A cable partial discharge sensor and monitoring device based on multi-stage resonance

Abstract

The present invention provides a cable partial discharge sensor and monitoring device based on multi-stage resonance. In the design of the cable partial discharge detection sensor, the present invention adds series inductance / parallel inductance to form a multi-stage resonant circuit, improves the capacitance type sensor transmission ratio, expands the signal pulse width, and increases the sensor sensitivity; in the acquisition circuit design, since the sensor resonance expands the signal pulse width, the detector can be used to reduce the device sampling rate requirement, thereby effectively controlling the overall power consumption and cost of the device, and since the device power consumption is effectively controlled, the cable partial discharge monitoring device can be powered by online energy extraction. At the same time, the design uses the energy extraction coil to construct a high-frequency boundary to suppress the influence of external interference.

Description

Technical Field

[0001] The invention belongs to the technical field of partial discharge monitoring of high voltage circuits, in particular to a cable partial discharge sensor and a monitoring device based on multi-stage resonance. Background Art

[0002] Improper operation of cables during production, construction, and installation may lead to local insulation defects in the cable body and accessories, resulting in partial discharge. In long-term operation, affected by electricity, heat, and stress, local defects continue to develop, eventually leading to overall breakdown. At present, the partial discharge detection of cables generally adopts the regular inspection method, but the regular inspection method requires power outage. There is a difference between the offline and running states of the cable, which affects the detection accuracy, and the cable state will also change during the regular inspection cycle.

[0003] The development trend of technology at home and abroad is to change from fixed inspection to condition maintenance based on online monitoring data. On the one hand, it can reduce the number of tests / power outages, and on the other hand, online monitoring can timely discover fault risk points. Cable partial discharge detection is achieved by detecting the electrical, thermal and acoustic signals emitted during the discharge process. From the detection quantity, it can be divided into non-electrical quantity detection method and electrical quantity detection method. The signal transmission distance of non-electrical quantity detection method (heat, sound, etc.) is limited, and it is only used for offline detection and cable accessory detection. The electrical quantity detection method realizes detection by analyzing the pulse voltage / current signal generated by partial discharge. According to the different signal frequency bands, electrical quantity detection is divided into two categories: high frequency / very high frequency (10kHz~300MHz) and ultra-high frequency (300MHz~3GHz). Ultra-high frequency detection has high sensitivity and good anti-interference performance, but it is affected by signal attenuation and is generally only used for offline detection and cable accessory monitoring; the high frequency / very high frequency detection band intersects with the interference signal band such as corona, and is greatly affected by external interference, but its monitoring range is large and can be used for full cable line monitoring, and the sensor is easy to install. It is the most widely used partial discharge detection / monitoring method.

[0004] A typical cable partial discharge online monitoring device consists of a sensor module, an acquisition module, a communication module, and a power supply module. The sensor module is responsible for extracting partial discharge signals and is the key to determining the sensitivity and reliability of cable partial discharge monitoring; the acquisition module is responsible for digital-to-analog conversion; and the power supply module is responsible for the overall power supply of the device. In terms of design, the features of the existing device are as follows:

[0005] (1) Sensor module

[0006] High frequency / very high frequency partial discharge detection sensors are divided into inductive and capacitive sensors according to different principles. Inductive sensors represented by high frequency current sensors (HFCT) have been widely used due to their ease of use, but they also have the following shortcomings:

[0007] The bandwidth is relatively narrow, especially the high-frequency response is weak, and the detection sensitivity is above 10pC;

[0008] Based on electromagnetic coupling, it has poor anti-interference ability and is more susceptible to interference such as white noise and non-periodic pulses. This is the main reason for the high false alarm rate of existing partial discharge detection, which seriously restricts the application effect.

[0009] In engineering, the following factors restrict the application of inductive sensors:

[0010] The price is relatively high. High-frequency current sensors have high requirements on the core material and structural design. Their price is relatively expensive, which limits their application in the field of online monitoring of medium and low voltage cable lines.

[0011] Sensor standardization. In most cases, inductive sensors are generally installed on the grounding wires at both ends of the cable. However, for long-distance cables, they need to be installed on the cross-connected wires or the cable body. At this time, the inner diameter of the inductive sensor needs to be customized according to the outer diameter of the cable, which is difficult to standardize. Individual customization further increases its cost and application limitations.

