WO2015185096A1

WO2015185096A1 - High voltage dc circuit breaker unit

Info

Publication number: WO2015185096A1
Application number: PCT/EP2014/061384
Authority: WO
Inventors: Markus Bujotzek; Angelos Garyfallos; Philipp Simka; Emmanouil Panousis; Nitesh Ranjan
Original assignee: Abb Technology Ag
Priority date: 2014-06-02
Filing date: 2014-06-02
Publication date: 2015-12-10

Abstract

An HVDC circuit breaker unit (1) with an interrupter branch (2) extending between a first node (3) and a second node (4) comprising a vacuum interrupter (5) connected to the first node (3), and a gas interrupter (6) connected to the second node (4) of the circuit breaker unit. The vacuum interrupter (5) is connected to the circuit breaker unit (1) at a third node (7) such that the gas interrupter (6) is electrically connected in series to the vacuum interrupter (5). A first movable contact member (11) of the vacuum interrupter (5) and a second movable contact member (12) of the gas interrupter (6) are operatable by at least one Thomson coil drive. An arrestor branch (8) comprising a first non-linear resistor (9)is connected to the first node (3) and the second node (4).

Description

High voltage DC circuit breaker unit

Technical Field

The invention relates to a circuit breaker for switching off high-voltage DC currents by employing a gas interrupter and a vacuum interrupter electrically con- nected in series to one another and an auxiliary circuit for causing a current zero required for interrupting the electric arc.

Background Art

Interruption of direct currents (DC) is one of the major challenges for the realization of multi-terminal DC networks (MTDC). Interrupting a DC current by causing a current zero, hereinafter abbreviated as CZ is known in the art since many years. Many approaches have been investigated in the past.

In WO2012/084693A1 , for example, a first approach is made by breaking an HVDC current path is performed by a breaker employing semiconductor elements. The drawback of such an approach resides in that the semiconductors will cause a steady loss in a closed state which steady loss lowers the overall electric efficiency. Besides for a rather complex overall structure of such a circuit breaker unit the controlling of all individual switching elements is very demanding.

A second approach resides in circuit breaker unit having a different type of a hybrid switching elements causing less steady losses and thus allow for achieving a better overall electric efficiency than the embodiments disclosed in WO2012/084693A1 . A representative of this approach is disclosed by "IEEE Transactions on Power Apparatus and Systems, Vol. PAS-103, No.3, 3 March 1984" where a vacuum interrupter and a gas interrupter are electrically con- nected in series to form an interrupter branch. A non-linear resistor and a voltage dividing capacitor are connected parallel thereto such that a voltage divider is formed. This hybrid breaker unit further comprises an LC circuit electrically connected in parallel to the interrupter branch. The LC circuit comprises a triggerable spark gap and a pre-charged capacitor and an inductance for injecting a counter current pulse into the interrupter branch. Since the vacuum interrupter and the gas interrupter employed in said approach are standard high voltage AC circuit breakers their movable contact members can be activated only comparatively slow such that said circuit breaker unit can be used for handling fault currents in a classic HVDC system such as a line commutated converter (LCC), for example. As a result, the circuit breaker unit of this second type is not applicable in a modern HVDC system comprising voltage source converters (VSC) requiring short switching times.

WO2013/014041 A1 is a further representative of the second approach. A disad- vantage of this solution resides in that the gas interrupter is stressed already at the very beginning of the interruption process, i.e. at CZ. Only gas interrupters of the top range and top quality can meet such requirements. Since such gas interrupters are more complicated than standard gas interrupters they are far more expensive. Furthermore a mere upscaling to higher voltages is not possible since the voltage over the vacuum interrupter is not limited. In addition, the second approach allows for upscaling the voltage over the vacuum interrupter only to some limited extent.

General disclosure of the invention

The object to be solved by the present invention resides in providing an economic circuit breaker unit that is able to protect a VSC-based HVDC system against damage from fault currents, e.g. short circuit currents.

Hereinafter the term HVDC is understood as a direct current with a voltage of at least 40kV, in particular more than 80kV, for example 160kV or 320kV. An HVDC system can be formed by a point-to-point HVDC link or network, a multi terminal HVDC system comprising at least three stations whereof one station is provided just for tapping a HVDC current, or a so-called HVDC grid comprising a plurality of power senders and receivers. The term fault current is understood and used herein interchangeably as the line current in case of a fault. Thus the present breaker can also be employed for interrupting a nominal current in the the nom- inal line of a HVDC system.

It was found that eventually it is the mechanics of the circuit breaker unit that dictates the abilities of an HVDC system. More particular the key element for solving the above-mentioned object resides in establishing a high switching speed for preventing an excessive and thus undesired and harmful rise in current in the DC system already at an early point in time after a fault current is detected. Owing to the high switching speed the mechanical interrupters can be kept as basic as possible. This allows for profiting from existing standard AC experience and interrupting devices as well as for economic solutions.

In a most basic embodiment the inventive HVDC circuit breaker unit comprises, an interruptible high voltage current path extending between a first node and a second node. Further it comprises an interrupter branch comprising a vacuum interrupter connected to the first node, and a gas interrupter connected to the second node of the circuit breaker unit, wherein the vacuum interrupter is connected to the circuit breaker unit at a third node such that the gas interrupter is electrically connected in series to the vacuum interrupter. The interrupter branch forms the nominal current path through the circuit breaker unit. A first movable contact member of the vacuum interrupter and a second movable contact member of the gas interrupter is operatable by at least one Thomson coil drive. The skilled person will recognize that the term Thomson coil drive' denotes the type of drive employing an electromagnetic repelling force and is not limited to a drive from a specific manufacturer of such drives. Anyway, employing a Thomson coil drive for putting the movable contact members into motion to open the interrupters is advantageous since such a drive concept allows for accelerating the movable contact members of the interrupters of the present inventive concept about at least ten times faster from trip signal compared to acceleration times from standard AC high voltage interrupters.

