SE462144B - Process and device for determining the capacitance of an electrostatic filter during operation - Google Patents

Abstract

The invention shows a process and a device for inspection of electrostatic filters 10 for gas purification, comprising a filter housing 12 having earthed collecting electrodes 22 and emitting electrodes 24 connected to an alternating-current-connected transformer and rectifier unit 30,38,40. The state of the filter is established during operation through capacitance measurements. The capacitance of the filter is calculated during use of that alternating-current component of the filter current and filter voltage which is of a certain frequency, preferably the fundamental tone or double the mains frequency. The voltage across the filter electrodes 22,24 is measured. A fall in voltage proportional to the filter current across a precision resistor R11 arranged in series with the electrodes, between the rectifier and earth, is measured. The direct-current part of the filter current and filter voltage and unwanted alternating- current frequencies are separated out through the use of a band-pass filter 62. The capacitance of the filter is calculated from the acquired measurement values. <IMAGE>

Description

462 144 2 10 15 20 25 30 35 in the filter and a program for controlling changes in the filter current, in a regular or circumstantial manner. Examples of such changes are so-called pulse feeding, which means that the filter effect has peaks at certain intervals. Furthermore, we strive to keep the filter voltage as close to the filter's overvoltage voltage as possible. This is done by slowly increasing the filter voltage, from eg 95% of the overvoltage voltage until the overheating takes place, several times per minute. The filter control unit is connected to sensing means for the filter power in connection with the transformer and rectifier unit. To protect the control unit and other instruments, there is overvoltage protection, which comes into operation in the event of a breakdown.

Some dust particles with poor electrical conductivity do not emit their charges when they have accumulated on the collection electrodes.

These residual charges provide abnormally high electric field strength in the filter cake, which leads to glow charges in the filter cake, so-called "back eorona".

To achieve optimal gas purification, the voltage across and the current passage between the electrodes are carefully adapted to the dimensions of the electrode system and the nature of the treated dust. An important factor here is the prevailing capacitance between the wire-shaped emission electrodes and the flat-shaped collecting electrodes. This capability can be changed during operation, which leads to changed filter characteristics, deteriorated separation conditions and higher dust content in the emission. The changes in capacitance may be due to some of the plates settling and thus having a changed distance to adjacent emission electrodes. The capacitance can also be changed by poor cleaning due to wear of the shaking mechanism, for example worn bearings and general play, or by the shaking mechanism for a plate completely ceasing to function. The capacitance also changes if the dust discharge has stopped working, so that the dust in the underlying pocket reaches up to the electrode level.

Such a change in capacitance leads to impaired dust separation, which in turn requires a shutdown of the malfunctioning filter chamber, cooling it down and extensive troubleshooting. It is per se possible to perform capacitance measurements on the electrodes of the electrostatic precipitator in the respective section, but this requires, using now known technology, that the filter be switched off and the current cut off. Also according to SU 986 502 a method has been proposed for measuring the capacitance during operation by compensating the reactive component of the current in an electrostatic precipitator with current but by a standard capacitor, apparently in some form of bridge coupling. In this way, differential capacitance is measured, which is due to precipitated dust particles with residual charges, so-called "reverse" or "back corona". Based on the measured value, conditioning agents, such as ammonia, are then dosed into the gas intended for purification, which reduces the tendency for back corona and re-radiation. This device operates at the same high voltage as the electrostatic precipitator itself and therefore requires extensive and expensive safety installation.

Some of the above-mentioned changes, which lead to impaired capacitance and impaired filtering ability, occur slowly and gradually. It is then possible to rectify the faults in the event of a planned closure of the facility for general maintenance, provided that a warning has been received that deteriorating operating conditions are on the way.

Other errors, such as the sudden loss of a percussion instrument or the overfilling of a pocket, occur suddenly. It is then desirable to be informed immediately about this, so that action can be taken immediately.

