Oil-paper composite insulation PDC measurement system and method in insulating oil flowing state

Abstract

The invention relates to an oil-paper composite insulation PDC (Polycrystalline Diamond Compact) measuring system and method in an insulating oil flowing state, and a tested sample assembly is arranged between an upper electrode cover and a lower electrode cover and comprises an oil gap gasket for providing an oil gap channel, oil impregnated paper and oil flow flowing through the oil gap channel; the PDC measuring circuit comprises a high-voltage electrode arranged on the upper electrode cover and a measuring electrode arranged on the lower electrode cover, a high-voltage direct-current power supply is connected to the high-voltage electrode after being connected with a high-voltage relay and a current-limiting resistor in series, and the measuring electrode is grounded after being connected with an over-current protection module and a high-resistance meter in series; the insulating oil flow speed control oil way comprises an oil storage tank, and insulating oil in the oil storage tank is driven by the oil pump to flow into the upper electrode cover and the lower electrode cover through the pressure buffering tank, flows through the oil gap channel to form oil flow and then returns to the oil storage tank. By simulating conditions of different flow rates of the insulating oil, measurement of the oil-paper composite insulation PDC in the flowing state of the insulating oil is realized, and related information closer to the insulation state of a large oil-immersed power transformer in the actual working condition is obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of polarization current measurement, and specifically to an oil-paper composite insulation PDC measurement system and method under the flowing state of insulating oil. Background Technique

[0002] The statements in this part only provide background technical information related to the present invention, and do not necessarily constitute prior art.

[0003] As the main insulation inside the converter transformer, the insulation performance of the oil-paper composite insulation directly affects the safe and stable operation of the converter transformer and even the entire high-voltage DC transmission system. Its insulation state can be evaluated by non-destructive methods.

[0004] Among them, the polarization / depolarization current (PDC) method based on the time-domain dielectric response technology can sensitively reflect the moisture absorption, aging and temperature influence characteristics of the oil-paper insulation because of its small impact on the transformer and strong anti-interference ability. And it can also reflect the traps and defects in the oil-paper composite insulation medium and the distribution information of space charges, so as to obtain electrical information comprehensively reflecting the insulation performance and dielectric performance of the oil-paper composite insulation. Therefore, it has become an important means and research hotspot for testing and evaluating the insulation state of oil-paper composite insulation and has been widely applied.

[0005] However, in large oil-immersed power transformers such as converter transformers, the insulating oil, as an insulating and cooling medium, is forced to circulate. In the oil-paper composite insulation, the insulating oil in the flowing state will surely affect the ionization, injection, recombination, migration, extraction and other behaviors of charges in the composite insulation, and further affect the change of the polarization / depolarization current of the composite insulation. At present, most of the oil-paper composite insulation PDC measurement methods are designed for the static state of the insulating oil, so that the obtained PDC measurement results cannot accurately reflect the charge transport behavior and insulation state of the oil-paper insulation under the condition of "flowing insulating oil", which is closer to the actual working condition of large oil-immersed power transformers. Summary of the Invention

[0006] In order to solve the technical problems in the above background technique, the present invention provides an oil-paper composite insulation polarization / depolarization current (PDC) measurement system and method under the flowing state of insulating oil. This system realizes the measurement of the PDC of the oil-paper composite insulation under the flowing state of simulating different flow rates of the insulating oil, and obtains the PDC characteristics of the oil-paper composite insulation under the condition of "flowing insulating oil", which is closer to the actual working condition of large oil-immersed power transformers, so as to accurately reveal the insulation state of the oil-paper composite insulation under the actual working condition, as well as the internal connection and mechanism between them.

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

[0008] The first aspect of the present invention provides an oil-paper composite insulation PDC measurement system in the flowing state of insulating oil, comprising:

[0009] A test sample assembly, arranged between an upper electrode cover and a lower electrode cover, including an oil gap gasket providing an oil gap channel, oil-impregnated paper, and an oil flow flowing through the oil gap channel;

[0010] A PDC measurement circuit, including a high-voltage electrode arranged on the upper electrode cover and a measurement electrode arranged on the lower electrode cover. After a high-voltage DC power supply is connected in series with a high-voltage relay and a current-limiting resistor, it is connected to the high-voltage electrode, and the measurement electrode is grounded after passing through an overcurrent protection module and a high-resistance meter;

[0011] An insulating oil flow rate control oil circuit, including insulating oil stored in an oil storage tank. Driven by an oil pump, the insulating oil passes through a liquid mass flowmeter and a pressure relief tank, and according to the experimental requirements of measuring the current on the paper side or the oil side, the insulating oil is respectively regulated to flow into the upper electrode cover (for measuring the current on the paper side) or the lower electrode cover (for measuring the current on the oil side). After flowing through the oil gap channel to form an oil flow, it returns to the oil storage tank.

[0012] Further, the high-voltage relay has three contacts. Contact 1 is connected to the current-limiting resistor, contact 2 is connected to the high-voltage DC power supply, and contact 3 is grounded.

[0013] Further, when contacts 1 and 2 of the high-voltage relay are closed, the high-voltage DC power supply, the high-voltage relay, the current-limiting resistor, the high-voltage electrode, the test sample assembly, the measurement electrode, and the high-resistance meter are connected in series, and the grounding ends of the high-voltage DC power supply, the overcurrent protection module, and the high-resistance meter are grounded to form a polarization current test circuit.

