US2137216A - Protective circuit - Google Patents

Description

Nov. 15, 1938. o. P. M CARTY 2,137,216

PROTECTIVE CIRCUIT Original Filed Jan. 6, 1937 4 T0 VOLTAGE SENSITIVE CON7'/?0L FOR g Inventor: Orin P McCartg,

is Attorhegf Patented Nov. 15, 1938 PATENT ()FFICE.

PROTECTIVE CIRCUIT Orin P. McCarty, Pittsfield, Mass, assignor to General Electric Company, a corporation of New York Application January 6, 1937, Serial No. 119,234 Renewed March 17, 1938 15 Claims.

My invention relates to protective circuits and more particularly to means for protecting an impedance'which carries the main load current of a transformer during operation under load of a tap changer on the transformer.

Protective means of this type is disclosed and broadly claimed in my Reissue Patent No. 20,147, dated October 27, 1936. The present invention is particularly adapted for use withautomatic tap changing voltage regulators and boosters of the type disclosed and claimed in my application Serial No. 69,789, filed March 20, 1936 as a continuation-in-part of an application filed April 16,

i 1935. This is a continuation-in-part of my application Serial No. 47,425, filed October 30, 1935. The above referred to reissue patent and patent applications are all assigned to the assignee of this application.

When transformer taps are changed under load,

means are often provided for snapping the movable contact or contacts rapidly from one fixed contact or transformer tap to the next fixed contact or transformer tap. In certain arrangements of this type an impedance is provided for carrying the current during the switching operation. Between switching operations the impedance is substantially short-circuited by the closed contacts of the tap changing switch. In this way there is always a closed conducting path for the current and an objectionable momentary break in the. circuit during the switching operation is eliminated.

As a normal tap changing operation of the above type takes place relatively rapidly,'I have found that it is not necessary to provide an impedance which can carry the current for long periods or indefinitely and it is only necessary to provide an impedance which can carry the current safely and without overheating during the 40 relatively short interval of time during which the tap changing switch is going from one contactmaking position to the next. This permits a substantial saving in cost as an impedance which can carry the line current continuously is much more expensive and larger than an impedance which need only carry the line current momentarily. It sometimes happens, however, that the mechanism for driving the tap changing switch gets out of order so that for one reason or another the switch will stop in an intermediate position between contact making positions. When this hap pens the load current flowing continuously through the impedance causes it to overheat to such an extent as to cause failure of the device and sometimes even fire.

In accordance with my invention I provide improved means for protecting the impedance from the' effects of a. stopping of the tap changing switch in an off position. One feature of this means comprises relatively quick acting means for disconnecting and effectively-by-passing the impedance when an ordinary value of line current flow passes through the impedance for substantially longer than a normal tap changing operation combined with relatively slow acting means for short-circuiting the impedance in case the current flow is so low as to fail to cause operation of the first means and yet is high enough to cause eventual overheating of the impedance. Another feature is the provision of means for protecting the transformer against the effect of said bypassing or short-circuiting means when an interrupted tap change is eventually completed.

An object of my invention is to provide a new and improved protective means for a switching impedance.

Another object of my invention is to provide improved means for disconnecting and by-passing or short-circuiting an impedance which is adapted to carry the line current during a transformer tap changing operation, in the event of stopping the tap changing means in an o position.

A further object of my invention is'to provide a new and improved protective circuit and appae ratus for a transformer tap changing under load type of. regulator.

My invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims. 7

In the drawing, Fig. 1 is a diagrammatic showing of an embodiment of my invention and FI Z is a similar showing of a modification thereof.

Referring now to Fig. 1 of the accompanyin drawing, l is a switching impedance which is connected around a transformer tap changing switch 2. Switch 2 is arranged to change taps on a transformer winding 3, which is shown, by way of example as the series winding of an autotransformer having an exciting windin 4 nected across an alternating current circuit 5. The winding 3 is connected in circuit 5 by means of an adjustable connector 6 for selective engagement with the taps on the winding 3. The purpose of this is to vary the range of voltage adjustment produced by the tap changer 2. In the midposition of connector 6, as shown in the drawin the tap changer can give equal amounts of voltage buck and boost in the circuit 5. If the connector 6 is connected to one end of the winding 3 the tap changing means will cause the autotransformer to produce varying amounts of voltage buck, while if the connector 6 is connected to the other end of the winding 3 the tap changing means will cause the autotransformer to produce varying amounts of voltage boost. With the connector 6 connected to a tap intermediate the midtap and the end of the winding 3 the buck and boost ratio will be 2 and 7 /g, or vice versa. The arrangement can be operated with the power flow in the circuit 5 from right to left, as viewed in the drawing, or from left to right.