The object of the present invention is to achieve a safer operation of electrostatic filters and thereby reduced dust emissions by specifying a method for making capacitance measurements of the various sections of the electric filter during operation, thereby ascertaining slowly occurring or sudden changes. in the capacitance of each filter section and thus in the separating ability of the filter. A further object of the invention is to provide such a method which gives a safe and reliable measurement result under varying operating conditions.

The object of the invention is further to provide a device for carrying out the method. 462 144 10 15 20 25 30 35 4 In order to achieve this object, the method according to the invention is characterized for controlling an electrostatic filter of the above-mentioned type, above all of the features stated in the characterizing part of claim 1. A device according to the invention is characterized above all by the features specified in the characterizing part of the first device requirement.

The invention has the advantage over SU 986 502 that all associated parts outside the electrostatic precipitator and the rectifier unit are located on the ground side of the electrostatic precipitator and that all measurements take place at low voltages and voltage potentials.

The invention is based on the basic idea that by measuring the AC component passing through the filter, the current capacitance of the filter can be determined. This is done by measuring the voltage drop across a measuring resistor connected in series with the filter together with measuring the voltage drop across the filter at a certain frequency. From the measured values, the capacitance can then be calculated.

The invention is explained in more detail in the following with reference to the accompanying drawing, which schematically shows an electric filter provided with a device for measuring the capacitance according to the invention and the results of field tests performed. In the drawing: Fig. 1 shows a schematic overview view of an electric filter with a device for carrying out the method according to the invention, Fig. 2 'an electrical circuit diagram showing the principles of the method according to the invention, Fig. 3 shows details which are irrelevant to the invention. free circuit diagram comprising a device for carrying out the invention, Fig. 4 a diagram showing values of voltage and current measured in a field test, Fig. 5 a diagram showing the result of performed in another field test measurements, Fig. 6 a diagram showing a Fourier analysis of the alternating voltage part according to the diagram in Fig. 4, Fig. 7 a diagram showing a Fourier analysis of the alternating current part of Fig. 4, Fig. 8 a diagram showing a Fourier analysis of alternating voltage the part in Fig. 5, Fig. 9 a diagram showing a Fourier analysis of the alternating current part of the current in Fig. 5, and Fig. 10 a schematic diagram for a device according to the invention, so m serves two parallel filter chambers.

Fig. 1 shows an electrostatic filter 10 comprising a housing 12 with an inlet line 14 for polluted air A, an outlet 16 for clean air B and a lock valve-provided outlet 20 arranged below a bottom pocket 18 for discharging separated dust C. Platform-collecting electrodes 22 are arranged in the housing. , which are connected to ground, and wire-shaped emission electrodes 24. The emission electrodes are clamped in frame frames 26 between the collection electrodes 22 and are fixed by the frame frames substantially midway between the collection electrodes formed channels 28. The emission electrodes 24 are connected via an insulated housing passage. on a transformer and rectifier unit 30.

The transformer and rectifier unit 30 consists of an oil-filled transformer housing 36 containing a transformer 38 and a rectifier bridge 40. The transformer's input lines 42 are connected to a thyristor-controlled AC power source. The negative side of the rectifier bridge is connected to the above-mentioned line 29 and via a high-ohmic resistor R5 and a line 43 to a switching cabinet 44 arranged on the outside of the transformer. In the switchgear, as shown in Fig. 3, the line 43 connected to a measuring output 31. The positive side of the rectifier bridge 40 is connected to earth via a line 46 and the switch cabinet 44. The flat collecting electrodes 22 are also connected to ground, thus forming a closed circuit.

Fig. 2 shows the principle according to the invention For measuring the capacitance of the electrostatic filter 10. Between the input line 29 of the electrostatic filter and earth There is a voltage ul.

This voltage can be measured by means of a voltmeter 48 connected in series with the resistor R5 via the measuring output 31 of the switching cabinet 44 and a second measuring output 32, connected to earth. (Voltmeter 48 is actually an ammeter, which is calibrated in relation to the resistor R5 and gives a pointer or other output measurement signal specified as a voltage expressed in kV). In series with the electrostatic filter, a low-ohmic resistor R11 is arranged between the collecting electrodes 22 and ground. In parallel with the resistor R11, a voltmeter 50 is arranged with a very high resistance in relation to the resistor R11. In the case of electric filters, it is practical to use such low-ohmic resistors connected in parallel with a voltmeter for measuring the passing current. In this case, the voltmeter is calibrated in relation to the resistor R11 and gives a pointer or other output signal specified in mA.

