High-voltage direct-current cable state online monitoring method and system based on MMC low-frequency injection

Abstract

The invention discloses a high-voltage direct-current cable state online monitoring method and system based on MMC low-frequency injection. The method comprises the steps that an MMC controller injects a low-frequency three-phase alternating-current signal into a direct-current cable based on an MMC low-frequency injection control strategy; the MMC low-frequency injection control strategy comprises the steps that an 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 an outer voltage loop, and a differential-mode voltage Udiffdq * and a common-mode voltage Ucomdq * are calculated through an inner current loop, so that the MMC outputs a target voltage Ugoal signal to a direct-current cable; according to the method, active injection of the low-frequency alternating-current signal with the specific frequency and amplitude is achieved by setting the specific MMC control strategy, a hardware structure of a main loop of an MMC system does not need to be modified, and implementation is safe, easy, convenient and flexible; and meanwhile, online quantitative evaluation of the insulation state of the direct-current cable is realized, and the monitoring process is high in precision, simple, convenient and efficient.

Description

Technical Field

[0001] The present invention relates to the field of high-voltage DC cable status monitoring, and in particular to a method and system for online monitoring of the high-voltage DC cable status based on MMC low-frequency injection. Background Art

[0002] High-voltage DC cables are a crucial physical carrier of modular multilevel converter (MMC) flexible DC transmission systems. Mechanical damage and electrothermal damage can exacerbate the aging of polymer dielectric materials, leading to partial discharge and dielectric breakdown, resulting in insulation failures and even system outages. Therefore, developing an effective online cable insulation condition monitoring system to maximize the safety and reliability of HVDC cable operation is crucial for ensuring a safe and stable power supply.

[0003] However, most existing cable insulation status detection technologies are offline detection, and the existing cable insulation online monitoring technologies are often difficult to meet the requirements due to insufficient accuracy and susceptibility to interference:

[0004] For example, the invention patent with publication number CN110161391B discloses a method for online monitoring of cable insulation through reverse injection of low-frequency signals. A low-frequency voltage signal is injected through the open delta side of an electromagnetic voltage transformer connected to the busbar, and the current signal of the cable grounding wire and the voltage signal of the cable core are collected and processed in real time to determine the degree of cable aging. For example, the domestic invention patent application with publication number CN 106443371A discloses a new type of online cable insulation detection device. This method uses a signal source and an isolation transformer to load a low-frequency, low-voltage detection signal into the cable. After obtaining the low-frequency current and voltage signals of the cable, the insulation parameters such as the distributed capacitance and insulation resistance of the cable to the ground are calculated. However, these methods are based on injecting a low-frequency AC detection signal into the cable through a transformer. They are only applicable to AC cable monitoring and cannot be used as a means of online monitoring of the status of high-voltage DC cables.

[0005] A domestic invention patent application with publication number CN101975914A discloses a method and device for online monitoring of the insulation condition of power cables. When power systems are interconnected via tie lines, small disturbances can cause the generator rotors to swing relative to each other, causing the active power on the tie lines to fluctuate within a certain range at a very low frequency (0.1 to 2.5 Hz), known as low-frequency oscillation. Therefore, this method measures the oscillating current and voltage signals during low-frequency oscillations in the power system to determine the cable dielectric loss factor and achieve online monitoring of insulation aging. However, low-frequency oscillations occasionally occur in actual power systems, and the oscillation frequency is random and has a low amplitude, making it difficult to effectively extract usable low-frequency signals in real time, limiting the practical application of this method. Summary of the Invention

[0006] In view of this, the object of the present invention is to provide a method and system for online monitoring of the status of a high-voltage DC cable based on MMC low-frequency injection. By setting a specific MMC control strategy, active injection of a low-frequency AC signal of a specific frequency and amplitude is achieved without modifying the hardware structure of the main circuit of the MMC system. The implementation is safe, simple, and flexible. At the same time, online quantitative evaluation of the insulation status of the DC cable is achieved, and the monitoring process is highly accurate, simple, and efficient.

[0007] The technical solution adopted in the present invention is as follows:

[0008] A method for online monitoring of the status of a high-voltage DC cable based on MMC low-frequency injection is disclosed. The DC cable serves as an output cable on the DC side of the MMC and includes a DC cable shielding layer. The MMC includes an MMC controller and an MMC rectifier as a DC side output. The online monitoring method includes: the MMC controller injects a low-frequency three-phase AC signal into the DC cable based on an MMC low-frequency injection control strategy. The MMC low-frequency injection control strategy includes:

[0009] The MMC controller adopts a dual closed-loop control method consisting of an outer voltage loop and an inner current loop; the voltage error is calculated by the outer voltage loop, and the differential mode voltage U is calculated by the inner current loop. diffdq * and common mode voltage U comdq * , so that the MMC outputs the target voltage U to the DC cable goal Signal.

