RS20060050A - Electrostatic filter with over voltage protection - Google Patents

Abstract

The invention relates to an electrostatic filter arrangement comprising a switched power supply which is used to produce high voltage leading to corona discharge. In said switched power supply, a cascade of electronic switches is used. A circuit arrangement is associated with each electronic switch, ensuring that the switch achieves a conducting state when the voltage on the main line thereof exceeds a predetermined value. The invention also relates to a circuit arrangement which informs a central control and monitoring circuit whether the monitored switch element is in a blocked or conducting state.

Description

ELECTROSTATIC FILTER WITH HIGH VOLTAGE PROTECTION,

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SIEMENS AKTIENGESEELSCI1AFT [DE/DEJ 'BV

Electrostatic filters are used to separate particles from the gas stream on one outer surface. This allows the gas flow that needs to be purified to pass between the filter plates. between which there is a strong electric field. Using live brushes, ie. of a short-term increase in electric potential, a corona discharge is realized on the plates in order to transfer the discharge carrier to the particle that needs to be separated. After the voltage pulse stops, the charged particles move in the direction of the plate with reversed polarity.

The basic construction of such an electrostatic filter is shown, for example, by EP Ii 1 9 c> 12B 1.

A high voltage source is connected in parallel to the filter plates. Additional voltage pulses were realized using a series oscillator circuit. which is parallel to the filter. From an electrical point of view the filter creates a single capacitor with about 100 nF. The series oscillator circuit contains a capacitor of about I uF. which has a high voltage switch added on its side. When the feed capacitor is charged, it is applied across the inductance of the series oscillator circuit by an electronically controlled filter switch. Thus, a transient process occurs that charges the filter to a substantially higher voltage compared to the idle voltage. This achieves the previously mentioned corona discharge.

When the switching process is completed, there is a return transition of electrical energy through DC diodes, which ends when the voltage on the llter is equal to the voltage in the idle state. The supply capacitor has a voltage that is slightly lower than the voltage before the start of the transient process.

The electronic switch that is used to provide the circuit for the switching process must withstand a very high voltage. When IGBTs are connected here, many IGBTs must be connected in series. because each individually does not have sufficient cut-off voltage.

Two problem statements arise from this:

It must be ensured that all electronic switches are translated into conduction at approximately the same time. If one of the electronic switches is delayed, it is possible that there will be an overvoltage on it with the consequence of a breakdown, which makes the electronic built-in element unusable. However, efforts are being made to prevent this error with the help of light conductors and appropriate switches. In any case, the electronic built-in elements themselves show a certain different timing behavior when switching on.

Another difficulty consists in overloading the cascade of electronic switches, if one of the switches no longer returns to the state of protection. The total voltage must then be accepted by a smaller number of electronic switches.

Based on these facts, the task of the invention is to provide an electrostatic filter where the voltage of the protective circuit of individual electronic switches does not become overloaded as a result of the time-varying switching behavior.

According to the invention, this task is solved by a filter device with characteristics according to Claim I.

Furthermore, the task according to the invention is to create an electrostatic filter, which will detect the failure of an electronic switch in a timely manner.

According to the invention, this task is solved by an electrostatic filter device with characteristics according to Claim 2.

With the new electrostatic filter device, one voltage source is foreseen, which provides the basic voltage for the filter plates. The voltage peaks for charging particles in the aerosol are realized by means of another voltage source, which works depending on the type of power supply involved. From the voltage source, the power capacitor is first charged, which is connected via a chain composed of electronic switches and an inductance parallel to the filter. This results in an excessive voltage increase leading to corona discharge. According to the time constants of the thus formed parallel oscillatory circuit. additional charge is returned to the power capacitor through the direct diodes, which are located in parallel with the electronic switches.

In order to equalize the time differences between switching on individual electronic switches, each of the electronic switches has appropriate means, which, depending on the voltage, and which are connected to the main line of electronic switches, control any of the electronic switches. Errors in signal transmission or the like cannot, under these conditions, lead to one electronic built-in element remaining in the disconnected state for too long and being broken due to exceeding the disconnection voltage.