An arrestor branch is connected to the first node and the second node, wherein said arrestor branch comprises a first non-linear resistor and is electrically connected in parallel to the interrupter branch. The required high acceleration of the movable contact members is achieved in that the first movable contact member and the second movable contact member are actuated by the at least one Thomson coil drive such that a line current in the interrupter branch in case of a fault can be commutated within less than 20 milliseconds from a trip signal sent to at least one of the vacuum interrupter and the gas interrupter to the arrestor branch. The trip signal is understood as a signal from a control unit to at least one of the interrupters in the interrupter branch until the movable contact member is sufficiently moved to its opened position such that re-arcing is preventable in a reliable manner. In other words, the trip signal is a signal from a control unit to at least one of the two interrupters to perform the switching operation. The term 'within less than 20 milliseconds' is not to be misunderstood as the amount of time be- tween detection of the fault in the HVDC system and the full interruption of the fault current in the HVDC circuit breaker unit. It is not to be confused with the time until a fully open position of the interrupters is reached. The term 'actuating of a movable contact member' is understood as 'putting a movable contact member from stand still into motion' throughout this document.

The first non-linear resistor in the arrestor branch is provided for dissipating energy from the HVDC system and thus for protecting the interrupters from harmful overvoltage due to the fault current. Since the current can increase tremendously and quickly in case of a ground fault in an HVDC system the switching speed plays a major role. The faster the HVDC circuit breaker unit interrupts the line current in the interrupter branch in case of a fault the smaller the amount of fault current to be dissipated and the smaller the required first non-linear resistor can be. Thus the faster the interruption process the lower the amount of energy the interrupter branch has to bear. As an example, said amount of energy to dissipate may be in the order of MJ/ms. Having a higher acceleration of the movable contact members and thus a higher interruption speed in the interrupters eventually contributes to minimum ex- penses for the first non-linear resistor and thus for creating a more economical circuit breaker unit because one can save on varistors.

Striving for shortest possible arcing times is further advantageous since it reduces the amount of contact wear. This is not only advantageous for economic reason but also for electric reasons since it reduces the risk of an electric break- down of the circuit breaker unit because sensitive arcing chamber parts of the circuit breaker unit are less exposed to wear than in circuit breaker units having longer arcing times. Furthermore, it is in general beneficial for all components of the HVDC transmission system to have as short as possible fault stress.

If the electromagnetic repelling force of a single Thomson coil drive is too weak for putting both movable contact members into motion effectively, the basic embodiment of the HVDC circuit breaker unit may be adapted such that the first movable contact member of the vacuum interrupter is actuatable by a first Thomson coil drive whereas the second movable contact member of the gas interrupter is operatable by a second Thomson coil drive.

Dedicating separate drives to the different interrupters allows further to tailor and optimize the drives and optionally the drive gear to the different needs of the vacuum interrupter and the gas interrupter.

It is known that the maximum insulation distance for the rating is different in traditional vacuum interrupters and gas interrupters as it is larger in the latter than in the former. Hence it takes longer to move the second movable contact member of the gas interrupter to its fully open position than the first movable contact member of the vacuum interrupter. For reducing the arcing time of the vacuum interrupter and thus its contact wear it may be advantageous to actuate their movable contact members at slightly different moments in time by a displaced trip signal. Such an embodiment is achievable if the first Thomson coil drive and the second Thomson coil drive are connected to a control unit. Said control unit is set such that the first drive can be put into motion with a time delay after the second drive has been put into motion by the trip signal.

For low-cost solutions it might be advantageous if the first and the second Thomson coil drive share a current source. In such an embodiment the first and the second Thomson coil drive may share a power source such as a capacitor bank or a charged capacitor. In this case the movable contact members of the vacuum interrupter and the gas interrupter will start moving simultaneously.

An advantageous embodiment of the HVDC circuit breaker unit in terms of size versus breaking performance is achievable if the second movable contact mem- ber is actuated that fast that an insulation distance in the gas interrupter can be established in less than 10 milliseconds from the trip signal. That way said insulation distance is able to prevent re-arcing under voltage stress, i.e. to withstand the required voltage stresses.

A further reduction of the amount of energy from the HVDC system by way of dissipation in the arrestor branch is achievable if the second movable contact member is actuated that an insulation distance in the gas interrupter can be established in less than 7 milliseconds from the trip signal such that said insulation distance is able to prevent re-arcing under voltage stress.

It is known in the art of conventional circuit breaker drives that a sound balance between the inertia of moving parts for putting a movable contact member of an interrupter into motion and the maximum switching speed is desirable. When employing an electromagnetic repelling force (Thomson coil drive) for putting the movable contact members into motion the overall inertia of all moving parts required for putting a movable contact member of an interrupter into motion is even more decisive. This because the overall mass of said parts to be moved for putting the movable contact member into motion requires a powerful drive. Unfortunately the power of a conventional HVAC drive typically goes in line with the overall mass of said parts to be moved for putting the movable contact member into motion.

Contrary thereto are fast accelerations of the movable contact members and thus a satisfactory interruption performance achievable in an inventive embodiment where the first movable contact member and the second movable contact member are gearless connected to the at least one Thomson coil drive. It proved that stripping all conventional drive gear known from a circuit breaker is an important measure for achieving highest acceleration values of the movable contact members. One option to achieve such values resides in that the second movable contact member of the gas interrupter is rigidly connected to a piston of the second Thomson coil drive by a second drive rod along a linear switching axis. This allows for a most lean connection in between the Thomson coil drive and the dedicated movable contact member of the interrupter, for example.

