Methods of enhancing capacity of transformer equipment and of power flow control using phase-shifting transformers and series impedances in parallel arrangements

A method and apparatus for enhancing the capacity of a transformer for a polyphase AC network. The method and apparatus having a constant current transformer branch connected in parallel with at least one conventional transformer branches. The constant current transformer branch includes a phase-shifting transformer in series with reactive elements. Further, for power flow control between two synchronous polyphase AC network busses, two parallel branches are used in which one includes a phase-shifting transformer, while the other includes a reactive element. An additional phase-shifting transformer is added in series with the reactive element to enhance the power flow control flexibility.

FIELD OF THE INVENTION 
The invention relates to a method of enhancing capacity of transformer 
equipment for a polyphase AC network without augmentation of its 
short-circuit level, and an apparatus thereof. The invention also relates 
to a method of power flow control between two synchronous polyphase AC 
network busses, and an interconnecting apparatus therefor. 
Although the methods and apparatus are directed toward different purposes, 
they are implemented with equipment having the following characteristics 
in common: 
it is connected between two busses of a polyphase AC network or two 
synchronous polyphase AC transmission networks; 
it has at least two branches in parallel whose impedances, having very 
different values, are subjected to phase-shifted voltages; 
it can be constructed from an already existing branch in the network; it is 
then sufficient to add, in parallel with the existing branch, at least one 
branch constructed as described hereinafter; 
the existing branch, if it is the case, can be either a phase-shifting 
transformer or not. 
1. Description of Related Art 
The conventional technique for limiting fault currents in a transformer 
station consists typically to insert an inductance in series with a cable. 
However, such an addition of an inductance in series with the cable 
reduces the transmission capacity of the transformers in the transformer 
station. 
Presently, if a transformer station becomes overloaded and it is impossible 
to install a transformer in parallel with the existing transformer(s) 
without for example overloading the network breakers located downstream of 
the transformer station during faults, the network owner can add a 
conventional transformer and divide the transformer station and its loads 
in two groups. The operating flexibility and reliability of the station 
are however reduced. Alternatively, the network owner can add a 
conventional transformer and replace all the breakers that can possibly be 
subjected to overloads during faults. The operating flexibility and 
reliability are preserved but at a cost that can become prohibitive if a 
high number of breakers must be changed. 
Also known in the art are IPCs (Interphase Power Controllers), which are 
not designed nor intended for the purpose of enhancing the capacity of a 
transformer station. IPCs are rather used to force and/or regulate/control 
a power flow between two busses of a synchronous AC network while ensuring 
a limitation of the fault currents. IPCs involve the use of at least two 
reactive elements (a capacitor and an inductor) per phase, the reactive 
elements being external to and in series with the (phase-shifting) 
transformer used if necessary. The reactive elements are subjected to 
separately phase-shifted voltages. 
Considering the steady state of transmission lines, improper power flows 
can depend on their lengths which are too long. In such a situation, 
series compensation may provide a way to reduce the line impedance and 
increase the power flow. However, it happens sometimes that the phase 
angle .delta. is simply too small to obtain a proper power flow even with 
series compensation. In such a case, a phase-shifting transformer can be 
used to obtain a power flow. 
Known in the art, there is the document IEEE TRANSACTIONS ON POWER 
DELIVERY, vol. 8, no. 3, Jul. 1, 1993, pages 1420-1429, XP000403138 
NOROOZIAN M. Et al. "POWER FLOW CONTROL BY USE OF CONTROLLABLE SERIES 
COMPONENTS" which describes a mathematical model for solving power flow 
control problems and providing information used to construct regions of 
feasible power flows of control lines. The model is devised for a line 
compensated by a series capacitor or phase shifter (i.e. a capacitor or a 
phase-shifter in series with the line). A mathematical formulation of the 
two fundamental series power flow controllers (controllable series 
compensation and phase-shifting transformer) have been developed in order 
to be compatible with load flow calculation programs. To illustrate the 
approach, a simplified network consisting of two parallel lines and of one 
power flow controller in series with one of the two lines is used. 
Then, a general network model is presented to show that the new 
modelization allows the simultaneous use of many power flow controllers. 
Also known in the art, there is the document YOUSSEF R. D. "PHASE-SHIFTING 
TRANSFORMERS IN LOAD FLOW AND SHORT-CIRCUIT ANALYSIS: MODELLING AND 
CONTROL", Jul. 1, 1993, IEEE PROCEEDINGS c. GENERATION, TRANSMISSION, 
DISTRIBUTION, vol. 140, NR 4 T C, pp. 331-336, STEVENAGE G. B., which 
describes a new mathematical model of phase-shifting transformer for load 
flow and fault analysis programs that can be used to represent accurately 
the behaviour of electronic phase-shifters. 
Power flow control can be achieved using two conventional phase-shifting 
transformers connected in parallel to obtain transfer levels not 
attainable with a single phase-shifting transformer. However, since the 
leakage impedances of the conventional phase-shifting transformers are 
low, it is essential that they be identical and always adjusted at the 
same tap position to avoid current flows from one phase-shifting 
transformer to the other. The parallelling of two conventional 
phase-shifting transformers does not provide two degrees of freedom to 
control independently the active power and the reactive powers. 
In order to obtain the desired power flow, it is also known in the art to 
install a conventional phase-shifting transformer in series with a series 
compensation system to compensate a transmission line (electrically 
shortening the line) and force a phase angle at its terminals that is 
different from the phase angle .delta. that would normally be present. 
However, as in the previous case, such a system does not provide two 
degrees of freedom. 
The IPC technology offers another manner of controlling the power flow 
wherever traditionally the phase-shifting transformer technology is 
considered to increase the power flow in a transmission line. But the 
presently known IPCs are designed to interconnect networks while 
decoupling one network from the other, which can be detrimental to the 
network stability. Consequently, a need arises for an apparatus 
specifically adapted for transmission line power flow control in order to 
preserve the inherent synchronizing effect provided by a transmission 
line, need which is not presently filled. 
