US3254268A

US3254268A - Protective system for capacitance serially connected with inductive apparatus

Info

Publication number: US3254268A
Application number: US301500A
Authority: US
Inventors: Raymond J Radus; Jr John J Astleford
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1963-08-12
Filing date: 1963-08-12
Publication date: 1966-05-31
Anticipated expiration: 1983-05-31
Also published as: GB1028593A; DE1513099A1; ES303000A1; DE1513099B2

Description

May 31, 1966 R. J. RADUS ETAL PROTECTIVE SYSTEM FOR CAPAC ITANCE SERIALLY CONNECTED WITH INDUCTIVE APPARATUS Filed Aug. 12, 1965 Fig.3.

4 $heets-Sheet 1 LOAD LOAD LOAD LO AD LOAD INVENTORS Raymond J. Rodus and John J. Asfleford,Jr.

dmbui ATTO R NEY May 31, 1966 R. J. RADUS ETAL PROTECTIVE SYSTEM Filed Aug. 12 1963 FOR CAPACITANCE SERIALLY CONNECTED WITH INDUC'I'IVE APPARATUS 4 Sheets-Sheet 2 V max Vc Fig.4A.

B I M M V II I --V mc|x [A A A H948 0 p May 31, 1966 R. J. RADUS ETAL 3,254,268

PROTECTIVE SYSTEM FOR CAPACITANCE SERIALLY CONNECTED WITH INDUGTIVE APPARATUS Filed Aug. 12 1963 4 Sheets-Sheet 5 Fig.9.

| l 8. l I us 86 88 H2 H2 ll4' I I "6 H8 j III III/IIIIIIII III/III III I III III/II) y 1956 t R. J. RADUS ETAL 3,254,268

PROTECTIVE SYSTEM FOR CAPAC ITANCE SERIALLY CONNECTED WITH INDUCTIVE APPARATUS Filed Aug. 12 1965 4 Sheets-Sheet 4.

United States Patent f PROTECTIVE SYSTEM FOR CAPACITANCE SERI- ALLY CONNECTED WITH INDUCTIVE APPARA- TUS Raymond J. Radus, Monroeville, and John J. Astlcford, Ji'., Sharon, Pa., assignors to WestinghouseElectric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 12, 1963, Ser. No. 301,500 11 Claims. (Cl. 317-14) This invention relates in general to electrical apparatus, and more particularly to protective systems for electrical apparatus.

Voltage regulation of electrical distribution systems has always been a particularly difficult problem. One method employed to improve the voltage regulation in an electrical distribution system is to provide transformers with tapchanging apparatus, and either automatic or manual means for operating the tap-changing equipment. This method is expensive compared to the cost of the associated transformer and has the disadvantage of allowing voltage to only be changed in steps, plus the additional maintenance produced by the tap-changing system.

Another method of voltage regulation involves connecting a capacitor in series circuit relation with the primary winding or the winding connected to the alternating potential source, of the transformer. The voltage drop across the capacitor varies with the load current being carried by the transformer and compensates for at least. a portion of the voltage drop across the overall impedance of said transformer, and for at least a portion of the voltage drop across the feeder line connected to said transformer in an electrical distribution system. This method, however, was not practical for several reasons. One of the reasons is the abnormally large and distorted exciting currents that may be produced when a saturable inductance, such as a transformer, is connected in series circuit relation with a capacitor. The large currents produced by this phenomenon, which is commonly called ferro-resonance, are not transients, but persist as a steady state condition until the circuit is interrupted or the equipment is damaged. Another reason involves sub-synchronous motor operation. Under certain conditions an induction motor supplied through a power line containing a series capacitor will operate as a generator, supplying current of lower than line frequency, with the excitation being supplied by the capacitor. Thus, large sub-synchronous currents are generated due to the reduced reactance of the supply circuit to the lower frequency, resulting in large voltage swings. This phenomenon will generally occur at reduced motor speeds, such as during start-up or during overloads, causing the motor to lock into step at subsynchronous speed, vibrate excessively, produce large current pulsations and voltage swings.

Further, the high voltages that may be built up across the capacitor due to load changes and short circuit current made the'capacitor too costly, and an inexpensive means for limiting the capacitor voltage to a value which would make the capacitor cost attractive was not avail-, able.

With the advent of the type of transformer which utilizes metallic strip or foil to form its windings and which has at least a portion of its windings interleaved or arranged to form a predetermined capacitance between said windings, which is effectively connected in series circuit relation with the primary winding and therefore aiding voltage regulation, the problem of an inexpensive means to effectively dampen ferro-resonance and subsynchronous motor operation, and to limit the voltage across the capacitive portion of the transformer has be- 3,254,268 Patented May 31, 1966 1 V-afzdi where i is the primary current of the transformer thus, load changes, overloads and short circuits will produce high capacitive voltages.

Another factor which must be considered when utilizing a series capacitor is that unless some provision is made to remove the capacitance from the circuit upon short circuit, tremendous short circuit currents will flow. This is due to the fact the inductive reactance of the transformer has been reduced or cancelled by the capacitive reactance.

