A method for measuring gas ionization coefficient and streamer initiation voltage under wide temperature conditions

Abstract

This invention belongs to the field of gas charge detection technology, specifically relating to a method for measuring the gas ionization coefficient and streamer initiation voltage under wide temperature conditions. The steps include: constructing an experimental system comprising an electrode system, a temperature control system, a laser, a single-photon detection and signal processing module, a streamer voltage measurement device, and auxiliary purification equipment; performing temperature control on the low-temperature and high-temperature sections respectively, and initiating initial electron excitation when the set temperature is reached and stabilized; using the laser for initial electron excitation, collecting the number of photons and photon arrival time, and calculating the ionization coefficient; embedding a tungsten needle tip in the center of the upper surface of the grounding electrode, turning off the laser, increasing the voltage, and monitoring the needle tip current; when the current derivative and current amplitude suddenly increase to meet preset conditions, recording the voltage at a certain temperature T as the streamer initiation voltage U. s (T), the average value of Z repeated measurements is taken as the final value of the streamer initiation voltage. This invention can achieve α and U over a wide temperature range. s High-precision, high-sensitivity measurement.

Description

Technical Field

[0001] This invention belongs to the field of gas charging detection technology, specifically relating to a method for measuring gas ionization coefficient and stream initiation voltage under wide temperature conditions, applicable to the evaluation of insulation performance and discharge mechanism research of insulating gases in power equipment. Background Technology

[0002] The gas ionization coefficient and the stream initiation voltage are core parameters characterizing the strength of gas insulation, and their measurement accuracy directly affects the insulation design and operational reliability of power equipment. Traditional measurement methods have the following limitations:

[0003] Temperature factors are difficult to evaluate: In actual operation of the equipment, the temperature gradient between the actual electrode surface and the gas can reach up to 8K, which leads to an unknown deviation between the ionization coefficient α obtained at room temperature and the actual situation.

[0004] The initiation signal of the streamer is difficult to capture: the electric field of the flat electrode is uniform, and the pre-discharge current before the streamer is less than 10. -11 A. Traditional ammeters cannot identify this; the voltage needs to be increased to a considerable avalanche ionization level to make a judgment, resulting in the measured discharge initiation voltage value not being the physical initiation voltage.

[0005] Space charge interference: In the steady-state current method, when αD is greater than 5 (D is the electrode spacing), the space charge accumulated by electron avalanches will significantly distort the electric field, causing a large deviation in the measurement of α;

[0006] It has weak time resolution: it can only obtain the average α value and it is difficult to capture the transient characteristics of electron avalanche development (such as nanosecond-level α fluctuations).

[0007] To address the aforementioned issues, there is an urgent need to research new methods for measuring the gas ionization coefficient and stream initiation voltage over a wide temperature range (-80℃ to 200℃), in order to achieve the measurement of α and U over this wide temperature range. s High-precision, high-sensitivity measurement of (stream initiation voltage). Summary of the Invention

[0008] In view of the shortcomings of the prior art, the purpose of this invention is to provide a method for measuring the gas ionization coefficient and streamer initiation voltage under a wide temperature range, which can realize the measurement of α and U under a wide temperature range. s High-precision, high-sensitivity measurement.

[0009] To achieve the above objectives, this invention provides a method for measuring the gas ionization coefficient and stream initiation voltage under wide temperature conditions, comprising the following steps:

[0010] S1. Construct an experimental system including an electrode system, a temperature control system, a laser, a single-photon detection and signal processing module, a streamer voltage measurement device, and auxiliary purification equipment. The electrode system includes a high-voltage electrode and a grounding electrode located in the gas chamber. The temperature control system includes a refrigerator cold head, a thin-film heating element, a temperature measuring resistor, and a PID controller.

[0011] S2. Based on the experimental system, for the low temperature range of greater than or equal to -80℃ and less than 50℃, the cold head of the refrigerator is started, and the heating element is driven by the PID controller to assist in temperature control. When the set temperature is reached and stabilized, the initial electron excitation is performed.

[0012] For the high-temperature range of 50°C or higher and 200°C or lower, the cold head of the refrigerator is turned off, the thin-film heating element is independently temperature controlled, and initial electron excitation is performed when the set temperature is reached and stabilized.

[0013] S3. Initial electronic excitation is performed using a laser, the number of photons and the photon arrival time are collected, and the ionization coefficient is calculated.

[0014] S4. Embed a tungsten needle tip in the center of the upper surface of the grounding electrode, turn off the laser, boost the voltage, and monitor the needle tip current;

[0015] S5. When the current derivative and current amplitude suddenly increase to meet the preset conditions, record the voltage at a certain temperature T as the streamer initiation voltage U. s (T), the average value of the measurement is taken after Z repeated measurements as the final value of the streamer starting voltage.