In the simplest case, this tool is realized by built-in elements that have the characteristics of a Z diode. These built-in elements can be Z diodes or Avalanche diodes.

If higher power is required, there is a possibility to create means for controlling electronic switches using a transistor, in which one built-in element with the characteristics of a Z diode is connected in parallel between the control electrode and the main electrode.

According to another aspect of the invention, it is about proportional to the voltage, prevent the chain of electronic switches from being overloaded, if one of the switches has failed and which remains constantly in the conducting state. Thus, the total voltage would have to be distributed over several electronic built-in elements. There is a danger that further outages will allow the protective voltage on the individual electronic switch to increase so much that it will also be broken with the consequence of a short circuit in the voltage supply.

That is why the circuit is designed to monitor the switch-on status of each electronic switch. In a predetermined control circuit, such synchronization can be achieved if the indication of the on or off state of the corresponding switch corresponds to the signal, which the predetermined control circuit. for example. light conductor, transmits to individual electronic switches.

One very simple switch detection solution is to monitor the voltage drop on any of the electronic switches.

A single diode connected in series with the power supply can be used here. Current flow through. the diode is adjusted while the electronic switch is in the conducting state, and breaks when the protective voltage on the transistor exceeds a predetermined value. The switch-on threshold is determined by the voltage of the power supply without load.

In order to determine the current, a device with sensors is provided. This sensor device can determine the voltage across the power source terminals or the voltage drop across a single sensing resistor.

By means of a Schmitt exciter, the signal is processed and sent through the light conductor of a predetermined control circuit. The signal that originates from there is one binary signal.

In all cases, at least one power supply diode can be included for the series circuit from the capacity and chain of controllable electronic switches. It ensures that when charging the capacitor, the circuit is closed.

The return oscillation of the electrical energy from the filter occurs primarily through individual DC diodes, which are switched on from time to time in parallel with electronic switches.

The electronic switch can consist of one IGBT or one GTO.

Particularly fast control of individual electronic switches is possible via light conductors.

In doing so, it is expedient to add a drive circuit to each electronic switch. which contains its own galvanically isolated power supply. The power supply device can be realized using a single power supply.

Furthermore, other features of the invention are the subject of dependent Claims. It will be clear to one of ordinary skill in the art that a whole range of variations is possible when studying the exemplary embodiments.

The methods of carrying out the subject of the invention are shown in the figures, in which

51.1 represents the principle picture of the electrostatic filter device

51.2 represents the driving circuit for an electronic switch in a simplified scheme. 51.3 represents an alternative way of performing a device for the protection of any electronic switch.

On Si. 1 shows in a very schematic way the block diagram of the electrostatic filter device I. Main voltage source 2, auxiliary voltage source 3, filter 4, which is on NE, belong to electrostatic filter device I. And provided as a capacitor, as well as circuit 5 for management and control.

There is a choke 6 connected in series with the auxiliary voltage source 3. For series connection from the auxiliary voltage source 3 as well as the choke 6, a chain composed of electronic switches 7 in the form of IGBTs is connected in parallel. The number of IGBTs 7 is obtained from the required protection voltage divided by the maximum permissible protection voltage for each individual IGBT 7.

For each IGBT 7, there is one DC diode 8 in parallel.

One feed capacitor 9 connects the collector of the upper IGBT 7 with the anode of one feed diode 10, the cathode of which is connected to the positive terminal of the auxiliary voltage source 3, to which the emitter of the lower IGBT 7 from the chain of IGBTs is also connected. From the anode of the diode 10, a choke I I leads to the capacitor 12, which serves for the galvanic separation of the main voltage source 2 and the auxiliary voltage source 3. The other end of the capacitor 12 is connected to the connection of the filter 4, the other connection of which is connected to the cathode of the power supply diode 10.

Resistor 13, which is parallel to filter 4, should symbolize current losses in filter 4.

Finally, in relation to filter 4, there is a parallel switch from. of the main voltage source 2 and choke 14.

In order to achieve a higher voltage on the filter 4 in an impulse manner, a circuit 5 for central management and control is provided, which is connected to the power switch 16 via a light conductor 15, which controls the isolated input of the IGBTs 7 via the output side.

