Patent Description:
Electric power networks such as medium voltage distribution networks have various different protection and grounding configurations such as a resistance-earthed grounding configuration, an isolated network configuration and a resonant-earthed network configuration, which are used in different installations and have different advantages. The resonant-earthed configurations include a compensated network configuration, where a Petersen coil is used to provide inductive reactance that compensates capacitive earth faults in the network. Typically, such a configuration includes an earthing transformer, a Petersen coil and a high-power resistor.

As an example, <CIT> discloses a kind of inductive reactance value can change self-regulating arc suppression coil.

An objective is to improve performance of the current arrangements for grounding an electric power network, where the electric power network is a compensated network.

Moreover, it is an objective to provide a more efficient construction for grounding the electric power network.

The present disclosure involves an arc suppression coil, also known as a Petersen coil (ASC). The arc suppression coil is adapted to be operated in a three-phase electric power network, which may be a medium-voltage network and/or a distribution network. The arc suppression coil is adapted not only to provide inductive compensation for capacitive earth faults in the electric power network but also to facilitate grounding of the electric power network, in particular without a zigzag winding for grounding the three-phase electric power network. This allows the electric power network to be grounded without a separate earthing According to the invention there is provided an arc suppression coil for a tree-phase electric power network as defined in claim <NUM>.

Furthermore, also according to the invention, there is provided a method of grounding a three-phase electric power network as defined in claim <NUM>.

Further developments of the invention are defined by the dependent claims.

According to a first aspect, an arc suppression coil for a three-phase electric power network comprises a transformer core. The transformer core has three limbs, comprising a first limb, a second limb and a third limb, allowing one limb to be dedicated to facilitate grounding for each of the three phases of the electric power network. In addition to the three limbs, the transformer core defines an additional path for carrying magnetic flux between the opposite ends of the three limbs. As the additional path is outside the three limbs, it allows forming a return path for the magnetic fluxes going through the three limbs. The arc suppression coil comprises three separate phase windings comprising a first phase winding wound at the first limb of the transformer core, a second phase winding wound at the second limb of the transformer core and a third phase winding wound at the third limb of the transformer core. The phase windings are separate for the three phases of the electric power network allowing the arc suppression coil to be adapted for each of the three phases of the three-phase electric power network to be connected to ground through exactly one of the three phase windings.

As the phase component of the magnetic flux substantially cancels out outside the three limbs, when the magnetic fluxes going through each of the three limbs are joined, the additional path can be adapted to carry zero sequence flux. The positive sequence flux in the additional path can thereby be zero or substantially zero. When there is asymmetry in the phase voltages of the three-phase electric power network, such as in the event of an earth fault, leakage flux may be generated in the arc suppression coil. The structure of the arc suppression coil allows the transformer core to be adapted for leakage flux in the arc suppression coil to be directed through the additional path, thereby providing compensation with respect to the source of the asymmetry, e.g. an earth fault. The additional path allows controlling zero sequence impedance independently from positive sequence impedance of the arc suppression coil and/or the network. In addition to providing the inductive earth-fault compensation, which traditionally is provided by separate arc suppression coils, the structure allows the present arc suppression coil to function also as an earthing transformer for the electric power network so that there is no need for two separate apparatuses. In particular, since no earthing transformer is needed. At the same time, the traditional zigzag winding configuration of an earthing transformer can be replaced by the separate phase windings, which has been found to allow further reduction in the size of the transformer core. Even if some opposing windings were maintained on the three limbs, e.g. in a configuration reminiscent of a conventional zigzag winding having substantially equal number of turns in both layers of windings, the number of turns in two opposing windings would be substantially different, e.g. such that the ratio of turns in each two pairs of opposing windings is at least <NUM>-<NUM>, meaning each of the three limbs is essentially dedicated to windings corresponding to only one phase of the network. The reduction in the size of the transformer core, may allow notable reductions in material used since the mass of an earthing transformer in typical applications can exceed <NUM> or <NUM> tons.

