WO1988007413A1

WO1988007413A1 - A method for producing a variable d.c. voltage

Info

Publication number: WO1988007413A1
Application number: PCT/SE1988/000159
Authority: WO
Inventors: Per Anders Gustav Ranstad
Original assignee: Fläkt Ab
Priority date: 1987-04-01
Filing date: 1988-03-30
Publication date: 1988-10-06
Also published as: AU1599488A; SE8701367D0; SE8701367L

Abstract

The present invention relates to a method for generating from an alternating voltage (1), preferably of mains frequency, a variable d.c. voltage (3) which is applied to the electrodes of an electrostatic precipitator with a voltage level exceeding 30 kV. The alternating voltage (1) is converted (3) to a d.c. voltage, optionally subsequent to transforming-up the d.c. voltage, and the d.c. voltage (via switches ''A'' and ''B'') is supplied pulsewise to the primary winding (23) of a transformer (22), and the output voltage of the secondary winding (28a, 28b) of the transformer is rectified (33, 35, 34) and constitutes the variable d.c. voltage. The pulsated supply of the d.c. voltage to the primary winding of the transformer (via the switches ''A'' and ''B'') is effected at a frequency of between 20kHz and 200kHz.

Description

TITLE OF THE INVENTION; A method for producing a variable d.c. voltage.

TECHNICAL FIELD The present invention relates generaXlly to a method for generating a variable high d.c. voltage from an alternating voltage, preferably at mains frequency, said d.c. voltage being applied to the electrodes of an electrostatic precip¬ itator whose voltage exceeds 30 kV and whose power require- ment is normally from 1 to 100 k .

The present invention relates particularly, but not exclu¬ sively, to a method of this kind in which the alternating voltage is first converted to a first d.c. voltage and this first d.c. voltage is applied pulsewise to the primary winding of a transformer, such as to obtain a high alternating voltage in the secondary winding of the trans¬ former, after which this high alternating voltage is con¬ verted to a high d.c. voltage, which constitutes said variable high d.c. voltage.

It will be understood that in addition to enabling a gener¬ ated high d.c. voltage across the electrodes of an electrostatic precipitator to be controlled in a predetermined manner or to maintain the d.c. voltage constant at a given level, the present invention also en¬ ables a d.c. voltage to be varied in time quickly, in a desired manner.

Thus, the present invention has been primarily devised for generating practically any desired variable high d.c. volt¬ age such as to^"' be applied to the electrodes of an electro¬ static precipitator, thereby to enable time control of the • voltage between the electrodes. BACKGROUND PRIOR ART

It is known that a high d.c. voltage can be generated, e.g. for supplying the electrodes of an electrostatic precipitator, from, e.g., a mains voltage, by first step- ping up the voltage through a transformer and then recti¬ fying said voltage.

It is also known that this high d.c. voltage can be con¬ trolled manually by switching the transformer or with the aid of power controlling devices, such as transductors or thyristors, connected in the primary circuit. The use of power control devices enables slow variations of the d.c. voltage to be obtained -when desired, for instance in order to adapt the d.c. voltage to variations in operating con- ditions.

It is also known that a sufficiently high voltage between two electrodes in an electrostatic precipitator, of which electrodes one is intended to produce a corona discharge, provides an ion current between the electrodes which can be used to extract dust from gas flowing between the electrodes.

In the case of conventional electrostatic precipita ors, intended, e.g., for cleansing flue gases, voltages in the order of 50 kV are required. Consequently the standard oil impregnated tr.ansfor ers used together with the rectifiers often weigh more than 1000 kg. The major part of this weight is the transformer itself.

It is also known that the dimensions of the transformer are contingent on the frequency used for the applied voltage, and that a higher frequency enables a reduction in the weight of the transformer. It is also known that the eddy current losses increase markedl at high frequencies. Therefor the core of the transformer norm ly consists of a number of thin plates. In practice, plates ha a thickness of 0.3 mm can be used up to 500 Hz. The thinnest p capable of being handled can be used up to frequencies of between 1 and 3 kHz.

An example of an arrangement of this kind is described and illustrated in the German Offenlegungsschrift DE-A1-35 22 569, to which the reader is referred.

The aforementioned Offenlegungsschrif also discloses that the pulse frequency of the d.c. voltage can be increased. to 3 kHz.

SUMMARY OF THE PRESENT INVENTION

TECHNICAL PROBLEMS When considering the present state of this art it will be seen that a technical problem resides in reducing the weight and dimensions of primarily the transformer which is incorporated in a rectifying circuit and which is intended to produce a d.c. voltage in excess of 30 kV at powers of several kW.

A further technical problem resides in the need to control the d.c. voltage level in an electrostatic precipitator.

A particular technical problem in this art is one of en¬ abling the voltage in an electrostatic precipitator to be varied with a time constant which is much shorter than the period of the mains voltage used.

Another technical problem is one of being able to interrupt quickly the voltage supply to an electrostatic precipi¬ tator in the event of flashover between the precipitator electrodes, and therewith minimize the time during which the precipitator is inactive as a result of gas discharge (arc).

A further technical problem resides in the ability of quickly supplying energy to the electrostatic precipitator subsequent to extinguishing the gas discharge caused by flashover.

Another technical problem is one of realizing that problems of insulation and problems of capacitive storage of energy in a transformer can be greatly reduced by magnetizing the transformer solely in one direction.

A further technical problem resides in the realization that the transformer can be demagnetized by a purely passive circuit, even somewhat quicker than the time during which the active magnetization takes place.

A further technical problem resides in the realization that those gaps which occur in the voltage supply to a secondary loa as a result of demagnetizing the transformer passively can be filled with two transformers working in "counterphase".

