Claims:We claim:
1. A current transformer (500) with blast preventing capabilities, the current transformer (500) comprising:
a primary terminals (501) connected with primary line conductor (502) for transmission of power;
a ring core (504) in which the primary line conductor (502) runs;
a major insulation (503) is provided in between the ring core (504) and the primary line conductor;
an expansion shell (506) is provided on top end of the current transformer and covers the primary terminals (501), expansion shell (506 )
characterized in that
a spring loaded valve (507) is provided in the expansion shell (506) to release pressure by allowing passage of gas generated inside the current transformer (500).
2. The current transformer (500) as claimed in claim 1, wherein the current transformer (500) further comprises a canopy (508) and tray (511) provided on top of the spring loaded valve (507) to restrict oil spillage abruptly and to guide spewed gas and oil towards a particular defined safe path to avoid damage to any other equipment in vicinity.

3. The current transformer (500) as claimed in claim 1, wherein the current transformer (500) further comprises an electrical contact coupled with the spring loaded valve (507), the electrical contact SWITCH OFF the electrical system and transformer during system disturbance condition.

4. The current transformer (500) as claimed in claim 3, wherein the electrical contact

put the electrical system and transformer in system in SWITCH OFF condition when the spring loaded valve (507) is in open condition due to pressure; and
put the electrical system and transformer in system in SWITCH ON condition when the spring loaded valve (507) is in closed condition due to no extra high pressure inside the current transformer (500).

5. The current transformer (500) as claimed in claim 1, wherein the spring loaded valve (507) opens aperture to release gases and closes the aperture automatically when pressure is low due to spring action to protect the current transformer (500) from damage.

6. The current transformer (500) as claimed in claim 1, wherein the major insulation (503) comprises of large number of composite layers of cellulose paper wounded over each other covering the primary conductor and the ring core (504) is not damaged due to retention of hermetically sealing condition even after the current transformer is subjected to extra high pressure.

7. The current transformer (500) as claimed in claim 1, wherein the spring loaded valve (507) keeps the current transformer in hermetically sealed condition even after the extra high pressure is released.

8. The current transformer (500) as claimed in claim 1, wherein the current transformer (500) is insulated by porcelain insulator (505) and saved from getting damaged due to generation of extra high pressure.

9. The current transformer (500) as claimed in claim 1, wherein the current transformer (500) Switch OFF during system disturbance condition and protect and isolate the transformer and other electrical equipment from danger.