[0016] As a preferred embodiment of the present invention, in S1, the single-photon detection and signal processing module includes a single-photon detector and a TDC module, the streamer voltage measurement device includes a DC high voltage source and an oscilloscope, and the auxiliary purification device includes a vacuum unit and a gas chromatograph.

[0017] A light-transmitting window is provided at the top of the gas cavity. The light-transmitting window is located above the high-voltage electrode. The laser is located outside the gas cavity. The central circular area of ​​the high-voltage electrode is the laser incident area, corresponding to the light-transmitting window of the cavity.

[0018] The high-voltage electrode is located above the ground electrode. The electrode spacing between the high-voltage electrode and the ground electrode is adjusted by a lead screw slide. The temperature control system is directly connected to the ground electrode. The TDC module is connected to the single-photon detector, which is set on the side wall of the gas chamber. The DC high-voltage source is connected to the high-voltage electrode. The oscilloscope is connected to the current monitoring circuit of the tungsten needle tip through a high-frequency current transformer and signal leads. The vacuum unit and gas chromatograph are respectively connected to the gas chamber.

[0019] As a preferred embodiment of the present invention, the high voltage electrode is an oxygen-free copper plate with a diameter between 80 and 120 mm and a thickness between 4 and 6 mm, the diameter of the laser incident area is 5 to 8 mm, and the surface roughness Ra≤0.03 μm;

[0020] The temperature control system includes a chiller cold head, four distributed thin-film heating elements, four PT1000 temperature sensing resistors, and a PID controller.

[0021] The grounding electrode is a hollow oxygen-free copper plate with a diameter between 80 and 120 mm and a thickness between 9 and 11 mm. The grounding electrode is connected to the cold head of the refrigerator and four distributed thin-film heating elements. Four PT1000 temperature measuring resistors are embedded radially along the grounding electrode with a spacing of 20 mm ± 1 mm and an accuracy of ± 0.1℃. The grounding electrode can achieve body temperature control from -80℃ to 200℃.

[0022] The electrode spacing is adjusted by a lead screw slide, with an adjustment range of 0.5~50mm, an adjustment accuracy of ±0.01mm, and a parallelism error of ≤0.01mm / m;

[0023] The tungsten needle tip has a purity of 99.99%, a tip curvature radius of 50μm±5μm, a height of 0.5mm±0.05mm, and a cone angle of 30°±2°.

[0024] The laser is a 355nm ultraviolet pulsed laser, equipped with a quartz lens and a neutral density filter. The laser energy is adjusted by the neutral density filter and then focused by the quartz lens set in the light transmission window of the gas cavity before entering the central circular area of ​​the high voltage electrode.

[0025] The single-photon detector uses an SNSPD detector, and the single-photon detector is connected to the TDC module via a coaxial cable.

[0026] As a preferred embodiment of the present invention, the outer surface of the grounding electrode is wrapped with a polyimide film of 0.2mm ± 0.02mm thickness, and the high-voltage electrode and the grounding electrode are insulated by a polytetrafluoroethylene support.

[0027] As a preferred embodiment of the present invention, in S2, the cooling power of the refrigerator cold head is ≥50W@-50℃, the surface power density of the thin film heating element is ≥25W / cm², and the temperature control stability is ≤±0.1℃ / h.

[0028] In a preferred embodiment of the present invention, in step S3, the laser emitted by the 355nm ultraviolet pulsed laser has its energy adjusted by a neutral density filter with an attenuation accuracy of ±2%, and is then focused by a quartz lens located in the light-transmitting window of the gas cavity before entering the central circular region of the high-voltage electrode. The energy density uniformity difference does not exceed ±5%. The initial electron density n0 of the emitted laser is calculated using the formula n0=E·η / (hν·V), where E is the laser energy, η is the gas photoionization efficiency, hν is the photon energy, V is the laser interaction volume, and the adjustment range of n0 is 10. 4 ~10 6 cm -3 .

[0029] As a preferred embodiment of the present invention, in S3, the number of photons and the photon arrival time are collected by a single photon detector, and the laser trigger signal t0 and the photon arrival time sequence are recorded synchronously by a TDC module to construct a photon number-time distribution curve N(t).

[0030] Electron collapse development speed v e Through formula v e =D / (t peak -t0) is calculated, where t peak Let N(t) be the peak time and D be the electrode spacing.

[0031] Calculate the dynamic ionization coefficient α(t,E1):

[0032] α(t,E1)=[1 / v e ]·[dN(t) / dt] / N(t);

[0033] In the formula, E1 is the average electric field between the electrodes, t represents time, and d is the differential symbol;

[0034] Take t peak The average value within a ±2 ns time window is used as the ionization coefficient α under E1.

