Patent Description:
There is considerable ongoing interest in HVDC electrical power transmission and/or distribution systems (hereinafter referred to as simply HVDC systems). Presently, most such systems only allow for point-to-point connections. Systems with more complex networks of connections have many potential advantages but require suitable HVDC circuit breakers. Some of the desired properties of such circuit breakers are:.

There has been considerable research in relation to HVDC circuit breakers, but only a relatively small number of practical devices have been developed. See, for example, <NPL>.

For example, <CIT> describes a hybrid (solid-state and mechanical) HVDC circuit breaker. It includes a series of power semiconductor switches (referred to as the main breaker) and, in parallel with the main breaker, a mechanical switch in series with at least one power semiconductor switch (referred to as the auxiliary breaker). In use, the auxiliary breaker is first opened, thereby commutating a fault current to the main breaker, then the mechanical switch is opened, and then the main breaker is opened, thereby commutating the fault current to a non-linear resistor. The mechanical switch is a disconnector, which has a high operating speed but is unable to sustain arcing and so cannot break any current and can only open at zero current. The mechanical switch bypasses the main breaker during closed-state operation. Hence the circuit breaker has a high operating speed (~<NUM>-<NUM>) and low closed-state losses. However, the circuit breaker has a high cost and a high peak fault current (~<NUM>-10kA in a 320kV system). Even though it only lasts for a very short time, this current may be transferred to, any may adversely affect, other parts of the HVDC system. A high peak fault current also leads to high fault energy dissipation (~<NUM>-30MJ).

<CIT> describes a hybrid HVDC circuit breaker which has a similar topology to that described above, except that it uses power thyristors rather than power semiconductor switches. Hence the circuit breaker has higher current capabilities and may have a lower cost than the circuit breaker described above, but still essentially has all of the above described shortcomings.

<NPL> describes a fully-mechanical HVDC circuit breaker. It operates by injecting an oscillating current from a pre-charged capacitor into a DC fault current, thereby creating zero crossings. The circuit breaker has a mechanical switch corresponding to a standard AC vacuum circuit breaker with a faster driving mechanism. This switch has contacts that are capable of sustaining an arc and an arcextinguishing chamber, and this switch can break current. In all similar DC CB topologies, arcing normally lasts for <NUM>-<NUM> until resonant current is injected to create zero crossing. As the circuit breaker has no semiconductor devices, it is expected to have a moderate cost. Furthermore, the closed-state losses are negligible as the load current only passes through a mechanical switch. However, compared to the above described hybrid circuit breakers, it has a much longer operating time (~<NUM>) and higher fault energy dissipation (~<NUM>-60MJ). Similar operating principle (arcing and a resonant circuit to interrupt the arc) is observed in W2015/<NUM>, <CIT>, <CIT>, <CIT>, <CIT> and others.

<CIT>, <CIT> and <CIT> describe a DC CB that employs two switches in series (a mechanical interrupter and a semiconductor) and a parallel capacitor which limits voltage rise and reduces opening time. However this DCCB requires a mechanical interrupter switch to break the current. Also, an essential semiconductor switch is required which may have high ratings (ratings not specified) and may increase costs substantially.

<CIT> discloses a circuit breaker having first and second switches in series with a current path of a high voltage DC transmission system,to be interrupted. A current rise limiter is included in series to limit the rate of current rise. A capacitor and inductance are provided in series with the first switch, which has an arc gap, so that an arc is formed in the arc gap when the first switch opens. As the current increases, oscillations occur in the current through the arc gap until such time that a zero crossing is generated so that the second switch can be activated.

<CIT> discloses a fast acting switching device to suppress an occurrence of an arc.

According to the present invention there is provided a circuit breaker configured for interrupting a direct current in a current path of a high voltage DC transmission system, the circuit breaker comprising:- a first switch having a disconnecting contact; a second switch; wherein the first and second switches are operatively connected in series in the current path; a capacitor operatively connected in parallel with the first switch;.

Thus, the circuit breaker has a new circuit breaker topology and seeks to achieve some or all of the above described desired properties of HVDC circuit breakers.

The circuit breaker may be configured such that the switch opening control operates the second switch to interrupt the current at or near a first zero crossing of the alternating current waveform.

The circuit breaker may further comprise an inductor in series with the first and second switches.

