EP2767996B1

EP2767996B1 - Switching device for an on-load tap changer

Info

Publication number: EP2767996B1
Application number: EP13155347.1A
Authority: EP
Inventors: Tommy Larsson
Original assignee: ABB Schweiz AG
Current assignee: ABB Schweiz AG
Priority date: 2013-02-15
Filing date: 2013-02-15
Publication date: 2017-09-27
Anticipated expiration: 2033-02-15
Also published as: EP2767996A1; CN105308703A; CN105308703B; WO2014124771A1

Description

Technical field

The present invention relates to the field of power transmission, and in particular to tap changers for controlling the output voltage of a transformer.

Background

Tap changers are used for controlling the output voltage of a transformer by providing the possibility of switching in or switching out additional turns in a transformer winding. A tap changer comprises a set of fixed contacts which are connectable to a number of taps of a regulating winding of a transformer, where the taps are located at different positions in the regulating winding. A tap changer further comprises at least one moveable contact which is connected to a current collector at one end, and connectable to one of the fixed contacts at the other end. By switching in or out the different taps, the effective number of turns of the transformer can be increased or decreased, thus regulating the output voltage of the transformer.
A mechanical tap changer is typically arranged so that upon changing taps, the new tap is physically connected before the old tap is disconnected. In order to avoid high circulating currents between the new and the old tap while both taps are connected, many tap changers include a switching device, by which a large transition resistor can be series-connected between the old and the new tap during the tap changing event.
Traditionally, changing of taps has been performed in the order of a few times per day. Recently, new applications of on-load tap changers have been proposed, wherein a more frequent switching will occur. Although the heat loss in the transition resistor(s) is small since the duration of the switching action is short, frequent switching would possibly still involve a risk of over-heating. A switching device for an on-load tap changer is known e.g. from the document WO2011/033254 A2 . Thus, there is a need for a tap changer design which can safely be operated more frequently.

Summary

A problem to which the present invention relates is how to obtain a tap changer which can safely be frequently operated.
One embodiment provides a switching device for an on-load tap changer which tap changer includes at least two fixed contacts according to claim 1. The main switch is a vacuum interrupter or a semiconductor switch, which are switch types that can block a high transient recovery voltage over a short isolation distance. Such switch types are commonly referred to as compact-break switches. By providing a transition path having a transition impedance which is mainly inductive, the tap changing operations can safely be performed more frequently since the heat losses involved in the tap-changing operations will be significantly reduced. However, when the main switch is opened and parts of the load current starts to flow in the transition path, the transition inductor will produce a transient recovery voltage across the main switch. By providing the main switch in the form of a compact-break switch, complete commutation of the load current to the transition path can be ensured despite the transient recovery voltage, without the need for additional equipment for creating a current-zero.
The impedance of the transition path of the inventive switching device falls within the range of $\frac{1}{10} \cdot \frac{U_{step}}{I_{R}} to 10 \cdot \frac{U_{step}}{I_{R}},$
where U_step is the expected voltage between two adjacent taps of the on-load tap changer, and I_R is the rated load current of the tap changer. This ensures that the voltage drop across the transformer, upon a tap changing operation, will be within an acceptable level for the end user.
The switching device can comprise further main paths and further transition paths, wherein the main path(s) and transition path(s) are associated with each other on a one-to-one basis to form path pairs. The transition path of a path pair forms a well-defined path to which the load current is commutated, if the main switch of the main path of the pair is opened in a current carrying state.
The invention further relates to a tap changer comprising an inventive switching device.
Further aspects of the invention are set out in the following detailed description and in the accompanying claims.

