A method and system for on-line monitoring of the insulation condition of an MMC DC cable

Abstract

The application discloses a kind of MMC DC cable insulation state online monitoring method and system, comprising: S1, by MMC controller to DC cable injection low-frequency three-phase alternating current signal;S2, the leakage current I of DC cable shielding layer is measured, after filtering processing, by Fourier analysis extraction low-frequency fundamental component in the leakage current I, the leakage current phase angle φ I It is calculated;S3, the leakage current phase angle φ I Calculated in S2 is compared with DC cable insulation aging threshold, according to the comparison result to realize the online evaluation of DC cable insulation state;Compared with the traditional cable insulation state monitoring mode, the number of sensors is reduced, the system computing resource demand is reduced, with the advantages of safety, simple and convenient;At the same time, the influence of the measurement phase difference between multiple sensors caused by transmission distance, model, response speed and other reasons is avoided, and the precision of DC cable insulation state online monitoring is further improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of DC cable insulation state monitoring, and particularly relates to a MMC DC cable insulation state online monitoring method and system. BACKGROUND

[0002] High-voltage DC cable is an important physical carrier of the modular multilevel converter (MMC) flexible DC power transmission system. However, under the long-term action of thermal stress, electric stress and mechanical stress, a large number of intramolecular chemical bonds and intermolecular cross-linking bonds of the cross-linked polyethylene dielectric material will appear to be cracked and destroyed, resulting in partial discharge and dielectric breakdown of the DC cable, and even threatening the safe and stable operation of the power system. Therefore, constructing an effective DC cable insulation state online monitoring system can improve the safety and reliability of the operation of the high-voltage DC cable, and has important significance for ensuring the safe and stable operation of the cable.

[0003] However, the existing DC cable insulation online monitoring technology has problems such as insufficient detection accuracy and being easily interfered by the electromagnetic environment of the power transmission system. In order to significantly improve the detection performance, the existing monitoring system needs to be restructured, including replacing high-precision sensors, deploying multi-band electromagnetic shielding cavities, and introducing a new adaptive filtering algorithm. However, such system-level modification faces technical adaptation problems, such as conflicts between different sensor interface protocols, contradictions between electromagnetic shielding and heat dissipation design, etc., and even if the modification is completed, limited by the nonlinear characteristics of the cable insulation aging mechanism, the monitoring accuracy improvement is difficult to achieve a substantial breakthrough.

[0004] The patent application with the publication number CN106990341A discloses a power distribution cable insulation diagnosis ultra-low frequency cosine square wave high-voltage generator and method. A 0.1Hz period change ultra-low frequency cosine square wave high voltage with a peak value of 30kV is applied to the test cable, and the cable dielectric loss tangent value is calculated through the partial voltage and current signals of the rising and falling edges. This method is an offline testing method, and the cable must be disconnected from the running state during the testing process, and a special high-voltage signal generator is required, which does not meet the requirements of the MMC DC cable insulation state online monitoring. In addition, it is difficult to directly perform Fourier transform on the partial alternating voltage and current signals to obtain the accurate dielectric loss tangent value.

[0005] The patent for invention with publication number CN102735970B proposes a cross-linked polyethylene cable insulation monitoring and life prediction method, taking one of the leakage current, insulation resistance and dielectric loss angle of the cross-linked polyethylene cable as the detected quantity alpha. The detected quantity alpha and the mutation rate beta of the detected quantity alpha are calculated in real time, so as to realize the monitoring of the cable insulation deterioration process; the average value of the detected quantity alpha and the mutation rate thereof is obtained every day, every month and every year through calculation, and the insulation life of the cable is calculated through formula; but when facing the scene of online monitoring of MMC DC cable insulation state, it is unknown how to implement application.

[0006] The patent application for invention with publication number CN117970050A discloses a cable dielectric loss factor online monitoring method and system based on grounding current, which solves the cable dielectric loss factor by measuring and calculating the injection cable voltage and leakage current phase respectively; however, this method needs to complete the test signal synchronization of the current transformer and the voltage transformer based on GPS or Beidou synchronous timing, which has too high requirements for system hardware resources. SUMMARY

[0007] Therefore, the purpose of the present application is to provide an MMC DC cable insulation state online monitoring method and system, which only measures and evaluates based on single variable, reduces the number of sensors compared with the traditional cable insulation state monitoring method, reduces the system calculation resource demand, has the advantages of safety and simplicity; at the same time, it avoids the influence of measurement difference caused by transmission distance, model, response speed and other reasons among multiple sensors, thereby improving the precision of the DC cable insulation state online monitoring of the present application.

[0008] The technical solutions adopted by the present application are as follows:

[0009] An MMC DC cable insulation state online monitoring method, the DC cable being the output cable of the MMC DC side, comprising a DC cable shielding layer; the MMC comprising an MMC controller and being provided with an MMC rectifier as the output of the DC side; the online monitoring method comprising at least the following steps:

[0010] S1, injecting a low-frequency three-phase alternating current signal into the DC cable through the MMC controller; wherein the MMC controller adopts a double-loop control mode composed of an outer voltage loop and an inner current loop; the voltage error is calculated through the outer voltage loop, and the differential mode voltage U diffdq * and the common mode voltage U comdq * are calculated through the inner current loop, so that the MMC outputs the target voltage U goal signal to the DC cable;

[0011] S2, measure the leakage current I of the DC cable shielding layer by the current sensor, after filtering, extract the low-frequency fundamental component in the leakage current I by Fourier analysis, and calculate the leakage current phase angle φ I ;

[0012] S3, compare the leakage current phase angle φ I calculated in step S2 with the DC cable insulation aging threshold, and realize online evaluation of the DC cable insulation state according to the comparison result.

