DC circuit breaker with counter current generation

Abstract

A circuit-breaker device, comprising a main branch and an auxiliary branch electrically in parallel with the main branch, wherein the main branch comprises at least one mechanical switch-disconnector in series with a breaker cell constituted of at least one semiconductor switch, and a snubber circuit in parallel with the at least one breaker cell, the snubber circuit including an energy storage element, wherein the mechanical switch-disconnector is switchable to selectively allow current to flow in the main branch in a first mode of operation or commutate current from the main branch to the auxiliary branch in a second mode of operation, characterized in that the snubber circuit further comprises a bleeder resistor arranged to create a counter current in the main branch when current is commutated from the main branch to the auxiliary branch by discharging the energy storage element.

Claims

1. A circuit-breaker device, comprising a main branch and an auxiliary branch electrically in parallel with the main branch, wherein the main branch comprises at least one mechanical switch-disconnector in series with a breaker cell constituted of at least one semiconductor switch; and a snubber circuit in parallel with the at least one breaker cell, the snubber circuit including an energy storage element, wherein the mechanical switch-disconnector is switchable to selectively allow current to flow in the main branch in a first mode of operation or commutate current from the main branch to the auxiliary branch in a second mode of operation, wherein the snubber circuit further comprises a bleeder resistor arranged to create a counter current in the main branch while current is commutated from the main branch to the auxiliary branch by discharging the energy storage element.

2. The circuit-breaker device of claim 1, wherein the snubber circuit comprises a diode connected in series with the energy storage element, the bleeder resistor being arranged in parallel with the diode.

3. The circuit-breaker of claim 1, further comprising a surge arrester in parallel with the at least one breaker cell.

4. The circuit-breaker of claim 1, wherein the auxiliary branch further comprises at least one thyristor in series with a switching-assistance module constituted by a parallel connection of a capacitor, a resistor, and a surge arrester.

5. A power system comprising: a transmission line arranged to carry direct current; and a circuit-breaker device comprising: a main branch and an auxiliary branch electrically in parallel with the main branch, wherein the main branch comprises at least one mechanical switch-disconnector in series with a breaker cell constituted of at least one semiconductor switch; and a snubber circuit in parallel with the at least one breaker cell, the snubber circuit including an energy storage element, wherein the mechanical switch-disconnector is switchable to selectively allow current to flow in the main branch in a first mode of operation or commutate current from the main branch to the auxiliary branch in a second mode of operation, wherein the snubber circuit further comprises a bleeder resistor arranged to create a counter current in the main branch while current is commutated from the main branch to the auxiliary branch by discharging the energy storage element coupled to the transmission line to controllably effect discontinuation of flow of direct current in the transmission line.

6. The power system of claim 5, comprising a High Voltage Direct Current power transmission system.

7. The circuit-breaker device of claim 1, wherein the counter current created by the bleed resistor is lower than a current to be interrupted.

8. The circuit-breaker device of claim 1, wherein the mechanical switch-disconnector is configured to switch at zero current.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other aspects, goals, advantages and features of the invention will appear more clearly on reading the following detailed description of embodiments thereof, given by way of non-limiting example and with reference to the accompanying drawing in which:

(2) FIGS. 1 and 2, already discussed above, show circuit breaker devices of the prior art;

(3) FIGS. 2A, 2 B, and -2C illustrate operating modes of another prior art circuit breaker device;

(4) FIG. 2D shows the current commutation from the main branch to the auxiliary branch of the prior art circuit breaker device of FIGS. 2A-2C;

(5) FIG. 3A shows a circuit breaker device;

(6) FIG. 3B shows the current commutation from the main branch to the auxiliary branch of the circuit breaker of FIG. 3A.

DETAILED DESCRIPTION OF THE INVENTION

(7) FIGS. 2A-2C illustrate operating modes of a prior art circuit breaker device 1 with no countercurrent generation for accelerating arc extinction. FIG. 3A shows a circuit breaker device 10 according to a possible embodiment of the invention. As will be apparent from the below description, the circuit breaker device 10 has the same topology as the prior art circuit breaker 1 but with an additional bleeder resistor that allows creating a counter current. For this reason, common elements in the circuit breaker devices 1 and 10 share the same references.

(8) The circuit breaker devices 1, 10 are each adapted to be coupled to a transmission line of a power system, arranged to carry direct current, for controllably effecting discontinuation of flow of direct current in the transmission line. The power system may comprise a High Voltage Direct Current power transmission system.

(9) Each of the circuit breaker devices 1, 10 comprises a main branch B.sub.M in which, in use, the current flows under steady conditions and an auxiliary branch B.sub.A electrically in parallel with the main branch. Each of the main and auxiliary branches B.sub.M, B.sub.A extend between a first and a second terminal 2, 3 which, in use, are connected to a DC electrical network 4.

(10) The main branch B.sub.M comprises at least one mechanical switch-disconnector S1, S2 in series with at least one breaker cell constituted by at least one semiconductor switch Q.sub.P. The at least one mechanical switch-disconnector S1, S2 is for instance a vacuum interrupter. Each semiconductor switch Q.sub.P can, for example, be a silicon-based insulated gate bipolar transistor (IGBT). Alternatively, other types of turn-off semiconductor device such as a JFET, MOSFET or bipolar transistor can be used, as could other wide-band-gap semiconductor materials such as silicon carbide or gallium nitride.

(11) The circuit breaker devices 1, 10 further comprise a switching control unit 5 to control switching of the at least one mechanical switch-disconnector S1, S2 and of the at least one semiconductor switch Q.sub.P.