In an embodiment, the arc suppression coil comprises means for adjusting reluctance of the additional path. This allows the arc suppression coil to act as tunable inductance for the electric power network so that the inductance provided by the arc suppression coil for compensating capacitive earth faults can be adjusted based on changing requirements for the capacitance to be compensated. The requirements may change, for example, depending on the location of the earth fault or when the size or the properties of the network change. In particular, the means for adjusting reluctance may be adapted for adjusting reluctance of the additional path even when an earth fault is present in the electric power network. This allows adjusting the inductance of the electric power network, also when the earth fault is present. This, in turn, allows situation-specific adjustment for optimizing the compensation provided by the arc suppression coil, for example by allowing the inductance of the network to be adjusted based on the actual capacitance of the network during the earth fault. This allows minimizing the fault current even when the capacitance of the network differs depending on the location of the earth fault. The means for adjusting reluctance of the additional path may comprise a virtual air gap adapted for adjusting the reluctance of the additional path. The means such as a virtual air gap allow substantially linear operation of the arc suppression coil.

It is noted that other means for varying the impedance of the arc suppression coil can also be used for tuning the arc suppression coil, including stepwise variation of impedance and other means. For example, impedance of the arc suppression coil may be varied by the arc suppression coil having one or more variable impedance windings, for example at the additional path, which can be adapted for varying the zero sequence impedance of the electric power network. Such a variable impedance winding may be formed, for example, by a switched capacitor bank. The one or more variable inductance windings may comprise, for example, one or more capacitors adapted for adjusting the impedance of the arc suppression coil, or zero sequence impedance of the electric power network in particular.

In an embodiment, the means for adjusting reluctance of the additional path comprise one or more control windings for the additional path. This allows electrical control of the reluctance of the additional path and thereby the inductance of the arc suppression coil for compensating capacitive earth faults. The one or more control windings may be adapted for magnetically saturating the material of the additional path to increase reluctance. The one or more control windings may thus be adapted for forming a virtual air gap for the additional path, for example at a limb, such as a fourth limb, and/or a yoke of the transformer core.

In an embodiment, the transformer core comprises a fourth limb and the additional path goes through the fourth limb. This allows the additional path to be integrated in the transformer core as a simple structure that can be efficiently formed and whose properties can be easily controlled. A four limb earth fault compensating reactor can be designed using the practices for designing ordinary three phase reactors. For example, the differences can be limited to allowing magnetic flux in the limbs to be half of what is normally allowed in reactor design. This is due to the magnetic flux generated in the transformer core due to fault current of the earth fault. This gives a straightforward framework for transformer manufacturers to work on.

In an embodiment, the reluctance of one or more, or even all, of the three limbs is larger than the minimum reluctance of the additional path. This allows the additional path to carry more flux than the corresponding limb or limbs, thereby optimizing the additional path for savings in material resources. For example, the cross-sectional area of one, two or three of the three limbs for defining the reluctance of the corresponding limb may be larger than the cross-sectional area of the additional path for defining the reluctance of the additional path. Since the reluctance of the additional path can be adjustable, it may be possible to still adjust the reluctance of the additional path to exceed the reluctance of any or all of the three limbs.

In an embodiment, the number of turns in the three phase windings is equal or substantially equal. In another embodiment, the reluctance of the first limb, the second limb and the third limb is equal or substantially equal. These allow mitigating asymmetries in the arc suppression coil. The three limbs may have equal or substantially length and/or cross-sectional area.

According to the invention, the arc suppression coil comprises an additional winding for the additional path. The additional winding is wound in direction of subtractive polarity with respect to the three phase windings allowing the zero sequence flux in the transformer core to be compensated even if the number of turns in the phase windings is increased. The additional winding is connected in series with respect to a combined output of the three phase windings allowing it to be connected between the phase windings and the ground of the electric power network. The additional winding provides an additional degree of freedom for compensating reactive power in the electric power network. In particular, it allows the number of turns in the phase windings to be increased, to decrease positive sequence flux, without decreasing the zero sequence flux, which works to compensate the reactive power in the electric power network. The number of turns in the phase windings and in the additional winding can therefore be adapted to obtain a desired ratio of positive sequence flux and zero sequence flux in the arc suppression coil, e.g. from one, implying even amounts, to substantially zero, implying negligible positive sequence flux. In an embodiment, the additional winding is wound around the transformer core, for example around a limb, such as the fourth limb, and/or a yoke of the transformer core.

