US2825824A - Electric circuit breaker for interrupting capacitive circuits - Google Patents

Description

March 4, 1958 M. c. PEROLINI 2,825,824

ELECTRIC CIRCUIT BREAKER FOR INTERRUPTING CAPACITIVE CIRCUITS Filed Oct. 16, 1956 2 Sheets-Sheet 1 J T T,

A U I Q U2 V r U U Ur w r A a Marcel Perolini, b5 4 His btcn-neg.

March 4, 1958 M. c. PEROLINI 2,825,824

ELECTRIC CIRCUIT BREAKER FOR INTERRUPTING CAPACITIVE CIRCUITS Filed Oct. 16, 1956 2 Sheets-Sheet 2 Fi .4. C l

Inventor: Marcel 'Perolini,

by m

His orneg.

rear. 4, 15958 Free ELECTRE'C (CRSUIT BREAKER FOR INTERRUPT= ENG CAPAQITEVE ClRCUlTS Marcel C. Perolini, laris, France, assignor to Societe Generale dc Constructions Electriques & Mechanraues (Aisthorn), Paris, France Application @ctoher o, 12856, Serial No. dlefliifi Claims priority, application France (October 18, B55

3 Claims. (Cl. 37-2l33) This invention relates to an electric circuit breaker and, more particularly, to a circuit breaker which is especially suited for the interruption of predominantly capacitive alternating-current power circuits.

it is known that the interruption of an A.-C. circuit is considerably facilitated by the fact that the current goes through zero at each half-cycle.

But this advantageous characteristic of the alternating current is not fully utilized in the ordinary circuit breaker whose contacts are opened at any point of the current wave. Arrangements, however, have already been proposed to synchronize the separation of the contacts with the natural passage of the current through zero, so as to prevent the formation of an arc. Such arrangements are of special interest in opening short circuits.

The present invention is concerned with an improvement applicable especially to breakers intended also for opening predominantly capacitive circuits such as those consisting of open circuit lines or of banks of capacitors. In carrying out the present invention in one form, the separation of the contacts of the breaker, at least when the breaker has to open capacitive circuits, is synchronized with the current wave or the voltage wave in such a way that this separation takes place systematically with a considerable lead with respect to current zero.

I For a better understanding of the invention, reference may be had to the following specification taken in connection with the accompanying drawings, wherein:

Fig. 1 is a one-line diagram illustrating a capacitive power circuit in which the circuit breaker of my invention is especially useful.

Fig. la is a graphical representation illustrating certain electrical relationships which are present when a capacitive power circuit is interrupted.

Fig. 2 is a graphical representation similar to that of Fig. 1 but illustrating the manner in which restrikes can take place during the interruption of a capacitive power circuit.

Fig. 3 is a graphical representation similar to that of Fig. 2 but illustrating how restrikes of a more severe nature can occur during the interruption of a capacitive power circuit.

Fig. 4 is a graph illustrating the manner in which the improvement of the present invention operates to lessen the tendency toward restriking.

Fig. 5 is a schematic diagram illustrating one form of synchronizing device for carrying out the present invention.

Fig. 6 is a schematic diagram illustrating another form of synchronizing device for carryin out the present invention.

Referring now to Fig. 1, there is shown an open singlephase line Zll fed by an alternatingcurrent source 22 with the capacitance of the line being schematically represented by a capacitor 24 shown connected between the line and ground. The voltage of the source is represented by a sinusoidal curve A in Fig. 1a. Since the charging current for the line will lead the vol tage by 90, this current passes through zero at each half cycle at the same instant that the voltage goes through its maximum designated U in Fig. 1a. Consider now a circuit breaker 25 having its contacts 26 connected in series relationship between the source and the line 2%. When the breaker is tripped open, an arc is drawn and at the first current zero (say at T) which follows separation of the contacts, the voltage 53 of the open line it) remains equal to the maximum voltage U of the source 22, while the voltage of the source continues to oscillate at the normal frequency following the curve A. The voltage across the contacts of the open breaker follows the sinusoidal variation of the wave of the voltage of the source to reach after half a cycle TT a value Ur equal to twice U.

