DESC:CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of Indian Provisional Application No. 3263/MUM/2013, filed on Oct. 17, 2013. The entire disclosure of the above application is incorporated herein by reference.

FIELD OF INVENTION

[0002] This invention generally relates to power transmission and distribution systems and protection of an electrical equipment against overvoltage. Particularly, this invention relates to disconnecting device for use in association with surge arresters used in protecting an electricalequipment against overvoltage.

BACKGROUND OF THE INVENTION

[0003] Surge arresters are extensively utilized in power transmission and distribution systems for protection of the power system equipment against transient over voltages caused due to lightning and switching activities. Surge arresters offer high resistance to nominal system voltage across power lines, but offer low resistance to surges generated by transient over voltages. Once the surge disappears, the voltage may drop and the surge arrester typically returns to the high resistance state. However, in case of surge arresterfailure or malfunction, the high resistance state is not resumed and the surge arrester continues to provide electrical path from the power line to the electrical ground to which the surge arrester is connected. This may result in short circuit condition causing breakdown of associated circuitry. In such a scenario, the surge arrester needs a replacement.

[0004] Disconnecting devices are commonly used in conjunction with the surge arresters. The disconnecting devices are employed to isolate an associated surge arrester from the system in the remote event of failure. The disconnecting devices are connected in series with the surge arrester through the earth conductor for preventing a persistent fault.

[0005] However, prior art disconnecting devices used in conjunction with the surge arresters are slow in operation compared to high speed earth fault power system circuit breakers. The operation of these high speed circuit breakers prior to the operation of the disconnecting device causes undesired interruption of the power supply to the associated electrical circuitry. Furthermore, prior art disconnecting devices have a lower energy handling capability and thus are used for low energy rating applications limited to distribution systems.

[0006] Yet another disadvantage associated with some of the disconnecting devices described in the prior art include usage of cartridge that uses hazardous chemicals that are subject to statutory regulation.

[0007] Hence, there is a need for a disconnecting device that exhibits high operating speed that prevents unsolicited operation of the circuit breaker thereby avoiding undesired interruption in the power supply. Further, it is desired that such a disconnecting device also avoids usage of hazardous chemicals and has an increased energy handling capacity that extends the usage of the disconnecting device to include transmission and substation applications apart from distribution application.

BRIEF DESCRIPTION OF THE INVENTION

[0008] In one embodiment, a disconnecting device for a surge arrester is provided.The disconnecting device comprises a first portion and a second portion, the first portion coupled to the surge arrester through a first terminal and the second portion coupled to an electrical ground through a second terminal, a bobbin extending between the first terminal and the second terminal, a resistive element wound around the bobbin, the resistive element configured for carrying a fault current generated within the surge arrester, in an event of arrester failure or on the verge of arrester failure, and a chemical agent disposed between the first portion and the bobbin such that heating up of the resistive element due to the fault current initiates a chemical reaction that generates pressure so as to separate the second portion from the first portion resulting in isolation of the surge arrester from the ground. Further, the chemical agent may comprise oxidizing agent such as potassium chlorate.

[0009] In another embodiment, a surge arrester assembly is provided that comprises a surge arrester configured for grounding transient over voltages and a disconnecting device coupled in series with the surge arrester, the disconnecting device configured for isolating the surge arrester in response to detecting a fault current.The disconnecting device comprises a first portion coupled to the surge arrester via a first terminal, a second portion coupled to an electrical ground via a second terminal, a bobbin located within the first portion, a resistive element wound spirally around the bobbin, the resistive element configured for carrying the fault current and a chemical agent disposed between the first portion and the bobbin and in direct contact with the resistive element such that the fault current flowing through the resistive element triggers a chemical reaction so as to generate pressure in order to separate the second portion from the first portion, thereby enabling the disconnecting device to isolate the surge arrester from the ground. In one embodiment, the chemical agent comprises an oxidizing agent such as for example potassium chlorate.

[0010] Various other features and advantages of the invention will be made apparent from the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

[0012] FIG. 1 shows a block diagram depicting a surge arrester assembly connected across an electrical equipment; and

[0013] FIG. 2 is a cross sectional view of a disconnecting device as described in one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] It shall be observed that system components described in accordance with exemplary embodiments have been represented by known symbols in the figures, showing only specific details that are relevant for an understanding of the present disclosure. Further, details that are readily apparent to those skilled in the art may not have been disclosed.

[0015] An object of the present invention is to provide a disconnecting device for a surge arrester which is least sensitive to transient over voltages and operable when exposed to an alternating currentfor a long duration of time.

[0016] Another object of the present invention is to provide a disconnecting device that functions with an increased operation speed to disconnect a surge arrester from an electrical ground in the event of surge arrester failure or potential surge arrester failure.

