WO2004112966A1 - 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

description

Electrostatic filter with surge protector

Electrostatic filters are used to separate particles from a gas stream on a surface. To do this, the gas stream to be filtered is passed through between filter plates, between which there is a strong electric field. With the help of superimposed voltage bursts, i.e. After a brief increase in the electrical potential, a corona discharge is generated on the plates in order to transport charge carriers onto the particles to be separated. After the disappearance of the voltage pulse, the charged particles move towards the plate with the opposite polarity.

The basic structure of such an electrostatic filter is shown, for example, in EP 1 119 912 B1.

A high voltage source is connected in parallel to the plates of the filter. The additional voltage pulses are generated with the help of a series resonant circuit that is parallel to the filter. From an electrical point of view, the filter forms a capacitor with approx. 100 nF. The series resonant circuit contains a charging capacitor with approx. 1 μF, which in turn is assigned a high-voltage charging circuit. As soon as the capacitor is charged, it is applied to the filter via the inductance of the series resonant circuit with the aid of a controlled electronic switch. There is a changeover process, which charges the filter to a much higher voltage than the rest voltage. This creates the corona discharge mentioned above.

As soon as the switching process is complete, the electrical energy returns via freewheeling diodes, which ends when the voltage across the filter is equal to the open circuit voltage. The charging capacitor instructs then a voltage that is slightly lower than the voltage before the start of the reversal process.

The electronic switches used to create the circuit for the reversal process have to withstand a very high voltage. If IGBTs are used for this purpose, a large number of IGBTs must be connected in series because each individual does not have a sufficient reverse voltage.

Two problems arise from this:

It must be ensured that all electronic switches change to the conductive state at about the same time. If one of the electronic switches lags behind, a voltage overload may occur on it, with the result that the breakdown occurs, which makes the electronic component unusable. Efforts are being made to avoid these errors with the aid of optical fibers and corresponding driver circuits. However, the electronic show

Components themselves a certain different switching behavior over time.

The second difficulty is overloading the cascade of electronic switches if one of the switches does not return to the locked state. The total voltage must then be taken up by a smaller number of electronic switches.

Proceeding from this, it is an object of the invention to provide an electrostatic filter arrangement in which no reverse voltage overload of the individual electronic switches occurs as a result of switching behavior which differs over time.

This object is achieved by the filter arrangement with the features of claim 1. Furthermore, it is an object of the invention to provide an electrostatic filter arrangement in which the failure of an electronic switch is recognized in good time.

This object is achieved with the electrostatic filter arrangement with the features of claim 2.

In the new electrostatic filter arrangement, a voltage source is provided which supplies the basic bias for the plates of the filter. The voltage peaks for charging the particles in the aerosol are generated with the aid of a second voltage source, which works in the manner of a switching power supply. A charging capacitor is first charged from the voltage source, which is connected in parallel to the filter via the chain of electronic switches and an inductance. In this way, a voltage surge is achieved, which leads to a corona discharge. In accordance with the time constant of the parallel resonant circuit formed in this way, the charges then run back into the charging capacitor via free-wheeling diodes which are parallel to the electronic switches.

In order to compensate for switching time differences in the individual electronic switches, each electronic switch is provided with associated means which, depending on the voltage applied to the main path of the electronic switch, control the respective electronic switch. Under these circumstances, signal transmission errors or the like cannot lead to an electronic component remaining in the blocking state for too long and breaking down due to the blocking voltage being exceeded.

In the simplest case, these means are formed by components which have Z-diode characteristics. Such components can be zener diodes or avalanche diodes. If higher powers are required, it is possible to form the means for opening the electronic switch by means of a transistor, to which a component with a Z-diode characteristic is connected in parallel between the control electrode and the one main electrode.

According to the second aspect of the invention, it is a matter of avoiding voltage overloading of the chain of electronic switches if one of the switches has failed and it remains in the conductive state at all times. This would mean that the total voltage would have to be divided among fewer electronic components. There is a risk that further failures will cause the blocking voltage on individual electronic switches to rise to such an extent that they will also be shot through, with the result of a short circuit in the voltage supply.

