Phenomena generated by splitter plates on low-voltage electric arc dynamics

ric DEBELLUT, Denis CAJAL, Francis GARY and Alain LAURENT Laboratoire dlectrotechnique de Montluon

Avenue Aristide Briand, BP 2235 03101 Montluon Cedex, France

cajal@moniut.univ-bpclermont.fr, gary@moniut.univ-bpclermont.fr

Abstract The breaking of an electric arc is obtained by its lengthening, its splitting up and its cooling in the quenching chamber made up of fins called splitters. However, the latter sometimes have unwanted effects. The presented study deals with the phenomena generated by their presence and their influence on the arc behaviour. The arc is observed thanks to a tool called magnetic camera based on the measurement of the magnetic field outside the device. For a given time, the magnetic picture allows the reconstitution of the average current line that represents the arc. Several tests were carried out with a quenching chamber consisting of iron splitters and prospective peak currents varying from 1370A to 6300A. Two configurations of positioning were studied. The first one consists in putting three tilted splitters located at the same level ; the second one is made up of four splitters, each with a shifted position compared to the others. Other parameters are also analyzed : arc voltage, current limiting, thermal stress, energy, break time. In general, the arc enters the quenching chamber where it is split. However, for high currents, it does not put out quickly after penetrating the splitters but oscillates for a few moments in front of them.

Electric break arc; arc behaviour; splitter plates; magnetic camera

I. INTRODUCTION In order to know how an electric break arc behaves without

impeding it, the Laboratory has developed two diagnostics methods for several years, both being based on magnetic measurements outside the apparatus studied. These methods are complementary and are jointly used. They are respectively called inverse method and magnetic camera. The inverse method uses the values of the magnetic induction measured at about ten points, then an inverse numerical solution allows the retracing of the arc dynamics. Thanks to the inverse method, we showed particular arc motions when side walls made of different materials were used [1].

For the magnetic camera, many more probes are placed but only a singular value of the induction is processed. It is mainly used when we want to focus on the analysis of the arc motion in a limited area. Moreover, this means of diagnostics enabled us to highlight a phenomenon unfavourable to the realisation of the break in a circuit breaker : the re-strike during which several arcs exist [2]. We also showed that this diagnostics remained valid in presence of magnetic materials [3].

It should be noticed that other global magnetic methods were developed : the magnetic planimeter among others [4]. In other respects, we compared the results and noted a good agreement between two means of diagnostics using optical and magnetic measurements [5]. Further, we should point out that theses equipments are heavy to bring into operation. This is one of the reasons laboratories are developing diagnostics methods based on the arc modelling [6] [7]. Previous studies [8, 9, 10] have investigated the influence of arcs runners on the arc motion.

In this paper, we aim to study the phenomena generated by the presence of several splitters on the arc motion. Indeed, the circuit breaker manufacturers lay out several splitters made of magnetic materials in the quenching chamber. The splitters are known for having current arc attraction and cooling properties. However, splitters are sometimes suspected of having harmful effects on the break. The arc may not penetrate the splitters entirely but go round them; in this way they become an obstacle to the arc. When the arc is in the splitters, a significant fusion of the metal can also occur. When the metal solidifies, it can short-circuit the splitters and involve a reduction of the arc voltage.

II. EXPERIMENTAL DEVICE

A. Electric arc production system The principle consists in establishing the discharge of a

battery of capacitors, initially loaded at a voltage U0, through an inductance L by triggering a power thyristor. The system produces a 50Hz half wave of current with a peak value which can vary from 1000 to 7300A according to the value of U0.

B. Breaking system [2] This device causes the opening of the electric circuit in

order to create the arc. The breaking system used is represented on Fig.1.

Two ceramics side walls, placed against both electrodes, confine the arc during its rise. The ceramics used is made of stumatite, a material known for having a neutral behaviour towards the arc [11].

When the splitters are made of iron, the magnetic field created by the arc is deformed. Iron being more permeable than

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the air, the lines of field tend to be confined inside the splitters. Hence, as the magnetic induction gradient is steered towards the splitters, the electromagnetic force that the arc undergoes propels it towards the quenching chamber.

Figure 1. Experimental device, side view of arc chamber.

In industrial circuit breakers, the splitters have V-shaped cuttings. This particular arrangement is supposed to increase the magnitude and the range of the magnetic forces. Actually, preliminary tests were carried out to compare the effects on both full and cut splitter plate. With more than one full splitter, the arc is stopped. This is why the splitters used for this study are also cut in order to ease the insertion of the arc (Fig.2). They are 1mm thick, 8mm wide, 120mm high (Fig.3).

Figure 2. Lines of induction near a splitter.

