WO2003103842A1

WO2003103842A1 - A method for flame detection and for fast voltage recovery after a voltage cutoff in a purification chamber

Info

Publication number: WO2003103842A1
Application number: PCT/NO2003/000168
Authority: WO
Inventors: Øyvind WETTELAND; Bjarte Kvingedal; Arild Nesse
Original assignee: Applied Plasma Physics Asa
Priority date: 2002-06-06
Filing date: 2003-05-23
Publication date: 2003-12-18
Also published as: NO20022666L; NO316556B1; AU2003234363A1; NO20022666D0

Abstract

A method for enabling fast reconnection of a high-voltage purification filter (1) after having cut off the current supply to the high-voltage purification filter (1) due to arcing, the electrical circuit of the high-voltage purification filter (1) comprising a direct-current supply (2) and a converter (4), in which a capacitor (38) is placed between current supply poles of the converter (4), and in which the capacitor (38) essentially must be fully discharged before reconnection of the high-voltage filter (1) can occur, wherein the converter (4) immediately is provided with a current-limiting function after disconnection of the high-voltage purification filter (1) has occurred, whereby energy located in the capacitor (38) may be directed to the high 15 voltage purification filter (1) without damaging components of the electrical circuit.

Description

A METHOD FOR FLAME DETECTION AND FOR FAST VOLTAGE RECOVERY AFTER A VOLTAGE CUTOFF IN A PURIFICATION CHAMBER

This invention relates to a method for detecting a flame in a purification chamber, and for providing a fast recovery of voltage after a voltage cutoff. More specifically, it concerns a method for the detection of relatively weak arcs in a high-voltage purification chamber, and the achievement of a fast recovery of voltage after the high voltage has been cut off due to an arc. The invention also includes a device for practising the method.

As pollution authorities have introduced requirements for purification of industrial emissions, for example smell and particles from waste gases, purification methods and equipment to remove at least part of the unwanted content from the gases have been developed. High-voltage purification installations that clean with the aid of so-called cold plasma has become increasingly more employed for purifying discharges from, for example, manufacturing works. It has been shown that a significant proportion of the content of smell of the gases, for example, can be converted to less smelly substances by sending the gas through a cold plasma zone. Cold plasma is characterised in that a specific energy quantity is supplied to the gas to separate the component molecules of the gas into a collection of ions, electrons, neutrally charged gas molecules and other forms having different degrees of influence. Cold plasma is distinguishable from so called warm plasma in that it is not supplied with enough energy for a thermal equilibrium to be achieved, in which ions and electrons on average have the same temperature. An electrical arc is an example of a condition where warm plasma is created.

In one embodiment, the reaction chamber of a high-voltage purification filter comprises a number of parallel through- flow channels for gas . Running concentrically through each of the channels, a rod or thread is arranged. In each channel/rod unit, the channel constitutes the one electrode, while the rod constitutes the other electrode.

Cold plasma is generated between the rod and the channel along the whole length of the channel when electric high voltage is applied between the electrodes. A direct-current voltage is used to maintain a unidirectional electron flow between the electrodes .

It is necessary to maintain as high a voltage as possible between the electrodes to achieve a satisfactory purification effect. Preferably, the voltage between the electrodes thus must be held at a level just below the voltage level at which arcing occurs . Due to deposition of pollutants in the form of electrostatically charged particles and dust on the channel wall, it is necessary to clean the channels. The purification process does not remove all the material, and therefore extra thick layers of pollution may build up on the inside of the channel .

Some forms of the accumulated pollutants have a chemical composition making them electrically semi-conductive . In the event of a spark/arc between the rod and the channel, a relatively high electrical resistance in the pollutant located on the channel wall will limit the current in the arc to a relatively low value relative to a short-circuit current. Thus, an arc of this type may not be registered by metering equipment arranged to register arcs of the short- circuit type .