The first and second switches may be mechanical switches.

The first switch may be a disconnector having non-zero contact velocity at the contact separation instant.

The second switch may be an AC circuit breaker.

The circuit breaker may further comprise a varistor operatively connected in parallel with the capacitor to limit the voltage across the capacitor.

In one embodiment, the circuit breaker may further comprise one or more further modules in series with the first switch and the or each module comprising:- a further first switch, a further capacitor operatively connected in parallel with the further first switch, and a further varistor operatively connected in parallel with the further capacitor to limit the voltage across the further capacitor; wherein the switch opening control operates to start to open each first switch while the second switch is closed.

A third switch may be provided in series with the first switch and both in parallel with the capacitor; wherein the switch opening control operates to open the third switch around the instant of contact separation of the disconnecting contact of the first switch to aid commutation from the first switch to the capacitor.

The third switch may be a semiconductor-based switch.

The circuit breaker may further comprise a transformer with primary winding connected in series with the capacitor, the secondary windings connected to an auxiliary pulse generator configured to generate a pulse around the instant of contact separation of the first switch to aid commutation from the first switch to the capacitor.

The circuit breaker may further comprise a controller configured to control operation of the circuit breaker opening.

The present invention encompasses a system comprising a current path of a high voltage DC transmission system; and a circuit breaker as herein above described.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-.

Referring to <FIG>, a first example of a circuit breaker <NUM> which is outside the scope of the claimed invention will now be described to provide information relevant for understanding the invention. The circuit breaker <NUM> includes two switches <NUM>, <NUM> (hereinafter referred to as first and second switches), an inductor <NUM>, and a capacitor <NUM>. The first and second switches <NUM>, <NUM> and the inductor <NUM> are connected in series in a current path <NUM>. The capacitor <NUM> is connected in parallel with the first switch <NUM>. The current path <NUM> is between two terminals 5a, 5b (hereinafter referred to as first and second terminals). In typical use in an HVDC system, the current path <NUM> forms part of a larger current path (not shown) for a DC current I (hereinafter referred to as the circuit current). In this example, the circuit current I flows via the second switch <NUM>, the inductor <NUM>, and then the first switch <NUM>. However, the first switch <NUM> (with the parallel capacitor <NUM>), the second switch <NUM>, and the inductor <NUM> may be connected in any order.

The first switch <NUM> is a mechanical switch. In this example, the first switch <NUM> is a disconnector and, more specifically, a fast disconnector. Fast disconnectors are generally preferred to other types of switches as they have a relatively high operating speed and a relatively low cost. However, while being fast (e.g. an opening time of ~<NUM>) enables the size, weight and costs of other parts of the circuit breaker <NUM> to be reduced, this is not essential and the first switch <NUM> may be a simple disconnector. It is an important property of this invention that arcing is practically non-existent, therefore a disconnector will suffice, which sets this invention apart from most other mechanical DC CBs which have an arcing period. Arcing may occur only as a consequence of parasitic inductances, and in such case it is for an extremely short period which may last several micro-seconds and is practically unmeasurable.

Referring to <FIG>, a fast disconnector <NUM> which may be used as the first switch <NUM> will now be described. The fast disconnector <NUM> includes two Thomson coil actuators <NUM>, <NUM>. Each actuator <NUM>, <NUM> includes two coils 11a, 11b, 12a, 12b and an actuator disc 11c, 12c. Each actuator disc 11c, 12c is mechanically connected to a contact <NUM>, <NUM>. When the fast disconnector <NUM> is fully closed (see <FIG>), the contacts <NUM>, <NUM> overlap by a distance OL.

A very large force on the actuator disks 11c, 12c can be generated by applying a current pulse to the appropriate coils (11b, 12b when opening). As the contacts move from standstill, they accelerate (increase speed) while still conducting current. The contacts <NUM>, <NUM> separate after the actuator discs 11c, 12c have moved a distance x = OL/<NUM>. Hence the contacts <NUM>, <NUM> can be separated at non-zero velocity. After the contacts <NUM>, <NUM> have separated (see e.g. <FIG>), the distance between the contacts <NUM>, <NUM> is z = 2x - OL.

As will be appreciated, there are many different types of switches with different driving mechanisms, insulating media, contacts, etc. which can instead be used as the first switch <NUM>.