At first glance, it would seem that these conditions are not as difficult as those encountered in the opening of an inductive circuit, since the voltage amplitude across the terminals of the breaker is of the same order of magnitude and the rate of recovery of the voltage is definitely lower. But when opening an inductive circuit, the conditions of voltage recovery are repeated exactly at each current zero, while the conditions can be entirely different for successive current zeros in the case of the opening of a capacitive circuit. In fact, in the latter case, the conditicns illustrated in the diagram of Fig. la might occur only once during the opening process, that being when the current goes through zero for the first time after the separation of the contacts of the breaker. if the breaker is capable of increasing the insulation between contacts at such a rate as to support the voltage wave applied between the terminals, the circuit will be definitely opened. Otherwise, referring to Fig. 2, the arc strikes again, for example at the instant 1" more or less close to T, and the line will discharge through the source following an oscillation E whose amplitude Us might reach 3U, as indicated in the diagram of Fig. 2. Such overvoltages endanger the insulation of the equipment; transformers, lightning arresters, etc., and in modern circuit breakers attempts are made to suppress or limit these overvoltages which might rise even above 3U if the breaker is exposed to a series of extinctions and restrikings of the arc.

in very large, very high voltage systems, more complex phenomena may be encountered which lead to even greater stresses in the circuit breaker and in equipment in general. Fig. 3 illustrates an example of a known condition of this nature. Referring to Fig. 3, when the circuit is fed from a source of relatively low short circuit power having a normal voltage represented by the curve A, with a normal maximum U, the voltage at the circuit breaker will rise to some value such as U (curve A) as a result of the flow of capacitance current through the source inductance. in this same regard, the voltage increases all along the line to reach a value of U (curve B) at the open end of the line.

These voltage differences cannot subsist as soon as a breaker opens a line, for example, at the instant T. The initial voltage U then tends to assume the normal value U through an equalizing oscillation of the voltage on the line. At the end of half a cycle, the voltage across the terminals of the breaker becomes Ur, which may reach 3U. If the arc restrikes, for example at the instant T, an overvoltage Us of the order of 4U is induced in the system, endangering the insulation of the transformers and in certain cases the lightning arresters. The highest stresses occur, by the way, when the contacts of the circuit breakor separate just when the current goes through Zero or near that point. in such case, the increase in insulation of the gap between the contacts corresponds to the course covered by the contacts during half the cycle.

Ieretofore, the only way of avoiding any dangerous overvoltage when opening a capacitive circuit was to equip the circuit breakers with means for withstanding the electric stress developed beginning with the first zero current which follows the separation of the contacts. Modern circuit breakers, especially oil impulse breakers or air blast breakers, provided with several breaks in series, are frequently capable of withstanding such stresses; but in cases of the type illustrated in Fig. 3, the dimensions that the breaker will require lead to great complications in the design and to an increase in cost.

The application of the improvement proposed by the invention avoids or reduces the complications in design and the increase in cost, as will be shown in connection with the diagram of Fig. 4.

In this figure, A is the voltage wave of the source and C is the current fed by this source to a capacitive circuit, the current leading the voltage by 90.

It will first be assumed that the contacts are separated at the instant T (current zero, maximum voltage), as may be encountered in ordinary breakers, the voltage between the separated contacts of the breaker being shown separately by the curve I. The circuit will be opened if the dielectric strength of the fluid between the contacts is increased as represented, for example, by the line L. If the increase in the dielectric strength of the fluid follows the line M, the arc will restrike at the point N and produce the overvoltages mentioned before.

If, on the contrary and according to the invention, the separation of the contacts of the circuit breaker is synchronized with the current Wave or the voltage wave of the current to be interrupted in such a way that this separation will take place systematically considerably in advance of the current zero, for instance at the instant t, a lead of a quarter cycle, the increase in the dielectric strength of the fluid between the contacts of the breaker will initially follow a much steeper line such as OP beginning with the extinction of the arc (current Zero), then the line Q, parallel to M, but one-quarter cycle ahead of M. In a circuit breaker so synchronized, the rate of increase of the dielectric strength of the fluid between the contacts required to extinguish the arc may thus be re duced for example as indicated by the line Q.

