US2341932A - Protective system - Google Patents

Description

Feb. 15, 1944. MacCARTHY PROTECTIVE SYSTEM Filed Jan. 2, 1941 A Mm -|& 5 5 F .00....

Inv ehto r Donne! l' D. FIacCaTt I192 b g .37 2%: H iZ /Z-ILQP neg Patented Feb. 15, 1944 PROTECTIVE SYSTEM Donnell D. MacCarthy. Pittsfield, Mass assignor to General Electric Company, a corporation of New York Application January 2, 1941, Serial No. 872,780

6 Claims. (Cl. 175-294) gized from a suitable source, such as through a constant current transformer. In order to prevent all the lamps which are connected in series from failing to light when one of the lamps in the circuit is burned out, there are usually provided suitable devices, such as film cutouts which are connected in shunt with each of the lamps. These film cutouts are designed to breakdown at a voltage slightly higher than the rated voltage of the lamp so that when a lamp burns out its cutout will provide a shunting circuit for the damaged lamp. It has been found, however, that in operation film cutouts are broken down or short circuited by impulse voltages that appear on the feeder due to lightning. Thus, lamps are put out of service although they are not burned out. It has, therefore, been the practice after each lightning storm to check the film cutouts and replace those which have burned out.

It is, therefore, an object of my invention to provide an improved system for protecting a low voltage device from impulse failure.

Another object of my invention is to provide an improved system for protecting film cutouts which may be employed to shunt serially connected street lamps from becoming short circuited when a high voltage wave passes over the line.

A further object of my invention is to provide an improved protective device for street lamps which is simple in construction, reliable in operation, and which is easy to install.

Further objects and advantages of my invention will become apparent from th following description referring to the accompanying drawing, and the features of novelty which characterize my invention will be pointed out with particu larity in the claims annexed to and forming a part of this specification.

In the drawing, Fig. 1 illustrates a plurality of serially connected lamps, such as may be found in a street lightning circuit, each of the lamps having connected therewith a protective system which is provided with an embodiment of my invention, and Fig. 2 illustrates the protective system employed in Fig. 1.

In the arrangement illustrated in the drawing, I have provided each lamp with one of my protective devices for preventing high voltage waves from being impressed across and damaging the lamp or the film cutout which would put the lamp out of service. This protective device or system includes a coil connected in series with one 'of the lines feeding the lamp, and a spark gap and a capacitor connected across the lines, the capacitor being connected across the feeder lines between the coil and the low voltage device or cutout and the gap being connected across the feeder lines on the other side of the coil.

Referring more particularly to Fig. l of the drawing, I have illustrated a circuit including a plurality of lamps III which are connected in series, such as may be found in conventional street lighting circuits. The lamps are connected in series with the feeder lines II and i2, these lines being in turn connected to any suitable source of power. Thus, these lines are connected to a secondary winding I 3 of a constant current transformer I which has a primary winding 15 connected to a suitable source of potential. A conventional lightning arrester indicated generally by the numeral "5 may have one side thereof connected to any suitable point in the series system, such as at H, through a conductor IS, the other side of this arrester being grounded. Since the lamps it are connected in series, a suitable low voltage device is connected across the lamp terminals, and may be contained in the lamp sockets, so that if one of the lamps should burn out the low voltage device will are over and pro- .vide a shunting circuit therearound in order that the remaining lights may be supplied with power. Such a low voltage device may be a conventional film cutout which is indicated by the numeral IS, in Fig. 2. Since street lamps are usually designed to operate at relatively low voltages, any device which is to provide a shunting path for the lamp in case it burns out must also be a low voltage device. In order, therefore, to accomplish this shunting purpose in case the lamp burns out, the film cutout must be designed to breakdown at a voltage not greatly in excess of the normal lamp operating voltage, and it must not breakdown at voltages equal to or less than the operating voltage of the lamp. Thus, film cutouts have been designed to breakdown at voltage ranges, such as from 50 to volts, from to volts, and from 250 to 300 volts, depending upon the rating of the lamp which it shunts. It will be seen, therefore, that a high voltage wave traveling over the feeder lines will tend to cause impulse failure of the film cutouts. Such high voltage waves are usually caused when lightning strikes the feeder lines in some place or result from induction due to a stroke near the lines. However switching may cause travel- Y ing waves that will damage the lamps or film film cutout. In order, therefore, to provide an arrangement for protecting a low voltage device, such as a film cutout from impulse failure. I have provided a system which, as shown in Fig. 2, includes an inductance coil 20, a capacitor 2|, and a suitable spark gap arrangement 22. This protective system or device is placed electrically relatively close or adjacent to the film cutout in order that any traveling wave will have to pass through the protective system before it can be impressed on the film cutout l9. In order that the protective device may be easily installed across each of the serially connected lamps it is placed in a suitable casing indicated generally by the letter A to provide a unitary structure and has line leads 23 and 24 and load leads 25 and 26 extending therefrom. The load leads, therefore, may be connected across the terminals of the lamp film cutout combination while the line leads 23 and 24 may be connected to the feeder lines. Thus, as illustrated in Fig. 1, the line lead 23 is connected to-the incoming line H while the other line lead 24 is connected to a cable running to the next serially connected lamp. It will-be seen, therefore, that when'a protective device is connected across a film cutout lamp combination that the inductance 20 is connected in series with or between line lead 24 and-load lead 26 or in series with one of the lines, and the capacitor 21 is connected across the lines and between the inductance and the film cutout, while the spark gap 22 is connected across the lines on the side of the inductance remote from the lamp and film cutout to be protected.