Referring again to <FIG>, the second switch <NUM> is also a mechanical switch. The second switch <NUM> may be any suitable type of mechanical switch since it experiences current which naturally passes through zero. The second switch <NUM> is preferably a standard AC circuit breaker or a switch that functions in a similar way - i.e. a switch that is able to sustain and then extinguish an arc. The second switch <NUM> also preferably has a relatively high operating speed as this will help increase the overall operating speed of the circuit breaker <NUM>.

The inductor <NUM> has a particular inductance L and the capacitor <NUM> has a particular capacitance C. As will be explained below, the values of L and C can be selected based on several criteria.

The circuit breaker <NUM> also includes a controller <NUM>. The controller <NUM> is operatively connected to each switch <NUM>, <NUM> to provide control signals thereto. The controller <NUM> is also preferably operatively connected to a trip unit (not shown) to receive signals therefrom. For simplicity, the controller <NUM> is omitted from <FIG>, <FIG>, <FIG>.

The circuit breaker <NUM> can have a relatively low cost because it has only two mechanical switches (<NUM>, <NUM>) and two passive components (<NUM>, <NUM>) and, in particular, has no energy absorbers. The capacitor <NUM> is likely to be the most costly component. However, as will become apparent, the circuit breaker <NUM> can function with a reasonable value of capacitance C and hence a reasonably-low-cost capacitor <NUM>.

Referring to <FIG>, operation of the circuit breaker <NUM> will now be described. Initially (at S0), the first and second switches <NUM>, <NUM> are both closed and the circuit breaker <NUM> is conducting a circuit current I. At this time, the circuit current I corresponds to a normal load current. The closed-state losses are negligible as the circuit current I passes through two mechanical switches (<NUM>, <NUM>) and an inductor (<NUM>). The capacitor <NUM> is discharged as it is shorted by the first switch <NUM>. At a first step S1, the controller <NUM> receives a trip signal from the trip unit and (immediately) provides a control signal to the first switch <NUM> which causes the first switch <NUM> to start to open. The trip signal may have been provided by the trip unit in response to the trip unit detecting a fault condition - for example, that the circuit current I is unduly high and corresponds to a fault current.

When the first switch <NUM> is at least partly open, the circuit current I (i.e. fault current) experiences a resonant network - in particular, a series LC network. The series LC network is made up (at least primarily) of the inductor <NUM> and the capacitor <NUM>. The series LC network causes the circuit current I to change from a DC current to an AC current. This will now be explained in more detail with reference to a simplified circuit <NUM>.

Referring to <FIG>, the circuit <NUM> includes the same elements as the circuit breaker <NUM> except that it does not include the second switch <NUM>. Also, the circuit <NUM> includes a power supply <NUM> across the first and second terminals 5a, 5b, which supplies a (constant) DC voltage VDC.

It is assumed that, initially, the first switch <NUM> is closed and there is a DC current I<NUM> (hereinafter referred to as the initial current) flowing around the circuit. At t = <NUM>, the first switch <NUM> is opened, thereby producing a series LC circuit. The current flowing around the circuit I (hereinafter referred to as the circuit current) is given by: <MAT> <MAT> <MAT>.

The voltage across the capacitor (hereinafter referred to as the capacitor voltage) is given by: <MAT>.

The time derivative of the capacitor voltage is given by: <MAT>.

<FIG> show the circuit current I, the capacitor voltage Vc and the time derivative of the capacitor voltage <MAT> calculated using equations <NUM>-<NUM> for a DC voltage VDC of 900V, an initial current I<NUM> of 100A, an inductance L of 7mH, and a capacitance C of 150µF. It can be seen that, after the first switch <NUM> is opened at t = <NUM>, the circuit current I rises from its initial value I<NUM> and reaches a peak IP (hereinafter referred to as the peak fault current) at t = t<NUM>. Furthermore, the capacitor voltage VC rises from zero and reaches a peak VCP (hereinafter referred to as the peak capacitor voltage) at t = t<NUM>.

When the first switch <NUM> is at least partly open, the circuit current I is an AC current with a mean value of zero - i.e. no DC component. Hence the circuit current I will have zero crossings for any value of DC voltage VDC and any value of initial current I<NUM> (including a low value of initial current and even zero initial current). The peak fault current IP, the peak capacitor voltage VCP, the initial rate of rise of the capacitor voltage, and the timing of the zero crossings depend, amongst other things, on the values of L and C.

