High energy dissipation harmonic filter reactor

Abstract: An air core inductor (10) having mounted thereon a band (20) of material of selected characteristics providing only an electromagnetically coupled resistance that reflects back into the windings of the inductor for filtering applications.

Description: FIELD OF INVENTION This invention relates generally to air-core reactors for power transmission systems and more particularly concerns an air-core reactor in combination with a resistive element mounted on the reactor in spaced relation with respect thereto and electrically insulated therefrom. The resistive element is preferrably physically in the form of a band and made preferrably from a high resistance, temperature stable material. The resistive element is responsive only to electromagnetic fields generated by the coil windings of the reactor. The resistive element performs two tasks, one of which is to act as a resistor in a filter circuit and the other is to act as a thermal dissipator. An arrangement showing the features of the preamble of Claim 1 of the present invention is known from US-A-3'264'590. Reactors of the present invention are characterized by having a very low quality factor at a selected frequency, or band of frequencies which are higher than the power system frequency and are required to absorb extremely large energies at this frequency or band of frequencies. BACKGROUND OF INVENTION Power system reactors are often used in combination with resistors and capacitors to perform filtering functions, to control the inrush and outrush from capacitor banks, etc. In many of these applications the parallel combination of a reactor and a resistor appear. The purpose of this combination is to alter the native characteristics of the reactor at frequencies higher than the system frequency such that the combination presents a much lower quality factor to these frequencies and absorbs very large power at these frequencies. The combination of a reactor and resistor in parallel is used for example in series with a capacitor to form a filter which presents a high impedance to the power frequency but a much lower impedance to a band of harmonic frequencies. Another arrangement is where the capacitor is also in parallel with the coil and resistor. This latter filter circuit presents a high impedance to a band of harmonic frequencies and a low impedance to the power frequency. In both cases, the resonant frequency is established primarily by the inductance and capacitance of the circuit and the bandwidth primarily by the resistance. A third combination which consists of the foregoing arrangements in series results in a filter which presents a low impedance to two selected frequencies, these frequencies being established by the choice of the LC combinations for the two parts of the filter. All three of the above combinations are often used to filter out harmonics generated by power semiconductor switching devices on power systems, for example, in DC to AC transformations and for controlling the reactor power flow in static compensator systems. The combination of a reactor and a resistor in parallel is also used to control the inrush and outrush from large capacitor banks when these are switched in and out of power systems. The resistors conventionally used in parallel with reactors are separate devices and in the case of outdoor installations must be housed in waterproof enclosures. The chief advantage of the separate resistor is the fact that the dissipation depends only on the voltage across the resistor (and therefore, the voltage across the reactor) and is independent of the frequency. The use of the separate parallel resistor for dissipation of large amounts of energy, however, is costly both in terms of the equipment itself and in terms of installation space required. A filter choke capable of handling high power levels is disclosed in United States Patent 3,808,562, issued April 30, 1974 and includes a choke coil and an active resistance element connected in parallel with the choke coil. The active resistive element is magnetically neutral, neither generating a magnetic field to influence the choke coil nor is it noticeably influenced by the magnetic field of the choke coil. SUMMARY OF THE INVENTION A principal object of the present invention is to provide an air core reactor with a resistive element that is only electromagnetically coupled during use and which is also capable of dissipating high energy levels. The invention provides apparatus for use in electric distribution systems, in accordance with Claim 1. The air core reactor preferrably includes coaxial, coextensive, cylindrical coils embedded in a rigidly set glass fiber reinforced resinous material such as an epoxy and a multi-arm spider at at least one end of the reactor for connecting the coil windings in parallel and permitting fractional turns for the different windings. The resistance element, which is electrically insulated from the coils, is responsive only to electromagnetic fields generated thereby during use thereof inducing therein 1 2 R losses which are reflected back into the coils causing the quality factor Q to be lowered. The resistance element is responsive only to an electromagnetic field and therefore is an electromagnetically coupled resistance element. The resistance element comprises one or more bands of resistive material each in the form of a closed loop coaxial with, in close proximity to and radially spaced from the coil(s). The band(s) is mounted by band mounting means that retains the same in fixed relation relative to the coil and electrically insulates the same therefrom. Each band is made of a