Protective circuits of the prior art for series connected capacitors have disadvantages in not protecting the capacitance and associated apparatus against all of the abnormal conditions. that may be produced by series connected capacitance. For example, the protective system may adequately protect the capacitance against over voltage due to load changes, but may not dampen ferroresonance, allowing associated transformer apparatus to be damaged. Or, if the protective system dampens ferroresonance, the system may be deficient in not damping the circuit conditions which cause sub-synchronous operation of motors connected to the'power distribution system.

Therefore, it is desirable to have an inexpensive, static protective means for quickly damping unstable circuit conditions, such as ferro-resonance and sub-synchronous motor operation, and for shorting out the effect of the capacitance when the voltage across said capacitance reaches a predetermined magnitude. Further, the protective means must be able to continuously provide protection without deterioration due to repeated operation. of the protective means, and the protective means must allow the transformer to be restored to normal operation upon the termination of an abnormal condition.

Accordingly, it is an object of this invention to provide new and improved protective means for electrical apparatus.

Another object of the invention is to provide a new and improved protective system for inductive apparatus.

Another object of this invention is to provide a new and improved protective system for capacitive apparatus.

A further object of this invention is to provide new and improved protective means for a system utilizing inductive apparatus, and series connected capacitance to aid voltage regulation.

Still another object of this invention is to provide a new and improved protective system for inductive apparatus of the type which has its windings arranged to provide a series capacitance which effectively aids voltage regulation.

Another object of this invention is to provide a new and improved protective system for power distribution systems utilizing series connected capacitance which prevents over voltages due to load changes and short circuits and which dampens unstable circuit conditions.

Briefly, the present invention accomplishes the above cited objects by providing a protective system for power distribution systems which utilize series capacitance, and for inductive apparatusof the type which utilize series capacitance in the primary circuit, such as transformers. The capacitance in the power distribution system may be used to regulate the feeder voltage to a plurality of distribution transformers. The capacitance in the primary circuit of an individual transformer may be effective capacitance, produced by certain types of windings and winding arrangements, or actual capacitance produced by physically connecting a capacitor in the primary circuit of said transformer. In general, the protective system shunts the capacitor, or capacitor section of the transformer windings, at the instant the voltage across the capacitor exceeds a predetermined maximum. The capacitance is discharged by the shunting action of the pro tective system, and then the shunt-ing act-ion automatically ceases, allowing the capacitor to recharge.

If the condition which caused the high current and consequently the high voltage across the capacitance still exists, the capacitance will recharge and the protective system will again shunt the capacitance at the predetermined maximum voltage, causing the capacitance to again discharge. This action continues until the abnormal condition ceases, allowing normal operation with the capacitance connected in the circuit, or until other protective apparatus such as circuit breakers or fuses associated with the transformer removes the transformer from the feeder circuit.

More specifically, the protective system comprises a pair of electrodes, with a predetermined spacing or gap between them, connected across or in shunt with the series capacitance. When the voltage across the capacitance, and hence across the electrodes, reaches a voltage which causes an are between the pair of electrodes, the capacitance will be effectively shunted from the inductive circuit, causing the capacitance to disharge, and allowing the full leakage reactance of the inductive apparatus to oppose the flow of current. In order to prevent the arc from continuing after the capacitor has substantially discharged, thus damping any unstable circuit conditions, as

well as preventing burning and pitting of the electrodes, which would cause unpredictable operation on subsequent protective operations, a magnetic field is disposed perpendicular to the arc. The magnetic field causes the arc to move across the face of the electrode and hence prevents the arc from overheating any one particular area or spot. The size of the electrodes are such that the heat produced by the arc and current flowing through the electrode is insufficient to substantially affect the electrode spacing or gap, and insufficient to initiate thermionic emission which reduces the breakdown voltage of the gap. Thus, the maximum voltage at which the protective system will operate u-pon repeated operations is substantially unaffected. More specifically, the size of the electrodes depends upon the speed of the arc across the face of the electrodes. The faster the movement of the arc, the longer the electrodes will have to be, but the depth of the electrodes can be lessened, as the heating of any one particular area has been reduced. The proper depth and length of the electrodes are very important in that they must not be too small. To obtain the unusual, unexpected results of the protective system, the depth must provide a heat sink which is adequate in preventing excessive heating, and the length must be sufficient to :blow out or extinguish the arc.

The are power required to sustain the arc must be maintained at a maximum, thus starving the arc and quenching it immediately upon discharge of the capacitance, to prevent follow current from the electrical system from flowing through and sustaining the arc. The rapid quenching of the arc is due to the combination of maintaining the electrodes at a relatively cool temperature preventing thermionic emission, and the rapid movement of the are over a sufiicient length of electrode. An are moving rapidly over a relatively cool surface requires a maximum of energy to sustain the arc. Therefore, when the capacitance discharges, the arc extinguishes, as the energy required to sustain the arc is greater than available from the circuit. I

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention an embodiment of the invention;

FIGS. 2 and 2A are schematic diagrams illustrating another embodiment of the invention;

FIG. 3 is a schematic diagram illustrating another embodiment of the invention; v

FIGS. 4A, 4B, and 4C illustrate graphically certain voltage-current relationships explanatory of the operation of the invention;

FIG. 5 shows oscillograms illustrating abnormal capacitive voltage and capacitive discharge current when a transformer is excited through series capacitance;

FIG. 6 is a schematic diagram illustrating another embodiment of the invention;

FIG. 7 is a side elevation, partially in cross section, illustrating one embodiment of the invention;