[0012] Capacitive sensors extract high-frequency signals based on the principle of capacitive voltage division and have the following characteristics:

[0013] Capacitive sensors with large bandwidth and high sensitivity can reach 1pC or even lower sensitivity.

[0014] It has good anti-interference performance. It is based on the principle of electrostatic coupling. The interference can only enter the sensor through conduction, and conducted interference is easier to shield.

[0015] In practical applications, the main problem restricting capacitive sensors is the inconvenience of installation. Existing capacitive sensors generally cut the outer sheath of the cable, or are installed in a pre-buried manner, which makes it difficult to use them in existing lines. A few researchers have proposed capacitive sensors that do not require the outer sheath to be damaged, but the detection sensitivity is greatly affected by the shielding layer. Compared with inductive sensors, capacitive sensors only require metal electrodes, so their cost is almost negligible. For online monitoring equipment that needs to be widely deployed, cost is an important factor affecting the promotion and application of technology.

[0016] (2) Acquisition Module

[0017] The acquisition module realizes the digital-to-analog conversion of the sensor output signal. There are two types of digital-to-analog acquisition and recording of power detection:

[0018] For over-limit acquisition, take UHF detection as an example. When the sensor detects that the UHF signal is over-limit, it records the signal amplitude and time, but does not record the signal time domain waveform. Therefore, the sampling rate requirement for over-limit acquisition is relatively low.

[0019] Time domain waveform acquisition, the existing high frequency / very high frequency acquisition generally adopts time domain waveform acquisition, which requires recording the original signal waveform of a certain length (a complete cycle or multiple cycles). The time domain waveform of a complete cycle is of great value for subsequent state analysis and partial discharge source location. Since the original waveform needs to be recorded, the sampling rate requirement is relatively high (generally above 20MHz), and the storage and other related hardware configurations are also correspondingly high, resulting in high power consumption and cost of the device.

[0020] (3) Power supply module

[0021] As mentioned above, due to the high sampling rate of the cable partial discharge online monitoring device, high requirements are placed on the high-speed acquisition chip, control chip and storage space, making it difficult to reduce the power consumption of the device. Existing cable partial discharge online monitoring devices are generally powered by 110 / 220V AC / DC power supplies, which greatly restricts the installation location of the device.

[0022] From the perspective of field application, the cable partial discharge online monitoring device should be able to be deployed on-site, wirelessly networked, and powered online, but the high sampling rate requirement makes it difficult for the device to meet the above requirements. Summary of the invention

[0023] The purpose of the present invention is to provide a cable partial discharge sensor and monitoring device based on multi-stage resonance, aiming to solve the problems of low sensor sensitivity and high overall power consumption in the prior art, thereby increasing the sensor sensitivity and reducing the overall power consumption and cost.

[0024] In order to achieve the above technical objectives, the present invention provides a cable partial discharge sensor based on multi-stage resonance, the sensor comprising:

[0025] A multi-stage voltage divider circuit consisting of the main insulation capacitor C0, the waterproof sheath capacitor C1, the shielding layer impedance R, and the patch electrode-to-ground stray capacitance C2;

[0026] A parallel inductor is set at both ends of the stray capacitor C2 to form a parallel resonance;

[0027] A series inductor is set between the stray capacitor C2 and the grounding point to form a series resonance;

[0028] The voltage across the stray capacitor C2 is the sensor output voltage.

[0029] Preferably, the output signal of the sensor is:

[0030]

[0031] Among them, Z C0 is the main insulation equivalent impedance, Z CL The stray capacitance C2 and the parallel inductance L form the equivalent impedance of the parallel branch.

[0032] Preferably, the output signal of the sensor is:

[0033]

[0034] Among them, Z C0 is the main insulation equivalent impedance, and R is the shielding layer characteristic impedance.

[0035] Preferably, the series inductor and the parallel inductor are configured as follows:

[0036]

[0037]

[0038] Among them, C represents the equivalent capacitance value, L is the length of the patch electrode, R is the radius of the outer electrode, r is the radius of the inner electrode, and ε is the dielectric constant.