OBJECTS OF THE INVENTION 
It is therefore an object of the invention to provide a method of enhancing 
capacity of transformer equipment for a polyphase AC network without 
significant augmentation, or even with reduction of its short-circuit 
level, and an apparatus thereof, which are of considerable simplicity, 
reliability and economically advantageous with respect to the prior art. 
It is another object of the invention to provide such a method of enhancing 
capacity and apparatus that can be implemented over an existing 
transformer station. 
It is another object of the invention, in addition to such a capacity 
enhancement, to provide for an apparatus that can generate or absorb 
reactive power to contribute to the support of the network voltages, 
wherein the capacity enhancement is achieved in a completely passive 
manner. 
It is another object of the invention to provide a method of power flow 
control between two synchronous polyphase AC network busses, and an 
interconnecting apparatus therefor, which have the faculty to adjust the 
transfer level of active power between the busses, and which are of 
considerable simplicity and economically advantageous with respect to the 
prior art. 
It is another object of the invention to provide such a method of power 
flow control and interconnecting apparatus which, in addition to the 
active power, have the faculty to adjust the generation or absorption of 
reactive power in the system in order to provide a support of the line 
voltages and to increase the transfer capacity of the system above its 
natural limit or simply to comply with the voltage criteria fixed by the 
network owner. 
It is another object of the invention to provide a method of power flow 
control and an interconnecting apparatus which are especially adapted to 
preserve the inherent synchronizing effect provided by a transmission 
line. 
It is another object of the invention to provide such a method that can be 
applied to uprate a phase-shifting transformer and reduce losses therein, 
by addition of reactive elements in parallel with it. 
SUMMARY OF THE INVENTION 
According to the invention, there is provided a transformer apparatus for a 
polyphase AC network, comprising one or several conventional transformer 
branches in parallel, each including opposite ends for connection with the 
AC network and a transformer having a small leakage impedance so that 
current in the conventional transformer branches produces at the ends 
thereof a small phase angle .delta..sub.sr slightly varying depending on a 
load level of the AC network characterized in that a constant current 
transformer branch is connected in parallel with the conventional 
transformer branches, the constant current transformer branch including a 
phase-shifting transformer to produce at ends thereof a phase angle .psi. 
substantially greater in absolute value than the phase angle 
.delta..sub.sr, and reactive elements in series with the phase-shifting 
transformer, for increasing an impedance of the constant current 
transformer branch, the reactive elements being capable to sustain at ends 
thereof a phase angle equal to .delta..sub.sr -.psi.. 
According to the invention, there is also provided a method of enhancing a 
capacity of transformer equipment for a polyphase AC network, the 
transformer equipment including one or several conventional transformer 
branches in parallel, each including opposite ends for connection with the 
AC network, and a transformer having a small leakage impedance so that 
current in the conventional transformer branches produces at the ends 
thereof a small phase angle .delta..sub.sr slightly varying depending on a 
load level of the AC network, the method being characterized in that it 
comprises the step of: 
adding a constant current transformer branch in parallel with the 
conventional transformer branches, the constant current transformer branch 
including a phase-shifting transformer to produce at ends thereof a phase 
angle .psi. substantially greater in absolute value than the phase angle 
.delta..sub.sr, and reactive elements in series with the phase-shifting 
transformer, for increasing an impedance of the constant current 
transformer branch, the reactive elements being capable to sustain at ends 
thereof a phase angle equal to .delta..sub.sr -.psi.. 
According to the invention, there is also provided an interconnecting 
apparatus for power flow control between two synchronous polyphase AC 
network busses, characterized in that it comprises: 
a first branch having opposite ends for connection respectively with the 
two AC network busses, the first branch including an adjustable 
phase-shifting transformer to produce at ends thereof an adjustable phase 
angle .psi..sub.1 ; and 
a second branch in parallel with the first branch, including reactive 
elements to transfer conjointly with the first branch, active power 
between the two AC network busses depending on the phase angle 
.psi..sub.1. 
The second branch may further include in series with the reactive elements, 
a second adjustable phase-shifting transformer to produce at ends thereof 
an adjustable phase angle .psi..sub.2, the reactive elements having a 
substantially higher impedance than leakage impedances of the first and 
second phase-shifting transformers. 
According to the invention, there is also provided a method of power flow 
control between two synchronous polyphase AC network busses, characterized 
in that it comprises the steps of; 
connecting between the two AC network busses, a first branch including an 
adjustable phase-shifting transformer to produce at ends thereof an 
adjustable phase angle .psi..sub.1 ; 
connecting a second branch in parallel with the first branch, the second 
branch including reactive elements to transfer conjointly with the first 
branch, active power between the two AC network busses depending on the 
phase angle .psi..sub.1 ; and 
adjusting the phase angle .psi..sub.1 for controlling a quantity of active 
power transferred between the two AC network busses or a quantity of 
reactive power absorbed or generated by the reactive elements. 
The method may comprise the additional steps of: 
connecting in series with the reactive elements, a second adjustable 
phase-shifting transformer to produce at ends thereof an adjustable phase 
angle .psi..sub.2, the reactive elements having a substantially higher 
impedance than leakage impedances of the first and second phase-shifting 
transformers (78,86); 
adjusting the phase angle .psi..sub.1 for mainly controlling a quantity of 
active power transferred between the two synchronous AC network busses; 
and 
adjusting the phase angle .psi..sub.2 for mainly controlling a quantity of 
reactive power generated or absorbed by the reactive elements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following description and in the drawings, like reference characters 
refer to like or corresponding parts throughout the several views. 
The two methods and apparatuses hereinbelow described allow to achieve 
classical functions relative to networks in a novel and original manner. 
The following section relates to the method of enhancing capacity of 
transformer equipment for a polyphase AC network without augmentation of 
its short-circuit level, and the apparatus thereof. 
Referring to FIG. 1, there is shown a schematic diagram of a transformer 
station whose short-circuit level is lower to what it would be if 
constructed only with conventional transformers. The transformer station 
is made of; 
one or several transformers 2 in parallel. In the following description, 
the combination of these transformers 2 is represented by an equivalent 
circuit branch referred to as the conventional transformer branch 4; 
one branch 6, in parallel with the conventional transformer branch 4, 
comprising an optional transformer 8 (for step down purposes) and a 
phase-shifting transformer 10 in series with a reactive (inductive or 
capacitive) element 12 which is intended to increase the impedance of the 
branch 6. The combination of this branch 6 is represented by an equivalent 
circuit branch hereinafter referred to as the constant current transformer 
branch, or simply the constant current transformer. 