[0014] Further, when contacts 1 and 3 of the high-voltage relay are closed, the high-voltage relay, the current-limiting resistor, the high-voltage electrode, the test sample assembly, the measurement electrode, and the high-resistance meter are connected in series, contact 1 of the high-voltage relay is grounded, and the grounding end of the high-resistance meter is grounded to form a depolarization current test circuit.

[0015] Further, a sealed space for accommodating the oil-impregnated paper, the oil gap gasket, and the insulating oil is formed between the upper electrode cover and the lower electrode cover. The oil-impregnated paper is in close contact with the oil gap gasket, and the insulating oil flows through the oil gap channel formed by the oil gap gasket in the sealed space to form an oil flow.

[0016] Further, a guard electrode is arranged outside the measurement electrode, and the two are insulated from each other. The measurement electrode is connected to a protection module and a high-resistance meter, and the guard electrode is grounded.

[0017] Further, the insulating oil flow rate control oil circuit further includes an oil flow inlet and an oil flow outlet respectively arranged on the upper electrode cover and the lower electrode cover, and the oil flow inlet and the oil flow outlet are connected to corresponding control valves.

[0018] Further, when the oil-impregnated paper is in close contact with the measuring electrode and the shielding electrode sides, the oil gap spacer and the oil flow are in close contact with the high-voltage electrode side, the control valves on the oil flow inlet and outlet of the upper electrode cover are opened, and the control valves on the oil flow inlet and outlet of the lower electrode cover are closed. The insulating oil flows through the upper electrode cover but not through the lower electrode cover, realizing the measurement of the polarization / depolarization current on the paper side.

[0019] Further, when the oil-impregnated paper is in close contact with the high-voltage electrode side, the oil gap spacer and the oil flow are in close contact with the measuring electrode and the shielding electrode sides, the control valves on the oil flow inlet and outlet of the upper electrode cover are closed, and the control valves on the oil flow inlet and outlet of the lower electrode cover are opened. The insulating oil flows through the lower electrode cover but not through the upper electrode cover, realizing the measurement of the polarization / depolarization current on the oil side.

[0020] Further, the overcurrent protection module and the high resistance meter are grounded, and the high resistance meter is connected to the data processing unit through the data acquisition module. The data processing unit can be a computer.

[0021] Further, the outlet of the oil storage tank is connected to the oil pump, the liquid mass flow controller and the pressure relief tank through pipelines. The pressure relief tank is respectively connected to the oil flow inlets of the upper electrode cover and the lower electrode cover through pipelines, and the oil flow outlets of the upper electrode cover and the lower electrode cover are connected to the inlet of the oil storage tank through pipelines.

[0022] Further, the oil pump is connected to a frequency converter for adjusting the flow rate.

[0023] The second aspect of the present invention provides a method for realizing measurement based on an oil-paper composite insulation PDC measurement system under the flowing state of insulating oil, including the following steps:

[0024] Pretreat the insulating oil and inject it into the groove of the lower electrode cover. According to the experimental requirements of measuring the current on the paper side or the oil side, place the oil-impregnated paper and the oil gap spacer into the groove insulating oil in different orders, and assemble the upper electrode cover and the lower electrode cover to obtain a sealed oil flow channel.

[0025] Start the oil pump and circulate for a set period of time to eliminate the air bubbles in the insulating oil flow rate control oil circuit.

[0026] Start the PDC measurement circuit. By controlling the closing of a pair of contacts of the high-voltage relay, the polarization current measurement circuit is actuated. By controlling the closing of another pair of contacts of the high-voltage relay, the depolarization current measurement circuit is actuated.

[0027] By adjusting the rotation speed of the oil pump and combining with the liquid mass flow controller, the regulation of the oil flow rate is realized. By adjusting the voltage amplitude of the high-voltage DC power supply, the applied voltage to the specimen is controlled, and the PDC curve under different oil flow rates and different voltages of the insulating oil is obtained.

[0028] Compared with the prior art, the above one or more technical solutions have the following beneficial effects:

[0029] 1. By controlling the oil flow path through the insulating oil flow rate, the insulating oil is circulated between the upper and lower electrode covers of the transformer and the oil storage tank, simulating the forced flow state of the insulating oil in the actual working conditions of a large oil-immersed power transformer. Further, a pressure relief tank is used to absorb the oil flow fluctuations, thereby achieving the effect of stable regulation of the insulating oil flow rate, and enabling the oil flow rate error to be controlled within an as low as possible range.

[0030] 2. In the PDC measurement circuit, through the design of the measuring electrode and the guard electrode structure, the surface leakage current along the edge of the specimen and other dielectric leakage currents are excluded, improving the accuracy of the PDC measurement of the oil-paper composite insulation.

[0031] 3. In the PDC measurement circuit, by switching the contacts of the high-voltage relay pair, a polarization current test circuit and a depolarization current test circuit are formed to realize the test of the polarization / depolarization current under different applied voltage amplitudes. By switching the control valves in the insulating oil flow rate control oil path, when the oil-impregnated paper is attached to different positions, the test of the polarization / depolarization current is realized. Thus, it is as close as possible to the actual working conditions of a large power transformer, expanding the measurement range and improving the measurement accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings forming a part of this invention are used to provide a further understanding of the invention. The schematic embodiments and descriptions thereof of the invention are used to explain the invention and do not constitute an improper limitation to the invention.