In an embodiment the number of turns in the additional winding is at most <NUM> percent, for example <NUM>-<NUM> percent, of the total number of turns in the three phase windings. This allows saving material in the winding while ensuring the functioning of the winding for the compensating zero sequence flux. In an embodiment the number of turns in the additional winding is <NUM>-<NUM> percent of the total number of turns in the three phase windings. In particular, an upper limit of at most <NUM> percent has been found to allow notable improvement in controllability of the arc suppression coil so that the tradeoff of having a smaller portion of positive sequence flux may be accepted for many applications. All values quoted above for the number of turns are particularly appropriate in a typical situation where the number of turns in the three phase windings is substantially equal.

According to a second aspect, an arrangement for grounding a three-phase electric power network comprises an arc suppression coil according to the first aspect or any combination of its embodiments. The arc suppression coil comprises an output connection, for example directly from the phase windings or from the additional winding as described above. The arrangement comprises means for connecting the three-phase electric power network to ground directly from the output connection.

According to a third aspect, a method of grounding a three-phase electric power network is disclosed. The three phase lines of the three-phase electric power network are connected to ground through an arc suppression coil comprising a transformer core having a separate limb and a separate phase winding for each of the three phases of the three-phase electric power network for facilitating a grounding connection. The transformer core defines also an additional path for carrying magnetic flux between the opposite ends of each of the separate limbs allowing a return path to be formed for the magnetic flux for each of the separate limbs. In an embodiment, the arc suppression coil is directly connected to ground. A single apparatus can therefore be used for both grounding the electric power network and providing the earth fault compensation. In an embodiment, the arc suppression coil is the arc suppression coil according to the first aspect or any combination of its embodiments.

The construction of the arc suppression coil allows various features to be easily integrated in the arc suppression coil. For example, the arc suppression coil may comprise a power supply connection adapted for transmitting power from the electric power network through the arc suppression coil to another network such as a low-voltage network. The power supply connection comprises, for example, a set of windings, such as low-voltage windings, wound at the three limbs of the transformer core for drawing power from the electric power network. The set of windings are adapted for grounding and they may be in zigzag configuration with respect to the three limbs of the transformer core. The set of windings has an output, which can be a three-phase output, for drawing power from the electric power network. The power supply connection can be maintained even during an earth fault, when only phase voltages change and the line voltages are not affected. This can be guaranteed by the set of windings having a zigzag configuration.

As another example, the arc suppression coil may be adapted for connecting a resistive circuit to the arc suppression coil for drawing power from the electric power network. The resistive circuit can be coupled to the additional path through a winding so that current can be induced into the resistive circuit in from the magnetic flux flowing through the additional path. This increase resistive current can be detected by a protective relay of the electric power network resulting in tripping of the relay protecting a feeder with an earth fault. The resistive circuit may comprise a switch for closing the circuit, allowing the circuit to be selectively activated, for example if the earth fault has persisted for a threshold time.

It is to be understood that the aspects and embodiments described above may be used in any combination with each other. Several of the aspects and embodiments may be combined together to form a further embodiment.

The accompanying drawings, which are included to provide a further understanding and constitute a part of this specification, illustrate embodiments and together with the description help to explain the principles of the invention. In the drawings:.

Like references are used to designate equivalent or at least functionally equivalent parts in the accompanying drawings.

The detailed description provided below in connection with the appended drawings is intended as a description of the embodiments and is not intended to represent the only forms in which the embodiment may be constructed or utilized. However, the same or equivalent functions and structures may be accomplished by different embodiments.

<FIG> shows an example of what a general system for grounding a three-phase electric power network could look like when schematically described in view of the present solution. The network <NUM> is connected to ground <NUM> through an earthing transformer <NUM>. Typically to delta-connected networks, the earthing transformer has a zigzag winding <NUM>. In a compensated network, the system comprises a Petersen coil <NUM> for compensating capacitive earth faults. In addition, the system comprises means <NUM> such as an oil-immersed high-power resistor to be connected in parallel with the Petersen coil <NUM>.