[0010] 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 includes a target voltage generation module, an outer loop controller module, an inner loop controller module and a pulse generation module;

[0011] The target voltage generation module sends the target voltage U of the MMC to the outer loop controller module.goal Signal;

[0012] The outer voltage loop is controlled by the outer loop controller module and tracks the target voltage U of the MMC. goal Signal, after calculating the voltage error, calculate the d-axis current command value I vd * and q-axis current command value I vq * ;

[0013] The inner current loop is controlled by the inner loop controller module to track the d-axis current command value I vd * and q-axis current command value I vq * After calculating the current error, the differential mode voltage U is generated diffdq * and common mode voltage U comdq * .

[0014] Preferably, the control process of the target voltage generating module includes: the frequency of the low-frequency three-phase AC signal is f, the frequency f range is ≦50Hz, the low-frequency three-phase AC signal U sin The target voltage amplitude is calculated using the following formula:

[0015] Where U dc It is represented by the original DC voltage output by the MMC, k is the proportional constant, k≦1%; t is the time variable;

[0016] After the MMC starts and reaches a stable DC output, the original DC voltage U output by the MMC dc The low-frequency three-phase AC signal is superimposed on the base, and the generated synthetic signal is used as the target voltage U of the MMC. goal , and its synthesis process uses the following formula:

[0017] .

[0018] Preferably, the control process of the outer loop controller module includes:

[0019] Real-time detection of the original DC voltage U output by the MMC dc And each phase output voltage U abc , each phase output voltage U abc After Parker coordinate transformation, it is converted into d-axis voltage U sd , q-axis voltage U sq ;

[0020] The real-time detection value of the MMC DC side output voltage is compared with the received target voltage U goalThe signals are compared and the voltage error is calculated;

[0021] The voltage error is adjusted by the PI controller to output the d-axis current command value I vd * ;

[0022] Where 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;

[0023] By calculating the reactive power of the MMC in real time and regulating it through negative feedback through the PI controller, the q-axis current command value I is output. vq * ;

[0024] Where, Qs * is the reactive power target value of MMC, Q s It is the real-time calculated value of reactive power of MMC;

[0025] Send the d-axis current command value I to the inner loop controller module vd * and q-axis current command value I vq * .

[0026] Preferably, the control process of the inner loop controller module includes:

[0027] Real-time detection of the target voltage U output by the MMC to the DC cable goal The current of each phase of the signal I abc , after Parker coordinate transformation, the d-axis current I is obtained vd and q-axis current I vq ;

[0028] The d-axis current I vd , q-axis current I vq Respectively with the received d-axis current command value I vd * , q-axis current command value I vq * Compare and calculate the current error;

[0029] The current error is adjusted by the PI controller to generate a differential mode voltage U diffdq * ;

[0030] ;

[0031] Where U diffdq* byU diffd * and U diffdq * Composition, w is the angular frequency, L ac is the inductance of the AC power cable at the MMC sending end, 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, s is the Laplace integral term;

[0032] The MMC outputs a target voltage U to the DC cable by real-time detection goal Internal circulation current of each phase of the signal I cirabc After Parker coordinate transformation, the internal circulation I of d-axis and q-axis is obtained cirdq , set the internal circulation target value I of the d-axis and q-axis cirdq * Suppressing it to 0 is the control target, generating a common mode voltage U comdq * ;

[0033] ;

[0034] Where U comdq * byU comd * and U comq * Composition; K P3 is the d-axis phase circulation ratio coefficient, K I3 is the d-axis interphase circulation integral coefficient, K P4 is the q-axis phase circulation ratio coefficient, K I4 is the q-axis interphase circulating current integral coefficient, s is the Laplace integral term;

[0035] Send differential mode voltage U to the pulse generation module diffdq * and common mode voltage U comdq * .

[0036] Preferably, the control process of the pulse generation module includes:

[0037] The received differential mode voltage U diffdq * and common mode voltage U comdq * After 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 signaldiffj * And the common mode voltage U comj * ;

[0038] Based on the differential mode voltage U diffj * And the common mode voltage U comj * Calculate the modulation wave U of the upper bridge arm pj * And the modulation wave U of the lower bridge arm nj * ;

[0039] ;

[0040] Modulation wave U based on the upper bridge arm pj * And the modulation wave U of the lower bridge arm nj * Outputting a PWM pulse control signal to the MMC converter station;

[0041] The MMC converter station outputs a target voltage U to the DC cable based on the PWM pulse control signal. goal Signal.

[0042] Preferably, after the low-frequency three-phase AC signal injection is completed, the output voltage of the MMC DC side is measured by a voltage sensor, and the leakage current I of the DC cable shield is measured by a current sensor. leak ; Then the dielectric loss angle of the DC cable is calculated; the insulation condition of the DC cable is evaluated based on the change in the dielectric loss angle; among them, as the degree of insulation degradation of the DC cable deepens, the test dielectric loss angle shows an increasing trend.

