A method and apparatus for detecting back corona in an electrostatic precipitator

Abstract: In an electrostatic precipitator for cleaning of flue gases from industrial plants, comprising one or more precipitator sections (7) powered from a separate traditional or intermittent DC high voltage supply, the back corona discharge in the dust layer precipitated during the cleaning process on the collecting plates is detected periodically by raising the precipitator current in each DC high voltage supply unitl sparkover occurs, and where, after sparkover (F) or possibly a blocking period (B) (in case of no sparkover condition), the minimum value (U 2min , U 3min ) of the precipitator voltage is compared with that (U 0min ) before the sparkover, the latter value being corrected by a selected detection sensitivity factor (k).

Description: The present invention relates to electrostatic precipitators which comprise several precipitator sections, and which are used for purifying flue gases from industrial plants. More particularly it relates to a method and an apparatus for detecting the occurrence of back corona, i.e. electric discharges in the dust precipitated on the collecting electrodes of such an electrostatic precipitator. In such precipitators the rate of precipitation increases with increasing power input, under no back corona conditions. Occurrence of back corona is detrimental to the degree of precipitation and it is therefore of importance to be able to detect the occurrence of back corona to make it possible to control power input to the precipitator sections with a view to optimal cleaning of the flue gases. US-A-4390835 deals with detection of back corona based on changes in the slope of the current/voltage characteristic curve. The mean current is utilised as a function of the mean value of the precipitator voltage according to this patent. Similarly, according to US-A-4311491, the mean current is utilised as a function of the average value of the minimum values of the precipitator voltage. In recent years it has become more and more common to utilise, in addition to the traditional DC high voltage supply, a so-called intermittent voltage supply, to increase the efficiency of the precipitator, see for example US-A-4410849, according to which the power supplied to the high voltage transformer is interrupted periodically for a specific number of half periods of the mains frequency. A method for detection of back corona with intermittent voltage supply is disclosed in DE-A-3525557, where measurement of the minimum precipitator voltage is performed over four consecutive half periods of the mains frequency, after a deliberate interruption of the power supply which, however, limits the measurements to a certain part of the functioning of the precipitator, namely after said interruption or blocking period and therefore does not contribute to prevent considerable dust emission through undetected back corona before such a period. It is therefore an object of the invention to provide a method and an apparatus for reliable detection of the occurrence of back corona whether a precipitator section operates on traditional or intermittent DC high voltage supply with a superimposed ripple voltage, based on measuring the precipitator voltage before and after a sparkover. In an electrostatic precipitator of the type referred to in the introduction, this is achieved by increasing the mean current in a precipitator section at selected intervals by ignoring the preset upper current limit until sparkover occurs and detecting the occurrence of back corona by means of control equipment which for each precipitator section compares the minimum value of the precipitator voltage before and after a sparkover and thereby indicates the possible occurrence of back corona, subject to an accurate controlled recovery of the precipitator voltage after the sparkover. This method is based on a rise of the precipitator voltage to a level equal to the mean voltage before the sparkover, within a maximum of three half-periods of the mains frequency, regardless of the value of the precipitator mean current. At preselected time intervals (for instance 1 to 60 min.) the DC high voltage supply goes through a detection procedure, during which the precipitator current, ignoring the normal preset upper limit, is increased towards its nominal value until sparkover occurs. The minimum value of the precipitator voltage, preferably measured as one of the last 3 minimum values before sparkover (U 0min ) is compared with a minimum value of the precipitator voltage after sparkover which, typically, corresponds to the second minimum value after sparkover (U 2min ). Back corona is determined if U 2min is greater than a preselected sensitivity factor k (typical value: 1 to 1.5) times U 0min , and lack of back-corona is determined if U 2min is less than or equal to k.U 0min . The sensitivity factor k is used to vary the sensitivity of the back corona detection procedure and is chosen in accordance with the degree of back corona found to be optimum in the actual case (i.e. it is dependant