<FIG> shows an example of grounding an electric power network <NUM> with an arc suppression coil <NUM> (ASC). The electric power network <NUM> (below also "the network") is a three-phase network, typically a medium voltage and/or a distribution network, having an operating frequency, e.g. <NUM>-<NUM>, which may be substantially constant. The network <NUM> may comprise a bus such as a substation bus for connecting the ASC <NUM> to the network <NUM>. Each of the three phase lines <NUM>', <NUM>'', <NUM>‴ of the network <NUM> are separately connected to the ASC <NUM>, for example from the bus. The ASC <NUM> provides the grounding connection to the network <NUM> and it may be directly connected to ground <NUM> by means for grounding such as grounding connectors. The ASC <NUM> can be located at an electrical substation having a substation primary transformer for converting high voltage to medium voltage. In a typical example, the network <NUM> is a delta-connected network, i.e. the substation primary transformer is delta connected. Many of the advantageous effects of the present arc suppression coil are applicable particularly to delta-connected networks but the ASC <NUM> can be applied also together with other network configurations such as with a star-connected network.

The arc suppression coil <NUM> comprises a transformer core <NUM>, which may be formed as a monolithic piece. The transformer core <NUM> is made of material capable of supporting a magnetic flux, for example of metal, such as iron. The transformer core <NUM> may be a laminated core, in which case the laminates of the core may be formed as monolithic pieces. The transformer core <NUM> comprises three limbs <NUM>: a first limb <NUM>', a second limb <NUM>'' and a third limb <NUM>‴. Each of the limbs creates a path for magnetic flux φ, as in a conventional three-limb reactor. The magnetic flux φi, ii, iii in different limbs may be substantially equal in magnitude but it may also vary. The three limbs <NUM> can be made of substantially equal size and material so that their reluctance is substantially equal. The transformer core <NUM> typically comprises also one or more yokes <NUM> for connecting the limbs to each other. A first yoke may be connected the three limbs at the first end of the limbs <NUM>, whereas a second yoke may connect the three limbs <NUM> at the second end of the limbs <NUM>, e.g. the opposite end of the limbs <NUM>. Consequently, the transformer core <NUM> can be substantially shaped as two conjoined loops with a central limb <NUM>'' joining the two loops and the two other limbs <NUM>', <NUM>‴ forming the two sides of the two loops. With respect to the three limbs <NUM>, the transformer core can thereby be shaped substantially as a conventional three-phase reactor. While the transformer core <NUM> generally forms closed paths for the magnetic flux, it is noted that, in some configurations, the transformer core may comprise one or more physical discontinuities, such as air gaps, so that at least a part of one or more closed paths is outside the transformer core <NUM> or in a part of the transformer core having different magnetic properties with respect to an adjacent part of the magnetic core <NUM> on the closed path. Such discontinuities may be used to alter the reluctance of the transformer core <NUM>.

The arc suppression coil <NUM> comprises three separate phase windings <NUM>, such as medium-voltage windings, for facilitating a grounding connection for the network <NUM>. The phase windings <NUM> are windings adapted for coupling to one of the three phase lines of electric power network <NUM> through a conductive connection. Moreover, they are adapted to facilitate grounding of the phase line. A first phase winding <NUM>' is wound around the first limb <NUM>', a second phase winding <NUM>'' is wound around the second limb <NUM>'' and a third phase winding <NUM>''' is wound around the third limb <NUM>'''. Each of the three phase windings <NUM> is dedicated to a single phase of the three-phase electric power network <NUM>. Correspondingly, each of the three limbs <NUM> can be dedicated to facilitate a grounding connection for exactly one phase of the three-phase electric power network <NUM>. The structure of the ASC <NUM> allows such a straight grounding connection to be made, in contrast to a conventional zigzag winding for delta-connected medium-voltage networks. This has been found to allow notable reduction in the required size of the transformer core <NUM> in comparison to a situation where the ASC would have zigzag windings. This can be seen, for example, as reduced requirement for iron in the transformer core <NUM>. In any case, and even if the first limb <NUM>' remains coupled to two or more phases of the network <NUM>, the number of turns of all phase windings wound at the first limb <NUM>' and dedicated for one of the three phases of the three-phase network <NUM>, such as the first phase winding <NUM>', can be at least <NUM>-<NUM> percent of the total number of turns of all phase windings, regardless of phase, wound around the first limb <NUM>', while still allowing an essential reduction in the required mass for the transformer core <NUM>. The same holds for all of the three limbs <NUM>, including the second limb <NUM>'' and the third limb <NUM>'''. Consequently, the number of turns of all phase windings wound at exactly one of the three limbs <NUM> and dedicated for exactly one of the three phases of the three-phase network <NUM> can be at least <NUM>-<NUM> percent of the total number of turns of all phase windings, regardless of phase, wound around the corresponding limb, while still allowing an essential reduction in the required mass for the transformer core <NUM>.