The details of the mechanism for driving the tap changing switch 2 form no part of my present invention and the movable contact may be operated by hand if desired. However, the present preferred form of automatic means for driving the movable contact of the tap changing switch 2 in response to the voltage of the circuit 5 is disclosed in my copending application, Serial No. 69,789, filed March 20, 1936, and assigned to the assignee of the present application. Such anarrangement produces an automatic voltage regulating device which is well adapted for automatically holding substantially constant voltage on branch feeder circuits.

It is immaterial to my invention, in its broader aspects, that the impedance I be of any particular type and it may be an ordinary resistor or a self-saturating iron core reactor, or a well known non-linear ferrc-resonant circuit or net work of any suitable type. However, I prefer to use a resistor of ceramic resistance material known as "Thyrite, the composition of which is disclosed and claimed in Patent No. 1,822,742, granted September 8, 1931, on an application of K. B. McEachron and assigned to the assignee of the present application. Thismaterial has the property of markedly reducing its electrical resistance substantially instantaneously with increases in voltage applied thereto or increases in current flowing therethrough. With such a resistor its electrical resistance is relatively very high when the contacts of the tap changing switch 2 are closed, as the voltage across the resistor is then low. Consequently, the current through the resistor is very low resulting in minimum heating andminimum losses in the resistor. When the contacts of the tap changing switch 2 separate, the voltage across the resistor l builds up and the current therethrough increases. This causes its resistance to decrease greatly at the same time so that the voltage drop across it is not as great as it would be across an ordinary linear resistance. Consequently the switching voltage, that is, the voltage across the contacts of the switch 2, when they perform a tap changing operation, is held to a reasonably low value. The above characteristic of resistor I is sometimes referred to as a negative resistance current characteristic.

For protecting the resistance I against the injurious effect of continuous approximately full load line current flow therethrough in the event of accidental stopping of the switch 2 in an off or intermediate position, as indicated by the dotted position of the movable contact, I provide a switch having a fixed contact 1 and a movable blade contact 8 which is biased towards fixed contact! by means of any suitable device, such as a spring 9. The switch is held open by means ,of a tension fuse I I].- The connections are such that fuse I0 is in series with resistor I, and the switch, when it closes, will by-pass, bridge 01' effectively short circuit the normally conducting paths through the resistor I, switch 2 and series winding 3.

The resistor I isv shown in the shape of a hollow cylindrical member through the center II of which is passed a bolt I2 carrying end nuts thereon for clamping the resistor between end conducting plates I3 mounted on a suitable insulating support I4. During a switching operation the current flows through a conductor I5 to one of the plates I3 thence through the resistance material I to the other plate I3, from there through a conductor I6, through the tension fuse I0 and the blade switch 8 in series, and then through a conductor IT to the other side of the tap changing switch 2.

In normal operation, the fuse II) can carry the line current during the period of time required for an ordinary tap changing operation. If, however, the tap changing switch stops in an off" position the current flowing through the fuse I 0 melts it, thereby allowing the spring 9 to cause closure of the switch by causing engagement between the blade contact 8 and the stationary contact I. By this action the resistor I is automatically disconnected and the normal current path through it is short-circuited. It is to be noted that when the fuse I0 melts, the motion of the blade 8 issuch as to increase the distance between the fused or broken ends of the fuse I0 thereby effectively preventing any continuance of an are at this point.

It is difficult to provide a fuse which will not blow or melt when momentarily carrying full load line current during an ordinary tap changing operation and which at the same time will melt when relatively low current is passed therethrough continuously. Such relatively low current may, however, cause injury and overheating of the resistor I if continued long enough. Consequently, there is provided suitable means in the form of a small fusible metallic cylinder or ring I8 surrounding the bolt I2. This fusible means I8 responds to the temperature of the resistor I so that if the current flowing through the fuse I0 is low enough not to cause it to melt but the current is still high enough to cause overheating of the resistor I then the fusible means I! will melt and in so doing will spread out and interconnect the conducting members I3, thereby short-circuiting the resistor I. The fusible means I8 is relatively slow acting compared to the relatively quick acting means operated by the fuse I0.

Experience has shown that vibration or sudden J'arring will sometimes cause the switch 2 to complete a switching operation after it has stopped in an "off" position. If this happens after the protective apparatus has by-passed or short-circuited any part of the series winding 3, a heavy short-circuit current will usually be induced in that part of the series winding 3 by the common winding 0. The one exception is when the movable contact 2 makes connection to the same tap that connector 6 is connected to for then no part of the series winding 3 is shortcircuited. If this short-circuit current persists the transformer will be injured and its series winding will probably be burned out.