[0035] As a preferred embodiment of the present invention, in S4, the voltage is boosted by 0.2kV / step ± 0.02kV / step, and the tip current is monitored by an oscilloscope with a sampling rate of 5GS / s and a bandwidth of 1GHz.

[0036] As a preferred embodiment of the present invention, in S5, the preset conditions are di / dt > 10A / μs and the current amplitude increases by ≥ 100 times, i is the needle tip current, t represents time, and d is the differential symbol;

[0037] When Z is set to 5, the standard deviation is ≤0.5kV when the measurement is repeated 5 times.

[0038] The beneficial effects of this invention are:

[0039] This invention significantly improves the gas ionization coefficient α and the streamer initiation voltage U over a wide temperature range. s Measurement performance: Employing a grounded electrode embedded cooling / heating structure and PID precise temperature control technology, it achieves high-precision temperature control within a wide temperature range of -80℃ to 200℃ with ±0.3℃, completely eliminating temperature gradient interference up to 8K in traditional methods, and improving electrode-gas interface temperature uniformity by 10 times; introducing 355nm ultraviolet laser for quantitative excitation of initial electrons with fluctuations of only ±2%, replacing traditional radioactive isotopes (fluctuations ±15%), and combining single-photon detection technology and a TDC module (time resolution ≥50ps), the lower limit of α measurement is lowered from 0.1cm.-1 Reduced to 0.01cm -1 It achieves accurate measurement of low α values ​​at -80℃ deep cryogenic temperature, and captures dynamic ionization coefficient curves at the 1ns level, overcoming the problems of weak time resolution and inability to capture transient characteristics of electron avalanche in traditional methods.

[0040] This invention boasts significant advantages in measurement reliability, anti-interference capabilities, and practicality. By embedding a tungsten needle tip into the grounding electrode to achieve local electric field distortion, it enhances the streamer initiation signal by 50 times. Combined with the dual criteria of "di / dt > 10 A / μs and a sudden increase in current amplitude ≥ 100 times," it further strengthens U... s Measurement error was reduced from the traditional 5% to 1%, enabling precise capture of the physical initiation voltage; the combination of laser quantitative initial electron and single-photon detection technology reduced the α deviation caused by space charge interference from 15% to 3%, and the influence of dark noise was effectively suppressed (dark count rate < 10 counts / s); the same system was used to coordinate the measurement of dynamic α and U. s The experimental efficiency is increased by 3 times, eliminating the need to build two additional sets of equipment. It is also suitable for the full-temperature performance evaluation and discharge mechanism research of insulating gases in power equipment, demonstrating significant application value. Attached Figure Description

[0041] Figure 1 This is a flowchart illustrating the principle of this invention;

[0042] Figure 2 This is a schematic diagram of the experimental system in this invention.

[0043] In the diagram, 1. Temperature control system; 2. Laser; 3. High voltage electrode; 4. Grounding electrode; 5. Tungsten needle tip; 6. Oscilloscope; 7. Single photon detector; 8. TDC module; 9. DC high voltage source; 10. Vacuum unit; 11. Gas chromatograph; 12. Quartz lens; 13. High frequency current transformer. Detailed Implementation

[0044] The embodiments of the present invention will be further described below with reference to the accompanying drawings:

[0045] Example 1: As Figure 1 As shown, a method for measuring the gas ionization coefficient and stream initiation voltage under wide temperature conditions includes the following steps:

[0046] S1. Construct an experimental system including an electrode system, a temperature control system 1, a laser 2, a single-photon detection and signal processing module, a streamer voltage measurement device, and auxiliary purification equipment. The electrode system includes a high-voltage electrode 3 and a grounding electrode 4 located in the gas chamber. The temperature control system 1 includes a refrigerator cold head, a thin-film heating element, a temperature measuring resistor, and a PID controller (adjustment cycle not exceeding 100ms).

[0047] S2. Based on the experimental system, for the low temperature range of greater than or equal to -80℃ and less than 50℃, the cold head of the refrigerator is started, and the heating element is driven by the PID controller to assist in temperature control. When the set temperature is reached and stabilized, the initial electron excitation is performed.

[0048] For the high-temperature range of 50℃ or higher and 200℃ or lower, the cold head of the refrigerator is turned off, and the thin-film heating element is independently temperature controlled (heating rate 5℃ / min ± 0.5℃ / min). When the set temperature is reached and stabilized, initial electron excitation is performed.

[0049] S3. Initial electronic excitation is performed using laser 2, the number of photons and the photon arrival time are collected, and the ionization coefficient is calculated.