[0033] Figure 1 It is a schematic diagram of an oil-paper composite insulation PDC measurement system in the flowing state of insulating oil provided by one or more embodiments of the invention;

[0034] Figure 2 It is a schematic diagram of the upper electrode cover plane of the measurement system provided by one or more embodiments of the invention;

[0035] Figure 3 It is a schematic diagram of the oil gap gasket plane of the measurement system provided by one or more embodiments of the invention;

[0036] Figure 4 It is a schematic diagram of the lower electrode cover plane of the measurement system provided by one or more embodiments of the invention;

[0037] Figure 5 It is a waveform diagram of the paper-side leakage current in different oil flow states (stationary and flowing) of the insulating oil of the measurement system provided by one or more embodiments of the invention;

[0038] Figure 6 It is a waveform diagram of the paper-side PDC in the oil flow state (0.3 m / s) of the measurement system provided by one or more embodiments of the invention;

[0039] Figure 7 It is a schematic diagram of the principle of the PDC measurement circuit in the measurement system provided by one or more embodiments of the present invention.

[0040] In the figure: 100 high-voltage DC power supply, 101 high-voltage relay, 102 current-limiting resistor, 103 high-voltage electrode, 104 measurement electrode, 105 protection electrode, 106 overcurrent protection module, 107 high-resistance meter, 108 data acquisition card, 109 computer, 110 upper electrode cover, 111 lower electrode cover, 112 oil gap gasket, 113 oil-impregnated paper, 114 oil gap oil flow, 115 sealing ring at the lower edge of the upper electrode cover, 116 screw, 117 nut, 118 spring lock washer, 119 stainless steel rectangular frame, 120 oil storage tank, 121 oil pump, 122 frequency converter, 123 liquid mass flow controller, 124 pressure stabilizing tank, 125 oil flow inlet valve at the upper electrode cover, 126 oil flow inlet of the upper electrode cover, 127 oil flow outlet of the upper electrode cover, 128 oil flow outlet valve of the upper electrode cover, 129 oil flow inlet valve of the lower electrode cover, 130 oil flow inlet of the lower electrode cover, 131 oil flow outlet of the lower electrode cover, 132 oil flow outlet valve at the lower electrode cover, 133 oil flow pipeline. Detailed implementation manners

[0041] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0042] It should be noted that the following detailed description is exemplary and is intended to provide further illustration of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0043] It should be noted that the terms herein are only for describing specific implementation manners and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "include" and / or "comprise" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0044] Term explanation:

[0045] Destructive Methods

[0046] It refers to the detection means that will cause irreversible damage to the oil-paper composite insulation structure or change its original performance during the test process. Such methods usually require sampling or applying stresses beyond the normal operating conditions of the equipment (such as high voltage, high temperature, or mechanical damage) to directly evaluate the ultimate performance or deterioration degree of the insulating material.

[0047] Non-Destructive Methods (NDT)

[0048] Refers to detection techniques that do not cause damage to the oil-paper composite insulation during testing and can maintain its original performance. These methods usually measure electrical, physical, or chemical signals to indirectly evaluate the insulation state and are applicable to on-line or off-line detection.

[0049] Common destructive methods include sampling and analysis of insulating paper, breakdown tests of oil-paper insulation systems, thermal aging tests, and mechanical strength tests, etc. Common non-destructive methods include dissolved gas analysis (DGA) in oil, partial discharge detection methods, frequency response analysis (FRA) methods, dielectric response test methods, and oil quality analysis methods, etc.

[0050] Non-destructive methods, such as DGA, partial discharge detection, etc., are generally applicable to on-line monitoring and preliminary evaluation.

[0051] Destructive methods, such as sampling and analysis of insulating paper, breakdown tests, etc., are generally applicable to accurate evaluation, but require power outage and damage to the insulation.

[0052] As introduced in the background technology, most of the current measurement methods for the oil-paper composite insulation PDC are designed for the case where the insulating oil is in a static state, resulting in a certain deviation between the obtained insulation state results and the actual insulation state.

[0053] Specifically, in large oil-immersed power transformers (especially converter transformers), the insulating oil, as a key insulating and cooling medium, flows in a forced circulation manner in the oil-paper composite insulation system. This flow state will significantly affect the dynamic behavior of charges in the composite insulation system, including processes such as charge ionization, injection, recombination, migration, and extraction, etc., thereby further changing the polarization / depolarization current characteristics of the composite insulation system. However, most of the current measurement methods for the polarization / depolarization current (PDC) of oil-paper composite insulation are designed based on the static state of the insulating oil, resulting in the measurement results being difficult to accurately reflect the charge transport characteristics and its insulation state under the actual operating conditions of "insulating oil flow", which is closer to the actual operating conditions of large oil-immersed power transformers. This limitation may affect the accuracy of the assessment of the transformer insulation state.

[0054] Therefore, the following embodiments provide an oil-paper composite insulation PDC measurement system and method under the insulating oil flow state. By simulating the conditions of different flow rates of the insulating oil, the measurement of the oil-paper composite insulation PDC is realized. The obtained measurement results are generated by the charge transport behavior under the actual operating conditions of insulating oil flow, which helps to establish the relationship between the polarization / depolarization current and the insulation state under the actual operating conditions, thus more truly reflecting the insulation state under the actual operating conditions.

