POLARITY EFFECTS IN VACUUM CIRCUIT BREAKERS
S. W. ROWE
Schneider Electric, Centre de recherche A2, 38050 Grenoble cedex 09, France.
Abstract
The influence of the arc current and recovery voltage polarities on vacuum circuit breaker behaviour has been studied.
Polarity sensitive behaviour is illustrated for, arc voltage, post arc current and shield current.
Interactions with the fixed potential shield are shown to be at the origin of these effects.
1 Introduction
The behaviour of the vacuum arc, under various experimental conditions, has been studied by many authors over the last 30 years. This work has clarified the influence of the majority of experimental variables on arc voltage, chopping current, post-arc current, shield current, interruption capability and dielectric breakdown.
Concurrently more fundamental studies have clarified many of the physical phenomenon governing the general behaviour of the arc and in particular have attempted to separate out anode, cathode and column contributions.
The majority of these investigations were carried out either in rigorously symmetrical conditions or concentrated on one polarity only.
Industrial applications However, frequently employ asymmetrical structures, in which the vapour shield surrounding the contacts is electrically connected to the fixed contact.
This design strategy implies a strong geometrical asymmetry, modifying firstly the electrical field distribution during and after arcing and more importantly the interaction between the arc plasma and its bounding walls.
The purpose of our work was thus to investigate the ways in which the presence of a fixed potential shield could influence vacuum circuit breaker behaviour, as a function of polarity.
2 Experimental
The contact material used for the experiments described here was exclusively CuCr 75/25, machined to diameters of either 20mm or 57mm.
Experiments were mainly carried out using demountable vacuum bottles, Fig.1. An axial magnetic field could be applied by using a high current solenoid wound around the outside of the arcing chamber.
The magnetic field obtained had a maximum value corresponding to about 36mT/kA for the 20mm diameter contacts and 24mT/kA for the 57mm diameter contacts, and was adjusted by varying the proportion of the 50HZ arc current flowing through the coil, Fig.2.
Fig.1: Demountable vacuum circuit breaker,
(20mm contacts).
The bottle bakeout cycle was 300°C / 8 hours and was followed by a current conditioning procedure including twenty interruptions at 7-10kA peak.
Fig. 2: General experimental circuit diagram.
The nominal electrode separation and opening speed were respectively 8mm and 1m/s.
Currents were measured using Rogowski coils whilst the post arc currents were measured using
sensitive triple-stage, diode clipped shunts[1].
Finally, a remotely controlled, high-vacuum gate-valve allowed pumping-down between tests to about 10-7 mbar, using turbo-molecular pumps.
3 RESULTS
In this section we present a selection of the results obtained, chosen for their representativity of the general effects observed.
3.1 Arc voltage
In all cases arc voltage polarity is given with respect to the mobile contact, Fig. 1.
The variation of the arc voltage with arc current, in the absence of an external magnetic field, is shown in Fig.3. For these 20mm diameter contacts we notice a clear difference of arcing voltage depending on polarity. After an initial increase, the arc voltages stabilise at values differing by about 20%.
Fig. 3: Amplitudes of the Arc voltages and cathode drop versus arc current, in absence of axial magnetic field, = positive; = negative, (contact diameter = 20mm).
The cathode voltage drop was taken as the voltage step at the instant of contact separation.
When a magnetic field of 36mT/kA is applied, the observed polarity effect disappears, as does the rapid initial arc voltage rise, Fig.4.
For the 57mm diameter contacts the arcing voltages are 10 to 20 volts lower over the same range of currents.
3.2 Post arc Current
The polarity of the post arc current (Ipa), is defined such that negative polarity Ipa, corresponds to the case where a negative transient recovery voltage (TRV) is applied to the fixed contact with respect to the mobile contact, Fig. 1. Positive ions will therefore be attracted to the fixed contact and the shield.
The variation of the amplitude and duration of
the post arc current as a function of arc current was found to be very sensitive to polarity. The peak amplitudes can be 3 times as high in the negative polarity than in the positive one. At the same time, the overall duration of this current is between 2 and 3 times shorter Fig.5, Fig.6.
Comparison of the post arc charge amplitudes, by integration of the post arc current data, shows that important differences exist. In fact more than twice as much charge is extracted for the negative polarity, (where the shield and the fixed contact are post arc cathodes), than for the positive polarity Fig.7.
Finally, the presence of an axial magnetic field is seen to have little effect on the negative polarity case but to have a clear influence for positive polarity, especially below 8kA rms.
Fig4: Amplitude of the arc voltages versus current with a 36mT/kA axial magnetic field, , = positive; = negative.
Fig.5: Post arc current versus arc current, (contact
diameter = 20mm).
3.3 Shield Current:
Polarity of shield current (Is) is defined such that for negative polarity the voltage on the shield is negative with respect to the mobile contact Fig. 1; (positive ions will be collected by the fixed contact and shield surfaces).