Due to the level of current to the high-voltage purification filter being insignificantly higher than during normal operation, an arc of this type may progress without the current supply being cut off. However, if the arcing is allowed to continue, which is similar to that of a short- circuit type arc, the arc will damage the rod and cause operational disturbances.

After the current to the high-voltage purification filter has been cut off due to arcing, it is of great importance that the power is reconnected as fast as possible in order for the purification process to be restored. In the following, the time lapsing from voltage cutoff until restoring thereof is referred to as "VRT" (voltage recovery time) . Due to the improvement in purification effect occurring at increased voltage, it may be advantageous for the voltage in the high-voltage purification filter to be sufficiently high to account for a number of arcs to occur every minute, for example. This improvement in the purification effect, however, will be reduced if the VRT after each arc is too long. VRT for prior art voltage generators is typically between 10 and 100 milliseconds. Relatively speaking, this amount of time will considerably reduce the efficiency of the high-voltage filter, inasmuch as the voltage must be held at a level at which relatively few arcs may be expected.

The object of the invention is to remedy the disadvantages of the prior art .

This object is achieved according to the invention, and by means of the features disclosed in the description below, and in the following patent claims.

The necessary voltage supplied to the electrodes of the high- voltage purification filter may be in the order of several ten thousands of volts. It has been shown that a full bridge, a so called "Hard Switch Converter" and referred to below as "converter", is well suited for controlling the current supply to a high-voltage filter of this type, the converter being arranged to allow immediate current cutoff in response to a signal, thereby discontinuing an arc.

A direct-current supply provides the converter with a voltage-regulated direct-current. The converter, which is controlled from a control system via two so-called half bridge IGBT drivers (IGBT - Insulated Gate Bipolar Transistor) , is arranged to convert the direct-current to alternating-current. The alternating-voltage is fed to a transformer, in which it is transformed up to a higher voltage before it is rectified and fed to the electrodes of the high-voltage purification filter. The direct-current supply is also controlled by the control system via a half bridge IGBT driver.

A current meter connected between the direct-current supply and the converter is arranged to be able to register even very small fluctuations in the amperage of the high-voltage purification filter. The measured" amperage is converted to a voltage, which then is compared to another regulated voltage, possibly that of the control system. If the measured voltage exceeds the voltage it is compared with, the current supply to the high-voltage filter is cut off .

With a converter of this type, in which a direct and instantaneous connection between the current supply and the high-voltage transformer is made, it is necessary, due to the danger of harmful excess current and/or voltage at the moment of connection, to ensure that the voltage on both sides of the converter is approximately equal at the moment of connection. After an arc, the voltage going into the converter thus must be approximately zero.

In order to sufficiently even out the voltage into the converter, among other reasons, it is necessary arrange a relatively large capacitor between the poles on the current supply side of the converter. After a power cut due to the above-mentioned reasons, this capacitor must be discharged before the current can be reconnected. Discharge by means of a discharge resistor connected in parallel will result in a significantly increased consumption of energy, inasmuch as input voltage of the converter will be applied to the discharge resistor during operation. It is possible to arrange the discharge resistor in a way allowing it to be disconnected during normal operation, and wherein it only is connected during a current supply cut to the high-voltage filter. Experience has shown that such a solution is costly and unsuitable due to the relatively large quantity of energy that must be consumed in a short time.

According to the invention, the converter immediately is provided with a current-limiting function when an arc occurs, and at the same time as the current supply to the converter is cut off, thereby discharging the capacitor via the converter of the high-voltage filter. In a preferred form of the method, the energy located in the capacitor while the arc in the high-voltage filter is still "burning" , is fed to the high-voltage filter for energy consumption in the ongoing arc, the energy being fed via the converter, the transformer and the rectifier at an amperage not exceeding the amperage at which the converter, the transformer and the rectifier are arranged to tolerate. After the arc is extinguished, the current is reconnected to the converter and the high-voltage purification filter.