material in which the resistance is substantially unaffected by temperature change herein referred to as a temperature stable material. The material may for example be a nickel chromium alloy such as known by the Trade-Mark NICHROME. Adequate results have been obtained using stainless steel which is substantially cheaper and more readily available in a wider range of widths and thicknesses than the nickel chromium alloy. LIST OF DRAWINGS The invention is illustrated by way of example in the accompanying drawings wherein: Figure 1 is an oblique diagrammatic view showing the physical arrangement of a filter provided in accordance with the present invention; Figure 2 is an oblique view illustrating modifications to the resistance element of the filter shown in Figure 1; Figure 3 is a partial sectional, oblique view illustrating in more detail an air core reactor with a band of high resistance material mounted thereon to provide a filter in accordance with the present invention for handling high power levels; Figure 4 is a circuit diagram of an illustrative embodiment of a conventional filter arrangement designed to pass the 11th and 13th harmonics; Figure 5 is a circuit diagram of the present invention designed for the same parameters as in Figure 4; and Figure 6 is a graph showing the input impedance curves for the respective arrangements of Figures 5 and 6. DESCRIPTION OF PREFERRED EMBODIMENT Illustrated in Figures 1 and 3 is a rigid open ended cylindrical coil unit having two multi-armed spiders with one being located at one end and the other at the opposite end. If desired there may be only one multi arm spider located at one end of the coil unit. The coil unit 10, illustrated in Figure 3, consists of a plurality of rigid cylindrical coils designated 10A, 10B, 10C, 10D and 10E disposed coaxially and they are radially spaced from one another by spacers designated S providing air channels therebetween. Spiders 11 and 12, at the opposite ends, provide means for connecting the coils in parallel and also for terminating the coil windings at different circumferential positions allowing for partial, i.e., fractional turns as is known in the art. Coils and spiders of this general construction and variations thereof are known from the teachings of applicant's United States Patents 3,902,147 issued August 26, 1975; 3,225,319 issued December 21, 1965; 3,264,590 issued August 2, 1966; British Patent 1,017,029 published January 12, 1966, and British Patent 1,007,569 dated May 29, 1962. The spiders 11 and 12 at opposite ends of the coil unit, each have a central hub H from which radiate a plurality of arms A. The spiders at opposite ends are tied together by suitable tie means (not shown). The coils 10A, 10B, 10C, 10D and 10E may consist of one or more layers (radially side-by-side), designated for example LA1, LA2 and LA3 in Figure 3, of windings of insulated conductor each having a beginning at one end of the unit and an ending at the opposite end with such opposite ends being connected respectively to spiders 11 and 12. Spider 11 can be omitted if desired and replaced by a mounting means for the reactor and suitable connection means connecting the coil windings at such end. Each layer may be one or more conductors high (axial direction of the coil) with all of the windings being helical and of insulated conductor. Figure 1 illustrates the present invention in its simplest form and comprises an electromagnetically coupled resistance element 20 in the form of a band of material coaxial with and radially spaced from a coil unit 10 1 . Coil 10 1 preferably is essentially the same as coil unit 10 described above but in its simplest form could be a single cylindrical coil (air core). Band 20 is a thin band of high resistance. material such as a nickel alloy, for example NICHROME* or the like temperature stable material in the form of a continuous closed loop. The band is mounted on the reactor by way of example by supports 21 located at the outer end of the arms A of the spider. Supports 21 may be pads mounted directly on the ends of the arm as seen in Figure 1 or attached thereto by brackets 22 as shown in Figure 3. The band 20 in the embodiment illustrated in Figure 1 is located at one end of the coil unit. It can however be variously located at different selected positions along the axis of the coil unit depending upon the coupling factor desired. In most cases a close coupling is desired the results of which may be achieved by the location shown in Figure 1. Arms A of the spider are electrically conductive and mounting supports 21 are therefore of necessity made of insulative material (or at least mounted on the arms by insulating means) electrically insulating band 20 from the spider arms. Instead of a single band as shown in Figure 1, two or more bands may be used and the two or more bands may be variously arranged and variously positioned. Referring to Figure 2 one arrangement is illustrated which consists of a group of co-axial radially spaced bands three individual bands being illustrated and designated 20A, 20B and 20C. These bands are radially spaced and connected one to the next by radial spacers 23. The spacers are metallic and welded or otherwise solidly secured to the bands making the plurality of bands a strong rigid integral unit. The spacers need not be made of an insulating material since they do not affect the operation of the apparatus. It is, however, essential that the bands be electrically insulated from the electrically conductive portion of the spider arms which, as previously mentioned, serve to connect the multiple coil windings in parallel and also serve to provide fractional turns for the