MEASURING DEVICE FOR ACTING CURRENTS AND HIGH VOLTACES OF PHYSICAL PLASMAS

Description

TECHNICAL AREA OF INVENTION

[0001] The invention relates to a measuring device for alternating currents and high voltages of physical plasmas, in particular of cold atmospheric pressure plasmas, as well as to a plasma generator with an alternating high voltage source, a plasma electrode connected to the alternating high voltage source via a high voltage line and such a measuring device.

[0002] To control the ignition of physical plasmas and / or to monitor their state, it is useful to measure the alternating high voltages applied to a plasma electrode and the alternating currents flowing across it. The product of these alternating high voltages and currents is electrical power, which is essentially converted within the physical plasma.

[0003] This specifically concerns the ignition and monitoring of physical plasmas, which are generated by applying an alternating high voltage to a plasma electrode and the resulting gas discharges relative to a reference potential such as the electrical ground. More specifically, it concerns cold atmospheric pressure plasmas in which the gas discharges are dielectrically hindered. STATE OF THE ART

[0004] High-voltage AC sources for plasma generators often include a transformer to convert an AC voltage applied to the primary winding of the transformer into an AC high voltage. For example, WO 2010 / 098 779 A1 discloses the method of measuring the AC voltage drop across the primary winding of such a transformer and the current flowing through it using separate voltage transducers and current transformers. From this, it is possible to deduce the AC high voltage drop and current flowing through the secondary side of the transformer, which is affected by the physical plasmas generated on the secondary side. However, this indirect measurement of the AC currents and voltages of physical plasmas proves to be inaccurate.

[0005] Current transformers are used for measuring alternating currents through conductors without potential differences. A current transformer has a toroidal core that encloses the conductor. A measuring winding is arranged on the toroidal core. An alternating current flowing through the conductor induces an alternating magnetic field in the toroidal core, which generates a voltage between the ends of the measuring winding. The measuring AC voltage across a measuring resistor connected between the ends of the measuring winding is a monotonically increasing function of the current flowing through the conductor. Various current transformers are offered on the website www.pearson.com, some with a hinged toroidal core and / or with a guide for the conductor through the toroidal core. TASK OF INVENTION

[0006] The invention is based on the objective of demonstrating a measuring device for alternating currents and high voltages of physical plasmas, in particular of cold atmospheric pressure plasmas, as well as a plasma generator with such a measuring device, in which the alternating currents flowing across the plasma electrode and the alternating high voltages applied to it are measured as directly and accurately as possible with as little effort as possible. SOLUTION

[0007] The object of the invention is achieved by a measuring device with the features of independent claim 1 and by a plasma generator with the features of dependent claim 9. Dependent claims 2 to 8 and 10 relate to preferred embodiments of the measuring device and the plasma generator according to the invention. DESCRIPTION OF THE INVENTION

[0008] A measuring device according to the invention for alternating currents and high voltages of physical plasmas comprises a current transformer with a toroidal core and a measuring winding on the toroidal core, a conductor guide configured to guide a high-voltage line running from an alternating high-voltage source to a plasma electrode through the toroidal core in a defined relative arrangement, a measuring resistor connected between the ends of the measuring winding, and a measuring capacitor connected between a center tap of the measuring winding and a reference potential connection.A first AC measurement voltage, drop across the measuring resistor and accessible there, is a strictly monotonically increasing function of the current of the alternating currents flowing through the high-voltage line to the plasma electrode; and a second AC measurement voltage, drop across the measuring capacitance, is a strictly monotonically increasing function of the waveform of the alternating high voltage applied to the plasma electrode via the high-voltage line relative to a reference potential connected to the reference potential terminal. With a typical design of the current transformer's toroidal core, these strictly monotonically increasing functions are linear functions of the current of the alternating currents and the waveforms of the alternating voltages, respectively. Furthermore, the frequencies of the AC measurement voltages coincide with those of the alternating currents and alternating high voltages.If phase shifts exist between the measuring AC voltages and the alternating currents or AC voltages, which may be frequency-dependent, these can be detected once and then easily compensated.

