An ultra-short cable ultra-low frequency dielectric loss measurement platform and method

Abstract

The present application relates to a kind of super short cable ultra-low frequency dielectric loss measurement platform, including ultra-low frequency high voltage power supply, transmission cable, cable to be measured, voltage acquisition module, current acquisition module, dielectric loss measurement module and calibration module.The present application also relates to a kind of super short cable ultra-low frequency dielectric loss measurement method, including S1, error calibration;S2, dielectric loss measurement.The present application is scientifically reasonable, can be realized at lower voltage grade to high voltage cable insulation ultra-low frequency dielectric loss measurement, to complete the state evaluation of its cable insulation.Compared with existing complete ultra-low frequency dielectric loss measurement device, the present application fills the blank of ultra-low frequency dielectric loss measurement equipment in short sample cable field, can be completed to the collection of very small resistive leakage current, shielding external high voltage interference.

Description

Technical Field

[0001] This invention belongs to the field of cable safety operation technology, specifically relating to a platform and method for measuring the dielectric loss of ultra-short cables at ultra-low frequencies. Background Technology

[0002] With the continuous development of my country's economy and the increasing size of urban areas, the construction of power facilities is also ongoing. Power cables are a crucial factor determining the safety and stability of the power system. Cross-linked polyethylene (XLPE) insulated power cables were invented by General Electric Company in the United States around the 1950s. Compared with traditional cables, XLPE insulated power cables have the following advantages: 1) Good electrical performance (high breakdown electric field strength, strong dielectric properties, and high insulation resistance); 2) Good thermal and mechanical properties, high allowable operating temperature, and large current carrying capacity; 3) Convenient installation and maintenance, simple structure, and low power loss; 4) Not only can they be widely used in medium and low voltage power transmission and distribution networks, but they are also suitable for high voltage and ultra-high voltage power transmission systems.

[0003] Cables operate under conditions of strong electric fields, high temperatures, and humidity for extended periods. The combined effects of electrical, thermal, mechanical, and chemical factors cause physical and chemical changes in the insulation, accelerating its deterioration and ultimately leading to cable breakdown and insulation failure. Thermal aging is a key factor influencing the aging state of cables. Many scholars have conducted accelerated thermal aging tests to study the aging state of insulation; however, most studies rely on the characteristics of test results from a single diagnostic method to determine the cable's condition, and these results are often affected by various factors. Therefore, it is necessary to conduct accelerated thermal aging tests on cable insulation and use multiple characteristic parameters to comprehensively evaluate the cable's condition, which is of great significance for improving the safe and reliable operation of power systems.

[0004] Commonly used methods for testing the operating status of cables include non-electrical parameter methods and electrical parameter methods. Non-electrical parameter methods diagnose the operating status by testing the physical and chemical properties of the cable, and are mainly used for overall aging life assessment of the cable, such as testing the elongation at break (EAB) and compression modulus of the cable material. Electrical parameter testing methods for cables mainly include measuring cable insulation resistance, withstand voltage testing, leakage current testing, and dielectric loss detection.

[0005] However, the aforementioned electrical methods are currently more commonly used in distribution network cables, while their application in high-voltage cables is insufficient. In recent years, the ultra-low frequency dielectric loss measurement method has been widely promoted in the testing of distribution network cables nationwide due to its advantages such as miniaturization and portability, thus facilitating the daily operation and maintenance of cables with voltage levels of 35kV and below in my country.

[0006] However, in actual operation, it has been found that the current sampling method of ultra-low frequency dielectric loss measurement equipment is to sample the current and voltage on the high-voltage side, and there are clear requirements for the minimum length (and minimum load capacitance value) of the cable under test. The minimum measurement length of the mainstream equipment is about 50m. When using ultra-low frequency dielectric loss to measure shorter cable samples (less than 10m), the following common problems exist: (1) The sample capacitance value is insufficient, the ultra-low frequency voltage waveform cannot work normally, and the output voltage waveform has a large error compared with the standard sine wave; (2) During the measurement process, because the resistive current ratio in the current component is extremely low, it is easily affected by the noise of the ultra-low frequency high-voltage power supply, resulting in a large measurement error.

[0007] Therefore, in order to meet the requirements for ultra-low frequency dielectric loss measurement of short-sample cables in cable condition assessment, an ultra-low frequency dielectric loss measurement platform needs to be developed to address the above issues and realize ultra-low frequency dielectric loss measurement for cable samples with a minimum length of 1m. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide an ultra-short cable ultra-low frequency dielectric loss measurement platform and method, which is suitable for ultra-low frequency dielectric loss measurement of short-length cables. It can be used to measure short cable samples under laboratory or field conditions. Moreover, the measurement platform is easy to operate, has a short measurement cycle, and effectively improves the ability to evaluate the insulation status of cables.