FIG. 8 is a front elevation, partially cut away, of the embodiment of the invention shown in FIG. 7;

FIG. 9 is a top view, with the cover removed, of the embodiment of the invention shown in FIGS. 7 and 8;

FIG. 10 shows a front elevation of a transformer, partially cut away and partially schematic, showing one method of mounting the protective system of FIGS. 7, 8 and 9 relative to a transformer;

FIG. 11 is a front elevation, in section of another embodiment of the invention; and

FIG. 12 is a front elevation, in section of still another embodiment of the invention. 1

Referring now to the drawings, and FIG. 1 in particular, there is shown a schematic diagram illustrating the protective system 10 connected to prevent the voltage across the capacitive section 12 of transformer 14 from exceeding a predetermined maximum. The construction of transformers, such as shown schematically in FIG. 1, whereby a predetermined capacitance is formed by various types of windings and winding arrangements, such as interleaving sections of foil constructed primary windings, is described in a copending application by H. W. Book, Serial No. 248,839, filed December 27, 1962, and assigned to the same assignee as the present application.

FIGURE 1 illustrates a transformer having primary winding

sections

16 and 18 and secondary winding 20 inductively disposed on a magnetic core 22. The

primary sections

16 and 18 have certain portions interleaved to form a predetermined capacitance 12 between the windings and insulation. The capacitance 12 is effectively connected in series circuit relation with primary winding 18 from terminal 24 of primary winding 16 to terminal 26 of primary winding 18, and therefore, the primary current flowing from alternating

current input terminals

28 and 30 flows through capacitor 12. The primary current flowing through series capacitance 12 produces a voltage across capacitance 12 which offsets or cancels at least a portion of the voltage drop across the effective inductance of the transformer, aiding voltage regulation.

Some protective means, however, must be provided to dampen unstable circuit conditions, such as ferro-resonance and sub-synchronous motor operation, and to limit the maximum voltage across capacitance 12 and the portion of the

primary windings

16 and 18 arranged to produce said capacitance. The capacitive voltage can reach dangerous and damaging magnitudes when the primary current increases, such as during inrush transients, load changes or steps, and short circuits. It is essential that the capacitance or capacitor section of the transformer be protected from the overvoltages to prevent puncture or failure of insulation.

The protective means should be inexpensive to manufacture, since the distribution transformers they will be called upon to protect are relatively inexpensive, and the protective means must restore the effect of the capacitance to the circuit when unstable circuit conditions have ceased and the capacitor voltage drops below a predeter mined minimum. Further, the protective action should be repeatable without substantially impairing the effectiveness of the protective means, and should provide protection instantly during the first half cycle of over-voltage.

A protective means having the desired characteristics is shown schematically in FIG. 1 at 10. Protective means is connected to effectively shunt the effect of the capacitance 12 and protect capacitance 12 from overvoltage -when the voltage drop across said capacitance reaches a certain magnitude. Since, as hereinbefore stated, the equivalent circuit .of FIG. 1 would show capacitance 12 lumped into one capacitance and connected between

terminals

24 and 26 as shown in FIG. 2, the protective means 10 may be connected to

terminals

24 and 26.

In general, protective means 10, which is shown in an enlarged view in FIG. 1A, comprises

electrodes

32 and 34 connected to

terminals

24 and 26, through

conductors

36 and 38, respectively.

Electrodes

32 and 34, which may be blocks of copper, or any other suitable conductive material, are disposed with similar faces adjacent to each other to form a small air gap or spacing 40. The size of gap 40 is determined by the voltage across capacitance 12 at which it is desired to shunt said capacitance. The breakdown voltage across gap 40 is a direct function of the gap length at substantially 140 volts per .001" in the range of .010" to .015". For example, at a gap length of .015"; the gap breakdown voltage is substantially 2000 volts peak. To prevent a substantial change in breakdown voltage due to the heating and expansion of

electrodes

32 and 34 after several successive operations or voltage breakdowns, the electrode should have a depth *sufiicient to provide a substantial heat sink effect. For example, in one application rectangular-blocks of copper 3 long, /2 wide and 1%" deep have been found to provide an adequate heat sink to prevent the air gap from substantially changing due to the expansion of the electrode, as well as preventing thermionic emission and burning and pitting of the electrodes.

The surfaces of the

electrodes

32 and 34 which face each other to form the gap 40 may be substantially flat, or they may be slightly tapered from the center of the facing surfaces to the edges, or may have any other suitable configuration.

The use of

electrodes

32 and 34 alone, however, to shunt capacitance 12 upon the voltage across the capacitance 12 reaching a certain maximum magnitude is not sufficient. The are will not extinguish when the capacitance 12 discharges, but continues to carry the current flowing through primary winding 18 until the current alternation goes to zero. If the arc is not quickly extinguished after the capacitance discharges, unstable circuit conditions such as ferro-resonance and sub-synchronous motor operation are not dampened. Further, the heat produced by the arc is sufficient to melt the electrodes with resulting pits and burrs if left to concentrate on one small area of the electrode face. Thus, accurate repeatability of the operation of the protective system 10 would not be possible, and the electrodes would have to be replaced after only a few voltage discharges across the gap, and the transformer and associated apparatus may be damaged since ferro-resonance and sub-synchronous motor operation would continue. In order to prevent the are from continuing after discharge of capacitance 12, and to prevent concentration of the are on a small surface area of the faces of the

electrodes

32 and 34, a magnetic field is created, which is substantially perpendicular to the are, by

magnet members

42 and 44.