The above-described synchronization of the separation of the circuit breaker contacts with the current wave or the voltage wave of the current to be interrupted may be obtained by any suitable means, especially by utilizing presently known synchronization devices, which make it possible to give an order at a given instant of a current wave or a voltage wave, this order being transmitted to the solenoid or any other tripping device of the circuit breaker at a selected instant, to obtain the above-defined separation of the contacts.

Referring to Figs. 5 and 6, examples will be described in which known synchronizing devices are used to carry out the invention. It is to be understood that other suitable synchronizing devices may also be used without departing from the scope of the invention.

In the example of Fig. 5, there is shown a conventional circuit breaker 2.5 which is to have the separation of its contacts 26 synchronized with the current wave or voltage wave in the single-phase circuit comprising the two conductors 26a and 28b. The contacts 26 of the breaker are biased toward open position by suitable spring means 30 and are normally held in closed position by a suitable latch 31 controlled by a tripping solenoid 1. This tripping solenoid is assumed to be connected in series with a suitable direct-current source 2.

The synchronizing device comprises suitable means 3 for deriving from the power circuit 20a, 2% an alternating voltage which makes a known and constant phase angle with the sinusoidal current to be interrupted. the disclosed embodiment, this means 3 takes the form of a suitable potential transformer connected across the two conductors 20a, 2% of the single phase power circuit.

The output terminals of this potential transformer 3 are connected to a'phase advancer of conventional form,

(fat such as the bridge-type phase advancer schematically shown at 4. This phase advancer comprises one set of opposite arms, each of which is constituted by a variable resistor 4a, and another set of opposite arms each of which is constituted by a variable condensor 4b. The junction point between one of the resistors and one of the condensors is connected to one output terminal of the potential transformer 3, and an opposed junction point is con nected to the other output terminal of the potential transformer 3. A sinusoidal voltage appears across the other two terminals of the bridge type advancer, and the phase relation of this voltage to the output voltage of the potential transformer 3 can be regulated by suitably adjusting the impedances of the various arms of the phase-advancer.

This sinusoidal output voltage from the phase advancer 4 is transformed into pulses by means of a conventional peaking transformer 5 or any other suitable peak generator. The positive pulses, if they have a sufiiciently large amplitude, are used to unblock a thyratron s which closes the circuit of the tripping coil 1 of the breaker.

The tripping order to the circuit breaker is given by amplifying the voltage peaks supplied by the peaking transformer 5 to the grid 6a of the thyratron.

This amplification is accomplished, for example, by closing a set of contacts 7 which are connected in shunt with a portion of a resistor 70 connected across the output terminals of the peaking transformer 5. When the contacts 7 are open, the voltage pulses supplied by the peaking transformer 5 to the thyratron grid 6a are of insufficient amplitude to fire the thyratron. Closing these contacts 7, however, eliminates the voltage drop which had previously taken place across the now-shunted part of the resistor 7a and, accordingly, abruptly increases the voltage of the lead 7b, which is connected to the thyratron control grid. This increased voltage is sufficient to fire, or unblock, the thyratron 6. Unblocking of the thyratron immediately allows sufiicient current to flow from the source 2 through the tripping solenoid 1 so as to cause the solenoid to operate to release the latch 31, thereby allowing the spring 30 to separate the contacts 26.

The bias of the thyratron grid 6a is appropriately controlled by means of a potentiometer 12 having its positive terminal connected to the cathode 6b of the thyratron and its voltage tap 12a connected to the thyratron grid through the resistor 7a. By adjusting the setting of this voltage tap 12a of the potenetiometer, the amplitude of the voltage pulse which will be required to fire the thyratron can be suitably adjusted.