[0009] The technical problem solved by this invention is achieved through the following technical solution:

[0010] A platform for measuring the dielectric loss of an ultra-short cable at ultra-low frequency is characterized by comprising an ultra-low frequency high-voltage power supply, a transmission cable, a cable under test, a voltage acquisition module, a current acquisition module, a dielectric loss measurement module, and a calibration module. The ultra-low frequency high-voltage power supply, transmission cable, and cable under test are connected sequentially. The cable under test is externally shielded, with a first shield electrode and a second shield electrode short-circuited on both sides of the cable under test, and short-circuited with the shielding shell. The current acquisition module includes a current sampling resistor, a current signal voltage follower, a current signal low-pass filter, and a current signal high-pass filter connected sequentially. The current sampling resistor is connected to the cable under test via switch K1. The voltage acquisition module includes a voltage sampling resistor R1, a voltage sampling resistor R2, a voltage signal voltage follower, a voltage signal low-pass filter, and a voltage signal high-pass filter. The voltage sampling resistors R1 and R2 are connected in parallel and then in series. The secondary connection includes a voltage follower, a low-pass filter, and a high-pass filter. The voltage sampling resistor R1 is connected to the first shielding electrode of the cable under test via switch K2. The dielectric loss measurement module includes a current signal zero-crossing comparator, a voltage signal zero-crossing comparator, an AND gate, and a first oscilloscope. One end of the current signal zero-crossing comparator is connected to the current signal high-pass filter, and the other end is connected to the AND gate via switch K3. One end of the voltage signal zero-crossing comparator is connected to the voltage signal high-pass filter, and the other end is connected to the AND gate via switch K4. The AND gate is connected to the first oscilloscope. The calibration module includes a signal generator and a second oscilloscope. The signal generator is connected to the cable under test via switch K1, and the second oscilloscope is connected to the current signal zero-crossing comparator and the voltage signal zero-crossing comparator via switches K3 and K4, respectively.

[0011] Furthermore, the ultra-low frequency high voltage power supply has a frequency of 0.01 to 0.1 Hz, a voltage of 0 to 80 kV, and a maximum load current of 40 mA.

[0012] Moreover, the transmission cable is a 50m long, 35kV power cable with a maximum withstand voltage of over 80kV.

[0013] Furthermore, the length of the cable under test is 1 to 50 m, and the shielding shell is made of aluminum foil, copper foil, or stainless steel.

[0014] Furthermore, the current sampling resistor is a non-inductive resistor with a resistance of less than 1 MΩ, the current signal voltage follower uses an integrated operational amplifier with an input resistance greater than 1 GΩ, the upper cutoff frequency of the current signal low-pass filter is 0.5 Hz, and the lower cutoff frequency of the current signal high-pass filter is 0.005 Hz.

[0015] Furthermore, the voltage sampling resistor R1 is a non-inductive resistor with a resistance of 1 GΩ, the voltage sampling resistor R2 is a non-inductive resistor with a resistance of 1 MΩ, the voltage signal voltage follower uses an integrated operational amplifier with an input resistance greater than 1 GΩ, the upper cutoff frequency of the voltage signal low-pass filter is 0.5 Hz, and the lower cutoff frequency of the voltage signal high-pass filter is 0.005 Hz.

[0016] Moreover, the response times of the current signal voltage follower, voltage signal voltage follower, voltage signal zero-crossing comparator, current signal zero-crossing comparator, and AND gate are all less than 10ns.

[0017] A method for measuring the dielectric loss of ultra-short cables at ultra-low frequencies, characterized in that the method comprises the following steps:

[0018] S1, Error Calibration: Set switch K1 to 1, switch K2 to 1', switch K3 to 4, and switch K4 to 4' to turn on the signal generator to generate a sine wave signal from 0.1Hz to 0.01Hz. The signal generator is simultaneously connected to the current acquisition module and the voltage acquisition module. The rectangular waves output by the zero-crossing comparator of the current signal and the zero-crossing comparator of the voltage signal are connected to the second oscilloscope, and the rise time difference Δt of the rectangular waves is observed on the second oscilloscope.

[0019] S2, Dielectric Loss Measurement: Set switch K1 to 2, K2 to 2', K3 to 3, and K4 to 3' to enter the dielectric loss measurement mode. The current signal and voltage signal first enter the current signal zero-crossing comparator and the voltage signal zero-crossing comparator respectively to convert the sine wave signal into a rectangular wave digital signal. Then, the two rectangular wave signals are connected to the two input terminals of the AND gate to generate a level comparison signal, which is displayed on the first oscilloscope.