Magnet members

42 and 44, which are disposed on opposite sides of gap 40 with their respective poles being arranged to attract each other, may be permanent magnets or electromagnets, with permanent magnets being preferred since they do not require any external connection.

By introducinga strong magnetic field across gap 40 by

magnets

42 and 44, when an arc is produced across

electrodes

32 and 34, the arc will move across the surface of the electrode due to the reaction between the magnetic field and electric current. The arc is like a conductor of electricity in an electric motor which moves when a current is passed through it while it is being subjected to a mag netic field. The intensity of the magnetic field and are current will determine how fast the arc moves. The intensity of the magnetic field, however, is not critical, requiring substantial changes in field intensity to produce noticeable changes in arc movement. For example, in one application utilizing a 2000 volt gap, a magnetic field of 1000 gauss was found to give sufiicient arc mobility. However, situations where a stronger field is required will be examined hereinafter. With the movement of the arc across the adjacent surfaces of

electrodes

32 and 34, no overheating is experienced and the energy required to sustain thearc is maintained at a maximum. The are is promptly extinguished after the capacitance 12 has substantially discharged as the circuit is unable to supply the energy required to sustain the arc. The electrodes are, therefore, able to perform over long periods with accurate repeatability and little or no damage to the adjacent surfaces forming the gap 40.

In describing the operation of protective means 10 and how it protects transformer 14 by shunting the capacitance 12 when the capacitive voltage reaches a predetermined maximum, references will be made to the graphical representations shown in FIGS. 4A, 4B, and 4C. FIG. 4A is a graph illustrating the voltage waveform across the capacitance 12, and, therefore, across the gap 40 of

electrodes

32 and 34, for various transformer primary current magnitudes, as shown in FIG. 4B. As illustrated, the first cycle of primary current I produces a volt-age V across capacitance 12 which is within the maximum capacitor voltage V max., as determined by the spacing of

electrodes

32 and 34 of protective system 10. The second and third cycles of primary current I increase in magnitude sharply, which tends to produce a capacitive voltage drop V which would exceed the maximum capacitive voltage V. max., as indicated by the dotted portions of the second and third cycles of voltage -V in FIG. 4A above line V max. The voltage V rises along the sine wave until the maximum capacitive voltage V max. is reached, at which point the voltage across gap 40 breaks down and establishes an are between

electrodes

32 and 34. The capacitance 12 quickly discharges, dropping the capacitive voltage V to substantially zero at point 52. The magnetic agitation or movement of the are due to

magnet members

42 and 44 cooperates with the heat sink effect of the

electrodes

32 and 34 to extinguish the arc, and the capacitance 12 immediately starts to recharge until the capacitive voltage V again reaches V max. at point 54. The capacitance 12 discharges to point 56, the arc is extinguished by the magnetic movement of the arc, and the capacitance again charges to the maximum capacitive voltage V. at point 56. The capacitance 12 again discharges to point 58 and begins to charge again, but this time the voltage sine wave envelope has dropped below the maximum capacitive voltage V allowing the capacitive voltage to only reach point 60. At point 60 the capacitance discharges and follows a sine wave configuration to zero and polarity reversal, where the same process continues in the negative portion of the cycle. The number of times the maximum voltage V max. is reached during a half cycle is a direct function of current magnitude, as evidenced by formula This protective action of protective system 10 continues as long as the capacitive voltage tends to exceed the maximum capacitive voltage V as determined by the protective system.

FIG. 4C illustrates the voltage V across capacitance 12 utilizing a protective system similar to protective system 10 except with the

magnet members

42 and 44 relow the maximum capacitive voltage V max.

moved. The voltage during the second cycle rises until the maximum capacitive voltage V max. is reached at point 62, which causes capacitance 12 to discharge and drop the voltage across capacitance 12 to substantially v zero at point 64. However, instead of the arc extinguishing, as shown in FIG. 4A, which allows the capacitive voltage V to again build up, the arc continues. The energy required to sustain the arc has been reduced by thermionic emission and expansion of the electrodes, and line current flows through the arc until the alternation of the line current goes through Zero at point 66. The are is extinguished as the current alternation goes through zero and the same sequence repeats itself during the following half cycles until the capacitive voltage V drops b The value of

magnet members

42 and 44, and their cooperation with

electrodes

32 and 34 in protective system is, therefore, apparent. The

magnet members

42 and 44 extinguish the are after capacitance 12 has substantially discharged by moving the arc across the face of

electrodes

32 and 34 preventing thermionic emission and prevent burning damage to the electrode surfaces. Without

magnet members

42 and 44 the arc would not only be stationary, causing burning and melting of the electrodes, but the arc would be allowed to continue, due to the reduced amount of power required to sustain the are, from the point of the maximum capacitive voltage V max. until the current alternation goes through zero. Operation without

magnet members

42 and 44 has no damping effect on the unstable circuit conditions of ferro-resonance and sub-synchronous motor operation. Also, after a few voltage cycles, the electrodes would no longer be useful as a part of the protective system. The pits and burrs caused by the burning arcs would immediately change the effective gap and, therefore, completely change the breakdown voltage of gap 40.

