Method for detecting partial discharge of cable joints under multiple stresses

Abstract

The present invention provides a method for detecting partial discharge of cable joints under multi-element stress, which belongs to the technical field of partial discharge detection. The method aims at the influence of cable operation impulse voltage on its insulation state, studies the evolution characteristics of partial discharge of cable intermediate joints under multi-element stress of AC superimposed operation impulse voltage, and further develops an integrated sensor capable of measuring partial discharge of cable joints and operation impulse voltage at the same time and place, develops a detection device, realizes insulation state detection and evaluation under complex working conditions of actual cables, and realizes accurate evaluation of their health status, which is of great significance for early detection and prevention of cable faults, avoiding possible power outages, and ensuring the stable operation of power grids.

Description

Technical Field

[0001] The invention belongs to the technical field of partial discharge detection, and in particular relates to a method for detecting partial discharge of a cable joint under multi-element stress. Background Art

[0002] During the operation of the cable, the cable needs to withstand the multi-element electrical stress of the superposition of AC voltage and operating impulse voltage. The multi-element voltage will have a complex effect on the cable joint, and it will stimulate and promote the defects that may exist inside the cable. Therefore, in actual operation, there have been many cases where the cable failed after being subjected to operating impulse voltage. For example, a 110kV substation tripped the longitudinal differential protection of the switch during the closing operation of a 110kV cable line. After on-site inspection, it was found that the intermediate joint of the A-phase cable had a breakdown failure. Further analysis of the cause of the accident found that the intermediate joint of the cable generated an electric tree during operation. Due to the overvoltage of the closing operation, the electric tree developed rapidly, resulting in insulation breakdown. In general, the root cause of this type of fault is that the high-amplitude impulse voltage will cause partial discharge, and the partial discharge will continue to develop under the continuous operating voltage, eventually leading to the breakdown of the cable insulation material, thereby causing serious power accidents.

[0003] The cable joint is the weakest link in the cable system, and its electrical and mechanical properties are directly related to the safety and reliability of the entire cable line. The internal structure of the cable joint is complex, and the interface between the insulation layer, conductor layer and semiconductor layer is prone to electric field distortion, which leads to an increase in local electric field strength, and often causes partial discharge. During the long-term operation of the cable, the partial discharge phenomenon will continue to accumulate. If it is affected by the superimposed operating impulse voltage, it will quickly damage the cable insulation material and eventually lead to insulation breakdown. Therefore, studying the partial discharge characteristics of cable joints under superimposed voltage and understanding the excitation and promotion of superimposed voltage on cable defects are the theoretical keys to ensure the safe operation of cables.

[0004] At the measurement level, in order to more accurately grasp the state of the cable under superimposed voltage, it is particularly important to develop new sensors that can simultaneously measure transient voltage and partial discharge. This type of sensor can not only capture the voltage changes of the cable under different working conditions, but also detect tiny partial discharge signals, providing a powerful tool for the correlation analysis between superimposed voltage and partial discharge. Through these high-precision measurement methods, it is possible to accurately perceive the state of the actual stress of the operating cable, timely and effectively evaluate the operating status of the cable joint, and avoid the occurrence of cable accidents.

[0005] At present, domestic and foreign researchers have conducted a lot of research on the partial discharge characteristics of cables and terminals under various external and internal conditions, but the relevant research mainly focuses on single voltage stress, including special stresses such as low frequency, square wave, and impact, and there is no relevant report on the research on the partial discharge characteristics under the multi-factor stress of AC superimposed impact. At present, the partial discharge characteristics of cables and terminals under this special stress need further research. At present, the high-frequency pulse current method is the most commonly used for partial discharge detection in cables. There is no relevant detection method for the operating overvoltage on the cable, especially there is no method to achieve the synchronous measurement of partial discharge signals and overvoltage signals, which cannot support the detection and analysis of the insulation status of cables and terminals under the multi-factor stress of AC superimposed operating impact. Summary of the invention

[0006] The technical problem to be solved by the present invention is that it is difficult to effectively detect partial discharge of cable joints under multi-element stresses using conventional detection methods.