[0055] Embodiment 1:

[0056] The PDC measurement system for oil-paper composite insulation under the flowing state of insulating oil includes a test sample assembly, a PDC measurement circuit, and an insulating oil flow rate control oil circuit.

[0057] The test sample assembly is arranged between the upper electrode cover and the lower electrode cover, and includes an oil gap gasket providing an oil gap channel, oil-impregnated paper, and an oil flow flowing through the oil gap channel;

[0058] The PDC measurement circuit includes a high-voltage electrode arranged on the upper electrode cover and a measurement electrode arranged on the lower electrode cover. After a high-voltage DC power supply is connected in series with a high-voltage relay and a current-limiting resistor, it is connected to the high-voltage electrode, and the measurement electrode is grounded after passing through an over-current protection module and a high-resistance meter;

[0059] The insulating oil flow rate control oil circuit includes insulating oil stored in an oil storage tank. The insulating oil is driven by an oil pump through a liquid mass flow controller and a pressure-relief tank, and according to the experimental requirements of measuring the current on the paper side or the oil side, it is regulated to flow into the upper electrode cover or the lower electrode cover. After flowing through the oil gap channel to form an oil flow, it returns to the oil storage tank.

[0060] As shown in Figures 1-4 Figure, the test sample assembly (112-114) includes an oil-paper composite insulation sample composed of an oil gap gasket 112 providing an oil gap channel, oil-impregnated paper 113, and an oil flow 114 flowing through the oil gap channel. This composite insulation sample is placed between the upper electrode cover 110 and the lower electrode cover 111. As the raised part of the upper electrode cover 110 is placed into the groove of the lower electrode cover 111, sealing is achieved through the sealing ring 115 along the lower edge of the upper electrode cover 110.

[0061] The PDC measurement circuit includes a high-voltage DC power supply 100, a high-voltage relay 101, a current-limiting resistor 102, a high-voltage electrode 103, a measurement electrode 104, a protection electrode 105, an over-current protection module 106, a high-resistance meter 107, a data acquisition module 108, and a computer 109.

[0062] As a further implementation manner, after the high-voltage DC power supply 100 is connected in series with the high-voltage relay 101 and the current-limiting resistor 102, it is connected to the high-voltage electrode 103.

[0063] As a further implementation manner, the high-voltage electrode 103 is located at the top end of the upper electrode cover 110.

[0064] As a further implementation manner, a measurement electrode 104 is provided at the bottom end of the lower electrode cover 111, and the measurement electrode 104 is grounded after passing through the over-current protection module 106 and the high-resistance meter 107.

[0065] As a further implementation manner, the high-voltage relay 101 has three contacts. Contact 1 is connected to the current-limiting resistor 102, contact 2 is connected to the high-voltage DC power supply 100, and contact 3 is grounded.

[0066] As a further embodiment, the overcurrent protection module 106 and the high resistance meter 107 are grounded, and the high resistance meter 107 is connected to the computer 109 through the data acquisition module 108.

[0067] Among them, the current limiting resistor 102 can prevent the high-voltage DC voltage from grounding due to the breakdown of the test sample.

[0068] Among them, the protection electrode 105 can eliminate the influence of the surface current along the edge of the test sample and the leakage current of other media outside the test sample.

[0069] As a further embodiment, the measuring end of the high resistance meter 107 is connected to the measuring electrode, the grounding end is grounded, and the collected polarization / depolarization current is transmitted to the computer through the data acquisition module 108.

[0070] In this embodiment, the schematic diagram of the principle of the PDC measurement circuit is as Figure 7 shown. Among them, the high resistance meter 107, the internal of the overcurrent protection module 106 and the measuring electrode 104 are in series relationship, while the protection electrode 105 is connected to the outer shell of the overcurrent protection module 106 and the grounding end for shielding and grounding.

[0071] In this embodiment, the working principle of the PDC measurement circuit is as follows:

[0072] When the contacts of the high-voltage relays 1 and 2 are closed, the high-voltage DC power supply 100, the high-voltage relay 101, the current limiting resistor 102, the high-voltage electrode 103, the test sample assembly, the measuring electrode 104 and the high resistance meter 107 are in series, and the grounding ends of the high-voltage DC power supply 100, the overcurrent protection module 106 and the high resistance meter 107 are grounded to form a polarization current test circuit;

[0073] When the contacts of the high-voltage relays 1 and 3 are closed, the high-voltage relay 101, the current limiting resistor 102, the high-voltage electrode 103, the test sample assembly, the measuring electrode 104 and the high resistance meter 107 are in series, the contact 1 of the high-voltage relay 101 is grounded, and the grounding end of the high resistance meter 107 is grounded to form a depolarization current test circuit.

[0074] As a further embodiment, the test sample assembly includes an oil gap gasket 112, oil-impregnated paper 113 and an oil gap oil flow 114.

[0075] As a further embodiment, the insulating oil flow rate control oil circuit includes an oil storage tank 120, an oil pump 121, an inverter 122, a liquid mass flow controller 123, a pressure relief tank 124, an oil flow inlet valve 125 at the upper electrode cover, an oil flow outlet valve 128 at the upper electrode cover, an oil gap gasket 112, and an oil pipeline 133.