[0013] Preferably, the MMC is provided with an MMC converter station, and the MMC rectifier is composed of an upper bridge arm and a lower bridge arm; the MMC controller comprises a target voltage generation module, an outer loop controller module, an inner loop controller module and a pulse generation module;

[0014] The target voltage generation module sends a target voltage U goal signal of the MMC to the outer loop controller module;

[0015] The outer voltage loop is controlled by the outer loop controller module, and tracks the target voltage U goal signal of the MMC, calculates the voltage error, and then calculates the d-axis current command value I vd * and the q-axis current command value I vq * ;

[0016] The inner current loop is controlled by the inner loop controller module, and tracks the d-axis current command value I vd * and the q-axis current command value I vq * , calculates the current error, and generates the differential mode voltage U diffdq * and the common mode voltage U comdq * .

[0017] Preferably, in step S1, the control process of the target voltage generation module comprises: the frequency of the low-frequency three-phase alternating current signal is f, the frequency f range ≦ 50 Hz, and the target voltage amplitude of the low-frequency three-phase alternating current signal U sin is as follows:

[0018] ; In the formula, U dc represents the original DC voltage output by the MMC, and k is a proportional constant, k ≦ 1%;

[0019] After the MMC is started to reach stable DC output, the original DC voltage U dcThe low-frequency three-phase AC signal is superimposed on the above, and the resulting composite signal is used as the target voltage U of the MMC. goal Its synthesis process uses the following formula:

[0020] .

[0021] Preferably, in step S1, the control process of the outer loop controller module includes:

[0022] Real-time detection of the raw DC voltage U output by the MMC dc and the output voltage U of each phase abc The output voltage U of each phase abc After Parker coordinate transformation, the voltage U is converted to the d-axis voltage. sd q-axis voltage U sq ;

[0023] The real-time detected value of the MMC DC side output voltage is compared with the received target voltage U. goal The signals are compared, and the voltage error is calculated.

[0024] The voltage error is adjusted by a PI controller, which outputs the d-axis current command value I. vd * ;

[0025] In the formula, K p K is the proportional gain of the PI controller. I Here, s represents the integral coefficient of the PI controller, and s is the Laplace integral term.

[0026] The reactive power of the MMC is calculated in real time and adjusted through negative feedback using a PI controller, outputting the q-axis current command value I. vq * ;

[0027] In the formula, Qs * Q is the reactive power target value for MMC. s This is the real-time calculated value of reactive power for MMC;

[0028] Send d-axis current command value I to the inner loop controller module vd * and q-axis current command value I vq * .

[0029] Preferably, in step S1, the control process of the inner loop controller module includes:

[0030] Real-time detection of the target voltage U output by the MMC to the DC cable goal The phase current I of the signalabc , and q-axis current I vd after Park coordinate transformation vq ;

[0031] The d-axis current I vd , q-axis current I vq is compared with the received d-axis current instruction value I vd * , q-axis current instruction value I vq * The current error is calculated;

[0032] The current error is adjusted by a PI controller to generate a differential mode voltage U diffdq * ;

[0033] ;

[0034] In the formula, U diffdq * is composed of U diffd * and U diffdq * , w is the angular frequency, L ac is the inductance of the MMC sending end power cable, L0 is the inductance of the upper and lower bridge arms, K P1 is the d-axis current proportional coefficient, K I1 is the d-axis current integral coefficient, K P2 is the q-axis current proportional coefficient, K I2 is the q-axis current integral coefficient, and s is the Laplace integral term.

[0035] By real-time detection of the MMC output target voltage U goal signal of each phase internal loop current I cirabc , after Park coordinate transformation, the d-axis and q-axis internal loop currents I cirdq are obtained, the d-axis and q-axis internal loop current target values I cirdq * are suppressed to 0 as the control target, and the common mode voltage U comdq * is generated;

[0036] ;

[0037] In the formula, U comdq * is composed of U comd * and U comq * ; K P3 is the d-axis interphase loop current proportional coefficient, KI3 K is the integral coefficient of the d-axis interphase circulation. P4 K is the q-axis phase-to-phase circulation proportionality coefficient. I4 is the q-axis phase-to-phase circulation integral coefficient, and s is the Laplace integral term;

[0038] Send differential voltage U to the pulse generation module diffdq * and common-mode voltage U comdq * .