It is to be understood that although the voltage and current wave forms shown in FIGS. 4A, 4B, and 4C are shown in phase, in actual practice the voltage and current may be out of phase.

The protective system 10 shown in FIG. 1 protects against excessive capacitive voltage caused by overloads,

load changes, short circuits'andhas been proven to be completely adequate in damping the conditions which produce and sustain sub-synchronous motor operation and ferro-resonance. FIGURE 5 shows oscillograms of capacitor voltage V and capacitor discharge current I illustrating the damping of ferro-resonace. When the capacitor voltage V reaches the breakdown voltage of the protective system 10, such as at points 51 and 53, the capacitor discharges, as shown at

points

55 and 57. Without the protective system 10, the capacitor voltageV would reach damaging magnitudes, and the initial voltage wave form V would be sustained as a steady state condition. However, with the protective system 10, the maximum capacitor voltage V is controlled, and the unstable condition is quickly dampened, as shown by the reduced capacitor voltage wave forms 59 and 61, and the substantial elimination of disturbance at 63.

The theory behind the operation of protective system 10, and why

magnet members

42 and 44 cooperate as they do with

electrodes

42 and 44 to extinguish the arc after the discharge of capacitance 12, stopping any flow of 60 cycle primary current through the electrodes, is due to deionization or magnetic blow-out. The rapid movement of the are over cool electrodes maintains the power required to sustain the arc at a maximum, thus quickly extinguishing the arc when the circuit power falls below the sustaining value. The exact theory however, behind the excellent results obtained by the protective system described herein in damping unstable circuit conditions is not known. However, in addition to the arc

movementby magnet members

42 and 44, the discharge time of the capacitance 12 has been found to be one of the controlling factors. If the capacitive discharge time is very short, arc extinguishment closely follows the discharge of the ca- 8 pacitance. If the capacitive discharge time is lengthened, the arc is not extinguished until much later in the half cycle, and the circuit does not dampen ferro-resonance and sub-synchronous motor operation.

In examining the effect of capacitive discharge time, reference will be made to FIGS. 2 and 2A. FIG. 2 is a schematic diagram which may be the equivalent circuit of FIG. 1, whereby the distributed capacitance between

windings

16 and 18 is lumped into one capacitance 70 connected in series circuit relation with primary Winding 18, or capacitance 70 may represent an actual capacitor connected in series circuit relation with primary winding 18 of transformer 14, as illustrated in FIG. 2A. The discharge time of capacitance 70 is determined by the discharge circuit resistance 72, the value of the capacitance.

in the discharge circuit, and the value of the inductance in the discharge circuit. The inductance can certainly be neglected where capacitance 70 represents an actual capacitor, and is insignificant when capacitance 70 is produced by transformer winding arrangements because of the magnetic symmetry in the transformer windings. The resistance 72 of the discharge circuit is variable and may be changed to observe the effect of different dis-charge times of capacitance 70. Since resistance 72 represents the total circuit resistance, its minimum value is the internal resistance of the capacitance. For example, in a 7200 volt system, a capacitive reactance of 5%, an air gap of .015, and using

magnetic members

42 and 44 which establish a 1000 gauss magnetic field perpendicular to the are, it was determined that the resistance of the discharge path should be kept below 2 ohms, as starting at substantially 2 ohms the system will not reliably recover from the ferro-resonance that occurs at no load inrush. No load inrush is the most severe test that the protective system 10 must handle, demonstrated by the fact that with a discharge circuit resistance of 2 ohms the protective system will still handle load changes satisfactorily;

When the discharge circuit resistance was increased to approximately 4 ohms, with a 1000 gauss magnetic field, a change of load very often puts the system into ferroresonance and recovery was not reliable. However, to illustrate the complex interaction between circuit discharge resistance, capacitive discharge time, and strength of the magnetic field, it was found that recovery from no-load inrush ferro-resonance could be made'reliable in the above example by increasing the magnetic field to 2900 gauss. Therefore, in this example, if the resistance of the discharge circuit, including the resistance of the interleaved sections of the transformer windings, if the capacitance is created by winding configurations and arrangements, or the internal resistance of the capacitor if an actual capacitor is used, is below two ohms, a 1000 gauss magnetic field may be used and the protective system will operate satisfactorily under all conditions, including no-load inrush, load changes and short time short circuit currents.

It is to be understood that the specific air gap, magnetic field, critical resistance values etc. given herein are for specific examples. Therefore, if capacitors having a different capacitive reactance are used, or the system voltage is different, the air gap required may change. If the air gap changes, the critical value of discharge circuit resistance may change, as well as the desired magnitude of the magnetic field.

If the discharge circuit resistance cannot be reduced below the critical value, the discharge current pulse width is increased and the arc between

electrodes

32 and 34 of protective system 10 is not extinguished at the end of the capacitive discharge, allowing 60 cycle per second current or line current to be conducted through the gap 40. Increasing the magnetic field strength aids in extinguishing the arc, as hereinbefore shown. However, instead of resorting to very high magnetic fields with a more violent arc movement and blowout action to extinguish the are when the discharge circuit resistance exceeds the critical value, another method has been found to be more suitable, which is illustrated schematically in FIG. 3.