An HVDC system including the circuit breaker <NUM> will behave similarly to the above described circuit <NUM>. In this case, the above described DC voltage VDC corresponds to the voltage of the HVDC system, and the above described initial current I<NUM> corresponds to the circuit current I when the first switch <NUM> starts to open (i.e. its contacts separate). Since the circuit breaker <NUM> preferably has relatively large values of L and C, the influence of any inductance or capacitance in the HVDC system will be relatively small. Hence the behaviour of the circuit breaker <NUM> can be predicted sufficiently reliably using expressions such as those given above.

This can help in selecting a suitable set of properties of the circuit breaker <NUM> - for example, suitable values of L and C and suitable properties of the first switch <NUM>. For example, the values of L and C can be selected to determine the initial rate of rise of the capacitor voltage VC. A higher rate can help the HVDC system voltage to recover from a fault, but the rate cannot be too high, as will be explained below.

The properties of the circuit breaker <NUM> are selected such that, when the first switch <NUM> is partly or fully open, the capacitor voltage Vc remains below a withstand voltage of the first switch <NUM> - i.e. a maximum voltage VM that the first switch <NUM> is able to withstand without arcing and/or failure. VM may also be called a switch recovery voltage. The withstand voltage VM of the first switch <NUM> depends on the distance z between its contacts (e.g. the contacts <NUM>, <NUM>) and the dielectric strength d of the medium between the contacts. In particular, the following linear relationship can be assumed: <MAT>.

By way of example, the dielectric strength of air is ~3kVmm-<NUM> and the dielectric strength of SF<NUM> gas is ~<NUM>. 5kVmm-<NUM>. For the capacitor voltage VC to remain below the withstand voltage VM of the first switch <NUM>, the following condition should be satisfied: <MAT>
where t is time and is measured relative to the point at which the first switch <NUM> starts to open (hereinafter referred to as the point of contact separation). If equation (<NUM>) is satisfied there will be no arcing across S1 contacts.

Depending on the properties of the first switch <NUM> and circuit breaker <NUM> generally, the following further condition (obtained by differentiating equation <NUM> with respect to time) which involves contact velocity v=dz(t)/dt, may also need to be satisfied: <MAT>.

Typically, the contacts of the first switch <NUM> will move with a substantially constant relative velocity v=dz(t)/dt during opening. A suitable such velocity v can be relatively easily selected using the above described predicted behaviour of the capacitor voltage VC (see e.g. <FIG>) and the above described conditions (see e.g. equations <NUM> and <NUM>).

Equation (<NUM>) should be satisfied at all times including the instant of contact separation. This can be achieved if the contacts of switch S1 have non-zero velocity at the instant of separation, This conclusion is deduced from equation (<NUM>) since voltage derivative is non-zero. This condition can be achieved for example if S1 has a construction with lateral overlapping contacts (OL in <FIG>). This enables contacts to accelerate while sliding (and conducting current) and to achieve some non-zero velocity at the separation instant. When contact velocity is adequate according to equation (<NUM>) current is commutated to capacitor and there is no need for current interruption. This sets the invention apart from most prior art. In the prior art, it is required to interrupt current which is achieved with an arcing mechanical interrupter in <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT> or semiconductor switch in <CIT> and others.

Moreover, the capacitor voltage VC may reach its peak value VCP after the first switch <NUM> has fully opened. Hence the first switch <NUM> should also be selected such that the maximum distance zM between its contacts satisfies zMd > VCP (cf. equation <NUM>).

Moreover, it can be seen from <FIG> that capacitor voltage Vc initially rises from zero to its peak value in a time of approximately ⅓-½ of the period of the LC oscillations (this period is <NUM>/f<NUM>).

The first switch <NUM> preferably has an opening time (i.e. the time taken for the distance z between its contacts to go from <NUM> to zM) that is the same as or similar to this time, i.e. also ~⅓-½ of the period of the LC oscillations.

The above described considerations can help in selecting suitable values of L and C and suitable properties of the first switch <NUM>, e.g. voltage rating and opening speed.