In order to fulfil the objectives of the invention according to the method 
herein described, the constant current transformer branch 6 must 
necessarily be installed in parallel with the conventional transformer 
branch 4 and it is this combination that forms the system herein 
described. 
The term "station" is herein used in a very large sense; it designates any 
installation, of any dimension, where transformers operate in parallel. 
The case of transformer stations in AC networks constitutes a privileged 
application for the equipment herein described (with however no limitation 
in this respect). 
In FIG. 1, the environment of the transformer station is represented by an 
infinite bus 18 and an equivalent network or load 20. 
In normal operation, the current in the conventional transformer branch 4 
produces at the ends thereof a phase angle .delta..sub.sr between the 
voltages at its terminals 14, 16 that varies with the load level, due to 
the equivalent leakage impedance of the conventional transformers 2. 
However, this phase angle remains small because the equivalent leakage 
impedance is small. The phase angle applied at the terminals of the 
reactive element 12 of the constant current transformer 6 is equal to the 
sum of .delta..sub.sr minus the phase angle .psi. of the phase-shifting 
element of this branch. Since .psi. is much greater in absolute value than 
the phase angle .delta..sub.sr the variations of .delta..sub.sr with the 
load current affect very little the phase angle .delta..sub.sr -.psi. and 
the current in the constant current transformer branch 6 varies little 
whatever the load level in comparison with the current variation in the 
conventional transformer branch 4. Since the current in the constant 
current transformer branch 6 varies little, this equivalent transformer 
can also be qualified as a constant power transformer. 
In short-circuit operation, when the reactive element 12 is an inductor, 
this inductor is selected so that the short-circuit current has the same 
order of magnitude as the load current in this branch 6; this current is 
thus far smaller than the short-circuit current of the conventional 
transformer branch 4 and the total short-circuit current of both branches 
4, 6 does not increase significatively with the paralleling of the two 
branches 4, 6. For this reason, it is possible to say here that the 
apparatus herein described enhances the capacity of transformation without 
increasing the short-circuit level. 
When the reactive element 12 is a capacitor, this capacitor is chosen so 
that its short-circuit current has the same order of magnitude or is 
greater than the load current in this branch 6. Since this current is, at 
the time of a fault, phase-shifted by almost 180.degree. with respect to 
the short-circuit current in the conventional transformer branch 4, the 
total short-circuit current of both branches 4, 6 is reduced. Thus, the 
apparatus herein described enhances the transformation capacity of the 
station while reducing the short-circuit level. 
Whether the reactive element 12 of the constant current transformer branch 
6 is inductive or capacitive, the series impedance of the constant current 
transformer 6 is always much greater than the series impedance of a 
conventional transformer 4. Indeed, during a fault, the constant current 
transformer 6 is subjected to the phase-to-ground voltage of the network 
20 and its series impedance must therefore be higher than the impedance of 
a conventional transformer 4 in order that the current in the series 
impedance has the same level as the load current. For this reason, the 
constant current transformer 6 is said to have a high impedance with 
respect to a conventional transformer 4. 
The phase angle .delta..sub.sr -.psi. between the terminals of the reactive 
element 12 of the constant current transformer 6 implies that this element 
12 absorbs or generates reactive power depending on whether this element 
12 is inductive or capacitive respectively. It is thus possible to choose 
the value of reactive power absorbed or generated by this branch 6. As for 
the active power, the reactive power remains relatively constant whatever 
the level of current flowing through the conventional transformers 2. In 
normal operation, the power factor of the constant current transformer 6 
is thus fairly constant. 
In the case where the conventional transformer branch 4 is already 
energized and the constant current transformer branch 6 is then switched 
in service, the constant current transformer 6 relieves the conventional 
transformers 2 in current and in power, and the phase angle .delta..sub.sr 
slightly varies. 
In normal operation, the fact that the non-conventional branch 6 is 
different from the conventional transformer branch 4 is thus invisible by 
the network 20 from the standpoint of active power. It is during a fault 
that the effect of the non-conventional branch 6 appears. At this time, 
the non-conventional branch 6 supplies the fault by means of a current 
having a value that is determined by the network voltage and the series 
impedance of the non-conventional branch 6. For this reason, among the 
properties of constant current, constant power, high impedance and 
constant power factor, it is the property of constant current that is 
retained in this text to characterize the non-conventional branch 6. 
The current in the constant current transformer branch 6 has therefore 
predetermined values during normal and short-circuit operation. Both 
values can be different or similar depending on the parameters of the 
system. 
The possibility of enhancing the capacity of an existing transformer 
station best illustrates the advantages that can bring the addition of a 
constant current transformer 6 in parallel with conventional transformers 
2. 
Referring to FIG. 2, there is shown a schematic diagram of a transformer 
station equipped with a conventional transformer 2 in parallel with a 
constant current transformer 6. The reactance X.sub.f corresponds to the 
leakage impedances 22 of the conventional transformer 2 and of the 
phase-shifting transformer 10 in the constant current transformer branch 
6. The reactance X.sub.eq represents the sum of the impedances 12, 22 of 
the constant current transformer branch 6. In the particular case of FIG. 
2, the impedance 12 of the constant current transformer 6 is purely 
capacitive. The transformer 24 (which may provide step down features) 
depicts the ideal part of the conventional transformer 2, while the 
transformer 26 depicts the ideal part of an optional step down transformer 
in the constant current branch 6. The phase-shifting transformer 10 and 
the step down transformer 26 may conveniently be combined in a single 
transformer providing both functions. 
