|Title of Invention
|ABSTRACT A compensator for balancing an ac voltage network with a load (8) connected between two phases (B, C), comprising a voltage-source converter (4) and a balancing device (9), wherein the balancing device comprises an inductance (10) connected between the phases (A, B) before the load. (Figure 1)
Balancing device TECHNICAL FIELD
The present invention relates to a device for power compen¬sation of an ac voltage network with an uneven load. Such a device is known under the acronym FACTS (Flexible AC Trans¬mission System). In particular, the invention relates to a device for balancing a transmission network with a plurality of phases and with a load between two phases only. The load is usually both resistive and inductive, such as, for example, an electrically rotating machine connected to the network via a converter.
It is known that the transmission power of an ac voltage network is reduced during reactive load and during uneven load of the three phases. Usually, the load is reactive and symmetrical. In this context, it is known to arrange a com¬pensator comprising capacitors connectible to the network. It is also known to connect to the network a static compen¬sator, usually designated SVC (Static Var Compensator) for balancing unsymmetries caused by unbalanced loads. Typical apparatus that may cause an unbalance in an ac network are, for example, arc furnaces. Another such load that may cause an unbalance is power supply of railways. Railway loads are often connected between two phase in the power network. To distribute the power output evenly between all the three phases of the power network, it is known to connect a static compensator, also called a load balancer. The compensator transfers power between the phases in such a way that the railway load together with the compensator is perceived by the power network as a symmetrical three-phase load.
A compensator is traditionally based on thyristor-controlled reactors, thyristor-controlled capacitors and fixed capaci¬tor banks/filters. This technique is well established. A
compensator may also be based on a voltage-source converter (VSC). Such a VSC comprises semiconductors with turn-off means and has a large field of operation. Use of this type of compensator, VSC, is also known.
Various devices for application within FACTS are known from, for example, US 6,963,187.
SUMMARY OF THE INVENTION
The object of the invention is to suggest means and ways of improving the balancing of an unevenly loaded polyphase network.
This object is achieved according to the invention by a compensator according to the characteristic features de¬scribed in the characterizing portion of the independent claim 1 and by a method according to the characteristic features described in the characterizing portion of the independent method claim 5. Preferred embodiments are de¬scribed in the characterizing portions of the dependent claims.
According to the invention, the object is achieved by connecting to the polyphase network a compensator comprising a voltage-source converter, the working point of which is displaced so that the operating range covers substantially the necessary control range. This results in the advantage that a converter with a smaller operating range may be uti¬lized. Such a displacement of the working point of the con¬verter is achieved by causing the compensator to comprise a balancing device including reactive and capacitive elements connected between the phases in the network. These elements together form an offset bank for the negative-sequence current.
A voltage-source converter is able to create a negative-sequence current with an arbitrary phase position in rela-
tion to the positive-sequence system. The operating range of the converter may be described as a vector in a coordinate system where the possible control space is described by the surface of a circle oriented around the origin of coordina¬tes . The negative-sequence current from a load containing active and reactive components and connected between two phases usually occurs in one and the same circular sector. If this sector is smaller than a quadrant, it is uneconomi¬cal to use a converter, the operating range of which com¬prises the whole circle. More than three-quarters of the operating range of the converter then do not fulfil any function in such an application.
By supplementing the converter with a balancing device com¬prising inductors and capacitors, the working point of the converter is displaced. A displacement of the working point corresponding to about half of the maximum balancing re¬quirement permits the operating range of the converter to be reduced and its rated power to be halved. A balancing device comprising an offset bank consisting of discrete components containing inductors and capacitors connected between the phases creates a negative-sequence current with a fixed amplitude and direction.
An ac voltage network with three phases comprises a first phase, a second phase and a third phase, whereby, for example, a load is connected between the second and third phases. The balancing device according to the invention then comprises an inductance connected between the first and second phases and a capacitance connected between the first and third phases. The power value of the inductance and the capacitance is preferably equal. If the load contains an inductive component, such as usually an electric motor, the balancing device also comprises a supplementary capacitance connected between the second and third phases, that is, in parallel with the load.