[0017] It is yet another object of the present invention to provide a disconnecting device that exhibits high energy withstanding capacity.
[0018] It is yet another object of the present invention to provide a disconnecting device that does not use a hazardous chemical that is subject to statutory regulation during transportation and/or handling.

[0019] A disconnecting device is used in conjunction with a surge arrester. Surge arresters are extensively utilized in power transmission and distribution systems for protection of the power system equipment/electrical equipment against transient over voltages.

[0020] Accordingly, in one embodiment as shown in FIG. 1, a surge arrester assembly 100 connected across an electrical equipment 102 that is to be protected is provided. The electrical equipment 102 may be for example a distribution transformer or a power transformer. The surge arrester assembly 100 may be connected to a power line along with aseries connection to an electrical ground. Further, the surge arrester assembly 100 is connected to the power line in parallel with the electrical equipment 102 as shown in FIG. 1. The surge arrester assembly 100 comprises a surge arrester 104 configured for grounding transient over voltages and a disconnecting device 106 coupled in series with the surge arrester 104, the disconnecting device 106 being configured for isolating the surge arrester 104 in response to a fault currentbeing detected.The fault current includes a current generated at a surge arrester that has failed anda leakage current exceeding the standard leakage current value, generated at a surge arrester that is on the verge of failing. The fault current is typically in the order of several amperes to kilo amperes. The disconnecting device 106 is further explained conjunction with FIG. 2.

[0021] Transient overvoltage may occur in multiple situations that may lead to system disturbances, including but not limited to fault condition, switching surges activated during operation of the circuit breaker (not shown) and induced over voltages resulting from lightning strikes and other such natural phenomena. These transient over voltages greatly exceed the voltage levels experienced by the surge arrester 104 during normal operating conditions.

[0022] The electrical characteristics of the surge arrester 104 are such that, the surge arrester 104 permits virtually no conduction under normal steady state operating voltages. However, transient over voltages that exceed a clamping voltage limit of the surge arrester 104, can be rapidly directed to an electrical ground thereby protecting the electrical equipment 102 connected across the surge arrester assembly 100.

[0023] Each surge arrester 104 is designed to exhibit high energy handling capacity. Prolonged exposure to abnormally high voltages and/or transient over voltages may cause the surge arrester 104 to fail leading toa short circuit. The failure of surge arrester 104 may enable the operation of a circuit breaker (not shown) in select cases.

[0024] Thecircuit breaker (not shown) is an automatically operated electrical switch designed to protect an electrical circuit and/or equipment 102 from damage caused by overload or short circuit. The basic function of a circuit breaker (not shown) is to detect a fault condition and interrupt current flow. The circuit breaker (not shown) can be reset either manually or automatically to resume normal operation.

[0025] Turning now to FIG. 2,a disconnecting device 200 for a surge arrester 104 is provided. The disconnecting device 200 is similar in construction to the disconnecting device 106 described with respect to FIG. 1. During normal, non-fault operation of the surge arrester 104, little or no-current passes through the disconnecting device 200. When subjected to overvoltage conditions, the surge arrester 104 experiences one or more high current pulses which pass through thedisconnecting device 200 to the electrical ground.

[0026] Surge arrester 104 is conventional and thus not described in detail. The disconnecting device 200 comprises a first portion 204 and a second portion 206, the first portion 204 coupled to the surge arrester 104 through a first terminal 208 and the second portion 206 coupled to an electrical ground through a second terminal 210.

[0027] The second portion 206 may be physically coupled to the first portion 204 using an adhesive such as epoxy. Suitable alternatives for the coupling may however be used. The first portion 204 and the second portion 206 are formed of any suitable insulating material such asthermo plastic or thermoset plastic materials. The line lead connecting the first terminal 208 to the surge arrester 104 is insert molded onto the first portion 204.

[0028] The disconnecting device 200 further comprises a bobbin 212 extending between the first terminal 208 and the second terminal 210 and a resistive element 214 wound around the cylindrical shaped bobbin 212. The resistive element 214 is configured for carrying a fault current generated within the surge arrester 104 in an event of an actual arrester failure or a potential arrester failure. In an exemplary embodiment, the bobbin 212 is made of an insulating material such as a thermoplastic or a thermoset plastic and the resistive element 214 may be formed using ferrous chromium aluminum alloy.

[0029] The first terminal 208 is structured so as to receive a stud from the surge arrester 104.An externally screw threaded stud that is provided at the lower terminal of the surge arrester 104 is received within an internally screw threaded first terminal 208. Further, at least a portion of the first terminal 208 may be molded to the inner wall of the first portion 204 of the disconnecting device 200. The disconnecting device 200 further comprises an arc pin 216 coupled to the first terminal 208.The arc pin 216 is configured for transmitting the transient over voltage from the surge arrester 104 to the ground.