Brief description of the drawings

Fig. 1: schematically illustrates an embodiment of a prior art diverter-switch type tap changer which includes a diverter switch.
Fig. 2: schematically illustrates another prior art diverter switch.
Fig. 3: schematically illustrates an embodiment of a prior art selector-switch type tap changer which includes a selector switch unit.
Fig. 4a: schematically illustrates an embodiment of a diverter switch according to the invention.
Fig. 4b: schematically illustrates an embodiment of a diverter switch according to the invention.
Fig. 5: illustrates a switching sequence of the diverter switches of Figs. 1, 4a and 4b.
Fig. 6a: schematically illustrates an embodiment of a diverter switch according to the invention.
Fig. 6b: schematically illustrates an embodiment of a diverter switch according to the invention.
Fig. 7: illustrates a switching sequence of the diverter switches of Figs. 2, 6a and 6b.
Fig. 8a: schematically illustrates an embodiment of a selector-switch type tap changer according to the invention.
Fig. 8b: schematically illustrates an embodiment of a selector-switch type tap changer according to the invention.
Fig. 9: illustrates a switching sequence of the selector switch unit of Figs. 3, 8a and 8b.
Fig. 10: is a schematic illustration of a control unit for triggering and control of a tap changing event.

Detailed description

There are numerous designs of switching devices which are arranged to provide electrical connection between at least one moveable contact and an external output of a tap changer, and by which smooth switching between different taps of the tap changer can be performed. Here, the term switching device is used to refer to a device capable of transferring the load current, I_load, in a tap changer from one tap to another tap under continuous operation of the tap changer, i.e. without any interruption in the power transmission. In one type of tap changer, referred to as the diverter-switch type, the tap changer comprises a switching device referred to as a diverter switch, as well as a tap selector. Switches of a diverter switch can be sequentially operated to perform a switching operation between two taps (cf. Figs. 5 and 7). In a diverter-switch tap changer, the tap selector is used to select the tap to which the load current is to be transferred, while the diverter switch is used to perform the commutation of the load current from the presently connected tap to the tap selected by the tap selector. Examples of diverter-switch tap changers are shown in Figs. 1, 2, 4a & 4b and 6a & 6b. Another type of tap changer, referred to as the selector-switch type, comprises a switching device referred to as a selector switch unit is provided to perform the switching between two taps, where switches of the selector switch unit are sequentially operated to perform a switching operation between two taps (cf. Fig. 9). Examples of a selector-switch tap changer are shown in Fig. 3 and Figs. 8a & 8b. In a selector-switch type tap changer, the selector switch unit is used to perform both the selection of the tap and the commutation of the load current to the selected tap.
A switching device is designed to be part of the electrical connection between a fixed contact and an external contact of a tap changer. In a selector-switch type tap changer, the switching device typically forms the entire connection between a fixed contact and the external contact, while in a diverter-type tap changer, the electrical connection between the fixed contact and the external contact is typically formed by the switching device together with a tap selector.
The switching device can provide at least one pair of paths wherein one of the paths of such pair, the main current path, is of lower impedance than the other path, the transition current path. A main current path will in the following be referred to as a main path, while a transition current path will be referred to as a transition path. Both the main path and the transition include a switch, so that the path can be opened or closed. The switching device is arranged so that when the switching device is in use, both the main path and the transition path will be in electrical connection with a fixed contact of the tap changer at a first end, and with the external contact of the tap changer at the other end.