In the modification shown in Fig. 2, I provide means for protecting the transformer against the above-described short-circuit current con dition. This means consists of an auxiliary circuit maker and breaker for deenergizing the shunt winding 4 of the autotransformer at times when the series winding 3 of the autotransformer is short-circuited by the protective means. In this way the production of any induced voltage in series winding 3 by transformer action iseliminated. A simple way to construct this auxiliary circuit maker and breaker and to secure its proper operation is to combine it with the tension fuse-operated switch. Thus, as shown in Fig. 2, the upper terminal of the shunt winding 4 of the autotransformer is connected to an auxiliary contact l8 which is normally engaged by the movable contact arm 8. When the fuse Ill melts as a result of the stopping of the tapchanging switch 2 in an "off" position, the sprin 9 will cause the arm 8 to separate from the contact l8 thereby breaking the circuit through the shunt winding 4.

Aside from the rearrangement of the tension fuse-operated switch and its additional contact I8, Fig. 2 differs from Fig. 1 in a number of other particulars. The first is that the negative resistance current characteristic impedance, the details of which are shown in Fig. 1 but merely for simplicitys sake are not shown in Fig. 2, has a saturable core reactor 19 connected in parallel therewith. For the range of current handled by transformer regulators of this type and with the permissible voltage drop allowed in the by-pass impedance, a reactor and negative characteristic resistor combination is less expensive than an equivalent resistor alone. This is because for equal current ratings, the reactor is less expensive than the negative resistor and with the parallel combination the reactor carries most of the current and the negative resistor merely limits the high voltage peaks across-the reactor during times when the flux in the reactor core is below saturation values.

Another difierence between Fig. 1 and Fig. 2 is that in Fig. 2, the by-pass circuit which contains the fuse l and resistor l is connected between the load circuit side of the tap-changing switch 2 and the electrical midpoint of the series winding 3, whereas in Fig. 1 the by-pass circuit is connected across both the tap-changing switch 2 s and the winding 3. The result is that in Fig. 2, the maximum continuous normal voltage which can be applied to the impedance I is one-half the voltage of the series winding 3 whereas in Fig. 1 when the range-adjusting terminal 6 is connected to either of the outermost taps of winding 3, the full voltage of the series winding 3 can be applied continuously to the by-pass impedance I.

Still another difference between the figures is that in Fig. 2, a lightning by-pass 20 is connected between the mid-point of winding 3 and the supply side of the circuit. The by-pass 20 is preferably made of the lightning arrester material Thyrite previously referred to. With such a connection a high voltage surge, such as would result from a lightning stroke on tthe circuit 5, can readily pass through the negative characteristic resistors or impedances 20 and I in series as these devices will lower their resistances very markedly with increase in voltage applied there-- to. Consequently the transformer windings are prevented from injury due to over-voltage as a result of such surges.

The switching duty imposed on the tension fuse-operated switch in Fig. 2 is relatively greater than that in Fig. 1, because this switch now actually removes a certain number of kilovolt am peres from the circuit. This switching principle is also somewhat different and may be described as follows:

If, with the parts in the positions shown in the drawing and with the circuit energized, the switch 2 is stopped in an off position, as indicated by the dotted line, the load current will be by-passed through the switch arm 8, tension fuse H), negative resistor l and reactor is in parallel, and the upper half of the series winding 3. When the fuse -Ill melts, the spring 9 carries the switch blade 8 toward contact 1, atthe same time opening the common winding excitation contacts at [8. Arcs are then drawn at the melting fuse, and at the excitation contacts. Should the are at the excitation contacts tend to inter-' .not exceed a value which can easily be interrupted under oil. The excitation are readily in terrupts and completely removes the regulator from the line.

In Fig. 2, there is also shown a tertiary winding 2| on the transformer for supplying current to any suitable voltage sensitive control means for driving the tap-changing switch 2.

It should be understood that in practice, the negative resistance I of Fig. 2 can be constructed as in Fig. 1 with the fusible element f8 if desired. While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that changes and modifications may be made in my invention and I,

therefore, aim in the appended claims to cover all such changes and modifications as fali 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. The combination, with an impedance for carrying the current during operation of a transformer tap changer under load, of protective means for said impedance comprising anormally open switch for short-circuiting the current path through said impedance, means for biasing said switch closed, and current responsive means for holding said switch open unless the current through said impedance exceeds a given value for a given time.

2. The combination, with a circuit including an impedance for carrying the current during operation of a transformer tap changer under load, of protective means for said impedance comprising a normally open switch for bridging a portion of said circuit containing said impedance when said switch is closed, means for biasing said switch closed, and a fusible element normally in circuit with said impedance for holding said switch open, said switch and fusible element be ing so arranged that the closing of said switch increases the distance between the terminals of said fusible element.