[0007] In order to achieve the above technical objectives, the present invention adopts the following technical solutions:

[0008] A method for detecting partial discharge of cable joints under multi-element stresses, the method using a fusion sensor to simultaneously perform transient voltage-partial discharge measurements on the cable joint grounding wire; the fusion sensor adopts a dual-winding structure, the dual-winding structure includes a left half magnetic core and a right half magnetic core, wherein the number of turns of the left half magnetic core is more than that of the right half magnetic core, a damping resistor is connected in parallel between the turns of the left half magnetic core, the left half magnetic core measures low-frequency signals, and the right half magnetic core measures high-frequency signals; the outputs of the windings on both sides are connected to a three-coaxial BNC head to form two outputs, and the sampling resistors of the two outputs are both chip resistors.

[0009] Preferably, the chip resistor is connected by welding, and the chip resistor is welded on the BNC connector as a sampling resistor.

[0010] Preferably, the fusion sensor is optimized by the following steps before performing transient voltage-partial discharge measurement: the mutual inductance coefficient is calculated based on the magnetic induction intensity, and then the sensor is equivalent to a transmission line model with distributed parameters, and then the transfer impedance of the sensor is obtained by solving the frequency equation of the transmission line model, and the correlation between the coil structure, size, material parameters and sensor response characteristics is established.

[0011] Preferably, in the transmission line model, the distributed mutual inductance adopts the calculation result considering the displacement current, the distributed inductance considers the influence of the leakage inductance, the winding impedance considers the skin effect, and the distributed capacitance considers the influence of the stray capacitance.

[0012] Preferably, the method further comprises the following steps: constructing a fusion detection system using a fusion sensor, and performing transient voltage-partial discharge measurement using the fusion detection system.

[0013] Preferably, the fusion detection system includes a broadband signal acquisition system, a detection and analysis system, and a communication system; the broadband signal acquisition system includes a fusion sensor, a range dynamic adjustment unit, a signal conditioning unit, and a digital-to-analog conversion unit; and the detection and analysis system is a microprocessor.

[0014] Preferably, the method further comprises the following steps: constructing a power frequency superimposed impulse voltage experimental platform, using the experimental platform to test actual cable equipment, and determining the local discharge characteristics of cable joints under the power frequency superimposed operating impulse voltage.

[0015] Preferably, the circuit of the power frequency superimposed impulse voltage experimental platform includes an impulse circuit, a power frequency test system, a measuring device and a high-voltage ball gap synchronous trigger.

[0016] Preferably, a Rogowski coil is used in the double-winding structure. When the current to be measured flowing through the current-carrying conductor changes, a changing magnetic field is generated around the conductor, causing electromagnetic induction in the Rogowski coil and generating an induced electromotive force at the output end.

[0017] As a preference:

[0018] When the current-carrying conductor passing through the center of the Rogowski coil is one turn and the current to be measured is I(t):

[0019] ∮H·dl=I(t)

[0020] When the magnetic field strength inside the ring is equal everywhere, the magnetic field strength at a radius r from the current-carrying conductor is:

[0021]

[0022] The relationship between magnetic induction intensity B and magnetic field intensity H is B = μ0·H, and the magnetic induction intensity B is obtained as:

[0023]

[0024] Where: μ r is the relative magnetic permeability of the skeleton material; μ0 is the magnetic permeability of vacuum;

[0025] When the magnetic field on the cross-sectional area S of the coil frame is equal everywhere, the total number of turns of the coil is N, and the total magnetic flux linkage Ψ of the coil linkage is:

[0026]

[0027] In the above formula, the induced electromotive force at both ends of the coil is proportional to the differential of the current to be measured, and this proportionality coefficient is the mutual inductance coefficient M between the current-carrying conductor to be measured and the coil;

[0028] When the self-integration condition is met When , ignoring the voltage drop caused by the induced electromotive force on the coil internal resistance and the sampling resistor, we get:

[0029]

[0030] When the number of coil turns is N:

[0031]

[0032] At this time, the current i(t) flowing through the coil is:

[0033]

[0034] Lianlide:

[0035]

[0036] That is, the output voltage waveform of the coil forms a linear relationship with the original current wave.