During a tap changing operation, there will be a stage when two taps (the "old" tap and the "new" tap) are connected to the external contact at the same time: one tap will be connected via a transition path of high impedance, while the other is connected via a main path of low impedance or via a second transition path (cf. stage III of Fig. 5, stage IV of Fig. 7 as well as stage IV of Fig. 9). Such stage will here be referred to as the transition stage. The transition path serves as a temporary connection between the external contact and one of the taps involved in the tap changing operation (old or new), until the new tap is connected to the external connection point via a main path. In the transition stage, a circulating current will flow, via the switching device, between the two taps involved in the tap changing operation. The transition path includes a transition switch, which will, upon opening, break the circulating current. The transition path also includes a transition impedance in order to ensure that currents circulating between the new and old taps during the transition stage will be of limited magnitude. The impedance of the main path, on the other hand, is low enough so that the main path can serve as the steady-state current path, which continuously carries the load current when no tap changing operation is performed.
An example of a diverter-switch tap changer 100 for connection to a regulating winding 105 of a transformer is schematically illustrated in Fig. 1 . The diverter-switch tap changer 100 comprises a switching device in the form of a diverter switch 115. The diverter switch 115 is connected between an external contact 155 and a tap selector 120. The tap changer 100 of Fig. 1 comprises a tap selector 120 having two current collectors 125, two moveable contacts 130 and a set of fixed contacts 135. The regulating winding 105 has a set of different taps 110, and each of the fixed contact 135 is connectable to one of the taps of the regulating winding 105. The regulating winding 105 is shown in Fig. 1 for illustrative purposes, and is normally not seen as part of the tap changer 100.
The diverter switch of Fig. 1 comprises two branches 160, each branch 160 comprising a series connection of a main switch 140 and a transition switch 145, with a transition resistor 150 connected in parallel with the main switch 140. Each branch is, at one end, connected to a respective one of the two current collectors 125, and, at the other end, connected to an external contact 155 of the tap changer 100. A connection point between a current collector 125 and a branch 160 of the diverter switch 115 is indicated in Fig. 1, for illustration purposes, by reference numeral 170. In Fig. 1, the main switch 140 and the transition switch 145 of one branch are shown to be open, while the main switch 140 and the transition switch 145 of the other branch are shown to be closed.
Depending on in which branch 160 the main switch 140 and transition switch 145 are closed, one or the other of the moveable contacts 130 will be connected to the external contact 155, and thus provide an electrical path through the tap changer 100. Upon performing a tap changing operation, the main and transition switches 140, 145 are switched in a predetermined sequence, temporarily connecting the transition resistors 150 between the "new" and "old" taps in order to avoid high short circuit currents between adjacent taps. The transition resistors 150 are only connected during the tap changing operation, and will be disconnected when the new main path is closed. Thus, each branch 160 of the diverter switch 115 of Fig. 1 provides a main path formed by the series connection of the main switch 140 and the transition switch 145, as well as a transition path formed by the series connection of the transition switch 145 and the transition resistor 150 of the branch 160. A switching sequence corresponding to the one used in the diverter switch 115 of Fig. 1 is illustrated in Fig. 5.
Another design of a diverter switch 115 is shown in Fig. 2 . In a tap changer 100, the diverter switch 115 of Fig. 2 would typically be connected to a tap selector 120, cf. Fig. 1. The diverter switch 115 of Fig. 2 comprises two branches, referred to as main branch 160a and transition branch 160b, respectively. Main branch 160a comprises a series connection of a main switch 140 and a four-way contact 240a. Transition branch 160b comprises a series connection of a four-way contact 240b, a transition switch 145, and a transition resistor 150, while main branch 160a does not include any transition resistor. The transition branch 160b will only carry current during the tap changing process. A first contact point of the four- way contacts 240a, 240b is