[0030] As the first terminal 208 extends towards thesecond portion 206, the first terminal 208 is separated by the first portion 204 through the bobbin 212. The shape of the first terminal 208 is tapered towards the second portion 206 so as to give rise to an air gap between the first terminal 208 and the bobbin 212. This air gap is essential in building up appropriate clearance for arriving at a measured spark gap 217.

[0031] During normal operation of the surge arrester 104, the arc pin 216 is configured to divert the overvoltage to the electrical ground. As illustrated in FIG.2, space within the first portion 204 between arc pin 216 and the bobbin 212 provides thespark gap 217 through which high current impulses travel to the electrical ground. Within the disconnecting device 200, the high current pulses travel through the arc pin 216 and spark gap 217 for connection to the second terminal 210 and subsequently to the electrical ground.

[0032] The arc pin 216 and the first terminal 208 may be formed of a conductive material such as a metal or a metal alloy. In one exemplary embodiment, the first terminal 208 is formed of aluminum or aluminum alloy and the arc pin 216 is formed of stainless steel. Suitable alternatives may however be used.

[0033] The second portion 206 may be formed of an insulating material such a thermoplastic or a thermoset plastic. However, the second terminal 210 employed for connecting to the ground may be formed of a conductive material such as a metal or a metal alloy. The second terminal 210 may comprise internal threading to receive a cable that connects the second terminal 210 to the ground. Further, the ground lead connecting the second terminal 210 is also insert molded onto the second portion 206.In one exemplary embodiment, the second terminal 210 is formed of aluminum or aluminum alloy.

[0034] Still referring to FIG. 1, the disconnecting device 200 further comprises a resistive element 214 spirally would around the non-conductive bobbin 212.The resistive element 214 is configured for carrying a fault current generated within the surge arrester 104. A chemical agent 218 is disposed between the first portion 204 and the non-conductive bobbin 212, such that the chemical agent 218 is in direct contact with the resistive element 214. In one preferred embodiment, the chemical agent 218 comprises an oxidizing agent such as potassium chlorate.

[0035] In the event of a sustained over voltage and/or failure of the surge arrester 104, a fault current willflow through the disconnecting device 200. In the process,the resistive element 214heats up the chemical agent 218initializing a chemical reaction. The chemical reaction thus activated generates a mechanical force that expels the second portion 206 from the first portion 204. This action electrically disconnects the surge arrester 104 from the ground and also provides a visual indication of the need for surge arrester 104 replacement.

[0036] When the surge arrester 104 is normally functioning while withstanding transient over voltage, impedance of the resistive element 214 is so graded that the fault current does not flow through the resistive element 214 and therefore the transient voltage is applied across the spark gap 217 making the impulses flow through the spark gap 217 to the ground. However, if the surge arrester 104 fails to withstand the transient voltage, arcs may be generated over a sufficiently longer duration to activate a chemical reaction causing the mechanical separation of the portions 204 and 206 from one another. This action electrically disconnects the surge arrester 104 from the electrical ground and provides a visual indication of the need for surge arrester 104 replacement.

[0037] Further, the disconnecting device 200 is configured to be operated in a similar fashion, when experiencing alternating current (AC) for a time period longer than a predetermined duration. A continuous flow of alternating current heats up the resistive element 214, that upon reaching a threshold temperature induces a reaction in the chemical agent 218 that surrounds the resistive element 214. The pressure generated due to chemical reaction propels the second portion 206 to separate from first portion 204 thereby severing the current path.
[0038] Advantageously, the potassium chlorate dissociates quickly even at lower fault currents, thereby operating the disconnecting device 200 to disconnect the surge arrester 104 from the system within a short time period. The surge arrester 104 thus disconnected prevents unsolicited operation of the circuit breaker. This way, power supply of the system continues without any interruption thereby avoiding unscheduled power cut.

[0039] Though, the disconnecting device 200 is described as having a combination of resistive element 214 formed of ferrous-chromium-aluminum alloy and chemical agent 218 comprising potassium chlorate, suitable alternatives that when combined give rise to similar characteristic chemical reaction, fall within the scope of the invention.

[0040] The disconnecting device 200 of the present invention has a high sensitivity and a high operating speed that enables isolation of the surge arrester 104 from the ground even at a lower fault current level thereby preventing unsolicited operation of the circuit breaker.

[0041] Further, construction of the disconnecting device 200 also provides mechanical strength compatible with applications such as transmission line arrester (TLA) installation and operation.

