The present invention concerns an electronic circuit suited to generating a direct voltage upon which a pulse voltage is superimposed

Said electronic circuit is the basic component of a power supplier for an electro-static precipitator It is known that electrostatic precipitators are used to remove solid particles from the fumes produced in industrial plants The operation of these electrostatic precipitators is based on the fact that the fumes loaded with dusts pass through a chamber where the single dust particles are electrically charged by absorplion of free ions generated by R high voltage ionizing field and such free ions pass from the emitting electrodes to the capturing electrodes dragging said particles that are held by means of molecular electric attraction

Pulse power suppliers apply a direct voltage (direct base voltage) to the insulated precipitator electrodes (emitting electrodes) upon which brief high voltage pulses (between a few tenths to a few hundredths of micro-seconds) are superimposed with a repetition frequency that generally reaches a few hundred pulses per second

Currently such power suppliers are realized by means of circuits that generate the pulses at relatively low voltage, raising the value downstream by means of elevator transformers, or else without the latter, but making use of appropriate protective coils

Said techniques present the drawback of being very costly because they employ expensive elevator transformers, or large coils, also leading to a greater weight and considerable size ofthe power suppliers

The present invention concerns a circuit with only high voltage components in which the pulse is produced by a discharge current from a storage-capacitor, previously charged and located in an oscillating circuit of which the precipitator capacity is an integral part and in which the protective coils are replaced with free-wheeling diodes,

DSAS (Symmetrical Silicon Overvoltage Suppressor) and/or BOD ( Break Overvoltage Diode)

Hence, the invented electronic circuit is suited to generating high-voltage electric pulses in the insulated electrodes of an electrostatic precipitator, as claimed herein, the circuit is an oscillating circuit comprising a) a first group of at least one static switch formed by a number of tyristors in series so as to form a column, each of which is protected by DSAS or BOD diodes, while the entire column is protected by free-wheelinμ diodes in parallel. b) a second group of at least one storage-capacitor at direct current defined by a storage-capacitor, at least one capacitor for the separation of the basic direct current, at least one coil and the precipitator capacity, said first and second groups producing a high voltage pulse every time a firing pulse is applied to the static switch that allows a current to pass in the oscillating circuit, said current being sensibly formed by a single positive semi-wave and by a single negative semi -wave

In said electronic circuit the charge of the storage-capacitor may be conducted by the same source that supplies the basic continuous voltage

Alternatively, the charge for the storage capacitor in said electronic circuit may be provided by a separate source of direct voltage One advantage of the second solution is provided by the independence obtained in regulating the basic direct voltage and the pulse, so that these regulations can be realized by means of two sources in series or in parallel to one another

Putting the two sources in parallel is especially advantageous when extremely high voltages are used (e g 80-1 10 kV of direct basic voltage and the same for pulse), so that one can have both sources on a common connector and earthing potential

The invention is described in detail below with reference to the drawings in which Fig 1 is the diagram of an electronic circuit, Fig 2 is a first diagram, Fig 3 is a diagram ofthe connections, Fig 4 is a second diagram,

Fig 5 is a third diagram, Fig 6 is a fourth diagram,

Fig 7 is a diagram showing the main components ofa power-unit Independently from the type of setting, Figure 1 shows the oscillating circuit through which the storage-capacitor discharges

The figure shows

- C 1 , the storage-capacitor kept charged by a direct current source

- 1 and M2, respectively the positive terminal (generally with earthed potential) and the negative terminal to the power supplier that keeps the said capacitor charged - C2, the direct basic voltage block capacitor upon which the pulse voltage is superimposed

- M3 and M4, respectively the positive and negative connectors of capacitor C2

- M5, the connector for the voltage electrodes ofthe precipitator

- M6, the connector for the precipitator electrodes that generally have earthed potential

- R, the equivalent resistor that bears the energy dissipated within the precipitator (owing to the corona effect in the circuit, etc.)

- L, the coil ofthe oscillating circuit

- Th, a group of thyristors in series, the firing of which periodically starts the discharge current from the storage-capacitor C 1

- Dl, return diodes, anti-parallel, for the reverse circulation of said oscillating discharge current

- D2, free-wheeling diodes,

- PREC, the electrostatic precipitator The circuit functions as described below

A firing pulse at the thyristor gates produces a discharge oscillating current formed by only two semi-periods and a pulse voltage at the ends of the precipitator that have curve shapes represented in Fig 2 in which they aie shown as

- SI the semi-wave of "direct" current crossing the thyristors, - S2, the semi-wave of "inverted" current crossing the diodes,

