EP0209714A1 - Method for operating an electrostatic filter - Google Patents

Abstract

1. A method for the operation of an electrostatic filter, in which an alternating current converted by an electronic regulator is stepped up and is rectified via a bridge-connected rectifier for the purpose of generating a basic direct voltage for the filter upon which high voltage pulses are superimposed if necessary, characterised in that the converted alternating current is produced by switching a direct current, with alternating sign, drawn from an intermediate circuit to the alternating current outputs of the converter in the cycle of a converter frequency of a few kHz, and in that the intermediate circuit is fed by a controllable rectifier arrangement (DR) with a controllable intermediate circuit current which in one period of the converter frequency is practically independent of the superimposition upon the alternating current outputs, and in that in the event of a flashover in the filter the direct current is conducted via a no-load path (TQ) past the alternating current outputs of the inverter.

Description

The invention relates to a method for operating an electrostatic filter with the features of the preamble of claim 1 (DE-AS 19 23 952).

For exhaust gas cleaning or more generally for the separation of foreign substances from a flowing medium, electrostatic filters are often used, the plates and spray wires of which are supplied with such a high DC voltage that in the medium passed between the plates and spray wires ionization of the foreign substances contained and their separation on the plates occurs. In the interest of a high degree of separation, the DC voltage (supply voltage) of the plates and spray wires is chosen to be as high as possible. On the other hand, with a high supply voltage, ionization processes also take place in the gas itself, which lead to a permanent discharge of the filter up to a corona discharge on the spray wires.

If the supply voltage rises above a limit value, the filter discharges via short breakdowns or even voltage breakdowns up to a stationary arc if the direct current supplied by the voltage supply is not interrupted. No significant foreign matter separation is then possible until the subsequent reconstruction of a high DC voltage. In addition, these processes cause wear on the filter, in particular its spray wires, and short downtimes of the entire device.

The ionization processes and thus the mentioned limit value of the supply voltage depend on the distribution of the electric field strength between the plates of the electrostatic filter. Insulating layers of foreign substances deposited on the plates must be knocked off, collected and removed at certain intervals - if necessary with the supply voltage switched off for as short a time as possible. In addition, ionization creates space charges with strong distortions in the potential profile between the plates, and the voltage gradient and the spray direction can even be reversed between plates and space charges.

The limit value mentioned is therefore not constant during operation. For a good separation, the supply voltage of the filter should be kept as close as possible to this practically uncontrollably changing limit value.

Commercial electrostatic precipitators contain a voltage supply that is connected to two phases of a three-phase network and takes an alternating current from the network via an electronic actuator. The output voltage of the actuator is controlled by the firing angle and supplies a line-frequency alternating current which is phase-shifted with respect to the input voltage and which then feeds the electrostatic filter as a pulsating continuous current after step-up and rectification. To approximate the optimal working conditions of the filter it is proposed in DE-AS 19 23 952 to ramp up the voltage on the electrostatic filter after a certain ramp-up function via the gate control in the actuator until the limit value corresponding to the current state of the filter is reached and there is a voltage breakdown or a similar sudden discharge of the filter occurs.

As a rule, the AC power controller must first be blocked after a breakdown in order to avoid an arc and to wait for the deionization of the plasma formed. The currentless minimum pause is determined by the frequency of the actuator, i.e. the mains frequency. The result of this is that the filter is fed by a direct current which flows practically without gap with a ripple corresponding to the mains frequency and is interrupted after a breakdown. For the filter voltage fed by this current, there is an undulating course which rises until it breaks down.

Electrostatic precipitators have also been proposed in which the filter is not supplied with such a practically seamless direct current which is taken from the supply network by a mains-frequency alternating current regulator, is highly transformed and rectified. Rather, the filter is charged by a sequence of individual voltage or direct current pulses. In order to supply the charge that flowed across the medium during the pulse pauses, the frequency and / or duration of the individual pulses are specified such that the mean current intensity of these isolated direct current pulses assumes a filter current setpoint value that is adapted to the respective filter state. This creates a filter voltage that is rippled in accordance with the pulse repetition frequency, the value of which is as far as possible below the breakdown limit.

