US3617765A

US3617765A - Variable closing resistor

Info

Publication number: US3617765A
Application number: US721666A
Authority: US
Inventors: Alfred E Kilgour; James H Harlow
Original assignee: Allis Chalmers Corp
Current assignee: Allis Chalmers Corp
Priority date: 1968-04-16
Filing date: 1968-04-16
Publication date: 1971-11-02
Anticipated expiration: 1988-11-02

Abstract

A source of high-voltage electrical power and an electrical system, comprising transmission lines, which is connectable to the source. Switching means are provided to effect such connection. Impedance means, comprising closing resistors, are provided for insertion in series between the source and system upon closing of the switching means to reduce voltage surges in the system. Each closing resistor has a variable resistance value, i.e., a predetermined initial value when first inserted which is decreased at a predetermined rate within a predetermined time after insertion to effect improved reduction of voltage surges.

Description

United States Patent [72] inventors Alfred E. Kilgour Brookfieid; James 11. Harlow, Hales Corners, both of Wis. [21] Appl. No. 721,666 [22] Filed Apr. 16, 1968 [45] Patented Nov. 2, 1971 [73] Assignee Allis-Chalmers Manufacturing Company Milwaukee, Wis.

[54] VARIABLE CLOSING RESISTOR 9 Claims, 4 Drawing Figs.

[52] U.S. Ci 307/93, 317/20, 200/148 [51] 1102b 3/22 [50] 307/93, 136, 98, 92; 317/20, 16; 318/228; 200/148 [56] References Cited UNITED STATES PATENTS 3,291,947 12/1966 Van Sickle ZOO/148.8

614,394 11/1898 Kahlenberg 318/228 730,108 6/1903 Girault 3l8/228X 916,343 3 1909 Lum 307/98 1,113,289 10/1914 Barnum... 3l8/228X 1,649,391 11/1927 Davis 317/20x 2,639,357 5/1953 Kesselring... 307/93x 2,963,637 12/1960 Osborn 307/93x 3,193,711 7/1965 Western 307/93x Primary Examiner-Herman J. Hohauser Attorneys-Thomas F. Kirby, Robert B. Benson and Lee H.

Kaiser ABSTRACT: A source of high-voltage electrical power and an electrical system, comprising transmission lines, which is connectable to the source. Switching means are provided to effect such connection. Impedance means, comprising closing resistors, are provided for insertion in series between the source and system upon closing of the switching means to reduce voltage surges in the system. Each closing resistor has a variable resistance value, i.e., a predetermined initial value when first inserted which is decreased at a predetermined rate within a predetermined time after insertion to effect improved reduction of voltage surges.

1 VARIABLE CLOSING RESISTOR SUMMARY OF THE INVENTION This invention relates generally to closing resistors which are used when connecting electrical systems to a power source to limit voltage surges in the system. More particularly, the inv vention relates to variable closing resistors which are decreased in ohmic value after insertion in series circuit between the source and system.

Closing resistors (so called low ohmic value" resistors) are usedin high-voltage electrical transmission systems to limit cuit between source and system as the switch is closed and is subsequently stepped out when the switch is fully closed. Closing resistors could be used in any electrical system of any voltage level but are most useful in high voltage or extra high voltage transmission systems on the order of 230 kv. and having line lengths on the order of many miles. In the absence of closing resistors, surge voltages may be as high as five-times-normal system peak voltage and extra costly insulation is therefore required. Heretofore, it was the practice to apply single step closing resistors which, for example, in systems of 230 kv. and above were on the order of 300 to 500 ohms and were inserted for a period of about 8 milliseconds (one-half cycle). although resistor values and insertion times were not unduly critical. Such single-step closing resistors were effective to limit surge voltage magnitude to about 210 percent of normal system peak voltage. It is desirable to provide improved closing resistors which limit surge voltages even more so as further reduce the amount of costly insulation required, especially as system voltages reach extra high voltage levels.

In accordance with the present invention, it is possible to limit surge voltage magnitude to less than 210 percent of normal system peak voltage by employing improved variable closing resistors which have variable rather than fixed value. Such variable resistors have a predetermined high value when initially inserted and decrease in value at a predetermined rate in a predetermined time after insertion. The preferred predetermined high value of resistance should be on the order of four times the normal line surge impedances so as to damp the initial voltage surge and the resistance value should then be decreased at a predetermined rate to dampen the subsequent voltage surges which are reflected from the system after full round trip travel time until the resistance is finally stepped out.