4. The combination, with an impedance for carrying the current during operation of a transformer tap changer under load, of protective means for said impedance comprising a relatively quick acting element for short-circuiting the current path through said impedance when a current above a predetermined value flows through said impedance for a given time, and an additional relatively slow acting element for short-circuiting said impedance if a current of less than said predetermined value flows through said impedance for longer than said given time.

5. The combination, with an impedance for carrying the current during operation of a transformer tap changer under load, of protective means for said impedance comprising a relatively quick acting current responsive element for shortcircuiting the current path through said impedance when a relatively high current flows through said impedance for substantially longer than the time required by normal tap changing operation,

and relatively slow acting means responsive to the temperature of said impedance for short-circuiting said impedance in case a current which is of lower value than is required to operate said relatively quick acting element but-which is sufficient to cause injurious heating of said impedance continues to flow in said impedance.

6. Protective means for a load current by-pass resistor in a transformer tap changer comprising, in combination, a normally open blade switch having a fixed. contact, a spring for urging the blade of said switch into engagement with said fixed contact, a tension fuse electrically and mechanically connected to the blade of said switch for holding said switch open, a conductor connecting one side of said resistor to said fixed contact, and a conductor connecting the other side of said resistor to said fuse whereby a current can flow through said resistor, fuse and blade in series and upon melting bf said fuse by a relatively long continued relatively high current said resistor will be disconnectedand said switch will close thereby short-circuiting the current path through said resistor and fuse.

7. In combination, regulating means including a single movable contact switch for changing transformer tapsunderload,anegative resistancecurrent characteristic resistor connected in shunt with said means, and separate means responsive respectively to the effects of relatively high and low currents flowing continuously through said resistor for effectively short-circuiting it.

8. In combination, an alternating current circuit, an auto-transformer connected therein, a switch for changing taps on the series winding of said autotransformer, a resistor whose resistance decreases instantaneously with increases in current therethrough connected to carry the circuit current during operation of said tap changing switch, a normally open protective switch for short-circuiting the current path through said resistor, a spring tending to close said protective switch, a fusible element for holding said protective switch open against said spring, said fusible element being normally connected in circuit with said resistor.

9. In an electric circuit, a pair of spaced contacts, an impedance, switching means having two operating positions closing a circuit around said contacts excluding and including respectively said impedance, and means responsive to current through said impedance above a given value for'a given time for causing said switching means to exclude said impedance.

10. In an electric circuit, a pair of spaced-contacts, an impedance, switching means having two operating positions for closing circuits around said contacts excluding and including respectively said impedance, means responsive to current through said impedance above a given value for a given time for causing said switching means to exclude said impedance, and means for shortcircuiting said impedance when said switching means is in a position to include said impedance. 11. In combination, an alternating current closed circuit, a transformer winding connected therein, means for changing taps on said winding, a current conducting by-pass path for carrying the load current of said circuit during a tap change on said winding, said by-pass comprising a common portion connected to a movable switch contact and two parallel branch portions, one of pair of normally separated contacts so connected that when they engage they complete a by-pass connection around said impedance, means for urging the contacts of said switch together, and a fuse in circuit with said impedance for holding said switch contacts apart. said fuse being so proportioned as to ,melt when the tap changing mechanism of the regulator stops in an off position while said alternating current circuit is carrying a substantial load current whereby said impedance will be open-circuited by the melting of said fuse and substantially simultaneously therewith said protective switch will close and complete a bypass connection around said impedance for said circuit current.

13. In combination, an alternating current load circuit, a transformer tap changing switch. a.

transformer having a tapped winding adapted to be connected in said circuit by means of said switch, an exciting winding on said transformer, an impedance for carrying the circuit load current during a tap changing operation, means for protecting said impedance against overloading should said tap changing switch stop in an intermediate position. said protective means being operative to complete a relatively low impedance short circuit around at least a portion of saidchanging operation, and protective means for said impedance operative as a result of thestopping of said tap changing means in an "021, positionwhen said circuit is carrying a substantial load current for substantially simultaneously closing a relatively low impedance by-pass connection around the normal current path through said impedance and deenergizing the exciting winding of said transformer.

15. In combination, an alternating current load circuit, a voltage regulator for said circuit comprising an autotransformer connected in said circuit and provided with means for changing taps on the series windings thereof, an impedance for carrying the circuit load current during a tap changing operation, and protective means for said impedance operative as a result of the stopping of said tap changing means in an off position when said circuit is carrying a substantial load current for substantially simultaneously opening the circuit through said impedance, closing a relatively low impedance by-pass connection around the normal current path through said impedance and opening the circuit of the common winding of said transformer.

ORIN P. McCARTY.

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