[0009] For measuring alternating currents, the measuring device according to the invention utilizes the known principle of a current transformer. For measuring alternating high voltages, the measuring device according to the invention exploits the fact that the primary and secondary windings of a transformer, and thus also the high-voltage line leading through the toroidal core and the measuring winding, are always coupled not only inductively but also capacitively in the measuring device according to the invention, i.e., without any capacitor being connected between the primary winding or high-voltage line on the one hand and the secondary winding or measuring winding on the other. The second measuring AC voltage results from this inherent capacitive coupling and is dropped across the measuring capacitor connected between the center tap of the measuring winding and the reference potential.The capacitance of the inherent capacitive coupling between the high-voltage line and the measuring winding remains constant as long as the relative arrangement of the toroidal core and the high-voltage line passing through it remains the same, which is ensured by the routing of the measuring device according to the invention. Although the capacitance of the inherent capacitive coupling is small, typically on the order of a few picofarads, i.e., for example, between 1 and 10 pF, this allows a capacitive voltage divider to be formed with a relatively small measuring capacitance in the nF range. Thus, at AC high voltages on the order of 10,000 V, a readily usable second AC measuring voltage in the volt range is available across the measuring capacitance.

[0010] The measuring capacitance also means that the first AC measurement voltage has a fixed reference to the reference potential. The measuring capacitance can be implemented by a single capacitor or by several capacitors connected together.

[0011] The conductor guide of the measuring device according to the invention can be designed, in particular, to guide the high-voltage line coaxially through the toroidal core. The conductor guide can be a shaped body that is supported on one side by the toroidal core or a housing of the toroidal core and on the other side by supporting the high-voltage line. This shaped body can be 3D-printed to adapt it to the respective toroidal core or its housing and the respective high-voltage line. The insert can be multi-part to allow assembly around the respective high-voltage line. The toroidal core can also be hinged to allow assembly around the respective conductor guide and the respective transformer line. It is understood that the conductor guide is to be made of a non-magnetic and non-electrically conductive material and can, in particular, be made of plastic.

[0012] In the measuring device according to the invention, a first low-pass filter can be connected between the terminals of the measuring resistor and the tap points for the first AC voltage. This low-pass filter can, for example, comprise an RC circuit. A first cutoff frequency of the first low-pass filter can be selected depending on a sampling rate of a first A / D converter for the first AC voltage, so that when sampling a signal at the cutoff frequency, the Nyquist-Shannon criterion is maintained, according to which the sampling rate for error- and artifact-free sampling of a signal with a signal frequency must be at least twice the signal frequency.

[0013] A second low-pass filter can be connected between the terminals of the measuring capacitor and the tap points for the second AC voltage measurement. This second low-pass filter can include a second RC network. A second cutoff frequency of the second low-pass filter can be matched to a sampling rate of a second A / D converter for the AC voltage measurement, whereby the Nyquist-Shannon criterion must also be observed.

[0014] Typically, the first cutoff frequency of the first low-pass filter and the second cutoff frequency of the second low-pass filter will be the same, just as the first sampling rate of the first A / D converter for the first AC measurement voltage and the second sampling rate of a second A / D converter for the second AC measurement voltage will be the same. Furthermore, the first and second A / D converters can be synchronized with respect to their sampling rates.

[0015] The electrical power of the respective plasma can be calculated from the first and second AC measurement voltages tapped at the measuring device according to the invention. However, it is also possible to provide an analog multiplier for the first and second AC measurement voltages in the measuring device according to the invention, which outputs a product voltage. This product voltage is then directly proportional to the electrical power of interest of the plasma. It is understood that a further low-pass filter and / or a further A / D converter may be provided for the product voltage.

[0016] In a plasma generator according to the invention, comprising an alternating high-voltage source configured to output an alternating high voltage relative to a reference potential at a high-voltage line connection, a plasma electrode configured to ignite a physical plasma in a working gas by electrical discharge relative to the reference potential when alternating high voltage is applied, and a high-voltage line connected to the high-voltage line connection and the plasma electrode, configured to apply the alternating high voltage to the plasma electrode, a measuring device according to the invention for alternating currents and voltages of the physical plasma is provided.

[0017] In the plasma generator according to the invention, a dielectric shield of the plasma electrode can be configured to dielectrically impede the electrical discharges relative to the reference potential. Alternatively, dielectric impediment of the electrical discharge can also be achieved by capacitive coupling of the plasma electrode to the high-voltage line. Due to this dielectric impediment, the plasma generator according to the invention is particularly suitable for generating cold atmospheric pressure plasmas.