- S3, the portion of semi-wave of "direct" current that runs through the diodes Dl in the "inverse" direction owing to the effect ofthe so-called "residual charge",

- S4, the voltage pulse at the ends ofthe precipitator

Also with reference to Fig 1, note that owing to the dissipation of energy essentially due to the corona effect (dissipator component R), semi-wave S2 has a lower average value than semi-wave SI and hence, also owing to current S3, at the end ofthe discharge a residual voltage is left in the precipitator and is added to the basic direct voltage

Should the residual charge in diodes Dl be absent, the discharge would cease in correspondence with the zero crossing of semi-wave S2, and the phenomenon would terminate with no interruption over-voltages because it would cease with zero current through coil L

Instead, owing to the residual charge, considerable over-voltages appear at the ends of the thyristor column (Th) and of the diodes (Dl) that must necessarily be absorbed with resistances and coils set in parallel Bearing in mind that with the pulse voltage values and the capacity values ofthe large

precipitators the amplitude ofthe current semi-waves SI and S2 are of many hundreds or even more than one-thousand amperes, the intensity of the interruption currents of the residual charge is very high and consequently protection devices in parallel with the thyristors dissipating very high power are necessary, with great difficulties in dissipating the heat developed and a low performance ofthe overall power-unit

This draw-back is eliminated by the free-wheeling diodes technique that avoid the sudden cancelling of the residual charge current Said diodes (shown with D2) are arranged as shown in Fig I

They ensure that - the potential at connector M3 is always linked to the earth potential for any over¬ voltage taking place between connectors M3 and M 1 with positive polarity at connector 3

- over-voltages of any kind between connectors M 1 and M3 with negative polarity on connector M3 are also practically with no effect on the thyristors and diodes, because these have an opposite polarization to the voltage existing at the ends of the capacitor

Cl

Fig 3 shows the method for setting the DSASs and the BODs

Fig 4 shows the characteristic v-i ofthe DSASs that are special bi-directional devices made up of back-to-back connected couples of avalanche diodes Fig 5 shows how, when the DSASs act with an over-voltage present, there is a cut- down of voltage peaks above their action threshold

For what concerns the BODs these are special diodes that set the thyristors to conduction in the presence of over-voltages at their ends

Fig 6 is a scheme ofthe special technique adopted for the protection ofthe thyristors, in order to avoid damage to the same arising from the application of a direct voltage

directed at their ends during the "turn-ofT state: this solution consists in applying a second gate trigger so as to set the thyristors on stage at the second passage of the zero current.

Fig 7 is a scheme ofa high voltage power-unit that generates a direct basic voltage to which brief pulses are superimposed with variable repetition frequency.

All the components of the power-unit are fitted into an oil-dip box for cooling and insulation.

The scheme is self-explanalory; Ihe main components are listed below:

- electro-static precipitator 1 , - high voltage transformer for continuous basic voltage 2,

- a first high voltage rectifier bridge 3,

- a high voltage transformer for the direct voltage generating the pulses 4 (that with transformer 2 realizes the alternative described above),

- a second high voltage rectifier bridge 5, - columns of thyristors-diodes for the generation of the high voltage pulse 6,

- capacitors 7,

- voltage measuring dividers 8,

- free-wheeling diodes 9,

- resonance circuit coil 10. Within the circuit an alternate 380 V voltage is raised by means of the two elevating transformers 2 and 4 and the voltages exiting are both rectified by means of the two rectifier bridges 3 and 5; the direct voltage exiting bridge 3 supplies the "basic direct voltage", while the direct voltage exiting rectifier bridge 5 charges the capacitors 7 (equivalent to the "storage capacitor" as a whole). By triggering the gates of the thyristors column 6 the resonance circuit is closed and,

as previously described, one has pulse voltages available at the ends ofthe precipitator

As stated, the drawing in Fig 1, is provided purely as an example; in industrial realizations there may be many non substantial variations, however holding firm the characteristics ofthe present invention that comprises: - the generation of voltage pulses by means of high voltage components and making up the above described unit, without the inclusion of voltage elevator pulse transformers,

- the use of free-wheeling diodes for the protection ofthe thyristor column;

- the use of DSASs and BODs for the protection of single thyristors,

- the re-trigger technique to avoid damages when subjected to a direct voltage during the switching off stage

More especially the components that form the oscillating circuit of which the precipitator capacitor is a part, may be set in a different order from the one shown.

Moreover, it is understood that, instead of the thyristors, as static switches may be used components such as GTOs (Gate Turn Off Thyristors), IGBTs (Insulated Gate Bi- polar Transistors), MCTs (Mos Controlled Thyristors) or similar suited to opening and closing the circuit.