Two sorts of polarity sensitive behaviour have
been observed. Firstly, the amplitudes of the shield currents are up to 5 times greater in the positive polarity for axial magnetic fields above 170mT.
A second difference can be observed by plotting positive and negative shield currents separately as functions of axial magnetic field. In this case a marked difference appears Fig.8 and Fig.9. For positive polarity the shield current falls progressively with magnetic field increase, as indicated by the majority of studies, [2]. However, in negative polarity, the shield current presents a sharp drop at a well defined value of axial magnetic field, similar to some recently published work [3].
Another important observation is that, although the average values of the shield current for zero magnetic field may be close to the often quoted limit of 8-12%, the peak values often reach 20 to 40% of the circuit current, [4]. This is especially the case for the 20mm diameter contacts.
Fig.6: Duration of the post arc current versus Arc current, (contact diameter = 20mm).
Fig.7: Post arc charge versus arc current, with and without applied axial magnetic field (contact diameter = 20mm).
4 Discussion
Discussion of the results in this section will be limited to those dealing with the post arc current. The shield current and arc voltage are dealt with in detail in the companion papers [5, 6].
The standard explanation of the origins of post arc current is based on the progressive extraction of the freely falling ions produced just prior to arc current zero[7]. Immediately following the current zero, the electrode which was originally the anode, becomes the new cathode under the influence to the transient recovery voltage.
Electrons in the decaying plasma are repelled from the cathode surface and a plasma sheath is formed. The positive ions, emitted by the original cathode continue to fall through the neutral plasma, then the sheath, and eventually reach the new cathode. As each ion reaches the cathode an electron can leave the plasma at the anode, [8].
The sheath hence expands towards the anode at a speed controlled by the velocity of the positive ions and current ceases once all the residual ions have reached the new cathode. The sheath will have effectively bridged the entire gap at this time [9].
In most cases however, models of post arc current consider symmetrical plane parallel electrodes and assume the plasma to be confined to the inter electrode region.
In the practical conditions described here, neither of these assumptions hold.
In fact the neutral plasma is likely to extend into the electrode-shield region. This is especially the case when the axial magnetic field is low, either because the actual peak value is low or because a small phase lag is instrumental in bringing the field to a low value as current zero is approached.
Fig.8: Negative shield current versus axial magnetic field.
In order to explain our experimental observations, we propose the following:
4.1 Negative post arc current
For cases where the fixed contact becomes the post arc cathode, the fixed potential shield acts as a very large extension of the latter, Fig1. A plasma sheath will hence form at its surface, in an identical manner to that explained above, and the ions initially directed towards it will be extracted directly by the shield and contribute to the measured post arc current.
The overall result will be to produce an efficient and rapid extraction process, giving rise to a high but short post arc current pulse. This greater surface area and speed of extraction may also reduce the impact of recombination processes and hence account for the higher post arc charge observed Fig.7, [10].
Fig9: Positive shield current versus axial magnetic field.
4.2 Positive post arc current
When the mobile contact becomes the post arc cathode, the active collecting surface area is restricted to its arcing surface and part of its sides only. In this case any positive ions initially directed away from the mobile contact surface will continue to follow their initial trajectories, [7].
Several things can happen to these ions.
- Firstly, some of them may intercept the positive shield from which they will be repelled and will eventually reach the cathode, somewhere along the sides of the mobile contact, Fig. 1. The average trajectory length will be considerably greater than the inter-contact distance and volume recombination will be increased.
- Secondly, some of these ions may recombine at the shield surface or, less probably, at the surface of the numerous metal droplets.
- Thirdly, some of them may move into high electrical field regions either by moving outside the main plasma, or by being caught up with, by part of the expanding sheath. In these cases they will immediately be vigorously accelerated, along the electrical field lines to the corresponding part of the cathode surface. In general, the time of arrival of this part of the ion population, will be delayed considerably with respect to the inter-contact ions.
If the above hypothesis were true we would expect the post arc current pulse to become more spread out in time. We would also expect the global charge extracted to fall in this polarity.
Both these points are observed experimentally, Figs.6, 7.
Furthermore, assuming a unique ion surface recombination process, the comparison of the two polarities allows an estimation of the recombination time constant. This was found to be roughly 10µs, which is in agreement with most published data, [10].
5 Conclusion
Our experiments have shown that several polarity sensitive effect can occur in vacuum circuit breakers. A proposition has been made to explain qualitatively the strong post arc polarity effects observed. Arc voltages and especially shield current are also shown to be strongly polarity sensitive and are treated in detail elsewhere [5,6].
6 Acknowledgements:
The author would like to extend thanks to Dr. J Maftoul for the post arc charge calculations and Dr. M. Barrault , Dr. H. Schellekens for their help and encouragement. Thanks also to D. Perret and P. Leclercq for their decisive contributions to the experimental side of this work.
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