In an alternative method that may be used when there is sufficient capacitance in the high-voltage filter, the converter is provided with a current-limiting function when connecting the current to the high-voltage filter after an arc. This alternative method may be used because the capacitance of the high-voltage filter causes a relatively slow build-up of voltage in the high-voltage filter. It is essential that the converter assume this special current- limiting function only for a short time period, and then assumes its normal operating condition in which it may immediately cut off the current supply in the event of an arc .

According to the invention, and in order to be able to discover weak and non-short-circuiting arcs in the high- voltage purification filter, a continual analysis of the power consumption of the high-voltage purification filter is used with the help of a control system. It is necessary for the current meter of the high-voltage purification filter to have high sensitivity to changes in the amperage to the high- voltage purification filter, as well as being robust. A galvanically isolated Hall-effect sensor has been shown to have sufficient resolution with little drift and noise. The method is explained in the description specification with reference to the attached drawings .

In the following, a non-limiting example of a preferred method and embodiment is described, and is illustrated in the accompanying drawing, in which:

Figure 1 shows a simplified circuit diagram of the high- voltage filter;

Figure 2 shows graphically current and voltage in the power circuit of the high-voltage filter and voltage across the input capacitor of the converter; and Figure 3 shows graphically current and voltage in the power circuit of the high-voltage filter.

On the drawings, reference numeral 1 denotes a high-voltage purification filter being supplied a high-voltage electric current from a direct-current supply 2 via a converter 4, a transformer 6 and a rectifier 8, all known per se . The devices 1, 2, 4, 6 and 8 are mutually connected by means of wires 9 and 11 respectively.

During normal operation, the current supply is connected by means of a control signal from a control system (not shown) via a first half bridge IGBT driver 10. The converter 4 is controlled by a second and a third half bridge IGBT driver 12 and 14 from the control system when a pulsed bell signal is supplied via the wire 16 and is divided between two inverters 18, causing the bell signal in the wire 16 to alternately switch the drivers 12 and 14 on and off. Via wires 20 the drivers 12 and 14 alternately connect to the switches of the converter 4 and convert, as a so-called full bridge, and in a manner known per se, direct-current to alternating-current at a frequency thus controlled by the bell signal frequency in the wire 16.

A current meter 22 is arranged to measure the amperage in the wire 9 connecting the direct-current source 2 with the converter 4. Preferably, the current meter 22 is a galvanically isolated Hall-effect sensor provided with a converter in order to convert a measured current to a corresponding voltage. The voltage being emitted by the current meter 22 is fed via a wire 24 to a comparator 26 known per se, in which the voltage in the wire 24 is compared with a reference voltage supplied via a wire 27. The reference voltage may be constant or adjustable, for example by means of the control system.

When the electricity drawn from the converter 4 causes the voltage from the current meter 22 to exceed the reference voltage in the wire 27, the comparator 26 emits a signal to the preset input of a toggle switch 28 via a wire 30. The toggle switch 28 thereby changes the status of an output connected to all half-bridge IGBT drivers 10, 12 and 14 via wires 32 and 34 and a logical "AND" element 36. All half- bridge IGBT drivers 10, 12 and Ity thereby immediately stop their respective controlled direct-current supply sources 2 and converter 4, so that any arc in the high-voltage filter is cut off before damage can occur.

A capacitor 38 is arranged between the poles on the input side of the converter 4. In order to avoid damage to the system's components, it is necessary for the capacitor 38 to be discharged before reconnecting the voltage to the high- voltage purification filter. To be able to reconnect the current as soon as possible after an arc, it is desirable to discharge the capacitor as fast as possible.