It is therefore provided in the circuit to monitor the switching status of each electronic switch. It can then be synchronized in the higher-level control circuit whether the notification of the conducting or blocked state of the electronic switch in question matches the signal which the higher-level control circuit transmits to the individual electronic switch, for example via optical fibers.

A very simple circuit arrangement for detecting the switching state consists in monitoring the voltage drop at the respective electronic switch.

A diode can be used for this purpose, which is connected in series with an electrical power supply. The current flow through the diode begins as soon as the electronic switch is in the conductive state and stops when the reverse voltage on the transistor has exceeded a predetermined level. The switching threshold is determined by the open circuit voltage of the power supply source. A sensor device is provided to determine the current. This sensor device can detect the terminal voltage at the electrical energy source or the voltage drop at a sensor resistor.

With the help of a Schmitt trigger, the signal is evaluated and reported to the higher-level control circuit via an optical fiber. The signal that is given there is a binary signal.

In all cases, at least one charging diode can be connected in parallel with the series connection of charging capacity and the chain of controllable electronic switches. It ensures that the circuit is closed when charging the charging capacitor.

The oscillation of the electrical energy from the filter is preferably done via individual freewheeling diodes, which are each connected in parallel to the electronic switches.

The electronic switch can be formed by an IGBT or a GTO.

Particularly fast control of the individual electronic switches is possible via optical fibers.

Each electronic switch is expediently assigned a driver circuit which contains its own galvanically isolated power supply. The power supply device can be formed by a switching power supply.

In addition, further developments of the invention are the subject of dependent claims. When studying the exemplary embodiments, it becomes clear to the person skilled in the art that a number of modifications are possible. Exemplary embodiments of the subject matter of the invention are shown in the drawing. Show it:

1 shows the basic circuit diagram of the electrostatic filter arrangement.

Figure 2 shows the driver circuit for an electronic

Switches in a simplified block diagram and

Figure 3 shows an alternative embodiment for the protective device of the respective electronic switch.

1 shows an electrostatic filter arrangement 1 in a highly schematic block diagram. The filter arrangement 1 includes a main voltage source 2, an auxiliary voltage source 3, a filter 4, which is shown in the circuit diagram as a capacitor, and a control and monitoring circuit 5th

A choke 6 is connected in series with the auxiliary voltage source 3. A chain of electronic switches 7 in the form of IGBTs is connected in parallel with the series connection from the auxiliary voltage source 3 and the choke 6. The number of IGBTs 7 results from the required reverse voltage divided by the maximum permissible reverse voltage for each individual IGBT 7.

A free-wheeling diode 8 is connected in parallel to each IGBT 7.

A charging capacitor 9 connects the collector of the top one

IGBT 7 with the anode of a charging diode 10, the cathode of which is connected to the positive connection of the auxiliary voltage source 3, to which the emitter of the lowest IGBT 7 from the chain of IGBTs is also connected. A choke 11 leads from the anode of the diode 10 to a capacitor 12, which is used for electrical isolation between the main voltage source 2 and the auxiliary voltage source 3. The other end of the separation condenser tors 12 is connected to one terminal of the filter 4, the other terminal of which is connected to the cathode of the charging diode 10.

A resistor 13, which is parallel to the filter 4, is intended to symbolize the leakage currents in the filter 4.

Finally, the parallel connection of the main voltage source 2 and a choke 14 is parallel to the filter 4.

In order to generate a higher voltage on the filter 4, the central control and monitoring circuit 5 is provided, which is coupled via optical waveguides 15 to driver circuits 16 which control the insulated gate of the IGBTs 7 on the output side.

The circuit described so far works as follows:

With the help of the main voltage source 2, a basic DC voltage of approximately 30 kV is applied to the filter 4.

As long as the IGBTs 7 remain in the blocking state, the charging capacitor 9 is charged to the voltage of the auxiliary voltage source 3 via the charging diode 10, that is to say a voltage of approximately 30 kV is applied to it. As can be seen from the circuit diagram, the two voltage sources 2 and 3 are connected in series, so that a voltage of approx. 30 kV is present across the isolating capacitor.