(12) Hence, by means of the switching control unit 5, the at least one mechanical switch-disconnector S1, S2 is switchable to selectively allow current to flow in the main branch B.sub.M in a first mode of operation or commutate current from the main branch to the auxiliary branch B.sub.A in a second mode of operation.

(13) The main branch B.sub.M further comprises a snubber circuit Sn1, Sn2 in parallel with the at least one breaker cell Q.sub.P. The snubber circuit includes a diode D.sub.P electrically in series with an energy storage element such as a capacitor C.sub.P, itself electrically in parallel with a discharge resistor R1.

(14) The energy storage element C.sub.P controls the rate of increase of the voltage at its terminals when the breaker cell Q.sub.P is switched to the OFF state by the switching control unit 5. The diode D.sub.P prevents violent discharging of the capacitor C.sub.P when the breaker cell Q.sub.P begins to conduct. Finally, the discharge resistor R1 enables slow discharging of the energy storage element C.sub.P.

(15) Hence, the snubber circuit protects the breaker cell to which it is associated by controlling the rate at which the voltage across its terminals increases when it switches from the conducting (ON) state to the non-conducting (OFF) state. This limitation of the rate of voltage increase also has a beneficial effect for switching the current from the main branch to the auxiliary branch, in the sense that it contributes to controlling the di/dt of the current in said branch.

(16) Also electrically in parallel with the breaker cell Q.sub.P is a surge arrester R.sub.P. It is designed to limit the voltage to a value less than the withstand voltage of the breaker cell Q.sub.P.

(17) The auxiliary branch B.sub.A is provided in parallel with the main branch B.sub.M and comprises at least one thyristor T1 in series with a switching-assistance module constituted by the parallel connection of a capacitor C, a resistor R to discharge the capacitor, and a surge arrester P. The switching control module 5 is further configured to switch the at least one thyristor T1 between its non-conducting (OFF) state and its conducting (ON) state.

(18) When the circuit breaker devices 1, 10 switch from the main into the auxiliary branch, the surge arresters RP, P are used one after the other in both branches.

(19) FIG. 2a illustrates the first mode of operation with current I.sub.P flowing in the main branch B.sub.M.

(20) Upon appearance of an electrical fault on the network 4, current I.sub.P increases in the main branch B.sub.M. In order to eliminate this fault, current has to be interrupted. Current interruption is performed with the switching control unit 5 implementing the following sequence of operations.

(21) As shown on FIG. 2B, first the at least one breaker cell Q.sub.P in the main branch B.sub.M is switched off and the current is diverted to the snubber circuit Sn1. The snubber circuit Sn1 limits the rate of rise of voltage and charges its energy storage element C.sub.P until the parallel surge arrester R.sub.P conducts.

(22) In a second step, as shown on FIG. 2C, the at least one thyristor T1 in the auxiliary branch B.sub.A is switched to its conducting state. Simultaneously, opening of the at least one mechanical switch-disconnector S1, S2 in the main branch is started.

(23) The current is therefore diverted to the auxiliary branch B.sub.A. FIG. 2D shows this current commutation from the main branch to the auxiliary branch, with I.sub.M designating the current value in the main branch over time and I.sub.A designating the current value in the auxiliary branch over time.

(24) Following this commutation, the capacitor C in the auxiliary branch B.sub.A gets charged until the parallel surge arrester P conducts. This last surge arrester P limits the voltage to a smaller value than the first surge arrester R.sub.P. The energy storage element C.sub.P of the snubber circuit Sn1 discharges slowly through the parallel discharge resistor R1. Depending on the voltage difference and technology used (highly non-linear transient voltage suppressor versus non-linear surge arrester) a current still passes in the same direction as before the second step through the at least one mechanical switch-disconnector S1, S2 in the main branch B.sub.M of the prior art circuit breaker device 1, and an electrical arc 6 is created.

(25) In order to establish a current zero in the branch and ensure that the arc extinguishes, an embodiment of the invention makes use of the charges stored in the energy storage element C.sub.P of the snubber circuit to generate a counter current in the main branch. As shown on FIG. 3a which represents the circuit breaker device 10 according to an embodiment of the invention, the snubber circuit Sn2 further comprises a bleeder resistor R2 arranged to create a counter current Ic in the main branch B.sub.M when current is commutated from the main branch to the auxiliary branch by discharging the energy storage element Cp.

(26) Upon complete discharge of the energy storage element Cp, the counter current gets stabilized at zero.

(27) As shown on FIG. 3a, the bleeder resistor R2 may be arranged in parallel with the diode D.sub.P of the snubber circuit Sn2.

(28) The bleeder resistor R2 may be identical to the discharge resistor R1, but can be chosen smaller to adjust the counter current Ic.

(29) The effect of adding the bleeder resistor R2 on the current in the at least one mechanical switch-disconnector S1, S2 can be seen on FIG. 3B which represents the current value I.sub.M in the main branch over time and the current value I.sub.A in the auxiliary branch over time. A peak of negative current is observed, of a few tens of amperes but of very low duration (less than a millisecond), which correspond to the counter current Ic created in the main branch B.sub.M. This counter current Ic is of very low value compared to the current to be interrupted, but of sufficient value to create the zero crossing and accelerate extinction of the arc. Lifetime of the contacts of the at least one mechanical switch-disconnector is therefore improved, while the counter-current only necessitates small capacitors to be generated.

(30) This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.