[0050] S4. Embed a tungsten needle tip 5 in the center of the upper surface of the grounding electrode 4, turn off the laser 2, boost the voltage, and monitor the needle tip current through the oscilloscope 6.

[0051] S5. When the current derivative and current amplitude suddenly increase to meet the preset conditions, record the voltage at a certain temperature T as the streamer initiation voltage U. s (T), the average value of the measurement was taken after 5 repeated measurements as the final value of the streamer starting voltage.

[0052] like Figure 2 As shown, in S1, the single-photon detection and signal processing module includes a single-photon detector 7 and a TDC module 8; the streamer voltage measurement device includes a DC high-voltage source 9 and an oscilloscope 6; and the auxiliary purification equipment includes a vacuum unit 10 and a gas chromatograph 11.

[0053] A light-transmitting window is provided at the top of the gas cavity. The light-transmitting window is located above the high-voltage electrode 3. The laser 2 is located outside the gas cavity. The central circular area of ​​the high-voltage electrode 3 is the laser incident area, corresponding to the light-transmitting window of the cavity.

[0054] The high-voltage electrode 3 is located above the ground electrode 4. The electrode spacing between the high-voltage electrode 3 and the ground electrode 4 is adjusted by a lead screw slide. The temperature control system 1 is directly connected to the ground electrode 4. The TDC module 8 is connected to the single-photon detector 7. The single-photon detector 7 is set on the side wall of the gas chamber. The DC high-voltage source 9 is connected to the high-voltage electrode 3. The oscilloscope 6 is connected to the current monitoring circuit of the tungsten needle tip 5 through the high-frequency current transformer 13 and the signal lead (the circuit composition sequence is: tungsten needle tip 5 (generates current) → lead wire inside the ground electrode 4 → high-frequency current transformer 13 (series coupling signal) → signal lead → oscilloscope 6). The vacuum unit 10 and the gas chromatograph 11 are respectively connected to the gas chamber.

[0055] The high-voltage electrode 3 is made of oxygen-free copper plate with a diameter between 80 and 120 mm (preferably 100 mm ± 2 mm) and a thickness between 4 and 6 mm (preferably 5 mm ± 0.1 mm). The diameter of the laser incident area (central circular area) is 5 to 8 mm, and the surface roughness Ra ≤ 0.03 μm.

[0056] Temperature control system 1 includes a chiller cold head, four distributed thin-film heating elements, four PT1000 temperature sensing resistors, and a PID controller;

[0057] The grounding electrode 4 is a hollow oxygen-free copper plate with a diameter between 80 and 120 mm (preferably 100 mm ± 2 mm) and a thickness between 9 and 11 mm (preferably 10 mm ± 0.1 mm). The grounding electrode 4 is connected to the cold head of the refrigerator and four distributed thin-film heating elements (20 mm × 20 mm PI film, 25 W power per element). Four PT1000 temperature measuring resistors are embedded radially along the grounding electrode 4 with a spacing of 20 mm ± 1 mm, an accuracy of ± 0.1℃, and a response time of < 100 ms. The grounding electrode 4 can achieve body temperature control from -80℃ to 200℃ (accuracy ± 0.3℃).

[0058] The electrode spacing is adjusted by a precision lead screw slide, with an adjustment range of 0.5~50mm, an adjustment accuracy of ±0.01mm, and a parallelism error of ≤0.01mm / m;

[0059] The tungsten needle tip 5 has a purity of 99.99%, a tip curvature radius of 50μm±5μm, a height of 0.5mm±0.05mm, and a cone angle of 30°±2° (preferably 0.5mm in height and 28° in cone angle); a near-conical morphology of the needle tip is preferred. The tungsten needle tip 5 is connected to the grounding electrode 4 by silver soldering (soldering strength ≥5MPa), and the coaxiality error between the needle tip axis and the central hole axis of the high-voltage electrode 3 is ≤0.1mm.

[0060] Laser 2 is a 355nm ultraviolet pulsed laser 2 (pulse width 8ns±1ns, single pulse energy 1~50μJ adjustable, repetition frequency 1kHz±10Hz), equipped with quartz lens 12 (or collimator) and neutral density filter. The laser energy is adjusted by the neutral density filter, and then focused by the quartz lens 12 set in the light transmission window of the gas cavity and shot into the central circular area of ​​the high voltage electrode 3.

[0061] The single-photon detector 7 is an SNSPD detector, and the single-photon detector 7 is connected to the TDC module 8 via a 50Ω coaxial cable (length ≤ 2m).

[0062] The outer surface of the grounding electrode 4 is wrapped with a 0.2mm±0.02mm thick polyimide film (thermal conductivity 0.12W / (m·K)±0.01W / (m·K)). The high voltage electrode 3 and the grounding electrode 4 are insulated by a polytetrafluoroethylene support (breakdown field strength ≥20kV / mm).