[0018] In general, the measuring device according to the invention, with its current transformer, can only measure alternating currents and voltages. It is not suitable for measuring direct currents and voltages of plasmas and corresponding plasma generators operating with direct voltage. The measuring device according to the invention also cannot detect direct current superimposed on an alternating current with a change in direction of flow, nor can it detect direct voltage superimposed on an alternating voltage with a change in polarity. However, the measuring device according to the invention is suitable for any type of alternating current and voltage, i.e., not only for sinusoidal currents, but also for alternating currents and voltages with any time waveform, and in particular for pulsed alternating currents and voltages.

[0019] It is understood that both the capacitance of a dielectric shield for the plasma electrode and of a capacitive coupling of the plasma electrode to the high-voltage line must be taken into account when determining the plasma's AC voltage from the second measurement voltage. However, if the AC voltage is directly multiplied by the AC current and the resulting product is averaged, the effects of any capacitances on the AC voltage are eliminated, because these only result in reactive power with alternating polarity, but no active power. If no resistive elements are electrically connected in series with the plasma, the active power determined from the AC voltage and the AC current in this or any other way is that of the plasma.

[0020] Advantageous further developments of the invention result from the patent claims, the description and the drawings.

[0021] Regarding the disclosure content—not the scope of protection—of the original application documents and the patent, the following applies: Further features can be derived from the drawings—in particular, the geometries depicted and the relative dimensions of several components to one another, as well as their relative arrangement and functional connection. The combination of features from different embodiments of the invention or from features of different claims is also possible, deviating from the chosen cross-references of the claims, and is hereby encouraged. This also applies to features that are illustrated in separate drawings or mentioned in their description. These features can also be combined with features from different claims.Likewise, features listed in the patent claims may be omitted for further embodiments of the invention, but this does not apply to the independent patent claims of the granted patent.

[0022] The features mentioned in the claims and the description are to be understood, with regard to their number, as meaning that exactly that number or a greater number than the stated number is present, without the need for the explicit use of the adverb "at least". Thus, for example, if a plasma electrode is mentioned, this is to be understood as meaning that exactly one plasma electrode, two plasma electrodes, or more plasma electrodes are present. The features listed in the claims may be supplemented by further features or may be the only features that the subject matter of the respective claim possesses.

[0023] The reference numerals contained in the patent claims do not constitute a limitation of the scope of the subject matter protected by the patent claims. They merely serve the purpose of making the patent claims easier to understand. BRIEF DESCRIPTION OF THE FIGURES

[0024] The invention will now be further explained and described with reference to preferred embodiments shown in the figures. Fig. 1 is a circuit diagram of a plasma generator according to the invention with a measuring device according to the invention, wherein a plasma generated with the plasma generator is represented by an equivalent circuit diagram; and Fig. 2 is a schematic representation of a more concrete embodiment of the measuring device according to the invention. FIGURE DESCRIPTION

[0025] The in Fig. 1The plasma generator 1 shown has an AC high-voltage source 2 that generates an AC high voltage UHV relative to a reference potential 3, for example, electrical earth or another electrical ground. The AC high voltage UHV is applied to a plasma electrode 5 via a high-voltage line 4. The plasma electrode 5 is provided with a dielectric shield, which in a plasma model 6 is connected to a shielding capacitance CD of a capacitor 7. A capacitor 8 of the plasma model 6 represents an electrical capacitance CG that forms above a discharge gap before the plasma is ignited. When the plasma is ignited, a switch 9 of the plasma model 6 is closed. A resistor 10 connected in series with the switch 9 represents the ohmic resistance NP of the plasma, which is ignited by the AC high voltage UHV applied to the plasma electrode 5 relative to the reference potential 3.

[0026] A measuring device 11 according to the invention for measuring the alternating current and voltage of the plasma comprises a current transformer 12, which is represented here as a transformer with a primary winding 14 and a secondary winding 15. In practice, the current transformer 12 has a Fig. 1The toroidal core (not shown) on which the secondary winding 15 is arranged as the measuring winding 16, and through which the high-voltage line 4 runs, thus forming a primary winding 14 without a single complete turn. Nevertheless, the primary winding 14 has an inductance LCT,Pri, which is inductively coupled via the toroidal core to an inductance LCT,Sec of the secondary winding. Furthermore, the primary winding 14 and the secondary winding 15 are inherently, i.e., without the actual presence of a capacitor, capacitively coupled with a capacitance CCT,Coup. A measuring resistor 19 with an ohmic resistance RIMs is connected between the ends 17 and 18 of the measuring winding 16. For a first measuring voltage UIP,Ms dropping across the measuring resistor 19, a first low-pass filter 20 is provided with an RC network 21 consisting of a resistor 22 with an ohmic resistance N1 and a capacitor 23 with a capacitance CC.For the low-pass filtered first AC measurement voltage UIP,Measures applied to tap points 24 and 25, a first A / D converter 26 is provided, which outputs a first measurement signal 27. The first measurement signal 27 is proportional to the alternating current flowing through the high-voltage line 4 to the plasma electrode 5 and thus through the plasma.

