The invention relates to transformers and, more particularly, to a fast transient mitigator circuit integrated into a cast transformer.
Fast transients, due to system switching or environmental impact can cause damage to the transformer windings and reduce the life thereof. Conventional “snubber” circuits include a resistor and a capacitor that are connected externally to the transformer terminal. The resistor is always in the circuit and adds additional losses to the transformer. Since the resistor and capacitor components are external to the transformer winding structure, they are required by ANSI standards to be disconnected during impulse tests and while the transformer winding structure is tested with fast transients associated with impulse voltage.
Thus, there is a need to integrate a fast transient mitigator circuit into a transformer.
An object of the invention is to fulfill the need referred to above. In accordance with the principles of an embodiment, this objective is achieved by providing a transformer having a ferromagnetic core; winding structure mounted on the core; electrical terminals connected to the winding structure; a fast transient mitigator circuit including an impedance circuit serially connected between one of the terminals and the winding structure, and a capacitor connected from the one terminal to external ground. The mitigator circuit is constructed and arranged to reduce the frequency spectrum and magnitude of fast transients. An encasement, of an insulating resin, commonly encapsulates the core, the winding structure, and at least the impedance circuit of the mitigator circuit.
In accordance with another aspect of an embodiment, a method provides a fast transient mitigator circuit integrated within a transformer. The method provides a ferromagnetic core. A winding structure is mounted on the core. Electrical terminals are connected to the winding structure. A fast transient mitigator circuit is provided and includes an impedance circuit serially connected between one of the terminals and the winding structure, and a capacitor connected from the one terminal to external ground, to reduce the frequency spectrum and magnitude of fast transients. The core, the winding structure, and at least the impedance circuit are encapsulated in a one insulating resin.
Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification.
The features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:
It should be noted that in the detailed description that follows, identical components have the same reference numerals, regardless of whether they are shown in different embodiments of the present invention. It should also be noted that in order to clearly and concisely disclose the present invention, the drawings may not necessarily be to scale and certain features of the invention may be shown in somewhat schematic form.
The present invention is directed to a dry-type transformer 10 to provide power to residences and small businesses. As such, the transformer 10 is a step-down transformer that receives an input voltage and steps it down to a lower, output voltage. The transformer preferably has a rating from about 16 kVA to 500 kVA, with an input voltage in a range from 2,400 to 34,500 Volts and an output voltage in a range from 120 to 600 Volts. The transformer 10 generally includes a winding structure preferably including a plurality of winding modules 12. The winding modules 12 are mounted to a ferromagnetic core 14 and all of which are disposed inside an encasement 16 formed from one or more resins, as will be described more fully below. The core 14 and the winding modules 12 mounted thereto are cast into the resin(s) so as to be encapsulated within the encasement 16.
The encasement 16 includes a generally annular body 18 joined to a base 20. The body 18 has a center passage 21 extending there-through. A pair of frusto-conical high voltage bushings 22, 22′ extends upwardly and outwardly from a top portion of the body 18. A low voltage terminal pad (not shown) is joined to a front surface of the body 18, above the center passage 21.
The core 14 is composed of a ferromagnetic material, such as iron or steel, and has an inner opening and a closed periphery. The core 14 may have a rectangular frame shape or an annular shape (as shown), such as a toroid. The core 14 may be comprised of a strip of steel (such as grain-oriented silicon steel), which is wound on a mandrel into a coil. Alternately, the core 14 may be formed from a stack of plates, which may be rectangular or annular and of the same or varying width or circumference, as the case may be.
As shown in
The winding modules 12 may be wound directly on the core 14. Alternately, the winding modules 12 may be formed on a mandrel and then mounted to the core 14 if the core 14 is formed with a gap or is formed from several pieces that are secured together after the winding modules 12 are mounted thereto.
The low voltage winding segments 30 of the winding modules 12 are electrically connected together (either in series or in parallel) by conductors to form a low voltage winding. Similarly, the high voltage winding segments 32 are electrically connected together (either in series or in parallel) by conductors to form a high voltage winding.