[0076] As a further implementation, the insulating oil in the oil storage tank 120 flows toward the PDC three-electrode oil inlet 126 under the push of the oil pump 121 , and the flow rate is roughly adjusted under the control of the inverter 122 .

[0077] As a further implementation, the insulating oil flows through the liquid mass flow controller 123 , and the flow rate is finely adjusted under the control of the liquid mass flow controller 123 .

[0078] As a further implementation, the insulating oil flows through the pressure relief tank 124 to effectively eliminate oil flow fluctuations, and passes through the valve 125 at the oil flow inlet of the PDC three electrodes to flow into the oil flow inlet 126 of the upper electrode cover.

[0079] As a further implementation method, the insulating oil flows through the oil gap at the oil gap gasket 112 of the test sample to form an oil flow 114, forming an oil-paper composite insulation in which the insulating oil is in a flowing state, and returns to the oil storage tank 120 through the upper electrode cover oil flow outlet 127 and the upper electrode cover oil flow outlet valve 128 to realize the oil flow circulation.

[0080] As a further embodiment, the oil storage tank 120 is grounded to facilitate the tested oil flow 114 to be fully relaxed after returning to the oil storage tank 120 .

[0081] As a further embodiment, the upper electrode cover 110 and the lower electrode cover 111 are respectively provided with an upper electrode cover oil inlet 126 and a lower electrode cover oil inlet 130, corresponding upper electrode cover oil inlet valve 125 and lower electrode cover oil inlet valve 129, as well as an upper electrode cover oil outlet 127 and a lower electrode cover oil outlet 131, corresponding upper electrode cover oil outlet valve 128 and lower electrode cover oil outlet valve 132, in order to realize the separate measurement of the current on the oil side and paper side of the oil-paper composite insulation.

[0082] When the oil-immersed paper is close to the measuring electrode 104 and the protective electrode 105, the oil gap gasket and the oil flow are close to the high-voltage electrode side, close the oil flow inlet valve 129 at the lower electrode cover and the oil flow outlet valve 132 at the lower electrode cover, open the oil flow inlet valve 125 at the upper electrode cover and the oil flow outlet valve 128 at the upper electrode cover to realize the measurement of the polarization / depolarization current on the paper side.

[0083] When the oil-impregnated paper is close to the side of the high-voltage electrode 103, the oil gap gasket 112 and the oil flow 113 are close to the sides of the measuring electrode 104 and the protective electrode 105, the oil flow inlet valve 125 and the oil flow outlet valve 128 at the upper electrode cover are closed, and the oil flow inlet valve 129 and the oil flow outlet valve 132 at the lower electrode cover are opened to realize the measurement of the polarization / depolarization current on the oil side.

[0084] The above system can achieve the following functions:

[0085] (1) Free adjustment of the thickness of oil and paper samples;

[0086] (2) Free regulation of the applied voltage;

[0087] (3) Stable and precise regulation of different oil flow velocities in the oil gap of the oil-paper composite insulation test sample under oil flow conditions;

[0088] (4) Precise measurement of the polarization / depolarization current on the oil side and paper side of the oil-paper composite insulation under oil flow conditions.

[0089] The working principle of the system is as follows:

[0090] Adjust the speed of the oil pump through a frequency converter to roughly control the liquid flow rate and push the insulating oil in the oil tank towards the measuring electrode direction, and further control the insulating oil flow rate through a liquid mass flowmeter;

[0091] The oil flow passes through a pressure stabilizing tank, and the compressed gas is used to effectively absorb the fluctuating kinetic energy of the liquid, further optimizing the oil flow stability, and flowing into the PDC three-electrode system to achieve the control of the insulating oil flow rate;

[0092] The oil flow flowing out of the PDC three-electrode system returns to the storage oil tank to form an oil flow cycle.

[0093] The PDC measurement circuit includes a high-voltage electrode, a measurement electrode, and a protection electrode;

[0094] Among them, the high-voltage electrode is connected to a current-limiting protection resistor, a high-voltage relay, and a high-voltage DC power supply, the protection electrode is connected to the ground wire, the measurement electrode is connected to an over-current protection module and a high-resistance meter, and the current signal measured by the high-resistance meter is transmitted to the computer acquisition system through a data acquisition card and data processing is performed to obtain the PDC curve of the insulating oil under different flow velocity states.

[0095] In the prior art, the insulating oil is limited to a static state, while in the actual operating conditions of large oil-immersed power transformers such as converter transformers, the insulating oil is forced to circulate. Therefore, in this embodiment, the speed of the oil pump is roughly adjusted through a frequency converter, and the flow rate is finely adjusted through a liquid mass flowmeter, and further the pressure relief tank is used to absorb the oil flow fluctuations, thereby achieving the effect of stable regulation of the insulating oil flow rate, and successfully designing an oil flow circulation path with adjustable speed. Among them, the oil flow rate error can be controlled within the accuracy of 1 mL / min.

[0096] Based on the fact that the flow of insulating oil in power transformers mainly belongs to the laminar flow state, therefore, through the design of the internal oil gap and the structure and size of the inflow and outflow ports of the PDC measurement circuit, the insulating oil flow in the tested oil-paper composite insulation sample is always in the laminar flow state, so as to achieve experimental simulation conditions closer to the actual working conditions.

