WO2000052720A1

WO2000052720A1 - Circuit breaker

Info

Publication number: WO2000052720A1
Application number: PCT/SE2000/000385
Authority: WO
Inventors: Lars Liljestrand; Lars Paulsson
Original assignee: Abb Ab
Priority date: 1999-03-01
Filing date: 2000-02-28
Publication date: 2000-09-08
Also published as: SE9900727D0; AU3686400A

Abstract

The invention relates to a vacuum circuit breaker for alternating current. The breaker has two contact electrodes (6, 7), which are arranged to be separated from each other at the opening of the breaker. According to the invention, synchronising means (9) are arranged to start the separation movement of the contact electrodes (6, 7) within a certain time window (T). The time window ranges from the moment of one zero-crossing of the current until 1 ms before the next zero-crossing. By this synchronisation, re-strikes are avoided which otherwise might occur if the opening is started shortly before a zero-crossing. The invention also relates to a corresponding method of interrupting an alternating current, to the use of the invented breaker and to an electric power network including the invented breaker.

Description

CIRCUIT BREAKER

Technical field

The present invention relates to a vacuum circuit breaker of the kind spe- cified in the preamble of claim 1 and to a method as specified in the preamble of claim 5. The invention also includes the use of the invented vacuum breaker as claimed in claim 9 and to a power network including the invented circuit breaker as claimed in claim 10.

Background art

When using a vacuum circuit breaker to operate inductive loads, multiple re-strikes followed by voltage escalation may occur at the opening of the breaker.

Re-strikes occur when the contacts of the breaker have not been separated enough at the first zero-crossing of the current after the start of the separa- tion. After the re-strike at the zero-crossing, the current is interrupted and the voltage between the contacts increases, causing a second re-strike and so on. Hundreds of re-strikes within one ms can be observed. The amplitude of the voltage at the load side of the breaker increases for every re-strike. Depending on the characteristics of the connected circuit, very high voltages can be generated as well as large and fast voltage changes which will be unevenly distributed over the load. If the load is e.g. a motor, about 90 % of the change in voltage might fall on the first winding turn and destroy the insulation.

Also for many other reasons, the multiple re-strikes are dangerous for motor windings. The change in polarity might result in voltage steps twice the surge arrester protective level across the stator winding. The number of transients at each occasion is high, and the time between two transients is short, and the voltage transients occur not only at the re-striking phase but on all three phases due to the capacitive coupling between the phases. Voltage transients occur not only on the motor side of the breaker, but also on the bus-side at multiple re- strikes. In the case when other motors are running on the same bus, they will also be exposed to voltage transients when multiple re-strikes occur at one motor. This kind of problem may occur not only in motors, but in any winding. It is thus desirable to avoid re-strikes at the opening of a vacuum circuit breaker.

For multiple re-strikes to occur, there are a number of conditions to be fulfilled: 1. There must be a high frequency current with zero-crossings through the breaker at every re-strike.

2. The breaker must be able to interrupt that current.

3. The time derivate of the recovery voltage must be higher than the increase in the dielectric strength of the breaker contacts. 4. The contact separation in the breaker of the first interrupting phase must start just before (< 1 ms) a current zero-crossing. Due to the short contact distance in the breaker, the dielectric strength is too low to withstand the recovery voltage, and a re-strike will occur.

If one of the conditions above is not fulfilled, multiple re-strikes will not oc- cur. Therefore, there are different measures against multiple re-strikes. A well selected RC-combination can prevent multiple re-strikes, though not the first re- strike, by damping the oscillating current. The correct values of the RC-combination must be chosen for each system, depending on the type of cables, the length of cables, etc. However, the combined effect of these parameters is hard to esti- mate.

Other attempts to reduce or eliminate the risk for re-strikes include affecting the opening speed of the contacts and the use of particular materials for the contacts. However, no successful solution to overcome the problem with re- strikes, has been achieved by means of these techniques. DE 41 05 697 discloses a vacuum circuit breaker but is focused on another problem than those related to re-strikes. The device of the disclosure provides the opening of the breaker to be synchronised to a time window just before zero-crossing of the current. The time window is defined to be within 2 ms before zero-crossing. Since this time window allows for opening also during the period when re-strikes occur, the device of this disclosure does not present a solution to the problem of avoiding re-strikes. Description of the invention

