AN INDUCTION REGULATOR FOR POWER FLOW CONTROL IN AN AC TRANSMISSION NETWORK

FIELD OF INVENTION

The present invention relates to an induction regulator for controlling the voltage amplitude and/or phase angle of a polyphase electric transmission network exhibiting a primary side with a primary voltage and a secondary side with a controllable secondary voltage, which induction regulator is connected between the primary side and the secondary side of the transmission network and comprising a stator with stator windings and stator poles and a rotor with rotor windings and rotor poles.

The invention also relates to a method of controlling the voltage and the phase angle of a polyphase transmission network for high voltage with an induction regulator.

The invention also relates to use of an induction regulator for power-flow control for a polyphase transmission network for high voltage.

DESCRIPTION OF THE BACKGROUND ART

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

X

where Ui 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 in a non-optimal way with regard to the production costs. With controllability in the network, it will therefore be poss- ible 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 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 redu- cing 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 also 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 proportional 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 re- gulator, 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 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 α in electrical degrees (one pole pitch = 180 electrical degrees), then the secondary voltage vector corresponding 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 induction regulator is that the possibilities of control are limited to the voltage change that the secondary voltage vector achieves and this while chang- ing the phase angle of the voltage. This renders parallel connection of lines and apparatus difficult since an angular rotation brings about a voltage change and hence a circulating reactive effect.

According to a first aspect the present invention seeks to provide an improved induction regulator for power flow control in high voltage ac polyphase transmission networks.

According to a second aspect the present invention seeks to provide an improved method of controlling voltage and phase angle of such polyphase transmission network for high voltage with an inductor regulator.

According to a third aspect the present invention seeks to provide use of an induction regulator for improved controlling of voltage and phase angle of such polyphase transmission network for high voltage.

SUMMARY OF THE INVENTION

According to the fist aspect of the present invention there is provided an induction regulator for controlling the voltage and the phase angle of a polyphase 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 2-15.

According to the second aspect of the present invention there is provided a method of controlling voltage and phase angle as specified in claim 16.

Appropriate embodiments of the invention according to this second aspect will become clear from the subsequent subclaims 17 - 18.

According to the third aspect of the present invention there is provided the use of a power flow control for a three- phase transmission network according to claim 19.

By influencing the magnetic flux region in the volume between the stator and the rotor pole according to the invention, the magnetic flux in the volume may be controlled, which means that the amplitude of the secondary voltage vector may be controlled, and this without mechanical rotation of the rotor occurring. The control function of the induction regulator is thus achieved by the volume comprising a controllable magnetic flux region.

In the following, the concepts rotor and rotor pole are used although the rotor according to the invention is not movable but stationary in relation to the stator. The reason for this designation is that it facilitates comparison with in- ductor regulators according to the prior art.

The control of the magnetic flux in the volume takes place by a change of temperature of one or more magnetic regions in the volume, whereby the respective region contains a material that provides a considerable change of the relative permeability in relation to the temperature change. According to an embodiment of the invention, it has proved that the element gadolinium (Gd) is a material that is especially suited in the magnetic region. This is based on the realization that gadolinium, which is a ferromagnetic material, has the unique property that its Curie temperature is

low, actually 292°K, which corresponds to 19°C. The Curie temperature is the limit above which a ferromagnetic material exhibits normal paramagnetic performance. This implies that the permeability of gadolinium is changed when its temperature varies around the Curie temperature. It is realized that for gadolinium, therefore, the permeability may be controlled if the temperature varies around room temperature and above. A special property of gadolinium is the considerable change in permeability that occurs also with small temperature variations in the interval above the Curie point. For example, the relative permeability may be changed in the order of magnitude of from 1000 to 1 by a change in temperature from 20°C to 40°C.

Gadolinium belongs to the group of rare-earth metals and occurs in several minerals but does not exist freely in nature .

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be 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,

Figure 3 schematically shows an induction regulator accor- ding to an embodiment of the invention,

Figures 4 a-d schematically show the build-up of the volume in various layers and segments, respectively.