[0039] The present invention also provides a cable partial discharge monitoring device based on multi-stage resonance implemented by using the sensor, the device comprising:

[0040] The sensors, conditioning circuits, detection circuits, MCU and communication modules are connected in sequence;

[0041] The MCU is powered by an energy taking coil connected in series on the cable body.

[0042] Preferably, the detection circuit reduces the sampling frequency to 5 MHz.

[0043] Preferably, the energy extraction coil is equivalent to an inductor connected in series with the cable body.

[0044] The effects provided in the content of the invention are only the effects of the embodiments, not all the effects of the invention. One of the above technical solutions has the following advantages or beneficial effects:

[0045] Compared with the prior art, the present invention adds series inductance / parallel inductance to the design of cable partial discharge detection sensor to form a multi-stage resonant circuit, improve the capacitance type sensor transfer ratio, expand the signal pulse width, and increase the sensor sensitivity; in the acquisition circuit design, since the sensor resonance expands the signal pulse width, the detector can be used to reduce the device sampling rate requirements, thereby effectively controlling the overall power consumption and cost of the device, and since the device power consumption is effectively controlled, the cable partial discharge monitoring device can be powered by online energy extraction. At the same time, the design uses the energy extraction coil to construct a high-frequency boundary to suppress the influence of external interference. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1A schematic diagram of a capacitive sensor structure provided in the prior art;

[0047] Figure 2 A schematic diagram of the wiring principle of a cable partial discharge sensor based on multi-stage resonance provided by an embodiment of the present invention;

[0048] Figure 3 A side view of the principle wiring of a cable partial discharge sensor based on multi-stage resonance provided by an embodiment of the present invention;

[0049] Figure 4 A front view of the wiring principle of a cable partial discharge sensor based on multi-stage resonance provided by an embodiment of the present invention;

[0050] Figure 5 A schematic diagram of the principle circuit of a cable partial discharge sensor based on multi-stage resonance provided by an embodiment of the present invention;

[0051] Figure 6 A schematic diagram showing the comparison between the resonant signal and the original signal provided in an embodiment of the present invention;

[0052] Figure 7 A principle block diagram of a cable partial discharge monitoring device based on multi-stage resonance provided by an embodiment of the present invention;

[0053] Figure 8 A schematic diagram of on-site deployment of a cable partial discharge monitoring device based on multi-stage resonance provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0054] In order to clearly illustrate the technical features of the present solution, the present invention is described in detail below through specific implementation methods and in conjunction with the accompanying drawings. The disclosure below provides many different embodiments or examples for realizing different structures of the present invention. In order to simplify the disclosure of the present invention, the components and settings of specific examples are described below. In addition, the present invention may repeat reference numbers and / or letters in different examples. This repetition is for the purpose of simplification and clarity, and does not itself indicate the relationship between the various embodiments and / or settings discussed. It should be noted that the components illustrated in the accompanying drawings are not necessarily drawn to scale. The present invention omits the description of known components and processing techniques and processes to avoid unnecessary limitations on the present invention.

[0055] A cable partial discharge sensor and monitoring device based on multi-stage resonance provided by an embodiment of the present invention will be described in detail below in conjunction with the accompanying drawings.

[0056] The embodiment of the present invention discloses a cable partial discharge sensor based on multi-stage resonance, the sensor comprising:

[0057] A multi-stage voltage divider circuit consisting of the main insulation capacitor C0, the waterproof sheath capacitor C1, the shielding layer impedance R, and the patch electrode-to-ground stray capacitance C2;

[0058] A parallel inductor is set at both ends of the stray capacitor C2 to form a parallel resonance;

[0059] A series inductor is arranged between the stray capacitance C2 and the grounding point to form a series resonance.

[0060] The basic design of the existing external capacitive partial discharge detection / monitoring sensor is as follows: Figure 1 As shown, the patch electrode is covered on the cable body. At this time, the cable core, main insulation, shielding layer, waterproof sheath, patch electrode and ground form a multi-stage voltage divider circuit. Among them, the main insulation capacitor C0, the waterproof sheath capacitor C1, the shielding layer impedance R (the shielding layer impedance is related to the cable parameters, generally above 100 ohms), and the patch electrode to ground stray capacitance C2 form a multi-stage voltage divider circuit. The sensor outputs the voltage across C2, and the sensor transfer ratio (sensitivity) is determined by the voltage divider circuit voltage divider ratio. In the following description, the series-parallel circuit composed of C1, R, and C2 is called a detection circuit.