The object of the present invention is to find a way to effectively eliminate or at least substantially reduce the risk for re-striking when opening a vacuum circuit breaker. This has been achieved by providing a vacuum circuit breaker with the specific features specified in the characterizing portion of claim 1 and by a method including the specific measures specified in the characterizing portion of claim 5. The invention thus focuses on the fact that the risk for re-strikes only prevail when the contact separation occurs less than 1 ms before a zero-crossing of the current. By synchronising the opening so that it starts outside that period, the risk for re-strikes is eliminated to a large extent, and this independently of the system parameters. For a one-phase system, the time window for opening thus will be 9 ms. For a three-phase system, it will be shorter. The time specified in ms in the claims relate to a 50 Hz system. For other frequencies, the time has to be modified accordingly and corresponds to an electric angle of 0 < ωt < 0,9π.

Since the time window is relatively large, the synchronising means needs not to be designed to be very accurate, which allows to attain the function without high costs for achieving stable mechanical opening times.

In one embodiment of the invention, the time window ends 2 ms before the next zero-crossing of the current. This gives an extra security margin and assures the elimination of re-strikes even at lower separating speeds.

In still a further embodiment of the invention, the start of the time window is 0,5 ms after a zero-crossing of the current. Thereby, a security margin is attained also at this end of the range. In yet another embodiment, synchronising is arranged for starting separation within a time window that is a predetermined part of the general range. Although this smaller time window increases the demand on the synchronising device, it offers a possibility to consider other influences on the system for optimising the synchronisation. The permissible time window for a synchronised opening of a three-phase circuit breaker will be reduced compared to a one-phase system. The time between two current zero-crossings is 3,33 ms in a three-phase 50 Hz system. The permissible time window for opening the breaker poles will be 2,3 ms (3,33 - 1 ms).

The interruption of the currents in a three-phase system differs depending on the type of grounding for the system. The arching time of the first interrupting pole will be given by the synchronising and will be independent of the grounding. The arcing times of the second and third interrupting pole of the circuit breaker are always longer than for the first interrupting pole. The arcing times are also depending on the grounding. The arcing time for the second and third interrupting pole will be equal in a high impedance grounded system. The third interrupting pole will have the longest arcing time in a low impedance grounded system.

The discussion above is based on the fact that the contact opening of the three poles occurs simultaneously. A combination of preventing multiple re-strikes and short arcing times for all three poles can be achieved by using individual opening for all three poles. The contacts of the first interrupting pole can be opened at one instant, before the second and third poles are opened. The delay in opening the second and third interrupting poles allows the selection of a large time window for the opening of the first interrupting pole, in the same manner as for the one-phase system.

The first interrupting pole can be opened during a first time window, and the second and third interrupting pole during a subsequent time window, with the two windows overlapping each other.

The overlap in the time windows constitutes the window used when three poles open simultaneously.

The delay in opening the poles may be arranged either by a fixed delay by means of a mechanical device using one actuator for all three poles, or a controllable delay by means of using three separate actuators, one for each pole.

The invented method and the preferred embodiments thereof offer advantages similar to those for the invented device as described above.

The invention will be explained in more detail with reference to the en- closed drawings. Brief description of the drawings

Figure 1 schematically illustrates the principle of the invention, and

Figure 2 is a graph representing a half-period of a 50 Hz AC.

Description of preferred embodiments

In Figure 1 , a vacuum circuit breaker 4 is connected at one side to a conduit 1 leading to the power network, and at the other side to a conduit 2 leading to a load, e.g. a motor 3. The breaker is sealed to maintain a vacuum within the chamber 5. The conduit 1 is electrically connected to a first contact electrode 6 and the conduit 2 to a second contact electrode 7. The fist electrode 6 is stationary, while the second electrode 7 is movable. During normal operation, the two contact electrodes are kept in contact with each other, and power is supplied from the network to the load. Usual materials for the contact electrode are Cu or special alloys thereof. For the present invention, however, the material is no critical aspect so that any material usually applied in a vacuum circuit breaker can be considered.

When the power supply has to be interrupted for some reason, the second contact electrode 7 is rapidly moved downwards, and an arc develops which initially burns in the metal vapour vaporised from the electrodes before extinguish- ing. The movable electrode 7 extends outside the chamber 5 through a sealed penetration, e.g. sealed by a bellow 8. Movement of the electrode 7 is effectuated via the extension outside the sealed chamber by any sort of conventional device, e.g. a spring arrangement 12, symbolically represented by spring 12 in Fig. 1. Important is that the movement is fast enough so that the distance between the elec- trodes will be to large for resulting in a re-strike at the next zero-crossing of the current. A sufficient distance can be attained within 1 ms by conventional means effectuating the movement.