Figure 5 schematically shows in detail the volume between the stator and the rotor in an induction regulator according to an embodiment of the invention as well as control means,

Figure 6 schematically shows how the control range of an embodiment of the invention may be controlled,

Figure 7 shows in the form of a diagram the ferromagnetic Curie point and the Neel point, respectively, of a few rare- earth metals versus the absolute temperature.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the description and the figures below, the invention is by way of example only exemplified for control of a three- phase network with the phases r, s, t, since three-phase distribution networks are the most commonly used in practice, whereas the embodiments of the invention as such is applicable also to a polyphase network with a different number of phases.

In Figure 1, 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 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 pri- mary vector, both ahead of and after the same.

Figures 2 a-d illustrate how the secondary voltage vector δE 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 δE 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 other 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 .

The magnitude of depends substantially on the magnetic flux induced in the stator and the rotor winding and on the relative permeability in the air gap between the stator and the rotor poles.

An induction regulator according to an embodiment of the present invention, to achieve control without any mechanical rotation of the rotor, is schematically shown in Figure 3.

Here, the primary side of the three-phase network with its phases 3r, 3s, 3t is connected to the respective stator win- dings 6r, 6s, 6t via their respective primary connections 8 and connected to the respective secondary side via the se-

condary connections 9. 11 denotes a stator pole. 10 denotes a stationary rotor with rotor windings 7. 12 denotes a rotor pole. A volume 14 is shown between the stator pole and the rotor pole. The rotor and its poles are fixed in relation to the stator and its poles.

For the sake of clarity, the figure only shows one pair of poles for the respective phase, whereas in practice the number of pole pairs may be higher (= a multiple of the number of phases) .

An electric line 13 connects the secondary connection 9 of the respective stator winding to the associated rotor winding 7. The rotor windings are interconnected in their res- pective secondary connections 15 to a common neutral point 16.

To provide control of the induction regulator, the volume 14 substantially comprises one or more magnetic regions, which exhibit (s) a relative permeability that is temperature- dependent, whereby the magnitude and direction of the secondary voltage vector δE are controlled by influencing the relative permeability of the respective region by control of its temperature.

According to one embodiment, the magnetic regions in the volume are divided into layers or segments as will be clear from Figures 4a-4d.

In Figure 4a the volume 14 is divided into layers 17 that are arranged in planes substantially parallel to the end planes of the poles. Channels 22 are arranged to circulate a medium through the respective layer for individual control of the temperature of the respective layer.

In Figure 4b the volume 14 is divided into layers 17 that are arranged in planes substantially perpendicular to the end planes of the poles. Also here, channels 22 are arranged to circulate a medium through these for individual control of the temperature of the respective layer.

In Figure 4c the volume 14 is divided into segments 23 which, in a cross section, form a two-dimensional matrix. Here, channels 22 are arranged to circulate a medium through the respective segments for individual control of the tempe- rature of the respective segment.

In Figure 4d, the segments 23, in their turn, are divided into sub-segments 24, which form a three-dimensional matrix. Also here, channels 22 are arranged to circulate a medium through the segments, and for individual control of the temperature of the respective sub-segment, additional heat- supply means 25 to the respective sub-segment are required. These means may consist of heat-emitting units 25 arranged in the centre of the respective sub-segment, whereby the individual temperature control may take place by the medium flowing through the channel 22 ensuring a common lowest temperature for a whole segment whereas the heat-emitting units increase the temperature of the individual sub-segment to the desired level.

According to one embodiment, the magnetic layer contains the element Gd (gadolinium) , which exhibits the property that the relative permeability is very greatly temperature-dependent. If, for example, the temperature is controlled between 20 and 40°C, the relative permeability may be changed from 1000 to 1.

The embodiments of the invention makes possible a mode of operation whereby the control may be made without influencing the phase angle between voltage and current, or by a suitable selection between changing the phase angle and the amplitude of the phase voltage En. It is realized that this offers considerable operational advantages since, depending on the mode of operation, it is possible to choose to control the voltage linearly or with a reactive component for the desired phase compensation.