connectable to a first current collector 125 (not shown) via a first connection point 170a, while a second contact point of the four- way contacts 240a, 240b is connectable to a second current collector 125 via a second connection point 170b. Hence, both the main branch 160a and the transition branch 160b can be connected to two different current collectors 125, via four- way contacts 240a and 240b, respectively. The third and fourth contact points of four-way contact 240a are both connected to the main contact 140, while the third and fourth contact points of four-way contact 240b are connected to the series-connection of the transition resistor 150 and the transition switch 145. Thus, the diverter switch 115 of Fig. 2 can provide two different main paths and two different transition paths: The main branch 160a can provide two different main paths, depending on to which current collector 125 the four-way contact 240a of the main branch is connected. The transition branch 160a can provide two different transition paths, depending on to which current collector 125 the four-way contact 240b of the transition branch is connected. In Fig. 2, the main switch 140 is shown to be open, while the transition switch 145 is shown to be closed.
A switching sequence corresponding to the one used in the diverter switch 115 of Fig. 2 is illustrated in Fig. 7.
In Fig. 3 , an example of a tap changer 100 of the selector-switch type is schematically illustrated. In a tap changer 100 of selector-switch type, a selector device in the form of a selector switch unit 300 is connected between a current collector 125 and the fixed contacts 135 in a moveable manner, so that the selector switch unit 300 can selectively provide connection between a selected one of the fixed contacts 135 and the external contact 155 via the current collector 125. The external contact 155 is connected to the current collector 125. Thus, the external contact 155 can, via the selector switch unit 300, be connected to one of the fixed contacts 135 at a time. The selector switch unit 300 of Fig. 3 comprises two branches: a main branch 160a and a transition branch 160b. Both the main branch 160a and the transition branch 160b are connected to the current collector 125 at a first end 305, and both comprise a moveable contact 130 at another other end (referred to as moveable contacts 130a and 130b, respectively). The moveable contacts 130a,b are each connectable to one of the fixed contacts 135 at a time.
The main branch 160a comprises, between its moveable contact 130a and the first end 305, a series connected main switch 140. The transition branch 160b comprises, between its moveable contact 130b and the first end 305, a series connection of a transition switch 145 and a transition resistor 150. In Fig. 3, the main switch 140 is shown to be open, while the transition switch 145 is shown to be closed. By switching the main switch 140, switching the transition switch 145 and moving the moveable contacts 130a,b in a predetermined manner, the fixed contact 135, which is in contact with the external contact 155, can be changed. The transition branch 160b of Fig. 3 will only carry current during the tap changing process. The switching device of Figs. 3 can only provide one main path through the switching device 300, formed by main branch 160a. Similarly, the switching device of Fig. 3 can only provide one transition path, formed by transition branch 160b. An example of a switching sequence corresponding to the one used in the selector switch unit 300 of Fig. 3 is illustrated in Fig. 9.
The different embodiments of switching devices 115, 300 illustrated in Figs. 1-3 share the common feature of comprising at least one transition resistor 150, which is connected in parallel with a main switch 140 in a manner so that the transition resistor 150 will only be connected in the load current path during a tap changing operation. Numerous other designs of switching devices for tap changers are also available and possible. However, regardless of which diverter switch design and switching sequence are used, a transition resistor 150 will never form part of the main path established when the tap changing operation is complete - the resistive losses would be too large.
As mentioned above, there is a need for tap changers which can cope with frequent switching without risking that the switching device will be damaged from over-heating.
According to the invention, a switching device for an on-load tap changer is provided, where the switching device provides:

a main path including a main switch which is series connected in the main path;
a transition path including a transition inductor and a transition switch, the transition switch and the transition inductor being connected in series; wherein
the impedance of the transition path is higher than the impedance of the main path;
the impedance of the transition path is mainly inductive; and
the main switch is a compact-break switch;
the main switch and the transition inductor are connected in parallel; so that upon opening of the main switch when the transition switch is closed, a load current flowing through the main path will be commutated to the transition path.

By providing a transition path having a transition impedance which is mainly inductive, tap changing operations can safely be performed more frequently since the heat losses involved in the tap-changing operations will be significantly reduced. However, when the main switch is opened and parts of the load current starts to flow in the transition path, the transition inductor will produce a transient recovery voltage across the main switch. By providing the main switch in the form of a compact-break switch, complete commutation of the load current to the transition path can be ensured despite the recovery voltage. The transient recovery voltage typically appears over a time period of 0.1-3 ms, including oscillations, although in some implementations, transient recovery voltages with other time durations may occur.
A main path is typically associated with one transition path only, said associated transition path forming a well-defined path to which the load current can be commutated if the main switch of the main path is opened. The impedance of the transition path is higher than the impedance of the associated main path.
A compact-break switch is a switch which can block the transient recovery voltage over a short isolation distance, typically over an isolation distance corresponding to 1 mm/kV, or less. Expressed in an alternative way, a compact-break switch used in the invention can typically block a transient electric field of higher magnitude than 1 kV/mm. Often, a compact-break switch which provides an isolation distance for transient voltages corresponding to less than 0.1 mm/kV, i.e. a switch which can block a transient electric field of higher magnitude than 10 kV/mm, will be used in the invention. By use of a compact-break switch, commutation of the load current to the inductive transition path will occur directly at current zero after the switch has been opened (or even before current zero, if a semiconductor component of turn-off capability is used, such as an IGBT). No additional equipment for creating a current-zero will be required. Efficient commutation of the load current can thus be achieved with a compact design of the switch device.
Examples of compact-break switches include vacuum interrupters and semiconductor switches.
The main switch can for example be an arcing switch capable of extinguishing the arc even in the presence of a transient recovery voltage set up across the inductor during a tap changing operation. A vacuum interrupter is an example of such arcing switch. Such switch can reliably perform fast commutation of the load current also in the presence of a high transient recovery voltage.
Alternatively, the main switch of compact-break switch type can be a semiconductor switch. Such semiconductor switch could e.g. include a thyristor, an IGBT (insulated-gate bipolar transistor), a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), an IGCT (integrated gate-commutated thyristor), a Bimode Insulated Gate Transistor (BIGT) or a GTO (gate turn-off thyristor). The main switch should preferably be a bi-directional switch, and could, when based on semiconductor technology, for example include two series connected parallel connections of a switch of transistor type and an anti-parallel (so called free-wheeling) diode; or two switches of thyristor type connected in anti-parallel, etc. In Figs. 4b, 6b and 8b, the main and transition switches are shown as triacs.
The main switch and the transition inductor are connected in parallel. This allows the use of a two-way switch as the main switch. In some embodiments, the parallel connection will be such that the main switch is connected in parallel with a circuit in which the transition inductor forms a part, and/or that the transition inductor is connected in parallel with a circuit in which the main switch forms a part (cf. Figs. 6a, 6b, 8a, 8b). In other embodiments, the parallel connection will be such that the main switch and the transition inductor are directly connected to each other (cf. Figs. 4a and 4b).
The switching device 115, 300 is arranged to open, during a tap changing operation, the main switch 140 in order to commutate the load current from the main path to the transition path. This commutation takes place when the transition switch 145 is in a closed state. In other words, the switching device 115, 300 is arranged so that the commutation of the load current from the main path to the transition path is performed by opening the main switch. This process is typically performed by natural commutation, where the load current through the main switch is commutated at the next current zero occurring after the main switch has been triggered to open.
The main switch will, upon opening, commutate the load current to an inductor, i.e. to the inductive transition path comprising the transition inductor 400. The transition switch, on the other hand, will, upon opening, break an inductive circuit. The transition switch could therefore also advantageously be a compact-break switch.