on the particular process in question). The minimum value after sparkover may likewise be selected as the third minimum value of the precipitator voltage (U 3min ) or as the mean value of the second and third minimum values, i.e. ½(U 2min + U 3min ). In case the precipitator mean current cannot be increased further due to the fact that the nominal value or the earliest firing angle for the phase control thyristors of the precipitator is reached, and sparkover has still not occurred, the power supply is blocked for a selected time interval (typical value 0.1 to 5 s). Immediately after this blocking period, the precipitator voltage is raised in the same manner as after a sparkover, and a minimum value as described above after the blocking period is compared with the minimum value before the blocking period. Back corona is detected if a minimum value after the blocking period (U 2min ) is higher than the minimum value before it (U 0min ) multiplied by the sensitivity factor k. Back corona is determined not to be present if U 2min is less than or equal to k.U 0min . The invention is based on the recognition that the back corona, which starts by discharges in the precipitated dust on the collecting plates and liberates ions of opposite polarity to the ions generated by the discharge electrodes, and which reduces the precipitator mean voltage due to the increased conductivity of the gas in the electrode space and reduction of the space charge, develops within a certain relaxation time. In the presence of sparkover the precipitator voltage drops to 0 V, and this causes the back corona to cease. Therefore, during the subsequent recovery period of the precipitator voltage, the precipitator is able, during a short period, to tolerate a higher voltage than before the sparkover until back corona develops again. One example of the method and apparatus of the invention will now be described with reference to the accompanying drawings, in which:- Fig. 1 shows, in schematic form, a precipitator section with associated DC high voltage supply and control equipment; Fig. 2 shows the behaviour of the precipitator voltage at sparkover with and without back corona as applying to a traditional high voltage supply; Fig. 3 shows the behaviour of the precipitator voltage before and after a blocking of the corona power input to a precipitator section as applied to a traditional high voltage supply, and Fig. 4 shows the precipitator voltage at sparkover with and without back corona in the case of an intermittent high voltage supply. In Fig. 1 the AC voltage of the mains supply is conducted via a main contactor 1 to a thyristor phase control unit 2 comprising two inverse parallel thyristors and then to a high voltage transformer 3. The high voltage winding of the transformer is connected via a bridge rectifier circuit 4 to a precipitator section 7 and a voltage divider 6, and there is likewise inserted a shunt resistor 5. These two devices measure the precipitator voltage and the current, respectively. The signals from the voltage divider and shunt are conducted via the connectors 8 and 9 and interface circuits 11 to the control unit 12. The firing angle of the thyristors 2 is computed in the control unit 12 according to measurements and the control strategy incorporated in the microprocessor memory and is transmitted in digital form to the thyristor unit 2 via gate drive amplifiers 13. The signal from the voltage divider 6 is also conducted to a back corona detector 10. The detector 10 is connected to the control unit 12 via a two-way connection 14. The back corona detector 10 receives control signals from the control unit 12 in order to sense the minimum value of the precipitator voltage at the right instants and determine the minimum value before the sparkover (or a blocking period of the power input to the precipitator section in the absence of a sparkover) (U 0min ) and after the sparkover (or a blocking period of the power input to the precipitator section in the absence of a sparkover) (U 2min ). This latter value, which is the second minimum value after sparkover (or a blocking period of the power input to the precipitator section in the absence of a sparkover) is the preferred value, but it can be selected in the control unit 12 to be the third minimum value (U 3min ) or the arithmetic average value of U 2min , and U 3 min , i.e. ½(U 2min + U 3min ). The back corona detector 10, taking into account the detection sensitivity factor k (1 to 1.5) compares the minimum value of the precipitator voltage before and after the sparkover. Via the connection 14 the result is transmitted from the detector 10 to the control unit 12. The latter is connected to a control panel 15 having a keyboard and display from which preset parameter values, forming part of the control and of the high voltage power supply, can be changed and read. Normally, a single control unit 12 controls a single power supply, powering one precipitator section 7. The control unit 12 may, however, be connected to a central control unit 16, which is common to several DC high voltage supplies. The