[0061] The existing external capacitive sensors have the following shortcomings:

[0062] In the low frequency band, due to the low shielding layer impedance R, the sensor transfer ratio and detection sensitivity are both low, which is also an important reason for the limited application of external capacitive sensors;

[0063] In the high frequency band, due to the reduction of the equivalent impedance of the patch electrode to the stray capacitance C2 of the ground, it will affect the high frequency band transfer ratio and detection sensitivity of the sensor.

[0064] Depend on Figure 1 It can be seen from the sensor principle circuit shown that the low-frequency capacitive sensor transfer ratio is mainly determined by the shielding layer impedance R, and the high-frequency capacitive sensor transfer ratio is mainly determined by the C1 and C2 branches. To increase the sensor transfer ratio, a series resonance should be formed in the low frequency band, and a parallel resonance should be formed in the high frequency band. Therefore, in the embodiment of the present invention, a series inductor and a parallel inductor are added to form a multi-stage resonant circuit, such as Figure 2-5 shown.

[0065] In the low frequency band (less than 10MHz), adding series inductance to construct the series resonance of the detection circuit reduces the impedance of C1 and C2 branches, but due to the formation of series resonance, the voltage across C2 still increases significantly. When the amplitude of the pulse signal is U1, the sensor output signal U0 is as follows:

[0066]

[0067] In the formula, Z C0 is the main insulation equivalent impedance, ZCL The stray capacitance C2 and the parallel inductance L form the equivalent impedance of the parallel branch;

[0068] In the high frequency band (greater than 10MHz), by adding parallel inductance and stray capacitance to form parallel resonance, the sensor output signal U0 is as follows:

[0069]

[0070] In the formula, Z C0 is the main insulation equivalent impedance, and R is the shielding layer characteristic impedance.

[0071] The resonant signal is compared with the original signal. Figure 6 shown.

[0072] The inductance values ​​of the series inductor and the shunt inductor are determined by the stray capacitance resonant frequency band.

[0073] The equivalent capacitances C0 and C1 of the main insulation and waterproof sheath can be calculated using the cylindrical capacitance calculation formula:

[0074]

[0075] Among them, C represents the equivalent capacitance value, L is the length of the patch electrode, R is the radius of the outer electrode, and r is the radius of the inner electrode.

[0076] Taking YJV22 8.7 / 15kV XLPE single-core 185 cable as an example, the thickness of the cable protective sheath is about 1.8mm, the outer diameter of the cable conductor is 16.1mm, the distance between the conductor and the outer shielding layer is 14.8mm, the outer diameter of the cable body is 33.9mm, the length of the conductive film is 95mm, and the air dielectric constant between the inner and outer shielding layers is 2.3. The main insulation capacitance C0≈4.44nF, the waterproof sheath capacitance C1≈209.97Pf, and the stray capacitance of the patch electrode to the ground is related to the distance from the ground, so it needs to be calculated using ANSYS. When the distance from the ground is 1m, C2≈10Pf.

[0077] In terms of resonant frequency selection, considering that typical corona interference is roughly between 3 and 10 MHz, the low-frequency series resonant frequency should be less than 3 MHz, and the high-frequency resonant shielding should be greater than 10 MHz. At this time, the inductance value configuration is as follows:

[0078]

[0079] Assuming C2≈40Pf, the series inductance is about 2.5mH ​​and the parallel inductance is about 1mH.

[0080] Compared with the existing capacitive sensors, the capacitive cable partial discharge detection sensor described in the embodiment of the present invention adds a series inductor at the sensor grounding point to form a series resonant circuit, thereby amplifying the signal and extending the signal pulse width, and adds a parallel inductor on the stray capacitance side to form a parallel resonant circuit, thereby amplifying the signal and extending the signal pulse width, thereby significantly improving the sensor sensitivity.