OBJECTS OF THE INVENTION It is an object of the present invention to provide improved closing resistors which maintain the lowest possible switching surge overvoltages.

Another object is to provide improved closing resistors which have variable rather than fixed resistance values.

Another object is to provide improved variable closing resistors which limit surge voltages to less than 210 percent of normal system voltage.

Another object is to provide improved variable closing resistors which permit even further reduction of insulation levels and its associated equipment as compared to prior art closing resistors.

Another object is to provide improved variable closing resistors which drain any charge trapped on the line in accordance with a preferred rate of decay.

Other objects and advantages of the invention will hereinafter appear.

DRAWINGS The accompanying drawings illustrate several preferred embodiments of the invention, but it is to be understood that the embodiments illustrated are susceptible of modifications with respect to details thereof without departing from the scope of the appended claims.

FIG. 1 is a schematic diagram showing an electrical installation comprising a multiphase source of electrical power, a transmission system which is adapted to be connected thereto and means comprising switches and variable closing resistors in accordance with the invention;

FIG. 2 is a graph wherein resistance value is plotted against time and which illustrates the performance characteristics of three types of variable closing resistors in accordance with the present invention;

FIG. 3 is a schematic showing of another type of closing switch and variable closing resistor usable with the source and system shown in FIG. I; and

FIG. 4 is a schematic showing of a third type of closing switch and variable closing resistor usable with the source and system shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown an electrical installation comprising a source 10 of electrical power and an electrical system 20. Source 10 is understood, for example, to be a three-phase high-voltage source which is grounded on one side as at 12 and is provided on its other side with three

terminals

14, 16, 18. The terminals l4, 16, I8 of source I0 are adapted to be connected and disconnected from system 20 which is understood, for example, to be a threephase highvoltage transmission system comprising three lines 22, 24 and 26, respectively. It is to be understood. for example, that source 10 generates a voltage on the order of 230 kv. or above, and that system 20 is of some predetermined length, for example, miles and that the system including its associated equipment has inductance designated 28 and capacitance designated 30 which can be determined by calculation. In FIG. 1, the inductance in line 22, for example, is hereinafter represented by the symbol L and the capacitance is hereinafter represented by the symbol C.

Means are provided to connect and disconnect system 20 to source 10 and such means take the form of

suitable switches

32, 34, and 36 in the lines 22, 24 and 26, respectively, which in practice would be circuit breakers. disconnect switches, air break switches or the like. The switches are shown in FIG. I in open condition.

In accordance with the invention,

variable closing resistors

38, 40, 42 are shown associated with the lines 22, 24 and 26. respectively. The variable closing resistors are adapted to be inserted in series circuit between the system and the source for a predetermined length of time after the source and system are connected. For purposes of discussion, terminal 14, switch 32 and variable closing resistor 38 associated with line 22 may be considered as typical of corresponding components detailed description of the operation of the components associated associated with line 22 will be sufficient to illustrate operation of the others.

As FIG. I shows, variable closing resistor 38 comprises a resistive portion 380 and a contact portion 3812. It is to be understood that as switch 32 is moved to close, it first makes contact with contact portion 38b of closing resistor 38 and sweeps smoothly through the full range of predetermined resistance values (from high to low values without steps) built into resistive portion 38a of resistor 38 in a predetermined amount of time, as FIG. 2 shows in connection with line 39.

In the embodiment shown in FIG. 1, resistance value declines at an exponential rate from a predetermined high-resistance value which is about four times normal line surge impedance ofline 22, as shown in FIG. 2, until it is stepped out of the system. Normal line surge impedance of line 22 is calculated as VZITC=Z0 and is so designated in the graph in FIG. 2, In practice, a 230 kv. system having a line surge impedance of Zo=3000 would use a variable closing resistor having a high value of about 1200.0. which decreases exponentially to a low value of about 1600. at which point it is removed from the system and the switch is fully closed. A high value of 4XZo is selected because it will overdamp the first voltage surge of the system and it is decreased exponentially because such rate of decline closely relates to the impedance associated with subsequent, progressively smaller voltage surges which are reflected from the system. The time during which resistance value declines exponentially is preferably selected to be about 8 milliseconds, the time of a normal half cycle in the line. However, this time as a practical value may vary depending on the nature of the system and the practical limits governing switch-closing operations.