The values: 
______________________________________ 
V.sub.b = 120 kV, 
S.sub.3.phi. b = 100 MW, 
Zyb = 144 .OMEGA., 
X.sub.f = 0.1 p.u., 
X.sub.C = 0.511 p.u., 
n = 2.63, 
.psi. = 30.degree., 
V.sub.r = 1.angle.0.degree., 
V.sub.s 1 = 1.angle.5.74.degree., 
V.sub.s2 = 1.angle.-24.26.degree., 
V.sub.s = 2.63.angle.5.74.degree., 
S.sub.1 = 1 - j0.05, 
S.sub.2 = 1 + j0.215, 
and S.sub.r = 2 + j0.165 
______________________________________ 
are typical of a 315/120 kV transformer station across which a total of 200 
MW transfer. As shown in FIG. 2, the transformer station delivers 16.5 
MVars on the receiving side. Since the voltages are equal on both sides of 
the station, it also delivers 16.5 MVars on the source side. 
Referring to FIGS. 3 and 4, there is illustrated the behavior of the 
transformer station shown in FIG. 2, when subjected respectively to a 
fault 28 on the source side and a fault 30 on the receiving side. In the 
case of FIG. 3,, due to the fault 28: 
V.sub.s =0, I.sub.s =3.04.angle.-98.76.degree., I.sub.1 
=3.8.angle.-90.degree., I.sub.2 =0.93.angle.120.degree., I.sub.r 
=7.57.angle.-90.degree.. 
In the case of FIG. 4, due to the fault 30: 
V.sub.r =0, I.sub.s =1.09.angle.-84.26.degree., I.sub.i 
=3.8.angle.-84.26.degree., I.sub.2 =0.93.angle.65.74.degree., I.sub.r 
=3.04.angle.-75.5.degree.. 
From the above results, it can be demonstrated that the total fault current 
with an installation comprising a conventional transformer 2 and a 
constant current transformer 6 is around 40% of what it would be with a 
pair of conventional transformers and around 80% of what it would be with 
a single conventional transformer. 
In the case where the constant current transformer 6 is connected in 
parallel with several conventional transformers 2 instead of one as herein 
described, the percentages obtained here will naturally be higher but 
there will always be a reduction of the fault currents. 
Thus, the method and system herein described enhance the capacity of a 
transformer station while avoiding overload of the downstream or upstream 
equipment current during the faults. 
The phase-shifting transformer 10 of the constant current transformer 6 
must produce a phase-shift .psi. and, when required, a change in the 
voltage level at its terminals. These two functions can be achieved in 
multiple ways, as shown in FIGS. 5A to 5E which illustrate several 
transformer configurations achieving this purpose. FIG. 5A represents a 
zigzag combined step down and phase-shifting transformer configuration 32. 
FIG. 5B represents a Y-.DELTA. combined step down and phase-shifting 
transformer configuration 34. FIG. 5C represents a combined step down and 
phase-shifting autotransformer configuration 36. FIG. 5D represents a 
Y-zigzag combined step down and phase-shifting transformer configuration 
38. FIG. 5E represents a Y--Y step down transformer configuration 40 and a 
squashed-delta phase-shifting transformer configuration 42. Although not 
illustrated, the two functions of these transformer configurations can be 
made adjustable for example by means of mechanical or electronic tap 
changers so that the apparatus may be adapted to the network requirement. 
To further enhance the apparatus flexibility, the impedance 12 of the 
constant current transformer 6 (as shown in FIG. 1) can be provided with 
mechanical or electronic means for varying its value or even changing its 
nature (inductive or capacitive). 
Referring back to FIG. 1, the present apparatus is entirely passive. It 
requires no external signal nor any operation for limiting the fault 
current when a fault occurs. It is thus completely different from the 
active systems presently existing or in development for limiting fault 
currents. 
The constant current transformer 6 shares some similarities with the 
following technologies, without however being implemented in the same way. 
The present apparatus differs from the conventional technique for limiting 
fault currents consisting typically to insert an inductance in series with 
a cable. In respect with the design of the present apparatus, in addition 
to a reactive element 6, as for the cables, there is a phase-shifting 
element 10 that is added in series with the reactance 6. Furthermore, from 
normal operation standpoint, the simple addition of a reactive element is 
not as advantageous as the addition of a phase-shifting element 10 in 
series with the reactive element 12, as in the present case. Indeed, the 
addition of an inductance in series with the cable reduces the 
transmission capacity whereas the present apparatus, by means of the 
phase-shifting element 10, allows to sustain or even increase, if need be, 
the transfer level through the transformer station. 
The method for enhancing the capacity of a transformer station involves the 
connection of at least two circuit branches 2, 6 in parallel, between two 
network busses 44, 46, wherein the impedances of the branches 2, 6 are 
subjected to voltages that are not in phase. In that sense, it shares 
certain similarities with a particular case of an IPC (Interphase Power 
Controller) that would be constructed as follows: the inductive branch 
would comprise a conventional transformer and a very small inductance 
whose role would be fulfilled by the leakage inductance of the transformer 
itself, while the capacitive branch would comprise a phase-shifting 
transformer, in series with a capacitor, as for a constant current 
transformer as herein described. 
However, the present art relating to the IPCs always involves at least two 
reactive elements external to and in series with the (phase-shifting) 
transformer used if necessary. Furthermore, the purpose of an IPC is 
completely different from the purpose of the present apparatus. An IPC is 
used to force a constant power flow between two busses of a synchronous 
network while ensuring a limitation of the fault currents whereas the 
present apparatus does not regulate at all the power between the two 
busses 44, 46 to which it is connected. It simply provides the possibility 
to implement a transformer station whose short-circuit level is lower than 
what it would be with conventional transformers only and this, without 
significant increase of the short-circuit level. 
Thus, according to the invention, if a transformer station becomes 
overloaded, the network owner can install, in parallel with the existing 
transformer(s) 2, a constant current transformer 6 as herein described. 
The operating flexibility is preserved and the apparatus can contribute to 
the voltage support for a minimal additional cost, which is the cost of 
the impedance 12 of the constant current transformer branch 6. 
The present apparatus thus constitutes a very interesting new tool for 
enhancing or preserving the performance and the operating flexibility of a 
network. 
The conventional transformer branch 4 is absolutely required in parallel 
with the constant current transformer branch 6 to maintain the phase angle 
.delta. between its terminals 48, 50 at an almost constant and very low 
value. 
On one hand, without the conventional transformer(s) 2 in parallel, the 
phase angle .delta. can increase substantially and the constant current 
property is lost. This results in important variations of reactive power 
generated or absorbed by the apparatus which can cause unacceptable 
voltage variations. 
