shi~ter 8.
~ig~ lc shows schematically an example of
the phase locked oscillator 6, the detail of which
is known from Proceedings of the IEEE Vol. 63, No. 2,
~ebruary 1975 pages 291 to 306, "Phase-Locked Loops"
by Someshwar C. Gupta. Symbols In and Out show input
and output terminals respectively. Numeral 61 shows`
a phase difference detector circuit for producing an
output voltage associated with the phase difference
between the input In and the output Out. Numeral 62
shows a smoothing circuit which produces an output
delayed by its time constant in response to the output
of the phase difference detector 61. Numeral 6~ shows
a voltage controlled oscillator which oscillates at
a frequency corresponding to the output volt~ge of
the smoothing circuit 62. By appropriately determining
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:105~4Sl
the constants of each circuit, it is possible to keep
the sienals In and Out in the same phase and to produce
a signal Out always, even in the transient absence
of signal In, in phase with the signal In just before
lt expires.
As will be understood from the foregoing
description, by constant steady application of a
phase signal from the ~C-DC connecting point to the
control apparatus of the well-known converter, the
operation of the direct current power transmission
system becomes possible simultaneously with the
restoration of the voltage which may have dropped
transiently at the AC-DC connecting point.
The voltage drop at the AC-DC connecting
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105~;451
1 point is attributable to various causes. Most of
them are the grounding or short-circuiting faults
of the alternating current power transmission line
occurring at such points as f, ~' and f" as shown
in Fig. la. These faults are eliminated by the
operation of appropriate protective relays not shown
in the drawing. In the case of the fault f in the
alternating current power transmission line AL2, for
instance, the circuit breakers CBl' and C~2' are
opened, thereby restoring the voltages at the AC-DC
connecting points tl and t2.
In Figs. 2a, 2b and 2c, the difference
angle e between the A~ voltages at the power stations
A and ~ is plotted horizontally, and the transmission
power P vertically. The graph of Fig. 2a concerns
the normal condition; the result obtained by the
control operation on the conventional principle is
shown in Fig. 2b; and the effect of the control
according to the invention is illustrated in Fig. 2c.
In the drawings, ~1, Q2 and ~3 show curves represent-
ing transmissible power through the alternating cur-
rent power transmission lines ALl and AI2 respectively.
In other words, the curve ~1 is associated with the
case where both the alternating current power trans-
mission lines AL1 and A~2 are operating normally;the curve Q2 the case where one of the alternating
current power transmission lines ALl and AL2 is not
operative; and the curve ~3 the case where the voltage
has dropped due to a fault of the alternating current
power transmission line. This transmissible power
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105f~4Sl
in tl~e alternating current power transmission line,
a~ well known, i~ 2xpressed as
Vl - V2
X sin e
where X is the reactance of the alternating current
power transmission system; Vl and V2 the voltages at
the terminals of the alternating current power trans-
mission line; and ~ the phase difference between
Vl and V2.
The graph of Fig. 2a shows the state where
the transmission power Po i8 distributed under the
normal condition between the alternating current power
transmission line and the direct current power trans-
mission line taking shares Pa and Pd respectively.
Under this condition, the phase difference between
the power stations A and B is expressed as ~0. If a
fault as shown by f occurs in the alternating current
power transmission line AL2, the result of the con-
2n ventional control is as shown in Fig. 2b. The voltageat the AC-DC connecting points tl and t2 drops. The
transmission power Pd is reduced naturally to zero,
while the power Pa is decreased also to the level b3.
The mechanical input to the rear power supplie~, on the other
~and, remains enough to supply-the transmission power
Po, so that the power supplies are accelerated. The phase
difference ~ begins to increase. If the fault is removed
when 40 reaches ~1~ transmission power is restored to the level
Q'2 but the direct current power transmission line has yet to begin
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- 1056451
1 transmission. When ~2 is reached subsequently, the
direct current power transmission line begins trans-
mission of power Pd. The phase difference, however,
continues to increase until transmission begins of
the deceleration competitive wlth the acceleration
occurred in the processes of changes from ~0 to ~1 to
2' namely, until the power higher than the input
begins to be transmitted. When ~3 is reached, the
acceleration is finally balanced with the deceleration
and the phase difference begins to decrease. In the
conventional method of control, it will thus be seen
that DC power transmission is not restored simultane-
ously with the elimination of a fault and therefore
the acceleration is increased as much.