Figs. 4a, 4b, 6a, 6b, 8a and 8b illustrate different embodiments of a switching device according to the invention. Figs. 4a and 4b illustrate diverter switches 115 based on the diverter switch 115 of Fig. 1, wherein the transition resistors 150 have been replaced by transition inductors 400, and the main switches 140 as well as the transition switches 145 are implemented as vacuum interrupters (Fig. 4a) or power semiconductor switches (Fig. 4b), respectively. The switching sequence of the diverter switch of Fig. 4a is illustrated in Fig. 5 , where the bold lines indicate the load current path at different stages I-V of the switching sequence. The same switching sequence can be used for the diverter switch unit of Fig. 4b.
Figs. 6a and 6b illustrate diverter switches 115 based on the diverter switch 115 of Fig. 2, wherein the transition resistor 150 has been replaced by a transition inductor 400, and the main switch 140 and the transition switch 145 are implemented as vacuum interrupters (Fig. 6a) or power semiconductor switches (Fig. 6b), respectively. The switching sequence of the diverter switch of Fig. 6a is illustrated in Fig. 7 , where the bold lines indicate the load current path at different stages I-V of the switching sequence. The same switching sequence can be used for the diverter switch unit of Fig. 6b.
The transition inductor 400 of Figs. 6a and 6b could be replaced with, or combined with, two inductors connected to a respective one of the first and second contact points of the four-way contact 240b, where the first and second contact points are for connection of the transition branch 160b to the first and second connection points 170a, 170b, respectively. If a transition path comprises two or more physically separated inductors, they will together form the transition inductor 400.
Figs. 8a and 8b illustrate selector-switch type tap changers 100 including selector switch units 300 based on selector switch unit shown in Fig. 3, wherein the transition resistor 150 has been replaced by a transition inductor 400, and the main switches 140 and the transition switch 145 are implemented as vacuum interrupters (Fig. 8a) or power semiconductor switches (Fig. 8b), respectively. The switching sequence of the selector switch unit of Fig. 8a is illustrated in Fig. 9 , where the bold lines indicate the load current path at different stages I-VII of the switching sequence. The same switching sequence can be used for the selector switch unit of Fig. 8b.
The transition inductor 400 in a switching device 115, 300 according to the invention could advantageously be dimensioned such that the impedance Z_tr of the transition path falls within the following range: $\frac{1}{10} \cdot \frac{U_{step}}{I_{r}} < Z_{tr} < \frac{1}{10} \cdot \frac{U_{step}}{I_{r}}$
where U_step is the voltage between two adjacent fixed contacts 135 of the tap changer in which the switching device is designed to be operated (this voltage referred to as the step voltage); and Ir is the rated load current of the tap changer. By providing an impedance Z_tr of the transition path that is approximately in the order of the ratio between the step voltage and the rated load current, the voltage drop across the transformer, upon a tap changing operation, will be within an acceptable level for the end user. Oftentimes, the transition inductor 400 will be dimensioned such that the impedance Z_tr falls within the following range: $\frac{1}{3} \cdot \frac{U_{step}}{I_{r}} < Z_{tr} < 3 \cdot \frac{U_{step}}{I_{r}}$
According to the invention, the inductive contribution to the transition impedance Z_tr is at least 50%, in order to reduce the heat loss in the transition path. Oftentimes, the inductive contribution will be 70%, or as high as 90%, or even higher. When the ratio of the inductive part of Z_tr to the entire Z_tr is M, M > 50%, the ratio of the resistance, R_tr, of the transition path to the inductance, L_tr, of the transition path can be expressed as: $\frac{R_{tr}}{L_{tr}} = \frac{\sqrt{(1 - M^{2})}}{M} \cdot 2 πf$
where f is the frequency of the transmission system (typically 50 Hz). The inductance L_tr of the transition inductor 400 can be expressed in terms of the transition impedance Z_tr and the resistance R_tr of the transition path as: $L_{tr} = \frac{\sqrt{{Z_{tr}}^{2} - {R_{tr}}^{2}}}{2 πf}$
The transition inductor 400 could for example be implemented by means of thin Al or Cu foils, or Al or Cu wire, which are wound in a number of turns. In one example of transition inductor 400, designed for a switching device 115, 300 arranged to operate at a step voltage of 4 kV and a load current of 1 kA, a 0.2 mm thick and 50 mm wide Al foil was wound into 75 turns, to yield an inductance of approximately 9 mH. The resistance if such transition inductor 400 was approximately 1 Ω, yielding a transition impedance Z_tr of approximately 3 Ω. This implementation of the transition inductor 400 is given as an illustrative example only, and other designs, yielding different or similar values of L_tr, R_tr and/or Z_tr, could be used.
The above discussion has been made under the assumption that the capacitive contribution to the transition impedance can be neglected. However, switching device designs where the transition impedance Z_tr has a capacitive contribution could also be contemplated.
As mentioned above, the impedance of the transition path, Z_tr, is higher than the impedance Z_main of the main path. Oftentimes, the ratio of the transition impedance Z_tr to the main path impedance Z_main is in the order of a thousand, or in the order of ten thousand, or more. The main path impedance Z_main is mainly resistive - typically, any inductive or capacitive components can be neglected. The dominant contribution to the main path impedance Z_main typically originates from the resistance of the compact-break switch(es) with associated contacts.