[0043] Preferably, the calculation process of the dielectric loss angle of the DC cable is as follows:

[0044] a1. Sensor signal A / D sampling: Perform A / D conversion on the collected data of the voltage sensor and the current sensor to obtain the output voltage data and leakage current I leak data;

[0045] a2. Extract the low-frequency fundamental components of voltage and current: Use filters to extract the output voltage data and leakage current I obtained above. leak The output voltage low-frequency fundamental signal of the MMC DC side and the DC cable leakage current I leak Low-frequency fundamental component, suppressing frequency band signals and interference noise that are irrelevant to the injected low-frequency three-phase AC signal;

[0046] a3. Low-frequency fundamental component data analysis and processing: The output voltage low-frequency fundamental signal and DC cable leakage current I are respectively converted into leak The low-frequency fundamental components are decomposed into the superposition of multiple sine waves, and then the output voltage low-frequency fundamental signal, DC cable leakage current I leak The phase information of the low-frequency fundamental component is used to obtain the low-frequency fundamental voltage phase φ on the DC side of the MMC U and the leakage current phase φ leak ; Wherein, the calculation process of the Fourier transform is:

[0047] ;

[0048] Where f(t) is the decomposition function, a0 is the DC component of the decomposition function, n is the harmonic order, w is the angular frequency, and a n is the amplitude of the nth cosine component, b n is the amplitude of the nth sine component; t is the time variable;

[0049] Then the same frequency items are merged according to the following formula:

[0050] ;

[0051] Where, φ n is the phase of the nth harmonic, A n is the amplitude of the nth harmonic;

[0052] The calculation process of the extraction is:

[0053] ;

[0054] According to the above process, the output voltage low-frequency fundamental signal and DC cable leakage current I are extracted respectively. leak Phase information of the low-frequency fundamental component;

[0055] a4. Calculate the phase angle difference θ using the following formula:

[0056] Where, φ leak is the leakage current phase, φ U is the low-frequency fundamental voltage phase on the DC side of the MMC;

[0057] a5. Calculate the dielectric loss angle of the DC cable: Use the following relationship between the dielectric loss angle δ and the phase angle difference θ to calculate the dielectric loss angle of the DC cable:

[0058] .

[0059] Preferably, a high-voltage DC cable status online monitoring system based on MMC low-frequency injection is provided, wherein the DC cable serves as the output cable of the MMC DC side, including a DC cable shielding layer; the MMC includes an MMC controller, and is provided with an MMC converter station and an MMC rectifier as the DC side output, wherein the MMC rectifier is composed of an upper bridge arm and a lower bridge arm; preferably, each bridge arm is composed of a submodule SM, and each submodule SM is composed of a switching device and a capacitor;

[0060] Wherein, the MMC controller includes a target voltage generation module, an outer loop controller module, an inner loop controller module and a pulse generation module;

[0061] The target voltage generation module is in communication with the outer loop controller module and sends the target voltage U of the MMC to the outer loop controller module. goal Signal;

[0062] The outer loop controller module is connected to the inner loop controller module for communication, and sends the d-axis current command value I to the inner loop controller module. vd * and q-axis current command value I vq * ;

[0063] The inner loop controller module is in communication with the pulse generation module and sends a differential mode voltage U to the pulse generation module. diffdq * and common mode voltage U comdq * ;

[0064] The pulse generation module is in communication 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 to the DC cable based on the PWM pulse control signal. goal Signal;

[0065] The MMC DC cable insulation status online monitoring system adopts the above-mentioned high-voltage DC cable status online monitoring method based on MMC low-frequency injection.

[0066] Preferably, the high-voltage DC cable status online monitoring system based on MMC low-frequency injection further includes:

[0067] Voltage sensor: installed between the positive and negative DC busbars on the MMC DC side to measure the output voltage of the MMC DC side;

[0068] Current sensor: Installed at the grounding point of the DC cable shield to measure the leakage current I of the DC cable shield leak ;

[0069] Filter: It is used to filter out the harmonics generated by the high-frequency switching of the MMC submodule IGBT and the fluctuation of the grid load end, and to extract the low-frequency fundamental signal of the output voltage of the MMC DC side and the DC cable leakage current I leak Low-frequency fundamental component;

[0070] Insulation status evaluation module: Based on the output voltage low-frequency fundamental signal of the MMC DC side extracted by the filter and the DC cable leakage current I leak The low-frequency fundamental component is used to estimate the dielectric loss angle of the DC cable, and the insulation condition of the DC cable is evaluated online based on its changing trend.

[0071] The beneficial effects of the present application are mainly as follows: the present application realizes the active injection of low-frequency AC signals of specific frequency and amplitude by setting a specific MMC control strategy, without modifying the hardware structure of the main circuit of the MMC system, and is safe, simple and flexible to implement; after completing the active injection of the low-frequency AC signal of specific amplitude and frequency, the insulation characteristic parameter of the DC cable is accurately calculated online: the dielectric loss angle, thereby realizing the online quantitative evaluation of the insulation status of the DC cable, and the monitoring process is highly accurate, simple and efficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 This is a structural diagram of a high-voltage DC cable status online monitoring system based on MMC low-frequency injection in a specific embodiment of the present application;

[0073] Figure 2 This is a schematic diagram of the control principle of injecting a low-frequency three-phase AC signal into a DC cable through an MMC controller in a specific embodiment of the present application;

[0074] Figure 3 Schematic diagram of the structure of the MMC rectifier according to the specific implementation method of the present application;

[0075] Figure 4 It is a structural diagram of the MMC simulation model in a specific embodiment of the present application;

[0076] Figure 5 This is a waveform diagram of the output voltage of the MMC DC side in a specific embodiment of the present application;