The advantageæ of the present invention are
illustrated in Fig. 2c. As in the case of Fig. 2b,
a fault occurs under the transmission of the power
Po (= Pa + Pd) at the phase difference of ~0 and the
fault is eliminated at the phase difference ~1 In
the case of the invented control shown in Fig. 2c,
not only the direct current power transmission is
restored but also the power Pd' begins to be trans-
mitted simultaneously with the elimination of the
fault at ~1 (Pd' is of course higher than Pd.)
As a result, the acceleration is not uselessly
increased on the one hand, and a great deceleration
is obtained on the other. The phase difference is
thus prevented from increasing more than ~3', greatly
contributing to stabili~ing the system.
In Fig. 2c, the DC power is increased to
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1056451
1 the level of Pd' only during the period of the in-
crease in the phase difference and restored to Pd
after ~3' is reached. In this way, a useless
fluctuation of the phase difference can be avoided.
As will be easily understood without special
reference to the drawings, in the face of the faults
such as f' and f" occurring outside of the AC-DC
parallel power transmission system, a control opera-
tion similar to that of Fig. 2c contributes to the
stable operation if the place of occurrence of the
fault is relatively near the system and the power
transmission of the direct current power transmission
line is stopped during the continuance of the fault.
In spite of the above-mentioned advantages,
; 15 the operation according to the invention may cause
the transmission power to be transiently increased
to Pd'. However, since the duration of this transient
power increase is very short J it will not cause any
trouble to operate the converters 1 and 2 and other
elements under overload during such occasion. ~o
implement this invention, therefore, it is not neces-
sary to provide a direct current power transmission
system with a rating capacity of Pd'.
~he diagram of Fig. ~ is for explaining
2~ to what degree the transmission power of the direct
current power transmission line should be increased
in eliminating a fault. In the drawing, the abscissa
represents the current Id of the direct current power
transmission line, while the ordinate shows the AC
voltage ET at the ~C-DC connecting point, the voltage
~056451
Vd of the direct current power transmission line,
the DC power Pd, the operating power factor Pf of the
converter~ and the phase difference ~ between the
AC voltages of the power stations A and ~. The graphic
presentation of Fig. ~ i9 bas~d on the calculations
made assuming that the tap positions on the trans-
formers TRl and TR2, hence the voltages of the rear
power supplies, are fixed and that the resulting
fixed power is shared by the alternating current power
transmission lines A~l and AL2 and the direct current
power transmission line D~ for the purpose of trans-
mission. Assume that the systems are operated at Id
(= Io). As will be seen ~rom the graph, in order to merely
increase the DC power Pd, it will be effective to increase the
DC current from Io to Idm. This, however, results in decreasing
not only ET and Vd, but also Pf, thereby incretasing the!)phase
difference ~. For the stable operation of the power
system, a smaller phase difference ~ is desirable.
Therefore, any increase of current from Id (= Io)
should be limited to Ido. The operation according
to the invention may utilize the fact that ET, Vd and
Pf assume their points of inflection at the point
Ido of the DC current.
A block diagram of an embodiment of the
invention covering only a power station is shown
in Fig. 4. In the drawing, like reference numerals
denote like component elements as in Fig. 1, and the
other reference numerals and characters will be first
described below.
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1 . Numerals 21 and 23 8hoW ~C current trans-
formers for detecting the currents flowing in the
alternating current power transmission line~ ALl and
AL2. Reference characters r and x show a resistance
and a reactance respectively similating the alternat-
ing current power transmission lines ALl and AL2.