Ends of the high voltage winding formed by the segments 32 are connected to leads 36, 36′, which extend through the body 18 and are ultimately secured to terminals 40, 40′, respectively, which are fixed to the ends of the high voltage bushings 22. A helical coil 38 may be disposed inside one of the high voltage bushings 22. The coil 38 is comprised of conductive wire that is helically wound to form a cylinder having a central passage 39. The conductive wire may or may not be encased in an insulating covering. The outer end of the conductive wire is secured to a terminal 40. The inner end of the conductive wire is folded inwardly so as to be disposed inside the central passage of the coil 38. The lead 36 extends through the central passage 39 of the coil 38. In this manner, the coil 38 is disposed around and spaced from the lead 36. The coil 38 controls the electrical fields that may be generated when current passes through the lead 36 and thereby reduce the dielectric stress on the resin material of the high voltage bushing 22.
As schematically shown in
Returning to
As shown in
The resistor 54 has a resistance in a range from about 20-150 Ohms to provide wave termination. The inductor 56 is non-saturable with the working current and has an impedance value that is selected such that the voltage drop at 50 Hz is small in order not to generate heat in the resistor 54. The impedance of the inductor 56 is greater than the resistance of the resistor 54 at frequencies greater than 10 kHZ. The capacitance of the capacitor 58 is relatively small, having a value of about 5-20 nanofarads (nF), more particularly about 10 nF.
The interconnected winding modules 12 mounted to the core 14, together with the leads 36, 42, 46, mitigator circuit 52 and the coil 38 form an electrical assembly that is cast into one or more insulating resins that is/are cured to form the encasement 16. The encasement 16 may be formed from a single insulating resin, which may be butyl rubber or an epoxy resin. In one embodiment, the resin is a cycloaliphatic epoxy resin, still more particularly a hydrophobic cycloaliphatic epoxy resin composition. Such an epoxy resin composition may comprise a cycloaliphatic epoxy resin, a curing agent, an accelerator and, optionally, filler, such as silanised quartz powder, fused silica powder, or silanised fused silica powder. The curing agent may be an anhydride, such as a linear aliphatic polymeric anhydride, or a cyclic carboxylic anhydride. The accelerator may be an amine, an acidic catalyst (such as stannous octoate), an imidazole, or a quaternary ammonium hydroxide or halide.
The encasement 16 may be formed from the resin composition in an automatic pressure gelation (APG) process. In accordance with APG process, the resin composition (in liquid form) is degassed and preheated to a temperature above 40 C, while under vacuum. The electrical assembly is placed in a cavity of a mold heated to an elevated curing temperature of the resin. The leads 36, 36′, 42, 46 and 59 extend out of the cavity through openings so as to protrude from the encasement 16 after the casting process. The degassed and preheated resin composition is then introduced under slight pressure into the cavity containing the electrical assembly. Inside the cavity, the resin composition quickly starts to gel. The resin composition in the cavity, however, remains in contact with pressurized resin being introduced from outside the cavity. In this manner, the shrinkage of the gelled resin composition in the cavity is compensated for by subsequent further addition of degassed and preheated resin composition entering the cavity under pressure. After the resin composition cures to a solid, the solid encasement 16 with the electrical assembly molded therein is removed from the mold cavity. The encasement 16 is then allowed to fully cure.
It should be appreciated that in lieu of being formed pursuant to an APG process, the encasement 16 may be formed using an open casting process or a vacuum casting process. In an open casting process, the resin composition is simply poured into an open mold containing the electrical assembly and then heated to the elevated curing temperature of the resin. In vacuum casting, the electrical assembly is disposed in a mold enclosed in a vacuum chamber or casing. The resin composition is mixed under vacuum and introduced into the mold in the vacuum chamber, which is also under vacuum. The mold is heated to the elevated curing temperature of the resin. After the resin composition is dispensed into the mold, the pressure in the vacuum chamber is raised to atmospheric pressure.
By integrating the mitigator circuit 52 directly into the encasement 16, all fast transients including those associated with impulse voltages can be controlled. Therefore, electrical stresses on air gaps are reduced and the air clearances may be reduced. The impact of dielectric stresses on solid insulation associated with switching surges and transient recovery voltages are decreased, allowing for a reduction in the insulation thickness.
The foregoing preferred embodiments have been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the spirit of the following claims.
Number | Date | Country | Kind |
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11193227.3 | Dec 2011 | EP | regional |