EP0108963B1 - Power supply circuit for an electrostatic dust separator - Google Patents

Abstract

The pulse voltage for the electrode (12) of a dust separator (10) is generated by a thyristor circuit (19) to be transferred to a dust separator through a transformer (16). It being possible that voltage arcings take place at the dust separator (10), there is the risk, in case of a blocked thyristor (20) that from the secondary of the transformer (16) a high voltage is produced at thyristor (20) to destroy the thyristor. To avoid such an occurrence, a detector (27) is provided which is only responsive to sudden voltage drops whereupon the thyristor (20) is enabled to become conductive so that the energy of the secondary circuit may be discharged to the storage capacitor (24) in the primary circuit.

Description

The invention relates to a supply circuit for an electrostatic dust collector with a transformer, the primary circuit of which contains a thyristor circuit and the secondary circuit of which is coupled to the electrode of the dust collector, and to a detector which is coupled to the electrode and which only detects rapid voltage changes which occur in the event of a flashover at the dust collector. responds and acts on the thyristor circuit.

Two sizes are required for electrostatically cleaning a gas from dust. On the one hand, charge carriers that are emitted as a result of a corona under the influence of high voltage from the spray electrode of the separator and accumulate on the dust particles that are carried in the gas. Secondly, a high-voltage field is required in which the charged particles are subjected to a force according to Coulomb's law, which drives them in the direction of the positive anode. This force, which is exerted on the particles in the high-voltage field, is proportional to the level of the electric field strength, i.e. also the level of the applied voltage, i.e. the faster the particles are moved, the smaller the filter can be designed. In this endeavor it was attempted to have the voltage as high as possible, which has the disadvantage with some dusts, especially with highly insulating dusts, that a very large excess of charge carriers is generated and, in critical cases, can lead to back spraying . These two functions, namely the generation of charge carriers and the provision of a high voltage, are separated in the pulsed energy supply. A short voltage pulse, which is above the breakdown voltage, is said to explosively generate charge carriers for a short time, while a second device generates a base voltage that is as smooth as possible, which only serves to accelerate the charge of the loaded dust particles. This base voltage should move as close as possible to the glow threshold voltage in order to avoid an excess of charge carriers. In order to be able to introduce pulses with a high power density into the filter, the pulses should be present as briefly as possible, i.e. in the range between 40 and 200 us. As a result of the short period of time, the voltage of these pulses can now be set very high above the normal breakdown voltage, because the time delay in the growth of the channel discharge, which leads to the arc, gives the voltage time to decay and thus the replenishment of energy into the discharge channel introduced is missing. The known circuit (DE-A-26 08 436) consists essentially of a separator capacitor and a thyristor circuit through which energy is transferred from a storage capacitor to the separator capacitor. As a result of the leakage inductances and a resistance contained in each circuit, this transition of the energy from the storage capacitor into the separator capacitor takes place in the form of a damped oscillation. The inductance of the transformer forms a resonant circuit with the capacitance of the dust collector and with a coupling capacitor, which causes the energy stored in the separator to be returned during the pulse of the voltage source. The returning energy is fed back to the storage capacitor by a so-called free-wheeling diode, so that only the losses in the circuit and the sprayed-off current have to be replaced.

Thyristors for short-term switching of high voltages and high powers are very expensive. In the case of an electrostatic precipitator in which very high voltages are briefly applied in order to generate the necessary charge carriers, it often happens that a flashover occurs during a voltage pulse. The voltage at the electrode of the dust separator breaks down suddenly because the dust separator is short-circuited to a certain extent. This short circuit creates an oscillating circuit, which now only consists of the transformer and the coupling capacitor. This vibration is transferred to the primary side of the transformer and generates a high current there, which tends to charge the charging capacitor. When the thyristor circuit is turned on, this current can flow freely to the storage capacitor. However, if the thyristor circuit is already blocked, a voltage spike is generated at it, which can destroy the thyristors and the free-wheeling diodes.

In a known dust collector of the type mentioned at the beginning (US-A-4 282 014), the transformer is supplied with AC voltage via the thyristor circuit. This alternating voltage is fed to the electrode of the dust collector via a rectifier. The electrode voltage is monitored by a detector, which detects rapid changes in the electrode voltage, such as those which take place in the event of a flashover, and sets the thyristor circuit in the blocking state in the event of a sparkover. Thereupon the voltage supply to the electrode is completely interrupted, so that the dust collector is inactive for a certain time. Since high voltages occur at the thyristor circuit in the event of a flashover, there is a risk that the thyristors will be destroyed in the blocking state.