[0063] Before the experiment, the gas chamber needs to be purified three times by vacuum unit 10 (ultimate vacuum ≤ 1 Pa, pumping speed ≥ 2 L / s) (each time the gas is filled to 0.1 MPa and then pumped to 1 Pa), and the gas purity is ≥ 99.999% (impurity content ≤ 5 ppm, detected by gas chromatograph 11).

[0064] In S2, the cooling power of the chiller cold head is ≥50W@-50℃, the surface power density of the thin film heating element is ≥25W / cm², and the temperature control stability is ≤±0.1℃ / h (within the range of -50℃ to 150℃).

[0065] In S3, the laser emitted by the 355nm ultraviolet pulsed laser 2 has its energy adjusted by a neutral density filter with an attenuation accuracy of ±2%, and then focused by a quartz lens 12 located in the light-transmitting window of the gas cavity before entering the central circular region of the high-voltage electrode 3. The energy density uniformity difference does not exceed ±5%. The initial electron density n0 of the emitted laser is calculated using the formula n0=E·η / (hν·V), where E is the laser energy, η is the gas photoionization efficiency, hν is the photon energy, and V is the laser interaction volume. The adjustment range of n0 is 10. 4 ~10 6 cm -3 .

[0066] The number of photons and photon arrival time are collected by single-photon detector 7, and the laser trigger signal t0 and the photon arrival time sequence (t1, t2, ..., t) are recorded synchronously by TDC module 8. n ), where n is the total number of effective photons captured by the single-photon detector 7 after a single laser trigger, and t is the total number of effective photons captured by the single-photon detector 7. n Let N(t) be the arrival time of the nth effective photon. Construct the photon number-time distribution curve N(t).

[0067] Electron collapse development speed v e Through formula v e =D / (t peak -t0) is calculated, where t peak Let N(t) be the peak time and D be the electrode spacing.

[0068] Calculate the dynamic ionization coefficient α(t,E1):

[0069] α(t,E1)=[1 / v e ]·[dN(t) / dt] / N(t);

[0070] In the formula, E1 is the average electric field between the electrodes, t represents time, and d is the differential symbol;

[0071] E1 can be calculated by E1=U / d1, where U is the applied voltage and d1 is the distance between the plates on the central axis.

[0072] Take t peak The average value within a ±2 ns time window is used as the ionization coefficient α under E1.

[0073] The calculation of α(t,E1) requires subtracting background noise, i.e., N(t) = N s (t)-N d , where N s (t) represents the measured number of photons, N d N is the dark count rate multiplied by the collection time. d ≤0.1 counts.

[0074] In S4, the voltage is boosted by 0.2kV / step ± 0.02kV / step, and the tip current is monitored by oscilloscope 6 with a sampling rate of 5GS / s and a bandwidth of 1GHz.

[0075] In S5, the preset conditions are di / dt > 10A / μs and the current amplitude increases by ≥ 100 times, where i is the needle tip current and t represents time.

[0076] When the measurement is repeated 5 times, the standard deviation should be ≤0.5kV (that is, the dispersion of the 5 sets of measurement data should be controlled within 0.5kV).

[0077] Taking the measurement of SF6 gas at -60℃ as an example, the following verification was performed:

[0078] Step 1: Configure the experimental system:

[0079] Electrode system: High voltage electrode 3 (oxygen-free copper, Φ100mm×5mm, Ra=0.03μm), ground electrode 4 (hollow oxygen-free copper, Φ100mm×10mm), spacing d=5mm±0.01mm, tungsten needle tip 5 (tungsten wire, radius 50μm±3μm, height 0.5mm, cone angle 28°).

[0080] Temperature control system 1: Refrigeration unit cold head (refrigeration power 55W@-50℃), thin film heating element (4 pieces of 20mm×20mm PI film, single piece power 25W), 4-channel PT1000 (accuracy ±0.05℃, response time less than 100ms);

[0081] Lasers and Detection: 2 355nm ultraviolet pulsed laser (8ns pulse width, 10μJ±0.5μJ energy), 7 superconducting nanowire single-photon detector (SNSPD, 2.1K cooling, dark count rate 0.3 counts / s, photon detection efficiency PDE=92%@800nm), 8 TDC module (1ps resolution);

[0082] Auxiliary equipment: DC high voltage source 9 (0~100kV, ripple ≤0.1%), vacuum unit 10 (ultimate vacuum 0.5Pa), gas chromatograph 11 (detection limit 1ppm).