'zero soon enough after gap breakdown to permit the arc to be extinguished before 60 cycle or line currentbegins to conduct through the gap 40. The addition of inductance 74 will effectively match a-discharge circuit having relatively high internal resistance (approximately to 12 ohms) to the protector spark gap 40, allowing the arc to, t be extinguished before 60 cycle per second line current begins to flow through the arc.

FIG. 6 illustrates an embodiment of the invention whereby a series capacitance 200 is utilized in a power distribution system, including power source 201 and load circuits 203, to aid voltage regulation, but is not directly associated with any one particular distribution transformer. More specifically, protective system 10 is connected to protect series capacitor 200, which is connected in power distribution system 202 to provide voltage regulation for a plurality of distribution transformers 204. Transformer 206 is shown merely to illustrate that there may be other voltage transformations before the voltage is applied to the distribution transformers 204. The operation of protective system 10 is as hereinbefore described. The fact that current for a plurality of distribution transformers, instead of for one particular transformer, flows through capacitor 200 does not affect the operation of the protective system.

FIGS. 7, 8 and 9 show side and front elevations and the top view, respectively, of a practical embodiment 78 of the invention. FIG. 7 is a side elevation, partially in section, showing

electrodes

80 and 82 disposed to have a gap 84 of a predetermined length between adjacent electrode surfaces.

Magnet members

86 and 88 are disposed relative to the gap 84 such that the magnetic field produced by

magnet members

86 and 84 will be substantially perpendicular to an are between

electrodes

80 and 82. The electrodes 80- and 82 and

magnet members

86 and 88 are disposed in a suitable casing 90, which may be constructed of a conductor of electricity such as aluminum or brass, or a non-conductor may be used if a separate ground lead is used, with said casing having a cover or top 92. Enclosure 90 may be disposed on the outside of the casing 93 of the inductive apparatus the protective apparatus is to protect. One electrical connection from the electrical inductive apparatus to one of the electrodes of the protective system 78 may be made through insulating bushing member 94 which extends through the casing 93 to the inductive apparatus. The conducting member 96 extends through the bushing member 94, with connection being made to a conductor from the inductive apparatus at one end of conducting member 96 by fastening means 98. The other end of conducting member 96 may be connected to electrode 80 through another conducting member 100 which may be secured to conducting member 98 by fastening means 102 and to electrode 80 by suitable holding and locating means, such as spring member 104. The remaining electrical connection may be made by properly grounding casing 90, which is directly connected to electrode 82.

FIGS. 8 and 9 show suitable hold-ing and locating means 110 and 112 which may be used to hold

electrodes

80 and 82 and

magnetic members

86 and 88 in the proper assembled relationship. FIG. 8 also shows how the

electrodes

80 and 82 may be tapered or rounded at points 114 and 114' to prevent arc concentration on sharp edges, and further cut back at

points

116, 116, and 118, 118'. This design of electrode is merely for illustrative purposes, however, with many different configurations being equally suitable. In fact, excellent results have been obtained with rectangular shaped electrodes which were not tapered, rounded or cut back in any way.

When the over FIG. 10 illustrates the protective system 78 shown in FIGS. 7, 8 and 9 disposed relative to a transformer 120 which is shown partially schematic. Transformer 120 may be of the conventional type having a high voltage winding 122 and a low voltage winding 124 inductively disposed on a magnetic core 126, and disposed in a suitable metallic casing or tank 128 containing the usual insulating dielectric and

low voltage bushings

130 and 132 and high voltage 134, along with a capacitor 136 connected in series circuit relation with high'voltage winding 122; or transformer 120 may be of the type whereby the series capacitance 136 is formed by using certain types and arrangements of conductors and insulations of various windings. In any event, the high voltage conductor 138 from high voltage bushing 134 is connected to terminal 140 of high voltage winding 132, and terminal 142 of high voltage winding 122 is connected to protective device 78 through conductor 144, as well as to ground 146 through capacitance 136. The protective device 78 is also connected to ground 146' such that when the voltage across capacitor 136 exceeds the breakdown voltage of protective device 78 the capacitance 176 will be effectively shunted, connecting terminal 142 of high voltage winding 122 directly to ground 146' for a time sufficient to allow capacitance 136 to discharge, as hereinbefore described.

While protective device 78 in FIG. 10 is shown mounted external to the casing 128 of transformer 120, it is to be understood that the protective device 78 may be appropriately sealed and mounted inside the casing.

Also, instead of air inside the protective casing of protective device 78, it may be evacuated, or a gas such as sulfur hexafluoride (SP hydrogen, or one of the inert gases, may be used.

FIGS. 11 and 12 illustrate further embodiments the protective device may take, using ring type magnets. FIG. 11 illustrates an arrangement shown in section, whereby

circular electrode members

220 and 222 are disposed with a predetermined gap 224 between adjacent surfaces, and ring

type magnet members

226 and 228 are disposed above and below the

electrodev members

220 and 222, with the polarities of the magnet members being as illustrated with the north poles being on the outside periphery, or the south. poles may be on the outside periphery, to produce a magnetic fieldv which will be substantially perpendicular to an arc between the electrode members 228 and 222'. An arc between

electrode members

220 and 222, under influence of the magnetic field produced by

magnet members

220 and 222 will move in a circle, around said circular electrode members.