The invention has for its object to provide a supply circuit of the type mentioned that with short-term pulses high voltage can be operated to achieve effective generation of charge carriers without the risk of destroying the thyristors or other electronic components.

To achieve this object, it is provided according to the invention that the primary circuit of the transformer is connected to a DC voltage source via the thyristor circuit, the thyristor circuit is controlled by a pulse generator, and the electrode is additionally connected to a DC voltage which supplies the pulses generated by the thyristor circuit via the transformer and a capacitor are supplied, and that the detector controls the thyristor circuit in the conducting state in the event of a flashover.

If the sudden voltage breakdown occurs due to arcing with a conductive thyristor circuit, the oscillating current transmitted from the secondary side of the transformer to the primary side can flow off to the charging capacitor. Otherwise, with a blocked thyristor circuit, such a high voltage would build up on it that the thyristors would possibly be destroyed. Such a voltage build-up is prevented by the detector, which does not react to the normal impulse transmission to the electrode of the dust collector, but does respond to a sudden voltage breakdown and then immediately switches the thyristor circuit into the conductive state. In this way, the thyristors are effectively protected against overvoltages and destruction by the action of the detector.

In the blocked state, however, the thyristors can also be endangered by other voltage influences that come either via the transformer or via the voltage supply. If such overvoltages occur, which have a slow voltage build-up and are not recognized by the detector, the thyristors are at risk. In order to prevent such hazards, a protective circuit is provided according to an advantageous development of the invention, which taps the potentials in front of and behind the thyristor circuit and controls the thyristor circuit to the conductive state when the potential difference exceeds a predetermined value. This protection circuit responds directly to the voltage between the main electrodes of the thyristors. It must have an extremely short response time of e.g. 1 µs to control the thyristors in the conductive state before voltage breakdowns can take place on the blocked thyristors.

In order to generate a sufficient amount of free charge carriers during the high-voltage pulses at the dust collector, it is important that the high-voltage pulses have a high voltage on the one hand, but are on the other hand very short in order to avoid flashovers. Short high-voltage pulses can only be achieved with a transformer if the leakage inductance of the transformer is as low as possible. The usual transformers have an iron core made of laminated metal sheets. The sheets do not form a continuous magnetic path, but have joints that are the cause of magnetic losses and scattering. To achieve short-term and sharply limited voltage pulses, it is provided according to a further aspect of the invention that the transformer is designed as a toroidal core transformer, the core of which carries the windings consists of a spirally wound sheet metal. This creates a continuous magnetic path free of impact points, the leakage inductance of which is limited to a minimum. With such a toroidal transformer, short high-voltage pulses can be generated on the secondary side. Because of the short pulse duration, it is possible to make the voltage higher than with the known transformers without increasing the risk of voltage flashovers on the dust separator.

Another problem that arises when generating high pulse voltages with a transformer is that the core of the transformer is magnetized in the same direction with each pulse. Each pulse leaves a residual magnetization or remanence in the core. The new magnetization builds up on this in the same direction, so that after a few pulses the magnetization of the core saturates and the pulses generated on the secondary side have ever smaller amplitudes. In order to avoid this effect, it is provided that the transformer, in addition to a secondary winding and a primary winding, has an auxiliary winding through which a rectified counter-magnetizing current flows, which generates a magnetic field which is opposite to that of the pulse current through the primary winding. This counter-magnetizing current is a direct current, which ensures after each transmitted pulse that the iron of the transformer is magnetized back to the operating point.

The counter-magnetizing current is preferably generated by the secondary coil of an auxiliary transformer, the primary coil of which is connected in series with the thyristor circuit. In this way, the magnitude of the demagnetizing direct current is generated as a function of the magnitude or frequency of the pulse current, so that the magnetization back is limited to the required extent. If the pulses are generated with a higher frequency, a larger counter-magnetizing current results than in the case that the pulses are generated at a lower frequency.

In the following an embodiment of the invention will be explained with reference to the drawings.

• Fig. 1 ein schematisches Schaltbild der Versorgungsschaltung für den Staubabscheider,
• Fig. 2 eine schematische Darstellung des Ringkerntransformators und
• Fig. 3 ein Strom-Spannungs-Diagramm der Impulse.

Show it:

• 1 is a schematic circuit diagram of the supply circuit for the dust collector,
• Fig. 2 is a schematic representation of the toroidal transformer
• Fig. 3 is a current-voltage diagram of the pulses.

