Composite voltage generation device and cable insulation performance evaluation system and method

Abstract

The invention discloses a composite voltage generation device and a cable insulation performance evaluation system and method, and relates to the technical field of high-voltage insulation testing. The composite voltage generating device comprises a synchronous controller, a direct current source, an impact source, an IGBT active protection unit and a magnetic isolation coupler, and by means of the excellent characteristics of the IGBT and the magnetic isolation coupler, the superposition time sequence accuracy of direct current and impact voltage is improved, and waveform distortion is reduced. The IGBT active protection unit comprises a first IGBT and a second IGBT, and direct-current voltage polarity switching can be realized through selective conduction of the IGBTs; the RC circuit is connected in parallel between the collector electrode of the first IGBT and the emitter electrode of the second IGBT, reverse current flowing back to the direct current source direction can be discharged through the RC circuit, the risk of energy flowing back is reduced, meanwhile, the magnetic isolation coupler can completely block direct current, and an impact source is protected against the influence of direct current voltage; according to the invention, the authenticity and reliability of cable insulation performance evaluation can be effectively improved.

Description

Technical Field

[0001] This invention relates to the field of high-voltage insulation testing technology, and in particular to a composite voltage generating device, a cable insulation performance evaluation system and method. Background Technology

[0002] In high-voltage direct current (HVDC) transmission systems, DC cables serve as the core carriers of electrical energy transmission, and their insulation performance directly determines the safety and reliability of system operation. However, in actual operation, DC cables not only bear pure DC voltage but also frequently encounter superimposed interference from impulse voltages such as lightning overvoltages and switching operation overvoltages, forming a composite voltage condition of "DC + impulse". Conventional tests on cable insulation performance often focus on pure DC voltage or single impulse voltage scenarios, neglecting the synergistic effect of space charge accumulation under DC voltage and instantaneous field strength distortion under impulse voltage. This synergistic effect accelerates the initiation and expansion of internal defects in the insulation material, resulting in a significantly higher rate of insulation performance degradation than under single voltage conditions. This leads to safety hazards such as insulation breakdown and cable faults, causing not only high equipment maintenance costs but also potential power transmission system shutdowns, affecting the stability of regional energy supply.

[0003] In recent years, research on the insulation performance of DC cables under combined voltage conditions has led to the design of DC superimposed impulse voltage experimental devices to simulate the "DC + impulse" voltage environment in actual operation. These devices share common characteristics in their circuit topology design: the DC source is designed as a low-voltage, high-current output, while the impulse source is a high-voltage, narrow-pulse output. The DC source side is typically equipped with a relay switch to control the on / off state of the DC circuit, and a current-limiting resistor is connected in series. The core function of the current-limiting resistor is to suppress the backflow current that may occur during the instantaneous loading of the impulse voltage, preventing damage to the DC source side components due to overcurrent. The impulse source side is generally equipped with a DC blocking capacitor connected in series. Through its "AC-passing, DC-blocking" characteristic, it blocks the DC component from penetrating to the impulse source side to protect the impulse source, while ensuring that the impulse voltage can be effectively coupled to the test circuit to achieve superposition with the DC voltage.

[0004] However, such devices have significant limitations: First, relay switches cannot achieve polarity switching; achieving DC current polarity switching requires manual adjustment of the rectifier circuit (taking tens of minutes), and cannot achieve dynamic polarity reversal, lacking the ability to test opposite polarities (e.g., DC negative voltage + impulse positive voltage); Second, relay switches have inherent action response delays (response time > 5μs, while the impulse wave rise edge is only 1.2μs), making it difficult to accurately control the superposition timing of DC voltage and impulse pulses, resulting in large deviations in the superposition time and failing to truly simulate the instantaneous coupling effect of voltage sudden changes under actual working conditions; Third, air gaps or oxide layers between relay contacts can form leakage paths, with leakage current reaching mA levels under high voltage. The equivalent capacitance of the cable discharges through this leakage current, causing the DC voltage to drop significantly before the impulse voltage is applied. When the impulse voltage is superimposed, stray inductance and capacitance in the circuit (such as stray inductance and capacitance caused by relay contacts) will oscillate freely during the relay's delayed turn-off period, resulting in overshoot or oscillation tails at the peak of the impulse wave, and the capacitive reactance of the DC blocking capacitor is high (e.g., 10). The 0pF capacitor has a capacitive reactance of approximately 15.9kΩ at 10kHz, which causes the leading edge of the impulse waveform to slow down, the peak value to decrease, and the high-frequency component to attenuate by more than 20%. These factors together result in an oscillation amplitude of the composite voltage after the impulse voltage is superimposed by more than 20% and a waveform distortion rate of more than 20%. Fourth, the mechanical characteristics of the relay cause the arc erosion from each turn-on and turn-off to accumulate, which can easily lead to poor contact or even welding at the contacts. The response delay of the relay will further increase. Moreover, the DC blocking capacitor, based on the capacitive reactance of the dielectric material, will inevitably generate leakage current under high voltage. This will cause the impulse source to be subjected to DC voltage for a long time, resulting in the deterioration or even damage of the internal electrical components. Therefore, it is impossible to guarantee the waveform consistency of multiple rounds of testing. Fifth, when the impulse source or the equivalent capacitance of the cable releases energy, a reverse current will be generated and backflowed into the DC source. The current-limiting resistor on the DC path is easily burned out (if a water resistor is used, the water resistor will vaporize and open the circuit). Therefore, the existing DC superimposed impulse voltage test device has low authenticity and reliability for cable insulation performance evaluation, cannot reflect the real working conditions, and will damage the device. Summary of the Invention

[0005] This invention provides a composite voltage generating device, a cable insulation performance evaluation system and method. The aim is to design a new composite voltage generating device to improve the accuracy of the superposition timing of DC and impulse voltages, reduce waveform distortion, reduce the risk of energy backflow, and establish a dynamic switching mechanism for DC voltage polarity. This facilitates multiple cyclic tests of DC superimposed impulse voltage on cable samples, thereby improving the authenticity and reliability of cable insulation performance evaluation.

[0006] The composite voltage generator provided by this invention includes a synchronous controller, a DC source, an impulse source, an IGBT active protection unit, a magnetic isolation coupler, and a common output terminal;

[0007] The output terminal of the DC source is electrically connected to the input terminal of the IGBT active protection unit, the output terminal of the impulse source is electrically connected to the input terminal of the magnetic isolation coupler, and the output terminal of the IGBT active protection unit and the output terminal of the magnetic isolation coupler are connected to a common output terminal.