The three phase windings <NUM> are generally wound in the direction of additive polarity with respect to each other. The phase windings <NUM>, as any other windings described herein, are made of electrically conductive material, for example of metal, such as copper. The number of turns in each of the three phase windings can be more than <NUM>. In an example, each of the phase windings <NUM> comprises at least <NUM> turns, for example <NUM>-<NUM> turns, and the number of turns in different phase windings <NUM> may be substantially equal.

The input of each of the three phase windings <NUM> is coupled, directly or indirectly, to one of the three phase lines of the network <NUM>, for example by a connection to the bus. The output of the three phase windings <NUM> may be connected to ground <NUM> separately or as a combined output <NUM>. The output of the three windings <NUM> may be combined for example by connecting the output from each of the three phase windings <NUM> to a connector <NUM> adapted to be connected to ground, for example by a grounding wire.

In addition to the three limbs <NUM>, the transformer core <NUM> defines an additional path <NUM> for carrying magnetic flux between the opposite ends of the three limbs <NUM>. The additional path <NUM> is outside the three limbs <NUM> provides a return path for the magnetic flux from the first of a limb to the second end of the limb for each of the three limbs <NUM>. This allows leakage flux to be directed into the additional path <NUM> during an earth fault to compensate for the capacitive earth fault. The additional path may be formed partially or wholly in the transformer core <NUM>, for example through a fourth limb <NUM> and/or one or more yokes <NUM> of the transformer core <NUM>. The additional path <NUM> may, for example, be formed as a loop with one open side connected to the three limbs <NUM>, e.g. as a substantially C- or U-shaped loop. The additional path <NUM> may be dimensioned for a desired amount of flux and its reluctance may be smaller than the reluctance of any or all of the three limbs <NUM>. The additional path <NUM> can be specifically adapted for carrying zero sequence flux, which may be taken account in the dimensioning of the additional path <NUM>.

The arc suppression coil <NUM> may optionally comprise means <NUM> such as an actuator for adjusting reluctance of the additional path <NUM>. The means <NUM> may be adapted for continuous adjustment of reluctance. The means <NUM> may be adapted for adjusting the reluctance when the earth fault is present in the network <NUM>. The means <NUM> may be adapted the reluctance to be adjusted electrically, wholly or at least partially. With electrical control of the reluctance of the additional path <NUM>, no motors and/or moving parts are required, which may allow the size, cost and maintenance requirements of the ASC <NUM> to be reduced. For example, the means <NUM> may be adapted to form a virtual air gap in the additional path <NUM> to for increasing the reluctance of the additional path <NUM>. The means <NUM> may be adapted for the reluctance of to be varied by at least a factor of <NUM>-<NUM>.

<FIG> shows an example of grounding an electric power network <NUM> with the ASC <NUM>. While the ASC <NUM> may otherwise be in accordance with any combination of examples above, it may also comprise an additional winding <NUM> for the additional path <NUM>. The additional winding <NUM> is wound at the additional path <NUM> and it can be wound around the additional path <NUM> for example around the fourth limb <NUM> and/or a yoke <NUM> of the transformer core <NUM>. While all three phase windings <NUM> are wound in the same direction with respect to each other, the additional winding <NUM> is wound in a direction opposite to that of the three phase windings <NUM>. This means that the additional winding <NUM> is wound in direction of subtractive polarity with respect to the three phase windings <NUM>, effectively allowing the ASC <NUM> to draw zero sequence flux through the additional path <NUM>. The additional winding <NUM> is connected in series with respect to the combined output <NUM> of the three phase windings <NUM> and it may be directly connected after the three phase windings <NUM>. The additional winding <NUM> is located between the phase windings <NUM> and the ground <NUM> of the electric power network <NUM> and it may be directly connected to ground <NUM>.