[0027] A measuring capacitor 29 with an electrical capacitance CU,Mess is connected between a center tap 28 of the measuring winding 16 and the reference potential 3. Together with the coupling capacitance C CT,Cuop, the measuring capacitor CU,Mess forms a capacitive voltage divider for the AC high voltage U HV. For a second measuring AC voltage U UP,Mess dropping across the measuring capacitor 29, a second low-pass filter 30 is provided with a second RC network 31 consisting of a second resistor 32 with an ohmic resistance R 2 and a second capacitor 33 with a capacitance C 2. A second A / D converter 36 is connected to second tap points 34 and 35 for the low-pass filtered second measuring AC voltage U UP,Mess, which outputs a second measuring signal 37. The second measuring signal 37 is proportional to the AC high voltage U HV applied to the plasma electrode 5.Additionally, the first measuring points 24 and 25 and the second measuring points 34 and 35 are connected to an analog multiplier 38, which outputs a product voltage 39. A further A / D converter 40 is provided for the product voltage 39, which outputs another measuring signal 41. The measuring signal 41 is proportional to the electrical power of the plasma.

[0028] Fig. 2Figure 1 shows a more detailed embodiment of the current transformer 12 with the toroidal core 42, on which the measuring winding 16 is arranged and through which the high-voltage line 4 passes. The toroidal core 42, together with the measuring winding 16, is embedded in a non-magnetic and non-electrically conductive potting compound 43, which forms a housing 44. A shaped element 46 of a guide 47 for the high-voltage line 4 is inserted into a central opening 45 of this housing 44. The shaped element 46 rests against the housing 44 and an insulation 48 of the high-voltage line 4, which encloses its electrically conductive core 49. The guide 47 guides the high-voltage line, specifically its electrically conductive core 40, coaxially through the toroidal core 42. [Not shown in Figure 44] Fig. 2 the low-pass filters 20 and 30, the multiplier 38 and the A / D converters 26, 36 and 40 of the measuring device 11.

[0029] In a specific embodiment of the measuring device 11, the inductance LCT,Pri of the primary winding 14 of the transformer formed by the current transformer 12 was 6.40 µH, the inductance LCT,Pri of the secondary winding 15 on each side of the center tap 28 was 3.10 mH, and the coupling capacitance CCT,Cuop was 6.0 pF. The ohmic resistance R1 of the measuring resistor 19 was, for example, 6, 12, or 24 ohms, typically less than 50 ohms. The number of turns of the secondary winding was 24. The ohmic resistances R1 and R2 of resistors 22 and 32 of RC elements 21 and 31 were 80 ohms, and the capacitances C1 and C2 of capacitors 23 and 33 of RC elements 21 and 31 were 10 pF. The measuring capacitance CU,Mess was 60 nF. With this measuring device, plasma currents and plasma voltages in the range of 10 A and 30,000 V were successfully measured with time resolution at a sampling rate of 500 MHz. REFERENCE MARK LIST