[0008] The output terminal of the DC source includes an upper DC bus and a lower DC bus. The IGBT active protection unit includes a first IGBT, a second IGBT, a first diode, a second diode, a first inductor, a second inductor, a first capacitor, and an RC circuit.

[0009] The first end of the first inductor is connected to the upper end of the DC bus, and the second end is connected to the collector of the first IGBT; the first end of the second inductor is connected to the lower end of the DC bus, and the second end is connected to the emitter of the second IGBT; the first capacitor is connected across the first end of the first inductor and the first end of the second inductor; the emitter of the first IGBT and the collector of the second IGBT are connected through a common node; the first diode is connected in anti-parallel with the first IGBT, and the second diode is connected in anti-parallel with the second IGBT; the RC circuit is connected in parallel between the collector of the first IGBT and the emitter of the second IGBT, and the RC circuit includes at least one resistor and at least one capacitor connected in series; the output terminal of the IGBT active protection unit is led out from the common node.

[0010] The synchronization controller is electrically connected to the gate of the first IGBT and the gate of the second IGBT, and is used to control the switching on and off of the first IGBT or the second IGBT in a preset timing sequence; the synchronization controller is also electrically connected to the control terminal of the impact source, and is used to trigger the impact source to output an impact voltage in a preset timing sequence.

[0011] Optionally, the RC circuit includes a first resistor, a second capacitor, a third inductor, and a third diode;

[0012] The first end of the first resistor is connected to the collector of the first IGBT, the second end of the first resistor is connected to the first end of the third inductor, the second end of the third inductor is connected to the first end of the second capacitor, and the second end of the second capacitor is connected to the emitter of the second IGBT; the anode of the third diode is connected to the first end of the first resistor, and the cathode of the third diode is connected to the second end of the first resistor.

[0013] Optionally, the magnetic isolation coupler includes a nanocrystalline magnetic ring, a dual-winding coil, and a protective housing;

[0014] The dual-winding coil includes a primary coil and a secondary coil, which are wound in parallel on the nanocrystalline magnetic ring.

[0015] The protective shell has a receiving cavity, in which the nanomagnetic crystal ring and the dual-winding coil are placed, and the receiving cavity is filled with silica gel.

[0016] The cable insulation performance evaluation system provided by this invention includes the composite voltage generating device described above, as well as a measurement module and an evaluation module;

[0017] The measurement module includes a simulation chamber and a high-speed monitoring unit. The simulation chamber is used to contain the cable sample, and the high-speed monitoring unit is used to collect test data of the cable sample. The test data includes the space charge distribution signal, leakage current signal, high-frequency transient current signal, ultrasonic signal, and temperature signal of the cable sample. The evaluation module is used to acquire the test data and output evaluation results based on the test data.

[0018] Optionally, the simulation chamber includes a temperature field control unit;

[0019] The temperature field control unit includes a medium-frequency induction coil and a spray ring. The medium-frequency induction coil is located inside the spray ring and is coaxially arranged with the spray ring. The medium-frequency induction coil is configured to generate an electromagnetic field that can be used to heat the conductor after being energized. The spray ring is configured to spray coolant towards the central axis of the spray ring.

[0020] A quartz glass partition is provided at the axial end of the spray ring. A through hole is opened on the quartz glass partition corresponding to the central axis of the spray ring. A fluororubber sealing strip is wrapped at the connection between the quartz glass partition and the spray ring.

[0021] Optionally, the simulation chamber further includes a mechanical stress loading unit;

[0022] The mechanical stress loading unit includes a hydraulic mechanism and multiple sets of clamps, which are configured to clamp the cable sample at preset intervals; the hydraulic mechanism is configured to drive the clamps to reciprocate in a preset direction at a preset frequency and amplitude.

[0023] Optionally, the high-speed monitoring unit includes a data acquisition card, a transient charge sensor, a high-precision electrometer, a high-frequency CT scanner, an ultrasonic transducer, and a temperature sensor.

[0024] The transient charge sensor is configured to acquire the space charge distribution signal of the cable sample insulation layer; the high-precision electrometer is configured to acquire the leakage current of the cable sample insulation layer; the high-frequency CT is configured to acquire the high-frequency transient current signal of the cable sample insulation layer; the ultrasonic transducer is configured to acquire the ultrasonic signal generated by the cable sample insulation layer; and the temperature sensor is configured to acquire the temperature of the cable sample. The acquisition card is communicatively connected to the transient charge sensor, the high-precision electrometer, the high-frequency CT, the ultrasonic transducer, and the temperature sensor.

[0025] Based on the cable insulation performance evaluation system described above, this invention proposes a cable insulation performance evaluation method, which includes the following steps:

[0026] S1, Place the cable sample in the simulation chamber. The cable sample includes a power connection end and a grounding end. The grounding end is grounded, and the power connection end is connected to the common output terminal of the composite voltage generator.

[0027] S2, turn on one of the IGBTs in the IGBT active protection unit, apply a DC voltage to the cable sample through a DC source, and detect the leakage current of the cable sample; when the rate of change of the leakage current is <0.1% / min, apply an impulse voltage to the cable sample through an impulse source, and at the same time turn off the IGBT active protection unit.

[0028] S3, Measure and record the target test signal of the cable sample;

[0029] S4, After applying the impulse voltage to the cable sample for a preset time, turn off the impulse source;

[0030] S5, repeat steps S2-S4 a preset number of times, calculate the field strength dependent conductivity of the cable sample insulation for each measurement, and calculate the remaining life of the cable sample based on the field strength dependent conductivity.

[0031] Optionally, the target test signal includes a space charge distribution signal, and the field strength-dependent conductivity is calculated using the following formula:

[0032] ;

[0033] In the formula, γ(E) is the field strength dependent conductivity, γ0 is the intrinsic conductivity of the cable sample insulation, α is the trap-assisted conduction coefficient, E is the local electric field strength related to space charge, and β is the field emission contribution coefficient.

[0034] The remaining life of the cable sample is calculated using the following formula:

[0035] ;

[0036] In the formula, L represents the remaining lifetime, L0 represents the initial lifetime, λ represents the aging acceleration factor, γ(E) is the field strength dependent conductivity, and E is the local electric field strength related to space charge.

[0037] Optionally, the target test signal further includes a high-frequency transient current signal, an ultrasonic signal, and a temperature signal; the evaluation method further includes the following step: constructing a visualization cloud map of the cable sample based on the high-frequency transient current signal, ultrasonic signal, temperature signal, field strength-dependent conductivity, and remaining lifetime.