[0039] Preferably, in step S1, the control process of the pulse generation module includes:

[0040] The received differential voltage U diffdq * and common-mode voltage U comdq * After performing the Parker coordinate transformation, the target voltage U output by the MMC to the DC cable is obtained. goal Differential mode voltage U of each phase in the signal diffj * and the common-mode voltage U of each phase comj * ;

[0041] Based on the differential mode voltage U of each phase diffj * and the common-mode voltage U of each phase comj * Calculate the modulation wave U of the upper bridge arm pj * Modulation wave U of the lower bridge arm nj * ;

[0042] ;

[0043] Based on the modulation wave U of the upper bridge arm pj * Modulation wave U of the lower bridge arm nj * Output PWM pulse control signal to the MMC converter station;

[0044] The MMC converter station outputs the target voltage U to the DC cable based on a PWM pulse control signal. goal Signal.

[0045] Preferably, in step S2, the leakage current phase angle φ I The calculation process is as follows:

[0046] ;

[0047] ;

[0048] ;

[0049] wherein I(t) is a function of the leakage current I and the time variable t; I0 is the DC component of the leakage current I; I an is the amplitude of the n-th harmonic cosine component, I bn is the amplitude of the n-th harmonic sine component; φ n is the phase angle of the n-th harmonic cosine component in the leakage current I; n is the harmonic number; ω is the fundamental frequency; φ I is the phase angle of the low-frequency fundamental component in the leakage current I, i.e. the phase angle of the leakage current; I a1 , I b1 are the amplitude of the low-frequency fundamental cosine component and the amplitude of the low-frequency fundamental sine component in the leakage current I, respectively, which are obtained by Fourier decomposition of I(t).

[0050] Preferably, in the online evaluation in step S3, when the leakage current phase angle φ I is equal to or greater than the DC cable insulation aging threshold value, it is determined that the DC cable line is in normal operation; when the leakage current phase angle φ I is less than the DC cable insulation aging threshold value, an alarm signal is issued to remind that the DC cable needs to be overhauled.

[0051] Preferably, an online monitoring system for the insulation state of a MMC DC cable, the DC cable being an output cable of a MMC DC side, comprising a DC cable shielding layer; the MMC comprising a MMC controller, and being provided with a MMC converter station and a MMC rectifier as an output of the DC side, the MMC rectifier being composed of an upper bridge arm and a lower bridge arm; wherein,

[0052] the MMC controller adopts a double closed-loop control mode composed of an outer voltage loop and an inner current loop; a voltage error is calculated by the outer voltage loop, and a differential mode voltage U diffdq * and a common mode voltage U comdq * are calculated by the inner current loop, so that the MMC outputs a target voltage U goal signal to the DC cable;

[0053] and further comprising a current sensor and an insulation state evaluation module, the current sensor being installed at the ground of the DC cable shielding layer for measuring the leakage current I of the DC cable shielding layer; the insulation state evaluation module being used for extracting a low-frequency fundamental component in the leakage current I, calculating a leakage current phase angle φ I , and online evaluating the insulation state of the DC cable according to the change of the leakage current phase angle φ I ;

[0054] The online insulation condition monitoring system for MMC DC cables adopts the online insulation condition monitoring method for MMC DC cables described above.

[0055] Preferably, the MMC controller includes a target voltage generation module, an outer loop controller module, an inner loop controller module, and a pulse generation module;

[0056] The target voltage generation module is communicatively connected to the outer loop controller module and sends the target voltage U of MMC to the outer loop controller module. goal Signal;

[0057] The outer loop controller module is communicatively connected to the inner loop controller module, and sends the d-axis current command value I to the inner loop controller module. vd * and q-axis current command value I vq * ;

[0058] The inner loop controller module is communicatively connected to the pulse generation module and sends a differential mode voltage U to the pulse generation module. diffdq * and common-mode voltage U comdq * ;

[0059] The pulse generation module is communicatively connected to the MMC converter station and outputs a PWM pulse control signal to the MMC converter station. The MMC converter station outputs a target voltage U to the DC cable based on the PWM pulse control signal. goal Signal.

[0060] The main advantages of this application are as follows: This application achieves active injection of low-frequency AC signals with specific frequencies and amplitudes by setting a specific MMC control strategy, without modifying the hardware structure of the MMC system's main circuit, making implementation safe, simple, and flexible; based on the specific MMC control strategy adopted in this application to achieve low-frequency AC signal injection, this application can directly extract the leakage current phase angle φ of the leakage current I at the grounding point of the DC cable shield. I The insulation condition of the DC cable is assessed by comparing it with the insulation aging threshold of the DC cable. Compared with traditional cable insulation condition monitoring methods, this method reduces the number of sensors and the system's computing resource requirements, and has the advantages of safety and simplicity. Moreover, this application only uses a single variable: a current sensor to measure the leakage current I of the DC cable shielding layer for measurement and evaluation, avoiding the influence of measurement differences caused by transmission distance, model, response speed, etc. between multiple sensors, thereby improving the accuracy of online monitoring of DC cable insulation condition in this application. Attached Figure Description

[0061] Figure 1This is a schematic diagram of the online monitoring system for insulation status of MMC DC cables according to a specific embodiment of this application;

[0062] Figure 2 This is a flowchart illustrating the implementation steps of the online monitoring method for the insulation status of MMC DC cables according to a specific embodiment of this application;

[0063] Figure 3 This is a schematic diagram illustrating the control principle of injecting low-frequency three-phase AC signals into a DC cable via an MMC controller, according to a specific embodiment of this application.

