TITLE: AN INDUCTION REGUIATOR AND USE OF SUCH REGULATOR

FIELD OF THE INVENTION

The present invention relates to an inductor regulator for controlling the active and/or reactive power flow in a three-phase electric transmission network, which inductor regulator is connected between a primary side and a secondary side of the transmission network comprising a stator with stator windings and a partial rotatable rotor with rotor windings and a controlling device arranged to turn the rotor in response to analyzed electric characteristics of the network.

The invention also relates to use of an inductor regulator according as a slowly Unified Power Flow Controller (UPFC) in the power network.

DESCRIPTION OF THE BACKGROUND ART

The active power flow in a lossless ac network is controlled by the equation

P= ' ² sin<? X

where Ul and U2 are the voltages at the transmitting and receiving ends, respectively, of the network. The line reactance is X and δ is the angle between the voltages.

One reason for having power flow control in a network is to be able to utilize the transport system (the network) from production (generators) to consumption (load) as well as possible. This means that generators with a low production cost may be utilized more. If unforeseen events occur in a network, it will require that the generators be planned or scheduled in a non-optimal way with regard to the production costs. With controllability in the network, it will

therefore be possible to produce electricity in a less expensive manner.

It is known that an active power flow in a network may be influenced in a plurality of ways by:

- decreasing or increasing voltage amplitudes

- changing the reactance by adding a series capacitor

- changing the angle δ for instance with a phase-shifting transformer

- adding a shunt and series voltage, usually named UPFC, SSSC, FACTS and the like acronyms, depending on the method,

- using an induction regulator.

When influencing the active power flow by reducing or increasing the voltage amplitudes, the control range for reducing or increasing the voltage amplitudes for voltage changes is limited because of the maximally allowed voltage level (usually ±5%) and the fact that operation at a high voltage level is preferable in order to reduce the losses in the network.

When influencing the active power flow by using a series capacitor, the reactance can only be reduced to a certain level and it is not possible to force the current irrespective of the state of the line since it is not possible in practice to overcompensate the line.

A phase shifter, on the other hand, can control the power flow and force any current by introducing a phase shift between its terminals.

A UPFC (Unified Power Flow Controller) makes use of power electronics to transmit power between a shunt-connected and a series-connected transformer. By proper control, it is possible to achieve an optional output voltage within the rated power of the apparatus. The disadvantages of existing UPFCs have primarily been associated with the difficulty of protecting the power electronics on the series-transformer

side. There have also been discussions whether a network point really needs full controllability both in amplitude and in phase position. Some network points require more voltage control and others more angular control to utilize the system in an optimum manner.

Induction regulators for controlling voltage and angle were used in laboratories and other locations where a very accurate and continuous voltage control was necessary. Today, induction regulators are used substantially as small units as regards voltage and power. An older field of use is voltage control of generators where the field winding on the rotor of the machine can be fed via an induction regulator. Nowadays, however, power electronics are used for the most part for this purpose.

The induction regulator is an asynchronous machine, which does not operate as a motor but with a stationary rotor, but where the rotor may be rotated in an angular direction to control the voltage.

The control is dependent on a mechanical movement of the rotor to connect a certain amount of flow from one phase to another. The power required to rotate the rotor is propor- tional to the power flowing through it. The control speed is low and considerable mechanical force is required to achieve the slow change of the output voltage. In the induction regulator, both a voltage change and an amplitude change occur. In this case, the output voltage follows a circle. An disadvantage of the induction regulator is that the voltage amplitude is changed when the angle δ is changed.

The power flow in a power line is controlled in essentially two different ways depending on the method.

One category of components transmits power between the phases and the other only influences the impedance in a specific phase.

The first category includes phase-shifting transformers, HVDC and UPFCs.

The second category includes mostly other categories of FACTS and series capacitors.

The first category has a considerable advantage since it is able to actively control the power flow without relying too much on the surrounding ac system. The second category is dependent on the other impedances of the ac network and may only to a certain extent influence the power flow. A device according to the first category is able to control the power flow within minimum and maximum ranges almost independently of the load states, whereas a control device according to the second category cannot always fulfil this requirement.