[0038] The present invention has the following beneficial effects:

[0039] The composite voltage generator designed in this invention uses an IGBT active protection unit to replace the relay and current-limiting resistor on the DC source side in the traditional scheme, and uses a magnetic isolation coupler to replace the DC blocking capacitor on the impulse source side in the traditional scheme, which has the following significant advantages:

[0040] The polarity of the DC voltage applied to the cable sample can be dynamically switched by controlling the commutation of the IGBT bridge arm (i.e., controlling the individual conduction of either the first or second IGBT). The IGBT is based on MOSFET gate control and has high-speed switching characteristics under voltage control, enabling turn-off within 100ns, fast dynamic response, and more accurate timing of composite voltage superposition. The IGBT is based on the reverse blocking of the semiconductor PN junction, with extremely low leakage current (typically <10μA), which can reliably block the reverse path of DC voltage and reduce waveform distortion. When the impulse voltage is triggered, the current flowing back into the DC source direction can be discharged through the RC circuit connected in parallel with the IGBT, realizing the active discharge of reverse energy and limiting the reverse current within a safe threshold to avoid burning out the IGBT.

[0041] Magnetic isolation couplers are based on the principle of electromagnetic induction, and their core function is to transmit changing magnetic fields. Their inductive reactance is much lower than that of traditional DC blocking capacitors, resulting in low frequency attenuation and phase shift of impulse voltages, which can reduce waveform distortion. The primary and secondary windings of the magnetic isolation coupler are completely electrically isolated, which fundamentally blocks DC reverse flow (the DC component generates a constant magnetic field, which cannot induce current in the secondary winding, so the DC is physically blocked, rather than blocked by the capacitive reactance of the DC blocking capacitor), thus avoiding the influence of DC voltage on the impulse source.

[0042] Based on the above advantages, the composite voltage generator used in this invention can effectively improve the authenticity and reliability of cable insulation performance evaluation. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0044] Figure 1 These are structural block diagrams of some embodiments of the composite voltage generating device of the present invention;

[0045] Figure 2 This is a schematic diagram of the circuit structure of the IGBT active protection unit of the present invention;

[0046] Figure 3 This is a schematic diagram of the timing control of the impact stage in some embodiments of the composite voltage generator of the present invention;

[0047] Figure 4 These are schematic diagrams of some embodiments of the magnetic isolation coupler of the present invention;

[0048] Figure 5 These are schematic diagrams of some embodiments of the temperature field control unit of the present invention;

[0049] Figure 6 These are structural block diagrams of some embodiments of the high-speed monitoring unit of the present invention;

[0050] Figure 7 This is a schematic diagram of the algorithm flow for evaluating cable insulation performance in some embodiments of the present invention.

[0051] Explanation of reference numerals in the attached figures:

[0052] 1. Synchronous controller; 2. DC power supply; 3. Impact source; 4. IGBT active protection unit; 5. Magnetic isolation coupler; 6. Common output terminal; 41. RC circuit; 101. Nanocrystalline magnetic ring; 102. Dual-winding coil; 103. Protective housing; 104. Silicon gel; 201. Medium frequency induction coil; 202. Spray ring; 203. Quartz glass partition. Detailed Implementation

[0053] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0054] The terms “first,” “second,” “third,” “fourth,” etc. (if present) in the specification and drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented, for example, in orders other than those illustrated or described herein. Furthermore, the terms “comprising” and “having,” and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0055] A first aspect of the present invention provides a composite voltage generating device for applying a DC superimposed impulse voltage to a cable sample.

[0056] The cable sample includes a power connection terminal and a grounding terminal. The grounding terminal is grounded, and the power connection terminal is connected to the common output terminal of the composite voltage generator. The cable sample is a DC cable. A typical cable sample usually includes, from the inside out, a conductor, an inner semi-conductive shielding layer, an insulation layer, an outer semi-conductive shielding layer, a shielding layer (metal shielding layer), an armor layer, and an outer sheath. The forms of DC cables include, but are not limited to, direct-buried DC cables, duct DC cables, submarine DC cables, and overhead DC cables.

[0057] See Figure 1 In this embodiment of the invention, the composite voltage generating device includes a synchronous controller 1, a DC source 2, an impulse source 3, an IGBT active protection unit 4 (IGBT: Insulated Gate Bipolar Transistor), a magnetic isolation coupler 5, and a common output terminal 6; the output terminal of the DC source 2 is electrically connected to the input terminal of the IGBT active protection unit 4, the output terminal of the impulse source 3 is electrically connected to the input terminal of the magnetic isolation coupler 5, and the output terminal of the IGBT active protection unit 4 and the output terminal of the magnetic isolation coupler 5 are both connected to the common output terminal 6.

[0058] The output terminals of the DC source include the upper end of the DC bus and the lower end of the DC bus (corresponding to the positive and negative terminals of the DC source, respectively). See [link / reference]. Figure 1 , 2 The IGBT active protection unit includes a first IGBT, a second IGBT, a first diode D1, a second diode D2, a first inductor L1, a second inductor L2, a first capacitor C1, and an RC circuit 41.

[0059] Specifically, the first end of the first inductor L1 is connected to the upper end of the DC bus, and the second end is connected to the collector of the first IGBT; the first end of the second inductor L2 is connected to the lower end of the DC bus, and the second end is connected to the emitter of the second IGBT; the first capacitor C1 is connected across the first end of the first inductor L1 and the first end of the second inductor L2; the emitter of the first IGBT and the collector of the second IGBT are connected through a common node; the first diode D1 is connected in anti-parallel with the first IGBT, and the second diode D2 is connected in anti-parallel with the second IGBT; the RC circuit 41 is connected in parallel between the collector of the first IGBT and the emitter of the second IGBT, and the RC circuit 41 includes at least one resistor and at least one capacitor connected in series; the output terminal of the IGBT active protection unit 4 is led out from the common node.

[0060] The synchronous controller 1 is electrically connected to the gate of the first IGBT and the gate of the second IGBT, and is used to control the switching on and off of the first IGBT or the second IGBT in a preset timing sequence; the synchronous controller 1 is also electrically connected to the control terminal of the impulse source 3, and is used to trigger the impulse source 3 to output impulse voltage in a preset timing sequence.

[0061] It should be noted that, in the embodiments of the present invention, the DC source and the impulse source can be obtained through market means or existing design schemes, and are not particularly limited here; generally, the DC source is a power device that can output a stable DC voltage or current, including units such as an input rectifier module, a filter module, a voltage and current stabilization module, and a protection module; the impulse source is a power device that can generate short-time high-voltage pulses (simulating lightning strikes, switching operation impulses, etc.), including units such as a high-voltage DC power supply, an energy storage module, a pulse generation unit, a waveform control module, a trigger control module, and a protection module.