[0037] The present invention provides a method for detecting partial discharge of cable joints under multi-element stress. The method targets the influence of cable operating impulse voltage on its insulation state, studies the evolution characteristics of partial discharge of cable intermediate joints under multi-element stress of AC superimposed operating impulse voltage, and further develops an integrated sensor capable of measuring partial discharge of cable joints and operating impulse voltage simultaneously and in the same place, develops a detection device, realizes insulation state detection and evaluation under complex working conditions of actual cables, and realizes accurate evaluation of their health status, which is of great significance for early detection and prevention of cable faults, avoiding possible power outages, and ensuring stable operation of power grids. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a circuit diagram of a power frequency superimposed impulse voltage experimental platform in the present invention.

[0039] Figure 2 It is a physical picture of typical man-made defects of cable accessories in the present invention.

[0040] Figure 3 It is a schematic diagram of the double winding structure in the present invention.

[0041] Figure 4 It is a flow chart of constructing a wide-band current sensor model in the present invention.

[0042] Figure 5 It is a structural diagram of the Rogowski coil in the present invention.

[0043] Figure 6 This is the concentrated parameter equivalent circuit diagram of the Rogowski coil in the present invention. DETAILED DESCRIPTION

[0044] The specific embodiments of the present invention will be described in detail below. In order to avoid too much unnecessary details, the well-known structures or functions will not be described in detail in the following examples. The approximate language used in the following examples can be used for quantitative expression, indicating that the quantity can be allowed to have a certain change without changing the basic function. Unless otherwise defined, the technical and scientific terms used in the following examples have the same meaning as those generally understood by those skilled in the art to which the present invention belongs.

[0045] 1. Partial discharge characteristics of typical defects in cable joints under power frequency superimposed operating impulse voltage

[0046] Under the action of power frequency superimposed transient voltage, high amplitude impulse voltage will trigger particle movement, which has an exciting and promoting effect on subsequent power frequency partial discharge. This physical process involves the combined action of two voltages, and it is necessary to consider the influence of free charge caused by nanosecond partial discharge excited by microsecond superimposed impulse on subsequent nanosecond partial discharge under the action of second-level continuous power frequency voltage. Compared with the partial discharge under the action of a single voltage, it has complex multi-scale time characteristics. At present, the characteristics and mechanism of partial discharge under superimposed voltage are still unclear. The present invention conducts research on the partial discharge characteristics of cable joints under power frequency superimposed impulse voltage through actual cable equipment tests.

[0047] First, build a power frequency superimposed impulse voltage experimental platform. Construct an equivalent circuit for the power frequency superimposed impulse voltage test, and through a combination of theoretical analysis and electromagnetic transient simulation calculations, propose protection measures for the impulse voltage generator and the power frequency test transformer in the superimposed voltage generator, study and analyze the protection effects of different measures, and achieve effective protection against voltage crosstalk and current damage during the superimposed voltage action process, as well as reliable output of the superimposed voltage; design and develop circuits and devices for accurately acquiring the power frequency phase, develop a multi-spherical gap synchronous triggering system such as an impulse voltage triggering spherical gap and a safety protection spherical gap, and achieve high-precision control output of the superimposed voltage under any power frequency voltage phase; design and construct a multi-parameter detection system such as partial discharge broadband current pulses, high-speed imaging, spectrum, ultra-high frequency, and light intensity, to achieve multi-directional and refined measurement of the same physical phenomenon. The circuit structure of the superimposed platform is as follows: Figure 1 shown.

[0048] Among the main causes of power cable insulation failure, electrical treeing is a common one. Bubbles, impurities and metal burrs can all cause the generation and development of electrical treeing in XLPE insulation, eventually leading to insulation breakdown. In order to obtain measurement data that is more consistent with the actual situation, the partial discharge characteristics of power cables are studied. Aiming at the weak link of cable insulation (cable terminal joint), the present invention sets an artificial defect model on the actual cable terminal joint, such as Figure 2 shown.