On the other hand, since the apparatus is optimized to operate only with a 
small phase angle .delta., the presence of the conventional transformer 
branch 4 ensures that the constant current transformer 6 will not be 
stressed by voltages or currents too important. 
For these reasons, the network owner must always make sure of leaving at 
least one conventional transformer 2 in parallel with the constant current 
transformer 6. 
In the case where the constant current transformer 6 generates an excessive 
amount of reactive power during low load conditions and this causes 
overvoltages, it is possible to operate the constant current transformer 6 
only during the peak periods or to provide suitable reactive power 
compensation. 
In the basic topology, the constant current transformer branch 6 comprises 
only one reactive impedance 12. The transfer of active power through the 
constant current transformer 6 can thus only take place with some 
generation or absorption of reactive power depending on whether the nature 
of the impedance 12 is capacitive or inductive. In certain cases, this 
substantial generation or absorption of reactive power can possibly cause 
a network operation problem. 
Of course, this situation can always be corrected by installing shunt 
compensation. But since a shunt or series compensation requires a current 
flow in a reactive impedance anyway, it is then more advantageous to use 
this current to carry an additional quantity of active power, thus 
relieving one or several transformers. The dimensioning of these 
transformers is then reduced as well. 
Referring to FIG. 6, there is shown a schematic diagram illustrating the 
general principle of the compensation applied to the constant current 
transformer branch 6. Here, two components are relieved: the phase-shifter 
10, because most of the current now flows through the compensation 
impedance 52; and the step-down transformer 8, because the power factor 
that it perceives is increased. 
In addition to illustrating the principle, FIG. 6 also shows how it can be 
applied when a phase-shifting transformer 10 distinct from the step-down 
transformer 8 is used. 
Referring to FIG. 7, there is shown another example of the apparatus, 
wherein a single polyphase transformer 54 fulfils the voltage step-down 
and phase-shifting functions. For each phase of the AC network, the 
transformer 54 has a first winding 56 fulfilling the step down function, 
to which is connected another winding 58 belonging to another phase, to 
provide a phase shift. The power flow can be adjusted using an optional 
tap changer 60. 
Referring to FIGS. 8A and 8B, there are shown different ways to apply the 
compensation principle introduced in FIG. 6. 
Referring to FIG. 9, there is shown another variant of the apparatus. In 
this variant, different reactive elements 12A, 12B, 52A, 52B are combined 
to reduce the global reactive power generated or absorbed. The phase-shift 
required to force the reactive elements 12A, 12B, 52A, 52B to transmit 
active power is provided by the voltage injected in the median branch 
where the equivalent sources V.sub.1 and V.sub.2 are located. Moreover, 
when the voltage phase angle across the terminals 48, 50 of the constant 
current transformer branch 6 is nil (or very low), the current (e.g. 
I.sub.1) provided by V.sub.1 and V.sub.2 is practically nil. Hence, very 
low power sources are sufficient for the good operation of the circuit. In 
the case where the reactive elements 12A, 12B, 52A, 52B form a conjugated 
pair of impedances, a reference potential applied to an internal node 
reference terminal 62 is required to avoid any indetermination within the 
circuit voltages (floating with respect to the ground). 
Referring to FIG. 10, there is shown an implementation of the aforesaid 
variant, wherein the sources V.sub.1 and V.sub.2 are supplied by a small 
series transformer 64, which is fed from an additional winding 66 of the 
principal transformer 68. A tap changer 70 can be used to change the power 
level by varying the voltage injected by the series transformer 64. The 
low voltage side of the series transformer 64 is .DELTA.-connected in 
order that the voltages V.sub.1 and V.sub.2 are in quadrature with the 
phase-to-ground voltage at the branch input. Indeed, such an injection in 
quadrature performs a voltage phase-shift. The additional winding 66 of 
the principal transformer 68 is Y-connected to provide the appropriate 
excitation signal to the series transformer 64. 
The following section relates to the method of power flow control between 
two synchronous polyphase AC network busses, and the interconnecting 
apparatus therefor. 
Referring to FIG. 14, there is shown a schematic diagram of a power flow 
control apparatus according to the invention. The apparatus comprises a 
conventional phase-shifting transformer 78 in parallel with a capacitor 
88. For the case where .delta..sub.sr and P.sub.r are of opposite sign, 
the capacitor 88 offers the following benefits without altering the power 
flow control capabilities of the phase-shifting transformer 78: 
current in the phase-shifting transformer 78. is significantly reduced at 
high power level since the capacitor 88 is then handling most of the line 
current; and 
losses in the phase-shifting transformer 78 are consequently much smaller. 
The reduction of the current stresses in the phase-shifting transformer 78 
can then be used to either: 
considerably increase the power capabilities of an existing phase-shifting 
transformer both in the modes of operation where it is limited by its 
current capability or its maximum phase shift; or 
control a nominal power transfer with a smaller phase-shifting transformer 
at a smaller overall cost that would do an installation using only a 
phase-shifting transformer. 
Moreover, the capacitor 88 produces reactive power that provides voltage 
support on both sides of the apparatus as opposed to a phase-shifting 
transformer used alone, which absorbs reactive power. 
The principle involved in the design of the power flow control apparatus 
can be explained with reference to a conventional generic Interphase Power 
Controller (IPC). The basic elements of such a generic IPC are an inductor 
and a capacitor subjected to separately phase-shifted voltages produced by 
corresponding phase-shifting transformers. 
The present invention can conveniently be used to increase the power flow 
in a transmission line. In this context, the inductor and the 
phase-shifting transformer in series with the capacitor of a generic IPC 
can be removed, for the reasons hereinafter explained. The inductor is 
removed to preserve the coupling effect of the line between its terminals. 
The only inductance remaining in the branch 76 is the leakage impedance 
L.sub.k (also denoted by numeral 82) of the phase-shifting transformer 78, 
numeral 80 representing the ideal part of the phase-shifting transformer 
78. It should be understood that the leakage impedance 82, although herein 
mentioned because it is normally inherent to structures involving windings 
as the phase-shifting transformer 78, does not provide any desirable 
effect nor fulfils any intended purpose in relation with the invention. It 
is in no way essential to the behaviour of the invention whose description 
could be made without it. The phase angle .delta. condition typically 
imposed by the network insures that the capacitor 88 will always work 
efficiently with the phase-shifting transformer 78 at high power transfer 
level. 