A set of a vacuum interrupter and a gas interrupter electrically connected in se- ries has been chosen deliberately for employing their particular benefits. There are also drawbacks and limitations to both the vacuum interrupter and a gas interrupter, for example given voltage limits. A satisfactory protection of the interrupters is achievable in an embodiment where

• a first capacitance is present between the first node and the third node; · a second non-linear resistor is present between the first node and the third node such that it is electrically in parallel to the first capacitance; and

• where a second capacitance is present between the second node and the third node such that a voltage divider is formed.

The third node ensures that the overall voltage drop over the HVDC breaker unit between the first node and the second node does not need to be borne by either the vacuum interrupter or the gas interrupter alone and is achieved by dividing the overall voltage drop according to a predefined scheme established by the voltage divider. Splitting the voltage drop into shares dedicated to the vacuum interrupter and to the gas interrupter allows for establishing a purely mechanical solution that fulfills the requirements of modern HVDC systems.

Owing to the voltage divider the vacuum interrupter and the gas interrupter are electrically coupled by current. A capacitance ratio of the first capacitance to the second capacitance needs to be dimensioned such that in case of the line current in the interrupter branch in case of a fault the vacuum interrupter carries a ma- jority of a voltage drop in between the first node and the second node in an initial stage of the interruption process until the second non-linear resistor becomes essentially conductive. The gas interrupter takes over a majority of a voltage drop in between the first node and the second node in a final stage of the interruption process of the circuit breaker unit once the second non-linear resistor became essentially conductive. The second non-linear resistor protects the vacuum interrupter from dielectric failure. Expressed differently said second non-linear resistor ensures that the vacuum interrupter is electrically not overstressed. If the voltage exceeds a predefined threshold said second non-linear resistor becomes essentially conductive and protects the vacuum interrupter shortly before the transient recovery voltage (TRV) reaches the maximum voltage limit of the vacuum interrupter. The term TRV is related to AC systems rather than DC system it is nonetheless used hereinafter as the behavior of the time-varying waveforms is the same. Said TRV has also been referred to in the art as transient interruption voltage (TIV) in the context of DC systems in order to draw a distinction to AC systems. Hereinafter the applicant sticks to the term TRV also for the HVDC circuit breaker units and systems disclosed in this document.

Anyway, said second non-linear resistor ensures that the major portion of the voltage is initially dropping over the vacuum interrupter and not over the gas interrupter because the vacuum interrupter is more suitable for handling harsh di/dt and du/dt conditions at the very beginning of the interruption process than the gas interrupter because the vacuum interrupter can typically bear a higher di/dt than a gas interrupter for the same ratings.

The first capacitance represents the capacitance of both the vacuum interrupter itself as well as of any resistive elements including non-linear resistors and the like electrically connected in parallel to the vacuum interrupter. Thus the first ca- pacitance may comprise a dedicated capacitor, if required. The second capacitance represents the capacitance of both the gas interrupter itself as well as any additional dedicated capacitance of the capacitor connected in parallel to the gas interrupter. Thus the second capacitance may comprise a dedicated capacitor, if required.

A good compromise between functionality and costs is achievable if the second capacitance is at least ten times as large as the first capacitance.

In the first few microseconds after reaching CZ conditions a residual current is flowing between the first node and the second node in the interrupter branch. That residual current hampers the dielectric recovery of the interrupters. Thus the gas interrupter is assisted in interrupting the residual current by the electric dimensioning of the second capacitance.

The second capacitance is dimensioned to be smaller than 1 μΡ, preferably smaller than 100 nF as such values are regarded as a fair balance of economics and technical function to the interrupter branch. Such a second capacitance contributes to extinguishing any residual current in between the first node and the second node in an operating state of the circuit breaker unit after reaching current zero.

A reliable way of inserting a countercurrent into the interrupter branch is achievable in that an LC circuit comprising an auxiliary switch is connected to the first node and to the second node such that the LC circuit extends parallel to the interrupter branch. A triggerable spark gap (for example an ABB CapThor™), a semiconductor switch or a dedicated fast closing switch may be used for forming said auxiliary switch. The LC circuit further comprises a pre-chargeable third capacitance and a first inductance, wherein the LC circuit is dimensioned such that upon closing of the auxiliary switch an oscillation is caused such that a counter- current is injectable into the interrupter branch in order to create a current zero in the interrupter branch. Said third capacitance can be pre-charged by a suitable power supply or from the HVDC line.

For protecting the vacuum interrupter and the gas interrupter better against steep slopes of the transient recovery voltage it is possible to provide a fourth capaci- tance in between the first node and the second node such that it extends over the interrupter branch. Said fourth capacitance is uncharged in an initial stage of the interruption process in the circuit breaker unit. Since the charging of the fourth capacitance consumes some time the capacitance of the fourth capacitance it contributes to smoothen the slope of the initial TRV such that du/dt can be limited to a few kV/με after CZ. Having a smoother TRV allows for some extra microseconds that are available for extinguishing the residual current from the interruption branch and for dielectric recovery of the arcing zones.

The fourth capacitance should be as small as possible for economic reasons but large enough to ensure proper functionality of the breaker. It is advantageous if the third capacitance is at least 4 times as large as the fourth capacitance for meeting the steepness of the slope di/dt of the TRV in order to lower the du/dt slope of the TRV. It is further advantageous and a good balance between functionality and costs that the third capacitance is charged to the HVDC system voltage.