In order to generate the required corona discharge in the filter 4, the chain of IGBTs 7 is briefly controlled by the central control and monitoring circuit 5. In order that the IGBTs 7 get into the conductive state at the same time as possible and because of the high voltage potential, they are controlled via the optical waveguides 15. The incoming optical signal is shown in the driver circuits 16 the electrical signal required to drive the IGBTs 7 implemented.

In order to avoid voltage overloading of individual IGBTs 7, the IGBTs 7 must simultaneously switch from the blocking state to the conductive state. This prevents an inadmissibly large reverse voltage from being applied to a slowly switching IGBT 7, which would overload the IGBT 7 in terms of voltage. A voltage overload leads to the IGBT 7 being alloyed.

When all IGBTs 7 are switched through, the charging capacitor 9 is connected in parallel with the filter 4 via the storage inductor 11. The electrical energy stored on the capacitor 9, which has a capacitance of approximately 1 μF, oscillates to the filter 4 via the resonant circuit now formed and charges the filter 4 to a voltage of greater than 30 KV. The end-of-charge voltage at the filter 4 results from the capacitance ratio of the charging capacitor 9 of 1 μF to the capacitance of the filter of approximately 100 nF. The isolating capacitor 12 is chosen so large that it does not influence the oscillation process.

The choke 6 is provided so that the auxiliary voltage source 3 is not short-circuited in the case of controlled IGBTs 7. It should only ensure that the current draw from the auxiliary voltage source 3 is almost constant. The same applies analogously to the choke 14, which is in series with the main voltage source 2. It also has no effect on the oscillation process for increasing the voltage on the filter 4.

As soon as the charge from the charging capacitor 9 has been completely transferred to the filter 4, a reverberation process begins. This reverberation process leads to a current flow through the freewheeling diodes 8 and ends as soon as the charging capacitor 9 is charged again to the original value at the beginning of the switching on of the IGBTs 7 minus the loss in the filter 4. In the vicinity of the zero current passage after the power line through the IGBTs 7, these are switched over again by the central control and monitoring circuit in the blocked state, so that the return current that flows from the filter 4 into the charging capacitor 3 via the freewheeling diodes 8 can flow.

A certain switching delay is provided for the IGBTs 7 in order to ensure that no switching peaks occur.

Although the signals coming from the central control and monitoring circuit 7 are simultaneously within very small tolerances, the IGBTs 7 differ in their

Switching behavior. In addition, different electrical transit times can be observed in the driver circuits 16 under certain circumstances.

In order to rule out with certainty that an IGBT 7 is overloaded because other IGBTs have already switched and are sufficiently in the conductive state, a Zener diode 17 is provided for each IGBT 7, which, as shown, between the collector and the gate of the respective IGBT 7. Its cathode is connected to the collector. Should the voltage between the collector and the gate of an IGBT 7 in question exceed the knee voltage of the Zener diode 17, a current begins to flow into the gate of the IGBT 7 in question and charges the gate capacitance there. The IGBT 7 becomes the leader. As the transistor becomes conductive, the voltage between the collector and emitter decreases, preventing voltage breakdown.

Nevertheless, individual IGBTs 7 can fail. To the extent that such transistors alloy, they do not participate in the blocking behavior of the entire chain, while the other functional transistors are in the blocking state pass. The alloying of a transistor means that the other properly operating transistors have to cope with a larger reverse voltage.

In order to recognize whether an IGBT 7 is alloyed or not, an additional circuit is provided in each driver circuit 16, as can be seen from the block diagram in FIG. 2.

Each driver circuit 16 contains its own switching power supply

18, which receives the electrical energy from a looped through power supply conductor 19. The switching power supply 18 with outputs 21 and 22 generates a DC voltage of approximately 10 V from the secondary side thereof. The negative voltage output 21 is connected to the emitter of the IGBT 7. The positive connection 22 is connected via a resistor 23 to the anode of a diode 24, the cathode of which is connected to the collector of the IGBT 7. In addition, the switching power supply 18 serves to supply power to a converter circuit 25, which converts the optical signal arriving via the optical waveguide 15 into an electrical signal, which is output at an output 26 to which the gate of the IGBT 7 is connected.

