A VOLTAGE CONVERTER

Abstract: The invention relates to a voltage converter arrangement which is intended to generate a high-voltage d.c. current for an electrostatic precipitator and which includes a transformer (T) which is encased in oil. The primary winding (23) of the transformer is supplied with a high-frequency voltage and the secondary winding (28a, 28b) of the transformer comprises a plurality of series-connected windings which have the form of plane spirals. The secondary winding of the transformer (T) is divided into two or more equal, or essentially equal, parts (28a and 28b) and the primary winding (23) of the transformer is placed between the secondary winding parts.

Description: TITLE OF THE INVENTION; A VOLTAGE CONVERTER TECHNICAL FIELD The present invention relates primarily to a voltage con¬ verter in which a high voltage direct current applied to an electrostatic precipitator is generated by rectifying voltage delivered from the secondary winding of a trans¬ former, the primary winding of which is supplied with a high-frequency voltage. The inventive voltage converter is particularly intended for use in equipment or apparatus in which a variable high d.c. voltage whose value exceeds 30 kV and whose power is greater than 1 kW but normally smaller than 100 kW is to be produced from an alternating voltage, preferably at mains frequency. The inventive converter may be based on an electric circuit arrangement which includes a first circuit which converts the alternating voltage to a first d.c. voltage, a device which applies the first d.c. voltage pulsewise to the pri¬ mary winding of a transformer, and a second circuit which is connected to the secondary winding of the transformer for converting a high alternating voltage generated there- on to a high d.c. voltage,wherewith there occurs on the output of said circuit a voltage which is capable of con¬ stituting the aforesaid variable high d.c. voltage. The invention has been developed primarily for the purpose of enabling practically any variable high d.c. voltage to be generated for application to the electrodes of an electrostatic precipitator in order to be able to control the variation of the voltage on the electrodes as a fun.c- - tion of time. BACKGROUND PRIOR ART It is known that the value of an alternating voltage ap- plied to the primary winding of a transformer can be in¬ creased or decreased through the secondary winding there¬ of. Thus, there can be obtained on the secondary winding a voltage which is 100 times higher than the value of the primary voltage. It is also known that this secondary voltage can be recti¬ fied, e.g. with the aid of diodes, to obtain a d.c. volt¬ age. It is also known to rectify the secondary voltage by per¬ mitting a current to pass through the secondary winding in solely one direction or, when current flows through the secondary winding in both directions , to rectify the current with the aid of bridge couplings which permit current to flow in both directions through the secondary winding. In the case of so-called half-wave rectification, i.e. current passes through the secondary winding in solely one direction, problems normally occur with regard to satura- tion in the core of the transformer, which results in cur¬ rent surge, or transients, in the primary winding and in a high rise in temperature. These problems may actually increase dramatically if high-grade core material is used, but are avoided by overdimensioning when a poorer grade of core material is used or if an air gap is incorporated in the magnet circuit. It is known that transformer cores can be configured from thin plates (0.3 mm), which are arranged in a high multiple of layers to form the core, or from powdered high-resis¬ tance material, e.g. ferrites, and that thick plate or solid iron will result in unreasonable eddy current losses, even at a mains frequency of 50 Hz. The conductors on both the primary and the secondary side normally consist of copper wire, or even of thick bonds or rails. Aluminium or other materials of good electrical conductivity may also be used in the windings. The higher the frequency with which the transformer works the thinner the conductors must be, due to the so-called skin effect, which causes the current to be confined to the outer sur¬ faces of the conductors. Consequently, multistrand con¬ ductors (Litzendraht wires) are often used so as to reduce the power lost in the windings due to the skin effect. It is also known that the size of the transformer is highly dependent on the frequency of the alternating voltage used. The higher the frequency, the smaller the transformer. The higher the voltage desired from the transformer, • the more turns required on the secondary winding. At voltages in the order of 50 kV this results in disturbances engen¬ dered by stray capacitances between the respective