[0049] Then, the local discharge characteristics of cable joints under power frequency superimposed operation impulse voltage are studied. The influence of experimental parameters such as superimposed parameters (impulse voltage waveform / amplitude ratio of power frequency and impulse voltage / superimposed phase / impulse voltage polarity) and defect type on the excitation and promotion of local discharge is studied; the distribution characteristics of local discharge related detection quantities (pulse amplitude / pulse polarity / pulse time / pulse interval / discharge duration / pulse number / etc.) under different experimental parameters are mastered, the distribution characteristics and distribution laws of local discharge detection quantities are statistically analyzed, the correlation between them and experimental parameters is established, and the macroscopic laws of the influence of superimposed voltage on the local discharge characteristics of cable joints are studied.

[0050] 2. Transient voltage-partial discharge integrated fusion sensor

[0051] According to the frequency and amplitude characteristics of partial discharge signals and operating impulse current signals, the present invention adopts a dual winding structure to measure high-frequency and low-frequency signals respectively. That is, the left half of the magnetic core is the low-frequency part - multiple turns, parallel damping resistors; the right half of the magnetic core is the high-frequency part - fewer turns. The outputs of the windings on both sides are connected to the three-coaxial BNC head to form two outputs. The sampling resistors of the two outputs are both chip resistors, such as Figure 3 shown.

[0052] The self-integrated frequency bands and gain impedances of the two windings are different, and they are responsible for signal detection in different frequency bands respectively, and finally merge the outputs. The specific discussion of the two winding structures is as follows.

[0053] 1) Low frequency part

[0054] The acquisition of low-frequency signals requires a large number of Rogowski coil turns, generally ranging from hundreds to thousands, to increase the coil inductance to increase the gain impedance in the low-frequency band. However, a large number of turns leads to a small gap distance between adjacent turns. Therefore, the stray capacitance between adjacent turns increases, which has a harmful effect on the induced signal of the coil. In addition, the coil winding and its grounded metal shield also form another distributed capacitance. All distributed capacitance and stray inductance of the coil can generate harmful resonant components that are parasitic in the real response in the coil output signal. Therefore, the output signal of the coil is not good, with large overshoot, resonant pulse top, long response time, and large swing.

[0055] The present invention adopts parallel damping resistor technology on a low-frequency multi-turn structure to achieve the effect of suppressing resonance. Because the low-frequency part pursues a frequency as low as 50Hz, the number of coil turns must increase. At this time, the stray parameters of the coil will increase, resulting in resonance, making the gain impedance curve of the coil no longer horizontal. Welding parallel damping resistors can solve this problem, but if a plug-in resistor is welded, the metal lead of the resistor will form a loop and couple with the magnetic field generated by the current to be measured, making the coil output greatly affected by the center offset of the conductor. If a PCB board is used to weld chip resistors, this problem can be solved.

[0056] 2) High frequency part

[0057] The structure with few turns can easily meet the self-integration condition in the high frequency band, and the stray capacitance is small, and no resonance point will appear in the self-integration frequency band. According to the above simulation results, this structure is selected to measure high-frequency signals. A key point of this structure is the welding method of the sampling resistor. Only by using a chip resistor welded on the BNC connector as a sampling resistor can a high sensitivity and flat frequency response curve be achieved. When a straight-insert resistor is welded on both ends of the coil as a sampling resistor, it cannot be achieved, and the sensitivity curve is deformed.

[0058] Different sampling resistor welding methods will also affect the eccentricity error during coil measurement. When the chip resistor is welded, the sensitivity of the coil will not change with the change of the center conductor position in the entire self-integration frequency band; on the contrary, the welding method of the through-hole resistor will cause the coil sensitivity to fluctuate with the change of the center conductor position.

[0059] The basic working principle of Rogowski coil for measuring current is the law of electromagnetic induction and Ampere's loop law. The current-carrying conductor passes vertically through the center of the skeleton. When the current to be measured flowing through the current-carrying conductor changes, a changing magnetic field will be generated around the conductor, causing electromagnetic induction in the Rogowski coil, generating an induced electromotive force at the output end. The entire measurement process is implemented according to the law of electromagnetic induction. Therefore, the measurement circuit of the Rogowski coil is coupled with the current-carrying conductor through the magnetic field. There is no direct electrical connection, which will not affect the measurement circuit of the coil, and it has high safety and reliability. The structure of the Rogowski coil is as follows: Figure 5 shown.