The above LC circuit has been described to deal with faults that may occur both at the side of the first node as well as at the side of the second node of the circuit breaker unit once connected to the HVDC system. Since the current to be broken is an HVDC current the HVDC circuit breaker unit must be able to cope with faults regardless the polarity of the fault to zero current. Hence in the above basic em- bodiment of the LC circuit the counter current injected into the interrupter branch is always that high that it must be able to cause a reliable CZ for any faults regardless its polarity. In an exemplary embodiment the counter current for a line current in case of a fault of 10 kA with a safety margin from about 20% regardless the polarity of the fault. In continuation of the earlier example said counter current leads to a total current of about 22kA.

If the polarity of the fault is detected at an early stage of the fault an advanced embodiment of a HVDC circuit breaker unit is achievable having two LC circuits dedicated to a single polarity of a fault current each. In such an embodiment the total current required can never exceed the fault current value, yet still achieving a reliable CZ in the interrupter branch.

This allows for choosing the interrupter branch elements to be dimensioned to handle lower total currents.

In any case for employing the advanced embodiment of a HVDC circuit breaker unit the polarity of the fault must be measured and the control unit must be able to trip the auxiliary switch of the designated LC circuit accordingly and sufficiently quick.

In other words, the advanced embodiment of a HVDC circuit breaker unit further comprises a further LC circuit comprising a further auxiliary switch being connected to the first node and to the second node such that the further LC circuit extends parallel to the interrupter branch. A triggerable spark gap (for example an ABB CapThor™), a semiconductor switch or a dedicated fast closing switch may be used for forming said further auxiliary switch. The further LC circuit further comprises a pre-chargeable fifth capacitance and a second inductance, wherein the further LC circuit is dimensioned such that upon closing of the further auxiliary switch an oscillation is caused such that a counter-current is injectable into the interrupter branch in order to create a current zero in the interrupter branch. Said fifth capacitance can be pre-charged by a suitable power supply analogous to the one for the third capacitor.

In short, using two LC branches each dedicated to a single polarity each contributes to a reduced stress of the interrupter branch compared to a solution with a single LC branch only. This supports a current upscale and a voltage upscale of the circuit breaker unit.

It is possible that the LC circuit and the further LC circuit share their inductance, if required. In such an embodiment the first inductance and the fifth inductance may be formed by the very same electric element. Thus the first inductance can be used as the fifth inductance.

It is advisable to connect a disconnector to a HVDC system side at its one end and to the first node or to the second node on its other end or at both nodes of the HVDC circuit breaker unit for establishing a galvanic separation. Such a galvanic separation is advantageous for preventing the arrestors of the circuit breaker unit from overheating as well as for carrying out maintenance duties. The current to be interrupted by the disconnector is in the range of some Amperes only and the operating speed of the disconnector is not time critical. Depending on the embodiment the disconnector may or may not be not be part of the circuit breaker unit.

Brief description of the drawings

The description makes reference to the annexed drawings, which are schemati- cally showing in

Fig. 1 an overall circuit of a first embodiment of the HVDC circuit breaker unit;

Fig. 2 an embodiment illustrating the main elements of the interrupter branch;

Fig. 3 a current-time and a voltage-time diagram of a line current and its interruption in case of a fault in the same time window around current zero; Fig. 4 a current-time oscillogramme of a rising line current in an interrupter branch of the HVDC circuit breaker according to fig. 1 and a subsequent injection of a counter current into the interruption branch such that a current zero is caused;

Fig. 5 a voltage-time diagram showing the impact of the size of the fourth ca- pacitance extending over the HVDC circuit breaker unit; and

Fig. 6 an overall circuit of a second embodiment of the HVDC circuit breaker unit.

In the drawings identical parts, currents and voltages are given identical reference characters.

Ways of working the invention:

A first embodiment of HVDC circuit breaker unit 1 is shown in fig. 1 along with fig. 2 whereas the latter figure illustrates the main elements of an interruptible interrupter branch 2 forming the nominal current path through the circuit breaker unit 1 . As to the currents indicated in fig. 1 one has to be aware that they will not run all in the very same moment in time but they are identified in fig.1 for the sake of a better overall understanding of the different branches of the HVDC circuit breaker unit 1 and their dedicated purpose in the description.

Said interrupter branch 2 extends between a first node 3 and a second node 4. The interrupter branch 2 comprises a vacuum interrupter 5 connected to the first node 3, and a gas interrupter 6 connected to the second node 4 of the circuit breaker unit 1 , wherein the vacuum interrupter 5 is connected to the circuit breaker unit 1 at a third node 7 such that the gas interrupter 6 is electrically connected in series to the vacuum interrupter 5. An arrestor branch 8 is connected to the first node 3 and the second node 4, wherein said arrestor branch 8 comprises a first non-linear resistor 9 and is electrically connected in parallel to the interrupter branch 2.

A first movable contact member 1 1 of the vacuum interrupter 5 and a second movable contact member 12 of the gas interrupter 6 are actuatable by a first Thomson coil drive 13 and a second Thomson coil drive 14, respectively, such that a fault current in the HVDC system can be commutated within a few milliseconds to the arrestor branch 8 for dissipation. The Thomson coil drives 13, 14 are able to accelerate all movable parts of the circuit breaker unit 1 to maximum speed from stand still within less than 7ms after a trip signal 15 is sent to at least one of the vacuum interrupter 5 and the gas interrupter 6. Having two Thomson coil drives 13 and 14 dedicated to each of the interrupters 5 and 6 allows for splitting and thereby lowering the inertia to be mastered for each Thomson coil drive compared to a solution where a single Thomson coil drive has to actuate both the first movable contact member as well as the second movable contact member.

Contrary to known HVAC circuit breakers the present HVDC circuit breaker unit 1 the second movable contact member of the gas interrupter 6 is rigidly connected to a piston of the second Thomson coil drive 14 by a second drive rod along a linear switching axis 16. The term rigidly connected is to be understood such that no flexible linkage such any gearing mechanism is required.