Hence the circuit breaker <NUM> is configured such that the first switch <NUM> opens without arcing. This is preferred as it results in faster operation and without wearing of the contacts. However because of parasitic inductances not considered above, a short arcing may occur at the instant of separation. This arcing would be extremely short in the range of few micro-seconds and practically unobservable. Therefore it can be considered that switch S1 sustains no arcing and does not break any current.

Referring again to <FIG>, at a second step S2, the controller <NUM> provides a control signal to the second switch <NUM> which causes the second switch <NUM> to open. The second step S2 is carried out when a particular period of time (hereinafter referred to as the time delay) has elapsed after the first step S1.

As explained above, at this time, the circuit current I (i.e. fault current) is an AC current with multiple zero crossings. This enables the second switch <NUM> to interrupt the circuit current I in the same or a similar way to an AC circuit breaker - in particular, to form an arc when the contacts of the second switch <NUM> separate, to sustain the arc, and then to extinguish the arc at or near a zero crossing of the circuit current I. Thus, the circuit current I is interrupted. As will be appreciated, the second switch <NUM> may be able to interrupt the circuit current I when it is a relatively low, non-zero value (e.g. up to ~<NUM> A or more), i.e. merely near to a zero crossing.

Preferably, the time delay between the first and second steps S1, S2 is such that the second switch <NUM> (which has a known opening time) fully opens just before a zero crossing of the circuit current I. Hence the arcing time in S2 can be minimised. Preferably, the opening will occur at the first zero crossing (i.e. t = t<NUM> in <FIG>) when the capacitor voltage VC is at its peak and circuit current I passes through zero for the first time. Interrupting the circuit current I at a zero crossing can be carried out with substantially zero energy dissipation. Hence the circuit breaker <NUM> can avoid the issue associated with typical HVDC circuit breakers of requiring large energy absorbers (and other components) due to the circuit current I having a high value when it is interrupted.

At a third step S3 (which may be omitted), the controller <NUM> resets the circuit breaker <NUM> by providing control signals to the first and second switches <NUM>, <NUM> which cause the first and second switches <NUM>, <NUM> to close. The third step S3 may be carried out in response to any suitable event - e.g. the controller <NUM> receiving a reset (or an operator close command) signal.

In some examples, the first switch <NUM> is closed before the second switch <NUM>. In this case, since the first and second steps S1, S2 leave the capacitor <NUM> in a charged state, action should be taken to avoid closing the first switch <NUM> across the charged capacitor <NUM>. For example, the capacitor <NUM> can be discharged via a resistor (not shown), which may be connected to the capacitor <NUM> either directly or via a controllable switch (not shown). Once the first switch <NUM> has been closed, the second switch <NUM> can simply be closed - for example, regardless of any circuit current I.

In other examples, the second switch <NUM> is closed before the first switch <NUM>. In this case, after the second switch <NUM> has been closed, the circuit current I experiences the above described series LC network and so, amongst other things, the capacitor voltage VC oscillates (see e.g. <FIG>). The first switch <NUM> is then closed during a period of time during which the capacitor voltage VC is relatively low. The timing of this can be based, for example the above described predicted behaviour of the capacitor voltage VC.

The above described steps S1, S2, S3 may then be repeated.

Referring to <FIG>, a second example of a circuit breaker <NUM> which is outside the scope of the claimed invention will now be described. The circuit breaker <NUM> is the same as the first example of the circuit breaker <NUM> except that it has an energy absorber - in particular, a surge arrester <NUM> - connected in parallel with the capacitor <NUM>. In other examples, the circuit breaker <NUM> may include a different type of (and/or a differently connected) energy absorber.

The circuit breaker <NUM> operates in the same way as the first example of the circuit breaker <NUM>, except that the surge arrester <NUM> limits the capacitor voltage VC to a particular value (hereinafter referred to as the capacitor voltage limit). Thus, the capacitor voltage VC can be limited to a value of, for example, ~<NUM>. 5pu (i.e. <NUM> × the DC voltage). Such a limit is used, for example, in <CIT>. This is lower than the peak capacitor voltage VCP of ~<NUM>-3pu in the first example of the circuit breaker <NUM>. The lowest capacitor voltage limit that enables the circuit breaker <NUM> to interrupt the circuit current I is just above 1pu.