FIG. 12 illustrates an arrangement whereby

circular electrode members

230 and 232 are disposed with a predetermined gap or spacing 234 between adjacent surfaces, with ring type magnet members 236 and 248 disposed relative to the internal and external diameters of

electrode members

230 and 232, respectively. The polarities of said magnet members should be such that a magnetic field is created across the gap 234 which will be substantially perpendicular to an are formed between

electrode members

230 and 232. Like the embodiment shown in FIG. 11, an arc between

electrode members

230 and 232 will be moved in a circle around said electrode members.

The protective system described herein has many advantages, in addition to its very low cost which makes its use with relatively inexpensive distribution transformers practical. One of the advantages is the fact that protective device effectively shunts the series capacitance and extinguishes the arc before 60 cycle or a line current can flow through the arc, thus immediately damping unstable circuit conditions. Another advantage is the fact that the protective device operates during the first half cycle of over voltage and can repeatedly and automatically perform its protective function without maintenance. voltage on the capacitance ceases, the

protective device no longer shunts the capacitance and is ready to await the next over voltage condition. Another advantage is the small size, lack of moving parts, and rugged construction of the protective system, which contribute to a substantially maintenance free system and allows it to be conveniently mounted relative to the apparatus it is to protect.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it'is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative, and not in a limiting sense.

We claim as our invention:

1. Electrical inductive apparatus comprising windings arranged to provide a predetermined capacitance, protective means connected across said capacitance, said protective means comprising electrode means having predetermined minimum dimensions and disposed to form a gap which will prevent electrical conduction below a predetermined voltage and allow conduction in the form of an arc above the predetermined voltage, and means disposed to move the arc in the gap provided by said electrode means along one of the dimensions of said electrode means, the predetermined minimum dimensions of said electrode means being selected to cause the arc to extinguish when said capacitance has substantially discharged, the predetermined minimum dimensions of said electrode means being determined by the discharge time of said capacitance and the speed of the arc.

2. Electrical inductive apparatus comprising windings interleaved to provide a predetermined capacitance, protective means connected across said capacitance, said protective means comprising electrode means having predetermined minimum dimensions and disposed to form a gap which prevents electrical conduction'below a predetermined voltage and allows conduction in the form of an arc above the predetermined voltage, and means disposed to provide a magnetic field in said gap which moves the are along one of the dimensions of said electrode'means, the predetermined minimum dimensions of said electrode means being selected to cause the arc to.

extinguish when said capacitance has substantially discharged, the predetermined minimum dimensions of said electrode means being determined by the discharge time of said capacitance and the strength of the magnetic field.

3. Electrical inductive apparatus comprising primary and secondary windings, capacitance means connected in series circuit relation with said primary winding, electrode means having predetermined minimum dimensions and connected across said capacitance means, said electrode means being disposed to provide a predetermined gap which prevents current fiow below a predetermined voltage and allows current flow in the form of an arc above the predetermined voltage, and means disposed to move the arc in the gap provided by said electrode means along one of the dimensions of said electrode means, the predetermined minimum dimensions of said electrode means being selected to cause the arc to extinguish when said capacitance means has substantially discharged, the predetermined minimum dimensions of said electrode means being determined by the discharge time of said capacitance means and the speed of the arc.

4. A transformer comprising primary and secondary windings for connection to a source of alternating potential and a load circuit, said primary winding being arranged to provide a predetermined capacitance in series circuit relation with said primary winding, electrode means having predetermined minimum dimensions and connected across said capacitance, said electrode means being disposed to provide a predetermined air gap, means disposed to provide a magnetic field in the air gap provided by said electrode means, the air gap provided by said electrode means breaking down and conducting electricity in the form of an are when the voltage across said capacitance reaches a predetermined magnitude, said arc being moved along the gap in the direction of one of the dimensions of said electrode means by the action of the magnetic field, the predetermined minimum dimensions of said electrode means being selected to cause the arc to extinguish when said capacitance has substantially discharged, the predetermined minimum dimensions of said electrode means being determined by the discharge time of said capacitance and the strength of the magnetic field.

5. A transformer comprising primary and secondary windings for connection to a source of alternating potential and a load circuit, capacitance means connected in series circuit relation with said primary winding, electrode means having predetermined minimum dimensions connected across said capacitance means and disposed to provide a predetermined gap, the gap provided by said electrode means breaking down and conducting electricity in the form of an arc when the voltage across said capacitance means exceeds a predetermined magnitude, means disposed to provide a magnetic field in the gap provided by said electrode means substantially perpendicular to the arc, the are being moved along the gap in the direction of one of the dimensions of said electrode means by the action of said magnetic field, the predetermined minimum dimensions of said electrode means being selected to cause the arc to extinguish when said capacitance means has substantially discharged, the predetermined minimum dimensions of said electrode means being determined by the discharge time of said capacitance means and the strength of the magnetic field.

6. A transformer comprising primary and secondary windings for connection to a source of alternating potential and a load circuit, capacitance means connected in series circuit relation with said primary winding, inductance means, electrode means having predetermined minimum dimensions and disposed to provide a predetermined gap, said inductance means and said electrode means being connected serially across said capacitance means, the gap provided by said electrode means allowing said capacitance means to discharge in the form of an arc when the voltage across said capacitance means reaches a predetermined magnitude, means disposed to provide a magnetic field in said gap having a field strength sufficient to move the arc in the gap provided by said electrode means along one of the dimensions of said electrode means, said inductance means causing the discharge current of said capacitance means to be oscillatory and having a predetermined natural frequency, the predetermined minimum dimensions of said electrode means being determined by the discharge time of said capacitance means and the strength of the magnetic field, the magnetic field and said electrode means extenguishing the are at a current zero of the oscillatory discharge current when said capacitance means has substantially discharged.