This creates the technical difficulty of using the short pulses to provide the filter with the required energy. For this purpose, it is proposed in US Pat. No. 3,641,740 to charge a series of capacitors by means of the rectified mains voltage, which are then connected via thyristors, high-voltage transformers and one Half-wave rectifier are placed on the electrostatic filter. The width of the current pulses reaching the electrostatic precipitator is, for example, 5% of the pulse pause between these pulses.

A combination is currently striven for as an optimal method in which the filter is initially biased via a rectifier with an already relatively high, practically constant basic DC voltage, which is then superimposed on an AC voltage or isolated individual voltage pulses in order to generate a wavy filter voltage.

According to US Pat. No. 3,984,215, the height of the filter is said to be considerably above the breakdown voltage of the filter, but a very short pulse duration means that no arc is formed when the filter is discharged. The duration, shape and pulse repetition frequency of these isolated individual pulses are adapted to the respective load condition of the filter. According to European patent specification 0 034 075, isolated current pulses are fed to the filter biased to the constant basic DC voltage, the maximum amplitude of which is controlled in accordance with a setpoint value for the filter current in such a way that the filter is thereby pulsed to a maximum voltage below the breakdown voltage. These current pulses are taken from an intermediate circuit fed by a rectifier by means of a resonant circuit converter dimensioned to the desired pulse width or a frequency-controlled converter with forced quenching and are transformed up. The ripple of the filter voltage is also ensured in that a diode suppresses one polarity of the highly transformed current pulse.

DE-OS 27 13 675 proposes a simple power supply in which the basic voltage is supplied by a gate-controlled AC power controller connected to two phases of a three-phase network with a transformer and rectifier connected downstream. The electrodes supplied with the basic direct voltage are connected to the secondary winding of a high-voltage transformer via a coupling capacitor, the primary winding of which is fed by a controllable rectifier device via an inverter in the center point circuit. As a result, a non-rectified AC voltage with a frequency that can be varied between 50 Hz and 2 kHz is superimposed on the basic voltage.

If these methods, which are determined by the properties of the deposition process, are to be used at the operating location of the filter, the requirements for the supply network must also be observed, for which increasingly stringent regulations apply. For example, Limits for the reactive current and harmonic load of the network as well as an asymmetrical load between the three-phase connections of the supply network must be observed. Finally, the installation costs are to be kept as low as possible.

The invention now provides a method which can be adapted as optimally as possible to the changing operating conditions and which responds quickly, particularly in the event of rollovers.

Since the filter has to transmit a relatively high power, those known methods are used to solve this problem, in which an alternating current redirected by means of an electronic actuator is transformed and used Generation of a basic DC voltage is rectified via a bridge rectifier. The filter is thus practically fed with a continuous direct current and therefore, when operated continuously, only has a ripple in its supply voltage that corresponds to the converter frequency. High-voltage pulses can still be superimposed on this basic DC voltage in a known manner, but in many cases these additional high-voltage pulses can also be dispensed with because of the conversion method according to the invention.

According to the invention, the alternating current is generated with an inverter frequency of a few kilohertz, in particular about 2 kHz or more.

While a frequency converter is only used in the prior art if a basic DC voltage for generating a ripple aimed for in the deposition process is an AC voltage with the converter frequency without rectification or a sequence of isolated individual pulses which result from the converter-frequency AC voltage by means of a diode by suppressing the negative voltage half-waves are formed, is superimposed, the high-frequency alternating current is used according to the invention in order to initially generate an initially low harmonic DC voltage via the bridge rectifier by means of a practically continuous direct current.

This has the advantage that in the event of a flashover the supply current of the filter can be extinguished within a fraction of a millisecond by blocking the converter clock in accordance with the high clock frequency.

It is advantageously provided that a DC link converter is used as the frequency converter, in which the converter connected to the three-phase supply network (e.g. a controllable three-phase rectifier or in particular a three-phase rectifier with a downstream DC converter for controlling the DC link direct current) supplies a direct current which to form the alternating current in time with the converter frequency, a high-frequency inverter with alternating sign is used to switch to the alternating current outputs of the converter. In the event of a breakdown, the inverter only needs to be blocked in order to switch off the filter current and the direct current is to be routed past the alternating current outputs of the inverter by means of an idle path using suitable means (e.g. a cross-thyristor between the direct current inputs of the inverter or suitable ignition pulses for series-connected inverter valves).

