EP0206160B1 - Current supply for an electrostatic filter - Google Patents

Description

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

For the purification of exhaust gases 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 constant 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 limit value of the supply voltage mentioned 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. Furthermore, ionization forms space charges with strong distortions in the potential course between the plates, whereby 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.

Commercially available 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 steepener. 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 start up the voltage on the electrostatic precipitator after the start-up control via the cut 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 which is interrupted after a breakdown. The filter voltage fed by this current results in 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 deliver the charge that flowed through the medium during the pulse pauses, the frequency and / or duration of the individual pulses are specified in such a way that the average current intensity of these isolated direct current pulses assumes a filter current setpoint value that is adapted to the respective filter condition. 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 capacitors are then connected to the electrostatic filter via thyristors, high-voltage transformers and a half-wave rectifier. The width of the current pulses reaching the electrostatic filter is e.g. 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 rippled 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 the European patent font 0 034 075 insulated 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 so 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 impulse.

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 processes, 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.

From DE-OS 29 29 601 it is known that steep edges of an approximately pulsed capacitor voltage can increase the yield. It is therefore proposed to dispense with an approximately constant basic DC voltage and to reload the capacitor as quickly as possible using current pulses with alternating polarity. This is achieved by inverters with a DC link and a single-phase bridge circuit that feeds the capacitor directly via a high-voltage transformer and conducts the DC link current by igniting valves in series via a freewheeling path during pulse pauses.

The object of the invention is to provide a power supply whose output voltage is practically optimally adaptable to the technology of the deposition process and the repercussions on the supply network are kept as small as possible. For example, a power factor of approximately cos <p = 1 for the supply network and a low breakdown frequency or avoidance of short-circuit overcurrents possible for the filter. This also results in a significant improvement with regard to the dimensioning of the components to be used and the stress on the spray wires.

This object is achieved by a power supply with the features specified in claim 1. The DC link makes it possible to adapt the power consumption from the grid to the requirements of the grid largely independently of the operation of the inverter and to shield it from the commutation perturbations of the inverter. In particular, the inverter can be operated at high frequency, which results in a favorable design of the power unit on the one hand, and an optimal adaptation to the deposition process on the other hand.

Advantageous developments of the invention are characterized in the subclaims and are explained using two exemplary embodiments.

In the figures, F denotes the electrostatic filter, between the plates 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, from a supply network N. should be supplied. For this purpose, the intermediate circuit of a converter is fed by the voltage of the supply network N with a network-side controllable rectifier arrangement and a filter-side inverter with a controllable freewheeling path for the intermediate circuit current. WP denotes the primary winding of a high-voltage transformer connected to the AC (or three-phase) output of the converter, the secondary winding WS of which feeds the electrodes of the filter F via a high-voltage rectifier GRH, preferably an uncontrolled rectifier bridge.

The controlled rectifier arrangement is, as shown in FIG. 1, an uncontrolled rectifier GR which is followed by a current actuator for the intermediate circuit direct current I which can be measured by means of a measuring element MI. If a DC regulator containing a free-wheeling diode FD with the control valve ST and a high-frequency operating frequency, preferably about 5 kHz, is used as the actuator, the downstream intermediate circuit choke ZI (together with an intermediate circuit capacitor ZK) need only be matched to the smoothing of this high frequency and decouples this Network N connected to the rectifier GR from possible repercussions of the inverter and the filter. For the network, there is practically only a symmetrical three-phase active load (cos ϕ = 1).

The intermediate circuit current, which can be regulated by a current controller IR and the control set SSt of the actuator ST to a setpoint l ^*, flows through the choke ZI - with the valve ST ignited from the mains and with the valve blocked via the freewheeling diode FD - practically constant, regardless of the switching state of the inverter.

1 consists of a bridge circuit of the valves Tr1, Tr2, Tr3 and Tr4. This a diode D1,... D4 is connected antiparallel, so that states are also possible in which the current flowing through the inductor WP generates a voltage opposite to the impressed direct current. Such states are characteristic of an actuator designed for 4-quadrant operation.

Such a circuit is customary as a pulse-controlled inverter, which connects a DC voltage impressed via correspondingly large DC link capacitors within a half cycle of a sinusoidal, low-frequency target output voltage in the form of sinusoidally pulse-width-modulated, high-frequency voltage pulses with alternating signs to the AC voltage outputs. With this voltage pulse, mutual interlocking must ensure that the DC voltage is not short-circuited by simultaneous current flow from valves in series.

However, this known circuit is operated here for the direct current impressed by the choke ZI and the controller IR in order to generate a high-frequency alternating current (working cycle preferably 1 to 3 kHz) by alternating switching of the direct current to the alternating current outputs.

If the valves Tr1 and Tr4 or Tr2 and Tr3 are simultaneously fired after a half cycle, current pulses flow through the connected winding WP, the length of which is equal to the half cycle and the amplitude is equal to the direct current. However, it is also possible to control an intermediate state within a half-cycle, in which a free-wheeling path is closed by simultaneous current flow of two valves in series (e.g. Tr1, Tr2 and / or Tr3, Tr4) or a separate cross valve that short-circuits the impressed direct current conducts past the AC connections and thus shortens the pulse duration of the high-frequency AC pulses; this means an additional fast control of the alternating current amplitude, which can already be set via the intermediate circuit direct current.