7. A transformer comprising a plurality of electrical windings, certain of said windings being interleaved to provide a predetermined capacitance, inductance means, electrode means having predetermined minimum dimensions and disposed to provide a predetermined gap, said inductance means and said electrode means being connected serially across said capacitance, the gap provided by said electrode means allowing said capacitance to discharge in the form of an are when the voltage across said capacitance reaches a predetermined magnitude, means disposed to provide a magnetic field in said gap having a field strength sufficient to move the arc in the gap provided by said electrode means along one of the dimensions of said electrode means, said inductance means causing the discharge current of said capacitance to be oscillatory and having a predetermined natural frequency, the predetermined minimum dimensions of said electrode means being determined by the discharge time of said capacitance and the strength of the magnetic field, the magnetic field and said electrode means extinguishing the arc at a current zero of the oscillatory discharge current when said capacitance has substantially discharged.

8. A power distribution system comprising a plurality of distribution transformers having primary and secondary windings, capacitance means, said capacitance means being connected in circuit relation with said plurality of distribution transformers, with the electrical energy flowing through said capacitance means also flowing through the primary windings of said distribution transformers, electrode means having predetermined minimum dimensions and disposed to provide a predetermined gap, said electrode means being connected across said capacitance means, the gap provided by said electrode means allowing said capacitance means to discharge in the form of an are when the voltage across said capacitance means reaches a predetermined magnitude, means disposed to move the are along one of the dimensions of said electrode means, the predetermined minimum dimensions of said electrode means being determined by the discharge time of said capacitance means and the speed of the arc, said electrode means and the movement of the arc cooperating to extinguish the are when said capacitance means has substantially discharged.

9. Protective means for damping unstable circuit conditions and preventing over-voltages when utilizing series connected capacitance in a power distribution system, comprising electrode means having predetermined minimum dimensions and disposed to provide a predetermined gap, said electrode means being adapted for connection across the capacitance, said electrode means discharging the capacitance in the form of an arc across the gap when the voltage across the capacitance reaches a predetermined magnitude, means disposed to move the arc in the gap provided by said electrode means along one of the dimensions of said electrode means, the predetermined minimum dimensions of said electrode means being determined by the discharge time of the capacitance and the movement speed of the arc, the movement speed of the arc and the predetermined minimum dimensions of said electrode means cooperating to cause the arc to extinguish when the capacitance has substantially discharged.

10. Protective means for damping unstable circuit conditions and preventing overvoltages when utilizing series connected capacitance in a power distribution system, comprising substantially circular shaped electrode means having predetermined minimum dimensions and disposed to provide a predetermined gap, said electrode means being adapted for connection across the capacitance, said means to extinguish when the capacitance has substantially discharged.

11. Protective means for capacitance connected serially with inductive apparatus, comprising electrode members having predetermined minimum dimensions and disposed to provide a predetermined gap, said electrode members being adapted for connection across the capacitance, said electrodemembers discharging the capacitance in the gap in the form of an arc when the voltage across the capacitance reaches a predetermined magnitude, magnet members, said magnet members being disposed to provide a magnetic field in said gap which moves the arc in the gap along one of the dimensions of said electrode members, the predetermined minimum dimensions of said electrode members being determined by the discharge time of the capacitance and the strength of the magnetic field, the magnetic field and said electrode members causing the arc in the gap provided by said electrode members to extinguish when the capacitance has substantially discharged.

References Cited by the Examiner UNITED STATES PATENTS 787,990 4/1905 Murphy 313 156 2,575,060 9/1951 Mathias. 2,664,525 12 1953 DlebOld 317- 12 2,677,032 4/1954 Wells. 2,725,446 9/1955 Slepian. 2,900,578 8/1959 Marbury 317-12 FOREIGN PATENTS 1,072,308 3/1954 France.

SAMUEL BERNSTEIN, Primary Examiner. R. V. LUPO, Assistant Examiner.

Claims

1. ELECTRICAL INDUCTIVE APPARATUS COMPRISING WINDINGS ARRANGED TO PROVIDE A PREDETERMINED CAPACITANCE, PROTECTIVE MEANS CONNECTED ACROSS SAID CAPACITANCE, SAID PROTECTIVE MEANS COMPRISING ELECTRODE MEANS HAVING PREDETERMINED MINIMUM DIMENSIONS AND DISPOSED TO FORM A GAP WHICH WILL PREVENT ELECTRICAL CONDUCTION BELOW A PREDETERMINED VOLTAGE AND ALLOW CONDUCTION IN THE FORM OF AN ARC ABOVE THE PREDETERMINED VOLTAGE, AND MEANS DISPOSED TO MOVE THE ARC IN THE GAP PROVIDED BY SAID ELECTRODE MEANS ALONG ONE OF THE DIMENSIONS OF SAID ELECTRODE MEANS, THE PREDETERMINED MINIMUM DIMENSIONS OF SAID ELECTRODE MEANS BEING SELECTED TO CAUSE THE ARC TO