The switch described so far works as follows:

Using the main voltage source 2, a basic voltage of about 30 kV is achieved on the filter 4.

As long as the IGBTs 7 remain in the off state, it is charged via the power diode 10 of the power capacitor 9 to the voltage on the auxiliary voltage source 3, which means that there is a voltage of about 30 kV on it. Both voltage sources 2 and 3 are connected in series, as can be seen from the diagram, so that a voltage of about 30 kV is generated via the separable capacitor.

In order for the necessary corona discharge to occur in the filter 4, the circuit 5 for central management and control is controlled for a short time by the chain made up of IGBTs 7. Thus, the IGBTs are brought into the conducting state for a short time if possible due to the high voltage potential. control occurs through the light conductor 15. In the drive switches 16, the incoming light signal is transformed into the necessary electrical signal for controlling the lGBTs 7.

In order to prevent the overvoltage of individual IGBTs 7, the IGBTs 7 must be simultaneously transferred from the off state to the conduction state. This prevents an impermissibly high voltage from occurring during one slow switch-on of the IGBT 7, which would overload the IGBT 7 from the voltage point of view. The voltage overload leads directly to the melting of the IGBT 7.

When all IGBTs 7 are switched on, a capacitor 9 is connected in parallel via choke I 1 for filter 4. Accumulated electrical energy on capacitor 9, which has a capacity of about I uf. it oscillates through the now created oscillatory circuits towards ti I tem 4 and supplies filter 4 with a voltage higher than 30 kV. The final voltage on filter 4 is obtained from the ratio of the capacity of Lade capacitor 9 of I uT and the capacity of filter 4 of 100 nF. Capacitor 12 is chosen so large that it does not affect the oscillation process.

In order for the controlled IGBTs 7 auxiliary voltage sources 3 not to be short-circuited, a choke 6 is provided. It only needs to ensure that the current intake from the auxiliary voltage source 3 is approximately constant. By itself, the same applies to choke 14, which is in series with the main voltage source 2. And it has no effect on the oscillation process due to the increase in the voltage on liquer 4.

When the capacitor 9 has completely transferred the charge to the filter 4, the reverse oscillation process begins. This reverse oscillation process leads to the flow of current through the DC diode and ends when the capacitor 9 is charged again to the original value from the start of the IGBTs switching on, rejecting the losses in the filter 4.

In the vicinity of zero current flow through the lines through the IGBT 7, these are switched by the circuit 5 for central management and control to the off state, so that the return current . which flows from the filter 4 to the supply capacitor 3. can flow through the directional diode 8.

For IGBTs 7, one cut-off delay is provided, to ensure that peaks do not occur during switching on or exclusion.

Despite the fact that the signals coming from the circuit 5 for central management and control are equal within very small tolerances, the IGBTs 7 differ during the switching process or. exclusions. In addition, the conditions of different electrical working times in the operating switches 16 should be taken into account.

In order to safely exclude the possibility of one IGBT 7 being overloaded from the voltage point of view, while the other IGBTs 7 are already switched on and are in the conduction state, one Z diode 17 is provided for each IGBT 7, which. as you can see, it is located between the collector and the input of each IGBT 7. This cathode is connected to the collector. If the voltage between the collector and the input of a corresponding IGBT 7 exceeds the basic voltage of the Z diode, the current in the input of the corresponding IGBT 7 starts to flow and fills the input capacity. IGBT 7 becomes a conductor. Depending on the degree to which the transistor becomes conductive, the voltage between the collector and the emitter decreases, which then prevents voltage breakdown.

In doing so, it may happen that some IGBT 7 fail. To the extent that such transistors are melted, they affect the protection process of the entire chain, while the other functionally capable transistors go into the protection state. The melting of one transistor leads to the fact that the remaining working transistors have to suffer a higher protection voltage.

To see if one IGBT 7 is melted or not. an additional circuit is provided in each of the drive circuits 16. as it follows from the diagram in Figure 2.