[0020] Due to the presence of capacitive current in the cable under test, the current signal leads the voltage signal. Based on the signal observed on the first oscilloscope, the half-cycle duration of the ultra-low frequency signal is T, the high-level duration of the rectangular wave output by the AND gate is t, and the time the current signal leads the voltage signal is Tt.

[0021] Therefore, the original dielectric loss angle in radians is calculated as follows:

[0022]

[0023] After considering calibration errors, the dielectric loss angle in radians is corrected as follows:

[0024]

[0025] Then the dielectric loss tangent can be further calculated as tgδ.

[0026] The advantages and beneficial effects of this invention are as follows:

[0027] 1. The ultra-short cable ultra-low frequency dielectric loss measurement platform of the present invention is easy to operate and can complete the measurement of short sample cables in a short time.

[0028] 2. The ultra-short cable ultra-low frequency dielectric loss measurement platform of the present invention overcomes the voltage waveform error caused by the short length of the cable under test and effectively avoids the interference of ultra-low frequency high voltage power supply on weak current signals.

[0029] 3. This invention can perform ultra-low frequency dielectric loss measurement on ultra-short cables and can also perform ultra-low frequency dielectric loss measurement on the insulation of high-voltage cables at lower voltage levels, thereby completing the condition assessment of the cable insulation. Compared with existing complete sets of ultra-low frequency dielectric loss measurement devices, this invention fills the gap in ultra-low frequency dielectric loss measurement equipment in the field of short cables, and can complete the acquisition of extremely small resistive leakage current while shielding external high-voltage interference. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the structure of the present invention;

[0031] Figure 2 This is a schematic diagram of the error calibration of the present invention;

[0032] Figure 3 This is a schematic diagram of dielectric loss measurement according to the present invention;

[0033] Figure 4 This is a diagram of the AND gate output signal of the present invention;

[0034] Figure 5 The voltage and current time-domain waveforms of this invention are shown. Detailed Implementation

[0035] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0036] like Figure 1 As shown, an ultra-short cable ultra-low frequency dielectric loss measurement platform is innovative in that it includes an ultra-low frequency high voltage power supply, a transmission cable, a cable under test, a voltage acquisition module, a current acquisition module, a dielectric loss measurement module, and a calibration module, with the ultra-low frequency high voltage power supply, transmission cable, and cable under test connected in sequence.

[0037] The ultra-low frequency high voltage power supply has a frequency of 0.01 to 0.1 Hz, a voltage of 0 to 80 kV, and a maximum load current of 40 mA.

[0038] The transmission cable is a 50m long, 35kV power cable with a maximum withstand voltage of over 80kV.

[0039] The cable under test is equipped with a shielding shell. The first shielding electrode and the second shielding electrode on both sides of the cable under test are short-circuited and short-circuited with the shielding shell. The two ends of the cable under test need to be treated as follows: remove the sheath and outer semiconductive layer on both sides to expose the main insulation, with an exposed length of not less than 15cm; then wrap copper foil tape around the outer semiconductive layer, and use a wire to short-circuit the first shielding electrode and the second shielding electrode on both sides and short-circuit with the shielding shell, and then ground.

[0040] The length of the cable being tested is 1 to 50 meters, and the shielding shell is made of aluminum foil, copper foil, or stainless steel.

[0041] The current acquisition module includes a current sampling resistor, a current signal voltage follower, a current signal low-pass filter, and a current signal high-pass filter connected in sequence. The current sampling resistor is connected to the cable under test through switch K1.

[0042] The current sampling resistor is a non-inductive resistor with a resistance of less than 1M ohms. The current signal voltage follower uses an integrated operational amplifier with an input resistance of greater than 1G ohms. The upper cutoff frequency of the current signal low-pass filter is 0.5Hz, and the lower cutoff frequency of the current signal high-pass filter is 0.005Hz.

[0043] The voltage acquisition module includes a voltage sampling resistor R1, a voltage sampling resistor R2, a voltage signal voltage follower, a voltage signal low-pass filter, and a voltage signal high-pass filter. The voltage sampling resistors R1 and R2 are connected in parallel and then connected in series to the voltage signal voltage follower, the voltage signal low-pass filter, and the voltage signal high-pass filter. The voltage sampling resistor R1 is connected to the first shielding electrode of the cable under test through switch K2.