[0062] IGBT, or Insulated Gate Bipolar Transistor, is a composite power semiconductor device that combines the voltage control characteristics of MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) with the high current carrying capacity of GTR (High-Power Transistor). Its core structure uses an insulated gate as the control terminal, and the main circuit is quickly controlled by the change of gate voltage. IGBTs are widely used in high-voltage, high-current power electronic systems. They have high-speed switching characteristics under voltage control, can be turned off within 100ns, have fast dynamic response speed, and more accurate timing of composite voltage superposition. Moreover, IGBTs are based on the reverse blocking of semiconductor PN junctions, with extremely small leakage current (typically <10μA), and can reliably block reverse paths.

[0063] A magnetic isolation coupler is an electronic device that transmits signals or energy through a magnetic field while simultaneously achieving electrical isolation between circuits. In this embodiment of the invention, the magnetic isolation coupler relies on the principle of electromagnetic induction of a transformer, achieving complete electrical isolation between the primary and secondary windings. Through electromagnetic induction, the input electrical signal of the primary winding (primary coil) is converted into a change in the magnetic field, and the secondary winding (secondary coil) generates an output signal by inducing the magnetic field. Therefore, when the impulse voltage is transmitted through the magnetic isolation coupler, the frequency attenuation and phase shift are extremely low. Furthermore, since DC generates a constant magnetic field, it is impossible to induce current in the secondary winding, thus fundamentally blocking DC current.

[0064] In this embodiment of the invention, the synchronization controller is a timing coordination unit used to control the conduction timing of the DC source and the impulse source; the synchronization controller is electrically connected to the control terminal (gate) of the IGBT element, and can precisely control the on / off state of the IGBT and the power of the DC voltage output to the common output terminal through pulse width modulation (PWM) (controlled by PWM duty cycle); the synchronization controller is also electrically connected to the control terminal of the impulse source to trigger the impulse source to output impulse voltage.

[0065] Depending on the requirements of testing accuracy, cost, and development cycle, the synchronous controller can be selected from microcontrollers (MCUs), digital signal processors (DSPs), complex programmable logic devices (CPLDs), application-specific integrated circuits (ASICs), or system-on-a-chip (SoCs). In some preferred embodiments, the synchronous controller is selected from field-programmable gate arrays (FPGAs). FPGAs implement signal processing based on hardware logic circuits, and the delay error of the trigger signal can be controlled in the range of 10-100ns, which can accurately match the high-speed switching characteristics of IGBTs and the pulse generation requirements of the impulse source.

[0066] In this embodiment of the invention, the RC circuit includes at least one resistor and at least one capacitor connected in series to discharge impact energy or residual energy of the cable sample; in some specific embodiments, the RC circuit may also be connected in series or in parallel with other components such as resistors, capacitors, inductors, and diodes.

[0067] See Figure 2 In some preferred embodiments, the RC circuit 41 includes a first resistor R, a second capacitor C2, a third inductor L3, and a third diode D3; the first end of the first resistor R is connected to the collector of the first IGBT, the second end of the first resistor R is connected to the first end of the third inductor L3, the second end of the third inductor L3 is connected to the first end of the second capacitor C2, and the second end of the second capacitor C2 is connected to the emitter of the second IGBT; the anode of the third diode D3 is connected to the first end of the first resistor R, and the cathode of the third diode D3 is connected to the second end of the first resistor R.

[0068] See Figure 3The timing control process for performing a DC superimposed impulse voltage test on a cable sample using the composite voltage generator provided in this embodiment of the invention is as follows:

[0069] First, connect the power terminal of the cable sample to the common output terminal of the composite voltage generator; turn on one of the IGBTs in the IGBT active protection unit, apply a DC voltage to the cable sample through the DC source, and detect the space charge and leakage current of the cable sample insulation; when the space charge of the insulation is stable and the rate of change of leakage current is <0.1% / min, apply an impulse voltage to the cable sample through the impulse source, and simultaneously turn off the IGBT active protection unit; measure and record the relevant test signals of the cable sample; after applying the impulse voltage to the cable sample for a preset time, turn off the impulse source, and the residual energy of the cable sample can be discharged through the tail resistor inside the impulse source and the grounding terminal of the cable sample (reference zero potential point).

[0070] See Figure 3 In one specific embodiment, the synchronous controller sends an IGBT turn-off command at 0μs, triggers the impulse source to output impulse voltage at 0.1μs, the impulse voltage reaches its peak at 1.2μs, and the impulse voltage ends at 50μs.

[0071] The principle of DC superimposed impulse voltage is as follows:

[0072] A DC voltage is applied to the cable sample by a DC source. The equivalent capacitance between the cable conductor and the outer shield can briefly maintain the DC voltage during the IGBT turn-off. For example, for a 1km long cable (equivalent capacitance C=200nF, R=10GΩ), the time required for the DC voltage to decay to 99% is approximately 2 seconds, which is much longer than the duration of the impulse voltage (≤100μs). Therefore, after triggering the impulse voltage, the voltage waveform applied to the cable sample is the DC voltage + the impulse voltage. The DC source needs to fully charge the cable before the IGBT turns off to ensure that the DC voltage fluctuation during the impulse is <0.1%.

[0073] The residual energy is discharged through the test circuit connected to the cable sample itself: the magnetic isolation coupler isolates stable DC, but has low impedance to changing voltages (such as the rising and falling edges of the shock wave). The attenuation of the shock wave tail is a relatively slow process. Therefore, the residual energy in the cable sample can be conducted through the magnetic isolation coupler to the tail resistor inside the shock source. When the charge flows through the tail resistor, the electrical energy is consumed as heat. In this discharge path, the tail resistor is the main component that consumes the residual energy and determines the discharge speed. Finally, the remaining residual energy in the cable sample flows to the ground, realizing the natural discharge of the residual energy.

[0074] The following is based on Figure 2Taking the circuit structure shown as an example, the circuit principle of the IGBT active protection unit in this embodiment of the invention will be explained in detail:

[0075] Taking the application of a positive DC voltage to a cable as an example, the power-on terminal of the cable sample is connected to the common output terminal of the composite voltage generator, and the grounding terminal of the cable sample is grounded. A preset PWM signal is applied by the synchronous controller to control the first IGBT to turn on. At this time, the second IGBT is turned off. The DC source forms a path with the grounded cable sample through the first IGBT, thereby charging the cable sample. The inductors (L1, L2, L3) play the role of energy storage and smoothing in this process, suppressing current surges to reduce switching losses and electromagnetic interference. At the same time, they also form an LC filter network with the capacitors (C1, C2) to smooth the pulse components in the DC voltage into a stable and pure output voltage. The first diode D1 connected in reverse parallel is a freewheeling diode, which provides a freewheeling path for the reverse electromotive force generated by the inductor when the IGBT is turned off, prevents voltage spikes from breaking down the first IGBT, and plays a clamping and isolation role.