The fundamental principle of induction regulators is described, for example, in "TRANSFORMERS for Single and Multiphase Currents" by Gisbert Kapp, London, Sir Isaac Pitman & Sons, LTD, 1925 (pp. 274-283).

Induction regulators are also known from GB 400.100 and GB 549,536.

Induction regulators may be single-phase or polyphase, but the present invention specific relates to polyphase induction regulators, preferably three-phase induction regulators .

The mode of operation of a three-phase induction regulator is described in the following. The induction regulator is used when it is desired to achieve a continuous rotation of the voltage vector, that is, a continuous change of the phase angle of the voltage. The rotor is kept stationary but is arranged so as to be rotatable through a certain angle in relation to the stator. The mechanical rotation suitably occurs via a worm gear. When the rotor is stationary, the direction of the EMF vectors of the rotor side depends on the position of the rotor in relation to the stator. If the machine is excited from the stator side, phase equality exists between the voltage induced in the stator winding and that induced in the rotor winding in those cases where the winding phases of the stator and the rotor are positioned

opposite to each other, but when the rotor is rotated forwards in the direction of rotation of the induction flux a certain angle cc in electrical degrees (one pole pitch = 180 electrical degrees) , then the secondary voltage vector cor- responding to the time angle is displaced in time (lags behind in phase) . If the rotor is rotated in the opposite direction, the vector of the secondary voltage will have a phase angle of the opposite sign compared to the preceding case.

A drawback of the prior art induction regulator is that the possibilities of control are limited to the voltage change that the secondary voltage vector achieves and this while changing the phase angle of the voltage. This renders paral- IeI connection of lines and apparatus difficult since an angular rotation brings about a voltage change and hence a circulating reactive power in the system causing losses and voltage drops.

According to a fist aspect the present invention seeks to provide an improved induction regulator for power flow control in high voltage ac three-phase transmission networks, with an increased regulation of the phase angle while keeping the voltage change over the device low.

According to another aspect the invention relates to use of an inductor regulator in a three-phase, transmission network for high voltage as a slowly Unified Power Flow Controller (UPFC) in the power network.

SUMMARY OF THS INVENTION

According to the first aspect of the present invention there is provided an induction regulator for controlling the active and/or reactive power flow in a three-phase electric transmission network as specified in claim 1.

Appropriate embodiments of the invention according to this first aspect will become clear from the subsequent subclaims 2 - 6.

According to the second aspect of the present invention there is provided use of an induction regulator in claim 7

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention is described below, by way of example only, in greater detail with reference to the accompanying drawings, where

Figure 1 schematically shows the connection of an induction regulator to a three-phase network according to the prior art,

Figures 2 a-d show the output controlled voltage as a vector sum of input voltage and the secondary voltage vector at different rotor positions of the induction regulator according to prior art,

Figure 3 schematically shows an induction regulator according to an embodiment of the invention,

Figure 4 schematically shows the connection of an induction regulator to a three-phase network according to an embodiment of the invention,

Figures 5 show the output controlled voltage as a vector sum of input voltage and the secondary voltage vector and angle according to an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An induction regulator according to prior art shall now be described with reference to Figure 1, which schematically shows the connection of an induction regulator to a three- phase network. 1 denotes a source of current that is connected to a consumer 2 via a three-phase network with its three phases 3r, 3s and 3t, said network exhibiting a primary side 3 and a secondary side 4. An induction

regulator 5 is connected between the current source and the consumer. Between the current source 1 and the induction regulator 5, the phase voltage Ea in the network is variable, whereas the phase voltage En in the network between the induction regulator and the consumer is maintained constant. The induction regulator exhibits three stator windings 6r, 6s and 6t and three rotor windings 7r, 7s and 7t. The respective stator winding exhibits a primary connection 8 that is connected to the primary side 3 of the network, and a secondary connection 9 that is connected to the secondary side 4 of the network. At the respective secondary connection 9, the respective stator winding is connected to the respective rotor winding 7 (r, s, t) . These, in turn, are interconnected, in their second connection, to the other rotor windings.