For the voltage ul, it consists of the sum of the voltage uz across the resistor R11 and the voltage drop across the electric filter itself.

In this case, the electric filter has a capacitive resistor CX and an ohmic resistor RX. With knowledge of the current measured with the aid of the measuring resistor R1, the following relationship can be established: 10 15 20 25 30 35 462 144 7 U U 1 - R2 <@kv. 1) R11 + Rx // CX ll This relationship can be developed as follows: R Rx // CX = o l l + jwRxCX _. R11 -_ U. R11 (1 * 5 "CxRx) 2“ 2 ~ “1 š" "'Rx" 1 R11 + jwRXR11cx + RX 11 *' 1¿jhnxcX 1 _. - C a _ 1 + JwCxRX Rx + Jw x 3 U = U = U 2 1. Rx 1 -1- + jwC- | -1- 1 + JwCXRX + fi; A RX x R11 u - U1 2 $ u1 4 2 "1_1 11 'R (3- _) T 1 _ 11 RX + JwCX - JWCXR11 UU 1 1,.

U2_ = 1 1 as 1 sz Ju1wCxR11 5 JWCXR11 JWCXR11 "2 Cxø _ '(eq. 2) 6" 1 W R11 Regarding the development of the relation according to equation l to the relation according to equation 2, the following is noted. In line 2 it has been introduced combined resistor For the parallel-connected resistors RX and CX, j a factor is due to the phase shift between an ohmic and a capacitive resistor and w the current frequency expressed in arc dimensions.In line 4, the term R11 has divided by Rx has been deleted because it is negligibly small compared to the term jwCX R11 For the same reason, in line 5 the term li denominator has been deleted because it is Negligibly small Finally, in equation 2 the phase shift between u2 and ul has been disregarded.

The voltage drop across the resistor R1 is negligibly small compared to the voltage drop between the electrodes in the electric filter, which is usually in the order of 20-50 kV or more. It is therefore possible to place the measuring resistor R11 in the switch cabinet 44 between a ground connection and the positive output line 46 of the rectifier bridge 40 without obtaining any appreciable change in the calculated capacitance. The two ends of the measuring resistor R11 are then connected via lines 56, 58 to measuring outputs 33, 34 on the switch cabinet 44.

Such measuring resistors R11 with associated measuring outputs 33, 34 are often already arranged on existing electrostatic filters in order to measure the total current through the electric filter. This total current consists of a large direct current part with a small superimposed irregular alternating current part with double the mains frequency. The measuring resistance R11 is then dimensioned, so that the voltage drop across the resistor becomes 1V at the maximum permissible current through the filter.

Equation 2 obtained above, however, does not relate to the measurement of a direct current but to the measurement of an alternating current, and moreover to the measurement of a sinusoidal alternating current of a certain frequency. According to the invention, it is therefore proposed that the superimposed alternating current portion for ul and u? is divided into a number of sinusoidal parts (fundamental and harmonics) with different frequencies and one of these parts with a certain frequency is used to perform the calculation of the capacitance of the electrostatic filter.

Mathematically, such a division of an irregular alternating voltage or alternating current can take place with the help of Fourier analysis in a known manner.

When directly measuring an electrical event, this division takes place by means of a so-called band-pass filter, a certain selected frequency. which only lets through The theories were confirmed by measuring in the field on two different filters and evaluating the measurement results by means of Fourier analysis. The measurements were performed by means of oscilloscopes using existing measuring outputs on the respective filter switch cabinets 44. Thereby, the voltage across the filter was measured over measuring outputs 31 and 32 connected to the respective resistors R5 and ground. The filter current was measured in a conventional manner for electric filters over measuring outputs 33 and 34 connected on each side of the resistor R11. 10 15 20 25 30 35 462 144 9 Measurement and calculation results are reported for filter a in Fig. 4, 6 0Ch 7 and for Filter b in rig. s, ß and 9. Filter a worked with frequently occurring glow charges or "back corona" in the dust cakes on the collection electrodes.