windings. The number of and location of spacers 23 can be varied dependent upon the strength required in the structure. The numer of and the location of the bands and the arrangement of the bands may be varied depending upon the physical and/or electrical results desired. There may for example be only two bands one being located as illustrated in Figure 1 and another for example band 20D mounted on spider 11 as indicated by broken line in Figure 3. Also while the bands are shown mounted on the spiders they can be mounted at any other position on the reactor by other means not shown. When the reactor is energized its magnetic field links the short circuited loop (or loops) provided by the band (or bands) of resistive material inducing currents in them. Since the bands are made of preferrably a high resistance material, an I 2 R loss is induced in them and this loss is reflected back into the coil causing the quality factor Q of the coil to be lowered. The number of bands, the material from which the bands are made, the thickness of the bands, their width, their diameter and the placement of the bands with respect to the coil midplane are chosen to accomplish the following results: (1) the power dissipated in the bands at the designed frequency must be as specified by the filter design; (2) the resistance of the bands should be sufficiently high that the current flowing in them is virtually in phase with the induced voltage, i.e., the inductive reactance of the bands at the specified frequency should be very much less than the resistance of the bands; (3) the number of bands used and their width in the axial direction of the coil are chosen so that the surface area presented by the dissipative element will be sufficient to ensure that its temperature rise does not exceed a specified maximum. For example, if the specified maximum temperature rise for the dissipative device is 200 degrees centigrade, then the total surface area of all of the bands should be sufficiently large that the power dissipation in the bands is not more than about 0.7 watts per square centimetre of surface area. The design of the dissipative element must be integrated with the design of the reactor. Most power reactors used for filtering applications consist of concentric helices which are connected in parallel by spider devices at the top and bottom of the reactor. The design of the rector itself is very complicated since all of the paralleled layers are coupled and interact with each other. In order to guarantee that the current will be shared appropriately among the different layers of the reactor this coupling must be taken into account during the design and the exact number of turns and partial turns for each layer are chosen to make sure that the proper current balance is established. When the dissipative element is added to the reactor, all of the bands are coupled to all of the layers of the reactor. When currents are induced in the bands of the dissipative element these currents interact with the main coil layers and will cause the current balance in them to change from the balance established if the coil is designed alone. Thus, the entire device, coil and dissipative element, must be designed with a program on an inter-active basis which results in the proper inductance of the coil, the proper balance of currents in the various layers of the coil, the appropriate total loss in the dissipative element at the designed frequency, sufficient surface area in the dissipative element in order to guarantee a temperature rise which does not exceed a specified maximum, and lastly the current flowing in the bands of the dissipative element must be virtually in phase with the induced voltage in the elements. This means that the resistance of each band of the dissipative element must be large compared to the effective reactance of each band at the specified operating frequencies. Applicant's dissipation system for power filtering applications has the following advantages: (1) the system can obtain levels of dissipation and resulting low Q factors for coils which is far in excess of that which can be obtained by eddy currents in the reactor itself or in surrounding structures; (2) the resulting characteristics of the reactor plus dissipative element are comparable to the case where a reactor is used in parallel with a separately designed resistor. However, the former is less expensive than the latter; (3) compared to the system in which a secondary is wound on a reactor to which is connected a resistor element, applicant's system is very much less expensive; (4) applicant's system is very simple and therefore much more maintenance free than existing systems; (5) because a dissipation element is incorporated with the reactor design, applicant's system takes up less space than other systems and is therefore less expensive to install; (6) applicant's system can be designed for very high BIL levels since the impulse level depends primarily on the design of the reactor and the dissipative element does not change the impulse withstand of the reactor significantly. This is in contrast to the use of a separate resistor where the resistor element also must be designed to withstand the high impulse levels and this impacts significantly on the cost of the resistor element; (7) no separate enclosure is required for the dissipation element in applicant's system whereas systems using separate resistors require housings for these resistors. A comparison of the present filter arrangement with a previously known arrangement by way of example only is illustrated in Figures 4 to 6. Figure 4 is a circuit diagram for a filter designed to pass the 11th