Each drive circuit 16 contains its own on-board power supply 18 which maintains galvanically isolated electrical energy from the power supply line 19 . The connected power supply 18 with outputs 21 and 22 thus produces a secondary DC voltage of about 10 V. The negative voltage output 21 is connected to the emitter of the IGBT 7. The positive connection 22 is located via the resistance 23 on the anode of the diode 24, the cathode of which is connected to the collector of the IGBT 7. In addition, the power supply 18 serves to supply current to the transformer circuit 25, which is connected via the light conductor 15 transforms the incoming optical signal into an electrical signal, which is given at the output 26. to which the input of the IGBT 7 is connected.

On the power supply side, only one line is useful, which connects the transformer circuit 25 to the ground of the circuit; the hot end is not shown. The ground of the circuit corresponds to the emitter of the IGBT 67 of each phase.

One Schmitt exciter 27 with two inputs 28 and 29 is connected on one side to the anode of the diode 24 and on the other side to the ground of the circuit. Its output 31 is located at the input 32 of a transformer circuit 33, which at the input 32 transforms the existing electrical signal into an optical signal, which then reaches the circuit for central management and control via the light conductor. Inputs 28 and 29 could be in parallel with resistor 23 to directly measure voltage losses.

The power supply of the transformer circuit 33 and the Schmitt exciter 27 is also carried out from the included power supply 18 via the outputs 21 and 22. Here, the connecting lines are not shown for the clarity of the image.

The switch solution from Figure 2 works as follows:

When the IGBT 7 is in the protection state, a high voltage prevails on it, which diode 24 raises the bias voltage in the direction of protection. No current can flow through diode 24. The voltage between the ground of the circuit and the anode of the diode 24 corresponds in this state essentially to the output voltage of the connected power supply 18. The Schmitt exciter 27 has its switching threshold selected appropriately, for example, the switching threshold is about 7 V, so that the Schmitt exciter 27 produces at its output a binary signal corresponding to the exceeding of the switching threshold. This binary signal is converted into an optical signal by the transformer 33 and transmitted to the central circuit 5 for management and control. It is able to check whether the time-correct moment of the protection state occurs.

When the IGBT 7 is controlled, the negative bias voltage on the diode 24 disappears. The cold end of the resistance 23 is placed across the diode 24 and the switched-on IGBT 7 on the ground of the circuit. The voltage on diode 24 is correspondingly small and is, for example. below the already mentioned value of 7 V, the switching threshold of the Schmitt exciter 27, which then changes to the second binary state. At its output 31, it delivers an electrical signal that corresponds to an overshoot from the lower side of the activation threshold at both inputs 28 and 29. This signal is converted back into an optical signal for the light conductor 34 via the converter 33.

Via the central management and control circuit 5, the filter can either be switched off completely or the maintenance service can be alerted to replace the faulty IGBT 7 or the group of IGBT 7 in which the faulty IGBT 7 is located.

When there is a sufficient number of IGBT 7, the drive of the filter device can be achieved with the other transistors until the next state occurs where repair is possible without affecting the drive. Figure 3 finally shows another alternative when the Z diode is undersized and probably overloaded due to the charging current for the IGBT 7 input. In this case, there is one additional IGBT 35 with its collector-emitter path between the input collectors of IGBT 7.

The input is via the already mentioned Z diode 17 on the collector of both IGBT 7 and IGBT 35. One discharge resistor 36 is connected between the input and the emitter of IGBT 35.

The joint switching of the Z diode 17 and the IGBT 35 acts as a single Z diode with a large power loss.

The invention can be foreseeably extended with IGBTs. A person skilled in the art will recognize without further ado that monitoring as well as forced excitation of electronic switches is not limited to this type of electronic built-in elements. What's more, lGTBs are only exemplarily shown. GTOs or other suitable electronic built-in elements can be placed in their place.

The electrical solution of the filter has a single power supply included to achieve the high voltage for corona discharge. A cascade of electronic switches is applied in this switched power supply. Each electronic switch has one such switch solution that ensures that the switch in the conducting state manages to exceed a predetermined value on its main path.

Furthermore, such a switch solution is provided, which serves to signal the control and control circuit whether the switch element under supervision is in the off or conducting state.