In Fig. 1 let us assume that a traveling wave originates at any point such as X on the feeder. This impulse will divide and part will go to the right and pass through the lamps or their shunt protective devices, until it is dissipated through the ground connection of the street lighting circuit somewhere to the right of the point X. The other part of the wave will travel to the left of X passing through the lamps or the protective devices that shunt them. One or more conventional lightning arresters may be connected between the-feeder line and ground as shown at point I! in Fig. 1. Such arresters can never in wholly effective indissipating the entire traveling wave to ground since an overvoltage is required to start the discharge in the arrester and because an overvoltage will exist when the arrester is discharging due to the impedance of the arrester and the impedance of the ground connection. This part of the surge will pass by such arresters and continue along the line, and will pass through the series lamps or their parallel protective devices. In the absence of a protective device, such as has been described, the part of the surge passing by the arresters would be capable of damaging the lamps or film cutouts. It is to be noted thatconventional arrestof 2 kv. per microsecond or slower.

ers, such as indicated by the numeral It in Fig.

1, are therefore incapable of protecting the street lights or film cutouts from impulse failure, even though a large number of these arresters were to be used. For this reason such arresters are usually not used on-the street lighting circuits at a distance from the power source. However, such arresters, if properly applied to the ungrounded terminal of the secondary winding of the constant current transformer l3, can protect this winding from damage even though these arresters are ineffective in protecting the street lights or film cutouts from damage. However, my protective device will be effective in preventing damage to the lamps or film cutouts, irrespective of whether or not conventional lightning arresters are connected between the feeder and ground. In the application of my inventionto a protective system such as for lamps which are seri ally connected as in street lighting (circuits, the spark gap may be of any suitable type. set to a small spacing to enable it to limit the voltage to as low a value as is possible. Mica spaced gaps have been found effective for this purpose due to the low impulse ratio. However, the spacing cannot be so small as to permit the gap to become short circuited by the droplets of metal melting from the electrode by discharges that the gap must carry in service. Thus, the gap will be set to breakdown consistentlyat approximately the same voltage or at about 2.3 kv., 60 cycles, or at about.3.8 kv., crest voltage at rates The maximum voltage which may be impressed across the cutout I9 is, of course, determined by the film cutout itself which is a relatively low value and the proper values of inductance 20 and capacitance 2! with a gap 22 set for as small a spacing as possible, therefore, may be determined by the following equations developed below:

The first part of the front of most impulse voltage waves can be approximated by assuming that the voltage rises uniformly. Such an assumption will be sufiiciently accurate over the range of voltage from 0 to about 4000 volts where the gap in this protective device will spark over. In calculating the necessary values of L and C for the protective device, it is also permissible to neglect the resistance capacity, and inductance of the film cutout and the lamp since ignoring these factors will result in a conservative design. The electrical circuit with which we are dealing may have the following symbols:

E=the impulse voltage at time t E1=rate of rise of voltage which is assumed to be constant Ee=voltage across capacitor, lamp, and film cutout at time t t=time in s L=inductance in h C=capacity in pf m t am at a (cot which is an approximation consisting oi'the first two terms or the infinite series expression for a sine function.

a-(a% (a) By combining Equations 1 and 3, Equation 4 is obtgined ind (a In calculating a suitable protection by the use of Equation 4, let us assume that the gap operates at an impulse voltage of E=4000 v. in the time t and that the voltage applied to the lamp and cutout, Es, should not exceed 100 v. Then it is possible to use Equation 4 to obtain the necessary values of the product of L and C which are included in the table below for tour diiiercut rates of voltage rise.

Rate oi rise Time required Necessary value oi impulse ior voltage E to of LXO to limit voltage reach 4.000 V. E. to 100 v.