The voltage, δE, induced between the stator winding and the rotor winding, is vector-added to the primary voltage Ea to form the secondary voltage En. In those cases where the sta- tor winding and the rotor winding are positioned right in front of each other, phase equality exists between the voltage induced in the stator winding and that induced in the rotor winding; however, if the rotor is rotated from this position, an arbitrary position may be imparted to the se- condary voltage vector in relation to the corresponding primary vector, both ahead of and after the same.

Figures 2 a-d illustrate how the secondary voltage vector δE at a prior art induction regulator is added to the primary voltage vector Ea to form the secondary voltage En. In Figure 2a, Ea has its lowest value and forms the voltage vector En together with δE. If the phase voltage Ea increases, as is clear from Figure 2b, the position of the rotor must be displaced, whereby the phase position of δE is displaced so that the sum of Ea and δξ is still constant En. When Ea has its highest value {Figure 2c) , the rotor must be displaced further to 180 electrical degrees in relation to Figure 2a, whereby δE is directed in a direction opposite to that of Ea. When the rotor is displaced in the opposite direction, δE will have an opposite direction, which is clear from Figure 2e.

It is clear from the above that the control range for a conventional induction regulator is, in principle, ± δE and the phase angle α.

An induction regulator 50 according to an embodiment of the present invention is schematically shown in Figure 3.

The inductor regulator 50 is connected between a primary side 30 and a secondary side 40 of the transmission network. For each phase the inductor regulator comprises a first stator winding 60a, 60b, 60c and a second stator winding 61a, 61b, 61c separated from each other.

The inductor regulator also for each phase comprises a first rotor winding 70a, 70b, 70c and second rotor winding 71a, 71b, 71c. The rotor windings are arranged on a rotor axis 210.

The induction regulator also comprises stator and rotor poles (not shown) with an air gap 14 between respective stator and rotor pole. As the movement of the rotor is slow it is possible to make the air gap very thin which gives reduced leakage flux.

The respective stator windings 60a, 60b, 60c are opposing the respective rotor windings 71a, 71b, 71c. The primary side 30 of the three-phase network with its phases 30a, 30b, 30c are connected to the respective first stator windings 60a, 60b, 60c via their respective primary connections 80a,

80b, 80c and connected to the respective secondary side 40 of the three-phase network with its phases 40a, 40b, 40c via the secondary connections 90a, 90b, 90c. The rotor and its first and second windings (and poles) are displaceable, by means of rotation, in relation to the stator and its windings (and poles), but preferably 180 electrical degrees at most, which, depending on the total number of poles, only implies a minor mechanical rotational movement in either

direction from the position where the windings in the stator and the rotor are positioned opposite to each other.

Electric lines 82a, 82b, 82c connect the secondary connections 81a, 81b, 81c of the respective first stator windings 60a, 60b, 60c to the associated first rotor windings 70a, 70b, 70c. The lines 82a, 82b, 82c are preferably arranged flexibly to follow the rotational movement of the rotor. Alternatively, the lines may be electrically connected to the rotor windings via slip rings (not shown embodiment) .

The secondary connections 81a, 81b, 81c of the first stator windings 60a, 60b, 60c are also connected in series with first connections 91a, 91b, 91c of the secondary stator windings 61a, 61b, 61c and the secondary connections 90a, 90b, 90c of the secondary stator windings are connected to the secondary side of the three phase network 40.

The electrical connections described above are thus similar for each phase.

The first rotor windings 70a, 70b, 70c are on their secondary connection each connected to the first connection of the second rotor windings, respectively, as illustrated in figure 3. The second rotor windings are connected in their respective secondary connections to a common neutral point 160.

Each first rotor winding 70a, 70b, 70c is herby connected to each second rotor winding 71a, 71b, 71c. The second rotor windings 71a, 71b, 71c are displaced compared with the first rotor windings. For example, in counter clockwise direction, the first rotor windings are connected in the order 70a, 70b, 70c while the second rotor windings are connected in the order 71a, 71c, 71b. This means that the second rotor winding 71a faces stator winding 61a, rotor winding 71b faces stator winding 61c and rotor winding 71c faces stator winding 71b.

Herewith the rotating magnetic flux in the first rotor windings in relation to the rotor axis 210 will be of opposite direction compared with the rotating magnetic flux in the second rotor windings.