By applying a sum of the outputs of the AC current
transformers 21 and 23 to the resistance-reactance
combination, a voltage simulating the phase signal
of the AC voltage at the power station B is obtained
thereacross. Numeral 20 shows a maximum difference
detector circuit. A "1" signal is produced from
this detector circuit 20 when the phase difference
between the simulation voltage of the other power
station obtained across the series-connected r and x
and the voltage of the power station at this end
obtained from the AC potential transformer 15 reaches
a maximum point, i.e., when the phase difference
which has so far been increased begins to decrease.
An example of the circuit under consideràtion will be
described more in detail later. Numeral 17 shows a
DC voltage transformer for introducing the voltage
Vd of the direct current power transmission line.
Reference numerals 22 and 32 show rectifier circuits
- 25 which rectify the voltages obtained at the AC potential
transformers 15 and 17 and produce them in the form
of DC voltages. Numeral3 24, 28 and 40 show comparator
circuits for comparing the reference voltages Vcl,
Vc2 and Vc3 with the input voltages el, e2 and e3
~0 respectively. Specifically, it is assumed that the
,
- 16 --
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1056451
l comparator 24 produces a "l" output when el is higher
than Vcl, that the comparator 28 produces a "l" out-
put when e2 is higher than Vc2, and that the comparator
40 produces a "l" signal when e3 is higher than Vc3.
Under the normal conditions of the power system, the
voltages Vcl and Vc2 are as high as about 50~0 of the
voltages el and e2 respectively while the voltage
Vc~ takes a positive value almost zero. As a result,
the comparators 24 and 28 naturally produce a "1"
signal when the voltage of the AC-DC connecting point
is normal. ~umeral 26 shows a flip-flop circuit,
which is set and produces a "1" signal in response
to an input at the set terminal S when the output
of the comparator circuit 24 change,s from "0" to "l",
namely, when the voltage at the AC-DC connecting point
less than 50% of the rating is restored to the rating.
The nip-flop circuit 26 is reset and produces a "0"
signal in response to an input to the reset terminal
R when the output of the maximum phase difference
detector circuit 20 changes from "0" to "1". ~umeral
34 shows a ramp voltage generator circuit which pro-
duces a continuously increasing voltage in response
to a "l" signal from the flip-flop circuit 26. ~Ihen
the output of the flip-flop 26 changes to "0", on the
other hand, the output of the circuit 34 is restored
to ~ero immediately. Numeral 36 shows a tracking and
holding circuit. When the flip-flop 38 produces a
"0" output, the output'of the ramp voltage generator
circuit 34 is followed by the output of the tracking
and holding circuit 36; whereas when the output of
- 17 -
1 the flip-flop 38 chang 4sStlo "1" the tracki d
holdin~ circuit 36 produce3 the same output as that
produced by the ramp voltage ~enerator circuit ~4
immediately before the change of the state of flip-
flop 38 to "1". Numeral 30 shows what is called aprimary delay circuit, or first order lag circuit,
~ith a comparatively short time constant. Numeral 27
shows an adder for taking a sum of the outputs of
the primary delay circuit 30 and the rectifier circuit
32 at the shown polarities. As long as the voltage
of the direct current power transmission line is
constant for stable operation, the output voltage e3
of the adder 27 is maintained at zero. If the voltage
of the direct current power transmission line sudden-
ly drops, however, the voltage e3 changes to positive,and vice versa. Therefore, assuming that the compa-
rator 40 compares the output of the adder 27 with a
reference voltage Vc~ which is substantially zero
and produces an output when e3 is higher than Vc3~
then the primary delay circuit 30, the adder 27 and
the comparator 40 are able to detect a point of sudden
voltage drop. Numeral 29 shows an AND circuit for
producing a "1" output in the presence of both the
outputs of the comparators 28 and 40 and an input "1"
applied to the input terminal 100. The "1" input is
applied to the AND circuit 29 only when the direct
current transmission system is under operation. By
this, the output of the comparator 40 is rendered
ineffective when the direct current transmission line
is separated from the transmission system. The presence
~- 18 -
Y~
. . ,
1~56451
of any output from the ~ND circuit 29 means therefore that a
voltage drop has occurred in the direct current power trans-
missi~n line under a high voltage level operation.