The withdrawing of the inductor of a generic IPC is best understood by 
considering the extreme case where the inductor becomes large enough to 
make the inductive impedance complex conjugated with the capacitor 
impedance. Then, the IPC works like a current source and the power flowing 
through is independent of the phase angle .delta..sub.sr. With only the 
small inductor L.sub.k as in the present case, the insertion of the 
apparatus in series with a transmission line does not affect significantly 
the coupling effect of the transmission line and therefore, the power 
varies greatly with the phase angle .delta..sub.sr. 
Consequently, the design and purpose of the invention is different from the 
presently known IPC technology. 
The capacitor 88 and the phase-shifting transformer 78 work in parallel in 
a similar manner to the capacitor 12 and the phase-shifting transformer 10 
of FIG. 8B. However, the phase-shifting transformer 10 has a phase shift 
.psi. that can be fixed or variable over a limited angle range as compared 
to the phase shift, .psi. of the apparatus shown in FIG. 14. Incidentally, 
the demonstration of the benefits provided by the capacitor 88 across the 
phase-shifting transformer 78 whose phase angle can vary over 
.+-.60.degree. is in no way obvious. Although the invention has a simple 
implementation, it took extensive network studies to demonstrate its 
compatibility to the network requirements. The unexpected aspect here is 
that the capacitor 88 can be kept constant while both the network and the 
phase-shifting transformer 78 can vary greatly. This constant capacitor 
property is of prime importance at high-voltage levels for network 
reliability and cost competitiveness. 
Referring to FIG. 15, there are shown the power characteristics of the 
power flow control apparatus according to the invention, and of its two 
branches 76, 84, as a function of the phase angle .delta..sub.sr across 
the apparatus terminals 112, 114. As shown, the capacitive impedance 88 
derived from curve 116 is much higher than the phase-shifting transformer 
leakage impedance 82 derived from curve 118. The overall apparatus 
characteristic represented by curve 120 and resulting from the addition of 
the branches characteristics 116, 118 is then mostly dictated by the 
phase-shifting transformer 78. It turns out that a phase shift .psi. 
produced by the phase-shifting transformer 78 will be seen as a phase 
shift .psi.' by the network, in the region of interest 122. Thus, the 
addition of the capacitor 88 enlarges the power flow control range of the 
phase-shifting transformer 78. 
Referring to FIG. 16, there is shown a simplified network representation 
illustrating the power flow control capability of the apparatus installed 
at the sending end of a transmission line 124. The apparatus and the line 
are connected to two infinite busses 126, 128 of the AC network 130. 
The following description relates to a non-limitative example of the 
apparatus according to the invention with a 500 kV transmission line 
represented only by a series reactance X.sub.L. Shunt admittances and 
losses are neglected here since they only slightly influence the power 
flow. The phase-shifting transformer 78 is of Mersereau type whose 
transformers each exhibits a 15.9% impedance. The phase-shifting 
transformer 78 can vary .psi. between .+-.25.degree., which makes X.sub.Lk 
varying between 0.0117 to 0.0228 p.u. The power basis is 100 MW. 
Referring to FIG. 17A to 17C, there are shown comparison results between 
the apparatus according to the invention and the phase-shifting 
transformer 78 when used alone. The results are plotted as a function of 
.delta..sub.sr (the phase angle difference between V.sub.s and V.sub.r as 
shown in FIG. 16) instead of .delta. (the phase angle difference imposed 
by the AC network 130 across the transmission line 124) as normally done, 
which constitutes a very effective approach for the power flow study of 
the apparatus since on the P-.delta..sub.sr plane, the apparatus and 
network limits are represented by straight lines whose slope m are almost 
constant and easy to calculate. Furthermore, comprehension of the 
apparatus, developed as a function of .delta..sub.sr, can be readily used 
in real network since .delta..sub.s r, imposed by the apparatus, is easy 
to vary whereas .delta. variations may be cumbersome to obtain. The 
following description of the phase-shifting transformer 78 indicates how 
to interpret the P-.delta..sub.sr plane. 
Referring in particular to FIG. 17A, there is shown a graph of the 
P-.delta.sr working area of the phase-shifting transformer 78 alone, in 
solid lines. This working area is delimited by three factors: 
the dispatch conditions and the contingencies imposed by the network 130 
represented by a .delta. variation of .+-.20.degree. (lines 132 and 134); 
the maximum phase shift of the phase-shifting transformer 78 (lines 136 and 
138); and 
the maximum winding current of the phase-shifting transformer 78 (lines 140 
and 142). 
Without the current limit, the phase-shifting transformer working area is 
then given by the dotted lines 144, 146. 
A variation of .psi. and .delta. results in a variation of the 
corresponding side of the working area. The thin straight lines 148, 150 
inside the working area show this when .psi.=-14.degree. and 
.delta.=10.degree. respectively. The operating point of the phase-shifting 
transformer 78 and the network 130 is given by the intersection of these 
straight lines 148, 150. Once the maxima of the phase-shift transformer 78 
and the network 130 are established, the steady-state operating point is 
necessarily inside the working area and the nominal ratings of the 
phase-shifting transformer 102 can be calculated. 
Referring to FIG. 17B, there is shown the working area of a power flow 
control apparatus according to the invention, resulting from the addition 
of a -0.05 p.u. capacitor 88 in parallel with the phase-shifting 
transformer 78. The power characteristic of this capacitor 88 is also 
presented. As shown by the arrows, the capacitor's value has been selected 
to exactly transit the power difference existing between the network limit 
and the current limit. In this condition, the action of the capacitor 88 
is equivalent to removing the current limit of the phase-shifting 
transformer 78. The shaded zones 152, 154 are the increase of power flow 
control capabilities of the apparatus over those of its phase-shifting 
transformer 78 working alone. 