A controlling device 200 is arranged to via the rotor axis 210 turn the rotors in response to analyzed electric characteristics of the network. Details of such controlling device does not need to be described here but it can be mentioned that such controlling device by way of example comprises a motor connected to a worm gearing giving the rotatable movement of the rotor.

According to an embodiment as schematically is shown with dotted lines in figure 3, there is a second controlling device 200a connected to an axis 200a. The controlling device 200 is thus divided in two parts 200 and 200a and arranged to turn the rotor parts of the rotor individually, which means that the first rotor windings 70a, 70b, 70c can be rotated with another angle compared with the second rotor windings 71a, 71b, 71c. The rotor axis 210 is herewith, by way of example, divided into two parts, 210 and 210a, each connected to and the controlling device 200 and 200a. Hereby the voltage and phase control possibilities of the inductor regulator are increased and this embodiment behaves similar to a slow Unified Power Flow Controller (UPFC) .

Figure 4 schematically shows the connection of an induction regulator 50 to a three-phase network according to a preferred embodiment. 10 denotes a source of current that is connected to a consumer 20 via the network with its three phases 30a, 30b and 30c and 40a, 40b and 40c.

A voltage, δEl, is induced between the first stator winding and the first rotor winding and a voltage δE2, is induced between the second stator winding and the second rotor winding .

Voltage δEl and voltage δE2 will be vector added to the primary voltage Ea to form the secondary voltage En.

As the rotating flux generated in the two rotor windings are of opposite direction, the direction of the voltage vector δEl is opposite the direction of the voltage vector δE2.

In those cases where the stator winding and the rotor winding are positioned right in front of each other, phase equality exists between the voltage induced in the stator winding and that induced in the rotor winding; however, if the rotor is rotated from this position, an arbitrary position may be imparted to the secondary voltage vector in relation to the corresponding primary vector, both ahead of and after the same.

Figures 5 illustrate how the voltage vector δEl and voltage vector δE2 are vector added to the primary voltage vector Ea to form the secondary voltage En.

When the rotor is rotated from its neutral position, the fist voltage vector δEl get phase angle αl and the second voltage vector δE2 get phase angle α2.

The sum αl and α2 gives the value of an equivalent reactance (phase angle between current and voltage) . One can also see this as a transformation of power between phases to achieve a voltage change in the phases causing a changed power flow in the system. When the first rotor windings and second rotor windings are rotated the same amount, which is the case when the rotors are mechanically connected to each other, αl and α2 are equal and the inductor regulator theoretically gives at total angle which is twice the angle of a prior art design.

However as, the rotating flux in the rotor windings are opposite, the direction of the vectors δEl and δξ2 also are opposite which give minimal voltage change.

By the invention device it is thus possible to get improved control range regulation of the reactance of the network, still with minor influence of the network voltage.

The induction regulator is primarily designed for control of three-phase transmission networks for high voltage, that is, networks with an operating voltage that provides a possibility of continuously substantially transporting electric power. In practice, the invention may be used for applications above 1 kV. Common operating voltages for transmission networks are 200-800 kV, for sub-transmission networks 70-200 kV, and for distribution networks 10-70 kV.

According to one embodiment a transformer can be utilized on one or both sides of the apparatus to adjust the power handling of the device to technically suitable voltage and current ratings .

According to one embodiment, the stator and/or rotor windings are constituted by cable windings fully insulated against ground, of the kind that is known from, for example, generators for high voltage described in patent document WO97/45919. It is advantageous that the temperature of the windings during operation then be kept at a relatively low temperature, for example, within a temperature interval of around 7O ⁰ C.

According to another aspect the invention also relates to use of a power-flow control for a three-phase, transmission network for high voltage. One field of use is to use the device as a slowly Unified Power Flow Controller (UPFC) in the power network.

In the description and the figures below, the invention is exemplified for control of a three-phase network with the phases a, b, c, since three-phase distribution networks are the most commonly used in practice, whereas the invention as such, as is apparent to the skilled person in the art, is applicable also to any polyphase network giving a rotating magnetic flux in the rotor windings .

Any range or device value given herein may be extended or altered without losing the effect sought, as will be apparent to the skilled person for an understanding of the teachings herein.