Numeral ~8 shows a flip-flop, which is set
to produce a "1" signal in response to a "1" signal
from the AND circuit 29 and reset to produce a "O"
signal in response to a "1" signal from the maximum
phase difference detector circuit 20. In this con-
nection, take note of the output of the tracking and
holding circuit 36 of the alternating current power
transmission system. Once the voltage is restored(The DC current is also restored) after it is decreased
(Of course, the voltage of the direct current power
transmission line drops also at the same tLme), the
tracking and holding circuit 36 produces the same out-
put as the ramp voltage generator circuit 34. At a
time point when the voltage of the direct current
power transmission line begins to drop and the flip-
flop 38 produces a "1" signal, the tracking and holding
circuit 36 holds the value of the output of the ramp
voltage generator 34 at that very instant. When thedifference reaches a maximum point, the maximum phase
difference detector circuit 20 produces a "1" output,
æo that the both the flip-flops 26 and 38 are reset
and the tracking and holding circuit 36 produces a
"O" output. The output of the tracking and holding
circuit 36 is applied to the adder 31 at the shown
polarity. In other words, the higher the output
voltage of the tracking and holding circuit ~6, the
19 .
~. .
lOS~451
1 current reference value Idp of the converters becomes
equivalently larger. It was already explained that
when the current of the direct current power trans-
mission line is increased to the level shown by Ido
in ~ig. 3, the tracking and holding circuit 36 detects
and holds the voltage drop of the direct current
power transmission line. Thus the output of the con-
verters 1 is controlled at an optimum value for stable
operation. It will also be understood from the fore-
eoing description that it is until the phase differencereaches ~3' in Fig. 2c that such an output is produced
from the tracking and holding circuit 36.
Explanation will be made of the other parts
of the circuit shown in Fig. 4. Numerals 33, 35, 37
and 39 show adders to which input signals are applied
at shown polarities respectively. The adder 33 is
impressed with the output of the adder 31 and, when
the switch SWl is turned on with the converter 1
inverter-operated, impressed also with the current
margin ~I. The adder 39 is impressed with the output
of the rectifier circuit 32 and the bias voltage
Vb. The bias voltage Vb is set at a level obtained
at the output of the rectifier circuit 32 when the
direct current power transmission line is in rated
condition. The adder 39 therefore produces a posi-
tive voltage as long as the direct current power
transmission line is operated with its voltage reduced
below a predetermined level. The adder 35 is impressed
with the output of the adder 39 at the shown polarity.
When the voltage of the direct current power
- 20 -
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1 transmission line i~ in the reduced condition, there-
fore, the function is apparently is equivalent to
that obtained when the current target of the converter
is reduced. The operation characteristics under such
a condition are shown in ~ig. 6. The solid lines in
the drawing show the operations at ratings. Assuming
that the system is operated at Vd and Idp and the
voltage of the alternating current power transmission
line drops to Vd', the current is also reduced to
I'dp thus causing the system to operate under the
characteristics shown by dashed lines.
' ~y reducing the current in accordance with
the voltage drop in this way, the reactive power
required of the converter l is also reduced, with the
re~ult that the voltage drop at the AC-DC connecting
- point is minimized. The adder 37 is for causing the
reference value of the current corrected on the basis
of various factors as mentioned above to be brought
face to face with the actual current of the direct
current power transmission line. Numerals 3 and 5,
like numeral lO, show voltage selector circuits. The
voltage selector circuit 3 compares the output of the
voltage selector circuit lO with a positive voltage
' of Va90 applied'thereto through the switch SW2, and
' 25 produces a lower one of them. Numerals 41 and 43
show polarity-reversing circuits for producing a
voltage of the same magnitude but of,different
~; polarity. The voltage selector circuit 5 is for
comparing the output of the polarity-reversing
circuit 41 with the negative voltage ~Va90 applied
- 21 -
105f~451
1 thereto through the switch SW3 and produces a signal
more negative than the other. The polarity-reversing
circuit 43 is for reversing the polarity of the output
of the voltage selector 5 and applying it to the auto-
matic pulse phase shifter 8. The voltages +V~gO and
~Va90 have an absolute value suitable for production
of a firing signal by the automatic pulse phase shifter
8 at a = 90. When the switches SW2 and SW3 are turned
on, therefore, a firing signal is produced from the
automatic pulse phase shifter 8 at the timing of
a = 90 regardless of the operation of each element,
as described later.