1, the housing 11 of a dust collector 10 is connected to earth potential. An electrode 12 protrudes into the pot-shaped housing 11, on which a high voltage with respect to the housing 11 is generated in a manner still to be explained. The voltage between the electrode 12 and the housing 11 is designated U _F.

A base voltage U _B of, for example, 35 kV is applied to the electrode 12 via a choke 13. This base voltage is a DC voltage that is supplied by a voltage source (not shown).

The electrode 12 is connected to one end of the secondary winding 15 of the transformer 16 via a coupling capacitor 14 of 1 μF. The other end of the secondary winding 15 is connected to ground potential.

The primary winding 17 of the transformer 16 is also connected to ground potential at one end and to the thyristor circuit 19 at the other end via an adjustable inductor 18. The thyristor circuit 19 consists of a plurality of pairs connected in parallel, each consisting of a thyristor 20 and an antiparallel to the thyristor 20 Diode 21. Only one of these pairs is shown for reasons of clarity. The thyristor circuit 19 is connected via the primary winding 23 of an auxiliary transformer 22 to the positive pole of the supply voltage U ₁ , which has a size of, for example, 7 kV. The storage capacitor 24, whose other electrode is connected to ground potential, is connected to the connection between the primary winding 23 and the thyristor circuit 19.

The circuit described so far is known. It works in such a way that the storage capacitor 24 is charged to the voltage U ₁ . When the thyristor 20 is driven into the conductive state by the application of a brief pulse from a control device 25 to its control connection, a current flows through the inductance 18 through the primary winding 17. This current induces a high voltage in the secondary winding 15. The winding ratio of primary winding 17 to secondary winding 15 is, for example, 1: 7. The secondary winding 15 forms a series resonant circuit with the capacitor 14 and the capacitance of the dust separator 10. The one shown in FIG. 3 is formed on the dust separator 10 ^. Voltage curve U _F , which has the function of a sine curve almost exactly. The maximum voltage of U _F is approximately 60 kV, and this pulse is superimposed on the base voltage U _e . The course of the current flowing in the series resonant circuit is also shown in FIG. 3. It can be seen that the current I first runs through a positive half-wave. At the time when the pulse voltage U _{F reaches} its maximum value, the current 1 goes through zero, and this is followed by a negative half-wave of the current I during the decay of the voltage U _F.

3 also shows the control voltage U _c , ie the pulse that the control circuit 25 generates in order to control the thyristor 20. This pulse U _c initially has a short voltage peak in order to open the thyristor 20, and this is followed by a region of lower voltage. The total duration of the pulse U _c is approximately 20 µs, and the total duration of the pulse U _F is approximately 120 µs. The times indicated in FIG. 3 are calculated from the time t _o at which the control of the thyristor 20 begins.

Voltage flashovers from the electrode 12 to the housing 11 can occur on the dust separator 10. This is symbolized by the spark gap 26 indicated by dashed lines. In the event of a voltage flashover, the voltage U _F suddenly drops to zero. The secondary-side resonant circuit of the transformer 16 then only consists of the secondary winding 15 and the capacitor 14. Although the voltage U _F has become zero, a high current flows through the radio link 26. This current generates a voltage across the primary winding 17, and this voltage discharges via the inductance 18 and the thyristor circuit 19 onto the storage capacitor 24, which is thereby recharged. This process is harmless if the thyristor 20 is still in the conductive state. If the thyristor 20 is already blocked, then a dangerously high voltage is generated on it by the short circuit of the spark gap 26.

At time t ₁ (FIG. 3), when the voltage U _{F reaches} the maximum value and the current I of the resonant circuit goes through zero, the thyristor 20 is still open, although the control voltage U _c has already ended. As is known, a thyristor is only extinguished when the thyristor current passes through zero. If a voltage flashover occurs at the dust separator 10 between times t _o and t _t , then the thyristor 20, which has been conductive until then, remains conductive, because after the voltage flashover it flows through in the same direction to charge the storage capacitor 24 as during the positive half-wave of the pulse current. However, if the voltage flashover occurs after the time t ₁ , for example at the time t ₂ , then the thyristor 20 is already in the blocking state, and a voltage is generated across it that has the opposite polarity to the diode 21 and can not flow through this diode.

In order to prevent the thyristor 20 from being destroyed in this state, a detector 27 is connected to the electrode 12, which supplies a pulse to a transformer 29 via line 28 when the voltage U _F drops sharply. The secondary winding of the transformer 29 is connected between the anode connection and the control connection of the thyristor 20. The transmitted on the secondary coil of the transformer 29 pulse from line 28 controls the thyristor 20 in the conductive state, so that the high primary voltage caused by the secondary short circuit of the transformer 16 can discharge through the conductive thyristor 20 to the storage capacitor 24.