The number of turns in the additional winding <NUM> is typically smaller or even much smaller than that in any of the phase windings <NUM>. For example, when the phase windings <NUM> have less than <NUM> turns in total, the additional winding may have less than <NUM>-<NUM> turns. When the phase windings <NUM> have less than <NUM> turns in total, the additional winding may have less than <NUM>-<NUM> turns. When the three phase windings <NUM> have less than <NUM>-<NUM> turns in total, the additional winding may have less than <NUM> turns.

As an additional or alternative feature, <FIG> illustrates one example for providing the means <NUM> for adjusting reluctance of the additional path <NUM>. In an example the means <NUM> comprise one or more control windings <NUM> wound at the additional path <NUM> for adjusting reluctance at the additional path <NUM>. The one or more control windings <NUM> may be wound partially or fully around the transformer core <NUM>, e.g. through the transformer core <NUM>. For example, a control winding <NUM> may be wound around or through a limb, such as the fourth limb <NUM>, and/or a yoke <NUM> of the transformer core <NUM>. The transformer core <NUM> may comprise one or more through-holes <NUM> adapted for winding a control winding <NUM> through the transformer core <NUM>. The one or more control windings <NUM> may be direct current (dc) windings but it is also possible to use alternating current (ac) windings for this purpose. The one or more control windings <NUM> are adapted for receiving current from a power supply unit <NUM>, which may be controlled by a controller such as a remote terminal unit (RTU). The controller may also be integrated with the power supply unit.

<FIG> shows an example of grounding an electric power network <NUM> with the ASC <NUM>. While the ASC <NUM> may otherwise be in accordance with any combination of examples above, it may also be adapted for connection with a resistive circuit <NUM>, where the resistive circuit <NUM> can be an external circuit <NUM> or an integral part of the ASC <NUM>. The resistive circuit <NUM> comprises a resistive element <NUM> for drawing active power from the network <NUM> to trip one or more protective relays of the network <NUM>. The resistive circuit <NUM> may also comprise a switch <NUM> adapted for closing the resistive circuit <NUM> so that it may draw power from the ASC <NUM>. The resistive circuit <NUM> can be coupled to the ASC <NUM> through a winding <NUM> adapted for inducing current into the resistive circuit <NUM> from the magnetic flux flowing in the ASC <NUM>. The winding <NUM> may be wound at the transform core <NUM>, for example at the additional path <NUM>. The ASC <NUM> may comprise the winding <NUM>, for example at a limb, such as the fourth limb <NUM>, and/or at a yoke <NUM> of the transformer core <NUM>.

As an additional or alternative feature, <FIG> illustrates one example for providing means in the ASC <NUM> for outputting power from the three-phase electric power network <NUM> through the ASC <NUM>. This can be used, for example, to provide low-voltage power supply from the ASC <NUM>. For these purposes, the ASC <NUM> may comprise a power supply connection <NUM> having a set of windings <NUM>, for example low-voltage windings, adapted for drawing power from the three-phase electric power network <NUM> through the transformer core <NUM>. The set of windings <NUM> may be wound around the three limbs <NUM> of the transformer core <NUM> allowing three-phase current to be generated through the set of windings <NUM>. The set of windings <NUM> may be formed as a zigzag winding. The set of windings <NUM> is coupled to one or more outputs <NUM> of the power supply connection <NUM> for transmitting power from the power supply connection <NUM>. The one or more outputs <NUM> may comprise three phase outputs <NUM>', <NUM>'', <NUM>''' adapted for outputting three-phase electric current. The one or more outputs <NUM> may comprise one or more electric sockets. The set of windings <NUM> are adapted for coupling to ground <NUM> for grounding. While the set of windings <NUM>. It is noted that while the set of windings <NUM> may be wound, at least partially, around one or more of the three limbs <NUM>, they need not be conductively connected to any of the three phase windings <NUM>. The three phase windings <NUM> are adapted for grounding the three-phase electric power network <NUM>, whereas the set of windings <NUM> of the power supply system serve as an additional option for drawing power from the ASC <NUM>, for example to be used at an electrical substation.