As shown in Figs. 4 and 5, there is a certain, small phase shift between the voltage u and the current I. This phase shift depends on the resistance and capacitance of the filter. When measuring sinusoidal alternating current to an ideal capacitor, the current has. maximum value, when the voltage increasing exceeds the average value. The current is said to be a quarter of a period before the voltage, which is mathematically denoted by "j".

Fourier analysis of the curves in Figs. 4 and 5 was performed on an IBM PC / XT computer in an integrated computer program SYMPHÛNY (Ver. 1.0).

To check the results obtained, sum curves were produced for the sine curves produced by the Fourier analysis, which sum curves were superimposed on the curves measured during the measurement. This overlay apparently shows a good agreement.

Figs. 6 and 7 show curves for filter a for the Actually measured values and superimposed curves S5, which constitute sum curves for the five lowest frequencies produced by the Fourier analysis. Furthermore, we see the obtained curves for the fundamental tone, 100 Hz, and the first two harmonics 200 Hz and 300 Hz, respectively. For the fundamental tone, a maximum voltage of 4.28 kV and a maximum current of 0.144 A were obtained. 8 shows the fundamental tone 100 Hz and also the first and second harmonics 200 Hz and 300 Hz, respectively, as well as a curve with a mains frequency of 50 Hz.

Figs. 8 and 9 show the corresponding for the filter b. In fig.

The measured measurement curve b is compared with a sum curve S6, which constitutes the sum of six superimposed sine curves with different frequencies.

Correspondingly, the 9 curves for 50 Hz, 100 Hz and 200 Hz as well as a sum curve S7 compared with the measured curve b are shown in fig.

For the fundamental tone 100 Hz, a maximum amplitude of 4.21 kV according to Fig. 8 and a maximum current of 0.0} 16 A according to Fig. 9 are obtained here. The maximum amplitudes obtained in the above manner for the fundamental tone 100 Hz was used to calculate the capacitance of each electric filter according to the above Formula. Furthermore, a capacitance measurement was performed on the two Filters, when these were switched off. This provides a check of the correctness of the hypothesis.

The experiments are summarized in the table below: Table l Filter a Filter b Dimensions L; B; tI (m) 2 x 4; 7.2; 9 2 x 3.75; 3.6; 5 Rectifier, max. Cape. _ 70 kV / 800 mA 70 kV / 400 mA Rll 1.25 ohm 2.5 ohm Gl (100 Hz) (peak value) 4.28 kv 4.21 kv Û2 (100 Hz) (peak value) 0.l5xl.25 (i2xRll) 0.032x2.5 (i2xRll ) Average current I 70 mA 20 mA Estimated capacitance CX 54 nF 12 nF Measured capacitance CX 55 nF 14 nF As can be seen, the calculated capacitance values and the actually measured capacitance values show a good agreement. the correctness of the hypothesis is confirmed.

Thus, it is of course also possible to perform the above calculations based on a higher Frequency than the fundamental tone. In this case, however, the amplitude and thus the accuracy becomes smaller. The curve obtained with Filter b with a mains frequency of 50 Hz is due to a disturbance and should not normally occur. This frequency cannot be used as a starting point for capacitance calculations.

For practical application of the invention, it is proposed that the selected Frequency For the calculations be separated even before the measurement of ul and u2 by means of so-called band-pass filters. This eliminates the direct current part and all frequencies except the selected measuring frequency. The output signal of the measuring instrument will thus directly without calculations show the desired magnitude.