[0064] Figure 4 This is a schematic diagram of the structure of the MMC rectifier according to a specific embodiment of this application;

[0065] Figure 5 This is a structural diagram of the MMC simulation model in a specific embodiment of this application;

[0066] Figure 6 This is a waveform diagram of the leakage current I of the DC cable shielding layer in a specific embodiment of this application;

[0067] Figure 7 The leakage current phase angle φ in the specific embodiment of this application I A schematic diagram showing the correspondence between insulation resistance and insulation resistance. Detailed Implementation

[0068] Please refer to the above. Figure 1 , Figure 2 , Figure 3 and Figure 4 As shown, this embodiment proposes an online monitoring system for the insulation status of MMC DC cables, including: a DC cable (i.e., a DC cable with insulation status of MMC DC cable). Figure 1 The "cable under test" in the text refers to the output cable of the MMC DC side, including the DC cable shielding layer; the MMC includes an MMC controller, which is connected to the sending-end power supply and the receiving-end power supply respectively; the MMC is equipped with an MMC converter station and an MMC rectifier (connected to the DC cable) as the DC side output. The MMC rectifier consists of an upper bridge arm and a lower bridge arm, and each bridge arm contains a submodule SM (see details). Figure 4 The SM1 to SM1 shown are shown. n It consists of ) and each submodule SM is composed of switching devices (such as Figure 4 The V shown T1 V T2 V D1 V D2 ) and capacitors (such as Figure 4 The C0 shown constitutes the three-phase AC signal U of the power supply at the sending end. sj (including such as) Figure 4 The U shown sa U sb U scThe AC input is connected to the AC input side of the MMC rectifier via an AC cable.

[0069] In this embodiment, the MMC controller adopts a dual closed-loop control method consisting of an external voltage loop and an internal current loop; the voltage error is calculated through the external voltage loop, and the differential voltage U is calculated through the internal current loop. diffdq * and common-mode voltage U comdq * This causes the MMC to output the target voltage U to the DC cable. goal Signal;

[0070] In this embodiment, the MMC DC cable insulation condition online monitoring system further includes a current sensor and an insulation condition assessment module. The current sensor is installed at the grounding point of the DC cable shield and is used to measure the leakage current I of the DC cable shield. The insulation condition assessment module is used to extract the leakage current I (i.e., the leakage current I). Figure 1 The "I" in leak The leakage current phase angle φ is calculated from the low-frequency fundamental component of the signal. I According to the leakage current phase angle φ I The changes in the insulation condition of the DC cable are assessed online.

[0071] Preferably, in this embodiment, the MMC controller includes a target voltage generation module (i.e., Figure 3 The marked "low-frequency injection target voltage generation module" and outer loop controller module (i.e. Figure 3 The marked "outer loop controller") and inner loop controller module (i.e. Figure 3 The marked "inner loop controller" and pulse generation module (i.e. Figure 3 (The marked "pulse generation");

[0072] In this embodiment, the target voltage generation module is communicatively connected to the outer loop controller module, and sends the target voltage U of the MMC to the outer loop controller module. goal Signal; the outer loop controller module communicates with the inner loop controller module, sending the d-axis current command value I to the inner loop controller module. vd * and q-axis current command value I vq * The inner loop controller module is communicatively connected to the pulse generation module, sending differential mode voltage U to the pulse generation module. diffdq * and common-mode voltage U comdq * The pulse generation module is communicatively connected to the MMC converter station, outputting PWM pulse control signals to the MMC converter station. Based on these PWM pulse control signals, the MMC converter station outputs the target voltage U to the DC cable. goalsignal (corresponding Figure 1 the "injection of low-frequency AC signal" is marked).

[0073] Please also refer to Figure 1 , Figure 2 , Figure 3 and Figure 4 It is shown that the embodiment proposes an online monitoring method as used in the above-mentioned MMC DC cable insulation state online monitoring system, including the following steps:

[0074] S1, inject low-frequency three-phase AC signal into the DC cable through the MMC controller (i.e. corresponding Figure 2 The first step: inject low-frequency AC signal, superimpose low-frequency AC signal on the output voltage of the MMC controller); wherein the MMC controller adopts a double closed-loop control mode composed of an outer voltage loop and an inner current loop; the voltage error is calculated through the outer voltage loop, and the differential mode voltage U diffdq * and common mode voltage U comdq * are calculated through the inner current loop, so that the MMC outputs the target voltage U goal signal to the DC cable; preferably, in this step S1, the target voltage generation module sends the target voltage U goal signal of the MMC to the outer loop controller module; the outer voltage loop is controlled by the outer loop controller module, and the target voltage U goal signal of the MMC is tracked, and after the voltage error is calculated, the d-axis current command value I vd * and the q-axis current command value I vq * are calculated; the inner current loop is controlled by the inner loop controller module, and the d-axis current command value I vd * and the q-axis current command value I vq * are tracked, and after the current error is calculated, the differential mode voltage U diffdq * and the common mode voltage U comdq * are generated;

[0075] Further preferably, in this embodiment, the control process of the target voltage generation module includes: the frequency of the low-frequency three-phase AC signal is f, the frequency f range ≦ 50Hz, the target voltage amplitude of the low-frequency three-phase AC signal U sin adopts the following formula:

[0076] ; in the formula, U dc represents the original DC voltage output by the MMC, k is a proportional constant, k ≦ 1%;

[0077] After the MMC is started to reach stable DC output, the original DC voltage U dc of the MMC output is superimposed with a low-frequency three-phase AC signal U sin , and a resultant signal is generated as the target voltage U goal of the MMC, and the synthesis process adopts the following formula:

[0078] ;

[0079] Further preferably, in the present embodiment, the control process of the outer loop controller module comprises:

[0080] The original DC voltage U dc of the MMC output and the output voltage U abc of each phase are detected in real time, and the output voltage U abc of each phase is converted into d-axis voltage U sd and q-axis voltage U sq through Park coordinate transformation;

[0081] The real-time detection value of the DC side output voltage of the MMC is compared with the received target voltage U goal signal, and a voltage error is calculated;

[0082] The voltage error is adjusted through a PI controller, and a d-axis current instruction value I vd * is outputted;

[0083] ; in the formula, K p is the proportional coefficient of the PI controller, K I is the integral coefficient of the PI controller, s is the Laplace integral term, that is, the transformation parameter of the Laplace integral, and belongs to a constant;

[0084] The reactive power of the MMC is calculated in real time, and a q-axis current instruction value I vq * is outputted through negative feedback adjustment of the PI controller;

[0085] ; in the formula, Qs * is the target value of the reactive power of the MMC, and Q s is the real-time calculation value of the reactive power of the MMC;

[0086] The d-axis current instruction value I vd * and the q-axis current instruction value I vq * are sent to the inner loop controller module.

[0087] More preferably, in this embodiment, the control process of the inner loop controller module includes:

[0088] Real-time detection of the target voltage U output by the MMC to the DC cable goal The phase current I of the signal abc After Parker coordinate transformation, the d-axis current I is obtained. vd and q-axis current I vq ;

[0089] The d-axis current I vd q-axis current I vq The received d-axis current command value I is respectively compared with vd * q-axis current command value I vq * By comparing the results, the current error was calculated.

[0090] The current error is adjusted via a PI controller to generate a differential voltage U. diffdq * ;

[0091] ;

[0092] In the formula, U diffdq * byU diffd * and U diffdq * Composition, w is angular frequency, L ac L0 is the inductance of the AC power cable at the MMC sending end, L0 is the inductance of the upper and lower bridge arms, and K is the inductance of the lower bridge arms. P1 K is the d-axis current proportionality coefficient. I1 K is the integral coefficient of the d-axis current. P2 K is the q-axis current proportionality coefficient. I2 is the q-axis current integral coefficient, and s is the Laplace integral term, which is the transformation parameter of the Laplace integral and is a constant;

[0093] The target voltage U is output to the DC cable by real-time detection of the MMC. goal Internal circulating current I of each phase of the signal cirabc After Parker coordinate transformation, the internal circulation I along the d-axis and q-axis is obtained. cirdq The target value I of the internal circulation along the d-axis and q-axis cirdq * Suppression of 0 is used as the control target (i.e., Figure 3 The "I" shown cirdq * =0”, generating common-mode voltage U comdq * ;

[0094] ;

[0095] In the formula, U comdq * byU comd * and U comq * Composition; K P3 K is the d-axis phase-to-phase circulation proportionality coefficient. I3 K is the integral coefficient of the d-axis interphase circulation. P4 K is the q-axis phase-to-phase circulation proportionality coefficient. I4 is the q-axis interphase circulation integral coefficient, and s is the Laplace integral term, which is the transformation parameter of the Laplace integral and is a constant;

[0096] Send differential voltage U to the pulse generation module diffdq * and common-mode voltage U comdq * ;

[0097] More preferably, in this embodiment, the control process of the pulse generation module includes:

[0098] The received differential voltage U diffdq * and common-mode voltage U comdq * After performing the Parker coordinate transformation, the target voltage U output by the MMC to the DC cable is obtained. goal Differential mode voltage U of each phase in the signal diffj * and the common-mode voltage U of each phase comj * ;

[0099] Based on the differential mode voltage U of each phase diffj * and the common-mode voltage U of each phase comj * Calculate the modulation wave U of the upper bridge arm pj * (That is, "u" in the following formula) pj * ") and the modulation wave U of the lower bridge arm nj * (That is, "u" in the following formula) nj * (”);

[0100] ;

[0101] Based on the modulation wave U of the upper bridge arm pj * Modulation wave U of the lower bridge arm nj *Output PWM pulse control signal to MMC converter station;

[0102] The MMC converter station outputs the target voltage U to the DC cable based on a PWM pulse control signal. goal The signal then outputs the original DC voltage U. dc and the injected low-frequency three-phase AC signal U sin It should be noted that, Figure 3 The "T" marked in the middle 3S-dq “T” dq-3s "All of these represent Parker coordinate transformations."