10. The current transformer (500) as claimed in claim 1, wherein the major insulation (503) is provided in between the primary line conductor (502) and ring core (504) due to large potential difference between them, protects itself and also the system from the effects of extra high pressure.
, Description:BLAST PROOF CURRENT TRANSFORMER
FIELD OF INVENTION:
[001] The present subject matter described herein, relates to oil type current transformer, and, in particular, to prevention of blasting of the Oil type Current transformers (CT) which is regularly manufactured for use in electric sub-stations for power generation and transmission. More particularly, the present subject matter relates to a blast proof oil type current transformers (CT) having capability to prevent blasting by reducing pressure building inside the oil type current transformer
BACKGROUND AND PRIOR ART:
[002] Electric power is the easiest form of energy that can be generated, transmitted and utilized for different segments of Society. It is important to protect the different equipment that is used for power transmission against different types of fault. During fault conditions, the voltage and current of the transmission system becomes unstable (i.e., transient) and reaches a value higher than the rated values of the system. These faults normally appear for a very short time before the different protection system gets activated and the system is switched off. However, these short time over voltage and current can make the system unstable and damage the equipment. On such events, it is important to protect the equipment from damaging even at the cost of isolating the equipment.
[003] As shown in figure 1a and figure 1b, a current transformer is connected in series with the equipment at the input and output terminals of the transformer. The sole objective is that the current transformer will sense the current in the power transmission line and generate appropriate signals for metering and protection. A current transformer is not designed to sense the voltage. But it will be subjected to the voltage appearing on the line. During system disturbance, the voltage of the transmission line becomes unstable. It could be very high, it could be fluctuating, such as increase and decrease very quickly; then the insulation provided in the current transformer will be more stressed. The stressed insulation will give rise to different types of abnormalities in the form of discharges, arc, spark inside the current transformer which is fully submerged in the oil. These abnormalities will generate heat and gas inside the current transformer and that will increase the pressure inside. With increase in pressure inside the current transformer it could break/blast.
[004] Further, the oil and insulation are under continuous stress during working and there is always some gradual deterioration in their property which is manifested by increase in the tan delta and power factor values. With gradual deterioration, the situation comes when the insulation fails. With such failure of insulation while power supply is still continuing, pressure will develop inside the current transformer and can result in blast.
[005] A blasting of the current transformer could lead to breaking of the insulator, gushing out of hot oil. The breakage of the current transformer endanger the power flow, since it will break the line conductor passing through it resulting a high sudden high stress on the connected power equipment leading to power failure. The insulator is basically porcelain. When it breaks under blasting condition, fragmented pieces of porcelain are thrown off in different direction at a very high speed .The breaking of insulation could damage the other equipment in it is vicinity and may even harm people working around.
[006] The oil splash owing to breakage of Current transformer in the sub-station could endanger the surroundings. If the hot oil falls on any electrically charged conductor/equipment like capacitor, bushing terminals or live conductor of the transmission line, it could cause fire incident which will be causing to catastrophic damage to the sub-station. With each transformer, there are at least 6 current transformers (1 on each phase of 3 incoming lines and 1 on each phase of 3 outgoing lines), so the probability of the hazard is multiplied many times.
[007] In art a method and device is required which can reduce the pressure inside the current transformer in the event of abnormal situation, so that blasting of the current transformer and all the associate problems can be avoided. Therefore, the present invention provides a current transformer which is blast proof and inexpensive.
OBJECTS OF THE INVENTION:
[008] The principal objective of the present invention is to provide a system for reducing pressure inside the current transformer to avoid blasting.
[009] Another object of the present subject matter is to provide a current transformer for sustaining the abnormal situation of the power system for very short unstable condition of very short duration.
[0010] Another object of the present subject matter is to provide a current transformer which sustains the abnormal situation (without blasting) of the power system during unstable condition for few milliseconds till the other signals from associated equipment trigger to isolate the system.
[0011] Another object of the present subject matter is to provide a current transformer which can avoid blasting during the deterioration of the insulation while power is still flowing.
[0012] Yet another object of the present invention is to provide a safety in working environment by avoidance of blasting of current transformer.
[0013] Yet another object of the present invention is to provide a system for avoiding oil spillage and fire hazards, which could be created due to blasting of current transformer.
SUMMARY OF THE INVENTION:
[0014] The subject matter disclosed herein relates to a current transformer with blast avoiding capabilities. The current transformer has a self-reclosing type spring loaded valve with canopy and tray in lieu of diaphragm to avoid blasting and spewing of hot oil. During the high pressure inside the current transformer, the spring loaded valve operates/opens and allow passage of hot gases. Due to inner high pressure, the gases pushes the spring valve in upside direction and by the movement of the spring valve an aperture is opened to allow passage of gases in environment. By releasing the gases in the environment, the pressure inside the current transformer reduces and thus avoids blasting of current transformer. Once the pressure inside the current transformer is reduce, the spring valve again closes the aperture and back into the hermetically sealed condition to avoid entering of foreign particles and moisture in the current transformer, thereby preventing deterioration of the insulation. Further, the present subject matter provides an electrical contact with the valve for switch ON and OFF of the electrical system and transformer during disturbance conditions to block flow of unstable current and voltage. Thus the present current transformer, protect the other electrical appliances and transformer from hazardous effects of unstable current and voltage.
[0015] In order to further understand the characteristics and technical contents of the present subject matter, a description relating thereto will be made with reference to the accompanying drawings. However, the drawings are illustrative only but not used to limit scope of the present subject matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system or methods in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:
[0017] Fig. 1a illustrates single line diagram and fig. 1b showing the location of the Current Transformer (CT) in the sub-station structure, in accordance with an embodiment of the present subject matter;
[0018] Fig. 2a and 2b illustrates toroidal ring core and secondary winding on the toroidal ring core of Current Transformer as known in the art;
[0019] Fig. 3 illustrates basic construction of Current Transformer as known in the art;
[0020] Fig. 4 illustrates construction of current transformer with Diaphragm for avoiding blast in the CT as known in the art;
[0021] Fig. 5 illustrates construction of current transformer with Pressure Release Valve with canopy and tray to avoid blasting of CT and spillage of oil abruptly, in accordance with an embodiment of the present subject; and
[0022] Fig. 6 illustrates the oil Tray placed below the Pressure Relief Valve, for collecting the spilled oil and guiding them to flow out in a decided path; so that the oil can be collected in a pit and the sub-station ground is not wet by the oil, in accordance with an embodiment of the present subject.
[0023] The figures depict embodiments of the present subject matter for the purposes of illustration only. A person skilled in the art will easily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.
DESCRIPTION OF THE PREFERRED EMBODIMENTS:
[0024] The subject matter disclosed herein relates to a current transformer with blast avoiding capabilities. The current transformer has a spring loaded valve with canopy and tray in place of diaphragm to avoid blasting and spewing of hot oil. During the high pressure inside the current transformer, the spring loaded valve operates and allow passage of pressurized hot gases. Due to inner high pressure, the gases pushes the spring valve in upside direction and by the movement of the spring valve an aperture is opened to allow passage of gases in environment. By releasing the gases in the environment, reduces the pressure inside the current transformer and thus avoids blasting of current transformer. Once the pressure inside the current transformer reduces, the spring valve again closes the aperture and back into the hermetically sealed condition to avoid entering of foreign particles like moisture in the current transformer. Further, the present subject matter provides an electrical contact with the valve for switch ON and OFF of the electrical system and transformer during disturbance conditions to block flow of unstable current and voltage. Thus the present current transformer, protect the other electrical appliances and transformer from unstable current and voltage.
[0025] It should be noted that the description and figures merely illustrate the principles of the present subject matter. It should be appreciated by those skilled in the art that conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present subject matter. It should also be appreciated by those skilled in the art that by devising various arrangements that, although not explicitly described or shown herein, embody the principles of the present subject matter and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the present subject matter and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. The novel features which are believed to be characteristic of the present subject matter, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures.
[0026] These and other advantages of the present subject matter would be described in greater detail with reference to the following figures. It should be noted that the description merely illustrates the principles of the present subject matter. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present subject matter and are included within its scope.
[0027] Current transformer is located between the equipment, i.e., transformer and the transmission line. They are connected in series with incoming and out going transmission line. The Current Transformer (CT) primary sees a voltage equal to the potential of the transmission line, i.e., 11 kV or 33 kV or 66 kV or 132 kV or 220 kV or 400 kV or even higher. The primary winding of the Current Transformer is connected to the transmission line that runs through the inner diameter of the ring core of the CT. As illustrated in the figure 2a and 2b, the ring core 201 is made of silicon steel rolled into a toroidal ring form. The ring core develops magnetization owing to the running of the primary line conductor of the current transformer through it. Power from the primary winding is transferred to the secondary winding 202 by virtue of magnetic induction activated by the magnetized ring core. The secondary terminal of the current transformer is connected to very low voltage metering and protection relay circuitry. The low voltage CT secondary winding is wound on the toroidal ring core 201 of the CT.