FIG. 3 schematically shows a combined switch 32 and a variable closing resistor 44 which may be used instead of the switch 32 and resistor 38 of FIG. 1. Resistor 44 comprises resistive portions 44a and contact points 4412. Resistor 44 is understood to be of a type where resistance values decrease in small incremental steps but at a substantially exponential rate, as shown by the line 45 in the graph of FIG. 2.

FIG. 4 schematically shows a combined switch 32 and a variable closing resistor 46 which may be used instead of the type shown in FIGS. 1 and 3. Resistor 46 comprises a resistive portion 46a, a contact point 46!), and means 46d whereby the resistor value is automatically varied before switch 32 is moved from contact point 46b to connect directly with line 22 at point 46c. In practice, resistor 46 might take the form of a wire-wound, carbon pile or liquid resistor. Preferably, resistor 46 decreases in value at an exponential rate as shown by line 39 in the graph of FIG. 2.

While it is preferable that the resistor values be varied at an exponential rate to get maximum surge reduction, voltage surges are reduced well below that obtained from conventional closing resistors, if resistor value decreases at a linear rate similar to the line 48 shown in the graph of FIG. 2, i.e., within the ohmic values and times specified in the graph. Superior performance is also achieved by a resistor having neither a true exponential nor a true linear drop but which has a rapid decrease initially and a somewhat slower decrease in value thereafter, as shown by line 50 in the graph of FIG. 2. 230 kv. and above and having line lengths 230 kv. and above were on the order of 300 to 500 It is to be understood, however, that while actual experiments with resistors having exponentially decaying resistance characteristics and resistors having linearly decaying resistance characteristics provided the results herein described, resistors having other nonlinear rates of decay (such as sinetype curves or hyperbolic-type curves) come within the purview of the invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrical power transmission installation comprising:

a source of electrical power,

a high-voltage electrical system comprising a plurality of spaced-apart transmission lines and having inductance and capacitance and tending to exhibit voltage surge which initially are several times normal system peak voltage during the transient period when energized from said source,

switch means for connecting said system to said source,

and variable impedance means adapted to be inserted in circuit between said system and said source by said switch means for a time interval less than one cycle of the fundamental system frequency to reduce said voltage surges to less than about 210 percent of the normal peak voltage of the system when said switch means connect said system to said source,

said switch means being adapted to decrease said variable impedance means from an initial predetermined value which is greater than about four times normal line surge impedance when first inserted through at least one value which is smaller than said initial value at a predetermined rate until said impedance means is removed from the circuit.

2. An installation according to claim 1 wherein said impedance means is decreased in value by said switch means at a constant rate.

3. An installation according to claim 1 wherein said impedance means is decreased in value by said switch means at an exponential rate.

4. An installation according to claim 1 wherein said impedance means is decreased in value by said switch means in incremental steps.

5. An installation according to claim 1 wherein said impedance means is decreased in value by said switch means first at a rapid rate and subsequently at a slower rate.

6. An installation according to claim I wherein said impedance means is decreased in value by said switch means at a rate which is related to the impedance of successive voltage surges in the system.

7. An installation according to claim 1 wherein said impedance means is decreased in value by said switch means at a constant rate.

8. An installation according to claim I wherein said impedance means is decreased in value by said switch means at an exponential rate.

9. An installation according to claim 1 wherein said impedance means is decreased in value by said switch means in incremental steps.

Claims

1. An electrical power transmission installation comprising: a source of electrical power, a high-voltage electrical system comprising a plurality of spaced-apart transmission lines and having inductance and capacitance and tending to exhibit voltage surge which initially are several times normal system peak voltage during the transient period when energized from said source, switch means for connecting said system to said source, and variable impedance means adapted to be inserted in circuit between said system and said source by said switch means for a time interval less than one cycle of the fundamental system frequency to reduce said voltage surges to less than about 210 percent of the normal peak voltage of the system when said switch means connect said system to said source, said switch means being adapted to decrease said variable impedance means from an initial predetermined value which is greater than about four times normal line surge impedance when first inserted through at least one value which is smaller than said initial value at a predetermined rate until said impedance means is removed from the circuit.

6. An installation according to claim 1 wherein said impedance means is decreased in value by said switch means at a rate which is related to the impedance of successive voltage surges in the system.

8. An installation according to claim 1 wherein said impedance means is decreased in value by said switch means at an exponential rate.