turns of a winding or parts thereof and between winding and core, and also between primary winding and secondary winding. This effect increases with increasing frequencies. It is known that the effects can be influenced by the wind¬ ing principle and a larger number of proposals are based on constructing the high voltage winding from plane spirals which are connected in series. Cross-couplings are often incorporated within each spiral, so that the current is able to pass alternately "in and out". It is also known that in the case of low powers, the wind- ings may be formed on circuit cards. In this case the high-voltage winding or the secondary windings are con¬ figured from a plurality of spiral-form conductors applied to a plurality of circuit cards which are stacked around the transformer core in suitably spaced relationship and the conductors on said cards mutually connected in series. By way of an example of the present state of the art in this regard, reference may be made to the arrangement illustrated and described in Swedish Published Specifica- tion No 426 115 and the arrangement illustrated and de¬ scribed in the US-PS Serial No 4 091 349. SUMMARY OF THE PRESENT INVENTION TECHNICAL PROBLEMS When considering the aforecited known state of the art it will be seen that a qualified technical problem is one of creating within the aforesaid electrostatic precipitator field, with the aid of simple means, conditions which will enable transformer constructions to be greatly simpli¬ fied and made less expensive, and in particular, to provide conditions which will enable transformers of very small external dimensions to be used. When considering this problem, it will be seen that a technical problem also resides in provisions which will enable the transformer, and particularly the transformer windings, to be given sufficient insulation distances, required when the variable d.c. voltage exceeds 30 kV and is as high as 120 kV. A further technical problem resides in the provision of a transformer construction for use in the aforesaid technical field which can be supplied with d.c. voltage pulses at a frequency far above 20 kHz, for instance frequencies in the vicinity of 50 kHz, while still effectively reducing the influence of stray capacitances in the transformer, such stray capacitances becoming highly manifest when in¬ creasing the frequency and particularly when increasing the voltage. It should be observed in this connection that such a trans¬ former will have a conversion ratio in the order of 1:100, with a few turns on the primary side and a considerable number of turns on the secondary side. A particularly qualified technical problem resides in the provision of an arrangement of the aforesaid kind which is particularly intended for generating a time-variable high d.c. voltage which is applied to the electrodes of an electrostatic precipitator, where the d.c. voltage shall be capable of varying between 30 kV and 120 kV. It will also be seen that in the case of circuits of this kind and in the case of their application within the aforesaid technical field, a further technical problem resides in the provision of conditions which will enable the voltage on the secondary side of the transformer to be used directly as the variable high d.c. voltage, despite a pulse-feed of d.c. voltage pulses. A further technical problem resides in the construction of a transformer, with the aid of simple means, which is active within a pulse frequency of between 20 kHz and 200 kHz, particularly at frequencies around 50 kHz, and to realize that the secondary winding of the transformer should preferably comprise a plurality of printed circuit cards on which the conductors have the form of electrically conductive layers and are "spirally" configured in the plane of the card, and where a plurality of cards provided with "spiral-shaped" conductors and placed side-by-side form the secondary winding of the transformer. Another technical problem is one of realizing that a trans¬ former may be provided advantageously with two or more mutually separated secondary windings and that the primary winding of the transformer should be positioned between those, two or more, secondary windings. A further technical problem resides in realizing that a possibility of effectively cooling the conductors and the cards is afforded by giving the secondary winding of the transformer the form of an electrically conductive layer on a number of cards arranged in side-by-side relationship. This enables the influence of the skin effect to be reduced. In this way there is obtained a flat thin conductor or winding which contains a limited amount of copper. SOLUTION The present invention relates to a voltage converter arrangement for producing from an alternating voltage, - preferably having a mains frequency, such as a frequency of 50 Hz or " 60 