[0103] S2. The leakage current I of the DC cable shield is measured by a current sensor (i.e., the corresponding current). Figure 2 The second step involves signal acquisition and measurement of the leakage current in the DC cable shield. After filtering, the low-frequency fundamental component of the leakage current I is extracted using Fourier analysis (to eliminate the influence of harmonics), and the leakage current phase angle φ is calculated. I (i.e., corresponding) Figure 2 The third step: Data processing to extract the low-frequency fundamental component from the leakage current and calculate its phase angle φ. I Preferably, in step S2, the leakage current phase angle φ I The calculation process is as follows:

[0104] ;

[0105] ;

[0106] ;

[0107] Where I(t) is the functional relationship between leakage current I and time variable t; I0 ​​is the DC component of leakage current I; I an The amplitude of the nth harmonic cosine component, I bn φ is the amplitude of the sinusoidal component of the nth harmonic; n φ is the phase angle of the nth harmonic cosine component in the leakage current I; n is the harmonic order; ω is the fundamental frequency; φ I The phase angle of the low-frequency fundamental component in the leakage current I is the leakage current phase angle; I a1 I b1 These are the amplitudes of the low-frequency fundamental cosine component and the low-frequency fundamental sine component in the leakage current I, respectively, obtained by Fourier decomposition of I(t).

[0108] S3. The leakage current phase angle φ calculated in step S2 is... I By comparing the DC cable insulation aging threshold with the comparison results, an online assessment of the DC cable insulation condition is achieved (i.e., corresponding to...). Figure 2Step 4 in the fourth step: whether the insulation state evaluation monitoring index is greater than the aging threshold value); preferably, in the online evaluation of the present step S3, when the leakage current phase angle φ I is equal to or greater than the DC cable insulation aging threshold value, it is determined that the DC cable line is in normal operation; when the leakage current phase angle φ I is less than the DC cable insulation aging threshold value, an alarm signal is sent out, reminding that the DC cable needs to be shut down for maintenance.

[0109] In order to enable personnel in the technical field to better understand the technical solutions in the present application, on the basis of the above embodiments, the following specific embodiments will be presented below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by the person of ordinary skill in the art without making creative efforts shall belong to the scope of protection of the present application.

[0110] In the following embodiments, a ±30kV MMC flexible DC power transmission system is taken as an example for simulation analysis, and the system Matlab simulation model is shown in Figure 5 , that is, the receiving end power source carried by the present specific embodiment is shown in Figure 5 ; in the simulation test process, the MMC flexible DC power transmission system works in the rated operating condition, and the output current is 300A; after the stable DC voltage is output at the MMC DC side, a low-frequency three-phase alternating current signal U sin is superimposed on the basis of the original DC voltage U dc ; in order to avoid that the amplitude of the injected low-frequency three-phase alternating current signal is too large to cause the power quality to decrease, the proportional constant k in the control process of the target voltage generation module is set to 1%; the frequency f of the injected low-frequency three-phase alternating current signal is set to 5Hz, and compared with the power frequency, the dielectric loss angle is increased by 10 times, so that the detection accuracy is improved; a current sensor is installed at the ground of the DC cable shielding layer, and the leakage current I of the DC cable shielding layer (i.e., the "cable shielding layer leakage current" marked in Figure 6 ) waveform is shown in Figure 6 , and it can be seen from Figure 6 that: the original DC voltage U dc waveform is in the steady state output for the first 10 seconds, and from the 10th second, the original DC voltage U dc waveform is superimposed with the low-frequency three-phase alternating current signal U sin to form the target voltage U goal signal.

[0111] In the present specific embodiment, the function relationship between the leakage current phase angle φ I and the DC cable insulation resistance R and the insulation capacitance C is as follows:

[0112] ;

[0113] In the formula, φ U The phase of the MMC output voltage does not change with operating conditions and is a fixed constant.

[0114] From this formula, we can determine the leakage current phase angle φ. I The degradation of DC cables continues to decrease as the degree of deterioration worsens.

[0115] Based on the above implementation, this embodiment simulates different insulation states of the DC cable in the simulation model by changing the insulation resistance R in the equivalent model of the DC cable (this insulation resistance R is set in the DC cable). The leakage current phase angle φ corresponding to different insulation states of the DC cable is also shown. I Please refer to Table 1 below:

[0116] Table 1. Leakage current phase angle φ under different DC cable insulation conditions I

[0117] Insulation resistance (MΩ / km) Leakage current phase angle φ I (°) 17 69.4118 3.40 67.1229 1.70 64.2746 1.50 63.5193 1.30 62.5346 1.00 60.3377 0.75 57.2153 0.59 53.9150

[0118] Table 1 above confirms that the leakage current phase angle φ I The degradation of DC cables continues to decrease as the degree of deterioration worsens.

[0119] See further Figure 7 The leakage current phase angle φ shown I (correspond Figure 7 The vertical axis "leakage current phase" and insulation resistance (corresponding to) Figure 7 The correspondence between the horizontal axis "insulation resistance" and the horizontal axis in the graph is as follows: Figure 7 In the diagram, the dashed curve represents the theoretical leakage current phase angle (i.e., the...). Figure 7 The relationship between the "theoretical leakage current phase" (marked in the figure) and the insulation resistance R is shown in the scatter plot, which represents the leakage current phase angle φ of the DC cable shielding layer calculated using this embodiment. I (that is) Figure 7 The solid line represents the DC cable insulation aging threshold (i.e., the phase of the test leakage current marked in the middle). Figure 7 (The "cable insulation aging threshold" marked in the middle).