Figure 3. Front view of a splitter (dimensions in mm).

For the location of splitters in the quenching chamber two configurations were selected. For the first one (Fig. 4a), the three splitters were placed so that the electrode gap was regularly shared. The bottom of the three splitters was positioned at a height of 83.5mm. For the second one (Fig. 4b), four splitters were laid out vertically in the breaking chamber. Each splitter was 10 mm up compared to the immediately below.

Figure 4. Splitter and probe location (dimension in mm).

For each configuration, series of three consecutive tests were carried out. Before each series, the electrodes, the splitters and the contact were changed to have similar conditions for the different tests. A series included only three consecutive shots because after, the material was too deteriorated to undergo other tests in such conditions.

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III. THE MAGNETIC CAMERA [12] An inverse numerical resolution was developed based on a

mathematical model taking into account the magnetic contributions of threadlike segments of current, which represent the various parts of the breaking system (current feed, electrodes and arc). The arc dynamics may be displayed using non-intrusive specific magnetic measurements.

96 probes are set so that they detect the magnetic component according to the y-axis, the axis perpendicular to plane (X, 0, Z) which is also the average evolution plane of the arc.

In a previous paper [3], the detail of the principle of the "magnetic camera" was presented. Simplifying the principle, the Y-component by(t) is cancelled when the arc moves in front of a considered probe. This singular point is called a zero. However, in a breaking device, magnetic contributions both of current feeds and of the arc bent must be taken into account. The points where by(t) equals zero being known, the aim is to find the shape of the corresponding electrical circuit for a given moment. With a matrix-shaped set of probes , for a given time, we can find the position of the zeros by interpolation between sensors running on the same line or column or diagonal line.

The arc is modelled by a succession of segments of current. In these conditions, the problem consists in seeking the minimum of the following criterion:

(1)

This quantity represents the quadratic error between measured induction and calculated induction. (xSi,zSi) are the co-ordinates of the zeros and x1,z1,,xN,zN the unknowns.

A solution is obtained by using an iterative method of descent : gradient method or steepest slope method [13].

As we wanted to focus our analysis on the arc motion inside a quenching chamber, this led us to place the probes just in front the splitters (Fig. 4).

During the whole break time, the signals of the sensors were recorded at a 1562.5 kHz sampling frequency and quantized on 8 bits (256 levels).

With regard to the performance, the sampling period being 0.64 s, the equivalent speed can reach 1 562 500 images a second. Moreover, as the probes are 5mm one from the others, the arc can be detected up to a 7812.5m/s speed.

We must note that the positions of the arc roots on each of the two faces of the splitters are also unknown, like the positions of the anodic and cathodic roots. Fig. 5 represents the choice of the sought polygonal line for each configuration of the splitters. It should be noticed that a splitter can conduct a current in the first configuration.

IV. EXPERIMENTAL RESULTS

A. Description of the tests For this study, the tests were carried out with a 400V initial

capacitor load voltage and a prospective peak current having successively the values 1370A, 2520A, 4000A, 4600A and 6300A.

The signals, digitized with a 0.64s sampling period, are recorded over an acquisition time of 10 ms.

Figure 5. Arc model and voltage measurement.

B. Current and voltage measurement (Fig. 5) The arc voltage measurement is carried out as close to the

electrodes as possible in order to minimize loop effects by means of a differential probe whose attenuation factor is 100.

The splitters behave like as many additional electrodes which produce voltage drops that are measured thanks to differential probes whose attenuation factor is 50.

The total current is measured using a current sensor with a constant coefficient equal to 10-3 V/A.

C. Splitter effects In order to better characterize the influence of the splitters,

several parameters such as the maximum value of the current Imax, the arc energy E, the break time darc and the thermal stress value CT were taken into account. These criteria are generally employed to appreciate the efficiency of the break. The study of these parameters could be undertaken starting from the analyses completed on more than 200 tests.

1) Current limiting effect The maximum value of current Imax allows the calculation

of the current limiting which is expressed by the ratio between the cut-off peak intensity and the prospective intensity Ip(Fig. 6). This short-circuit current limiting has become a need for electrical systems and system protection. Indeed, the very high current switching-off using a low-volume circuit breaker cannot be achieved without a significant reduction of energy dissipated in the breaking device.

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The current limitation is obtained by the arc voltage increase. However, the two principles to increase this voltage consist in, on the one hand, lengthening the arc in the breaking system and on the other hand, propelling it against splitter plates in order to split it and thus generate additional voltage drops.

The combination of these principles makes it possible to obtain a rather high arc voltage quickly, essential factor of the current limitation.

Figure 6. Average current limiting as a function of the prospective current.