Hz, a variable or constant high-voltage d.c. current having a d.c. voltage level or value above 30 kV. The invention is based on a voltage converting arrangement which is constructed to produce a high-voltage d.c. current and which includes a transformer which is surrounded by oil and the primary winding of which is supplied with high- frequency voltage. The secondary winding of the transformer comprises a plurality of windings which are mutually connected in series and which have the form of plane spirals. In accordance with the invention, the secondary winding of the transformer is subdivided into two or more equal, or essentially equal, parts and the primary winding of the transformer is arranged between said parts. In accor d ance with the invention, the secondary winding of . the transformer is subdivided into two or more equal, or essentially equal, parts and the primary winding of the transformer is positioned between said secondary winding parts that rectifying devices, e.g. diodes, are connected up-stream, downstream and centrally of the secondary winding, that the se condary winding comprises a plurality of mutually connected circuit cards, that rectifying devices are connected between mutually adjacent circuit cards and that the circuit cards are vertically or at least essentially vertically positioned. A particular advantage is afforded when the secondary winding comprises a plurality of mutually connected circuit cards each of which has a conductor path in the form, of a rectangular plane spiral. In view of the suggested frequency range, it is proposed that the transformer core is made of a ferrite material and comprises a plurality, preferably eight, U-shaped parts. According to one advantageous embodiment, a capacitor and a freewheel diode are connected in parallel across the arrangement. ADVANTAGES The advantages primarily characteristic of the inventive voltage converter,which can be used particularly for pro¬ ducing the high d.c. voltage to be applied to the elec- trodes of an electrostatic precipitator,reside in the provision of conditions which through substantially ele¬ vated frequencies enable the dimensions of the transformer to be reduced, in combination with a marked reduction in the deleterious effect from stray capacitances occurring in the transformer at this high frequency. The primary characteristic features of the inventive volt- age converter are set forth in the characterizing clause of the following claim 1. BRIEF DESCRIPTION OF THE DRAWING The following description is made with reference to the known state of the art with regard to varying the voltage applied to the electrodes of an electrostatic precipitator, and with reference to a voltage converter in accordance with the invention and a transformer constructed for high frequencies. In the accompanying drawings: Figure 1 illustrates an electric circuit of a variable high d.c. voltage generating device of known construction; Figure 2 illustrates a known arrangement of the upper part of an electrostatic precipitator in which the known device shown in Figure 1 is used; Figure 3 is a block schematic of a device constructed in accordance with the present invention; Figure 4 illustrates an arrangement in the upper part of an electrostatic precipitator in which the inventive device of Figure 3 is used; Figure 5 is a circuit diagram of the inventive device shown in Figure 3; Figure 6 illustra±es examples of pulse-width-modulated d.c. voltage pulses applied to the primary winding of the transformer; Figure 7 is a perspective view of a proposed transformer core; and Figure 8 is a perspective view of part of one secondary winding of the transformer. A DESCRIPTION OF : EMBODIMENTS AT PRESENT PREFERRED Figures 1 and 2 illustrate known arrangements. Figure 1 shows the circuitry of a device for generating a variable, high d.c. voltage having a value which, normally exceeds 30 kV and which appears on a conductor 2, The device is supplied with energy in form of alternating voltage, preferably having a mains frequency of 50 Hz, supplied on a conductor 1. The conductor 2 is connected to corona discharge electrodes in an electrostatic precipitator 10.' Figure 1 corresponds to Figure 6 of US Patent Specification Serial No 4 486 704, and the reader is therefore referred to this patent specification for a better understanding of the circuit illustrated in Figure 1 and the manner in which the d.c. voltage across the electrodes of an electrostatic precipitator is controlled. When the device illustrated in Figure 1 is dimensioned for use in an electrostatic precipitator, as illustrated in Figure 2, it is necessary to place on the roof of the precipitator, in each section, a unit which includes a transformer T of very large volume surrounded by an oil tray 11, and also an insulating arrangement 11a for conducting direct current to the emission