[0120] pass Figure 7 As shown, it can be seen that as the insulation resistance R decreases, the leakage current phase angle φ of the DC cable shielding layer calculated in this embodiment also decreases. I It will also decrease, showing an almost perfect relationship with the theoretical leakage current phase angle curve, thus verifying the leakage current phase angle φ of the DC cable shielding layer used in this application. I This study examines the feasibility and effectiveness of using changes in DC cable insulation status for online monitoring.

[0121] It will be apparent to those skilled in the art that the application is not limited to the details of the above-exemplified embodiments and that the present application can be implemented in other particular forms without departing from the spirit or essential characteristics of the present application. The presently disclosed embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the application being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. No feature of the application is to be construed as limiting the scope of the claims to the instant embodiment.

[0122] Furthermore, it should be understood that although the description above refers to particular embodiments, the description can include more than one independent technical solution, and the description is presented herein in a way of clarity only and should be considered as a whole, and the technical solutions in each embodiment can be combined appropriately to form other embodiments which can be understood by those skilled in the art.

Claims

1. A method for on-line monitoring of the condition of the insulation of a MMC DC cable, characterized in that The DC cable is an output cable of a MMC DC side, comprising a DC cable shielding layer; the MMC comprises a MMC controller, and is provided with a MMC rectifier as the output of the DC side; the online monitoring method comprises at least the following steps: S1, injecting a low-frequency three-phase alternating current signal into the DC cable through an MMC controller; wherein the MMC controller adopts a double closed-loop control mode composed of an outer voltage loop and an inner current loop; a voltage error is calculated through the outer voltage loop, and a differential mode voltage U diffdq * and a common mode voltage U comdq * is calculated through the inner current loop, so that the MMC outputs a target voltage U goal signal to the DC cable; The MMC is provided with a MMC converter station, and the MMC rectifier is composed of an upper bridge arm and a lower bridge arm; the MMC controller comprises a target voltage generation module, an outer loop controller module, an inner loop controller module and a pulse generation module; The target voltage generation module sends a target voltage U of the MMC to the outer loop controller module goal signal; The outer voltage loop is controlled by the outer loop controller module, tracking the target voltage U of the MMC goal The signal, after calculating the voltage error, calculates the d-axis current command value I vd * And the q-axis current command value I vq * ; The inner current loop is controlled by the inner loop controller module, tracking the d-axis current command value I vd * and the q-axis current command value I vq * After calculating the current error, the differential mode voltage U diffdq * and the common mode voltage U comdq * are generated. S2, the leakage current I of the DC cable shielding layer is measured by the current sensor, after filtering processing, the low-frequency fundamental component in the leakage current I is extracted by Fourier analysis, and the leakage current phase angle φ is calculated I ; S3. determining the phase angle φ of the leakage current calculated in step S2 I comparing the threshold value with the threshold value for the insulation of the DC cable, and implementing an online assessment of the state of the insulation of the DC cable on the basis of the comparison result.

2. The method for on-line monitoring of the insulation condition of the MMC DC cable according to claim 1, characterized in that, In step S1, the control process of the target voltage generation module includes: the frequency of the low-frequency three-phase alternating current signal is f, the frequency f is in the range of ≦50 Hz, and the target voltage amplitude of the low-frequency three-phase alternating current signal U sin adopts the following formula: ; where U dc is the raw DC voltage output as MMC, and k is a proportional constant, k < 1%. After the MMC is started to reach stable DC output, the original DC voltage U dc The low-frequency three-phase AC signal is superimposed on the basis of the above-mentioned low-frequency three-phase AC signal, and the generated composite signal is used as the target voltage U goal of the MMC, and the synthesis process adopts the following formula: 。 3. The method for on-line monitoring of the insulation condition of the MMC DC cable according to claim 1, characterized in that, In step S1, the control process of the outer loop controller module comprises: Real-time detection of the original DC voltage U of the MMC output dc and each phase output voltage U abc , each phase output voltage U abc is converted to d-axis voltage U sd , q-axis voltage U sq ; The real-time detection value of the MMC DC side output voltage is compared with the received target voltage U goal The signal is compared, and the voltage error is calculated. The voltage error is adjusted by a PI controller to output a d-axis current command value I vd * ; where K p is a proportional coefficient of the PI controller, K I is an integral coefficient of the PI controller, and s is a Laplace integral term By calculating the reactive power of MMC in real time and adjusting it by a PI controller, the q-axis current command value I vq * ; ; wherein Qs * is the reactive power target value of the MMC, Q s is the real-time calculated value of the reactive power of the MMC; sending a d-axis current command value I to the inner loop controller module vd * and a q-axis current command value I vq * .