The switches SW2 and SW3 are controlled by
the ~witch drive circuit 18. The switch drive circuit
18 is impressed with the output of the polarity-
reversing circuit 45 reversing the output of the com-
parator 24. As long as the alternating current power
transmission line is operating normally, the compa-
rator 24 produces a "1" signal and therefore the output
of the polarity reversing circuit 45 is in the state
of "0", thus keeping the output of the switches SW2
and SW3 in the off state. When the voltage of the
alternating current power transmission line is reduced
and the output of the comparator 24 becomes zero, by
contrast, the polarity reversing circuit 45 produces
a "1" signal so that the switches SW2 and SW3 are
turned on through the switch drive circuit 18. In
other words, when the voltage of the alternating
current power transmission line drops to such a degree
as to make normal converter operation impossible, the
.
- ~2 -
1056451
1 control function is made ineffective and a firing
signal continues to be applied to the converter.
In this way, voltage restoration is awaited.
The magnitude and variation of the signals
of the respective parts related to the foregoing
description are taken into consideration in the dia-
gram of Fig. 5 in which time is plotted horizontally.
In the drawing, character a shows the controlled
delay angle of the converter which is assumed to
take the value of al under normal conditions.
During the period from tl to t2 when the AC voltage
drops sharply, a is assumed to be 90. Until the
time point t3 where the phase difference reaches its
maximum, the delayed angle al is reduced by aO, thus
increasing the transmission power. For subsequent
periods such as between t4 and ts when the voltage
- drop is small, aO' is added to al thereby to lessen
the reactive component required by the converter.
An example of the maximum difference
detector circuit 20 and an outline of the operating
waveforms thereof will be explained with reference
to Figs. 7 and 8 respectively showing a block diagram
of the circuit 20 and waveforms of operation thereof.
' In Fig. 7, reference numerals 201 and 203 show zero
voltage detectors which are impressed with the voltages
Etl and Et2 at the AC-DC connecting points tl and
t2 and produce a pulse at a point where the above-
mentioned voltages are reduced to zero respectively.
The operation of the zero voltage detectors 201 and
203 is ~hown in Figs. 8a, 8b and 8c. Numeral 202
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~056451
1 shows a flip-flop circuit set and reset in response
to output pulses from the zero voltage detectors
201 and 203 respectively. (The setting is rendered
by a pulse associated with the zero value of the
voltage at the AC-DC connecting point on the power
station A side, while the resetting is accomplished
by a pulse associated with the zero value of the
simulation voltage at the AC-DC connecting point on
the power station ~ side.) ~he signal waveforms for
such operations are shown in Fig. 8d. Numeral 204
' shows an integrator for integrating the output of
the flip-flop 202. The output signal of the integrator
204 has a peak proportional to the phase difference,
and the signal waveform thereof is,shown in ~ig. 8e.
Numeral 205 shows a smoothing circuit impressed with
and smoothing the output of the integrator 204, the
signal waveform thereof being shown in Fig. 8f.