[0083] Step two, calibrate the experimental system:

[0084] Temperature calibration: Immerse grounding electrode 4 in a precision constant temperature oil bath (-60℃). The PT1000 readings are -60.02℃, -60.05℃, 60.03℃, and -59.98℃. The calibration curve fitting error is ≤0.05℃.

[0085] Laser energy calibration: Laser energy was measured using an energy meter. At 10 μJ, the energy density uniformity was ±4.2%, corresponding to an initial electron density n0 = 5 × 10⁻⁶. 5 cm -3 (The photoionization efficiency of SF6 is η = 0.02%).

[0086] SNSPD efficiency calibration: The SNSPD detector was illuminated with a 355nm laser attenuated to the single-photon level, and the photon detection efficiency PDE was measured to be 35% ± 2% (at a bias voltage of 57.5V).

[0087] Step 3, Gas Preparation and Temperature Control:

[0088] Gas purification: Vacuum unit 10 is used to evacuate to 0.5 Pa, 99.999% SF6 gas is introduced to 0.1 MPa, and the cycle is repeated 3 times. Gas chromatograph 11 is used to detect H2O≤3ppm and O2≤2ppm.

[0089] Grounding electrode 4 temperature control: Set the target temperature to -60℃. After the cold head of the refrigerator runs for 5 minutes, the temperature stabilizes. The PT1000 reading fluctuates by ±0.05℃ / 10min, ensuring that the gas temperature is consistent with the electrode (verified by the ideal gas law: P / T=constant, where P is pressure and T is temperature, with a deviation of 0.3%).

[0090] Step 4, Measurement of ionization coefficient (α):

[0091] High voltage application and signal acquisition: A DC high voltage of U=35kV±0.1kV is applied through DC high voltage source 9 to synchronously trigger the laser (repetition frequency 1kHz). TDC module 8 acquires 1000 photon time series, with a single acquisition duration of 100ns.

[0092] Perform data processing:

[0093] The main peak of the photon number-time distribution N(t) is located at t. peak =25ns (t0=5ns, laser triggering time), FWHM=3.5ns;

[0094] Electron avalanche velocity v e =d / (t peak -t0)=5mm / (20ns)=2.5×10 5 m / s ± 0.1 × 10 5 m / s;

[0095] The average electric field between the electrodes is E1 = U / d1 = 35kV / (5mm) = 7kV / mm ± 0.03kV / mm;

[0096] The photon number derivative dN(t) / dt reaches a peak value of 1.2 × 10⁻⁶ at t = 25 ns. 4 counts / ns, N(t) = 3 × 10 4 Use counts to calculate the ionization coefficient under this electric field (E1 = 7 kV / mm ± 0.03 kV / mm):

[0097] α(t,E1)=[1 / v e ]·[dN(t) / dt] / N(t)=(1 / 2.5×10 5 )·(1.2×10 4 / 3×10 4 =1.6cm -1 ±0.05cm -1 ;

[0098] Under this electric field (E1=7kV / mm±0.03kV / mm), the traditional current method measures α=1.55cm. -1 (3.2% deviation) The single-photon method is closer to the true value because it has no space charge interference.

[0099] Step 5, Streamer initiation voltage (U s )Measurement:

[0100] Voltage boost and current monitoring: A tungsten needle tip 5 is embedded in the center of the upper surface of grounding electrode 4, and the laser is turned off. The voltage is boosted at 0.2 kV / step. At a temperature of -60℃, when U = 48.3 kV, the oscilloscope 6 (bandwidth 1 GHz) captures the current waveform: di / dt = 12 A / μs, with the current amplitude increasing from 8.5 × 10⁻⁶. -12 A suddenly increased to 1.1 × 10 -9 A (increase of 129 times);

[0101] U s Confirmation: Repeat the measurement 5 times, U sThe values ​​at -60℃ are 48.3kV, 48.1kV, 48.5kV, 48.2kV, and 48.0kV, respectively, with an average of 48.2kV and a standard deviation of 0.2kV (0.4%). The traditional current criterion (breakdown voltage) is 49.5kV (2.7%).

[0102] Verification results and analysis:

[0103] Temperature characteristic of α coefficient (E1=7kV / mm): α=1.6cm at -60℃. -1 At 25℃, α = 0.95cm -1 (Literature value 0.93cm) -1 (deviation 2.1%), α = 0.72 cm at 150℃. -1 This conforms to the rule that the ionization coefficient of SF6 decreases with increasing temperature;

[0104] U s Temperature characteristics: U at -60℃ s =48.2kV, U at 25℃ s =42.5kV, U at 150℃ s =38.1kV, with a deviation from the theoretical calculation value of ≤1.5%;

[0105] Advantages of the single-photon method: at α=0.08cm -1 At -80℃, E1=4kV / mm, the traditional current method has an error greater than 30%, while the single-photon method has an error of only 4.8% (N(t)=300counts, dark count N). d =0.05 counts).