4. The method for on-line monitoring of the insulation condition of the MMC DC cable according to claim 1, characterized in that, In step S1, the control process of the inner loop controller module comprises: detecting in real time the target voltage U outputted by the MMC to the DC cable goal the respective phase currents I of the signal abc , obtaining d-axis current I vd and q-axis current I vq after park coordinate transformation; The d-axis current I vd , q-axis current I vq received d-axis current command value I vd * , q-axis current command value I vq * is compared with the received d-axis current command value I , q-axis current command value I , and the current error is calculated. The current error is adjusted by a PI controller to generate a differential mode voltage U diffdq * ; ； In the formula, U diffdq * U diffd * and U diffdq * w is the angular frequency, L ac is the inductance of the MMC sending end power AC cable, L0 is the inductance of the upper and lower bridge arms, K P1 is the d-axis current proportional coefficient, K I1 is the d-axis current integral coefficient, K P2 is the q-axis current proportional coefficient, K I2 is the q-axis current integral coefficient, and s is the Laplace integral term. by detecting in real time the target voltage U outputted by the MMC to the DC cable goal the internal circulating current I of each phase of the signal cirabc , and the internal circulating currents I of the d-axis and q-axis are obtained after the park coordinate transformation cirdq the target values I of the internal circulating currents of the d-axis and q-axis are generated cirdq * the common-mode voltage U is generated as a control target with the inhibition being 0 comdq * ; ； wherein U comdq * U comd * U comq * K P3 is a d-axis inter-phase circulating current proportional coefficient, K I3 is a d-axis inter-phase circulating current integral coefficient, K P4 is a q-axis inter-phase circulating current proportional coefficient, K I4 is a q-axis inter-phase circulating current integral coefficient, s is a Laplace integral term sending a differential-mode voltage U to the pulse generation module diffdq * and a common-mode voltage U comdq * .

5. The method for on-line monitoring of the insulation condition of the MMC DC cable according to claim 1, characterized in that, In step S1, the control process of the pulse generation module comprises: The received differential mode voltage U diffdq * and common mode voltage U comdq * After the Park coordinate transformation, the MMC output target voltage U goal The differential mode voltage U diffj * and common mode voltage U comj * of each phase in the signal based on the phase difference voltage U diffj * and the phase common-mode voltage U comj * the modulation wave U pj * and the modulation wave U nj * ; ； a modulation wave U of the upper bridge arm pj * a modulation wave U of the lower bridge arm nj * outputting a PWM pulse control signal to the MMC converter station; The MMC converter station outputs a target voltage U to the DC cable based on a PWM pulse control signal goal signal.

6. The method for on-line monitoring of the insulation condition of the MMC DC cable according to claim 1, characterized in that, In step S2, the leakage current phase angle φ I is calculated as follows: ； ； ； Wherein, I(t) is the function relation of the leakage current I and time variable t; I0 is the direct current component of the leakage current I; I an is the n harmonic cosine component amplitude, I bn is the n harmonic sine component amplitude; φ n is the n harmonic cosine component phase angle in the leakage current I; n is the harmonic number; ω is the fundamental frequency; φ I is the low frequency fundamental component phase angle in the leakage current I, that is, the leakage current phase angle; I a1 , I b1 are respectively the low frequency fundamental cosine component amplitude and the low frequency fundamental sine component amplitude in the leakage current I, which are obtained after Fourier decomposition of I(t).

7. The method for on-line monitoring of the insulation condition of the MMC DC cable according to claim 1, characterized in that, In the online evaluation of step S3, when the leakage current phase angle φ I is equal to or greater than the DC cable insulation aging threshold value, it is determined that the DC cable line is operating normally; when the leakage current phase angle φ I is less than the DC cable insulation aging threshold value, an alarm signal is issued to remind that the DC cable needs to be overhauled.

8. An on-line monitoring system of the insulation condition of a MMC DC cable, characterized in that The DC cable is an output cable of a MMC DC side, comprising a DC cable shielding layer; the MMC comprises a MMC controller, and is provided with a MMC rectifier as the output of the DC side, the MMC rectifier is composed of an upper bridge arm and a lower bridge arm; wherein, The MMC controller adopts a double closed-loop control mode composed of an outer voltage loop and an inner current loop; a voltage error is calculated through the outer voltage loop, and a differential mode voltage U diffdq * and a common mode voltage U comdq * are calculated through the inner current loop, so that the MMC outputs a target voltage U goal signal to the DC cable; The current sensor is installed at the ground of the DC cable shielding layer for measuring the leakage current I of the DC cable shielding layer; the insulation state evaluation module is used for extracting the low-frequency fundamental wave component in the leakage current I, and calculating the leakage current phase angle φ I The insulation state of the DC cable is evaluated on-line according to the change of the leakage current phase angle φ I . The MMC DC cable insulation state online monitoring system adopts the MMC DC cable insulation state online monitoring method according to any one of claims 1-7.

9. The MMC DC cable insulation condition on-line monitoring system according to claim 8, characterized in that, The MMC controller comprises a target voltage generation module, an outer loop controller module, an inner loop controller module and a pulse generation module; The target voltage generation module is in communication connection with an outer loop controller module, and sends the target voltage U of the MMC to the outer loop controller module goal signal; The outer loop controller module is in communication connection with the inner loop controller module, and sends a d-axis current instruction value I vd * and a q-axis current instruction value I vq * ; The inner loop controller module is in communication connection with the pulse generation module, and sends the differential mode voltage U to the pulse generation module diffdq * and the common mode voltage U comdq * ; The pulse generation module is in communication connection with the MMC converter station, and outputs a PWM pulse control signal to the MMC converter station; the MMC converter station outputs a target voltage U goal signal.

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