Numeral 206 shows a memory comprised of the primary
delay circuit which is impressed with the output
signal from the smoothing circuit 204 and produces
the same signal a predetermined time later. Numeral
207 shows an adder for adding the outputs of the
primary delay circuit 206 and the smoothing circuit
, 205 to each other at shown polarities. Numeral 208
shows a comparator which, in response to the output
', of the adder 207, produces an output signal. This
' output signal changes from "1" to "0" when the
polarity of the output signal of the adder 207 changes
from positive to negative, namely, when the phase
difference is reduced. Numeral 209 shows a
- 21~ -
105~45~
1 differentiating circuit for applying reset signals
to the flip-flops 26 and 38 at the time of change of
the output of the circuit 209 from "1" to "0". In
this way, the fact that the phase difference has
reached ~3' is detected in Fig. 2, whereupon the
DC transmission power is returned from Pd' to Pd.
The present invention is not limited to
the embodiment of Fig. 4 but may be embodied in
various modifications. As explained with reference
to Fig. 3, when the current in the direct current
power transmission line is increased beyond Ido,
not only the DC voltage Vdl but also the voltage at
the AC-DC connecting point and the power factor of
the converter undergo a sudden change. It will be
noted that this point of inflection may be detected
to set the flip-flop circuit 38 in ~ig. 4. This
concept is incorporated in the embodiments shown in
Figs. 9 and 10. These circuits are built around the
circuit elements used for setting the flip-flop 38,
leaving the other parts identical with like elements
in the other embodiments. Also, like numerals are
attached to those elements whose functions are similar.
As will be obvious by comparing the output of the
rectifier circuit 32 in Fig. 4 with that of the
rectifier circuit 22 in Fig. 9, the circuit arrange-
ment of this particular part is quite the same
e~cepting the detection of the point of inflection
of the voltage at the AC-DC connecting point. The
circuit of Fig. 10 is configured with special emphasis
on the change in the power factor of the converter 1
.
- 25 -
1056451
1 nnd ha~ an additional ~C current transformer 58.
The ou~puts V nnd I of the AC potential transformer
15 and AC current transformer 58 are applied to the
Hall converter 72 and the multiplier 74 respectively
for calculation of VIcos y and VI, where y is the
phase difference between V and I. The outputs of the
Hall converter 72 and the multiplier 74 are introduced
to the divider 70 to obtain a power factor expressed
by VIvIS ~ = c08 ~. This signal is applied to
the primary delay circuit 28, the adder 27 and the
comparator 40 thereby to detect a point of inflection.
The AND circuit 102 is provided to allow the output
of the comparator 40 to be applied to the flip-flop
38 only when the direct current transmission line is
under operation. In this case, if the voltage i9
zero, no power factor is calculated out and therefore
the need for the comparator 28 and the AND gate 29
as in the circuit of Fig. 4 is eliminated.
Further, instead o4 starting the increase
in the current reference value at the time of voltage
re~toration at the AC-DC connecting point after shoot-
ing the fault, the voltage restoration of the direct
current power transmission system may of course be
utilized, as illustratively shown in Fig. 11. By
comparing Fig. 11 with Fig. 4, it will be obvious
that in the case of Fig. 11 it sufficies if the out-
put of the rectifier circuit 22 in Fig. 4 is replaced
by that of the rectifier 32 as an input to the com-
parator circuit 24. In Fig. 11, the AND circuit 104
~ - 26 -
: .
~OS~451
1 is corresponding to the AND circuit 102 in Fig. 10
and its input 105 receives a "1" si~nal only when
the direct current transmission line is under
operation.
It will thus be seen that according to the
invention, in the event of a fault developed in the
power system including an AC-DC parallel power trans-
mission system and the resulting voltage drop at the
AC-DC connecting point, the converter is rendered
ready to enter into operation simultaneously with
the voltage restoration, in response to the voltage
drop. Also, when the converter resumes its operation,
a transmission power most suitable for stable opera-
tion of the power system is automatically determined.
Furthermore, if the voltage at the AC-DC connecting
- point drops if not to such a degree as to make the
converter ready for operation, the transmission power
of the direct current power transmission line may be
reduced accordingly. As a result, the use of the
present invention is considered to enhance the value
o~ the AC-DC parallel power transmission system among
other parts of the power system.
- 27