[0044] The dielectric loss measurement module includes a current signal zero-crossing comparator, a voltage signal zero-crossing comparator, an AND gate, and a first oscilloscope. One end of the current signal zero-crossing comparator is connected to the current signal high-pass filter, and the other end is connected to the AND gate through switch K3. One end of the voltage signal zero-crossing comparator is connected to the voltage signal high-pass filter, and the other end is connected to the AND gate through switch K4. The AND gate is connected to the first oscilloscope.

[0045] The calibration module includes a signal generator and a second oscilloscope. The signal generator is connected to the cable under test via switch K1, and the second oscilloscope is connected to the zero-crossing comparator of the current signal and the zero-crossing comparator of the voltage signal via switches K3 and K4, respectively.

[0046] The voltage sampling resistor R1 is a non-inductive resistor with a resistance of 1 GΩ, the voltage sampling resistor R2 is a non-inductive resistor with a resistance of 1 MΩ, the voltage signal voltage follower uses an integrated operational amplifier with an input resistance greater than 1 GΩ, the upper cutoff frequency of the voltage signal low-pass filter is 0.5 Hz, and the lower cutoff frequency of the voltage signal high-pass filter is 0.005 Hz.

[0047] The response times of the current signal voltage follower, voltage signal voltage follower, voltage signal zero-crossing comparator, current signal zero-crossing comparator, and AND gate are all less than 10ns.

[0048] The ultra-low frequency high voltage power supply, transmission cable, cable under test, current signal voltage follower, current signal low-pass filter, current signal high-pass filter, current signal zero-crossing comparator, voltage signal voltage follower, voltage signal low-pass filter, voltage signal high-pass filter, voltage signal zero-crossing comparator, and AND gate are all grounded.

[0049] An innovative method for measuring the dielectric loss of ultra-short cables at ultra-low frequencies is described in the following steps:

[0050] S1, Error Calibration: (e.g.) Figure 2 As shown, switch K1 is set to 1, K2 to 1', K3 to 4, and K4 to 4'. The signal generator is turned on to generate a sine wave signal from 0.1Hz to 0.01Hz. The signal generator is connected to both the current acquisition module and the voltage acquisition module. The rectangular waves output by the zero-crossing comparator of the current signal and the zero-crossing comparator of the voltage signal are connected to the second oscilloscope, and the rise time difference Δt of the rectangular waves is observed on the second oscilloscope.

[0051] S2, Dielectric loss measurement: such as Figure 3 As shown, switch K1 to 2, K2 to 2', K3 to 3, and K4 to 3' to enter the dielectric loss measurement mode. The current and voltage signals first enter the current signal zero-crossing comparator and the voltage signal zero-crossing comparator, respectively, to convert the sine wave signal into a rectangular wave digital signal. Then, the two rectangular wave signals are connected to the two inputs of an AND gate to generate a level comparison signal, which is displayed on the first oscilloscope. Figure 4 As shown;

[0052] The measured voltage and current signals are as follows Figure 5 As shown, this signal finally enters the corresponding signal processing circuit, resulting in the corresponding rectangular or square wave output signal. Figure 5 As shown, 10 cycles of signal can be acquired at once, resulting in 10 rectangular wave waveforms. After connecting the rectangular wave signal to an oscilloscope, the widths t1 to t10 of each rectangular wave can be calculated, and then their average value t can be obtained.

[0053] Due to the presence of capacitive current in the cable under test, the current signal leads the voltage signal. Based on the signal observed on the first oscilloscope, the half-cycle duration of the ultra-low frequency signal is T, the high-level duration of the rectangular wave output by the AND gate is t, and the time the current signal leads the voltage signal is Tt.

[0054] Therefore, the original dielectric loss angle in radians is calculated as follows:

[0055]

[0056] After considering calibration errors, the dielectric loss angle in radians is corrected as follows:

[0057]

[0058] Then the dielectric loss tangent can be further calculated as tgδ.