electrod . In addition, each precipitator section normally requires a control cabinet or panel 12 by means of which the d.c. voltage supplied to the electrostatic precipitator can be controlled and regulated, therewith to enable the d.c. voltage to be varied in a predetermined manner. The reader is referred to the aforementioned US patent specification for a better understanding of the manner in which this control can be achieved. Figure 3 is a block schematic illustrating a device con¬ structed in accordance with the invention. The device illustrated in Figure 3 comprises a circuit 3 which converts alternating voltage to a d.c. voltage,and a unit 4 which is operative in pulsating the d.c. voltage to the primary winding Tl of the transformer and which is controlled over a conductor 5. The inventive device further includes a circuit 6 which is connected to the secondary winding (T2) of the transformer T, in which circuit 6 the generated a.c. high voltage is converted to a high d.c. voltage, and the output signal of which cir¬ cuit 6 is preferably passed through a filter 7 and consti¬ tutes said variable high d.c. voltage on the conductor 2, this high d.c. voltage being supplied to the electrostatic -precipitator. t In accordance with the invention, the unit 4 which supplies the occurrent d.c. voltage pulsewise to the primary wind¬ ing Tl is constructed to impart to the d.c. voltage pulses supplied to the primary winding Tl of the transformer a pulse frequency of between 20 kHz and 200 kHz. As will be understood, in relation to prior art trans¬ formers,this will result in structural changes in, inter alia, the primary and secondary windings of the trans- former. In accordance with one particular embodiment of the inven¬ tive method, when the device is dimensioned and constructed for applying a variable high d.c. voltage to the- electrodes of an electrostatic precipitator, the d.c. voltage vari¬ ation lies within the range of 30 kV to 120 kV, preferably within the range of from 50 kV to 90 kV. In the case of this latter application in particular, the unit 4 is constructed so as to supply the d.c. voltage pulses to the primary winding Tl of the transformer at a pulse frequency of between 20 kHz and 100 kHz, suitably between 30 kHz and 70 kHz, and preferably at a pulse fre¬ quency around 50 kHz. In this latter case the transformer preferably has the confuguration illustrated in greater detail in Figures 7 and 8. In Figure 4 the reference 15 identifies an arrangement having a casing 15a which houses the requisite transformer in the bottom half thereof, the construction of which transformer is described in more detail hereinafter, and in the top half thereof houses the requisite control circuits for controlling switches or electrical contacts "A" and "B" in the desired manner. A comparison between Figures.2 and 4 will show the substan¬ tial simplification and the reduction in external measure¬ ments, and therewith the saving in weight, afforded by the present invention. Figure 5 illustrates an arrangement, or device, which is constructed in accordance with the principles of the pres¬ ent invention and which comprises an input three-phase cable 1 which is connected to a circuit 3 of known con- struction for converting the nominal alternating voltage to a d.c. voltage. If it is assumed that the alternating voltage supply on the conductors 1 is' a three-phase 380 V alternating voltage having a mains frequency of 50 Hz, there will appear on the output conductors 3a of the circuit 3 a d.c. voltage of approximately 500 V, which is supplied to a circuit 4. The circuit 4 includes two switch contacts "A" which, when activated simultaneously, effect a transfer of the d.c. voltage on conductors 3a pulsewise to the primary winding 23 of a first transformer 22. The control circuits which control the switches "A" are of a known kind and have not therefore been shown in detail here for the sake of simplicity. The switches may be arranged to be activated at for instance, frequencies from 20 kHz to 200 kHz. In the illustrated example the switches are activated at a frequency of 50 kHz. The supply time of each pulse is variable, so as to be able to vary the level of the generated variable d.c. voltage within the electrostatic precipitator 10. The circuit, or unit, 4 also includes two switches "B" which, similarly to the switches "A", are intended, when activated simultaneously, to connect the d.c. voltage present on the conductor 3a to the primary winding 27 of a second transformer 26 within a time section other than that in which the switches "A" connect voltage to the primary winding 23. The pulses W e also in this case applied at a frequency of 50 kHz and the duration of these pulses can also be regulated or controlled. The illustrated circuit 4 also includes, in accordance with the invention, four diodes, of which