[0076] When the space charge of the insulation is stable and the rate of change of leakage current is <0.1% / min, an impulse voltage is applied to the cable sample through an impulse source, while the first IGBT is turned off. At this time, the DC source no longer applies DC voltage to the cable sample. However, after the impulse voltage is superimposed on the cable sample, the instantaneous high voltage of the impulse source and the instantaneous high voltage of the cable sample will generate a reverse current flowing to the DC source side, which causes energy backflow to the DC source side. At the same time, the sudden turn-off of the IGBT will also generate an instantaneous reverse voltage spike. At this time, the reverse current is guided to the RC circuit through the first diode D1. The second capacitor C2, the third inductor L3 in the RC circuit, and the first inductor L1, the second inductor L2 and the first capacitor C1 that form the oscillation circuit can quickly absorb the voltage spike through energy storage, and then slowly discharge through the first resistor R (the third diode D3 is the clamping / freewheeling diode of the resistor R). Finally, it is consumed in the form of heat energy on the first resistor R (when the first resistor is a damping resistor). This realizes the active discharge of reverse energy, which can limit the reverse current within the safe threshold and avoid burning out the IGBT and damaging the DC source.

[0077] When a negative DC voltage is applied to the cable sample, the first IGBT is turned off and the second IGBT is turned on. The first and second IGBTs form a half-bridge topology. The subsequent control steps and implementation principles are the same as those described in the case of applying a positive DC voltage to the cable sample, and will not be repeated here.

[0078] The composite voltage generator designed in this embodiment of the invention uses an IGBT active protection unit to replace the relay and current-limiting resistor on the DC source side in the traditional scheme, and uses a magnetic isolation coupler to replace the DC blocking capacitor on the impulse source side in the traditional scheme, which has the following significant advantages:

[0079] The polarity of the DC voltage applied to the cable sample can be dynamically switched by controlling the commutation of the IGBT bridge arm (i.e., controlling the individual conduction of either the first or second IGBT). The IGBT is based on MOSFET gate control and has high-speed switching characteristics under voltage control, enabling turn-off within 100ns, fast dynamic response, and more accurate timing of composite voltage superposition. The IGBT is based on the reverse blocking of the semiconductor PN junction, with extremely low leakage current (typically <10μA), which can reliably block the reverse path of DC voltage and reduce waveform distortion. When the impulse voltage is triggered, the current flowing back into the DC source direction can be discharged through the RC circuit connected in parallel with the IGBT, realizing the active discharge of reverse energy and limiting the reverse current within a safe threshold to avoid burning out the IGBT.

[0080] Magnetic isolation couplers are based on the principle of electromagnetic induction, and their core function is to transmit changing magnetic fields. Their inductive reactance is much lower than that of traditional DC blocking capacitors, resulting in low frequency attenuation and phase shift of impulse voltages, which can reduce waveform distortion. The primary and secondary windings of the magnetic isolation coupler are completely electrically isolated, which fundamentally blocks DC reverse flow (the DC component generates a constant magnetic field, which cannot induce current in the secondary winding, so the DC is physically blocked, rather than blocked by the capacitive reactance of the DC blocking capacitor), thus avoiding the influence of DC voltage on the impulse source.

[0081] Based on the embodiments of the above-described composite voltage generator, see [reference]. Figure 4 In some preferred embodiments, the magnetic isolation coupler includes a nanocrystalline magnetic ring 101, a dual-winding coil 102 (only the dual-winding coil is shown wound on the nanocrystalline magnetic ring in the figure, not the actual number of turns and winding density), and a protective housing 103; the protective housing 103 forms a receiving cavity, in which the nanocrystalline magnetic ring 101 and the dual-winding coil 102 are placed, and the receiving cavity is filled with silicone gel 104.

[0082] Nanocrystalline magnetic rings are ring-shaped magnetic components made of nanocrystalline soft magnetic materials (such as iron-based and cobalt-based nanocrystalline alloys). Their core feature is that the grain size inside the material is precisely controlled at the nanometer level (usually 10-100 nm), and the grain boundaries are uniformly distributed and the microstructure is stable. These components rely on the unique magnetic properties of nanocrystalline materials, and have high permeability (far superior to traditional silicon steel sheets and ferrites, which is conducive to efficient coupling of weak magnetic fields), low high-frequency magnetic loss (hysteresis loss and eddy current loss are significantly reduced under high-frequency alternating magnetic fields, making them suitable for high-frequency power electronics scenarios), excellent magnetic saturation characteristics (able to withstand high magnetic induction intensity without easily saturating) and good temperature stability (small fluctuations in magnetic properties over a wide temperature range). Their ring structure can reduce magnetic field leakage and improve magnetic coupling efficiency.

[0083] The magnetic isolation coupler is based on the principle of a transformer. The dual-winding coil includes a primary coil and a secondary coil, which are wound in parallel on a nanocrystalline magnetic ring. The primary coil is connected to the output terminal of the impulse source, and the secondary coil is connected to the common output terminal of the composite voltage generator. The parallel winding method makes the DC resistance and AC impedance of the two coils highly symmetrical. The tightly wound structure can enhance the coupling between the windings, reduce leakage flux, and suppress electromagnetic interference through magnetic field interaction, thereby improving the energy transmission efficiency and anti-interference ability of high-frequency impulse voltage.

[0084] The protective shell provides physical protection and electromagnetic shielding for the nanomagnetic crystal ring and the dual-winding coil, blocking external electromagnetic interference. The protective shell is made of metal. In some preferred embodiments, the protective shell is made of aluminum alloy, which has the advantages of high thermal conductivity, lightweight and high strength. The silicone gel, as a filler material, mainly achieves the functions of heat conduction and heat dissipation, sealing and moisture prevention, cushioning and shock absorption and electrical insulation.

[0085] A composite voltage generator is used to apply a DC superimposed impulse voltage to a cable sample. The purpose of this test is to detect the performance change of the cable sample insulation under the DC superimposed impulse voltage. To this end, based on the above-mentioned embodiment of the composite voltage generator, a second aspect of the present invention proposes a cable insulation performance evaluation system.

[0086] In some embodiments, the cable insulation performance evaluation system includes the composite voltage generating device provided in the above embodiments, as well as a measurement module and an evaluation module.

[0087] The measurement module includes a simulation chamber and a high-speed monitoring unit. The simulation chamber is used to accommodate cable samples and provide a preset test environment. The high-speed monitoring unit is used to collect test data from the cable samples, including space charge distribution signals, leakage current signals, high-frequency transient current signals, ultrasonic signals, and temperature signals of the cable sample insulation. The evaluation module is used to acquire these test data and output evaluation results based on the test data.