[0106] This verification example demonstrates that the measurement method based on single-photon detection in this embodiment can effectively measure SF6 gas α (E1=7kV / mm) and U at -60℃. s High-precision measurement capability, α measurement error ≤5%, U s The measurement error is ≤1%, which is significantly better than traditional methods and can effectively support the study of gas insulation performance over a wide temperature range.

[0107] Example 2: The difference between this example and Example 1 is that the single-photon detector 7 adopts a fast silicon photomultiplier with an operating temperature of -80℃ to 150℃, an average dark count rate of <10 counts / s, a photon detection efficiency of >40%@550nm, and is equipped with a focusing system (numerical aperture 0.6±0.05, collection solid angle >2π / 3).

[0108] Example 3: Based on Example 1 or Example 2, when calculating α, an adaptive time window is used instead of a fixed time window. The specific steps are as follows:

[0109] Based on the laser trigger signal t0 and photon arrival time sequence recorded by TDC module 8, the photon number-time distribution curve N(t) is constructed, and the full width at half maximum (FWHM) of N(t) is calculated by Gaussian fitting algorithm.

[0110] With the peak time t of N(t) peak Centered on the window, an adaptive time window is determined according to the window width W1 = k × FWHM, where k is an adjustment coefficient ranging from 1.2 to 1.5, and the value of k is dynamically calibrated in the following way:

[0111] When the measured temperature T ≤ -40℃, k = 1.5; when -40℃ < T < 50℃, k = 1.3; when T ≥ 150℃, k = 1.2.

[0112] Within the adaptive time window, according to the formula α(t,E1)=[1 / v e The dynamic ionization coefficient at each time step is calculated using [dN(t) / dt] / N(t). Then, the weighted average of all dynamic ionization coefficients within the window is taken as the final ionization coefficient α under E1. The weighting is the proportion of N(t) at each time step to the total N(t) within the window.

[0113] By dynamically matching the half-width at half-maximum (WHM) time window width and combining it with the temperature-segmented calibration adjustment coefficient, a precise adaptation between the time window and the dynamic development characteristics of electron avalanche is achieved. Within the full temperature range of -80℃ to 200℃, the relative error of ionization coefficient measurement can be reduced, and the measurement stability in the low-temperature and high-temperature ranges can be improved.

Claims

1. A method for measuring the gas ionization coefficient and streamer initiation voltage under wide temperature conditions, characterized in that... Includes the following steps: S1. Construct an experimental system including an electrode system, a temperature control system, a laser, a single-photon detection and signal processing module, a streamer voltage measurement device, and auxiliary purification equipment. The electrode system includes a high-voltage electrode and a grounding electrode located in the gas chamber. The temperature control system includes a refrigerator cold head, a thin-film heating element, a temperature measuring resistor, and a PID controller. The single-photon detection and signal processing module includes a single-photon detector and a TDC module; the streamer voltage measurement equipment includes a DC high-voltage source and an oscilloscope; and the auxiliary purification equipment includes a vacuum unit and a gas chromatograph. A light-transmitting window is provided at the top of the gas cavity. The light-transmitting window is located above the high-voltage electrode. The laser is located outside the gas cavity. The central circular area of ​​the high-voltage electrode is the laser incident area, corresponding to the light-transmitting window of the cavity. The high-voltage electrode is located above the ground electrode. The electrode spacing between the high-voltage electrode and the ground electrode is adjusted by a lead screw slide. The temperature control system is directly connected to the ground electrode. The TDC module is connected to the single-photon detector, which is set on the side wall of the gas chamber. The DC high-voltage source is connected to the high-voltage electrode. The oscilloscope is connected to the current monitoring circuit of the tungsten needle tip through a high-frequency current transformer and signal leads. The vacuum unit and gas chromatograph are respectively connected to the gas chamber. S2. Based on the experimental system, for the low temperature range of greater than or equal to -80℃ and less than 50℃, the cold head of the refrigerator is started, and the heating element is driven by the PID controller to assist in temperature control. When the set temperature is reached and stabilized, the initial electron excitation is performed. For the high-temperature range of 50°C or higher and 200°C or lower, the cold head of the refrigerator is turned off, the thin-film heating element is independently temperature controlled, and initial electron excitation is performed when the set temperature is reached and stabilized. S3. Initial electronic excitation is performed using a laser, the number of photons and the photon arrival time are collected, and the ionization coefficient is calculated. S4. Embed a tungsten needle tip in the center of the upper surface of the grounding electrode, turn off the laser, boost the voltage, and monitor the needle tip current; S5. When the current derivative and current amplitude suddenly increase to meet the preset conditions, record the voltage at a certain temperature T as the streamer initiation voltage U. s (T), the average value of the measurement is taken after Z repeated measurements as the final value of the streamer starting voltage.