<FIG> is a schematic diagram of a system <NUM> for operating the ASC <NUM> according to an example. The system <NUM> is adapted to be electrically connected to the ASC <NUM>. Moreover, the system <NUM> can be adapted to be electrically connected to the network <NUM>, for example to the bus of the network <NUM>. The system <NUM> may be a local system, for example at an electrical substation or at a distribution transformer. Through its connection to the network <NUM>, for example through the bus of the network <NUM>, the ASC <NUM> may be connected to a transformer, such as a substation primary transformer or a medium voltage-low voltage transformer of the network <NUM>.

The system <NUM> comprises an apparatus <NUM>, such as a controller, for operating the ASC <NUM>. The apparatus <NUM> may be adapted to function as a stand-alone unit but for many typical applications, the apparatus <NUM> can be adapted to function as a part of a distributed system <NUM> for controlling the network <NUM>, e.g. it may be a remote terminal unit (RTU), such as an RTU of a SCADA system. The apparatus <NUM> may be adapted to be connected to the network <NUM>, e.g. to the bus, for determining information indicative of the status of the network <NUM>. The system <NUM> may comprise a gateway <NUM> for remote communication of the apparatus <NUM> with one or more external systems, e.g. with a distributed system <NUM> such as a SCADA system.

The apparatus <NUM> is electrically connected to the ASC <NUM> for operating the ASC <NUM>, for example for adjusting the inductance of the ASC <NUM>, e.g. by adjusting the reluctance of the ASC. The apparatus <NUM> may be adapted to adjust the inductance of the ASC <NUM> in the absence and/or presence of an earth fault in the network <NUM>. The apparatus <NUM> may be adapted to control one or more analog outputs (VCOMP) for controlling the inductance of the ASC <NUM>, for example by feeding current, such as dc current, to means <NUM> for adjusting the reluctance of the additional path <NUM>, e.g. to the one or more control windings <NUM> adapted for forming a virtual air gap <NUM> for adjusting the reluctance of the arc suppression coil. Voltage- and/or current-based control may be used. The system <NUM> may comprise a converter <NUM>, e.g. a dc-dc converter or an ac-dc converter, connected between the ASC <NUM> and the apparatus <NUM>. The apparatus <NUM> may be grounded to the ground <NUM> of the network <NUM>, which may correspond to the ground <NUM> of the ASC <NUM>, for example by a direct connection. The apparatus <NUM> may also be adapted to control the switch <NUM> for closing the resistive circuit <NUM> for drawing power from the network <NUM>. This allows the resistive circuit <NUM> to be closed based on any conditions defined for the apparatus <NUM>, for example after a threshold time from the onset of the earth fault and/or based on the fault current of the earth fault exceeding a threshold level.

Some examples for design parameters are given in the following, but it should be noted that the ASC <NUM> may be dimensioned also at other parameter ranges. The ASC <NUM> may be dimensioned, for example, for a maximum reactive current of <NUM>-<NUM> amperes per phase but also larger reactive currents may be supported. The three phase windings <NUM> may be adapted to support a current of at least <NUM> A each, for example <NUM>-<NUM> A per phase, on a continuous basis. The three limbs <NUM> may be dimensioned so that they are adapted to carry both zero sequence flux and positive sequence flux. This way, the ASC <NUM> may be dimensioned for both the reactive current and the fault current from the earth fault. The reluctance of the additional path <NUM> can be made, for example, at least <NUM>-<NUM> percent smaller than that of any or all of the three limbs <NUM>. Correspondingly, the cross-sectional area of the additional path <NUM> can be at least <NUM>-<NUM> percent larger than that of any or all of the three limbs <NUM> for providing a path of reduced reluctance at the additional path <NUM>.