In accordance with the present invention, the phase advancer 4 is so adjusted in relationship to the phase angle of the capacitive power current that the above-described voltage pulses fire the thyratron 6 at an instant which produces parting of the circuit breaker contacts 26 considerably in advance of current zero of the capacitive current. Thus, it will be apparent that the synchronizing device of Fig. 4 is sensitive to the phase angle of the capacitive current in the circuit 20a, 29b and acts to cause the contacts of the breaker 25, upon opening, to part at an instant considerably in advance of current zero of the capacitive current.

In the example shown in Fig. 6, use is made of a timed rotating contact 8 driven by a synchronous motor 9 suitably connected to the pulsation of the capacitive current to be interrupted. This contact 8 engages a conductive segment 8a during a portion of each one of its revolutions and is out of contact with the conductive segment 8a during the remaining portion of the revolution. Thus, it will be apparent that the contacts 8, 8a are sensitive to the phase angle of the capacitive current in the main power circuit and can be arranged to engage during any preselected portion of a cycle or half-cycle of such current.

As shown in Fig. 6, the contacts 8 and 8a are connected in series with a suitable control power source 2; a set of initiating contacts 7, which may be selectively or otherwise suitably controlled; the coil of a holding relay 1t) and the tripping solenoid 1 of the circuit breaker. When the contacts 7 are operated to closed position and the contacts 8, 8a make, a circuit is completed for the tripping solenoid 1, and the solenoid responds by tripping the breaker. The holding relay 10 is an instantaneously-acting relay which closes to establish a shunt path around the contacts 8, 8a as soon as the tripping coil is energized and thus assures that the tripping operation will be completed even though the contacts 8, 8a move out of engagement prior to such completion.

In accordance with the present invention, the contacts 8 and 8a make at a point 40 selected in relationship to the phase angle of the capacitive power current that the tripping solenoid trips the breaker to cause separation of the breaker contacts at a point considerably in advance of current zero of the capacitive power current.

In a three-phase circuit, the application of the present invention is especially easy when the breakers have separate poles, which is usually the case in very high voltage systems. In this case, each pole of the breaker is provided with its own tripping device, which may this be controlled by a separate synchronizing device for each phase.

The capacitive currents to be normally interrupted do not exceed a few hundred amperes. On the other hand, the use of a synchronizing device for interrupting short-circuit currents is of interest only when they exceed several thousand amperes. In carrying out my invention in one form, each pole of the circuit breaker may be provided with two synchronizing devices, one responding, for example, to currents up to 1000 amperes and covering the range of the capacitive currents, and the other responding only to currents exceeding 5000 amperes, for example. Depending upon the magnitude of the current at the instant when the breaker is to be tripped, the proper synchronizing device will go into action and cause the separation of the contacts considerably in advance of the current zero when a maximum current of 1000 amperes is to be interrupted, for example, or nearly at the current zero when a short circuit current has to be interrupted.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects and I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Means for isolating a predominantly capacitive power circuit from a source of alternating current comprising: a circuit breaker having contacts adapted to be connected in series relationship between said source and said circuit and operable upon opening to isolate said circuit from said source, and means sensitive to the phase angle of the capacitive current in said circuit for causing said contacts upon opening to part at an instant considerably in advance of current zero of said capacitive current.

2. In a circuit breaker for interrupting the current which flows in a predominantly capacitive alternatingcurrent power circuit, opening means for separating said contacts in response to the reception of a tripping impulse, and synchronizing means sensitive to the phase angle of the capacitive current in said circuit for delivering a tripping impulse to said opening means at such an instant that the opening means parts said contact at an instant considerably in advance of current zero of said capacitive current.

3. Circuit interrupting means for isolating a predominantly capacitive power circuit from a source of alternating current comprising: a circuit breaker having contacts adapted to be connected in series between said source and said circuit and operable upon opening to isolate said circuit from said source, and means including a synchronizing device sensitive to the phase angle of the capacitive current in said circuit for causing said contacts during an opening operation to part as an instant considerably in advance of current zero of said capacitive current.

No references cited.

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