Only one line of the power supply is shown, which connects the converter circuit 25 to the circuit ground; the hot end is not shown. The circuit mass corresponds to the emitter of the IGBT 7 of the respective stage.

A Schmitt trigger 27 with two inputs 28 and 29 is connected on the one hand to the anode of the diode 24 and on the other hand to the circuit ground. Its output 31 is connected to an input 32 of a converter circuit 33, which converts the electrical signal present at the input 32 into an optical signal that reaches the central control and monitoring circuit via an optical waveguide 34. The inputs 28 and 29 could also be parallel to the resistor 23 in order to measure the voltage drop directly.

The power supply to the converter circuit 33 and the Schmitt trigger 27 likewise takes place from the switched-mode power supply 18 via the outputs 21 and 22. The connecting lines for this purpose are not shown for reasons of clarity.

The circuit arrangement according to FIG. 2 works as follows:

When the IGBT 7 is in the blocking state, a high voltage is applied to it, which biases the diode 24 in the blocking direction. No current can flow through the diode 24. The voltage between the circuit ground and the anode of the diode 24 essentially corresponds to that in this switching state

Output voltage of the switching power supply 18. The Schmitt trigger 27 is selected with the switching threshold accordingly, for example the switching threshold is around 7 V, so that the Schmitt trigger 27 generates a binary signal at its output which indicates that the switching threshold of 7 has been exceeded V corresponds. This binary signal is converted into an optical signal by the converter 33 and transmitted to the central control and monitoring circuit 5. This can check whether the locked state occurs at the correct time.

When the IGBT 7 is turned on, the negative bias for the diode 24 disappears. The cold end of the resistor 23 is connected to the circuit ground via the diode 24 and the switched-through IGBT 7. The voltage at the diode 24 is correspondingly low and is, for example, below the aforementioned 7 V of the switching threshold of the Schmitt trigger 27, which then changes to the other binary state. At its output 31 it delivers an electrical signal corresponding to the falling below the switching threshold at the two inputs 28 and 29. This signal is in turn converted via the converter 33 into the optical signal for the optical waveguide 34. The central control and monitoring circuit 5 thus receives information about whether the IGBT 7 is in the blocked or in the conductive state. If the IGBT 7 remains in the conductive state even though the signal on the optical waveguide 15 which is intended to switch the IGBT 7 into the conductive state disappears, the signal on the optical waveguide 34 is an indication that the relevant IGBT 7 is chaining through the IGBTs is.

Via the central control and monitoring circuit 5, either the filter can be switched off completely or a maintenance service can be alerted to replace the faulty IGBT 7 or the group of IGBTs 7, to which the faulty IGBT 7 belongs.

If there is a sufficient number of IGBTs 7, the operation of the filter arrangement can be maintained with the remaining transistors until the next state occurs, in which maintenance can be carried out without affecting the operation. Finally, FIG. 3 shows an alternative if the Z diode 17 is dimensioned weaker and is possibly overloaded by the charging current for the gate of the IGBT 7. In this case, another IGBT 35 with its collector-emitter path lies between the collector and the gate of the IGBT 7.

The gate is connected to the collector of both the IGBT 7 and the IGBT 35 via the aforementioned Z diode 17. A discharge resistor 36 is connected between the gate and the emitter of the IGBT 35.

The interconnection of the Zener diode 17 and the IGBT 35 acts like a Zener diode with greater power loss.

The invention has been explained above in connection with IGBTs. It is also easy for the person skilled in the art recognizable that both the monitoring and the forced triggering of the electronic switch is not limited to this form of electronic components. Rather, the IGBTs 7 are only shown as examples. GTOs or other suitable electronic components can also be used in their place.

An electrostatic filter arrangement has a type of switching power supply in order to generate the high voltage leading to the corona discharge. A cascade of electronic switches is used in this switching power supply. Each electronic switch has a circuit arrangement which ensures that the switch becomes conductive when the voltage on its main line exceeds a predetermined value.

Furthermore, a circuit arrangement is provided which serves to report to a central control and monitoring circuit whether the monitored switching element is in the blocked or conductive state.