In a coordinate system a negative-sequence current in the second quadrant is controlled by a corresponding positive-sequence current with the opposite direction. This negative-sequence current thus occurs in a quadrant opposite to the second quadrant, that is, in the fourth quadrant. To correctly control a negative-sequence current occurring within a circular segment, the working point of the conver¬ter must be displaced in the opposite direction. This work¬ing point is defined by a vector composed of the reactance and capacitance of the balancing device, the magnitude of which is determined by the power value of these and the angle of which is determined by the power value of the com¬pensation capacitance. The effect of this procedure results in the coordinate system for the operating range of the con¬verter being displaced such that the origin of coordinates ends up in the control range. In one embodiment, a favour¬able working point of the converter is achieved by controll¬ing the power value of the capacitance and the inductance to about 1/2V3 (one by two times the root of three) of the maximum resistive load power. In a further embodiment, the power value of the compensation capacitance is controlled so as to correspond to half the angle of the circular segment.
In a practical embodiment, the capacitive and inductive ele¬ments between the phases may be designed as tuned harmonic filters. It is also part of the inventive concept to control the balancing device in fixed steps so that favourable work¬ing points for the converter are defined within the neces¬sary circular segment. In case of a control need that, for longer periods of time, comprises a smaller region within the circular segment, a converter with an additionally reduced operating range may thus be utilized.
According to a first aspect of the invention, the task is fulfilled by a compensator for balancing an ac voltage network with a load connected between two phases, comprising a voltage-source converter and a balancing device, the balancing device comprising an inductance connected between
the phases before the load and a capacitance connected between the phases after the load. In case of loads with an inductive component, the balancing device comprises a compensation capacitance connected in parallel with the load.
According to a second aspect of the invention, the object is achieved by a method for balancing an unevenly loaded poly¬phase network comprising a voltage-source converter, the method comprising displacing the control range of the con¬verter by introducing an inductance between the phases be¬fore the load and by introducing a capacitance between the phases after the load. According to one embodiment, the dis¬placement of the control range of the converter also compri¬ses a rotation of the control range by introducing a capaci¬tance in parallel across the load.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained in greater detail by de¬scribing an embodiment with reference to the accompanying drawing, wherein:
Figure 1 is an ac voltage network with a load between two of the phases as well as a voltage-source converter and a balancing device.
Figure 2 is the necessary control range, that is, within which the load should lie.
Figure 3 is the working range of a converter, and
Figure 4 is the control range resulting from the con¬verter and the balancing device.
DESCRIPTION OF PREFERRED EMBODIMENTS
Figure 1 shows an ac voltage network 1 with three phases A, B and C with substantially symmetrical loads 2. Between the second phase B and the third phase C, an additional load 8
is unsymmetrically connected. The load 8 is only symboli¬cally indicated and may include both resistive and inductive components. A compensation installation comprising a voltage-source converter 4 with-a capacitor bank 5, reactors 6 and a balancing device 9 is connected to the ac voltage network by way of a transformer 3. The balancing device comprises an inductance 10 connected between the first phase A and the second phase B, and a capacitance connected between the first phase A and the third phase C. In the example shown, the balancing device also comprises a capa¬citance 12 connected in parallel with the load.
Figure 2 shows, in a coordinate system 18, that range within which the negative-sequence current normally lies for a load connected between two phases. The load range 15 constitutes a circular segment limited by the origin of coordinates as well as by the horizontal coordinate axis and by the circle 16 indicating the maximally necessary control possibility. To be able to properly control the load within the load range 15, the balancing device must comprise a control range 14 that constitutes a mirror image of the load range. Figure 3 shows, in a second coordinate system 19, the operating range 17 of the voltage-source converter in the form of the negative-sequence current. Since a voltage-source converter is able to control the negative-sequence current in all directions, the operating range constitutes a circle centred about the origin of coordinates.