[0097] The oil flow inside an oil-immersed power transformer is mainly in a laminar flow state and may be in a turbulent flow state at positions with complex structures such as corners. Some existing studies believe that the flow in a power transformer mainly belongs to laminar flow, and the research on flow velocity should be limited to the laminar flow range. Whether the oil flow is in a laminar or turbulent state can be described by formula calculation.

[0098] It can be known through calculation that the flow velocity range simulated by the experimental system under the actual working conditions of a large oil-immersed power transformer is much lower than the laminar-turbulent critical flow velocity corresponding to the oil gap structure of this experimental system. Therefore, the flow velocity during the experiment in this system is within the laminar flow range.

[0099] Through the design of the measuring electrode and the protection electrode structures of the PDC three-electrode system, by designing the inner edge width of the protection electrode to be smaller than the inner ring width of the oil gap gasket, the surface current along the edge of the specimen and the leakage current of other dielectrics can be excluded, thus improving the accuracy of the PDC measurement of the oil-paper composite insulation under the oil flow state.

[0100] Specifically: The length and width of the outer ring edge of the oil gap gasket 112 and the oil-impregnated paper 113 are the same as the dimensions of the lower electrode groove or the upper electrode protrusion part. The protection electrode 105 is as flat as the lower electrode 104 and is on the same horizontal plane. The inner ring width of the protection electrode 105 is smaller than the inner ring width of the oil gap gasket 112 to ensure that the inner edge of the protection electrode 105 can protrude from the hollow part of the oil gap gasket 112, so that the wall current along the inner ring wall of the oil gap gasket 112, as well as the conductance current in the oil gap gasket medium and the oil-impregnated paper at this place, and even the current at the outer edge of the oil-impregnated paper and the oil gap gasket can be drained to the grounding electrode.

[0101] In summary, in a large oil-immersed power transformer (especially a converter transformer), the insulating oil flows in a forced circulation manner in the oil-paper composite insulation system. This flow state will significantly affect the dynamic behavior of charges in the composite insulation system, including processes such as charge ionization, injection, recombination, migration, and extraction, thereby further changing the polarization / depolarization current characteristics of the composite insulation system. However, most of the current measurement methods for the polarization / depolarization current (PDC) of oil-paper composite insulation are designed based on the static state of the insulating oil, resulting in the measurement results being difficult to accurately reflect the charge transport characteristics and its insulation state under the "insulating oil flow" condition, which is closer to the actual operating conditions of a large oil-immersed power transformer. This limitation may affect the accuracy of the assessment of the transformer insulation state.

[0102] The PDC measurement system for oil-paper composite insulation under the condition of insulating oil flow proposed in this embodiment realizes the measurement of PDC of oil-paper composite insulation by simulating the conditions of different flow rates of insulating oil. The measurement results are generated by the charge transport behavior under the actual working condition of insulating oil flow, which helps to establish the connection between polarization / depolarization current and insulation state under actual working conditions, so as to more truly reflect the insulation state under actual working conditions.

[0103] Embodiment 2:

[0104] The method of the PDC measurement system for oil-paper composite insulation under the condition of insulating oil flow includes the following steps:

[0105] Pre-treat the insulating oil and inject it into the groove of the lower electrode cover. Then place the oil-impregnated paper and the oil gap gasket into the groove, and assemble the upper electrode cover and the lower electrode cover to obtain a sealed oil flow channel.

[0106] Start the oil pump and circulate for a set period of time to eliminate the air bubbles in the oil flow velocity control oil circuit.

[0107] Start the PDC measurement circuit. By controlling the closing of a pair of contacts of the high-voltage relay, the polarization current measurement circuit is actuated. By controlling the closing of another pair of contacts of the high-voltage relay, the depolarization current measurement circuit is actuated.

[0108] By adjusting the rotation speed of the oil pump and combining with the liquid mass flow controller, the regulation of the oil flow velocity is realized. By adjusting the voltage amplitude of the high-voltage DC power supply, the applied voltage of the specimen is controlled; the PDC curves under different flow rates and different voltage states of the insulating oil are obtained.

[0109] Specifically:

[0110] (1) Place a part of the insulating oil sample after drying and vacuum treatment into the groove of the lower electrode cover 111. Secondly, place the oil-impregnated paper 113 sample and the oil gap gasket 112 into the groove successively according to the measurement needs. Then place the convex part of the upper electrode cover 110 into the groove of the lower electrode cover 111, and thus form a sealed oil gap oil flow 114 channel in the oil-paper composite insulation.

[0111] (2) Connect the outlet of the storage tank, the oil pump, the liquid mass flow controller, the pressure relief tank, and the valve at the inlet of the PDC three-electrode system in sequence. Connect the valve at the outlet of the PDC three-electrode system to the inlet of the storage tank to form a closed oil circuit. Start the oil pump to complete the elimination of air bubbles in the whole oil circuit.