The time to the first current zero crossing will be somewhat longer in the circuit breaker <NUM> than in the first example of the circuit breaker <NUM> - i.e. the first current zero crossing will occur at t > t<NUM> (see <FIG>). The time delay between the opening of the first and second switches <NUM>, <NUM> should account for this if it intended for the second switch <NUM> to fully open just before the first zero crossing of the circuit current I (to minimise arcing time). The cost of the circuit breaker <NUM> may be lower than that of the first example of the circuit breaker <NUM> - depending on the increased costs due to the surge arrester <NUM> versus the reduced costs due to the lower capacitor voltage rating, etc..

<FIG> shows simulation results for a circuit breaker corresponding to the second example of the circuit breaker <NUM>. The simulation was carried out with the following parameters:.

<FIG> shows (i) the capacitor voltage VC and (ii) the withstand voltage VM of the first switch <NUM>. It can be seen that VM > VC. <FIG> shows (i) the circuit current I and (ii) the current ISA in the surge arrester <NUM>.

The circuit current I starts at 2kA and then begins to rise. The circuit current I is <NUM>. 6kA at t = tA, when a trip signal is received and the contacts of the first switch <NUM> start to move, and is <NUM>. 4kA at t = tB, which is the point of contact separation. The circuit current I then reaches a peak value of <NUM>. 4kA before falling to zero. The peak fault current of <NUM>. 4kA is considerably lower than, for example, the above described known hybrid HVDC circuit breakers (see e.g. <CIT>), and so any adverse effect on other parts of the HVDC system should be reduced. Put simply, the circuit breaker <NUM> is able to quickly reduce the circuit current I because the capacitor <NUM> is introduced into the current path <NUM> almost immediately after the trip signal is received.

<FIG> shows (i) the current IS1 in the first switch <NUM>, (ii) the current IC in the capacitor <NUM>, and (iii) the control signal S<NUM> sent by the controller <NUM> to the first switch <NUM>. It can be seen that there is successful current commutation to the capacitor <NUM>. <FIG> shows (I) the displacement x of each of the contacts of the first switch <NUM> (e.g. the contacts <NUM>, <NUM>), (ii) the distance z between the contacts, and (iii) the relative velocity v of the contacts. It can be seen that there is a sufficiently high relative velocity, resulting in arc-less commutation at t = tB.

<FIG> show experimental results for a (scaled-down) prototype circuit breaker corresponding to the second example of the circuit breaker <NUM>. In this case, the parameters are:.

<FIG> shows (i) the capacitor voltage VC and (ii) the voltage VS2 across the second switch <NUM>. It can be seen that capacitor voltage VC rises to ~1100V, showing that the first switch <NUM> has opened and that the circuit current I has been commutated to the capacitor <NUM>. Since voltage rise is fast, there is no arcing. The behaviour of the voltage VS2 across the second switch <NUM> shows that the circuit current I is interrupted by the second switch <NUM>.

<FIG> shows (i) the circuit current I, (ii) the current IS1 in the first switch <NUM>, and (iii) the current IC in the capacitor <NUM>. The circuit current I starts at ~5A and then begins to rise. The circuit current I is ~40A when a trip signal is received and the contacts of the first switch <NUM> start to move, and is ~ 90A at the point of contact separation. It can be seen that the current IS1 in the first switch <NUM> drops abruptly to zero at the point of contact separation, showing that the circuit current I of ~90A is successfully commutated to the capacitor <NUM>. The (total) circuit current I reaches a peak value of 150A before falling to zero ~<NUM> after the trip signal is received. This confirms that the circuit current I can be interrupted by a simple AC circuit breaker (i.e. the second switch <NUM>).

<FIG> shows (i) the control signal S<NUM> sent by the controller <NUM> to the first switch <NUM>, (ii) the displacement x of each of the contacts of the first switch <NUM>, (iii) the distance z between the contacts, and (iv) the relative velocity v of the contacts. It is seen that the contact separation occurs at around <NUM>.

Referring to <FIG>, a third example of a circuit breaker <NUM> which is outside the scope of the claimed invention will now be described. The circuit breaker <NUM> is the same as the second example of the circuit breaker <NUM> except that it has a switch <NUM> (hereinafter referred to as a third switch) connected in series with the first switch <NUM> such that the first and third switches <NUM>, <NUM> are parallel with the capacitor <NUM>. Furthermore, the controller (not shown) is also operatively connected to the third switch <NUM> to provide control signals thereto. The third switch <NUM> is preferably a semiconductor-based switch such as an insulated-gate bipolar transistor (IGBT) or a metal-oxide-semiconductor field-effect transistor (MOSFET). A diode <NUM> is connected anti-parallel across the third switch <NUM>.