[0088] The cable insulation performance evaluation system proposed in this embodiment of the invention uses the composite voltage generating device provided in the above embodiments. Therefore, the cable insulation performance evaluation system also has the beneficial effects of the composite voltage generating device provided in the above embodiments, which will not be elaborated here. In addition, the simulation chamber of the cable insulation performance evaluation system can construct an environmental simulation unit related to the actual working conditions of DC cables (e.g., to simulate the ambient temperature and stress of DC cables), which can more accurately evaluate the actual performance changes of DC cables under DC superimposed impulse voltage. The high-speed monitoring unit can more completely and comprehensively reflect the performance changes of cable sample insulation after being subjected to impact by collecting a variety of test signals, providing a more accurate data source for the output of subsequent evaluation results.

[0089] In some preferred embodiments, the simulation chamber includes a temperature field control unit.

[0090] See Figure 5 The temperature field control unit includes an intermediate frequency induction coil 201 and a spray ring 202 (spray holes on the spray ring are not shown). The intermediate frequency induction coil 201 is located inside the spray ring 202 and is coaxially arranged with the spray ring 202. The intermediate frequency induction coil 201 is configured to generate an electromagnetic field that can be used to heat the conductor after being energized. The spray ring 202 is configured to spray coolant (e.g., water or liquid nitrogen) towards the central axis of the spray ring. A quartz glass partition 203 is provided at the axial end of the spray ring 202. The quartz glass partition 203 has a through hole at the position corresponding to the central axis of the spray ring 202. The connection between the quartz glass partition 203 and the spray ring 202 is covered with a fluororubber sealing strip.

[0091] During the test, the cable sample is placed into the cavity of the simulation chamber through a through hole in the quartz glass partition. The induction heating coil generates eddy currents inside the cable to rapidly heat the conductor part of the cable. The spray ring sprays coolant onto the outer layer of the cable sample to reduce the surface temperature of the cable sample. The combination of the two achieves a gradual change in the surface and internal temperature of the cable, thereby simulating the working environment of DC cables in seawater (for example, the conductor of the cable can be heated to 70°C by the induction heating coil, and the surface temperature of the cable can be cooled to 0°C by the coolant).

[0092] The quartz glass partition and fluororubber sealing strip are used to prevent radial heat conduction and convection of the cable sample, so as to make the temperature in the simulation chamber more stable and accurate.

[0093] In some other preferred embodiments, the simulation chamber further includes a mechanical stress loading unit; the mechanical stress loading unit includes a hydraulic mechanism and multiple sets of clamps, the multiple sets of clamps being configured to clamp the cable sample at preset intervals; the hydraulic mechanism is configured to drive the clamps to reciprocate in a preset direction at a preset frequency and amplitude (e.g., amplitude ±30mm, frequency 0.5Hz), thereby simulating the cable bending vibration caused by ocean currents in the environment where the submarine DC cable is located.

[0094] By constructing a temperature field control unit and a mechanical stress unit, synchronous loading of electro-thermal-mechanical stress can be achieved, which more realistically reflects the performance changes of the insulation layer of DC cables under impulse voltage in actual application environments. This is especially significant for the insulation performance testing and evaluation of submarine DC cables.

[0095] See Figure 6In some preferred embodiments, the high-speed monitoring unit includes a data acquisition card and a transient charge sensor, a high-precision electrometer, a high-frequency CT scanner, an ultrasonic transducer, and a temperature sensor; the data acquisition card is communicatively connected to each of the transient charge sensor, the high-precision electrometer, the high-frequency CT scanner, the ultrasonic transducer, and the temperature sensor.

[0096] Specifically, the transient charge sensor is configured to acquire the space charge distribution signal of the cable sample insulation layer. The transient charge sensor can be a PEA (pulse electroacoustic) thin-film electrode, which acquires space charge distribution data through the PEA method. The PEA method typically involves applying a short-duration high-voltage pulse to the signal electrode. The pulsed electric field causes the accumulated space charge (such as positive / negative charge clusters formed under DC voltage) in the sample to be subjected to Coulomb force or Lorentz force, generating a transient electroacoustic pulse. This acoustic pulse propagates inside the sample at a fixed sound velocity (determined by the properties of the insulating material). When it reaches the back electrode, it is reflected and transmitted to the piezoelectric sensor. The sensor converts the acoustic signal into a weak electrical signal. Subsequently, the electrical signal is amplified by a preamplifier, and the time-domain waveform is acquired by a high-speed oscilloscope. Combined with the known material sound velocity, sample thickness, and signal propagation time difference, the sample depth position corresponding to different time points is calculated. The space charge density at each depth is obtained by converting the signal amplitude. In some specific embodiments, the sampling parameters of the transient charge sensor are set to 1 GS / s.

[0097] A high-precision electrometer is configured to collect the leakage current of the cable sample, and a temperature sensor is configured to collect the temperature at different locations of the cable sample in a multi-point manner.

[0098] The high-frequency CT (high-frequency current transformer) is configured to acquire high-frequency transient current signals of the insulation layer of the cable sample. The high-frequency transient current signals include insulation polarization transient current, insulation defect partial discharge current, space charge migration transient current, insulation surface discharge current, etc. In some specific implementations, the sampling parameters of the high-frequency CT are set to 100MHz.

[0099] The ultrasonic transducer is configured to collect ultrasonic signals generated by the insulation layer of the cable sample to detect microscopic damage to the cable sample insulation under composite stress. Specifically, when a bending vibration test is performed using a mechanical stress loading unit, a healthy cable exhibits a regularly decaying sine wave, while a defective cable exhibits abnormal harmonics. When an impulse voltage is superimposed, due to the instantaneous electrostriction effect, the cable insulation excites a special ultrasonic frequency response. Intact insulation only produces a single pulse response, but aged insulation will show obvious oscillation tails due to the release of trapped charges.

[0100] It is understandable that the test data obtained by the transient charge sensor, high-precision electrometer, high-frequency CT, ultrasonic transducer, and temperature sensor are important data for evaluating the changes in the insulation performance of the cable. Furthermore, the evaluation module can be used to analyze this test data, simulate, evaluate, and output results on the performance changes and life prediction of the cable insulation. The evaluation methods and theoretical models used by the evaluation module can be obtained through existing technologies, or new methods and theoretical models can be designed according to actual conditions. The evaluation module can be implemented in an information-based and visualized form through software functional units.