[0028] Fig. 3 illustrates the construction of existing current transformers. The current transformer 300 has primary terminals 301, major insulation 302, porcelain insulator 303, toroidal ring core 304, flanges 305 and expansion shell 306. The current transformer is Hair pin type oil filled current transformer. The toroidal ring core 304 is at ground potential and the secondary windings are at very low potential where as the CT primary is at line voltage, i.e., 11 kV or 33 kV or 66 kV or 132 kV or 220 kV or 400 kV or higher. Owing to the large potential difference between the toroidal ring core and the Current transformer primary line conductor, high volume of insulation, i.e., major insulation 302 is provided between them. The primary and secondary windings are also to be isolated electrically which have a very large potential difference between them. The secondary voltage is in the tune of 5 volt to 2000 volts where as the primary line terminal is at rated line voltage up to kilo volts, i.e., 11 kV or 33 kV or 66 kV or 132 kV or 220 kV or 400 kV or higher. The major insulation 302 comprises of large number of composite layers of cellulose paper wound over each other covering the CT primary conductor and also the core of the Current transformer.
[0029] At the time of system disturbances, when transient conditions arises. Under the transient conditions, voltage fluctuates and could reach to very high value may be for short duration or prolonged duration of few seconds. The current also could be very high, i.e., 40 kilo-ampere or 63 kilo-ampere or higher depending upon the type of fault appearing on the transmission line. In some situations, even fault in some neighboring line, i.e., sympathetic unbalancing also causes flow of high current in the current transformers 300. These high current and fluctuating high voltages over stress the oil and major insulation 302 inside the CT 300. Owing to these high stresses, the major insulation 302 gets highly stressed causing arcs, sparks, and discharges generating gas. As a result high pressure is generated inside the CT. In the event of prolonged pressure or sustained abnormalities or sudden high pressure the CT could blast.
[0030] The CT 300 is continuously connected to the series with the power equipment and causing continuous stressing of the major insulation 302 inside the CT 300. The insulating property of the major insulation 302 gets deteriorated gradually due to stressing, resulting increase in Tan Delta and power factor values; which is an indication of the weakening of the insulations. At times for some reason the CT 300 could not be taken for maintenance /servicing to increase the strength of the insulation, the CT 300 is liable to fail even at normal working condition while the power flow is still continuing. This situation causes sudden generation of gas, developing high internal pressure in the CT 300 and blasting of the current transformer 300. The high internal pressure developed inside the current transformer 300 has to be reduced by releasing the gas generated inside the current transformer 300. The reduction in pressure avoids the blasting of the CT.
[0031] Figure 4 illustrates construction of current transformer with Diaphragm for avoiding blast in the CT as known in the art. Conventionally, a diaphragm 401 is provided in CT 400 (the CT 400 is similar to the CT 300). In the event of high pressure in the CT 400, the diaphragm 401 ruptures and allow passage of the gas generated inside the CT 400. Due to passage of gas from the CT 400, the pressure is reduced in the CT 400 and thus saves the CT 400 from blasting. But this configuration has several serious associated shortcomings. Firstly, even after the rupture of the diaphragm 401, the CT 400 continues to function and remain in line transmitting power under the unstable condition till other protections are activated to isolate the transformer which means the equipment is still operating even if the system has become unstable and the voltage and current parameters has become dangerous for the associated working equipment as well as the Current transformer itself.
[0032] Secondly, the CT 400 is hermetically sealed equipment. With the rupture of the diaphragm 401 the CT 400 is no longer hermetically sealed even if everything inside the CT 400 remains intact with lesser oil. Further, after the rupture of the diaphragm 401, the CT 400 is open to atmosphere and some of the oil moves out and moisture goes inside the CT 400 there by damaging its insulating properties. The CT 400 can only become operative and all the dielectric property of the insulating material can only be regained after processing; which necessarily requires time, money and outage of CT 400 and the complete system from service resulting disruption in power supply.
[0033] Thirdly, the CT 400 remains inactive causing power failure even after the system is stabilized and normal conditions are restored. So, the power system remains disconnected even after the fault is subsided owing to the non-functionality of the CT 400. Further, there is no mechanism to determine the time of rupture of diaphragm 401 and it cannot be linked to any other abnormal event in the system. Fourthly, the rupture of the diaphragm 401 spews out the gas and oil abruptly in any direction and that could create problem when it falls on any active/live power / current carrying equipment/conductor current that could initiate a fire hazard.
[0034] In order to overcome above mentioned all technical problems existing in the prior arts, the present subject matter provides a technical solution to avoid blasting of current transformer. In the present subject matter, the current transformer has a spring loaded valve with canopy and tray in place of diaphragm to avoid blasting and spewing of hot oil. During the high pressure inside the current transformer, the spring loaded valve operates and allow passage of hot gases. Due to inner high pressure, the gases pushes the spring valve in upside direction and by the movement of the spring valve an aperture is opened to allow passage of gases to external environment. By releasing the gases in the environment, the pressure reduces inside the current transformer and thus avoids blasting of current transformer. Once the pressure inside the current transformer is reduced, the spring valve again closes the aperture and back into the hermetically sealed condition avoiding entry of foreign particles in the current transformer. Further, the present subject matter provides an electrical contact with the valve for switch ON and OFF of the electrical system and transformer during disturbance conditions to block flow of unstable current and voltage. Thus the present current transformer, protect the other electrical appliances and transformer from unstable current and voltage.