The detector 27 consists of a series connection of a capacitor 30 and a resistor 31, which is connected to ground potential. The RC constant of the detector 27 is approximately 1 μs, so that only very brief changes in the voltage U _F can generate a signal on line 28, while the normal pulse voltage, which is shown in FIG. 3, does not cause a signal change on line 28 .

To protect the thyristor 20, a further protective circuit 32 is provided, which generates a control signal on line 45 as a function of the voltage that prevails between the main electrodes of the thyristor 20. This protective circuit consists of a first voltage divider consisting of resistors 33 and 34 and a second voltage divider consisting of resistors 35 and 36. The taps of the two voltage dividers are connected to one another and connected to the input of an amplifier 37. The output of amplifier 37 is connected to transformer 29 via line 45. The ohmic voltage divider 33, 34 has a reaction time which is too great due to the capacitances or inductances which the resistors necessarily have. For this reason, the voltage divider 35, 36 is provided in parallel with the ohmic voltage divider, which has a short response time due to the capacitor 36.

The structural design of the transformer 16 is shown schematically in FIG. 2. The transformer has a toroidal core 38 made from a spirally wound single sheet metal strip. The cylindrical toroid is wound with the primary winding 17 and the secondary winding 15 in the manner shown. In addition, the toroidal core 38 also carries an auxiliary winding 39 which is wound in the opposite direction to the primary winding 17, which is indicated in FIG. 1 by the points. The auxiliary winding 39 is connected to the two ends of the secondary coil 41 of the auxiliary transformer 22 via a rectifier 40.

During the pulses generated by the thyristor 20 under control by the control device 25, the primary winding 23 of the auxiliary transformer 22 is traversed by current. As a result, a voltage is generated in the secondary winding 41, which is rectified by the rectifier 40 and discharged via the auxiliary winding 39. In this way, a direct current is generated in the auxiliary winding 39, the magnitude of which depends on the frequency and strength of the pulses at the dust separator 10 and which generates a counter-magnetization in the core 38, thereby preventing the core 38 from gradually increasing due to the pulses the saturation is controlled. A capacitor (not shown) can be provided in parallel with the auxiliary winding 39.

To further protect the thyristor 20, a series circuit comprising a diode 42 and a capacitor 43 is connected between the thyristor and ground potential. A resistor 44 is connected in parallel to the capacitor 43. The diode 42 ensures that negative pulse jumps are kept away from the cathode connection of the thyristor 20. Such negative pulse jumps charge the capacitor 43 via the diode 42, which can then slowly discharge via the resistor 44. This protective circuit also helps to prevent sudden overvoltages on the thyristor.

Claims (5)

1. Power supply circuit for electrostatic dust separator comprising

a transformer (16) whose primary circuit includes a thyristor circuit (19) and whose secondary ciruit is coupled to the electrode (12) of the dust separator (10),

and a detector (27) coupled to the electrode (12), said detector being responsive only to quick voltage changes occurring in case of a spark-over at the dust separator (10) to act on the thyristor circuit (19),

characterized in that

the primary circuit of the transformer (16) is connected through the thyristor circuit (19) to the d.c. voltage source (U_l),

the thyristor circuit (19) is controlled by a pulse generator (25),

the electrode (12) is additionally applied to a d.c. voltage (U_B) to which the pulses produced by the thyristor circuit (19) may be fed via the transformer (16) and a capacitor (14),

and that, in case of a spark-over, the detector (27) drives the thyristor circuit (19) into the conductive state.

2. Power supply circuit as defined in claim 1, characterized by a protective circuit (32) tapping the potential ahead and downstream of the thyristor circuit (19) and driving the latter into the conductive state when the potential difference exceeds a predetermined value.

3. Power supply circuit as defined in claim 1 or 2, characterized in that the transformer (16) is a torodial transformer whose winding-carrying core (38) consists of a helically wound sheet metal.

4. Power supply circuit as defined in one of claims 1 to 3, characterized in that, in addition to a primary winding (17) and a secondary winding (15), the transformer includes an auxiliary winding (39) which is traversed by a rectified anti-magnetizing current generating a magnetic field being inverse to that of the pulse current by the primary winding (17).

5. Power supply circuit as defined in claim 4, characterized in that the anti-magnetizing current is produced by the secondary coil (41) of an auxiliary transformer (22) whose primary coil (23) is connected in series with the thyristor circuit (19).

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