According to one embodiment, the layers 17 contain Gd (gadolinium) doped with a substance that influences the structure of the crystal lattice, and/or doped with a substance that influences the magnetic coupling intrinsically

in the material, for influencing the temperature for its magnetic phase transition. The substances for doping are suitably one or more of the substances belonging to the group of rare-earth metals, such as La, Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu.

Figure 5 further schematically shows a device 18-20 for controlling the temperature of the magnetic layer or segments .

Here, the stationary rotor is schematically indicated by the arrow 10 and the corresponding stator with stator poles is indicated by the arrow 11. Two containers 18, 19 containing a gas or a liquid medium are provided, via feed lines 20a, with mixing valves 21 and return lines 20b adapted to circulate a gas or liquid medium through the magnetic layers or segments in the volume 14 through channels 22.

The gas or liquid is stored in two containers 18, 19, one with a high temperature, for example 70°C, and one with a low temperature, for example 20°C. Through a mixing valve, gas or liquid of the desired temperature is supplied to the respective layer or segment in the volume.

Means (not shown) for respectively heating/cooling are also suitably arranged for the circulating liquid medium, depending on the mode of operation, to be in the respective container. It is also realized that the number of containers and associated conduits and valves may be varied in a suit- able way to fulfil the requirements for controllability.

How the control range is extended by embodiment of the invention is clear schematically from Figure 6, which shows the controlled voltage as a vector sum of the input voltage Ea and the secondary voltage vector δE . At A, the magnetic volume has its lowest relative permeability, which occurs at a high temperature, whereas at B the volume has reached its highest relative permeability, which occurs when the volume has its lowest temperature within the temperature-control interval.

By individual control of the relative permeability within the layers and segments of the volume, respectively, a displacement of the magnetic flux may be achieved in relation to the centre line of opposite poles. The achieved control range is illustrated in Figure 6. In practice, embodiment of the invention achieves a phase shift similar to that which is achieved when rotating the rotor in a conventional induction regulator.

Figure 7 shows in the form of a diagram the magnetic Curie temperature of a few rare-earth elements. The absolute temperature ( ⁰ K) is shown on the Y-axis and on the X-axis elements belonging to rare-earth elements are stated according to the number 4f of electrons. These elements are La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dv, Ho, Er, Tm, Yb and Lu. The curve designated NP shows the Neel temperature and the curve designated FCP shows the ferromagnetic Curie temperature of these materials. The diagram shows, inter alia, that gadolinium is the substance that has the highest Curie tempera- ture of these substances, that is, around room temperature.

According to the embodiment of the invention, the device 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, but not information. In practice, embodiments of the invention may be used for applications above 1 kV. Common operating voltages for transmission networks are 200-750 kV, for sub- transmission networks 70-200 kV, and for distribution networks 10-70 kV.

According to one embodiment, the means for temperature control (18-21) are adapted to vary the temperature of the vol- ume 14 between 20°C and 150°C, preferably between 30°C and 70°C.

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. Especially when the temperature of the volume is maintained relatively low, for example between 30°C and 70°C, this does not either entail any heat transfer to the poles to any major extent, so its windings may also maintain 70°C in operation. This temperature, or a temperature interval around this, is exceedingly well suited for cable windings.

According to a second aspect the invention also relates to a method of controlling the voltage and the phase angle of a polyphase transmission network for high voltage with a device according to claims 16-18, wherein the control takes place by control of the controllable magnetic flux region in the volume while keeping the rotor fixed in relation to the stator .

According to a third aspect the invention also relates to use of a power-flow control for a three-phase transmission network for high voltage according to claim 19. One field of use is to use the device as a slowly Unified Power Flow Controller (UPFC) in the power network.

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.

Disclosures in Swedish patent application No. 0502169-6 of September 29, 2005 and Swedish patent application No. 0502716-4 of November 29, 2005, from which applications this application claims priorities, are incorporated herein by reference .