Fig. 10 shows a practical embodiment, where two pieces of electric filter 10 and 10 ', respectively, each with its own power supply system 30 and 30', respectively, an indicating device, 11 462 144 are operated by the same measuring and evaluation means. From one 10, the respective measuring outputs 31, 32, 33, 34 of lines 52, 54, 56, 58 emanate from the switch cabinet 44 to a selection and time delay unit 60. the other filter 10 'is connected to the selection and time delay unit 60 with lines. 52 ', 54', 56 ', 58'. Additional filter chambers and / or additional filter sections with separate electrics of the two filters Correspondingly, the risk system arranged in the same filter chamber can similarly be connected by additional lines to the selection and time delay unit 60.

From the selection and time delay unit 60, a single set of lines 52, 54, 56, 58 output via a band-pass filter 62 to a measuring unit 64. The measuring unit is in turn connected to an evaluation unit 66 connected to a result accounting unit. 68, logo or digital printers and / or other display devices. which can be of any kind, such as ana- Furthermore, the evaluation unit is connected to an alarm device 70, for example a warning lamp and / or an audible signal.

The function of the joint measurement and evaluation body is as follows. The selection and time delay unit 60 connects the lines 52-58 and 52'-58 ', respectively, from the respective filter chambers or filter section at regular time intervals and connects these via the band-pass filter 62 to the measuring unit 64. The band-pass filter 62 separates the direct current part and transmits only an alternating current of the selected frequency, preferably the fundamental tone, to the measuring unit 64. The measuring unit 64 determines the highest values for ul and uz or with these proportional average values. The values obtained are forwarded to the evaluation unit 66, the capacitance being calculated. where the obtained capacitance value is compared with setpoints for the capacitance preserved in the memory of the evaluation unit.

If the calculated value differs too much from the setpoint, the alarm limit is exceeded and alarms are triggered. At the same time, it is activated which shows which filter chamber or filter section caused the alarm. Regardless of whether an alarm is triggered or not, the measurement result is reported by means of the profit reporting unit 68.

The measurement results from the measurements on each individual filter chamber or 10 15 20 25 3D 35 462 144 order and order For each section Separately, a 12 clay section in a Filter Chamber is obtained and preserved by printing in the result reporting unit and / or in the evaluation unit's memory.

By studying the instantaneous value of the capacitance and its changes over a longer period of time, one can draw conclusions about the status of the electrofilter with regard to maintenance needs and filtration results. Minor deviations from the setpoint are nothing to worry about and do not appreciably affect Filtration result A slow-moving change in capacitance indicates a slow-moving change, such as general wear, dust build-up on the collecting electrodes or that one or more of the collecting electrodes starts to strike. A more sudden change, albeit a relatively small, immediate one. can, on the other hand, motivate In the same way, different assessments can be made, if a major change in capacitance occurs suddenly or has built up slowly.

Electrostatic Filters For large plants are divided into a number of chambers parallel to the gas flow and in each chamber the emission and collection electrodes are divided into sections in the direction of the gas flow, which sections are connected to their respective power supply systems, as described above.

By performing capacitance measurements in the manner described above in continuous monitoring of the status of the electrodes in each section of operation. Furthermore, in the event of a malfunction, an information is immediately received in which section the malfunction occurs. This significantly reduces the troubleshooting time when shutting down a filter chamber for repair. This can reduce the shut-down time and return the Filter to Full purification capacity more quickly.

When manufacturing an electric filter, capacitance measurements are made on the filter in assembled condition for each electrode section. This capacitance value constitutes the setpoint and is entered into the memory of the evaluation unit. Limit values are also entered in the memory For acceptable values of the capacitance. If these values are exceeded, alarms are triggered. With Fördel, you can enter two limit values and thus two alarm levels. The smaller deviation and the lower alarm level mean that it is time to prepare maintenance on the filter or the relevant filter section. The greater deviation and the higher alarm level, on the other hand, means that immediate action must be taken and the damaged or otherwise non-functioning electrode section must be repaired immediately.

The alarm level or levels depend on the nature of the pollution and acceptable emission levels. In general, however, a deviation in capacitance in the order of 5-15% does not cause any concern and does not require any action. A capacitance deviation of the order of 25%, on the other hand, indicates serious malfunctions and may give rise to immediate action.