As mentioned above, the impedance of the main path is typically low enough so that the main path can serve as the steady-state current path, which continuously carries the load current when no tap changing operation is performed.
However, in some implementations, when there is a desire to reduce the resistive losses of the switching device even further, a by-pass current path can be included in the switching device. The by-pass current path could e.g. be connected between a connection point 170, 170a, 170b and the external connection point 155 via a by-pass switch. Since a longer duration of the commutation of the load current to/from such by-pass path from/to the main path can be accepted, the by-pass switch can be designed in a number of different ways. Switching designs which provide slow and less controlled switching procedures could be used in the by-pass path, whereas in the main path, the switching procedure has to be fast and predictable, since the time should be minimized during which circulating currents occur between two simultaneously connected taps, at the same time as the switching sequence should be long enough to facilitate for the current to commutate at current zero.
A switching device 115/300 can further include a control unit for initiation and control of a tap changing event. In Fig. 10 , a control unit 1000 for controlling the main switch(es) 140 and transition switch(es) 145 of a switching device 115/300 is schematically illustrated. The control unit 1000 of Fig. 10 has an input interface 1005 configured to receive a trigger signal 1007 indicative of a desire to perform a tap changing operation, as well as an output interface 1010 configured to transmit output signals 1012 to the main and transition switches of the switching device. In one implementation, the output interface 1010 includes one output per switch in the switching device, so that a control input of each switch 140/145 of the switching device can be connected to an output of output interface 1010. The input interface 1005 can be configured to receive a trigger signal 1007 via a manual interface, or from an automatic control system. The control unit 1000 Fig. 10 further comprises a processor 1015 connected to a memory 1020. The processor 1015 is further connected to the input interface 1005 and the output interface 1010. The memory 1020 stores computer readable code means in the form of a computer program product 1025 which, when executed by the processor 1015, causes the control unit 1000 to send output signals 1012 which will cause the switching device 115/300 to perform a suitable switching sequence (cf. Figs. 5, 7 and 9). In particular, the memory stores computer readable code means operable to instruct the output interface 1010 to send, upon receipt of a trigger signal 1007 indicative of a desire to perform a tap changing operation, an output signal 1012 to the main switch 140 of the currently conducting main path, causing the main switch 140 to open, so that commutation of the load current from a currently conducting main path to a transition path including a transition inductor 400 will be performed. In a switching device design where the transition switch 145 does not form part of the main current path, in order to ensure that the transition switch 145 is closed, the memory can for example further store computer readable code means operable to instruct the transition switch 145 to close, or to check that the transition switch 145 is in a closed state. If desired, the input interface 1005 and/or the output interface 1010 could be implemented as I/O interfaces, so a two-way communication can occur between the control unit and the switching unit 115/300, and/or between the control unit and the trigger mechanism.
In an alternative design of the control unit 1000, the processor 1015 and the memory 1020 are replaced by suitable electronic circuitry.
The above discussed switching device is arranged to be used in a tap changer which is connected on the high voltage side of a transformer, also referred to as a European style tap changer. The design of tap changers has historically developed in two different directions in Europe and the US. In Europe, the tap changer is typically placed on the high-voltage side of the transformer, while in the US, the tap changer is typically placed on the low-voltage side of the transformer. Thus, when connecting the tap changer according to the European standard, currents flowing through the tap changer are comparatively smaller than in the US standard, while the voltage between adjacent taps is comparatively higher, and vice versa. Therefore, the requirements on European style tap changers and US style tap changers are very different.
The tap changer designs shown in Figs. 1, 3, 8a & 8b and Fig. 9 are given as examples only, and a switching device according to the invention can be used in any suitable tap changer design. For example, the switching devices disclosed above can be used in a tap changer 100 having any number of fixed contacts 135; the switching device 115, 300 can be of a different design, etc.
The above described switching device can be used in tap changers of any voltage rating, and in particular for tap changers rated for a system voltage of 5 kV or higher. It is particularly advantageous for frequently operating tap changers, where the heat loss in a traditional resistive transition impedance 150 would be high. Examples of applications in relation to which the inventive tap changer would be particularly advantageous include phase shifters, arc furnaces and HVDC systems.
One skilled in the art will appreciate that the technology presented herein is not limited to the embodiments disclosed in the accompanying drawings and the foregoing detailed description, which are presented for purposes of illustration only, but it can be implemented in a number of different ways, and it is defined by the following claims.