[0042] The disconnecting device 200 described herein has higher energy handling capacity and thus can be used in conjunction with surge arresters that have higher energy ratings. For example, surge arresters used in transmission and substation application.

[0043] Another advantage associated with the disconnecting device 200 described in the invention is usage of chemical agents 218 that are subject to lesser statutory regulations during transportation and usage thereby alleviating the need to use hazardous chemicals that are subject to higher statutory regulations. The usage of the chemical agent 218 thus provides for a safer option.
[0044] It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.
,CLAIMS:1. A disconnecting device (200) for a surge arrester (104), the disconnecting device (200) comprising:

a first portion (204) coupled to the surge arrester (104)through a first terminal (208);

a second portion (206) coupled to an electrical ground through a second terminal (210);

a bobbin (212) extending between the first terminal (208) and the second terminal (210);

a resistive element (214) wound around the bobbin (212), the resistive element (214) configured for carrying a fault current generated within the surge arrester (104)in an event of arrester failure; and

a chemical agent (218) disposed between the first portion (204) and the bobbin (212) such that heating up of the resistive element (214) due to the fault current initiates a chemical reaction that generates pressure so as to separate the second portion (206) from the first portion (204) resulting in isolation of the surge arrester (104)from the ground;

wherein the chemical agent (218) comprises potassium chlorate.

2. The disconnecting device (200) of Claim 1, further comprising:

an arc pin (216) coupled to the first terminal (208), the arc pin (216) configured for conducting a transient over voltage from the surge arrester (104) to the electrical ground through a spark gap (217).
3. The disconnecting device (200) of claim 1, wherein the resistive element (214) is formed of ferrous-chromium-aluminumalloy.

4. The disconnecting device (200) of claim 1, wherein the first portion (204) is coupled to the second portion (206) using an adhesive.

5. A disconnecting device (200) for a surge arrester (104), the disconnecting device (200) comprising:

a first portion (204) coupled to the surge arrester (104)through a first terminal (208);

a second portion (206) coupled to an electrical ground through a second terminal (210);

an arc pin (216) coupled to the first terminal (208) and configured for transferring a transient over voltage from the surge arrester (104)to the electrical ground via a spark gap (217);

a non-conductive bobbin (212) located within the first portion (204);

a resistive element (214) spirally would around the non-conductive bobbin (212), the resistive element (214) configured for carrying a fault current generated within the surge arrester (104); and

a chemical agent (218) disposed between the first portion (204) and the non-conductive bobbin (212), such that the chemical agent (218)which is in direct contact with the resistive element (214) triggers a chemical reaction, when the fault current flows through the resistive element (214), so as to generate a pressure in order to separate the second portion (206) from the first portion (204) resulting in the isolation of the surge arrester (104) from the ground.

6. The disconnecting device (200) of claim 5, wherein the chemical agent (218) comprises potassium chlorate.

7. The disconnecting device (200) of Claim 5, wherein the resistive element (214) is formed of ferrous-chromium-aluminum alloy.

8. The disconnecting device (200) of claim 5, wherein the first portion (204) is coupled to the second portion (206) using an adhesive.

9. A surge arrester assembly(100) comprising:

a surge arrester (104)configured for grounding transient over voltages; and

a disconnecting device (200) coupled in series with the surge arrester (104), the disconnecting device (200) configured for isolating the surge arrester (104)in response to detecting a fault current, the disconnecting device (200) comprising:

a first portion (204) coupled to the surge arrester (104)via afirst terminal (208),

a second portion (206) coupled to an electrical ground via asecond terminal (210);

a bobbin (212) enclosed within the first portion (204);

a resistive element (214) wound spirally around the bobbin (212), the resistive element (214) configured for carrying a fault current; and

a chemical agent (218) disposed between the first portion (204) and the bobbin (212) and in direct contact with the resistive element (214) such that the fault current flowing through the resistive element (214) triggers a chemical reaction so as to generate pressure in order to separate the second portion (206) from the first portion (204), thereby enabling the disconnecting device (200) to disconnect the surge arrester (104)from the ground;

wherein the chemical agent (218) comprises potassium chlorate.

10. The surge arrester assembly (100) of claim 9, wherein the surge arrester assembly is connected across an electrical equipment (102) that is to be protected.

11. The surge arrester assembly (100) of claim 9, wherein the disconnecting device (200) further comprises an arc pin (216) coupled to the first terminal (208), the arc pin (216) configured for transmitting the transient overvoltage from the surge arrester (104) to the ground through a spark gap (217).

12. The surge arrester assembly (100) of claim 9, wherein the resistive element (214) is formed of ferrous-chromium-aluminumalloy.

13. The surge arrester assembly (100) of claim 9, wherein the first portion (204) is coupled to the second portion (206) using an adhesive.