In such an intermediate circuit converter, the intermediate circuit current is impressed by the actuator. Its amplitude is practically unchangeable within a period of the converter cycle, so that even in the event of a breakdown there is no significant increase in current at the transformer and therefore no significant recharging of the filter. This prevents arcing and limits the duration of discharge phenomena, which leads to reduced wear on the spray wires and increased service life of the filter.

On the other hand, the converter cycle only needs to be blocked for just as many cycle periods as is necessary to deionize the gas. The entire intermediate circuit current is then available again for alternating and recharging the filter, the separating ability of which is therefore quickly restored, thereby the filter efficiency is increased. As a result of the high alternating current frequency, only a smaller transformer is required, which results in lower construction costs and power losses and more favorable transient behavior in the event of rapid changes in operation.

In addition, there is the possibility of providing such a deionization time from the outset after several converter cycle periods by temporarily temporarily blocking the actuator cycle in order to reduce the number of breakdowns. The practically gap-free direct current generated in the kHz cycle is thus divided into individual direct current packets with a lower repetition frequency. The duration of the currentless breaks that arise in this way can be adapted to the deionization time in the respective application.

The e.g. By controlling the DC link direct current, possible amplitude control of the high-frequency alternating current offers an additional degree of freedom for controlling the filter voltage. In this way, the inverter amplitude can be ramped up again after each breakdown or each inverter lock after a predefined start-up function.

In particular, the curve shape of the filter supply voltage, which is very different for the respective application, can be adapted via the amplitude control within the DC packets mentioned.

For example, An adjustable ripple dU / dt, e.g. the basic voltage U of the filter, e.g. depending on the residual content in the medium.

This is advantageous, for example, if the medium is already largely free of foreign substances and very is high impedance. In this case, the medium is only able to conduct a low current with a constant filter voltage and thus only absorb a low power. However, the energy consumption of the filter can be increased by a higher ripple in the supply voltage. It may also be necessary to control the average level of the filter voltage as a function of the residual impurity content; With a higher filter voltage, a separation of residual foreign matter can also be achieved in the outflowing, already largely cleaned gas.

The optimal operating point for feeding the filter can be specified in an iterative search process via the control. For example, the filter voltage can be increased automatically by increasing the supplying direct current, as long as the frequency of the breakdowns occurring is not significantly increased. As a result, a dependency of the filter voltage on the foreign matter content of the inflowing gas is automatically taken into account, but it can also be carried out by a predetermined function for controlling the alternating current amplitude, e.g. depending on the recorded breakdown frequency.

In particular, the periodicity of the entire voltage curve can also be controlled via the converter frequency and changed depending on the load condition or the foreign substance content of the inflowing gas.

Only the actuator itself acts as a load for the supply network, in the case of an uncontrolled three-phase bridge with a downstream DC regulator, a symmetrical load with a very low ripple of only about 4.2%. The power factor (cos ⇑) is included Practically 1 and the intermediate circuit largely protects the supply grid against grid effects of the inverter.

Furthermore, there is an overall reduction in the power consumed by the electrostatic precipitator, since after a flashover the duration of this short circuit is small because of the short time between the detection of the short circuit and the extinction of the high-frequency alternating current, so that only a little energy can flow into the short circuit.

• Figur 1 den prinzipiellen Aufbau und die
• Figuren 2 und 3 vorteilhafte Realisierungsmöglichkeiten einer Vorrichtung zur Durchführung der Erfindung.

Advantageous further developments are characterized in the subclaims. The invention is explained in more detail with reference to three figures. Show:

• Figure 1 shows the basic structure and
• Figures 2 and 3 advantageous implementation options of a device for performing the invention.

In the figures, F denotes the electrostatic filter, between the electrodes (plates and spray wires) of which the medium represented by an arrow M (for example flue gas or another exhaust gas) is passed and which has a voltage U, which is detected by a measuring element MU, to be supplied from a supply network N. For this purpose, the input of the filter is connected via a high-voltage rectifier GRH to the secondary winding WS of a transformer, the primary winding WP of which is at the output of the high-frequency clocked frequency converter HF.