Besides current limit elimination, the capacitor 88 increases the power 
flow control range. Used alone, the phase-shifting transformer 78 would 
need a tap changer set around -34.degree. to force the same power flow 
than the apparatus set at -25.degree.. 
Depending on the point of operation of the apparatus, the power flow in the 
capacitor 88 and the phase-shifting transformer 78 can be in the same 
direction or of opposite direction. Same direction power flow is ideal 
since the two branches 76, 84 are working together. Slight opposite 
direction power flow can be still attractive if the capacitor 88 is 
carrying almost all the current line while the phase-shifting transformer 
78 is slightly working against it. However, if the phase-shifting 
transformer 78 is working against the capacitor 88 up to the point where 
there is no net power flow, the apparatus is just dissipating losses 
without transit of power. Of course, no transfer of real power is 
something a phase-shifting transformer used alone can do best with minimal 
losses. 
Referring to FIG. 17C, there is shown a graph based on the limits where the 
phase-shifting transformer 78 is dissipating the same losses, whether used 
alone or with the capacitor 88. These limits are used to define the shaded 
zones 156, 158 inside the apparatus working area where the apparatus will 
have less losses than the phase-shifting transformer 78 used alone. During 
normal operation, the capacitor 88 is in service only for all the points 
of operation located inside the shaded zones 156, 158. The heavy lines 
160, 162 indicate the region where the phase-shifting transformer windings 
inside the apparatus will be subjected to their nominal current. 
The closer the operating point is of the dotted line 164 where there is no 
power flow in the phase-shifting transformer 78, the smaller will be the 
losses since the capacitor 88 is then carrying most of the line current. 
Moreover, the more reactive power will be produced by the apparatus. It 
thus supports the voltages at its terminals 112, 114 as opposed to the 
phase-shifting transformer 78 used alone that consumes reactive power. 
Outside the shaded zones 156, 158, it is more efficient to disconnect the 
capacitor 88 and use the phase-shifting transformer 78 alone. This 
transition from the apparatus to the phase-shifting transformer modes of 
operation can be done either when .delta..sub.sr =0.degree., when no 
voltage is applied across the capacitor 88, or when the power flow is low. 
In either case, the switching can be done without any significant 
transient in the network 130. 
For the example presented here, the rating of a capacitor design to work 
inside these shaded zones 156, 158 is calculated at .psi.=-25.degree. and 
.delta.=-20.degree.. 
It should be noted that power electronics can be added into the apparatus 
for transient purposes. Furthermore, depending on the AC network 
characteristics and the type of power flow control to achieve, an inductor 
may be used instead of the capacitor 88. Series impedances can be further 
added in series with the phase-shifting transformer 78. For instance, the 
reactive element 88 could be an inductor. Moreover, the reactive element 
88 could be conveniently embodied by a variable capacitor or inductor 
whose variation is obtained either mechanically or electronically. The 
phase-shifting transformer 78 can be designed as a phase-shifting and 
voltage-regulating transformer whose construction is based on mechanical 
or electronic tap changers, all this to provide more flexibility and/or 
degrees of freedom to the invention and enlarge its field of applications. 
Referring now to FIG. 11, there is shown a schematic diagram of the 
interconnecting apparatus for power flow control between two synchronous 
polyphase AC network busses 72, 74 wherein: 
a first branch 76 comprises a conventional phase-shifting transformer 78 
depicted by an ideal phase-shifting element 80 producing a phase shift 
.psi..sub.1 and a leakage reactance X.sub.f 1 also denoted by numeral 82; 
and 
a second branch 84, in parallel with the first branch 76, comprises a 
conventional phase-shifting transformer 86 in series with an impedance Z 
also denoted by numeral 88. The conventional phase-shifting transformer 86 
is depicted by an ideal phase-shifting element 90 producing a phase shift 
.psi..sub.2 in series with a leakage reactance X.sub.f2 also denoted by 
numeral 92. This branch 84 will be hereinafter referred to as the 
high-impedance phase-sifting transformer branch or simply high-impedance 
phase-shifting transformer since, by design, Z&gt;X.sub.f. The equivalent 
impedance Z.sub.eq (also denoted as numeral 94) of this branch 84 is thus 
principally determined by Z. This impedance can be inductive or 
capacitive. 
In order to fulfil the above-mentioned objectives according to the method 
hereinafter described, the high impedance phase-shifting transformer 84 
must necessarily be installed in parallel with a conventional 
phase-shifting transformer 78 and it is this combination that forms the 
interconnecting apparatus herein described. 
In order to facilitate the description of the operating principle of the 
apparatus, FIG. 12A shows a simplified diagram of the apparatus based on 
the following hypotheses: 
the impedance Z is purely capacitive and has a much higher value than that 
of the leakage impedance X.sub.f2 so that Z.sub.e q can be considered as a 
capacitive reactance X.sub.Ceq, also denoted by numeral 96; 
the leakage reactance X.sub.f1 is much lower than the capacitive reactance 
X.sub.Ceq so that, from the point of view of the balance of active and 
reactive powers in both branches 76, 84 of the apparatus, the active power 
of the apparatus is controlled by the ideal phase-shifter 80 while the 
reactive powers on both sides of the apparatus are controlled by the ideal 
phase-shifter 90 and the reactance X.sub.Ceq. It is thus possible to 
illustrate the operating principle of the system by setting X.sub.f1 =0; 
the voltages on both sides of the apparatus are equal such that the 
reactive powers Q.sub.s and Q.sub.r, defined as positive when outgoing 
from the apparatus, are equal. 
These hypotheses are not required to ensure the good operation of the 
apparatus: they are only used to facilitate the description. 
Referring to FIG. 12B, there is shown that the active power characteristic 
P.sub.r transmitted as a function of the phase-shift between the two 
network busses 72, 74 is a straight line having an infinite slope: a small 
variation of the phase angle .delta..sub.sr across the terminals of the 
apparatus produces an infinite variation of the power since the pure 
phase-shifter branch 76 has no impedance. The angular position of this 
characteristic is set by the pure phase-shifter 80; for a phase-shift 
.psi..sub.1, the power characteristic crosses the abscissa at 
.delta..sub.sr =.psi..sub.1. 