Due to parasitic inductances in the connecting cables, there may be a delay before the current IC to the capacitor <NUM> rises, and during this period a voltage may occur across the contacts of the first switch <NUM> which may cause arcing. As explained above, the conditions for arcless separation can be especially critical at this point.

The third switch 31is to help the first switch <NUM> commute the circuit current I to the capacitor <NUM>. In particular, the third switch <NUM> is opened for a period of time starting at or near the instant of contact separation of the first switch <NUM>, thereby removing current from the separating contacts of the first switch <NUM> (e.g. the contacts <NUM>, <NUM>). Hence the third switch <NUM> enables the first switch <NUM> to more effectively and/or more safely commute the circuit current I to the capacitor <NUM> by enabling the current IC to the capacitor <NUM> to stabilise. Hence the third switch <NUM> provides additional voltage blocking capability. The third switch is required only to overcome any parasitic inductance in the circuit, and it is rated for very low voltage which differentiates this design from <CIT>.

The voltage rating - and hence the cost - of the third switch <NUM> can be relatively low. By way of example, for a typical 320kV HVDC system (see the example described above with references to <FIG>), a small, 10kV-rated third switch <NUM> would stop the circuit current I for ~<NUM> - i.e. it takes the capacitor voltage VC ~<NUM> to rise to 10kV.

The circuit breaker <NUM> operates in the same way as the second example of the circuit breaker <NUM>, except that the controller (not shown) provides a control signal to the third switch <NUM> which causes the third switch <NUM> to open for a period of time as described above. The timing of this can be based, for example, on the above described predicted behaviour of the capacitor voltage Vc.

Alternatively, instead of using the third switch, various other methods can be employed to help current commutate to the capacitor branch at the instant of contact separation. One attractive method is to inject a current pulse from an auxiliary source which is coupled through a small transformer with the capacitor branch as shown in <FIG>. The transformer provides isolation of the low-voltage auxiliary unit which may be less costly than third switch. The pulse from the auxiliary unit is very short (below <NUM>) of limited current magnitude and achieves the same effect as the third switch. This commutating aid is only required if parasitic inductance is present, and therefore it has low rating.

The inductor <NUM> in the first example of the circuit breaker <NUM> limits the initial rate of rise of the circuit current I and also produces the series LC circuit which defines the initial rate of rise of the capacitor voltage VC and changes the circuit current I from a DC current to an AC current. Nevertheless, the inductor <NUM> need not be included. This is because the HVDC system may include one or more components that have some or all of these effects.

Accordingly, referring to <FIG>, a fourth example of a circuit breaker <NUM> outside the scope of the claimed invention is the same as the first example of the circuit breaker <NUM> except that it does not include an inductor. In this example, the circuit breaker <NUM> is suitable for use in an HVDC system including one or more components (e.g. transformers or cables) with internal inductances which perform a similar function to the inductor <NUM> in the first example of the circuit breaker <NUM>. For lower-voltage applications, the voltage tolerances need not be very strict and so a ~<NUM>-4pu voltage need not be of special concern and the circuit breaker <NUM> may operate with a relatively small inductance in the series LC network.

Referring to <FIG>, a fifth example of a circuit breaker <NUM> outside the scope of the claimed invention is the same as the second example of the circuit breaker <NUM> except that it does not include an inductor. Accordingly, the circuit breaker <NUM> corresponds to the fourth example of the circuit breaker <NUM> with a surge arrester <NUM>.

In this example, the circuit breaker <NUM> is suitable for use in an HVDC system which does not include components with (significant) internal inductances as described above, but which includes components (e.g. a converter) that limit the magnitude of the circuit current I. Such components can have the effect of limiting the initial rate of rise of the capacitor voltage Vc. In such a case, the capacitor voltage VC will rise until it reaches the limit set by the surge arrester <NUM> and the circuit current I will then reduce to low values. Since there is no series LC circuit, there will be no intrinsic zero crossings of the circuit current I. However, such zero crossings are not essential if the circuit current I reduces to sufficiently low values for interruption by the second switch <NUM>. The circuit breaker <NUM> may be particularly suitable in relatively-low-power circuits where a low-cost second switch <NUM> can readily interrupt non-zero currents.