[0077] Figure 6 is the leakage current I of the DC cable shielding layer in the specific embodiment of the present application leak Waveform graph;

[0078] Figure 7 This is a schematic block diagram of the dielectric loss angle calculation process of a DC cable in a specific embodiment of the present application;

[0079] Figure 8 This is a comparison of the theoretical value of the DC cable dielectric loss angle and the simulation test value in the specific embodiment of this application. DETAILED DESCRIPTION

[0080] Please refer to Figure 1 、 Figure 2 and Figure 3 As shown, this embodiment proposes a high-voltage DC cable state online monitoring system based on MMC low-frequency injection, the DC cable (i.e. Figure 1 The cable under test is marked as 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 as the DC side output (connected to the DC cable). The MMC rectifier consists of an upper bridge arm and a lower bridge arm, and each bridge arm is composed of a submodule SM (for details, see Figure 3 SM1.... to SM shown n ), each submodule SM consists of a switching device (such as Figure 3 The V T1 、V T2 、V D1 、V D2 ) and capacitors (such as Figure 3 C0 shown in the figure); the sending end power three-phase AC signal U sj (including Figure 3 The U shown sa 、U sb 、U sc ) is input to the AC input side of the MMC rectifier through an AC cable;

[0081] See further Figure 2 As shown, in this embodiment, the MMC controller includes a target voltage generating module (i.e. Figure 2 The "low frequency injection target voltage generation module" marked by the outer loop controller module (that is, Figure 2 The outer loop controller module (the inner loop controller module) Figure 2 The inner loop controller) and the pulse generation module (that is, Figure 2 labeled "Pulse Generation");

[0082] In this embodiment, the target voltage generation module is connected to the outer loop controller module for communication and sends the target voltage U of the MMC to the outer loop controller module. goal Signal; the outer loop controller module is 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 * The inner loop controller module is 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 * The pulse generation module is 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 (corresponding to Figure 1 labeled "Inject low-frequency AC signal");

[0083] Preferably, in this embodiment, the high-voltage DC cable status online monitoring system based on MMC low-frequency injection further includes:

[0084] Voltage sensor: Installed between the positive and negative DC busbars of the MMC DC side to measure the output voltage of the MMC DC side. Figure 1 As shown, the output voltage contains the original DC voltage U dc and the injected low-frequency three-phase AC signal U sin (corresponding to the "output voltage of each phase U abc "),correspond Figure 1 The marked “DC voltage online monitoring”;

[0085] Current sensor: Installed at the grounding point of the DC cable shield to measure the leakage current I of the DC cable shield leak ,correspond Figure 1 Marked "leakage current online monitoring";

[0086] Filter: can be set to Figure 1 The "Insulation Status Analysis Platform" shown is used to filter out harmonics generated by the high-frequency switching of the MMC submodule IGBT and the fluctuation of the grid load end, and to extract the low-frequency fundamental signal of the output voltage of the MMC DC side and the DC cable leakage current I leak Low-frequency fundamental component;

[0087] Insulation condition assessment module (i.e. Figure 1 Marked "Insulation Status Analysis Platform"): Based on the output voltage low-frequency fundamental signal of the MMC DC side extracted by the filter and the DC cable leakage current I leak The low-frequency fundamental component is used to estimate the dielectric loss angle of the DC cable, and the insulation condition of the DC cable is evaluated online based on its changing trend.

[0088] See also Figure 1 、 Figure 2 and Figure 3 As shown, this embodiment proposes an online monitoring method adopted by the above-mentioned high-voltage DC cable status online monitoring system based on MMC low-frequency injection, including: the MMC controller injects a low-frequency three-phase AC signal (including the following) into the DC cable based on the MMC low-frequency injection control strategy. Figure 3 The U shown sa 、U sb 、U sc ); Among them, the MMC low-frequency injection control strategy includes:

[0089] The MMC controller adopts a dual closed-loop control method consisting of an outer voltage loop and an inner current loop; the voltage error is calculated by the outer voltage loop, and the differential mode voltage U is calculated by the inner current loop. diffdq * and common mode voltage U comdq * , so that the MMC outputs the target voltage U to the DC cable goal Signal;

[0090] Preferably, in this embodiment, the target voltage generation module sends the 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 and tracks the target voltage U of the MMC goal Signal, after calculating the voltage error, calculate the d-axis current command value I vd * and q-axis current command value I vq * The inner current loop is controlled by the inner loop controller module and tracks the d-axis current command value I vd * and q-axis current command value I vq * After calculating the current error, the differential mode voltage U is generated diffdq * and common mode voltage U comdq * ;

[0091] Further preferably, in this embodiment, the control process of the target voltage generating module includes: the frequency of the low-frequency three-phase AC signal is f, the frequency f range is ≦50Hz, the low-frequency three-phase AC signal U sin The target voltage amplitude is calculated using the following formula:

[0092] Where U dc It is represented by the original DC voltage output by the MMC, k is the proportional constant, k≦1%; t is the time variable;

[0093] After the MMC starts and reaches a stable DC output, the original DC voltage U output by the MMC dc On the basis of superimposed low-frequency three-phase AC signal U sin The generated synthetic signal is used as the target voltage U of the MMC goal , and its synthesis process uses the following formula:

[0094] ;

[0095] Further preferably, in this embodiment, the control process of the outer loop controller module includes:

[0096] Real-time detection of the original DC voltage U output by the MMC dc And each phase output voltage U abc , each phase output voltage U abc After Parker coordinate transformation, it is converted into d-axis voltage U sd , q-axis voltage U sq ;

[0097] The real-time detection 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;

[0098] The voltage error is adjusted by the PI controller, and the d-axis current command value I is output. vd * ;

[0099] Where 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, which is a constant;

[0100] By calculating the reactive power of the MMC in real time and regulating it through negative feedback through the PI controller, the q-axis current command value I is output. vq * ;

[0101] Where, Qs * is the reactive power target value of MMC, Q s It is the real-time calculated value of reactive power of MMC;

[0102] Send the d-axis current command value I to the inner loop controller module vd * and q-axis current command value I vq * .

[0103] Further preferably, in this embodiment, the control process of the inner loop controller module includes:

[0104] Real-time detection of the target voltage U output by MMC to the DC cable goal The current of each phase of the signal I abc , after Parker coordinate transformation, the d-axis current I is obtained vd and q-axis current I vq ;

[0105] The d-axis current I vd , q-axis current I vq Respectively with the received d-axis current command value I vd * , q-axis current command value I vq * Compare and calculate the current error;

[0106] The current error is adjusted through the PI controller to generate the differential mode voltage U diffdq * ;

[0107] ;

[0108] Where U diffdq * byU diffd * and U diffdq * Composition, w is the angular frequency, L ac is the inductance of the AC power cable at the MMC sending end, 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, s is the Laplace integral term, that is, the transformation parameter of Laplace integral, which is a constant;

[0109] The target voltage U is output to the DC cable by real-time detection of the MMC goal Internal circulation current of each phase of the signal I cirabc After Parker coordinate transformation, the internal circulation I of d-axis and q-axis is obtained cirdq , set the internal circulation target value I of the d-axis and q-axis cirdq * Suppress to 0 as the control target (that is, Figure 2 The “I cirdq * =0"), generating a common-mode voltage U comdq * ;

[0110] ;

[0111] Where U comdq * byU comd * and U comq * Composition; K P3 is the d-axis phase circulation ratio coefficient, K I3is the d-axis interphase circulation integral coefficient, K P4 is the q-axis phase circulation ratio coefficient, K I4 is the q-axis interphase circulation integral coefficient, s is the Laplace integral term, that is, the transformation parameter of the Laplace integral, which is a constant;

[0112] Send differential mode voltage U to the pulse generation module diffdq * and common mode voltage U comdq * ;

[0113] Further preferably, in this embodiment, the control process of the pulse generation module includes:

[0114] The received differential mode voltage U diffdq * and common mode voltage U comdq * After Parker coordinate transformation, the target voltage U output by 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 comj * ;

[0115] Based on the differential mode voltage U diffj * And the common mode voltage U comj * Calculate the modulation wave U of the upper bridge arm pj * (This is the "u" in the following formula pj * ”) and the modulation wave U of the lower bridge arm nj * (This is the "u" in the following formula nj * ”);

[0116] ;

[0117] Modulation wave U based on the upper bridge arm pj * And the modulation wave U of the lower bridge arm nj * Output PWM pulse control signal to MMC converter station;

[0118] The MMC converter station outputs the target voltage U to the DC cable based on the PWM pulse control signal. goal signal, and then output the original DC voltage U dc and the injected low-frequency three-phase AC signal U sin ;

[0119] It should be noted that Figure 2 The mark "T 3S-dq ”, “T dq-3s ” both represent Parker coordinate transformation.

[0120] Further preferably, in this embodiment, after the low-frequency three-phase AC signal injection is completed, the output voltage of the MMC DC side is measured by the voltage sensor, and the leakage current I of the DC cable shielding layer is measured by the current sensor. leak ; Then calculate the dielectric loss angle of the DC cable; evaluate the insulation condition of the DC cable based on the change of the dielectric loss angle; wherein, as the degree of insulation degradation of the DC cable deepens, the test dielectric loss angle tends to increase; for specific preference, please refer to Figure 1 As shown, the DC cable insulation aging threshold can be set (i.e. Figure 1 The insulation status of the DC cable is determined by comparing the dielectric loss angle with the threshold. When the dielectric loss angle is equal to or less than the threshold, the DC cable line continues to operate. When the dielectric loss angle is greater than the threshold, an alarm signal is issued to remind that the DC cable needs to be shut down for maintenance.