[0101] In some preferred embodiments, the transient charge sensor, high-precision electrometer, high-frequency CT, ultrasonic transducer and temperature sensor of the high-speed monitoring unit are controlled by a synchronization controller (which may be the same as the synchronization controller of the composite voltage generator) to start 10μs before the impulse voltage is triggered, ensuring that the time alignment error is ≤1ns.

[0102] The acquisition card, transient charge sensor, high-precision electrometer, high-frequency CT, ultrasonic transducer, and temperature sensor involved in the above embodiments can all be obtained through market means or existing technical solutions, and no specific limitations are made here. It should be understood that those skilled in the art can select transient charge sensors, high-precision electrometers, high-frequency CT, ultrasonic transducers, and temperature sensors with appropriate parameters and performance according to actual needs. The cable insulation performance evaluation system constructed using the embodiments of the present invention is within the protection scope of the present invention.

[0103] Based on the composite voltage generating device and cable insulation performance evaluation system provided in the above embodiments, the present invention also proposes the following cable insulation performance evaluation method.

[0104] In some embodiments, the cable insulation performance evaluation method includes the following steps:

[0105] S1. Place the cable sample in the simulation chamber. The cable sample includes a power connection end and a grounding end. The grounding end is grounded, and the power connection end is connected to the common output terminal of the composite voltage generator.

[0106] S2 turns on one of the IGBTs in the IGBT active protection unit, applies a DC voltage to the cable sample through a DC source, and detects the leakage current of the cable sample insulation; when the rate of change of leakage current is <0.1% / min, an impulse voltage is applied to the cable sample through an impulse source, and the IGBT active protection unit is turned off at the same time.

[0107] S3, Measure and record the target test signal of the cable sample; the target test signal includes, but is not limited to, space charge distribution signal, high-frequency transient current signal, ultrasonic signal and temperature signal.

[0108] S4, after applying the impulse voltage to the cable sample for a preset time, turn off the impulse source.

[0109] The specific details and implementation methods of steps S1-S4 above have been fully discussed in the embodiments of the above composite voltage generating device and cable insulation performance evaluation system, and will not be repeated here.

[0110] S5. Repeat steps S2-S4 a preset number of times, calculate the field strength dependent conductivity of the cable sample insulation for each measurement, and calculate the remaining life of the cable sample based on the field strength dependent conductivity.

[0111] The conductivity of cable insulation materials (such as cross-linked polyethylene XLPE) is approximately constant in the low field strength range, but at high field strengths, the conductivity increases significantly with increasing field strength, exhibiting a nonlinear characteristic, thus forming a field strength-dependent conductivity. The conductivity of cable insulation can be calculated according to Ohm's law σ=I / SE (σ is conductivity, I is leakage current, S is the effective electrode area of ​​the high-precision electrometer, and E is the applied field strength). By fitting the characteristic curve of conductivity, a linear expression for the relationship between conductivity and field strength σ(E) can be obtained. Since the space charge inside the insulation causes the applied field strength to be different from the actual field strength, it is necessary to measure the space charge density of the cable insulation and calculate the actual local electric field strength based on Gauss's theorem. In this embodiment of the invention, the field strength-dependent conductivity can be calculated by the following formula:

[0112] ;

[0113] In the formula, γ(E) is the field strength-dependent conductivity, γ0 is the intrinsic conductivity of the cable sample insulation, α is the trap-assisted conduction coefficient, E is the local electric field strength related to space charge, and β is the field emission contribution coefficient; for XLPE cables, the typical value of the intrinsic conductivity γ0 of the cable sample insulation is 1×10⁻⁶. -16 The typical value for the trap-assisted conduction coefficient α is 0.032 mm / kV, and the typical value for the field emission contribution coefficient is 7.5 × 10⁻⁶. -7 S / (kV 2 ·m).

[0114] Furthermore, in this embodiment of the invention, based on the field strength-dependent conductivity, the remaining life of the cable sample can be calculated using the following formula:

[0115] ;

[0116] In the formula, L represents the remaining lifetime, L0 represents the initial lifetime, λ represents the aging acceleration factor, γ(E) is the field strength dependent conductivity, and E is the local electric field strength related to space charge; wherein, the initial lifetime L0 of the cable sample is obtained from the manufacturer's accelerated aging test data, the aging acceleration factor can be calibrated by the stepped impact test, and the integral range of dE is from the minimum field strength to the maximum field strength that the cable sample insulation can withstand.

[0117] Finally, based on test data such as high-frequency transient current signals, ultrasonic signals, and temperature signals obtained from multiple cyclic tests, as well as calculated data such as field strength-dependent conductivity and remaining lifetime, a visual cloud map of the cable sample can be constructed to intuitively present the spatial distribution of various physical quantities in a visual form; for example, in some specific implementation methods, see [reference needed]. Figure 7 The electric field strength is reconstructed by acquiring the space charge density (i.e., calculating the local electric field strength related to the space charge; those skilled in the art can use publicly available methods and related simulation software to reconstruct the electric field strength, and this application does not limit the specific methods). The leakage current is obtained to calculate the conductivity of the cable insulation layer. Based on the reconstructed electric field strength and conductivity, the field strength-dependent conductivity can be calculated, and then the remaining life of the cable can be calculated. At the same time, high-frequency transient current signals (partial discharge) that can reflect the defect situation are acquired. The defect types and corresponding high-frequency transient current signals are classified and categorized. These data are fused through a preset nonlinear parameter model and then visualized to obtain the lifetime probability cloud map of the cable sample. The simulation model and visualization tools related to this embodiment can be obtained through existing publicly available technologies or software platforms.

[0118] The cable insulation performance evaluation method proposed in this embodiment of the invention uses the composite voltage generating device and cable insulation performance evaluation system provided in the above embodiments. The cable insulation performance evaluation method also possesses the beneficial effects of the composite voltage generating device and cable insulation performance evaluation system, which will not be elaborated here.

[0119] In the several embodiments provided by this invention, it should be understood that the disclosed apparatus and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between apparatuses or units may be electrical, mechanical, or other forms.

[0120] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0121] Furthermore, the functional units in the various embodiments of this application can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0122] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this application, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions for executing all or part of the steps of the methods described in the various embodiments of this application through a computer device (which may be a personal computer, server, or network device, etc.). The aforementioned storage medium includes: USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, optical disks, and other media capable of storing program code.