Figure 4 shows the operating range of the combination of a voltage-source converter adjusted by the balancing device according to the invention. The figure shows three coordi¬nate systems, of which the lefthand system in the figure relates to the necessary control range, the central system relates to the operating range ,of the converter, and the righthand system relates to the total power of the converter and the balancing device. The necessary control range 14 in the first coordinate system 18 is the same as in Figure 2 and corresponds to the mirror image of the load range 15. In
the first coordinate system, a vector 20 is also indicated for displacing the working point of the converter that is created by the balancing device. The second coordinate sys¬tem 19 is, in principle, the same as in Figure 3 but in Figure 4 the operating range is reduced by selecting a con¬verter with a considerably lower rated power. The third co¬ordinate system, which is a combination of the first co¬ordinate system 18 and the second coordinate system 19, shows how the operating range 17 of the converter is dis¬placed with the aid of the displacement vector 20 such that the operating range 17 of the converter, which is introduced with a smaller rated power, covers the necessary control range 14.
Study a case with a load with the active power P and the power factor cosφ connected between the b and c phases. The network has the principal voltage V1-1 .
The voltage phase-to-ground in phase a is used as a refer¬ence phase and is assigned the angle zero. It is obvious that the phase currents are
Thus, with the aid of the general equations for symmetrical components
A system .of reactive elements connected between the phases -an offset bank - gives rise to the following sequence currents
To balance the unsymmetrical load, a negative-sequence cur¬rent in phase opposition to the negative-sequence component in the load current need be generated. The active component in the load current is balanced by connecting an inductor between phase A and phase B and a capacitor between phase C and phase A.
The reactive component of the load is balanced by connecting a capacitor between phase B and phase C
So as not to generate a positive-sequence current, all the components in the phases are adjusted in an inductive direc¬tion:
The offset bank is selected such that about half the need of maximum negative-sequence current is obtained. The angle is
selected to about half the maximum angle for the load range. The total instantaneous need is satisfied by the VSC conver¬ter contributing the difference between the need and the current of the offset bank. It is to be noted that to achieve an arbitrary point in the operating range after the displacement with the discrete components, the converter needs to be able to generate a negative-sequence current in all directions.
Although advantageous, the invention is not limited to the embodiments shown, but also comprises embodiments that are obvious to a person skilled in the art. Thus, as stated be¬fore, the invention comprises the case where the balancing device is controlled in a stepwise manner. Further, as indi¬cated above, the invention comprises the case where the balancing device is built together with a filter, if any, for filtering away system resonances and harmonics from the converter.
1. A compensator for balancing an ac voltage network with a
load (8) connected between two phases (B, C), comprising a
voltage-source converter (4) and a balancing device (9),
characterized in that the balancing device (9) comprises an
inductance (10) connected between the phases (A, B) before
the load and a capacitance (11) connected between the phases
(A, C) after the load.
2. A compensator according to claim 1, wherein the balancing
device comprises a compensation capacitance (12) connected
in parallel with the load.
3. A compensator according to claim 1 or 2, wherein the power value of the inductance (10) and the capacitance (11) is substantially equal.
4. A compensator according to any of the preceding claims, wherein the power value of both the inductance (10) and the capacitance (11) contains the maximum resistive load power divided by two times the root of three.
5. A method for balancing an unevenly loaded polyphase net¬work with a voltage-source converter (4), characterized in that the method comprises displacing the operating range of the converter by introducing an inductance (10) between the phases (A, B) before the load and by introducing a capaci¬tance (11) between the phases (A, C) after the load.
6. A method according to claim 5, wherein the displacement
of the working point also comprises a rotation of the con¬
trol range by introducing a capacitance (12) in parallel
|Indian Patent Application Number
|PG Journal Number
|Date of Filing
|Name of Patentee
|ABB TECHNOLOGY LTD
|AFFOLTERNSTRASSE 44, CH-8050 ZURICH
|PCT International Classification Number
|PCT International Application Number
|PCT International Filing date