two, referenced 20 and 21, belong to the primary circuit of the first transformer 22, whereas the remaining two diodes, referenced 24 and 25, belong to the primary circuit of the second transformer 26. The diodes 20, 21 and 24, 25 function as demagnetizing diodes, therewith enabling the transformer to be demagnet¬ ized by a current passed through the diodes subsequent to the supply of a d.c. voltage pulse through the primary winding of the transformer. It is important to the invention that the circuit or unit 4 can be controlled by a control data-processor 30 in a manner which will enable the duration of the d.c. voltage pulse to be varied up to the half-period of the frequency concerned, this control data-processor being of a known kind and therefore not described here. The magnetic core of the transformeris to be de-magnetized during the remaining part of the frequency period. It should be observed that the current directions for the d.c. voltage pulses through respective transformers are constantly oriented in the samedirec ion, and hence the voltage across respective switc-f s "A" and "B" and the transformer will be substantially lower than if an alter- nating polarity had been used. It is particularly preferred in accordance with the in¬ vention, that the unit 4 is so controlled by the data processor or control unit 30, through the intermediary of switches "A" and "B", as to effect a pulse width modulation of the individual, sequential pulses, therewith to form a time-variable, high d.c. voltage, which exhibits pulses having a pre-determined variation in time. This latter applies in particular to those cases when the electrostatic pre¬ cipitator is to be supplied with a d.c. voltage which shall be superimposed with one or more pulses. Figure 6 is a diagrammatic illustration of d.c. voltage pulses a and b which, via the switches "A" and "B", are supplied to respective transformers 22 and 26 during each half time-period, therewith transferring maximum energy to the electrostatic precipitator 10 connected to the point referenced 31, such as to achieve the theoretical maximum d.c. voltage. Figure 6 also illustrates pulses a_' and b_' which are sup¬ plied to the transformers 22 and 26 and the pulse-width- modulation of which is selected so that voltage is trans¬ ferred to the primary windings of respective transformers during solely 25 % of the time period. It is obvious that a lower voltage will then lie across the electrodes of the electrostatic precipitator. The lowermost row in Figure 6 illustrates pulses a," which have mutually different pulse widths in time and which wilL with this distribution in time, successively raise the voltage level in the electrostatic precipitator. It should be observed in this regard that although in Figure 6 the pulse modulation of the two signals ei and b has been shown to be equal, this need not necessarily be the case. It will also be understood that the voltage in the electrostatic precipitator 31 can be varied accurately in dependence on the prevailing pulse width modulation of the mutually sequential pulses. Returning to the circuit diagram shown in Figure 5, it will be seen that the secondary winding of each transformer comprises two, preferably similar windings 28a and 28b (respectively 29a and 29b), each of which is connected up¬ stream and downstream of its respective diode 33 and 34 (33a and 34a) and which has an intermediate diode 35 (35a) connected therebetween* These diodes are connected n^ a manner to control the current in the two windings for current flow in solely one direction. As will also be seen from Figure 5, a respective terminal point of each secondary winding is earthed in the system through a respective diode 34 and 34a, and that the two remaining terminal points of the secondary windings 28a, 29a of respective transformers are connected to a common point 36 through their respective diodes 33, 33a. A diode 37 and a capacitor 38 are earthed in parallel from the common point 36, across the two series-connected wind¬ ings and the three series-connected diodes, the diode 37 functioning as a freewheel diode. The system also includes an output choke 39 which is located between the common point 36 and the electrodes (not shown) of the electro¬ static precipitator 10. As will be understood, each diode symbol may represent, in practice, a plurality of diode units connected together in series. Figures 7 and 8 illustrate- ' a suitable transformer T for the inventive function, where part of the secondary winding of the transformer comprises an electrically con¬ ductive layer 40 applied to a circuit card 41. The layer 40 is applied in the form of a "plane spiral" on one side of the card. Seventeen turns of the full secondary winding are located on each circuit card. About twenty circuit cards are arranged in side-by-side relationship (41, 41a) such as