[0030] 1 Plasma generator 2 AC high-voltage source 3 Reference potential 4 High-voltage line 5 Plasma electrode 6 Plasma model 7 Capacitor 8 Capacitor 9 Switch 10 Resistor 11 Measuring device 12 Current transformer 13 Transformer 14 Primary winding 15 Secondary winding 16 Measuring winding 17 End of measuring winding 16 18 End of measuring winding 16 19 Measuring resistor 20 Low-pass filter 21 RC circuit 22 Resistor 23 Capacitor 24 First tap point 25 First tap point 26 A / D converter 27 First measuring signal 28 Center point tap 29 Center point of light measuring capacitance 30 Low-pass filter 31 RC circuit 32 Resistor 33 Capacitor 34 Second tap point 35 Second Tap point 36 A / D converter 37 Second measuring signal 38 Multiplier 39 Product voltage 40 A / D converter 41 Further measuring signal 42 Toroidal core 43 Potting compound 44 Housing 45 Opening 46 Molded body 47 Guide 48 Insulation 49 Core U HV AC high voltage CD Capacitance of a dielectric shield of the plasma electrode 5 CG Capacitance of the working gas in the discharge gap before plasma ignition RP Ohmic resistance of the ignited plasma L CT,Pri Inductance of the primary winding 14 L CT,Sec Inductance of the secondary winding 15 C CT,Coup Coupling capacitance between primary winding 14 and secondary winding 15 RI,Measuring Ohmic resistance of the measuring resistor 19 R 1 Ohmic resistance of the resistor 22 C 1 Capacitance of the capacitor 23 U IP,Measuring first measuring AC voltage CU,Measuring Electrical capacitance of the measuring capacitor 29 R 2 Ohmic resistance of the resistor 32 C 2 Capacitance of the capacitor 33 U UP,Measuring second measuring AC voltage

Claims

1. Measurement apparatus (11) for alternating currents and voltages of physical plasmas, particularly of cold plasmas at atmospheric pressure, comprising - a current transformer (12) having a ring core (42) and a measurement winding (16) on the ring core (42), - a cable guide (47) configured for guiding a high voltage line (4) running from an alternating high voltage source (2) to a plasma electrode (5) in a defined relative arrangement through the ring core (42), and - a measurement resistor (19) connected between ends (17, 18) of the measurement winding (16), - wherein a first measurement alternating voltage (UIP,Mess) dropping over the measurement resistor (19) is a first strictly monotonic increasing function of an amperage of the alternating currents flowing through the high voltage line (4) to the plasma electrode (5), characterized in - that a measurement capacitance (29) is connected between a center tap (28) of the measurement winding (16) and a reference potential connector, - wherein a second measurement voltage (UUP,Mess) dropping over the measurement capacitance (29) is a second strictly monotonic increasing function of an amplitude of an alternating high voltage (UHV) applied to the plasma electrode (5) via the alternating high voltage with respect to a reference potential (3) connected to the reference potential connector.

2. Measurement apparatus (11) of claim 1, characterized in that the cable guide (47) is configured for coaxially guiding the high voltage line (4) through the ring core (42).

3. Measurement apparatus (11) of any of the preceding claims, characterized in that a first low pass filter (20) is connected between connections of the measurement resistor (19) and first tap points (24, 25) for the first measurement alternating voltage (UIP,Mess).

4. Measurement apparatus (11) of any of the preceding claims, characterized in that a second low pass filter (30) is connected between connections of the measurement capacitance and second tap points (34, 35) for the second measurement alternating voltage (UUP,Mess).

5. Measurement apparatus (11) of claim 4, so far as dependent on claim 3, characterized in that a first cut-off frequency of the first low pass filter (20) and a second cut-off frequency of the second low pass filter (30) are equal.

6. Measurement apparatus (11) of any of the preceding claims, characterized in that a first A / D converter (26) for the first measurement alternating voltage (UIP,Mess) and a second A / D converter (37) for the second measurement alternating voltage (UUP,Mess) are provided.

7. Measurement apparatus (11) of any of the preceding claims, characterized in that an analogous multiplier (38) for the first measurement alternating voltage (UIP,Mess) and the second measurement alternating voltage (UUP,Mess) which puts out a product voltage (39) is provided.

8. Measurement apparatus (11) of claim 7, characterized in that a further A / D converter (40) for the product voltage (39) is provided.

9. Plasma generator (1) comprising - an alternating high voltage source (2) configured for outputting an alternating high voltage (UHV) with respect to a reference potential (3), - a plasma electrode (5) configured for igniting a physical plasma in a working gas with applied alternating high voltage (UHV) by electric discharges with respect to the reference potential (3), - a high voltage line (4) connected to the alternating high voltage source (2) and the plasma electrode (5) and configured for applying the alternating high voltage (UHV) to the plasma electrode (5), and - a measurement apparatus (11) of any of the preceding claims for alternating currents and voltages of the physical plasma.

10. Plasma generator (1) of the preceding claim, characterized in that a dielectric barrier of the plasma electrode (5) is configured for dielectrically barring the electric discharges.