[0060] It is usually assumed that the current-carrying conductor passing through the center of the Rogowski coil is one turn, and the current to be measured is I(t). According to Ampere's loop law, we have:

[0061] ∮H·dl=I(t)

[0062] Assuming that the magnetic field strength is equal everywhere in the ring, the magnetic field strength at a radius r from the current-carrying conductor is:

[0063]

[0064] The relationship between magnetic induction intensity B and magnetic field intensity H is B = μ0·H, so the magnetic induction intensity B is:

[0065]

[0066] Where: μ r is the relative magnetic permeability of the skeleton material; μ0 is the magnetic permeability of vacuum.

[0067] Assuming that the magnetic field on the cross-sectional area S of the coil frame is equal everywhere and the total number of turns of the coil is N, the total magnetic flux Ψ of the coil linkage is:

[0068]

[0069] It can be seen from the above formula that the induced electromotive force at both ends of the coil is proportional to the differential of the current to be measured, and this proportionality coefficient is the mutual inductance coefficient M between the current-carrying conductor to be measured and the coil.

[0070] The equivalent circuit of the Rogowski coil with concentrated parameters is as follows: Figure 6 As shown. I(t) is the current flowing through the coil and the sampling resistor R is connected in parallel to the output end t , L is the self-inductance, r is the resistance of the coil winding; C z is the inter-turn capacitance; C d It is the capacitance to ground generated between the coil winding and the grounded shielding shell.

[0071] The self-integration working mode means that the coil itself contains an integration link, and the differential signal can be directly restored to a signal proportional to the measured signal through the coil without the need for an external integrator. When , the voltage drop caused by the induced electromotive force on the coil internal resistance and the sampling resistor can be ignored, and we can get:

[0072]

[0073] If the number of coil turns is N, then:

[0074]

[0075] At this time, the current i(t) flowing through the coil is:

[0076]

[0077] Lianlide:

[0078]

[0079] That is, the output voltage waveform of the coil forms a linear relationship with the original current waveform, and there is no need to use an external integrator to restore the signal. The self-integration condition can also be simplified to ωL=(r+R t), it can be seen that the main influencing factors of the self-integration condition are the frequency of the signal to be measured and the self-inductance of the coil winding.

[0080] In order to realize the self-integration of the coil, it is necessary to strengthen the coupling ability of the coil to the current source I(t) to be measured. Using a magnetic core with high magnetic permeability as the coil core can meet this requirement.

[0081] In summary, according to the frequency band distribution of high-frequency partial discharge pulse current signal and impulse current signal under operating impulse voltage, the integrated fusion measurement of the two physical signals can be realized by using a wide-band self-integrated Rogowski coil.

[0082] After determining the structure of the fusion sensor, due to the difference in frequency and amplitude of the two physical signals, the improvement of the performance of the two is mutually restricted, so it is necessary to optimize and improve the performance of the sensor through mathematical means. In view of this, the present invention studies the space-time equation satisfied by the spatial magnetic induction intensity excited by the excitation current based on the full current theory, considers the boundary conditions satisfied by the equation, and solves and calculates to obtain the magnetic induction intensity passing through the inside of the coil. On this basis, the mutual inductance coefficient is calculated by the definition of mutual inductance when considering the displacement current; then, the sensor is equivalent to a transmission line model with distributed parameters, in which the distributed mutual inductance adopts the calculation result considering the displacement current, the distributed inductance considers the influence of leakage inductance, the winding impedance considers the skin effect, and the distributed capacitance considers the influence of stray capacitance; finally, the transfer impedance of the sensor is obtained by solving the frequency equation of the transmission line model, thereby establishing a correlation between the coil structure, size, material and other parameters and the sensor response characteristics. The process of building a wide-band current sensor model is as follows: Figure 4 As shown,

[0083] Based on the understanding of the influencing factors of different parameters on the sensor response characteristics, the sensor structure parameters are optimized and designed, and a new fusion sensor is optimized to perform broadband signal fusion detection such as impact current / high-frequency current at the cable joint grounding wire.