Such "cross firings", which temporarily release the freewheeling path of the direct current, are carried out according to FIG. 1 at least whenever a breakdown in the filter is detected. This can e.g. recognize a threshold value element SG from a breakdown of the filter voltage U. At the same time, the normal ignition pulses are blocked via the control rate WSt of the inverter.

A program part "program" controls the re-enabling of the inverter, in addition the ramping up of the AC amplitude and / or the inverter frequency itself, e.g. depending on the penetration frequency. and on the foreign matter content of the inflowing and outflowing medium can be controlled by the program part.

It is particularly advantageous that the current flowing into the transformer is always limited to the impressed direct current, even if there is a breakdown in the filter, but is also maintained during an inverter lock, so that the inverter feed into the transformer can be resumed as quickly as desired. The transformer itself has to be tuned to the high frequency of the inverter and is therefore very inexpensive.

In order to stabilize an operating point (which can be specified by the program part), an additional voltage limitation control is preferably provided, which limits the filter voltage to the filter voltage setpoint belonging to the specified operating point. For this purpose, the voltage setpoint U ^* set in the setpoint generator SS is compared with the actual voltage value U measured by the voltage measuring element MU and fed via a limiting regulator BR to a limiting circuit BG at the input of the current regulator IR.

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 be dependent on the raw material gas content, i.e. the load state of the filter. In addition, the controller can react very quickly to any voltage drop and to the start and end of a knocking process and the ripple of the voltage, i.e. 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 to smooth 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.

The valves ignite in the normal phase sequence TH1 and TH4 and the valves TH2 and TH3 each 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 passed 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 alternating output 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 foreign 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 raw material 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 dips occur on the measuring element MU.

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 which limits the current setpoint. In order, for example, to be able to ramp up the supply voltage according to a predefined curve profile after a breakdown, a ramp generator HG is provided at the setpoint input of the limiting controller PR, the final value (for example 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, further actual and setpoint relationships can be processed in accordance with the technology provided for the separation in order to control the ramp generator HG and / or the setpoint generator SS for every possible operating state, for example also in the event of a knocking process (removing the separated foreign substances) to enable an optimal intervention in the control of the alternating current. 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 a breakdown. 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 DC link choke.

Claims (10)

1. Power supply for an electrostatic filter (F) having a transformer, the primary winding (WP) of which is connected by way of a converter (Tr1 ... Tr4, D1, ... D4, GR) to the supply network (N), and the secondary winding (WS) of which feeds the electrostatic filter by way of a rectifier (GRH) on the filter side, characterised in that the converter in an indirect converter comprising a rectifier arrangement (GR, ST), controlled on the network side, for generating an impressed intermediate-circuit current (I) which continues to flow constantly, even in the case of a filter breakdown, and an inverter (TR1, D1, ..., Tr4, D4) having a controllable free-wheeling path for the intermediate-circuit current which continues to flow in the case of a filter breakdown (Figure 1).

2. Power supply according to claim 1, characterised in that the controlled rectifier arrangement consists of an uncontrolled rectifier (GR), and a current control element (ST) connected downstream, for the intermediate-circuit current (Figure 1).

3. Power supply according to claim 2, characterised in that the current control element is a d.c. chopper controller (ST) having a high-frequency working frequency, preferably approximately 5 KHz, and containing a free-wheeling diode (FD), and in that a d.c.-link reactor (ZI) tuned to the smoothing of this high frequency is connected upstream to the inverter input (Figure 1).

4. Power supply according to one of claims 1 to 3, characterised in that the inverter is a controller, in particular a bridge circuit comprising in each case a controllable valve (Tr1, ..., Tr4) with a back-to-back diode (D1, ..., D4), the free-wheeling path of which valve can be switched by means of conduction by bridge arms (Tr1, Tr2 or Tr3, Tr4) connected in series (Figure 1).

5. Power supply according to one of claims 1 to 3, characterised in that the inverter is designed for interphase commutation, and the free-wheeling path is a bypass valve (TQ) between its direct-current inputs (Figure 2).

6. Power supply according to one of claims 1 to 5, characterised by a setpoint generator (SS) for a current-setpoint (I^*) of the intermediate-circuit current, which current-setpoint is determined according to a current-voltage characteristic from a predetermined optimum voltage setpoint, and a current regulator (IR) for controlling the intermediate-circuit current (I) (Figure 1).

7. Power supply according to claim 6, characterised by a voltage clamping regulator (BR) which limits the actual current-setpoint in accordance with the deviation of the filter voltage (U) from a voltage value (U^*) tuned to the optimum current-setpoint (Figure 1).

8. Power supply according to one of claims 1 to 7, characterised in that the inverter connects the primary winding of the transformer to the direct-current intermediate-circuit, in each case inside a half- period of a pre-selected, high-frequency working cycle, preferably a working cycle of approximately 1 to 3 KHz, for a pre-selected pulse duration, and the transformer is proportioned to the high frequency of the working cycle (Figure 1).

9. Power supply according to one of claims 1 to 8, characterised in that, when there is a short-circuit within the filter, the current flowing into the transformer can be blocked temporarily.

10. Power supply according to one of claims 1 to 9, characterised in that the rectifier (GRH) on the filter side is an uncontrolled rectifier-bridge.

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