[0112] (3) Connect the high-voltage DC power supply, high-voltage relay, current-limiting resistor, and the high-voltage electrode of the PDC three-electrode system in sequence. Connect the measuring electrode of the PDC three-electrode system, high-resistance meter, and computer in sequence. Connect the grounding terminal of the high-voltage DC power supply, the grounding terminal of the protection electrode, the shell of the over-current protection module, and the grounding terminal of the high-resistance meter to the grounding wire respectively to complete the connection of the measuring circuit;

[0113] (4) Turn on the high-voltage DC power supply and control the closing of the contacts 1 and 2 of the high-voltage relay to complete the connection of the polarization current measuring circuit. Control the closing of the contacts 1 and 3 of the high-voltage relay to complete the connection of the depolarization current measuring circuit;

[0114] (5) Regulate the oil pump speed control module and set the flow rate of the liquid mass flow controller to control the oil flow velocity. Regulate the voltage amplitude of the high-voltage DC power supply to control the applied voltage on the specimen.

[0115] By adjusting the settings of the voltage output amplitude of the high-voltage DC power supply and the oil flow velocity, the polarization / depolarization currents on the paper side and oil side of the oil-paper composite insulation can be measured under different oil flow velocity conditions and applied voltage amplitude conditions.

[0116] To verify the usability and reliability of the measurement system, an oil-paper composite insulation specimen is constructed using a 0.25-mm-thick oil-impregnated paper specimen and a 0.35-mm-thick oil gap oil flow, and the oil-paper composite insulation current is measured under a flow velocity condition of 0.3 m / s.

[0117] Figure 5 Under a field strength condition of 10 kV / mm, keep the voltage applied continuously for 30 min to make the current reach a stable state. Then, under the condition of keeping the applied electric field unchanged, measure the leakage current on the paper side during the process when the insulating oil is first stationary, then flows (at 0.3 m / s), and then becomes stationary again. As can be seen from this figure, different flow velocities of the insulating oil will affect the current of this composite insulation.

[0118] Figure 6 This is the polarization (field strength 10 kV / mm) / depolarization current waveform diagram on the paper side under the condition that the flow velocity of the insulating oil is 0.3 m / s. As can be seen from this figure, the experimental system can measure the PDC waveform of the oil-paper composite insulation under the flowing state of the insulating oil.

[0119] The insulating oil circulates between the upper and lower electrode covers of the transformer and the oil storage tank through the insulating oil flow velocity control oil circuit, simulating the forced-flow state of the insulating oil in the actual working condition of a large oil-immersed power transformer. Further, a pressure relief tank is used to absorb the oil flow fluctuations, thereby achieving the effect of stable regulation of the insulating oil flow velocity and enabling the oil flow rate error to be controlled within an as low range as possible.

[0120] In the PDC measurement circuit, through the design of the measuring electrode and the guard electrode structure, the surface leakage current along the edge of the specimen and other dielectric leakage currents are excluded, improving the accuracy of the PDC measurement of oil-paper composite insulation.

[0121] In the PDC measurement circuit, by switching the contacts of the high-voltage relay pair, a polarization current test circuit and a depolarization current test circuit are formed to realize the test of polarization / depolarization currents under different applied voltage amplitudes. By controlling the switching of the control valve in the oil circuit through the flow rate of the insulating oil, the test of polarization / depolarization currents is realized when the oil-impregnated paper is attached to different positions. Thus, it is as close as possible to the actual working conditions of large power transformers, expanding the measurement range and improving the measurement accuracy.

[0122] In summary, in large oil-immersed power transformers (especially converter transformers), the insulating oil flows in a forced circulation manner in the oil-paper composite insulation system. This flow state will significantly affect the dynamic behavior of charges in the composite insulation system, including processes such as charge ionization, injection, recombination, migration, and extraction, thereby further changing the polarization / depolarization current characteristics of the composite insulation system. However, most of the current measurement methods for the polarization / depolarization current (PDC) of oil-paper composite insulation are designed based on the static state of the insulating oil, resulting in the measurement results being difficult to accurately reflect the charge transport characteristics and insulation state under the actual operating conditions of "insulating oil flow" that are closer to large oil-immersed power transformers. This limitation may affect the accuracy of the assessment of the transformer insulation state.

[0123] Through the solution proposed in this embodiment, the problem of testing the PDC of oil-paper composite insulation at different flow rates can be solved. Through the test research on the PDC under the flowing state of the insulating oil, the characteristics of the polarization / depolarization current of the oil-paper composite insulation, such as the speed and time constant of polarization / depolarization, current value, trapped current component and other characteristic parameters, can be obtained from the collected current data. Through the internal connection and mechanism between these characteristic parameters and the insulation state, important information such as moisture, aging, conductivity, and space charge behavior in the composite insulation under the oil flow state closer to the actual working conditions of large power transformers can be obtained, providing meaningful phenomena and theoretical basis for the insulation design and equipment maintenance of oil-immersed transformers.

[0124] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. Oil-paper composite insulation PDC measurement system under the condition of insulating oil flowing, characterized by: include: The test sample assembly is arranged between the upper electrode cover and the lower electrode cover, and includes an oil gap gasket providing an oil gap channel, oil-impregnated paper, and an oil flow flowing through the oil gap channel; The PDC measuring circuit comprises a high-voltage electrode arranged on the upper electrode cover and a measuring electrode arranged on the lower electrode cover. The high-voltage DC power supply is connected in series with a high-voltage relay and a current-limiting resistor and then connected to the high-voltage electrode. The measuring electrode is connected in series with an overcurrent protection module and a high-resistance meter and then grounded. The insulating oil flow rate control oil circuit includes the insulating oil stored in the oil storage tank. Driven by the oil pump, the insulating oil passes through the liquid mass flow meter and the pressure relief tank. According to the experimental requirements of measuring the paper side or oil side current, the insulating oil is regulated to flow into the upper electrode cover or the lower electrode cover respectively. After flowing through the oil gap channel to form an oil flow, it returns to the oil storage tank.