[0121] See Figure 7 As shown, further preferably, in this embodiment, the calculation process of the dielectric loss angle of the DC cable is as follows:

[0122] a1, sensor signal A / D sampling (corresponding to Figure 7 "A / D sampling of sensor signals": Perform A / D conversion on the collected data of the voltage sensor and the current sensor to obtain the output voltage data and leakage current I leak data;

[0123] a2. Extract the low-frequency fundamental components of voltage and current (corresponding to Figure 7 "Voltage and current filtering and extraction of low-frequency fundamental components"): Use filters to extract the output voltage data and leakage current I obtained above. leak The output voltage low-frequency fundamental signal of the MMC DC side and the DC cable leakage current I leak Low-frequency fundamental component, suppressing frequency band signals and interference noise that are irrelevant to the injected low-frequency three-phase AC signal;

[0124] a3、Analysis and processing of low-frequency fundamental wave component data (corresponding to Figure 7 "Analysis and processing of voltage and current low-frequency fundamental wave data"): The output voltage low-frequency fundamental wave signal and DC cable leakage current I are converted into leak The low-frequency fundamental components are decomposed into the superposition of multiple sine waves, and then the output voltage low-frequency fundamental signal, DC cable leakage current I leakThe phase information of the low-frequency fundamental component is used to obtain the low-frequency fundamental voltage phase φ on the DC side of the MMC U and the leakage current phase φ leak ; Among them, the calculation process of Fourier transform is:

[0125] ;

[0126] Where f(t) is the decomposition function, a0 is the DC component of the decomposition function, n is the harmonic order, w is the angular frequency, and a n is the amplitude of the nth cosine component, b n is the amplitude of the nth sine component; t is the time variable;

[0127] Then the same frequency items are merged according to the following formula:

[0128] ;

[0129] Where, φ n is the phase of the nth harmonic, A n is the amplitude of the nth harmonic;

[0130] The calculation process of extraction is:

[0131] ;

[0132] According to the above process, the output voltage low-frequency fundamental signal and DC cable leakage current I are extracted respectively. leak Phase information of the low-frequency fundamental component;

[0133] a4. Calculate the phase angle difference θ (corresponding to Figure 7 "Calculating the phase angle difference between low-frequency fundamental voltage and current":

[0134] Where, φ leak is the leakage current phase, φ U is the low-frequency fundamental voltage phase on the DC side of the MMC;

[0135] a5. Calculate the dielectric loss angle of the DC cable (corresponding to Figure 7 "Calculating Cable Dielectric Loss Angle" in the text: Using the following relationship between dielectric loss angle δ and phase angle difference θ, the dielectric loss angle of the DC cable can be calculated:

[0136] .

[0137] In order to enable those skilled in the art to better understand the technical solutions of the present invention, based on the above implementation scheme, the following specific embodiments will be presented in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0138] In the following specific embodiment, a ±30 kV MMC flexible DC transmission system is used as an example for simulation analysis. The system Matlab simulation model can be found in Figure 4 As shown, that is, the receiving end power supply carried by this specific embodiment is Figure 4 The simulation model shown in the figure; in the simulation test process, after the MMC DC side outputs a stable DC voltage, the original DC voltage U dc The low-frequency AC signal U is superimposed on the sin , realize the low-frequency AC signal injection on the MMC DC side; install a voltage sensor between the positive and negative DC busbars on the MMC DC side, install a current sensor at the grounding point of the DC cable shield layer, and save the measurement data to the workspace for subsequent data analysis.

[0139] To avoid the degradation of the transmission power quality caused by the excessive amplitude of the injected low-frequency three-phase AC signal, 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 AC signal is set to 5 Hz, which can ensure the real-time online monitoring of the dielectric loss angle. Compared with the power frequency, the dielectric loss angle is increased by 10 times, which improves the detection accuracy.

[0140] During the MMC startup phase, the original DC voltage output by the MMC is set to the system rated output DC voltage of 60kV. After the MMC outputs a stable DC voltage, a 600V, 5Hz sinusoidal AC signal is superimposed on the original DC voltage (i.e., a low-frequency three-phase AC signal U is injected). sin ); Output voltage waveform of the MMC DC side (that is, Figure 5 Please refer to the "MMC DC side output voltage" marked Figure 5 As shown, through Figure 5 It can be seen that the first 10 seconds is the original DC voltage U output in steady state dc Waveform, starting from the 10th second, the original DC voltage U dc The waveform is superimposed with a low-frequency three-phase AC signal to form the target voltage U goal signal; correspondingly, the leakage current of the DC cable shield I leak (that is, Figure 6 The waveform marked "cable shield leakage current") can be found in Figure 6 As shown;

[0141] The phase angle difference θ is calculated by the calculation process provided in this embodiment, and then based on the following relationship between the dielectric loss angle δ and the phase angle difference θ:

[0142] ;

[0143] The dielectric loss angle δ of the DC cable is calculated. It should be noted that the increase in the dielectric loss angle δ of the DC cable is the core warning signal of its insulation degradation. Its essence is the increase in conductivity loss, polarization loss or discharge loss, which corresponds to defective states such as aging, moisture, and discharge of the insulation material.