2. The method for measuring gas ionization coefficient and streamer initiation voltage under wide temperature conditions according to claim 1, characterized in that, The high-voltage electrode uses an oxygen-free copper plate with a diameter between 80 and 120 mm and a thickness between 4 and 6 mm. The diameter of the laser incident area is 5 to 8 mm, and the surface roughness Ra ≤ 0.03 μm. The temperature control system includes a chiller cold head, four distributed thin-film heating elements, four PT1000 temperature sensing resistors, and a PID controller. The grounding electrode is a hollow oxygen-free copper plate with a diameter between 80 and 120 mm and a thickness between 9 and 11 mm. The grounding electrode is connected to the cold head of the refrigerator and 4 distributed thin film heating plates. 4 PT1000 temperature measuring resistors are embedded radially along the grounding electrode with a spacing of 20 mm ± 1 mm and an accuracy of ± 0.1℃. The electrode spacing is adjusted by a lead screw slide, with an adjustment range of 0.5~50mm, an adjustment accuracy of ±0.01mm, and a parallelism error of ≤0.01mm / m; The tungsten needle tip has a purity of 99.99%, a tip curvature radius of 50μm±5μm, a height of 0.5mm±0.05mm, and a cone angle of 30°±2°. The laser is a 355nm ultraviolet pulsed laser, equipped with a quartz lens and a neutral density filter. The laser energy is adjusted by the neutral density filter and then focused by the quartz lens set in the light transmission window of the gas cavity before entering the central circular area of ​​the high voltage electrode. The single-photon detector uses an SNSPD detector, and the single-photon detector is connected to the TDC module via a coaxial cable.

3. The method for measuring gas ionization coefficient and streamer initiation voltage under wide temperature conditions according to claim 1, characterized in that, The outer surface of the grounding electrode is wrapped with a 0.2mm±0.02mm thick polyimide film, and the high-voltage electrode is insulated from the grounding electrode by a polytetrafluoroethylene support.

4. The method for measuring gas ionization coefficient and streamer initiation voltage under wide temperature conditions according to claim 1, characterized in that, In S2, the cooling power of the refrigerator cold head is ≥50W@-50℃, the surface power density of the thin film heating element is ≥25W / cm², and the temperature control stability is ≤±0.1℃ / h.

5. The method for measuring gas ionization coefficient and streamer initiation voltage under wide temperature conditions according to claim 2, characterized in that, In S3, the laser emitted by the 355nm ultraviolet pulsed laser has its energy adjusted by a neutral density filter with an attenuation accuracy of ±2%, and then focused by a quartz lens placed in the light-transmitting window of the gas cavity before entering the central circular region of the high-voltage electrode. The energy density uniformity difference does not exceed ±5%. The initial electron density n0 of the emitted laser is calculated using the formula n0=E·η / (hν·V), where E is the laser energy, η is the gas photoionization efficiency, hν is the photon energy, and V is the laser interaction volume. The adjustment range of n0 is 10. 4 ~10 6 cm -3 .

6. The method for measuring gas ionization coefficient and streamer initiation voltage under wide temperature conditions according to claim 1, characterized in that, In S3, the number of photons and the photon arrival time are collected by a single photon detector, and the laser trigger signal t0 and the photon arrival time sequence are recorded synchronously by the TDC module to construct the photon number-time distribution curve N(t); Electron collapse development speed v e Through formula v e =D / (t peak -t0) is calculated, where t peak Let N(t) be the peak time and D be the electrode spacing. Calculate the dynamic ionization coefficient α(t,E1): α(t,E1)=[1 / v e ]·[dN(t) / dt] / N(t); In the formula, E1 is the average electric field between the electrodes, t represents time, and d is the differential symbol; Take t peak The average value within a ±2 ns time window is used as the ionization coefficient α under E1.

7. The method for measuring gas ionization coefficient and streamer initiation voltage under wide temperature conditions according to claim 1, characterized in that, In S4, the voltage is boosted by 0.2kV / step ± 0.02kV / step, and the tip current is monitored by an oscilloscope with a sampling rate of 5GS / s and a bandwidth of 1GHz.

8. The method for measuring gas ionization coefficient and streamer initiation voltage under wide temperature conditions according to claim 1, characterized in that, In S5, the preset conditions are di / dt > 10A / μs and the current amplitude increases by ≥ 100 times, where i is the needle tip current, t represents time, and d is the differential symbol. When Z is set to 5, the standard deviation is ≤0.5kV when the measurement is repeated 5 times.

Images