The high-voltage rectifier GRH is designed as a full-wave rectifier, in particular as an uncontrolled rectifier bridge. An intermediate circuit converter is preferably used as the high-frequency frequency converter HF used. In the block diagram of FIG. 1, a switch Q indicates a switchable freewheeling path via which inductive currents which occur when the converter is blocked, for example the impressed intermediate circuit current when a current intermediate circuit is used in the converter, can continue to flow. The amplitude of the high-frequency alternating current (ia) can usually be controlled via a corresponding control input of the alternating current controller HF. NF indicates that the amplitude control can also be adjusted via a controllable mains frequency AC power controller.

PR summarizes the control and regulation of the high-frequency alternating current in FIG. 1, which, in addition to the actual value U of the filter supply voltage, also contains suitable actual and target values depending on the application. The amplitude, frequency and the "cross firings" of the freewheeling path Q can be used to control the interruptions in the current ia at the output of the AC power controller HF via the output signals.

For the operation of the filter, very different parameters can be taken into account and implemented in a correspondingly fast control and regulation. The operation of the filter can therefore be optimized in many ways. This adaptability is explained using an example in FIG. 2, but can also be implemented quite differently depending on the application.

For example, the foreign substance raw gas content (content of the inflowing medium of foreign substances) and / or foreign substance pure gas content (foreign substance content of the outflowing medium) can be used as input signals. The supply voltage and / or supply current of the filter can be optimized, in particular they can be controlled according to a predefined voltage / current characteristic. This characteristic can depending on the foreign substance raw gas content, ie the load state of the filter. In addition, the control can react very quickly to any voltage drop and to the start and end of a knocking process, and the ripple of the voltage, ie the fluctuation of the voltage between an upper and lower limit value, can be specified and optimized.

In this Figure 2, the controllable rectifier arrangement is shown schematically as a controllable three-phase rectifier bridge DR, which already contains the necessary means to change the intermediate circuit current I (measuring element MI) of an intermediate circuit converter and thus the amplitude of the high-frequency actuator output current with a certain control behavior regulate.

The intermediate circuit contains an intermediate circuit choke ZI, which is designed for the structure of the intermediate circuit current and is optionally supplemented by an intermediate circuit capacitor.

The downstream inverter AR generates the high-frequency alternating current. The suitable inverter shown in FIG. 2 is known as an inverter with "phase sequence deletion". A two-phase bridge is sufficient, although in principle three-phase and multi-phase bridges can also be possible and possibly also advantageous in order to obtain a direct current that is as complete as possible after step-up transformation and rectification.

In the normal phase sequence, the valves TH1 and TH4 and the valves TH2 and TH3 each ignite simultaneously and delete the previously ignited valves by reloading the commutation capacitors K1 and K2.

The transverse thyristor TQ is provided as a means for cross-ignition. In such a cross-ignition, the specified intermediate circuit current continues to flow through the choke ZI, but is conducted via the freewheeling path TQ past the primary winding WP, which therefore quickly de-energizes in every phase position of the inverter and, after blocking any number of converter clock pulses, is excited again with the full intermediate circuit current can be. After a breakdown, the required separation voltage can be quickly built up again. In other bridge circuits, such cross-firings can also be carried out by firing valves in series. They can also be provided in order to shorten the current carrying time of the valves fired in the normal clock sequence compared to a half period of the inverter output current. The impressed intermediate circuit current itself is practically not affected by these switching operations.

In the control PR, the operating point of the power supply is determined in that a setpoint generator SS specifies a setpoint I * for the intermediate circuit current or the amplitude of the output alternating current, the control deviation of which controls the control rate SDR for the controlling means of the controllable rectifier arrangement via a current controller SR . The setpoint I * can in particular be determined on the basis of a current / voltage characteristic stored in the setpoint generator SS, to which the value for the optimum voltage U * is specified by a current control program part PS. In this case, U * can be changed periodically, for example as a function of the residual substance content measured on a flue gas probe RG, in order to generate the aforementioned ripple in the filter supply voltage. The optimal basic level for U * can be determined by a flue gas probe EG depending on the foreign substance raw gas content or can be changed in an iterative search procedure so that on the one hand a high degree of separation, on the other hand, a low frequency of breakdowns and voltage drops at the measuring element MU occur.