This phenomenon is well illustrated by Fig. 6, where Imax is the maximum value of the total current measured with a current sensor. A better current limitation is noted in the presence of splitter plates. Indeed, the arc, once entered the splitter plates, involves an arc voltage increase implying a current reduction.

It must be noted that the higher the current is, the more there is limiting device effect. With high currents, the arc moves and lengthens more quickly in the breaking system involving a faster voltage growth.

2) Thermal stress The thermal stress, the electrical system is subjected to

during the short-circuit (Fig. 7), is a significant parameter for the evaluation quality of the break. Actually, a good break is characterized by a significant reduction of the peak current together with a small break time, which also results in a low thermal stress. This quantity is given by the following equation :

(2)

Figure 7. Average thermal stress CT as a function of the prospective current

Splitters induce a reduction of the thermal stress for prospective peak current of 1370A, 2520A and 4000A. Beyond these values, they have contrary effects with those wanted in circuit breakers.

We can observe that configuration 2 (four splitters) gives better results than configuration 1 (three splitters) regarding the value of the thermal stress. This can be explained by the positioning of the splitters in the quenching chamber. As a matter of fact, their lower ends are shifted, the closer to the cathode the splitter is, the higher the splitter is. The arc lengthens more before it penetrates the splitters entirely. It involves a reduction of the break time (see Fig. 8), producing a lower thermal stress compared to configuration 1.

3) Arc energy The arc energy represents the energy released in the

breaking volume of a circuit breaker (Fig. 9). This energy is mainly dissipated into heat (hot gases, splitter plate heating). It is expressed by the formula :

(4)

Figure 8. Average break time darc as a function of the prospective current

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Figure 9. Average energy E as a function of the prospective current

Observing the results of Fig.9, we notice a high increase of the arc energy for prospective currents of 4600A and 6300A, induced by the presence of splitters. Indeed the arc, once into the splitters, remains a few moments instead of quickly putting out. For these tests with high currents, the arc greatly erodes the splitters. The metal vapours and the emitted particles increase the plasma conductivity that sustains the arc in the splitters. Moreover, for high currents, the number of splitters used (three or four) represents an insufficient metal mass to allow a satisfactory cooling of the arc. Thus, with currents of 4600A and 6300A, the deion plates allow the splitting-up but do not manage -while heating- to absorb enough of the energy produced in the plasma column.

Photography Fig. 10 represents the splitters after tests with prospective peak currents of 1370A and 6300A. With high currents, the splitters are quickly damaged.

Figure 10. Splitter plate erosion for low and high currents

4) Break time The break time is defined as being the time which elapses

between the beginning of the arc and its complete extinguishing (Fig. 8). The beginning of the arc is roughly determined by the sudden growth of the arc voltage.

With regard to the value of the break time, configuration 2 gives better results than configuration 1. The lengthening of darc for configuration 1 is explained by the arc stagnation inside the quenching chamber. Indeed, once in the splitters, the arc remains sustained in the quenching chamber.

The arc production system is based on the discharge of a battery of capacitors through an inductance. Thus the system does not simulate perfectly an electrical supply network in short-circuit mode. The supplied initial energy can be regarded as constant for these tests. Although the arc voltage depends on arc chamber design, generated arc voltages have roughly the same magnitude, if Imax is more important without splitter plate then the break time must be weaker. Furthermore, in ours configurations, the number of splitter plates is weak, thus there is not a sudden increasing of the voltage, therefore the current does not decrease, and thus leads to an augmentation of the break time.

The objective of this part was the comparison of the two configurations of splitters positioning. The configuration without splitter plate was used as a reference but was not compared with the others configurations. The presented study relates to the comparison of quenching chambers in order to know which configuration has a better influence on the arc behaviour. Under these conditions, of the two configurations chosen, configuration 2 gives better results (higher arc voltage, weaker break time).

D. Analysis of the arc motion using the magnetic camera In this part, the study is undertaken with the configuration

2. Several tests were carried out for prospective peak currents of 1370A, 2520A, 4000A, 4600A and 6300A.

Fig. 11 and Fig. 12 display the arc dynamics using the magnetic camera for tests carried out with two extreme current values. Moments noted t1 and t2 correspond to the time interval during which the analysis of the arc evolution was performed by using the information delivered by the probes.

We can notice that the number of probes is limited to 96 (Fig. 4), and they are placed in a so manner that, when the arc reaches the top of the electrodes, data are too very few to display arc dynamics on the top of the breaker, which corresponds to what happens after the moment t2.

In a previous study [2], we showed that the material used for the splitter plates (iron or non magnetic material, such as copper) has not a significant effect on the measurement of the magnetic field.