Further, the circuit shown in fig. 1 has a first capacitance 17 arranged between the first node 3 and the third node 7 and a second capacitance 18 arranged between the second node 4 and the third node 7. Moreover a second non-linear resistor 19 is connected to the first node 3 and the third node 7 to be electrically parallel to the vacuum interrupter 5 and the first capacitance 17 such that a voltage divider is formed. The second capacitance 18 is at least ten times as large as the first capacitance 17.

An LC circuit 20 comprising an auxiliary switch 21 is connected to the first node 3 and to the second node 4 such that it extends parallel to the interrupter branch 2. The auxiliary switch 21 is a triggerable spark gap. The LC circuit 20 also comprises a pre-chargeable third capacitance 22 and a first inductance 23 such that a resonant circuit is formed. The pre-charging of the third capacitance 22 is done by means of a power supply (not shown in fig.1 ) in an operating state of the circuit breaker unit 1 . The LC circuit 20 is dimensioned such that upon closing of the auxiliary switch 21 an oscillation is caused such that a counter-current 24 is injectable into the interrupter branch 2 for creating a current zero in the interrupter branch 2.

The second capacitance 18 is dimensioned to be below 1 μΡ and is fifteen times as large as the first capacitance 17.

A fourth capacitance 25 is arranged in between the first node 3 and the second node 4 such that it extends parallel to the interrupter branch 2. The fourth capacitance 20 is uncharged in an initial stage of the interruption process in the circuit breaker unit 1 . The third capacitance 22 is preferably at least four times as large as the fourth capacitance 25.

A disconnector 26 is connected to a HVDC system side to the second node 4 for establishing a galvanic separation from the circuit breaker unit 1 to the HVDC system at the side of the second node 4.

Fig. 2 shows an embodiment illustrating more details of the interrupter branch 2 of the circuit breaker unit of fig .1 and represents a snapshot of the interruption branch in an initial state of the interruption process shortly after time U explained later on. The display of electric arcs in between the contacts of the vacuum interrupter 5 and the gas interrupter 6 have been omittet as it would not contribute to an improved overall understandability of fig. 2. The interruptible interrupter branch 2 comprises the vacuum interrupter 5 that is connected via the third node 7 in series to the gas interrupter 6. The vacuum interrupter 5 has the first movable contact member 1 1 for establishing an electrical connection in between the first node 3 and the third node 7 in its closed position. Said first movable contact member 1 1 is activatable such that it moves from that closed position to an open position and vice versa by way of the first Thomson coil drive 13.

The gas interrupter 6 has the second movable contact member 12 for establishing an electrical connection in between the third node 7 and the second node 4 in its closed position. Said second movable contact member 12 is activatable such that it moves from that closed position to an open position and vice versa by way of the second Thomson coil drive 14.

The first Thomson coil drive 13 and the second Thomson coil drive 14 can be activated such that their dedicated movable contacts members 1 1 and 12, respectively, get in motion by the trip signal 15 issued by the control unit 27. Said control unit 27 is wired to a first current detector 28 at the first node 3 and to a second current detector 29 at the second node 4 of the HVDC system for detecting any fault current. The control unit 27 is able to distinguish allowable deviations from nominal HVDC currents and fault currents.

Alternatively it is also possible to detect any fault current by one single current detector, too.

Fig. 2 is simplyfied in so far as the trip signals 15 are issued to the vacuum interrupter 5 and the gas interrupter 6 in their closed state and not in their open state as shown in fig. 2.

As briefly touched earlier on a residual current 39 is present in the interrupter branch 2 as there are electric arcs between the contacts of the vacuum interrupter 5 and the gas interrupter 6 in the initial interruption phase.

In an alternative embodiment (not shown) to the one shown in fig. 2 the first movable contact member 1 1 and the second movable contact member 12 may be actuatable by a common Thomson coil drive.

Function of such an embodiment:

After detection of a fault current in the nominal line 30 of the HVDC system the control unit 27 trips the vacuum interrupter 5 and the gas interrupter 6 to open (see fig. 2). The detection of the fault current takes place by conventional detection means like suitable current detectors 28, 29. The detection of the fault cur- rent takes place at time to (see figures 3 and 4) where a rising line current 36 in the interrupter branch 2 departing from a nominal system current 35 is present. The term nominal system current is understood as the nominal current of an HVDC system where no fault case is present such as before time to. Both the gas interrupter 6 and the vacuum interrupter 5 are opened in the initial phase by the trip signal 15 at time ti and t.2 respectively. The time span between ti and t.2 is in a range between 0 ms and about 3 ms.

In an initial phase of the breaking process the second movable contact member 12 is actuatable that fast that an insulation distance in the gas interrupter can be established in less than 7 milliseconds from the trip signal 15. Owing to the high voltage at the line current 36 in case of fault an electric arc is formed in the arcing zones of both the vacuum interrupter 5 and the gas interrupter 6 in between the movable contacts and their counter-contacts each such that a current keeps flowing in the interrupter branch 2. For explanatory purpose let us assume that said line current 36 entering the interrupter branch 2 from the nominal line 30 of the HVDC system rises from the nominal system current 35 constantly to about 10 kA such as shown in the lower diagram of fig. 3 as well as in fig. 4. Since the time window between to and t.3 or U extends over several milliseconds the slope of the waveform of the line current 36 is displayed as increasing in the lower diagram of fig.3. Different thereto the time window between U and te extends over several ten to several hundred microseconds only such that the waveform of the current 22 is displayed as being fairly constant. Again different, the time window between te and tg extends over several milliseconds again such that the waveform of the line current 36 is displayed as decreasing in the lower diagram of fig.3.