[0059] Although embodiments and drawings of the present invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A platform for measuring the dielectric loss of ultra-short cables at ultra-low frequencies, characterized in that: The system includes an ultra-low frequency high-voltage power supply, a transmission cable, a cable under test, a voltage acquisition module, a current acquisition module, a dielectric loss measurement module, and a calibration module. The ultra-low frequency high-voltage power supply, transmission cable, and cable under test are connected sequentially. The cable under test is externally shielded, with the first and second shielding electrodes on both sides of the cable short-circuited and short-circuited to the shielding shell. The current acquisition module includes a current sampling resistor, a current signal voltage follower, a current signal low-pass filter, and a current signal high-pass filter connected sequentially. The current sampling resistor is connected to the cable under test via switch K1. The voltage acquisition module includes a voltage sampling resistor R1, a voltage sampling resistor R2, a voltage signal voltage follower, a voltage signal low-pass filter, and a voltage signal high-pass filter. The voltage sampling resistors R1 and R2 are connected in parallel and then in series with the voltage signal voltage follower. The test module includes a voltage signal low-pass filter and a voltage signal high-pass filter. The voltage sampling resistor R1 is connected to the first shielding electrode of the cable under test through switch K2. The dielectric loss measurement module includes a current signal zero-crossing comparator, a voltage signal zero-crossing comparator, an AND gate, and a first oscilloscope. One end of the current signal zero-crossing comparator is connected to the current signal high-pass filter, and the other end is connected to the AND gate through switch K3. One end of the voltage signal zero-crossing comparator is connected to the voltage signal high-pass filter, and the other end is connected to the AND gate through switch K4. The AND gate is connected to the first oscilloscope. The calibration module includes a signal generator and a second oscilloscope. The signal generator is connected to the cable under test through switch K1, and the second oscilloscope is connected to the current signal zero-crossing comparator and the voltage signal zero-crossing comparator through switches K3 and K4, respectively.

2. The ultra-short cable ultra-low frequency dielectric loss measurement platform according to claim 1, characterized in that: The ultra-low frequency high voltage power supply has a frequency of 0.01 to 0.1 Hz, a voltage of 0 to 80 kV, and a maximum load current of 40 mA.

3. The ultra-short cable ultra-low frequency dielectric loss measurement platform according to claim 1, characterized in that: The transmission cable is a 50m long, 35kV power cable with a maximum withstand voltage of over 80kV.

4. The ultra-short cable ultra-low frequency dielectric loss measurement platform according to claim 1, characterized in that: The length of the cable under test is 1 to 50 m, and the shielding shell is made of aluminum foil, copper foil or stainless steel.

5. The ultra-short cable ultra-low frequency dielectric loss measurement platform according to claim 1, characterized in that: The current sampling resistor is a non-inductive resistor with a resistance of less than 1 MΩ, the current signal voltage follower uses an integrated operational amplifier with an input resistance of greater than 1 GΩ, the upper cutoff frequency of the current signal low-pass filter is 0.5 Hz, and the lower cutoff frequency of the current signal high-pass filter is 0.005 Hz.

6. The ultra-short cable ultra-low frequency dielectric loss measurement platform according to claim 1, characterized in that: The voltage sampling resistor R1 is a non-inductive resistor with a resistance of 1 GΩ, the voltage sampling resistor R2 is a non-inductive resistor with a resistance of 1 MΩ, the voltage signal voltage follower uses an integrated operational amplifier with an input resistance greater than 1 GΩ, the upper cutoff frequency of the voltage signal low-pass filter is 0.5 Hz, and the lower cutoff frequency of the voltage signal high-pass filter is 0.005 Hz.

7. The ultra-short cable ultra-low frequency dielectric loss measurement platform according to claim 1, characterized in that: The response time of the current signal voltage follower, voltage signal voltage follower, voltage signal zero-crossing comparator, current signal zero-crossing comparator, and AND gate is all less than 10 ns.

8. A measurement method for an ultra-short cable ultra-low frequency dielectric loss measurement platform as described in claims 1 to 7, characterized in that: The steps of the method are as follows: S1, Error Calibration: Set switch K1 to 1, switch K2 to 1', switch K3 to 4, and switch K4 to 4' to turn on the signal generator to generate a sine wave signal from 0.1Hz to 0.01Hz. The signal generator is simultaneously connected to the current acquisition module and the voltage acquisition module. The rectangular waves output by the zero-crossing comparator of the current signal and the zero-crossing comparator of the voltage signal are connected to the second oscilloscope, and the rise time difference Δt of the rectangular waves is observed on the second oscilloscope. S2, Dielectric Loss Measurement: Set switch K1 to 2, K2 to 2', K3 to 3, and K4 to 3' to enter the dielectric loss measurement mode. The current signal and voltage signal first enter the current signal zero-crossing comparator and the voltage signal zero-crossing comparator respectively to convert the sine wave signal into a rectangular wave digital signal. Then, the two rectangular wave signals are connected to the two input terminals of the AND gate to generate a level comparison signal, which is displayed on the first oscilloscope. Due to the presence of capacitive current in the cable under test, the current signal leads the voltage signal. Based on the signal observed on the first oscilloscope, the half-cycle duration of the ultra-low frequency signal is T, the high-level duration of the rectangular wave output by the AND gate is t, and the time the current signal leads the voltage signal is Tt. Therefore, the original dielectric loss angle in radians is calculated as follows: After considering calibration errors, the dielectric loss angle in radians is corrected as follows: Then the dielectric loss tangent can be further calculated as tgδ.

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