The reactive power characteristics Q.sub.s and Q.sub.r are determined by 
the capacitor's reactance X.sub.Ceq ; there is no reactive power absorbed 
by the pure phase-shifter 80. The angular position of these 
characteristics is set by the pure phase-shifter 90; for a phase-shift 
.psi..sub.2, the characteristics touch the abscissa at .delta..sub.sr 
=.psi..sub.2. 
Referring to FIG. 13A, there is shown a schematic diagram of the apparatus 
connected to two Thevenin equivalent circuits 98, 100. The above-mentioned 
hypotheses are also used here. The diagram models the operation of the 
system in network applications where it is connected in series with a 
transmission line or where it is used as an interconnecting circuit 
between two busses of the same network or between two busses of 
synchronous networks. 
Referring to FIG. 13B, the active and reactive power characteristics of the 
apparatus are superimposed on the same set of axes. The dotted straight 
line represents the active power characteristic P.sub.r of the network. 
The slope of this characteristic P.sub.r is determined by the sum of the 
external impedances X.sub.L /2 (shown in FIG. 13A) while the angular 
position of this dotted straight line is determined by the phase-shift 
between the Thevenin sources 98, 100 (shown in FIG. 13A); the line crosses 
the abscissa at .delta..sub.sr =.delta.. 
The operating point of the apparatus-network combination is given by the 
intersection of the active power characteristics P.sub.r located at 
.delta..sub.s r=.psi..sub.1. The reactive power generated by the system is 
directly given by the reactive power characteristic Q.sub.s and Q.sub.r at 
.delta..sub.s r=.psi..sub.1. 
It is thus possible to adjust independently the transfer of active power by 
means of .psi..sub.1 and the quantity of reactive power generated on both 
sides of the system by means of .psi..sub.2. The adjustment of the 
phase-shifts .psi..sub.1 and .psi..sub.2 is carried out by means of tap 
changers as in a conventional phase-shifting transformer. Thyristor-based 
phase-shifting transformers can also be used. 
Although not illustrated, the impedance of the high-impedance 
phase-shifting transformer 84 can be provided with means for varying its 
value or even changing its nature (inductive or capacitive) in order to 
enhance, if need be, the operating flexibility of the apparatus. A 
phase-shifting and voltage-regulating transformer can also be used to 
extend the possibilities of the apparatus. Moreover, power electronic can 
be used to further enhance the performance for both impedance and 
phase-shifting transformer variations. 
In the case where X.sub.f1 is not negligible: 
the power characteristic P.sub.r is not a straight line of infinite slope 
anymore, but a sinusoid segment around 0.degree.; 
the phase-shift .psi..sub.1 is chosen to be lower than .psi..sub.2, as 
shown in FIG. 13B, so that the transfer of active power in the 
high-impedance phase-shifter branch 84 is in the same direction as the 
transfer across the conventional phase-shifter branch 76 to relieve the 
latter. Furthermore, the reactive power balance of the apparatus must take 
into account the reactive power absorbed by the conventional 
phase-shifting transformer branch 76; 
for a given phase angle .delta..sub.sr, the reactive power generated by the 
apparatus is lower than the reactive power generated by the ideal 
apparatus shown in FIGS. 12A and 13A; 
for a certain range of .psi..sub.2 around .delta..sub.s r, it is possible 
to absorb a certain quantity of reactive power on both sides of the 
apparatus. 
The present system shares some similarities with the following systems 
without however being implemented in the same way. 
Two conventional phase-shifting transformers can be connected in parallel 
to obtain transfer levels not accessible with a single phase-shifting 
transformer. 
The design of the present apparatus is distinct from the aforesaid type of 
installation because there is, in series with one of the two 
phase-shifting transformers, a high impedance with respect to their 
leakage impedances. 
Moreover, since the leakage impedances of the conventional phase-shifting 
transformers are low, it is essential that they be identical and always 
adjusted at the same tap position to avoid current flows from a 
phase-shifting transformer to the other. The parallelling of two 
conventional phase-shifting transformers does not provide two degrees of 
freedom to control independently the active power and the reactive powers 
in comparison with the present apparatus. 
In order to obtain the desired power flow, it can be required to install a 
conventional phase-shifting transformer in series with a series 
compensation system to compensate a transmission line (electrically 
shortening the line) and force a phase angle at its terminals that is 
different from the phase angle that would normally be present. 
The design of the present apparatus differs from this type of installation 
since a second phase-shifting transformer 80 is connected in parallel with 
the phase-shifting transformer 90 equipped with a series impedance 88. 
It is worth noting that the phase-shifting transformer and the conventional 
series compensation constitute a system that does not provide two degrees 
of freedom as is the case for the present apparatus. 
The topology of the present apparatus is similar to the topology of a 
constant-current transformer (as described in the previous section) that 
uses a conventional transformer instead of a phase-shifting transformer. 
The design of the present apparatus differs from an interphase power 
controller for the same reasons as those mentioned for the apparatus and 
method for enhancing the capacity of a transformer station in the case of 
the constant-current transformer. 
The present apparatus is typically intended for network applications where 
independent control of the active and reactive powers is desired. It 
effectively allows to adjust the transfer of active power by means of the 
conventional phase-shifting transformer branch 76 while providing voltage 
support by means of the high-impedance phase-shifting transformer branch 
84 when the latter has a capacitive nature. 
The parallel installation of the two systems' branches 76, 84 is 
particularly useful in comparison with the installation of a conventional 
phase-shifting transformer and a shunt capacitor bank. In the present 
apparatus, the capacitor 88 carries a current having a component which 
contributes to the transfer of active power allowing to relieve the 
conventional phase-shifting transformer 78 and to reduce therefore its 
dimensioning. Furthermore, by using a large number of tap positions on the 
high-impedance phase-shifting transformer 86, a relatively precise 
adjustment of the generated or absorbed reactive power can be obtained. 
Therefore, the present apparatus provides the possibility of adjusting the 
active power at an advantageous cost while contributing to the support of 
the network voltages. 
Although the present invention has been explained hereinafter by way of 
preferred implementations thereof, it should be pointed out that any 
modifications to these preferred implementations are not deemed to change 
or alter the nature and scope of the present invention.