2. The oil-paper composite insulation PDC measurement system under the insulating oil flow state according to claim 1, characterized in that: The high-voltage relay has three contacts, contact 1 is connected to a current-limiting resistor, contact 2 is connected to a high-voltage direct current power supply, and contact 3 is grounded.

3. The oil-paper composite insulation PDC measurement system under the insulating oil flow state according to claim 2, characterized in that: When contacts 1 and 2 of the high-voltage relay are closed, the high-voltage DC power supply, high-voltage relay, current-limiting resistor, high-voltage pole, test sample component, measuring pole and high resistance meter are connected in series, and the high-voltage DC power supply, overcurrent protection module and high resistance meter ground terminal are grounded to form a polarization current test circuit.

4. The oil-paper composite insulation PDC measurement system under the insulating oil flow state according to claim 2, characterized in that: When the contacts 1 and 3 of the high-voltage relay are closed, the high-voltage relay, current-limiting resistor, high-voltage pole, test sample component, measuring pole and high resistance meter are connected in series, the high-voltage relay contact is grounded, and the grounding terminal of the high resistance meter is grounded to form a depolarization current test circuit.

5. The oil-paper composite insulation PDC measurement system under the insulating oil flow state according to claim 1, characterized in that: A sealed space for accommodating oil-impregnated paper and insulating oil is formed between the upper electrode cover and the lower electrode cover. An oil gap gasket is arranged outside the oil-impregnated paper. The insulating oil flows through the hollow oil gap channel of the oil gap gasket to form an oil flow.

6. The oil-paper composite insulation PDC measurement system under the insulating oil flow state according to claim 1, characterized in that: A protective electrode is arranged at the periphery of the measuring electrode, and the two are insulated from each other. The measuring electrode is connected to the protection module and the high resistance meter, and the protective electrode is grounded.

7. The oil-paper composite insulation PDC measurement system in the insulating oil flowing state according to claim 1 is characterized in that: The insulating oil flow rate control oil circuit also includes an oil flow inlet and an oil flow outlet respectively arranged on the upper electrode cover and the lower electrode cover, and the oil flow inlet and the oil flow outlet are connected to corresponding control valves.

8. The oil-paper composite insulation PDC measurement system in the insulating oil flowing state according to claim 1 is characterized in that: When the oil-impregnated paper is close to the measuring electrode and the protective electrode side, the oil gap gasket and the oil flow are close to the high-voltage electrode side, and the insulating oil flows through the upper electrode cover but not the lower electrode cover, the polarization / depolarization current on the paper side can be measured.

9. The oil-paper composite insulation PDC measurement system in the insulating oil flowing state according to claim 1, characterized in that: When the oil-impregnated paper is close to the high-voltage pole side, the oil gap gasket and the oil flow are close to the measuring pole and the protective pole side, the insulating oil flows through the lower electrode cover but not the upper electrode cover, thereby realizing the measurement of the polarization / depolarization current on the oil side.

10. The oil-paper composite insulation PDC measurement system in the insulating oil flowing state according to claim 1, characterized in that: The overcurrent protection module and the high resistance meter are grounded, and the high resistance meter is connected to the data processing unit through the data acquisition module.

11. The oil-paper composite insulation PDC measurement system in the insulating oil flowing state according to claim 1, characterized in that: The outlet of the oil storage tank is connected to the oil pump, the liquid mass flow controller and the pressure relief tank through a pipeline. The pressure relief tank is connected to the oil flow inlets of the upper electrode cover and the lower electrode cover through pipelines respectively. The oil flow outlets of the upper electrode cover and the lower electrode cover are connected to the inlet of the oil storage tank through pipelines.

12. The oil-paper composite insulation PDC measurement system in the insulating oil flowing state according to claim 1, characterized in that: The oil pump is connected to a frequency converter for adjusting the flow rate.

13. A method for measuring an oil-paper composite insulation PDC under the flowing state of insulating oil based on the measuring system of any one of claims 1 to 12, characterized in that: The following steps are involved: Pre-treat the insulating oil and inject it into the groove of the lower electrode cover, place the oil-impregnated paper and the oil gap gasket into the groove, install the sealing ring at the upper electrode cover, and assemble the upper electrode cover and the lower electrode cover to obtain a sealed oil flow channel; The oil pump starts and circulates for a set period of time to eliminate bubbles in the oil circuit; The PDC measurement circuit is started, and the polarization current measurement circuit is operated by controlling one pair of contacts of the high-voltage relay to close, and the depolarization current measurement circuit is operated by controlling the other pair of contacts of the high-voltage relay to close; By adjusting the oil pump speed and combining it with a liquid mass flow controller, the oil flow rate can be regulated. By adjusting the voltage amplitude of the high-voltage DC power supply, the external voltage applied to the sample can be controlled. The PDC curves of the insulating oil under different flow rates and voltage conditions can be obtained.

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