[0144] Based on the above implementation content, this embodiment simulates different insulation states of the DC cable in this simulation model by changing the insulation resistance R in the DC cable equivalent model (the insulation resistance R is set in the DC cable). The test results of the dielectric loss angle δ corresponding to different DC cable insulation states are shown in Table 1 below:

[0145] Table 1 Test results of dielectric loss angle δ under different DC cable insulation conditions

[0146] Insulation resistance (MΩ / km) The dielectric loss angle δ(°) obtained by the test 17 0.5962 3.40 2.8858 1.70 5.7340 1.50 6.4892 1.30 7.4736 1.00 9.6705 0.75 12.7926 0.59 16.0928

[0147] As can be seen from Table 1 above, as the insulation degradation of the DC cable deepens, the test dielectric loss angle δ shows an increasing trend, which is consistent with the aging law of cable insulation;

[0148] See further Figure 8 The comparison results of the DC cable dielectric loss angle theoretical value and simulation test value are shown in FIG; Among them, Figure 8 The dotted curve is the relationship curve between the theoretical dielectric loss angle and the insulation resistance of the DC cable (that is, Figure 8 The “theoretical dielectric loss angle” is marked); the scattered points are the test results of dielectric loss angle under different DC cable insulation conditions (i.e. Figure 8 "Simulated dielectric loss angle" marked in );

[0149] pass Figure 8 It can be seen that the test results of the dielectric loss angle are close to its theoretical dielectric loss angle, which verifies the feasibility and effectiveness of the online monitoring of the insulation state of the high-voltage DC cable based on MMC low-frequency injection in this application.

[0150] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be included therein. Any reference sign in a claim should not be construed as limiting the claim to which it relates.

[0151] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A method for online monitoring of high-voltage DC cable status based on MMC low-frequency injection, characterized in that: The DC cable serves as an output cable of the MMC DC side, including a DC cable shielding layer; the MMC includes an MMC controller and is provided with an MMC rectifier as a DC side output; The online monitoring method includes: an MMC controller injecting a low-frequency three-phase AC signal into the DC cable based on an MMC low-frequency injection control strategy; wherein the MMC low-frequency injection control strategy includes: The MMC controller adopts a dual closed-loop control method consisting of an outer voltage loop and an inner current loop; the voltage error is calculated by the outer voltage loop, and the differential mode voltage U is calculated by the inner current loop. diffdq * and common mode voltage U comdq * , so that the MMC outputs the target voltage U to the DC cable goal Signal.

2. The method for online monitoring of high-voltage DC cable status based on MMC low-frequency injection according to claim 1, characterized in that: 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 includes 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 the 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 and tracks the target voltage U of the MMC. goal Signal, after calculating the voltage error, calculate the d-axis current command value I vd * and q-axis current command value I vq * ; The inner current loop is controlled by the inner loop controller module to track the d-axis current command value I vd * and q-axis current command value I vq * After calculating the current error, the differential mode voltage U is generated diffdq * and common mode voltage U comdq * .

3. The method for online monitoring of high-voltage DC cable status based on MMC low-frequency injection according to claim 2, characterized in that: 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 is ≦50Hz, the low-frequency three-phase AC signal U sin The target voltage amplitude is calculated using the following formula: Where U dc It is represented by the original DC voltage output by the MMC, k is the proportional constant, k≦1%; t is the time variable; After the MMC starts and reaches a stable DC output, the original DC voltage U output by the MMC dc The low-frequency three-phase AC signal is superimposed on the base, and the generated synthetic signal is used as the target voltage U of the MMC. goal , and its synthesis process uses the following formula: 。 4. The method for online monitoring of high-voltage DC cable status based on MMC low-frequency injection according to claim 2, characterized in that: The control process of the outer loop controller module includes: Real-time detection of the original DC voltage U output by the MMC dc And each phase output voltage U sabc , the output voltage of each phase U sabc After Parker coordinate transformation, it is converted into 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 signals are compared and the voltage error is calculated; The voltage error is adjusted by the PI controller to output the d-axis current command value I vd * ; Where 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; By calculating the reactive power of the MMC in real time and regulating it through negative feedback through the PI controller, the q-axis current command value I is output. vq * ; Where, Qs * is the reactive power target value of MMC, Q s It is the real-time calculated value of reactive power of MMC; Send the d-axis current command value I to the inner loop controller module vd * and q-axis current command value I vq * .

5. The method for online monitoring of high-voltage DC cable status based on MMC low-frequency injection according to claim 2, characterized in that: The control process of the inner loop controller module includes: Real-time detection of the target voltage U output by the MMC to the DC cable goal The current of each phase of the signal I abc , after Parker coordinate transformation, the d-axis current I is obtained vd and q-axis current I vq ; The d-axis current I vd , q-axis current I vq Respectively with the received d-axis current command value I vd * , q-axis current command value I vq * Compare and calculate the current error; The current error is adjusted by the PI controller to generate a differential mode voltage U diffdq * ; ； Where U diffdq * byU diffd * and U diffdq * Composition, w is the angular frequency, L ac is the inductance of the AC power cable at the MMC sending end, 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, s is the Laplace integral term; The MMC outputs a target voltage U to the DC cable by real-time detection goal Internal circulation current of each phase of the signal I cirabc After Parker coordinate transformation, the internal circulation I of d-axis and q-axis is obtained cirdq , set the internal circulation target value I of the d-axis and q-axis cirdq * Suppressing it to 0 is the control target, generating a common mode voltage U comdq * ; ； Where U comdq * byU comd * and U comq * Composition; K P3 is the d-axis phase circulation ratio coefficient, K I3 is the d-axis interphase circulation integral coefficient, K P4 is the q-axis phase circulation ratio coefficient, K I4 is the q-axis interphase circulating current integral coefficient, s is the Laplace integral term; Send differential mode voltage U to the pulse generation module diffdq * and common mode voltage U comdq * .