In very high voltage applications, the requirements for a short opening time of the first switch <NUM> and a large maximum distance zM between the contacts of the first switch <NUM> can be high. These requirements can be met by using multiple first switches <NUM> connected in series and operated simultaneously. By way of example, if a blocking voltage of 450kV is required, this corresponds to zM = <NUM> (<NUM> × <NUM>. 5kV = 450kV) for a single first switch <NUM>, which may be difficult to achieve in an opening time of <NUM>. However, it can be achieved with reasonable accelerations and mechanical forces using six first switches <NUM> connected in series, each providing <NUM> of contact separation and opening in <NUM>.

Accordingly, referring to <FIG>, a sixth example of a circuit breaker <NUM> which is outside the scope of the claimed invention is the same as the second example of the circuit breaker <NUM> except that it includes a further first switch <NUM>' connected in series with the first and second switches <NUM> and also includes a further capacitor <NUM>' connected in parallel with the further first switch <NUM>' and a further surge arrester <NUM>' connected in parallel with the further capacitor <NUM>'. In other, similar examples, there may be a series of more than two 'modules', each of which includes a first switch, a capacitor and a surge arrester.

Referring to <FIG>, a seventh example of a circuit breaker <NUM> is the same as the second example of the circuit breaker <NUM> except that it includes a further first switch <NUM>' connected in series with the first and second switches <NUM>, and in parallel with the capacitor <NUM> and the surge arrester <NUM>. In other, similar examples, there may be a series of more than two first switches.

As mentioned in the third example, parasitics in the circuit may invalidate conditions for the arc-less contact separation. Instead of using the third switch, various other methods can be employed to help current commutate to the capacitor branch at the instant of contact separation. One attractive method, which is outside the scope of the claimed invention, is to inject a current pulse from an auxiliary source <NUM> which is coupled through a small transformer <NUM> with the capacitor branch as shown in <FIG>. The transformer provides isolation of the low-voltage auxiliary unit which may be less costly than third switch. The pulse from the auxiliary unit is very short (below <NUM>) of limited current magnitude and achieves the same effect as the third switch. This commutating aid is only required if parasitic inductance is present, and therefore it has low rating.

It will be appreciated that there may be many other variations of the above described embodiments. For example, each circuit breaker may be used in other types of DC networks or in AC networks. Amongst other things, this is because each circuit breaker can produce an AC circuit current I (with zero crossings) regardless of the values of VDC and I<NUM>.

Claim 1:
Circuit breaker (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) configured for interrupting a direct current in a current path (<NUM>) of a high voltage DC transmission system, the circuit breaker comprising:-
a first switch (<NUM>) having a disconnecting contact;
a second switch (<NUM>);
wherein the first and second switches (<NUM>,<NUM>) are operatively connected in series in the current path;
a capacitor (<NUM>) operatively connected in parallel with the first switch (<NUM>);
wherein:-
the capacitor (<NUM>) is selected such that, when the first switch (<NUM>) starts to open, the current in the current path (<NUM>) experiences a series inductor-capacitor network to cause a change in the current from a direct current waveform to an alternating current waveform; and
a switch opening control (<NUM>):-
operating to start to open the first switch (<NUM>) while the second switch (<NUM>) is closed thereby to cause the current flowing in the current path (<NUM>) to charge the capacitor (<NUM>) and to experience the series inductor-capacitor network hereby the charging of the capacitor causes the current to subsequently reduce; and
operating to open the second switch (<NUM>) at a time delay after starting to open the first switch (<NUM>) thereby to interrupt the current at a low current or a zero current in the alternating current waveform;
characterised in that the disconnecting contact of the first switch (<NUM>) has a contact velocity sufficient so that the voltage across the capacitor (<NUM>) is below a withstand voltage of the first switch (<NUM>) at substantially all times, and wherein the circuit breaker further comprises;
at least one further first switch (<NUM>') in series with the first switch (<NUM>) and both in parallel with the capacitor (<NUM>);
wherein the switch opening control (<NUM>) operates to start to open each first switch (<NUM>, <NUM>') while the second switch (<NUM>) is closed.