Claims

A switching device (115, 300) for an on-load tap changer (100) which includes at least two fixed contacts (135), the switching device providing:
a main current path comprising a main switch (140) which is series-connected in the main current path; and

a transition current path comprising a transition inductor (400) and a transition switch (145), the transition switch and the transition inductor being connected in series;

the impedance of the transition current path being higher than the impedance of the main current path, the switching device being characterized in that:
the inductive contribution to the impedance of the transition current path is at least 70 % at the frequency of a transmission system of which the switching device is arranged to form a part;

the impedance of the transition current path falls within the range of $\frac{1}{10} \cdot \frac{U_{step}}{I_{R}} to 10 \cdot \frac{U_{step}}{I_{R}},$
where U_step is the expected voltage between two adjacent taps of the on-load tap changer, and I_R is the rated load current of the tap changer;

the main switch is a compact-break switch; and

the main switch and the transition inductor are connected in parallel, so that upon opening of the main switch when the transition switch is in a closed state, a load current flowing through the main current path will be commutated to the transition current path.
The switching device of claim 1, wherein
the compact-break switch is a vacuum interrupter.
A switching device of claim 1 or 2, wherein
the switching device does not include any semiconductor switches.
The switching device of claim 1, wherein
the compact-break switch is a semiconducting switch.
The switching device of any one of claims 1-4, wherein
the impedance of the transition current path falls within the range of $\frac{1}{3} \cdot \frac{U_{step}}{I_{R}} to 3 \cdot \frac{U_{step}}{I_{R}},$
where U_step is the expected voltage between two adjacent taps of the on-load tap changer, and I_R is the rated load current of the tap changer.
The switching device of any one of the above claims wherein the switching device is a selector switch unit (300), the selector switch unit comprising:
a current collector (125) connected to an external output (155);

a main branch (160a) and a transition branch (160b), the main branch providing the main current path and the transition branch providing the transition current path; wherein

the main branch comprises the main switch (140) and a moveable contact (130a) connected in series, the moveable contact being arranged to make contact with a fixed contact of a tap changer;

the transition branch comprises a transition contact (145), a moveable contact (130b) and a transition inductor connected in series, the moveable contact being arranged to make contact with a fixed contact of a tap changer;

the main branch and the transition branch are connected at one end to the current collector;

the main branch and the transition branch are mechanically moveable so that the respective moveable contacts can, upon movement of the respective branch, be connected to or disconnected from a fixed contact.
The switching device of any one of claims 1-5, wherein the switching device is a diverter switch (115), the diverter switch comprising:
two connections points (170) for connection to a respective current collector of a tap selector; and

two branches (160) connected between the external contact and a respective one of the current collector connections; wherein each branch is capable of providing two different current paths for the load current.
The switching device of claim 7, wherein
each of the two branches comprises a transition contact (145), a main contact (140) and a transition inductor (400); wherein

for each branch, the main contact and the transition contact are connected in series, and the transition inductor of the branch is connected in parallel with the main contact, so that each branch can provide a main current path formed by the transition contact and the main contact, as well as a transition current path formed by the transition contact and the transition inductor.
The switching device of claim 7, wherein
said two branches are a main branch (160a) and a transition branch (160b); wherein

the transition branch comprises an inductor;

each of the main and transition branches comprises a series connection of a four-way contact and a compact-break switch, a first contact of the four-way contact being connected to the first current collector, a second contact of the four-way contact being connected to the second current collector, and a third and fourth contacts of the four-way contact being connected to the compact-break switch of the branch, so that the main branch can provide two alternate main current paths and the transition branch can provide two alternate transition current paths; and

the inductor of the transition branch is connected in series with the four-way contact and compact-break switch.
The switching device of any one of claims 1-9, the switching device further comprising a control unit (1000), the control unit being configured to:
perform, upon receipt of a signal indicative of a desire to perform a tap changing operation, commutation of the load current from a currently conducting main current path to a transition current path including a transition inductor (400) by opening, when the transition switch is in a closed state, the main switch of the currently conducting main current path.
The switching device of any of claims 1-10, wherein
the inductive contribution to the impedance of the transition current path is at least 90 % at the frequency of the transmission system of which the switching device is arranged to form a part.
A tap changer (100) comprising a switching device according to any one of the above claims.