[0084] 3. Cable joint transient voltage-partial discharge fusion sensing system

[0085] The present invention designs and develops a detection system based on the integrated fusion sensor of "transient voltage-partial discharge". According to the time scale of different signal information, a joint collection and processing method of multi-channel information is designed to ensure the comprehensiveness and synchronization of the collected information, and finally demonstrates and applies it on actual cable equipment.

[0086] The cable joint "transient voltage-partial discharge" fusion detection system should mainly consist of a broadband signal acquisition system, a detection and analysis system, and a communication system. The broadband signal acquisition unit includes a fusion sensor, a range dynamic adjustment unit, a signal conditioning unit, and a digital-to-analog conversion unit, which is mainly used to obtain broadband electromagnetic signals that can be used for detection in real time in a complex electromagnetic environment. Research the processing methods for signals of different frequency bands, design the corresponding conditioning circuits, and preserve the broadband signals without distortion while ensuring good anti-interference performance.

[0087] The detection and analysis system can be realized by a microprocessor, which processes and analyzes the signals obtained by the detection system in real time, and obtains the insulation status of the cable joint from the characteristics of signals in different frequency bands. It also includes a fault analysis unit to analyze abnormal detection data.

[0088] The embodiments of the present invention are described in detail above, but the contents are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the scope of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for detecting partial discharge of cable joints under multi-stress, characterized in that: The method uses a fusion sensor to simultaneously perform transient voltage-partial discharge measurement on a cable joint grounding wire; the fusion sensor adopts a double-winding structure, the double-winding structure includes a left half magnetic core and a right half magnetic core, wherein the number of turns of the left half magnetic core is more than that of the right half magnetic core, a damping resistor is connected in parallel between the turns of the left half magnetic core, the left half magnetic core measures a low-frequency signal, and the right half magnetic core measures a high-frequency signal; the outputs of the windings on both sides are connected to a three-coaxial BNC head to form two outputs, and the sampling resistors of the two outputs are both chip resistors; the chip resistors are connected by welding, and the chip resistors are welded on the BNC connector as sampling resistors; the method also includes the following steps: constructing a power frequency superimposed impulse voltage experimental platform, using the experimental platform to test actual cable equipment, and determining the partial discharge characteristics of the cable joint under the power frequency superimposed operation impulse voltage; using a Rogowski coil in the double-winding structure, when the current to be measured flowing in the current-carrying conductor changes, a changing magnetic field is generated around the conductor, so that the Rogowski coil is electromagnetically induced, and an induced electromotive force is generated at the output end.

2. The method for detecting partial discharge of cable joints under multi-stress according to claim 1, characterized in that: The fusion sensor is optimized through the following steps, and then transient voltage-partial discharge measurement is performed: the mutual inductance coefficient is calculated according to the magnetic induction intensity, and then the sensor is equivalent to a transmission line model with distributed parameters, and then the transfer impedance of the sensor is obtained by solving the frequency equation of the transmission line model, and the correlation between the coil structure, size, material parameters and sensor response characteristics is established.

3. The method for detecting partial discharge of cable joints under multi-stress according to claim 2, characterized in that: In the transmission line model, the distributed mutual inductance adopts the calculation result considering the displacement current, the distributed inductance considers the influence of the leakage inductance, the winding impedance considers the skin effect, and the distributed capacitance considers the influence of the stray capacitance.

4. The method for detecting partial discharge of cable joints under multi-stress according to claim 1, characterized in that: The method also includes the following steps: constructing a fusion detection system with a fusion sensor, and using the fusion detection system to perform transient voltage-partial discharge measurement.

5. The method for detecting partial discharge of cable joints under multi-stress according to claim 4, characterized in that: The fusion detection system includes a broadband signal acquisition system, a detection and analysis system, and a communication system; the broadband signal acquisition system includes a fusion sensor, a range dynamic adjustment unit, a signal conditioning unit, and a digital-to-analog conversion unit; the detection and analysis system is a microprocessor.

6. The method for detecting partial discharge of cable joints under multi-stress according to claim 1, characterized in that: The circuit of the power frequency superimposed impulse voltage experimental platform comprises an impulse circuit, a power frequency test system, a measuring device and a high-voltage ball gap synchronous trigger.

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