[0035] Fig. 5 illustrates construction of current transformer with Pressure Release Valve with canopy and tray to avoid blasting of CT and abrupt spillage of oil, in accordance with an embodiment of the present subject. The present current transformer 500 have primary terminals 501, primary line conductor 502, major insulation 503, toroidal ring core 504, porcelain insulator 505, expansion shell 506, spring loaded valve 506, a canopy 508, and tray 511, mounting flanges 509, and oil tank 510. The toroidal ring core 504 is at ground potential and the secondary windings are at very low potential where as the CT primary terminal 501 is at line voltage, i.e., 11 kV or 33 kV or 66 kV or 132 kV or 220 kV or 400 kV or higher. The oil tank 510 is provided around the toroidal ring core 504 and secondary windings to cool down the toroidal ring core 504 and the secondary windings. Owing to the large potential difference between the toroidal ring core 504 and the Current transformer primary line conductor 502, high volume of insulation, i.e., major insulation 503 is provided between them. The primary and secondary windings 504 are also to be isolated electrically which have a very large potential difference between them. The secondary voltage is in the tune of 5 volt to 2000 volts where as the primary line terminal is at rated line voltage up to kilo volts, i.e., 11 kV or 33 kV or 66 kV or 132 kV or 220 kV or 400 kV or higher. The major insulation 503 comprises of large number of composite layers of cellulose paper wound over each other covering the CT primary conductor 502 and also the ring core 504 of the Current transformer 500. The porcelain insulator 505 is provided at the outer side of the current transformer to keep it cool.
[0036] The CT 500 is continuously connected to the series with the power equipment and there is continuous stressing of the major insulation 503 inside the CT 500. Further, the spring loaded valve 507 is provided at aperture at top end of the current transformer in the expansion shell 506. At the time of system disturbances, when transient conditions arise, voltage fluctuates and could reach to very high value, which may be for short duration of few milliseconds or prolonged duration of few seconds. The current also could be very high, i.e., 40 kilo-ampere or 63 kilo-ampere or higher depending upon the type of fault appearing on the transmission line. These high current and fluctuating high voltages over stress the oil present in the current transformer and in the expansion shell 506 and the major insulation 502 inside the CT 500. The system disturbance causes sudden generation of gas, developing high internal pressure in the CT 500 and has a danger of blasting of the current transformer 500.
[0037] During the high pressure inside the current transformer 500, the spring loaded valve 507 operates and allow passage of hot high pressure gases. Due to inner high pressure, the gases pushes the spring loaded valve 507 in upside direction and by the movement of the spring loaded valve 507 an aperture is opened to allow passage of high pressure gases in environment. By releasing the gases in the environment, the pressure inside the current transformer reduces and thus avoids blasting of current transformer 500. Once the pressure inside the current transformer is reduced, the spring loaded valve 507 again closes the aperture and back into the hermetically sealed condition to avoid entering of foreign particles in the current transformer 500.
[0038] Fig. 6 illustrates the oil Tray 511 placed below the Pressure Relief Valve, for collecting the spilled oil and guiding them to flow out in a decided path; so that the oil can be collected in a pit and the sub-station ground is not wet by the oil, in accordance with an embodiment of the present subject. The tray 511 is placed below the Pressure Relief Valve to collect the spilled oil 512 directed by the canopy 508.
[0039] The current transformer 500 has a canopy 508 and tray 511 provided over the top of the spring loaded valve 507 which restrict spillage of oil abruptly in any direction during opening of the aperture. The canopy 508 and the tray 511 guide the spewed gases and oil towards a particular defined safe path so that no hazard is created. The canopy 508 and the tray 511 can be made of any material, such as metal or weather resistant plastic with high melting point.
[0040] Furthermore, the current transformer 500 is provided with an electrical contact coupled with the spring loaded valve 507. The electrical contact allows switch OFF of the electrical system and transformer during disturbance conditions to block flow of unstable current and voltage. During disturbance condition and during opening of the spring loaded valve 507, the electrical contact is activated in Switch ON condition. In the switch ON condition, the electrical contact SWITCH OFF the complete system, blocks the flow of power thereby saving the system and transformer from unstable current and voltage. Thus protects the other electrical appliances and transformer from unstable current and voltage. Therefore, this is no possibility that there is power flow under the system in disturbance condition.
[0041] In another embodiment of the present subject matter, after releasing the gas from the current transformer 500, the aperture closes automatically due to spring action of the valve, so the hermetical sealed condition is maintained. As result, there is no scope of deterioration of the insulating properties of the CT 500 and hence there is no need for any processing so there is saving in money, time and no outage of service on this account which means the availability of the system is higher.
[0042] Further, the timing of the operation of the contact can be identified and related events in the system can be linked and identified for further future preventive actions.
[0043] Although embodiments for the present subject matter have been described in language specific to structural features, it is to be understood that the present subject matter is not necessarily limited to the specific features described. Rather, the specific features and methods are disclosed as embodiments for the present subject matter. Numerous modifications and adaptations of the system/component of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the scope of the present subject matter.