The following equations are given to illustrate the functioning of the ASC <NUM>. It is noted that, as always, the equations provide a simplification of the actual physical phenomena taking place in the ASC <NUM> and it should be understood that they necessarily contain approximations. In the following, the number of turns in one of the three phase windings <NUM> is denoted by N<NUM> and it is assumed substantially equal in each of the three phase windings <NUM>. Similarly, the reluctance of one of the three limbs <NUM> is denoted by RL and it is assumed substantially equal in each of the three limbs <NUM>. In the following, the value for the reluctance of the additional path <NUM> is λRL and it may be adjusted by an adjustable reluctance ratio λ for the means <NUM> for adjusting the reluctance of the additional path <NUM>. The adjustable reluctance ratio may have values in the range of, for example, <NUM>-<NUM>, and when there is no adjustment λ=<NUM>. The number of turns in the additional winding <NUM> is denoted by N<NUM> and when N<NUM>=<NUM>, the situation from ASC <NUM> of <FIG> reduces to that of <FIG>. When the operating frequency of the network <NUM> is denoted by f, the magnitude of the zero sequence impedance of the network <NUM> can be approximated as <MAT> whereas the magnitude of the zero sequence flux in the transformer core, and through the additional path <NUM>, can be approximated as <MAT>.

The magnitude of the positive sequence impedance of the network <NUM> can be approximated as <MAT> and the magnitude of the positive sequence flux in the transformer core <NUM> can be approximated as <MAT>.

The magnitude of zero sequence voltage V<NUM> of the network <NUM> typically changes during an earth fault and it may be of several kilovolts, for example <NUM> kV, corresponding to the magnitude of the phase voltage of the network <NUM>. The magnitude of the positive sequence voltage V<NUM> of the network <NUM> is a parameter of the network <NUM>, which may be substantially equal to the magnitude of the phase voltage of the network <NUM>. From the equations it is seen that the inclusion of the additional winding <NUM> in the ASC <NUM> allows reducing the magnitude of the positive sequence flux φ<NUM> by increasing the number of turns N<NUM> in the phase windings <NUM> without decreasing the magnitude of the zero sequence flux φ<NUM>, as long as the increase is compensated by an increase in the number of turns N<NUM> in the additional winding <NUM>.

Regarding the flux distribution in the ASC <NUM>, it is noted that whereas the three limbs <NUM> are adapted for carrying both zero sequence flux (magnitude φ<NUM>) and positive sequence flux (magnitude φ<NUM>). The first limb <NUM>' may be adapted to carry a combined flux φi, the second limb <NUM>'' may be adapted to carry a combined flux φii and the third limb <NUM>''' may be adapted to carry a combined flux φiii. Assuming that the properties of the three limbs <NUM> and the three phase windings <NUM> are substantially equal, the combined flux in each of the three limbs is substantially equal φi=φii=ϕiii. In the absence of the additional winding <NUM>, the combined flux has a magnitude of φ<NUM>/<NUM>+φ<NUM>. However, when the additional winding <NUM> is included in the ASC <NUM> with N<NUM>><NUM>, the magnitude of the combined flux goes towards the value φ<NUM> as the number of turns in the additional winding <NUM> increases. Consequently, the magnitude of the positive sequence flux can be made negligible in the three limbs <NUM>. On the other hand, the additional path <NUM> can be adapted to carry only zero sequence flux φ<NUM>, regardless of the value of N<NUM>, and the magnitude of zero sequence flux may be maintained constant by simultaneously increasing both N<NUM> and N<NUM>.

The different functions discussed herein may be performed in a different order and/or concurrently with each other.

Any range or device value given herein may be extended or altered without losing the effect sought, unless indicated otherwise. Also any embodiment may be combined with another embodiment unless explicitly disallowed.

Claim 1:
An arc suppression coil (<NUM>) for a three-phase electric power network (<NUM>) comprising:
a transformer core (<NUM>) having three limbs (<NUM>), which comprise a first limb (<NUM>'), a second limb (<NUM>'') and a third limb (<NUM>‴), wherein the transformer core defines an additional path (<NUM>) for carrying magnetic flux between the opposite ends of the three limbs;
three separate phase windings (<NUM>) comprising a first phase winding (<NUM>') wound at the first limb of the transformer core, a second phase winding (<NUM>'') wound at the second limb of the transformer core and a third phase winding (<NUM>‴) wound at the third limb of the transformer core; and
an additional winding (<NUM>) for the additional path;
characterized by the additional winding being wound in direction of subtractive polarity with respect to the three phase windings and connected in series with respect to a combined output (<NUM>) of the three phase windings.