The movement is triggered by a synchronising means 9 via a signal line 10 to the spring arrangement 12 to be activated and start the contact separation. Via a first input signal line 11 , the synchronising means 9 reads the phase angle of the current in conduit 1. When the synchronising means 9, via a second input signal line 13, receives information that the power has to be interrupted, this in- formation is processed together with the information from the first input signal line 1 1 in order to trigger activation of the spring arrangement 12. The synchronising means 9 is programmed to allow activation during a certain range of a current period only. This range, the so-called time window, extends from the moment of a zero-crossing of the current until 1 ms before the next zero-crossing. Thus, there will always be at least 1 ms for the separating operation until the next zero-crossing occurs and therefore, the risk for re-strikes will be practically eliminated.

The synchronisation of the opening may be achieved in different manners. One way is to measure the current through the breaker poles and detect the in- stant of a current zero-crossing. The start of opening the contacts will then be initiated after a selected delay from the instant of the last current zero-crossing.

The precondition for synchronisation related to current zero-crossings is the presence of a current with a minimum amplitude through the breaker in order to be able to detect current zero-crossings. When synchronisation is applied to prevent multiple re-strikes, this precondition causes no problems since multiple re- strikes occur only when a high current flows through the breaker. Neither does this precondition cause any problem when synchronised opening is applied to reduce arcing time, since the arcing time is only of interest in case of high currents, not in case of a low current amplitude. The synchronisation can be overruled in case of a low current.

For synchronisation, the amplitude of the current may also be measured, detecting the current zero-crossings therethrough. In case the current exceeds a certain value, the current zero-crossing will be detected and the breaker will be opened after a pre-set delay from the current zero-crossing. In case the current is below the selected amplitude, the breaker will be opened without taking the current zero-crossings into account.

In order to attain an additional security margin or for taking account of other relevant aspects, a narrower time window may be pre-set, e.g. from 0,5 ms after a zero-crossing until 2 ms before the next zero-crossing. In Fig. 2, a half-period of the current is illustrated, i.e. a period of 10 ms when the frequency is 50 Hz. The shaded interval between 9 < t < 10 represents the period wherein re-strikes might occur if contact separation starts within that interval. For 0 < t < 9, i.e. during 90 % of the time, no re-strike will occur, and as explained above, the synchronising means is arranged to trigger the start of the separation during that interval.

In Fig. 2, the ranges for the time window corresponding to some of the embodiments of the dependent claims are indicated as well.

Claims

1. Vacuum circuit breaker for alternating current having first (6) and second (7) contact electrodes that are arranged to be separated from each other at the opening of the breaker, characterized in that synchronising means (9) are arranged to start the separation movement of the contact electrodes (6, 7) within a time window (T) ranging from the moment of one zero-crossing of the current until 1 ms before the next zero-crossing.

2. Vacuum circuit breaker according to claim 1 , characterized in that said time window (T¹) ends 2 ms before said next zero-crossing

3. Vacuum circuit breaker according to claim 1 or 2, characterized in that said time window (Tⁿ) starts 0,5 ms after said one zero-crossing.

4. Vacuum circuit breaker according to any of claims 1 - 3, characterized in that said time window (T^IM) is a predetermined part of said range.

5. A method for interrupting an alternating current by means of a synchro- nised vacuum circuit breaker having a first and a second contact electrode arranged to be separated from each other at the opening of the breaker, characterized by starting separation of the contact electrodes within a time window ranging from the moment of one zero-crossing of the current until 1 ms before the next zero-crossing.

6. A method according to claim 5, characterized in that said time window ends 2 ms before said next zero-crossing.

7. A method according to claim 5 or 6, characterized in that said time win- dow starts 0,5 ms after said one zero-crossing.

8. A method according to any of claims 5 - 7, characterized in that said time window is a predetermined part of said range.

9. The use of a vacuum circuit breaker according to any of claims 1 - 4 for interrupting an alternating current.

10. An electric power network including at least one vacuum circuit breaker according to any of claims 1 - 4.