In general, limiting the voltage to the predetermined value U * is advantageous. For this purpose, the setpoint / actual value difference of the supply voltage U is applied to a limit controller BR, which operates on a limit circuit BG that limits the current setpoint. To e.g. After a breakdown to be able to raise the supply voltage according to a predetermined curve, a ramp generator HG is provided at the setpoint input of the limit controller PR, the final value (e.g. depending on the frequency of the voltage breakdowns detected on the voltage measuring element MU) can be changed by a pulse program part PI. In the two program parts PS and PI, according to the technology provided for the separation, further actual and target value relationships can be processed in order to control the ramp generator HG and / or the target value transmitter SS for every possible operating state, e.g. to enable an optimal intervention in the control of the alternating current even in the case of a knocking process (removal of the separated foreign substances). In accordance with the specified operating point on the filter characteristic curve, the voltage limiting regulator BR enables stable operation of the power supply up to the vicinity of the breakdown point, as a result of which the breakdown frequency is reduced and the filter service life is increased.

The pulse program part PI also has the task of specifying the AC output frequency and thus the high frequency of the inverter AR by means of a corresponding, operationally dependent control signal for the inverter tax rate WSt. It also generates the switching signal for the freewheeling path (valve TQ) and the temporary stopping and restarting of the inverter after one Punch. In addition, the DC current drawn from the high-voltage rectifier GRH can be interrupted by periodic blocking ("packet formation") and thus a voltage ripple on the filter can also be forced.

By controlling the basic DC voltage of the filter, the use of additional, isolated high-voltage pulses is largely unnecessary. However, the coupling capacitor KK shown in FIG. 2 also facilitates the additional connection of such pulses which can be applied to the corresponding input terminals HFI of the filter.

The high frequency of the alternating current used enables considerable savings to be made on the transformer. Similar savings also result for the intermediate circuit choke if the circuit according to FIG. 3 is used as the controllable rectifier arrangement.

Accordingly, a DC chopper with a control valve ST and a freewheeling diode FD is used to control the impressed direct current I, the size of the choke FD being reduced by an actuator frequency in the kHz range (e.g. 5 kHz). The intermediate circuit decouples the network N from reactions of the inverter AD.

The input voltage of the direct current controller is advantageously supplied by an uncontrolled three-phase rectifier bridge, which thus represents a symmetrical, practically no reactive current load for the network N.

Claims (11)

1. A method for operating an electrostatic filter, wherein an alternating current converted by means of an electronic actuator is transformed up and rectified via a bridge inverter to generate a basic direct voltage for the filter, which may be superimposed with high voltage pulses, characterized in that the alternating current has an inverter frequency a few kHz is generated. 2. The method according to claim 1, characterized in that the converted alternating current is generated by switching a DC current taken from an intermediate circuit with alternating sign to the alternating current outputs of the converter in time with the converter frequency and that the intermediate circuit is fed by a controllable rectifier arrangement with a controllable intermediate circuit current which is practically independent of the connection to the AC outputs in a period of the converter frequency. 3. The method according to claim 1 or 2, characterized in that in the event of a flashover in the separator, the alternating current is switched off while blocking the converter frequency and is switched on again after a short pause coordinated with the deionization time by releasing the converter frequency. 4. The method according to claim 3, characterized in that the switching off and on again of the current is also carried out independently of the occurrence of a flashover after several periods of the converter cycle. 5. The method according to claim 3 or 4, characterized in that the AC amplitude after a shutdown of the converter frequency after a predetermined ramp-up function for the AC amplitude or the basic DC voltage is ramped up again. 6. The method according to any one of claims 1 to 5, characterized by a predetermined control program for controlling the reversal during a knocking process and / or in the event of undervoltage on the filter. 7. The method according to any one of claims 1 to 6, characterized in that the basic DC voltage is impressed by a change in the AC amplitude, a predetermined ripple. 8. The method according to any one of claims 1 to 7, characterized in that the converter frequency is changed depending on the operation. 9. The method according to claim 8, characterized in that the converter frequency is changed as a function of the load state or the foreign matter content of the medium flowing into the filter. 10. The method according to any one of claims 1 to 9, characterized in that the mean level and / or the ripple of the basic DC voltage is controlled in dependence on the foreign matter content of the medium flowing out of the filter by controlling the alternating current. 11. The method according to any one of claims 1 to 10, characterized in that the amplitude of the alternating current is controlled by a control variable which is determined according to a predetermined current-voltage characteristic curve from a target value for the optimal filter voltage.

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