As shown in the lower diagram of fig. 3 the line current 36 in the interrupter branch 2 rises quickly, i.e. in few milliseconds up to a few kilo amperes, for example to the above-mentioned 10 kA in case of the above mentioned fault current.

As shown in fig. 4 the line current 36 in the interrupter branch 2 rises until time t.3 at which the counter-current 24 is released by closing the auxiliary switch 21 of the LC-circuit 20. Shortly thereafter the counter-current 24 injected into the interrupter branch 2 causes a CZ at time t₄.

In the embodiment according to fig. 1 the LC circuit 20 must be able to deal with faults that may occur both at the side of the first node 3 as well as at the side of the second node 4 of the circuit breaker unit 1 in the HVDC system. Since the line current to be broken is an HVDC current the HVDC circuit breaker unit must be able to cope with faults regardless the polarity of the fault. Hence in the above basic embodiment of the LC circuit 20 the counter current 24 injected into the interrupter branch 2 is dimensioned such that it causes a reliable CZ for any faults regardless the polarity (see the total current peak 38 including the counter current 24 in fig. 4).

After CZ is achieved at U there are time-varying conditions in the circuit breaker unit 1 . Hence the first capacitance 17, the second capacitance 18 and the fourth capacitance 25 become players. It will be evident for the skilled reader that there will be a time-varying current 40 in the branch with the first capacitance 17, a time-varying current 41 in the branch with the second capacitance 18 as well as a time-varying current 42 the branch with the fourth capacitance 25. Once the limiting voltage level of the second non-linear resistor 19 is reached a time-varying current 43 will flow through the branch with the second non-linear resistor 19, too.

The capacitance ratio of the first capacitance 17 to the second capacitance 18 is dimensioned such that the initial voltage stress from the interruption at CZ is borne by the vacuum interrupter 5 and not by the gas interrupter 6. This is advantageous since vacuum interrupters are superior to gas interrupters in the initial phase of the interruption process because they can establish a dielectric insulation quicker. In the subsequent final stage of the interruption process the high voltage insulation is mainly done by the gas interrupter 6, since a gas interrupter is superior for that purpose as it has typically better insulation properties for higher voltages.

As shown in the voltage-time diagram of fig. 5 there would be a harsh voltage step 45 between the first node 3 and the second node 4 due to the injection of the counter current 24 if no fourth capacitor 20 were present. The harsh voltage step 45 of the voltage is displayed by a vertical solid line in fig. 5 is a consequence of the high di/dt of the counter current 24 measuring a few hundred Amperes per microsecond and would be too harsh for the vacuum interrupter 5 and the gas interrupter 6. The initial TRV is governed by the voltage step 45 (without the fourth capacitor 25) which voltage step 45 is equal to the product of the first inductance 23 multiplied by di/dt at CZ. The value of the fourth capacitor 25 has been chosen such that

• more time can be gained for a cooling of the insulation gas in the gas interrupter 6 such that a better dielectric recovery is achievable. A suffi- cient dielectric insulation is required for reliably preventing a re-arcing in the gas interrupter 6 in order to be able to take over the major portion of the voltage drop between the first node 3 and the second node 4 in the interrupter branch 2 after the limiting voltage 37 of the vacuum interrupter 5 ensured by the second non-linear resistor 19 is exceeded; and

· more time is available for extinguishing the residual current 39 in the interrupter branch 2.

As shown in the upper diagram of fig . 3 there is no significant voltage drop between the first node 3 and the second node 4 until time U where CZ creation and interruption take place.

As shown by the solid line (with no line markers) tagged with reference numeral 50 in the upper diagram of fig. 3 as well as in fig. 5 for the time shortly after CZ the TRV of the voltage across the circuit breaker unit 1 (between the first node 3 and the second node 4) drops about the voltage step 45 after CZ and rises in the subsequent recovery phase above the system voltage 46 until the limiting volt- age level of the first non-linear resistor 9 is reached and where the first non-linear resistor 9 becomes electrically conductive such that a current 47 flows in the arrestor branch 8. The first non-linear resistor 9 protects the circuit breaker unit 1 from failure of excessive line current 36 in case of fault until the system voltage 46 is reached again in that it dissipate excessive fault current energy.

The take-over of the main portion of the voltage drop over the interrupter branch

2 by the gas interrupter 6 from the vacuum interrupter 5 is explained with refer- ence to the upper diagram of fig. 3. After U the voltage across the circuit breaker unit 1 (i.e. between the first node 3 and the second node 4) is indicated by a solid line 50 (with no line markers). Said voltage 50 drops first sharply due to the injection of the counter current 24 into the interrupter branch 2 and starts oscillating back towards the system voltage 46. Since the gas interrupter 6 has not taken over the main portion of the voltage drop across the circuit breaker unit 1 from the vacuum interrupter 5 until time te the waveform of the voltage 51 across the vacuum interrupter 5 (indicated by a dashed line 51 ) follows the one of the voltage 50 across the circuit breaker unit 1 after CZ provided that the limiting voltage levels 37 of the vacuum interrupter 5 set by the second-non-linear resistor 19 allows it.

At time ts, i.e. shortly after CZ at U the gas interrupter 6 starts taking over a portion of the voltage drop over the interrupter branch 2. The time span between U and ts is employed for dielectric recovery of the gas interrupter 6. The residual current 39 is present in the interrupter branch 2 from time t₄ until time ts at the latest. The waveform of the voltage 52 across the gas interrupter 6 is indicated by a dotted line 52 in the upper diagram of fig. 3. After te the voltage 51 across the vacuum interrupter 6 stagnates again because the limiting voltage levels 37 of the vacuum interrupter 5 is reached again. From time te on the gas interrupter 6 takes over the main portion of the voltage drop across the circuit breaker unit 1 from the vacuum interrupter 5.