[0081] like Figure 7 , 8 As shown, the embodiment of the present invention further discloses a cable partial discharge monitoring device based on multi-stage resonance implemented by using the sensor described above, the device comprising:

[0082] The sensors, conditioning circuits, detection circuits, MCU and communication modules are connected in sequence;

[0083] The MCU is powered by an energy taking coil connected in series on the cable body.

[0084] Existing partial discharge monitoring devices generally use FPGA and CPLD to control AD ​​signal acquisition. According to the sampling theorem, the sampling frequency should be twice the signal frequency. Therefore, the sampling frequency of existing devices is generally greater than 20MHz. Even if over-limit acquisition is used, the sampling interval should be less than the signal pulse width.

[0085] When the output signal of the cable partial discharge sensor based on multi-stage resonance described above is used to form multiple resonances, the signal pulse width is extended, so that the partial discharge pulse width is extended from tens of ns to hundreds of ns, thus significantly reducing the sampling frequency requirements. At this time, the sampling frequency can be reduced to 5MHz or even lower by using the detection circuit, and the signal acquisition can be realized by using the built-in AD chip of the low-power MCU. When the detection circuit detects that the signal exceeds the limit, the sampling data is recorded.

[0086] In addition, regarding the power supply of the partial discharge monitoring device, the existing cable partial discharge monitoring device has a high sampling rate and the power consumption is generally greater than 5W, and is generally powered by a 110 / 220V AC / DC power supply.

[0087] The power supply of a typical online energy-taking module is generally no more than 3W, which is difficult to support the existing cable online monitoring device. The energy-taking coil can be used to supply power after the sampling frequency is reduced by using the detection circuit. In addition, the energy-taking coil is an inductor coil in principle. For high-frequency signals, when the inductor coil is connected in series with the cable body, it is equivalent to connecting the inductor in series with the cable body, which can suppress the influence of interference signals outside the monitoring range.

[0088] The embodiment of the present invention uses a detection circuit and online energy acquisition to realize partial discharge monitoring based on the cable partial discharge sensor based on multi-stage resonance as described above. Since the signal pulse width is extended by the resonance circuit, the detection circuit can be used to reduce the signal sampling rate requirement, thereby reducing the hardware configuration of the device, thereby reducing the power consumption and cost of the device, and realizing the local deployment of the device; the energy acquisition coil is used to power the device and construct a high-frequency boundary at the same time to suppress the influence of external interference.

[0089] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A cable partial discharge sensor based on multi-stage resonance, characterized in that: The sensor comprises: A multi-stage voltage divider circuit consisting of the main insulation capacitor C0, the waterproof sheath capacitor C1, the shielding layer impedance R, and the patch electrode-to-ground stray capacitance C2; A parallel inductor is set at both ends of the stray capacitor C2 to form a parallel resonance; A series inductor is set between the stray capacitor C2 and the grounding point to form a series resonance; The voltage across the stray capacitor C2 is the sensor output voltage; The output signal of the sensor is: ； in, is the main insulation equivalent impedance, The stray capacitance C2 and the parallel inductance L form the equivalent impedance of the parallel branch; is the sensor output signal, is the amplitude of the partial discharge pulse signal; The output signal of the sensor is: ； in, is the main insulation equivalent impedance, and R is the shielding layer impedance.

2. A cable partial discharge sensor based on multi-stage resonance according to claim 1, characterized in that: The configuration of the series inductor and the parallel inductor is as follows: ； ； Where C represents the equivalent capacitance value, L is the length of the patch electrode, is the outer electrode radius, r is the inner electrode radius, and ε is the dielectric constant.

3. A cable partial discharge monitoring device based on multi-stage resonance implemented by using the sensor according to claim 1 or 2, characterized in that: The device comprises: The sensors, conditioning circuits, detection circuits, MCU and communication modules are connected in sequence; The MCU is powered by an energy taking coil connected in series on the cable body.

4. The cable partial discharge monitoring device based on multi-stage resonance according to claim 3 is characterized in that: The detection circuit reduces the sampling frequency to 5 MHz.

5. The cable partial discharge monitoring device based on multi-stage resonance according to claim 3 is characterized in that: The energy extraction coil is equivalent to an inductor connected in series with the cable body.

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