Claims

claims 1. Electrostatic filter arrangement (1), with an electrostatic filter (4) forming a capacitor, with a first voltage source (2) which is connected in parallel with the electrostatic filter (4), with a series resonant circuit with at least one charging capacity (9 ) and at least one inductor (11), with a chain of controllable electronic switches (7), which forms a series circuit with the series resonant circuit (9, 11), which is connected in parallel with the electrostatic filter (4), with a second voltage source ( 3), which is parallel to the chain of electronic switches (7), with a control circuit (5) which is set up to periodically control the electronic switches (7) in the conducting and the blocked state, and by means (17, 35) which are set up to bring a respective electronic switch (7) into the conductive state independently of a signal from the control circuit (5) when the voltage drop a n the respective electronic switch (7) exceeds a predetermined value. 2. Electrostatic filter arrangement (1), with an electrostatic filter (4) forming a capacitor, with a first voltage source (2) which is connected in parallel with the electrostatic filter (4), with a series resonant circuit comprising at least one charging capacity (9) and at least one inductor (11), with a chain of controllable electronic switches (7), which forms a series circuit with the series resonant circuit (9, 11), which is connected in parallel with the electrostatic filter (4), with a second voltage source ( 3), which is parallel to the chain of electronic switches (7), with a control circuit (5), which is set up to periodically control the electronic switches (7) in the conductive and the blocked state, and with means (23, 24, 27, 33) that are set up to switch the switching state of a respective electronic Determine switch (7) and report it to the control circuit (5). 3. Filter arrangement according to claim 1, characterized in that the electronic switch (7) is such a component that is controlled in the on state in which a voltage with a polarity is applied to the control path, which has the same sign as the polarity in the forward direction. 4. Filter arrangement according to claim 1, so that the means (17) for independently converting the electronic switch (7) into the conductive state are formed by an electronic component that has Z-diode characteristics. 5. Filter arrangement according to claim 1, characterized in that the means (17.35) for converting the electronic switch (7) into the conductive state has a transistor (35) which is associated with an electronic component (17) with a Z-diode characteristic that is between the control connection and the collector. 6. Filter arrangement according to claims 4 or 5, so that the component (17) with a Z-diode characteristic is formed by a Z-diode or an avalanche diode. 7. Filter arrangement according to claim 2, characterized in that the Mit- tel (23, 24, 27, 33) for monitoring the switching state of the electronic switch (7) comprise the series circuit comprising an electrical energy source (18, 23) and a diode (24) which lead to the electronic switch (7 ) is in parallel such that a current flows through the diode (24) when the electronic switch (7) is in the conductive state. 8. The filter arrangement as claimed in claim 7, so that sensor means (27) are provided in order to detect the diode current. 9. The filter arrangement as claimed in claim 8, so that the sensor means (27) monitor the terminal voltage of the electrical energy source (18, 23). 10. The filter arrangement as claimed in claim 8, so that the sensor means (17) detect the voltage drop across a sensor resistor (23) which is connected in series with the diode. 11. The filter arrangement as claimed in claim 8, so that the sensor means (27) comprise a Schmitt trigger. 12. The filter arrangement as claimed in claim 8, so that the sensor means (27), depending on the current, generate a binary signal through the diode (24) that is transmitted to the control circuit (5) via an optical waveguide (33). 13. Filter arrangement according to claims 1 or 2, characterized in that at least one charging diode (10) is connected in parallel to the series circuit comprising the charging capacity (9) and the chain of controllable electronic switches (7). 14. Filter arrangement according to claims 1 or 2, so that each electronic switch (7) is bridged by a free-wheeling diode (8). 15. Filter arrangement according to claims 1 or 2, so that the electronic switch (7) is formed by an IGBT or a GTO. 16. Filter arrangement according to claims 1 or 2, so that the electronic switches (7) are actuated via optical waveguides (15). 17. Filter arrangement according to claims 1 or 2, so that each electronic switch (7) is assigned a driver circuit (16) which contains its own galvanically isolated power supply device (18). 18. Electrostatic filter arrangement according to claim 17, so that the power supply device (18) contains a switching power supply.