At time t₇, the voltage across the circuit breaker unit 1 has reached the limiting voltage level 53 of the first non-linear resistor 9 where the latter starts dissipating the energy from the line current in case of the fault (i.e. the fault current) until the voltage 50 across the circuit breaker unit 1 is allowed to reach the system voltage 46 at about time tg again.

The stepped waveform of the voltage 52 across the gas interrupter 6 is caused by the dimensioning of the non-linear voltage divider composed by the first capacitance 17, second capacitance 18 and the second non-linear resistor 19.

The second non-linear resistor 19 protects the vacuum interrupter 5 against die- lectric failure in that it limits the voltage over the vacuum interrupter 5 to a voltage limit 37. As soon as the non-linear resistor 19 becomes conductive a current 43 flows through the branch with said second non-linear resistor 19, too. As can be seen in fig. 3 the voltage limit 37 of the non-linear resistor 19 is not depending on the polarity of the line current 36.

The voltage-time diagram of fig. 3 reveals further that the first few microseconds after CZ are decisive for the success of the whole HVDC breaking process. Any residual current 39 needs to be extinguished from the interrupter branch 2 in between time U and time ts as it hampers the dielectric recovery of the gas interrupter 6. For that purpose the second capacitance 18 is dimensioned to be preferably smaller than 100 nF because such a value contributes best to extinguishing any residual current 39 from the interrupter branch 2.

Fig. 5 is a voltage-time diagram showing the impact of a different sized fourth capacitance 25 extending over the HVDC circuit breaker unit on the TRV waveform after CZ. As mentioned in the context of fig. 3 there is no voltage drop across the circuit breaker unit 1 until time U where CZ is reached. The voltage step 45 caused by the counter current injection as well as a portion of the linear slope of a TRV (lacking a fourth capacitance) towards the system voltage are indicated by a solid line in fig. 5. The voltage waveform of the actual transient recovery voltage and of the subsequent recovery voltage of a circuitry comprising a fourth capacitance 25 have been displayed by tightly dotted graphs, each. A first voltage waveform 54 (indicated by triangles) shows the TRV and recovery voltage of the voltage 50 across the circuit breaker unit 1 after CZ with a capacitance value C that may be 0.1 μΡ, for example. A second voltage waveform 55 (indicated by bullets) shows the TRV and recovery voltage after CZ with a capacitance value of ten times the value of C. A third voltage waveform 56 (indicated by diamonds) shows the TRV and recovery voltage after CZ with a capacitance value of a hundred times the value of C.

Fig. 5 shows that the larger a capacitance value for the fourth capacitance 25 the smoother the TRV slope becomes. Unfortunately capacitors with capacitance values of a hundred times the value of C are quite costly. Thus the choice of a capacitance value for the fourth capacitance is not only a technical one as ex- plained in the context of fig. 3 but also an economic one and has an impact on the design space required (overall dimensions).

A second embodiment of a HVDC circuit breaker unit 100 is shown in fig. 6. The circuit breaker unit 100 differs to the circuit breaker unit 1 in that it comprises a further LC circuit 60 in addition to the LC circuit 20. The conceptual main differ- ence to the first embodiment shown in fig. 1 resides in that the polarity of the line current in case of a fault is detected at an early stage of the fault and in that a separate LC circuit is dedicated to each polarity of a fault current. Analogously to the LC circuit 20 the further LC circuit 60 comprises a further auxiliary switch 61 that is connected to the first node 3 and to the second node 4 such that the further LC circuit 60 extends parallel to the interrupter branch 2. The further auxiliary switch 61 forms a further triggerable spark gap. The further LC circuit 60 comprises a pre-chargeable fifth capacitance 62 and makes use of the first inductance 23. A polarity of the pre-chargeable fifth capacitance 62 is opposite to the polarity of the third capacitance 22 when charged. The further LC circuit 60 is dimensioned such that upon closing of the further auxiliary switch 61 an oscillation is caused such that a further counter-current 63 is injectable into the inter- rupter branch 2 in order to cause a current zero in the interrupter branch 2.

List of reference numerals:

1 , 100 Circuit breaker unit

2 Interrupter branch

3 First node

4 Second node

5 Vacuum interrupter

6 Gas interrupter

7 Third node

8 Arrestor branch

9 First non-linear resistor (varistor)

1 1 First movable contact member

12 second movable contact member

13 First Thomson coil drive

14 second Thomson coil drive

15 Trip signal

16 Linear switching axis

17 First capacitance

18 Second capacitance

19 Second non-linear resistor (varistor)

20 LC circuit

21 Auxiliary switch

22 Third capacitance

23 First inductance

24 Counter current

25 Fourth capacitance

26 Disconnector

27 Control unit First current detector/sensor

Second current detector/sensor

Nominal line of HVDC system

Nominal system current

Line current in the interrupter branch

limiting voltage level of the vacuum interrupter

Total current peak including the counter current

Residual current

Current in the branch with the first capacitance after CZ

Current in the branch with second capacitance after CZ

Current in the branch with fourth capacitance after CZ

Current in the branch with second non-linear resistor after CZ

Voltage step

System voltage

Current in the arrestor branch

Voltage across circuit breaker unit after CZ

Voltage across vacuum interrupter after CZ

Voltage across gas interrupter after CZ

limiting voltage level of the first non-linear resistor

First voltage waveform

Second voltage waveform

Third voltage waveform

Further LC circuit

Further auxiliary switch

Fifth capacitance

Further counter current

Claims

Patent Claims

1 . An HVDC circuit breaker unit (1 , 100) comprising

an interruptible interrupter branch (2) extending between a first node (3) and a second node (4),