to form one part of the secondary winding of the transformer, for instance the winding part referenced 28a. The primary winding 23 of the transformer 22 may comprise one or two turns, which are positioned between the two secondary windings 28a, 28b, of which only one part of one secondary winding is shown in Figure 8. The circuit cards 41, 41a, and all remaining circuit cards, are arranged to embrace and be embraced by a transformer core 43, and each circuit card is positioned at a distance 44 from an adjacent circuit card, this distance preferably being from 1-2 mm. Both circuit cards and transformer cores are surrounded by cooling oil, which also covers the requisite diodes or diode units. A particular advantage is afforded when diodes, such as diodes 45, 46, are connected also between mutually adjacent circuit cards. In this way, a diode will connect each cir ¬ cuit card with another circuit card andtherewith reduce the influence of the capacitance against the core. Referring to the transformer core illustrated in Figure 7, the primary winding 23 is intended to be placed centrally on the transverse limb 43a ' of the transformer, with a secondary winding 28a on the left hand side and a secondary winding 28b on the right hand side. % The electrically conductive layer 14 applied to the circuit card 41 and forming a part of the secondarywinding is connected to corresponding layers or parts of the seconary winding on the circuit card 41, etc., through the intermediary of connecting lines not shown in Figure 8. Requisite diodes 33, 34 and 35 may be mounted adjacent the transformer on a plate not shown. The transformer core may be constructed from a ferrite material and may comprise eight standardized ϋ-shaped sec¬ tions. In the case of the embodiment illustrated in Figure 7, a first two such ϋ-shaped sections 50 and 51 are placed ad- jacent one another and a second two such U-shaped sections 52 and 53 are placed adjacent one another, such that the mutually adjacent limbs of said sections form one half of the illustrated transformer limb 43. The remaining four transformer sections are positioned in mirror image to the aforesaid four sections. Apertures formed centrally in respective circuit cards 41 and 41a are intended to accommodate the limb 43a of the transformer. It will be understood that it also lies within the scope of the invention to subdivide the secondary winding of the transformer into more than two equal or substantially equal parts. In the case of a secondary winding which comprises three equal parts, it is proposed that the primary winding of the transformer is subdivided into two equal parts and that each of said parts is placed between the secondary winding parts on the transformer limb 43a. It will be understood that the invention is not restricted to the described and illustrated exemplifying embodiments thereof and that modifications can be made within the scope of the inventive concept.

Claims: CLAIMS 1. A voltage converter arrangement intended for generating a high-voltage direct current for an electrostatic precipitator and which comprises a transformer which is encased in oil and the primary winding of which is supplied with a high-frequency voltage and the secondary winding of which comprises a plurality of mutually series-connected windings in the form of plane spirals, characterized in that the secondary winding of the trans¬ former is divided into two or more equal or essentially equal parts, that the primary winding of the transformer is positioned between said secondar winding parts, that rectifying devices, e.g. diodes, are connected uptrea downstream and centrally of the secondary winding, that the secondary win¬ ding comprises a plurality of mutually connected circuit cards, that said rectifying devices are connected between mutually adjacent circuit cards and that the circuit cards are essentially vertically positioned. 2. An arrangement according to claim 1, characterized in that the secon¬ dary winding comprises a plurality of mutually connected circuit cards eac of which has a conductor path in the form of a rectangular plane spiral. 3. An arrangement according to claim 1, characterized in that the core is a ferrite core. 4. An arrangement according to claim 1 or claim 3, characterized in that the transformer comprises a angetized core which is constructed from a plurality, preferably eight, of U-shaped sections. 5. An arrangement according to claim 1, characterized in that a freewheel diode is connected in parallel across the output connection of the arrange¬ ment. 6. An arrangement according to claim 1, characterized in that a capacitor is connected in parallel across the output connection of the arrangement.

Inventors: RANSTAD PER ANDERS GUSTAV, RANSTAD, PER, ANDERS, GUSTAV
Applicants: FLAEKT AB
Application year: 1988
Main classification: B03C3
IPC codes: H02M3