Claims (17)

I. The electrostatic filter device (I) is with one capacitor that forms an electrostatic filter (4), with one first source (2) of voltage, which is connected in parallel with respect to the electrostatic filter (4). with one series oscillator circuit consisting of at least one supply capacity (9) and at least one inductance (11 ), with one chain of controllable electronic switches (7) which with the series oscillator circuit (9. 1 I) form a series circuit, which is connected in parallel with respect to the electrostatic filter (4), with one other voltage source (3), which is located in parallel with respect to the chain of electronic switches (7), with one control circuit (5), which is designed so that the electronic switches (7) are periodically controlled so that they are in conductive and protective state, and with circuits (17, 35) which are designed to bring any of the electronic switches (7) into the conductive state independently of the signal from the control circuit (5), when the voltage drop on any of the electronic switches (7) exceeds a predetermined value. 2. Electrostatic filter device (I ). is with one capacitor that forms an electrostatic filter (4). with one first source (2) of voltage, which is connected in parallel with respect to the electrostatic filter (4). with one series oscillator circuit consisting of at least one supply capacity (9) and at least one inductance (1 I ). with one chain of controllable electronic switches (7) that form a series circuit with the series oscillator circuit (9, I I), which is connected in parallel with the electrostatic filter (4), with another voltage source (3), which is located in parallel with the chain of electronic switches (7), with one control circuit (5), which is designed to periodically control the electronic switches (7) so that they are in conductive and protective state, and with devices (23, 24, 27. 33) which are designed to determine the state of the circuit of any electronic switch (7) and report to the control circuit (5). 3. filter device, according to Claim 1, characterized by the fact that the electronic switch (7) is such a built-in element that is controlled in the breaking state, in which a voltage with one polarity is established on the control line, which has the same sign as the polarity of the breaking direction. 4. Filter device, according to Claim 1, indicated by the fact that the devices (17) for independent translation of the electronic switch (7) into conductive transmission are made of electronic built-in parts, which have the characteristics of a Z diode. 5. Filter device, according to Claim I, indicated by. that the devices (17, 35) for converting the electronic switches (7) into the conducting state have a transistor (35), to which an electronic built-in element (17) with the characteristics of a Z diode is added, which is located between the control connection and the collector. 6. Filter device, according to Claims 4 and 5, indicated by t i m e. which is a built-in element (17) with Z diode characteristics realized by one Z diode or one Avalanche diode. 7. Filter device. according to Request 2. the designated ti m e. that the devices (23. 24. 27. 33) for monitoring the switching state of electronic switches (7) include a series circuit consisting of one electrical source (18. 23) and one diode (24). which is parallel to the electronic switch (7), so that current flows through the diode (24), when the electronic switch (7) is in the conducting state. 8. Filter device. according to Claim 7, it is indicated that sensors (27) are provided to register the diode current. 9. Filter device, according to Claim 8, point 7, in that the sensors (27) register the clamp voltage of the electrical source (18, 23) of energy. 10. Filter device. according to Claim 8, it is indicated that the sensors (17) register the voltage drop by a sensor resistor (23), which is connected to the diode in series. 1 I. Filter device. according to Claim 8, characterized in that the sensors (27) contain one Schmitt exciter. 12. Filter device, according to Claim 8, characterized in that the sensors (27) produce one binary signal depending on the current through the diode (24). which is transmitted to the control circuit (5) via the light conductor (33). 13. The filter device, according to Claims I or 2, is characterized by the fact that at least one power supply diode is connected in parallel to the series circuit, which consists of the power supply capacity (9) and a chain of controllable electronic switches (7). 14. Filter device, according to Claims 1 or 2, characterized in that each electronic switch (7) is bridged by one DC diode (8). 15. Filter device. according to Claims 1 or 2, indicated by. which electronic switch (7) is designed as an IGBT or a GTO. 16. Filter device. according to Requirements 1 or 2. the control of the electronic switches (7) is realized by the light conductor (15). 17. Filter device, according to Claims 1 or 2, characterized in that a drive circuit (16) is added to each electronic switch (7). which has its own galvanically isolated device (18) for power supply. 18. [Electrostatic filter device, according to Claim 17, characterized in that the device (18) for power supply includes an included power supply 18