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Figure 11. Arc current lines dynamics for a prospective current of 2520A and four splitters

In all the cases, the arc penetrates the splitters where it is split and put out. However it should be noticed that the arc stagnates at the bottom of the splitters before entering the quenching chamber. We observe a correlation with the peaks of the arc voltage when the arc enters each splitter plate.

For prospective peak currents lower than 4000A, these stagnations are accentuated.

For currents higher than 4000A, the arc enters the splitters with more ease because of higher magnetic field.

Figure 12. Arc current lines dynamics for a prospective current of 6300A and four splitters

V. CONCLUSION The objective of this work was the study of the phenomena

caused by the presence of deion plates on the arc dynamics. These splitters intercalated on the arc course behave like as many additional electrodes. The interest is double : it allows the increase of the arc voltage by splitting it on the one hand, and the arc cooling on the other hand.

Nevertheless, it is advisable to be careful because we have shown that splitters could impede the path of the arc. Two types of splitter positioning were tested. Configuration 2 (four splitters) seems the most powerful. Indeed, the arc goes further

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in the quenching chamber leading to a faster quenching of the arc.

The study carried out in this paper showed that it is better to keep configuration 2 with a greater number of iron splitters. This would improve the break and allow the splitters to fulfil their function of splitting-up and arc cooling.

REFERENCES [1] Toumazet J.P., Velleaud G., Brdys C. and Servant S,"A study of the

influence of the wallsnature on the behaviour of a low-voltage arc breaker by means of an inverse method," J. Phys. D : Appl. Phys., vol. 32, 1999, pp. 121127.

[2] Debellut E., Gary F., Cajal D. and Laurent A, "Study of re-strike phenomena in a low-voltage breaking device by means of the magnetic camera,"J. Phys. D : Appl. Phys., vol. 34, 2001,pp. 1665-1674.

[3] Debellut E., Cajal D., Gary F. and Laurent A, "Study of the influence of the breaking-chamber structure on the electric arc behaviour thanks to the magnetic camera," Proc. 20th Int. Conf. On Electric Contacts, Stockholm, Sweden, 2000, pp. 73-78.

[4] Guillaumond F. and Haug R, "A compensation method using a planimeter to study the motion of an arc column in a disturbing environment," Meas. Sci. Technol., vol. 7, 1996, pp. 1054-1064.

[5] Toumazet J.P., Brdys C., Debellut E., Ponthenier JL, "Study of the arc dynamics by means of optical and magnetic measurements," Eur. Phys. J. A.P., vol 20, 2002, pp. 55-59.

[6] Daube T., Stammberger H., Anheuser M., Dehning C,"3D simulation of low voltage switching arc based on MHD equations," Proc. of the 9th Int. Conf. On Switching Arc Prhenomena, Lodz, Poland, 2001, pp. 168-173.

[7] Andre G., Schneider W., Rieder W, "Modelling of arc-gas flow interaction in magnet blast switching devices", Proc. 20th Int. Conf. On Electric Contacts, Stockholm, Sweden, 2000, pp. 79-84.

[8] Mc Bride J.W., Jeffery P.A., "The design optimisation of current limitting circuit breakers," 3rd Int. Conf. On Elect. Contacts., Arcs, Apparatus and their Applications, Xi'an, China, 1997, pp. 354360.

[9] Lindmayer M., Springtubbe M., "3D-Simulation od arc motion between arc runners including the influence of ferromagnetic material," 47th IEEE Holm Conf. On Electric. Contacts, Montreal, Canada, pp. 148153.

[10] Chen Degui, Yuan Haiwen, Chen Xu, "Experimental study of the interrupting process for the miniature current limiter," 3rd Int. Conf. On Elect. Contacts., Arcs, Apparatus and their Applications, Xi'an, China, 1997, pp. 361365.

[11] Toumazet J.P., Brdys C. , Velleaud G., Gary F., Cajal D., Servant S., "Inverse method applied to study the behaviour of a low voltage break arc. Influence of the breaking chamber walls nature", XIIIth Symposium on Physics of Switching Arc, Brno, Czech Republic, 1998, p. 111 - 114.

[12] Laurent A., Gary F., Cajal D., Velleaud G. and Mercier M, "A magnetic camera for studying the electric breaking-arc," Meas. Sci. Technol. 4, 1993, pp. 1043-1049.

[13] Cajal D., Laurent A., Gary F., Mercier M., Servant S., "A study of the various phases of the break in a low-voltage circuit breaker thanks to the magnetic camera," J. Phys. D : Appl. Phys., vol. 32, 1999, pp. 1130-1135.

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