A further technical problem is one of realizing that the power input to an electrostatic precipitator can be ad- vantageously controlled by supplying a high voltage trans¬ former with a pulse voltage having a high and fixed fre¬ quency, and by varying solely the pulse time.

A qualified technical problem in this regard is one of realizing that with a high frequency, pulse-width-modulated pri ary voltage the rectified secondary voltage can be caused to closely resemble a desired, more slowly varying course.

A further technical problem resides in the provision, with the aid of simple components, of a method which utilizes a pulse frequency of between 20 kHz and 200 kHz with regard to the d.c. voltage to be applied to the primary winding of the transformer, and therewith to create conditions, in the absence of excessive losses, such that the d.c. voltage occurring between the electrodes of an electrostatic pre¬ cipitator can be caused to vary in time in accordance with a predetermined desired pattern or in any desired manner.

Finally, a further technical problem is one of realizing that when practicing the aforementioned method, in which a configured d.c. voltage is applied pulsewise to the pri¬ mary winding of the transformer, the frequency can be selected advantageously between 30 kHz and 70 kHz.

SOLUTION

The present invention relates to a method for producing a variable or constant high d.c. voltage having a d.c. volt¬ age level or value in excess of 30 kV,from an alternating voltage, preferably at mains frequency, such as 50 Hz or 60 Hz.

The inventive method is based on the known technique of converting said alternating voltage to a d.c. voltage, optionally subsequent to transforming-up the voltage, and of applying this d.c. voltage pulse-wise to the primary winding of a transformer, and by rectifying the output voltage from the secondary winding of the transformer, this output voltage constituting said variable d.c. voltage. It is proposed in accordance with the invention that this pulsewise supply of d.c. voltage to the primary winding of the transformer is effected at a frequency of between 20 kHz and 200kHz, particularly between 30 kHz and 70 kHz, and preferably at about 50 kHz.

It is of particular importance that the transformer is mag¬ netized solely in one direction, by supplying to the primary winding a pulsed d.c. voltage of solely one polar¬ ity.

It is also suggested that the variable d.c. voltage is obtain by connecting two transformers in parallel and by activating said transformers alternately.

In accordance with one advantageous embodiment of the in^¬ vention, the primary winding of the transformer is supplied with a pulsed d.c. voltage of fixed frequency and the de^¬ sired variation ih the variable d.c. voltage is obtained by varying the time during which respective d.c. voltage pulses are supplied to the primary winding.

The transformer magnetizing time is controlled by active components, such as transistors or thyristors, whereas demagnetization of the transformer is effected with the aid of passive circuit elements, such as diodes for exam_¬ ple.

In accordance with the invention, the variable d.c. voltage can be advantageously obtained, by connecting two trans_¬ formers or transformer windings in parallel and by switching-i them alternatively.

ADVANTAGES The advantages primarily characteristic of a method according to the present invention which can be used to particular advantage for varying the high d.c. voltage applied to the electrodes of an electrostatic precipitator reside in the provision of conditions which enable the dimensions of the transformer to be substantially reduced while still achieving improved and more rapid control of the voltage between the precipitator electrodes while enabling the electrode voltage to be maintained with practically any desired time variation.

The primary characteristic features of an invention ac- cording to the present invention are set forth in the characterizing clause of the following claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the accompanying drawings there will now be described in more detail the state of the prior art with regard to variation of the d.c. voltage applied to the electrodes of an electrostatic precipitator and also an arrangement constructed in accordance with the invention and a transformer which is adapted for high frequencies; in which drawings

Figure 1 is an electric circuit diagram of a known device for generating a variable high d.c. voltage;

Figure 2 illustrates a known arrangement in the upper part of an electrostatic precipitator and operating with a device according to Figure 1;

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 illustrates 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 electrodes.

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. 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 are 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 same direction, and hence the voltage across respective switches "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 b¹ 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,, wi^th 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 a_ 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 in- 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 5 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 illustrated 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 U-shaped sec¬ tions.

In the case of the embodiment illustrated in Figure 7, a first two such U-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

1. A method for generating from an alternating voltage, preferably at mains frequency, a variable d.c. voltage 5 which is applied to the electrodes of an electrostatic precipitator having a voltage level in excess of 30 kV, and in which method the alternating voltage is converted to a d.c. voltage, optionally subsequent to transforming up said voltage, and said voltage is supplied pulsewise to

10 the primary winding of a transformer, and in which the output voltage of the secondary winding of said transformer is rectified and constitutes said variable d.c. voltage, characterized by supplying the d.c. voltage pulsewise to the primary winding of the transformer at a frequency of

15 between 20 kHz and 200 kHz. and by supplying the primary- winding with said d.c. voltage in forms of pulses of solely one polarity, such as to magnetize the transformer solely in one direction.

23⁾ 2 A method according to claim 1 characterized in that the variable d.c. voltage is obtained by connecting two transformers in parallel and by activating said trans¬ formers alternately.

5 3. A method according to claim 1 or 2, characterized by supplying said d.c. voltage at a frequency of between 30 kHz and 70 kHz.

4. A method according to claim 1 or 2, characterized by ³⁰ supplying a pulsated d.c. voltage of fixed frequency to the primary winding of the transformer and by achieving the desired variation in the variable d.c. voltage by varying the times during which respective d.c. voltage pulses are supplied to the primary winding- 35 1 3

5. A method according to claim 4, characterized by con¬ trolling the magnetizing period of the transformer with the aid of active components, such as transistors or thyristors.

6. A method according to claim 4 or 5, characterized by demagnetizing the transformer with the aid of passive circuit elements, such as diodes.