control unit 12 may be connected to a central control unit 16 via the connection 17 which transmits two-way information. The central control unit 16 may be programmed for simultaneous control and monitoring of several DC high voltage supplies and can control, totally or partially, the back corona detector of each high voltage supply to coordinate in an optimal way the detection procedure in the individual precipitation sections. The control unit 12 and the back corona detector 10 may be digital, analog or a combination thereof. The detector 10 may either serve a single precipitator section or be common to a plurality of sections. In the case where the control unit 12 works with a central control unit 16, the latter may be designed to control, totally or partly, the detection procedure and to coordinate the detectors for each precipitator section to avoid, for instance, simultaneous blocking periods of the precipitator voltage in various power supplies. Fig. 2 illustrates a comparison between the minimum value before and after a sparkover F in the case of a traditional high voltage supply, wherein the value before sparkover is designated U 0min and after sparkover U 2min , corresponding to the second minimum value, i.e. the value to which the precipitator voltage drops after the second pulse of the precipitator current after sparkover and just before initiation of the third current pulse. Fig. 2a shows the situation in the presence of back corona, and Fig. 2b the situation in the absence of back corona with an indication of the difference in magnitude between U 2min and U 0min . The ordinate in the upper curve indicates the absolute value of the precipitator voltage, and in the lower curve represents the current delivered by the high voltage power supply. The abscissa indicates the time. Fig. 3 shows the precipitator voltage before and after a blocking of the power supply as applying to a conventional high voltage supply, wherein U 0min is the voltage before the blocking period B and U 2min the minimum voltage after B. Fig. 3a shows a situation with back corona, while Fig. 3b shows a situation without back corona. This method is employed in the case where the precipitator mean current has reached the highest possible value, and sparkover still has not occurred. Fig. 4 represents a comparison between the minimum value before and after a sparkover F in the case of an intermittent high voltage supply and a cycle time C corresponding to three half periods of the mains frequency, where the thyristors are blocked for two half periods after a conduction interval of one half period. The other designations are the same as those used in Fig. 2. Fig 4a shows the precipitator voltage at sparkover when back corona occurs, while Fig. 4b shows the situation without back corona.

Claims: A method for detecting discharges, so called back corona, in the dust layer deposited on the collecting electrodes of an electrostatic precipitator (7) used for cleaning the flue gases from industrial plants, comprising one or more precipitator sections powered by a conventional continuous or an intermittent DC high voltage supply on which a further ripple voltage is superimposed, characterised in that the mean current in a precipitator section (7), at selected intervals, is increased by ignoring the preset upper current limit until sparkover occurs, and the occurrence of back corona is determined when a minimum value (U 2min ,U 3min ) of the precipitator voltage after sparkover is greater than the minimum value (U 0min ) before the sparkover multiplied by a detection sensitivity factor (k). A method according to claim 1, characterised in that the minimum value of the precipitator voltage after a sparkover which is used for measurement, is the second minimum value (U 2min ) after a sparkover, the third minimum value (U 3min ) after a sparkover, or the mean value (½(U 2min + U 3min )) of the second and third minimum values. Apparatus for carrying out the method of claim 1 or claim 2, comprising at least one precipitator section (7) having its own separate power supply (1-4) and control equipment (9-15) for adjusting the precipitator current, the control equipment including a microprocessor based control unit (12) for measurement and control of the high voltage supply, the control unit being connected via interface circuits (11) to the divider (5) and the shunt (6) of the control equipment; and a back corona detector (10) connected to the control unit (12), characterised in that the back corona detector (10) includes means for comparing measurements of the precipitator voltage before and after sparkover. Apparatus according to claim 3, characterised in that the control equipment further includes a central control unit (16) for comparing results from a plurality of precipitator sections and for central control and monitoring of a plurality of DC high voltage supplies in an electrostatic precipitator.

Inventors: REYES VICTOR M SC, REYES, VICTOR M. SC.
Applicants: SMIDTH & CO AS F L, F.L. SMIDTH & CO. A/S
Application year: 1987
Main classification: B03C3
IPC codes: B03C3