and 13th harmonics in a 50 CPS power system. The circuit as illustrated includes capacitors C 1 and C 2 , inductive coils L 1 and L 2 and a resistor R1 in a series parallel arrangement as illustrated. The resistor R1 has a rating of 350 kilowatts. The solid line curve in Figure 6 shows the input impedance (ohms) curve for such filter combination. The dotted line curve is for the equivalent filter constructed according to this invention where the total harmonic power of 320 kilowatts is dissipated in the electromagnetically coupled resistance elements R 1 / 1 and R 1 / 2 added to the two reactors L 1 / 1 and L 1 / 2 shown in the circuit diagram of Figure 5. The coupled resistor R 1 / 1 of reactor L 1 / 1 comprises six concentric NICHROME* rings, each 406,4 mm (16 inches) high, 2,16 mm (0.085 inch) thick and having diameters of 2,31 m, 2,36 m, 2,41 m, 2,46 m, 2,51 m, 2,57 m (91, 93, 95, 97, 99 and 101 inches). This unit dissipates 230 kilowatts at a temperature rise of 200°C. The coupled resistor element R 1 / 2 of coil L 1 / 2 comprises three concentric NICHROME rings, each 0,2 m (8 inches) high and 0,25 mm (0.01 inch) thick and having diameters of 2,03 m, 2,08 m and 2,13 m (80, 82 and 84 inches). This unit dissipates 90 kilowatts. By way of cost comparison the 320 kilowatts resistor unit R1 in Figure 4 is about $10,000.00 while the total cost of the coupled resistor units for reactors L 1 / 1 and L 1 / 2 is only about $5,000.00. While NICHROME was used for the foregoing and is the preferred material for the band for some applications its high resistivity makes it unsuitable for some applications. The material characteristics must be taken into account depending upon its application. It is important that the material be temperature stable. Some other alloys considered suitable are nickel-copper and chromium aluminium and stainless steel. The magnetic coupled bands of the filter has perceived disadvantages below certain Q values (quality factor). Tests have shown that attempts made at reaching a Q of 6 the current in the band was not in phase with voltage. It appears that as frequency goes up, for a given voltage, the power falls off and in some applications it may be more efficient to use known filter arrangements with a hard wired resistance.

Claims: Apparatus for use in an AC power transmission system with an air core reactor unit (10), characterised by (a) said air core reactor unit (10) having a resistance (R 1 1, R 1 2) for the reactor which is operative solely by electromagnetic coupling therewith, the resistance comprising a metallic element (20, 20D) electrically isolated from coil windings of the reactor and positioned in magnetic coupling with the windings, and (b) means (21,22) mounting the metallic element on the reactor in spaced relation therewith, permitting dissipation of large quantities of heat from the metallic element without adversely damaging the reactor, the metallic element being responsive to electromagnetic fields generated by coil windings of the reactor by inducing, during use, at a selected frequency or band of frequencies higher than a power frequency of the power transmission system I 2 R losses that reflect back into the windings causing the quality factor Q of the reactor to be lowered at the said selected higher frequency. Apparatus as claimed in Claim 1, in which the metallic element (20, 20D) comprises a closed loop thin band mounted on the reactor unit. Apparatus as claimed in Claim 1 or Claim 2, in which the air core reactor unit has a spider (11) on one end thereof, the spider comprising a hub (H) having a plurality of arms (A) radiating outwardly therefrom, and in which the band is mounted on outer ends of the arms, and is electrically insulated from them. Apparatus as claimed in Claim 2 wherein the air core reactor includes coaxial, coextensive, cylindrical coil windings (LA1, LA2, LA3) embedded in a rigidly set glass fiber reinforced resinous material and a multi-arm spider (12) at one end of the reactor for connecting the coil windings in parallel and permitting fractional turns for the different windings. Apparatus as claimed in Claim 1 in which the resistance element comprises one or more bands (20) of resistive material each in the form of a closed loop coaxial with, in close proximity to and radially spaced from the reactor (10). Apparatus as claimed in Claims 4 and 5, in which the band (20) is mounted on radial outer ends of the arms of the spider (12) and is electrically insulated from them. Apparatus as claimed in Claim 6, which includes one or more bands (20) of resistive material which are spaced with respect to one another, and means retaining the said bands in fixed spaced relation relative to the coil unit. Apparatus as claimed in Claim 7, in which the bands (20) are coaxial and radially spaced with respect to one another, and to the cylindrical coil unit and means retaining the bands in fixed radial spaced relation relative to one another. Apparatus as claimed in any one of Claims 6 to 8, in which the band (20) is located adjacent to one end of the cylindrical coil unit. Apparatus as claimed in any one of Claims 6 to 9, in which the resistance element is a thin band (20) of nickel alloy material. Apparatus as claimed in any one of Claims 5 to 10 wherein the band (20) is made of a material having a resistance which is substantially unaffected by temperature change. Apparatus as claimed in Claim 11 wherein said material is stainless steel.

Inventors: BURKE PATRICK E, PEWNY NORBERT, BURKE, PATRICK E., PEWNY, NORBERT
Applicants: BBA CANADA, BBA CANADA LIMITED
Application year: 1992
Main classification: H01F27
IPC codes: H01F38, H01F27, H01F30, H02H9