The capacitor 38 is discharged by immediately reconnecting the converter 4, allowing the energy located in the capacitor 38 to be fed to the ongoing arc in the high-voltage purification filter 1. When an arc occurs between the electrodes of the high-voltage purification filter 1, the voltage between the electrodes drops to approximately zero volt, whereby the amperage increases greatly. As the current meter 22, due to a high measured amperage, emits a voltage exceeding the reference voltage, see event "a" in figure 2, the converter 4 and the direct-current supply 2 are stopped by means of a signal in the wire 32, as described above. (References using lower-case latin letters refer to figure 2) . The output current from the converter 4 thereby drops toward zero. At the same time the control system receives a message via a wire 40 that an arc has occurred, and it alters the voltage level in a wire 42 of the first half-bridge IGBT driver 10, so that the driver 10 does not emit a connection signal to the direct-current source 2 when the toggle switch 28, due to a signal in a wire 44, reconnects the converter 4.

While the direct-current supply 2 is still disconnected, the control system immediately emits a signal via the wire 44 to a CLK-port of the toggle switch 28 in order to reconnect the converter 4, see event "b" in figure 2. Due to residual energy in the capacitor 38, the current in the converter 4 increases again, after which the current meter 22, due to the high amperage measured, again emits a voltage exceeding the reference voltage. The converter 4 is stopped again, and the amperage drops. A pulse is again supplied in the wire 42, whereby the converter is started, see event "c" in figure 2.

This sequence repeats over a predetermined time period, after which it may be assumed, from experience, that the capacitor 38 is discharged, see event "d" in figure 2. The converter 4 is started, and the direct-current supply 2 is caused to supply current by changing the voltage condition in the wire 42, see event "e" in figure 2. The value "U" in figure 2 represents the electrode voltage of the high-voltage purification filter 1, while the values "A" and "Ul" represent the corresponding amperage at the current meter 22 and the voltage across the capacitor 38, respectively.

The relatively fast switching of the toggle switch 28 by means of a voltage pulse in the wire 42 renders possible for the discharge of the stored energy in the capacitor 38 to be carried out through the ongoing arc in the high-voltage purification filter.

In an alternative method, which may be used when the capacitance of the high-voltage purification filter is sufficiently high, the current supply to the high-voltage purification filter is cut off, thereby immediately stopping the arc, see event "f" in figure 2. After extinguishing the arc, the current is reconnected again, see event "g" in figure 2. The charging of the relatively high capacitance of the high-voltage purification filter 1 causes the rise time of the amperage to become sufficiently long for the converter (4) to be disconnected when the amperage passes a predetermined level without any overload damages occurring, see event "h" in figure 2. Disconnection and connection of the converter 4 is repeated for a time period until the capacitor 38, based on experience, is sufficiently discharged, see event "i" in figure 2, after which the control circuit is set in an operating condition in which it will ensure that the current supply to the high-voltage purification filter 1 is cut off in the event of an arc. In figure 3, the upper part of the diagram shows the voltage of the high-voltage purification filter as a function of time, while the lower part of the diagram shows the corresponding current as a function of time. Events in figure 3 are denoted with roman numerals. During normal operation, the current supply to the high-voltage purification filter shows a relatively stable voltage and a current level wherein some noise is present, cf . the corresponding curves I. When a short-circuiting arc occurs, cf . event II, the voltage across the high-voltage purification filter drops to approximately zero, while the current increases rapidly to a short-circuit detection level immediately causing the control system to disconnect the current supply.

When the current is reconnected to the high-voltage purification chamber 1, cf. event III, the voltage and the amperage increase to the level they were at before the arcing.

A sudden small increase in electrical consumption, cf. event IV, indicates that a non-short-circuiting arc is occurring in the high-voltage purification filter 1. The current supply is cut off, cf . event V, and is reconnected at event VI. Electrical consumption of the high-voltage filter assumes the same level that it was at before event IV and thus confirms that the current change at event IV was due to a non-short- circuiting arc.

The amperage level when reconnecting the current, cf . event X, to the high-voltage purification filter 1 after having cut off the current at event IX due to a significant increase in amperage at event VIII, indicates that an arc not detected also existed at event VII. Via such an interpretation of changes, it is possible to automatically adjust the detection level .