6. The method for online monitoring of high-voltage DC cable status based on MMC low-frequency injection according to claim 2, characterized in that: The control process of the pulse generation module includes: The received differential mode voltage U diffdq * and common mode voltage U comdq * After 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 comj * ; Based on the differential mode voltage U diffj * And the common mode voltage U comj * Calculate the modulation wave U of the upper bridge arm pj * And the modulation wave U of the lower bridge arm nj * ; ； Modulation wave U based on the upper bridge arm pj * And the 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 the PWM pulse control signal. goal Signal.

7. The method for online monitoring of high-voltage DC cable status based on MMC low-frequency injection according to claim 1, characterized in that: After the low-frequency three-phase AC signal is injected, the output voltage of the MMC DC side is measured by the voltage sensor, and the leakage current I of the DC cable shield is measured by the current sensor. leak ; Then the dielectric loss angle of the DC cable is calculated; the insulation condition of the DC cable is evaluated based on the change in the dielectric loss angle; among them, as the degree of insulation degradation of the DC cable deepens, the test dielectric loss angle shows an increasing trend.

8. The method for online monitoring of high-voltage DC cable status based on MMC low-frequency injection according to claim 7, characterized in that: The calculation process of the dielectric loss angle of the DC cable is as follows: a1. Sensor signal A / D sampling: Perform A / D conversion on the collected data of the voltage sensor and the current sensor to obtain the output voltage data and leakage current I leak data; a2. Extract the low-frequency fundamental components of voltage and current: Use filters to extract the output voltage data and leakage current I obtained above. leak The output voltage low-frequency fundamental signal of the MMC DC side and the DC cable leakage current I leak Low-frequency fundamental component, suppressing frequency band signals and interference noise that are irrelevant to the injected low-frequency three-phase AC signal; a3. Low-frequency fundamental component data analysis and processing: The output voltage low-frequency fundamental signal and DC cable leakage current I are respectively converted into leak The low-frequency fundamental components are decomposed into the superposition of multiple sine waves, and then the output voltage low-frequency fundamental signal, DC cable leakage current I leak The phase information of the low-frequency fundamental component is used to obtain the low-frequency fundamental voltage phase φ on the DC side of the MMC U and the leakage current phase φ leak ; Wherein, the calculation process of the Fourier transform is: ; Where f(t) is the decomposition function, a0 is the DC component of the decomposition function, n is the harmonic number, w is the angular frequency, a n is the amplitude of the nth cosine component, b n is the amplitude of the nth sine component; t is the time variable; Then the same frequency items are merged according to the following formula: Where, φ n is the phase of the nth harmonic, A n is the amplitude of the nth harmonic; The calculation process of the extraction is: ； According to the above process, the output voltage low-frequency fundamental signal and DC cable leakage current I are extracted respectively. leak Phase information of the low-frequency fundamental component; a4. Calculate the phase angle difference θ using the following formula: Where, φ leak is the leakage current phase, φ U is the low-frequency fundamental voltage phase on the DC side of the MMC; a5. Calculate the dielectric loss angle of the DC cable: Use the following relationship between the dielectric loss angle δ and the phase angle difference θ to calculate the dielectric loss angle of the DC cable: 。 9. A high-voltage DC cable status online monitoring system based on MMC low-frequency injection, characterized in that: The DC cable serves as the output cable of the MMC DC side, including a DC cable shielding layer; the MMC includes an MMC controller, an MMC converter station, and an MMC rectifier as the DC side output, wherein the MMC rectifier consists of an upper bridge arm and a lower bridge arm; wherein the MMC controller includes 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 with 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 is connected to the inner loop controller module for communication, and sends 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 in communication with the pulse generation module and sends a differential mode voltage U to the pulse generation module. diffdq * and common mode voltage U comdq * ; The pulse generation module is in communication 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 to the DC cable based on the PWM pulse control signal. goal Signal; The high-voltage DC cable state online monitoring system based on MMC low-frequency injection adopts the high-voltage DC cable state online monitoring method based on MMC low-frequency injection according to any one of claims 1 to 8.

10. The high-voltage DC cable status online monitoring system based on MMC low-frequency injection according to claim 9, characterized in that: Also includes: Voltage sensor: installed between the positive and negative DC busbars on the MMC DC side to measure the output voltage of the MMC DC side; Current sensor: Installed at the grounding point of the DC cable shield to measure the leakage current I of the DC cable shield leak ; filter :Used to filter out fluctuations caused by high-frequency switching of IGBT of MMC submodule and load end of power grid The generated harmonics extract the low-frequency fundamental signal of the output voltage on the DC side of the MMC and the DC cable leakage current I leak Low-frequency fundamental component; Insulation status evaluation module: Based on the output voltage low-frequency fundamental signal of the MMC DC side extracted by the filter and the DC cable leakage current I leak The low-frequency fundamental component is used to estimate the dielectric loss angle of the DC cable, and the insulation condition of the DC cable is evaluated online based on its changing trend.

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