In the manufacture of electrostatic precipitators, the capacitance is determined and measured, respectively, and a guide value for normal value (setpoint value) is determined and entered into the evaluation unit's memory. A higher threshold level for deviating capacitance value upwards and / or downwards is determined and entered into the memory. Exceeding this threshold level indicates a serious error, which requires immediate action.

This is indicated by the triggering of a "large alarm" by means of the alarm device 7Ü. Furthermore, a lower threshold level is suitably determined for a so-called Vlitet alarm "for action at the next maintenance stop or possibly more immediately in the event of a sudden change.

For filters that include pulse control, a tunable bandpass filter must be used.

Claims (7)

14 462 144 P A T E N T K R A V A method of controlling electrostatic filters (10) for gas purification comprising a filter housing (12) with grounded collection electrodes (22) and emission electrodes (24) connected to an AC-connected transformer and rectifier unit (30, 38, 40), the status of the filter (10) is determined during operation by capacitance measurements, characterized in that the capacitance of the filter (10) is calculated using the alternating current component of the filter current and the filter voltage at a certain frequency, preferably the fundamental or double mains frequency, by the ter electrodes (22, 24) are measured, and that a voltage drop proportional to the filter current is measured across a measuring resistor (R11), which is arranged in series with the electrodes (22, 24) between the rectifier and ground, the filter current and the filter voltage being a direct part and not desired alternating current frequencies are separated by a so-called bandpass filter (62), and that the capacitance of the filter is calculated from the measured values obtained. 2. A method according to claim 1, characterized in that said capacitance measurements are performed intermittently during operation, preferably at regular time intervals. 3. A method according to claim 1 or 2, characterized in that said capacitance measurements are performed in a measurement and evaluation means (62, 10) common to several filter chambers and / or filter chamber sections (10, 10 '). 64, 66), which is connected to at least one result reporting unit (68) and / or at least one alarm device (70). __ A method according to claim 3, characterized in that said filter chamber or filter chamber sections (10, 10 ') are connected to the measuring and evaluation means (62, 64, 66) via separate lines (52, 54, 56). , 58 and 52 ', 54', 56 ', 58'), respectively, and a selection and time delay unit for periodically connecting respective lines 462 144 to the measuring and evaluating means (62, 64, 66). Device for carrying out the method according to one or more of claims 1-4 for determining the capacitance of an electrostatic filter with at least one filter chamber or section (10, 10 ') comprising a housing (12) with flat, grounded collecting electrodes ( 22) and emission electrodes (24) and an AC-connected transformer and rectifier unit (30, 38, 40), characterized in that the device comprises means (R5,48, 64) for measuring a voltage ul above the electrodes (22, 24) and means (50, 64) for measuring a voltage uz proportional to the filter current across a measuring resistor (R11) connected in series with the filter electrodes connected between the rectifier (40) and ground, that the device comprises at least one bandpass filter (62) , which allows the passage of a certain, selected measuring frequency, preferably the fundamental tone or double the mains frequency, while the direct current component of the voltage and alternating current components of another frequency are switched off, and that it comprises an evaluation unit (66) intended to calculate the capacitance of the filter or filter section (10, 10 ') from the measured current and voltage values at the selected frequency. Device according to claim 5, wherein the electro-static filter is divided into several chambers (10, 10 ') parallel in gas flow direction and / or sections located one after the other in the flow direction, each provided with a transformer and rectifier unit ( 30, 30 '), characterized by a common evaluation unit (66) for calculating the capacitance and a selection and time delay unit (60), for periodically connecting wires (52, 54, 56, 58 and 52 ', 54', 56 ', 5B') to the respective filter part and preferably also a common measuring unit (64) and / or a common band-pass filter (62). 462 144 16 Device according to one of Claims 5 or 6, characterized by a profit accounting unit (68), such as a digital or analogue printer and / or a digital or analogue, visually readable profit accounting unit, and / or by an alarm device (70). for emitting a visual or audible alarm signal in the event of exceeding or falling below a threshold level For the measured and calculated capacitance value and / or of a memory device (66, 68) for preserving measured and calculated capacitance values.