Changes in the high-voltage purification filter 1 due to environmental effects, for example due to changes in gas quantity or the chemical composition of the gas, cause a gradual change in the electricity consumption between events XI and XII.

The current supply to the high-voltage purification filter is disconnected at event XIII due to the level of current supply at event XII . After reconnection of the current to the high- voltage filter at event XIV, the current level is unchanged relative to that at event XIII. This implies that no arcing probably occurring at event XII .

Because the control system is programmed to carry out logical decisions concerning which conditions to be present in the high-voltage filter 1, and also concerning the preceding development of electricity consumption before disconnection of the current supply to the high-voltage purification filter 1 is carried out, the voltage level in the high-voltage purification filter 1 may gradually increase to a level at which an acceptable number of arcs per unit of time occur. When reconnecting the current supply, and based on the electricity consumption before and after disconnection, the control system program may analyse if an arc or an environmental change in the high-voltage filter is involved. By means of so-called "fuzzy logic" based on said analysis, the control system may change the control program to react in a different way, for example when environmental changes occur .

Claims

C L A I M S

1 . Method for enabling fast reconnection of a high-voltage purification filter ( 1 ) after having cut off the current supply to the high-voltage purification filter (1) due to

5 arcing, the electrical circuit of the high-voltage purification filter ( 1 ) comprising a direct-current supply (2) and a converter (4), in which a capacitor (38) is placed between current supply poles of the converter (4), and in which the capacitor (38) essentially must be o fully discharged before reconnection of the high-voltage purification filter (1) can occur, c ar ac ter i z ed i n that the converter (4 ) immediately is provided with a current-limiting function after disconnection of the high-voltage purification filter ( 1 ) s has occurred, whereby energy located in the capacitor (38) may be directed to the high-voltage purification filter ( 1 ) without damaging components of the electric circuit.

2 . Method according to claim 1, in which the amperage 0 between the direct-current supply (2) and the converter (4) is measured by means of a current meter (22), the current meter (22) being arranged to emit a voltage which is essentially proportional to the measured amperage, and in which the emitted voltage from the current meter (22) 5 is compared with a reference voltage in a voltage comparator £-?&->) , the voltage comparator (26) being arranged to emit a signal to a toggle switch (28) when the voltage from the current meter (^'gl) is the same or exceeds the reference voltage, whereby the toggle switch 0 (28) via necessary connections is arranged to immediately cut off the current supply to the high-voltage purification filter (1) as the current supply (2) and the converter (4) is stopped, c ha r a c t e r i z e d i n that the toggle switch (28) is supplied with a voltage changing its condition and thus reconnecting the converter while a control signal to the control of the current supply (2) prevents the current supply (2) from being reconnected.

Method according to one or more of the preceding claims, c h a r a c t e r i z e d i n that the method according to claim 2 is repeated until the capacitor (38) essentially is fully discharged.

Method according to one or more of the preceding claims, c h a r a c t e r i z e d i n that the method according to claim 2 is repeated until the input and output voltages of the converter (4) essentially are the same.

Method according to one or more of the preceding claims, c h a r a c t e r i z e d i n that non-short-circuiting arcs in the high-voltage purification filter (1) are discovered by monitoring the amperage being supplied to the high-voltage purification filter (1), inasmuch as a stepwise increase in amperage is assumed to indicate that a non-short-circuiting arc is present, while a slow increase in amperage indicates that environmental changes are occurring in the high-voltage purification filter (1) -

Method according to claim 5, c h a r a c t e r i z e d i n that the amperage to the high-voltage purification filter (1) after reconnecting the current, is compared with the amperage immediately before power cut off in order to determine if a non-short-circuiting arc was present before disconnecting the high-voltage purification filter (1).

Method according to claim 6, c h a r a c t e r i z e d i n that the voltage of the high-voltage purification filter (1) is controlled based on the comparison according to claim 5.