the interrupter branch (2) comprising a vacuum interrupter (5) connected to the first node (3), and a gas interrupter (6) connected to the second node (4) of the circuit breaker unit, wherein the vacuum interrupter (5) is connected to the gas interrupter (6) at a third node (7) such that the gas interrupter (6) is electrically connected in series to the vacuum interrupter (5), wherein a first movable contact member (1 1 ) of the vacuum interrupter (5) and a second movable contact member (12) of the gas interrupter (6) are operatable by at least one drive (13, 14),

and wherein an arrestor branch (8) comprising a first non-linear resistor (9) is connected to the first node (3) and the second node (4), wherein said arrestor branch (8) is electrically connected in parallel to the interrupter branch (2),

characterized in that the first movable contact member (1 1 ) and the second movable contact member (12) can be actuated by at least one Thomson coil drive (13, 14) such that a line current in case of a fault can be commu- tated within less than 20 milliseconds from a trip signal (15) to at least one of the vacuum interrupter (5) and the gas interrupter (6) from the interrupter branch (2) to the arrestor branch (8).

2. Circuit breaker unit, according to claim 1 , characterized in that the first movable contact member (1 1 ) of the vacuum interrupter (5) can be actuated by a first Thomson coil drive (13),

and in that the second movable contact member (12) of the gas interrupter (6) is operatable by a second Thomson coil drive (14).

3. Circuit breaker unit, according to claim 2, characterized by a control unit (27) to which the first Thomson coil drive (13) and the second Thomson coil drive (14) are connected, wherein said control unit (27) is set such that the first movable contact member (1 1 ) can be actuated with a time delay after the second movable contact member (12) has been actuated by the trip signal (15).

4. Circuit breaker unit, according to claim 2, characterized in that the second Thomson coil drive (14) and the first Thomson coil drive (13) are powered by the same current source.

Circuit breaker unit, according to any one of claims 2 to 4, characterized in that the second movable contact member (55) can be actuated that fast that an insulation distance in the gas interrupter (6) can be established in less than 10 milliseconds from the trip signal (14).

Circuit breaker unit, according to any one of claims 2 to 4, characterized in that the second movable contact member (12) can be actuated that fast that an insulation distance in the gas interrupter (6) can be established in less than 7 milliseconds from the trip signal (15).

Circuit breaker unit, according to claim 1 , characterized in that a second non-linear resistor (19) is connected to the first node (3) and the third node (7) for protecting the vacuum interrupter (5) against dielectric failure, and in that a first capacitance (17) is present between the first node (3) and the third node (7),

and in that a second capacitance (18) is present between the second node (4) and the third node (7) such that a voltage divider is formed,

wherein a capacitance ratio of the first capacitance (17) to the second capacitance (18) is dimensioned such that in case of the line current in case of a fault the vacuum interrupter (5) carries a majority of a voltage drop in between the first node (3) and the second node (4) in an initial stage of the interruption process until the second non-linear resistor (19) becomes conductive,

and wherein the gas interrupter (6) takes over a majority of a voltage drop in between the first node (3) and the second node (4) in a final stage of the interruption process of the circuit breaker unit (1 ) once the second nonlinear resistor (19) became conductive.

Circuit breaker unit, according to claim 7, characterized in that the second capacitance (18) is at least 10 times as large as the first capacitance (17).

Circuit breaker unit, according to claim 8, characterized in that the second capacitance (18) is dimensioned to be below 1 μΡ, preferably below 100nF.

Circuit breaker unit, according to any one of the preceding claims, characterized in that an LC circuit (20) comprising an auxiliary switch (21 ) is connected to the first node (3) and to the second node (4) such that the LC circuit (20) extends parallel to the interrupter branch (2),

wherein the LC circuit (20) comprises a pre-chargeable third capacitance (22) and a first inductance (23),

wherein the LC circuit (20) is dimensioned such that upon closing of the auxiliary switch (21 ) an oscillation is caused such that a counter-current (24) is injectable into the interrupter branch (2) in order to create a current zero in the interrupter branch (2).

1 1 . Circuit breaker unit, according to any one of the preceding claims, charac- terized in that a fourth capacitance (25) is arranged in between the first node (3) and the second node (4) such that it extends over the interrupter branch (2),

wherein the fourth capacitance (25) is uncharged in an initial stage of the interruption process in the circuit breaker unit (1 ).

12. Circuit breaker unit, according to claim 1 1 referring to claim 10, characterized in that the third capacitance (22) is at least 4 times as large as the fourth capacitance (25).

13. Circuit breaker unit, according to claim 1 1 , characterized in that a further LC circuit (60) comprising a further auxiliary switch (61 ) is connected to the first node (3) and to the second node (4) such that the further LC circuit (60) extends parallel to the interrupter branch (2),

wherein the further LC circuit (60) comprises a pre-chargeable fifth capacitance (62) and a second inductance,

wherein a polarity of the fifth capacitance (62) of the further LC circuit (60) has a direction opposite to a polarity of the third capacitance (22) of the LC circuit (20) when charged,

wherein the further LC circuit (60) is dimensioned such that upon closing of the further auxiliary switch (61 ) an oscillation is caused such that a further counter-current (63) is injectable into the interrupter branch (2) in order to create a current zero in the interrupter branch (2).

14. Circuit breaker unit, according to claim 13, characterized in that the first inductance (23) can be used as the fifth inductance.

15. Circuit breaker unit, according to any one of the preceding claims, characterized in that a HVDC system is connected to the first node or to the sec- ond node (4) via a disconnector (26).