[0123] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A composite voltage generating device, characterized in that, Includes a synchronous controller, DC power supply, impulse source, IGBT active protection unit, magnetic isolation coupler, and common output terminal; The output terminal of the DC source is electrically connected to the input terminal of the IGBT active protection unit, the output terminal of the impulse source is electrically connected to the input terminal of the magnetic isolation coupler, and the output terminal of the IGBT active protection unit and the output terminal of the magnetic isolation coupler are connected to a common output terminal. The output terminal of the DC source includes an upper DC bus and a lower DC bus. The IGBT active protection unit includes a first IGBT, a second IGBT, a first diode, a second diode, a first inductor, a second inductor, a first capacitor, and an RC circuit. The first end of the first inductor is connected to the upper end of the DC bus, and the second end is connected to the collector of the first IGBT; the first end of the second inductor is connected to the lower end of the DC bus, and the second end is connected to the emitter of the second IGBT; the first capacitor is connected across the first end of the first inductor and the first end of the second inductor; the emitter of the first IGBT and the collector of the second IGBT are connected through a common node; the first diode is connected in anti-parallel with the first IGBT, and the second diode is connected in anti-parallel with the second IGBT. The RC circuit is connected in parallel between the collector of the first IGBT and the emitter of the second IGBT. The RC circuit includes at least one resistor and at least one capacitor connected in series. The output terminal of the IGBT active protection unit is led out from the common node. The synchronization controller is electrically connected to the gate of the first IGBT and the gate of the second IGBT, and is used to control the switching on and off of the first IGBT or the second IGBT in a preset timing sequence; the synchronization controller is also electrically connected to the control terminal of the impact source, and is used to trigger the impact source to output an impact voltage in a preset timing sequence.

2. The composite voltage generating device according to claim 1, characterized in that, The RC circuit includes a first resistor, a second capacitor, a third inductor, and a third diode; The first end of the first resistor is connected to the collector of the first IGBT, the second end of the first resistor is connected to the first end of the third inductor, the second end of the third inductor is connected to the first end of the second capacitor, and the second end of the second capacitor is connected to the emitter of the second IGBT; the anode of the third diode is connected to the first end of the first resistor, and the cathode of the third diode is connected to the second end of the first resistor.

3. The composite voltage generating device according to claim 1, characterized in that, The magnetic isolation coupler includes a nanocrystalline magnetic ring, a dual-winding coil, and a protective housing; The dual-winding coil includes a primary coil and a secondary coil, which are wound in parallel on the nanocrystalline magnetic ring. The protective shell has a receiving cavity, in which the nanomagnetic crystal ring and the dual-winding coil are placed, and the receiving cavity is filled with silica gel.

4. A cable insulation performance evaluation system, characterized in that, It includes the composite voltage generating device according to any one of claims 1-3, as well as a measurement module and an evaluation module; The measurement module includes a simulation chamber and a high-speed monitoring unit. The simulation chamber is used to contain the cable sample, and the high-speed monitoring unit is used to collect test data of the cable sample. The test data includes the space charge distribution signal, leakage current signal, high-frequency transient current signal, ultrasonic signal, and temperature signal of the cable sample. The evaluation module is used to acquire the test data and output evaluation results based on the test data.

5. The cable insulation performance evaluation system according to claim 4, characterized in that, The simulation chamber includes a temperature field control unit; The temperature field control unit includes a medium-frequency induction coil and a spray ring. The medium-frequency induction coil is located inside the spray ring and is coaxially arranged with the spray ring. The medium-frequency induction coil is configured to generate an electromagnetic field that can be used to heat the conductor after being energized. The spray ring is configured to spray coolant towards the central axis of the spray ring. A quartz glass partition is provided at the axial end of the spray ring. A through hole is opened on the quartz glass partition corresponding to the central axis of the spray ring. A fluororubber sealing strip is wrapped at the connection between the quartz glass partition and the spray ring.

6. The cable insulation performance evaluation system according to claim 4, characterized in that, The simulation chamber also includes a mechanical stress loading unit; The mechanical stress loading unit includes a hydraulic mechanism and multiple sets of clamps, which are configured to clamp the cable sample at preset intervals; the hydraulic mechanism is configured to drive the clamps to reciprocate in a preset direction at a preset frequency and amplitude.

7. The cable insulation performance evaluation system according to claim 4, characterized in that, The high-speed monitoring unit includes a data acquisition card, a transient charge sensor, a high-precision electrometer, a high-frequency CT scanner, an ultrasonic transducer, and a temperature sensor. The transient charge sensor is configured to acquire the space charge distribution signal of the cable sample insulation layer; the high-precision electrometer is configured to acquire the leakage current of the cable sample; the high-frequency CT is configured to acquire the high-frequency transient current signal of the cable sample insulation layer; the ultrasonic transducer is configured to acquire the ultrasonic signal generated by the cable sample insulation layer; and the temperature sensor is configured to acquire the temperature of the cable sample. The acquisition card is communicatively connected to the transient charge sensor, the high-precision electrometer, the high-frequency CT, the ultrasonic transducer, and the temperature sensor.

8. A method for evaluating the insulation performance of cables, using the cable insulation performance evaluation system according to any one of claims 4-7, characterized in that, Includes the following steps: S1, Place the cable sample in the simulation chamber. The cable sample includes a power connection end and a grounding end. The grounding end is grounded, and the power connection end is connected to the common output terminal of the composite voltage generator. S2, turn on one of the IGBTs in the IGBT active protection unit, apply a DC voltage to the cable sample through a DC source, and detect the leakage current of the cable sample; when the rate of change of the leakage current is <0.1% / min, apply an impulse voltage to the cable sample through an impulse source, and at the same time turn off the IGBT active protection unit. S3, Measure and record the target test signal of the cable sample; S4, After applying the impulse voltage to the cable sample for a preset time, turn off the impulse source; S5, repeat steps S2-S4 a preset number of times, calculate the field strength dependent conductivity of the cable sample insulation for each measurement, and calculate the remaining life of the cable sample based on the field strength dependent conductivity.

9. The cable insulation performance evaluation method according to claim 8, characterized in that, The target test signal includes a space charge distribution signal, and the field strength-dependent conductivity is calculated using the following formula: ； In the formula, γ(E) is the field strength dependent conductivity, γ0 is the intrinsic conductivity of the cable sample insulation, α is the trap-assisted conduction coefficient, E is the local electric field strength related to space charge, and β is the field emission contribution coefficient. The remaining life of the cable sample is calculated using the following formula: ； In the formula, L represents the remaining lifetime, L0 represents the initial lifetime, λ represents the aging acceleration factor, γ(E) is the field strength dependent conductivity, and E is the local electric field strength related to space charge.

10. The cable insulation performance evaluation method according to claim 9, characterized in that, The target test signal also includes high-frequency transient current signal, ultrasonic signal and temperature signal; The cable insulation performance evaluation method also includes the following steps: A visualization cloud map of the cable sample was constructed based on the high-frequency transient current signal, ultrasonic signal, temperature signal, field strength-dependent conductivity, and remaining lifetime.

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