1 Where L is in microhenrles and C is in microisrads.

Thus it will be seen that in order to hold the voltage on the lamp and cutout to 100 v., the

product of L and C must be made larger it protection is desired at slow rates of voltage rise, such as 1 ,kv./]J.8 than for fast rates of rise such as 1000 kv./ .is. Available information on lightning transients indicates that only a small percentage of impulse voltages on feeders will have a slower rate of voltage rise than 1 kv./ns. Hence if a protective device were designed to have a LC product of the order of 100 or more, the voltage appearing on the lamp and cutout very rarely will exceed 100 v., or only in case the rate of voltage rise is slower than 1 kin/ s. The proper value of the LC product in a practical arrester is, of course, determined by a consideration of economics, since it is necessary to balance the value of protection obtained for very slow waves which is obtained by using larger values of L and C against the higher cost and diminishing returns which will be obtained.

It will be seen from Equation 4 that if a small value of capacity were used, a larger value of L will be necessary'in order to obtain the desired LC product. However, it will not be possible to omit the capacitor entirely since any practical form of inductance has a small but real value of terminal-to-terminai capacity. However, it the capacitor II were omitted, the charge passing through the terminal-to-terminal capacity oi the inductance would result in a voltage across the film cutout suflicient to cause failure. However, the terminal-to-terminal capacity of the inductance L will not be a cause oifailure provided a relatively large capacitor is connected in parallel with the lamp and cutout. However, in selecting the proper value of the capacitor, above the needed minimum value, to be connected in parallel with the lamp and cutout, the relative costs of capacity and inductance must be balanced and it will be found uneconomical to make L too large and C too small.

Modifications oi the particular arrangement which I have disclosed embodying my invention will occur to those skilled in the art, so that I do not desire my invention to be limited to the par- 78 ticular arrangement set forth and I intend in the appended claims to cover all modifications which do not depart from the spirit and scope oi my invention.

What I claim as new and desire to secure by Letters Patent 0! the United States is:

l. A protective system including line leads adapted to be connected to feeder lines, load leads adapted to be connected to a relatively low voltage device, a coil, a spark gap, and a capacitor, said coil only being connected in series with one of said line leads and one of said load leads, said capacitor being connected across said load leads on the load side of said coil, and only said gap being connected across said line leads on the line side of said coil in order to protect the device from impulse failure.

2. A system for protecting a relatively low voltage device from lightning surges including line leads adapted to be connected to feeder lines, load leads adapted to be connected to the device, a coil, a spark gap, and a capacitor, said coil only being connected in series with one or said line leads and one of said load leads, said capacitor being connected across said load leads on the load side or said coil, and only said gaD being connected across said line leads on the line side of said cell in order to protect the device from impulse railure.

3. A protective system for a device the normal operating voltage of which being within a, range oi approximately 50 to 300 volts including line leads adapted to be connected to feeder lines, load leads adapted to be connected to the device, a call, a spark gap. and a capacitor, said coil only being connected in series with one of said line leads and one 01 said load leads, said capacitor being connected across said load leads on the load side of said coil, and only said gap being connected across said line leads of the line side .of the coil in order to protect the device from impulse iailure. g 4. A protective system for a device the normal operating voltage of which being within a range of approximately 50 to 300 volm including line leads adapted to be connected to feeder lines. load leads adapted to be connected to the device, a coil, a spark gap, and a capacitor, said coil being connected in series with one of said line leads and one or said load leads, said capacitor being connected across said load leads on the load side of said coil, and said gap being connected across said line leads on the line side of said coil in order to protect the device from impulse failure, the approximate value oi the product oi the inductance of the coil and the capacitance of said capacitor being determined by the following equation:

i E 6L0 where Es equals the safe voltage which may be impressed across the device, E equals the impulse spark potential or said gap due to a voltage traveling along the lines assuming it has a constant rate 01 rise, t equals the time at which the voltage of the impulse reaches the value E, and L and C equal the values of the inductance and capacitance, respectively.

5. In a protective system for a street lighting circuit having a lamp and a film cutout, line leads adapted to be connected to feeder lines, load leads adapted to be connected to the lamp, a coil, a spark gap, and a capacitor, said coil only being connected in series with one of said line leads and one of said load leads, said capacitor being connected across said load leads on the load side of said coil, and only said gap being connected across said line leads on the line side or said coil in order to protect the film cutout from tailures due to lightning surges.

8. A rotectivedevice for a film cutout having feeder lines connected thereto including a coil, a spark gap, a capacitor, device to provide a unitary structure, a pair or line leads and a pair of load leads extending from said casing, said coil being connected between one of said line leads and one or said load leads,

a casing enclosing saidv said capacitor being-connected across said leads on'the load side oi sald coil, said spark gap being connected across said leads on the line side of said coil so as toprotect said cutout from impulse failure when said load leads of said protective device are connected to the cutout and said line leads are connected to the lines feeding the cutout, the product of the inductance of said cell in microhenries and the capacitance of said gsgacitor in microfa'rads being of the order of DONNELL D. MAOCAR'I'HY.

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