Method and system for on-line monitoring of high voltage dc cable condition based on low frequency injection of MMC

Abstract

The application discloses a kind of high-voltage direct current cable state on-line monitoring method and system based on MMC low-frequency injection, comprising: MMC controller based on MMC low-frequency injection control strategy to direct current cable injects low-frequency three-phase alternating current signal;Wherein, MMC low-frequency injection control strategy includes: MMC controller adopts the double closed-loop control mode consisting of outer voltage loop and inner current loop;Voltage error is obtained by outer voltage loop calculation, and differential mode voltage U diffdq * And common mode voltage U comdq * Make that MMC outputs target voltage U goal Signal to direct current cable;The application is realized by setting specific MMC control strategy that specific frequency and amplitude low-frequency alternating current signal is actively injected, without modifying the hardware structure of MMC system main loop, implementation safety, simple, flexible;While realizing on-line quantitative evaluation to direct current cable insulation state, monitoring process is high in precision, simple and efficient.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of high-voltage direct-current cable state monitoring, and particularly relates to a high-voltage direct-current cable state online monitoring method and system based on MMC low-frequency injection. BACKGROUND

[0002] The high-voltage direct-current cable is an important physical carrier of the modular multilevel converter (MMC) flexible direct-current transmission system. Mechanical external force damage and electric-thermal damage and other problems can aggravate the aging of the polymer dielectric material, and then cause partial discharge and dielectric breakdown, leading to insulation failure, and even causing system shutdown. Therefore, it is of great significance to build an effective cable insulation state online monitoring system, realize cable insulation state online monitoring, and maximize the safety and reliability of high-voltage direct-current cable operation, to ensure the safe and stable power supply of the cable.

[0003] However, the existing cable insulation state detection technology is mostly offline detection, and the existing cable insulation online monitoring technology is often difficult to meet the requirements due to insufficient accuracy and susceptibility to interference:

[0004] For example, the patent for invention with the announcement number CN110161391B discloses a method for online monitoring of cable insulation by low-frequency signal reverse injection, a low-frequency voltage signal is injected through the open delta side of the electromagnetic voltage transformer connected to the bus, and the current signal of the cable grounding wire and the voltage signal of the cable core are processed in real time, so as to judge the aging degree of the cable; and for example, the domestic patent application for invention with the publication number CN106443371A discloses a novel cable insulation online detection device, which uses a signal source and an isolation transformer to load a low-frequency low-voltage detection signal to the cable, and calculates the insulation parameters such as the distribution capacitance and insulation resistance of the cable to ground after obtaining the low-frequency current and voltage signals of the cable; however, these methods are all based on injecting a low-frequency alternating-current detection signal into the cable through a transformer, and are only applicable to alternating-current cable monitoring, and cannot be applied as an online monitoring means for high-voltage direct-current cable state.

[0005] The domestic invention patent application with the publication number CN101975914A discloses a kind of power cable insulation state on-line monitoring method and device, since power system is interconnected by tie line, under the action of small disturbance, relative swing will occur between generator rotor, active power on tie line fluctuates in a certain range with very low frequency (0.1-2.5Hz), it is called low-frequency oscillation;Therefore, the method measures the oscillation current and voltage signal when power system occurs low-frequency oscillation, obtains cable dielectric loss factor value, realizes insulation aging on-line monitoring. However, low-frequency oscillation occasionally occurs in actual power system, oscillation frequency has randomness and low amplitude, it is difficult to extract available low-frequency signal in real time effectively, limit the practical application performance of the method. SUMMARY

[0006] Therefore, the purpose of the present application is to provide a kind of high voltage DC cable state on-line monitoring method and system based on MMC low-frequency injection, by setting specific MMC control strategy, low-frequency AC signal of specific frequency and amplitude is actively injected, without modifying the hardware structure of MMC system main loop, implementation is safe, simple, flexible;At the same time, on-line quantitative evaluation of DC cable insulation state is realized, the precision of monitoring process is high, simple and efficient.

[0007] The technical solutions adopted by the present application are as follows:

[0008] A kind of high voltage DC cable state on-line monitoring method based on MMC low-frequency injection, DC cable is as the output cable of MMC DC side, including DC cable shielding layer;MMC includes MMC controller, and is provided with as the MMC rectifier of DC side output;On-line monitoring method includes: MMC controller based on MMC low-frequency injection control strategy to the DC cable injection low-frequency three-phase AC signal;Wherein, the MMC low-frequency injection control strategy includes:

[0009] The MMC controller adopts double closed-loop control mode composed of outer voltage loop and inner current loop;Voltage error is obtained by outer voltage loop calculation, differential mode voltage U diffdq * And common mode voltage U comdq * Make MMC output target voltage U goal Signal to DC cable.

[0010] Preferably, the MMC is provided with MMC converter station, and the MMC rectifier is composed of upper bridge arm and lower bridge arm;The MMC controller includes target voltage generation module, outer loop controller module, inner loop controller module and pulse generation module;

[0011] The target voltage generation module sends the target voltage Ugoal signal;

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

[0013] 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, generating the differential mode voltage U diffdq * and the common mode voltage U comdq * .

[0014] Preferably, 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 range ≦ 50Hz, and the target voltage amplitude of the low-frequency three-phase alternating current signal U sin is as follows:

[0015] ; In the formula, U dc represents the original DC voltage output by the MMC, k is a proportional constant, k ≦ 1%, and t is a time variable;

[0016] After the MMC is started to reach a stable DC output, the low-frequency three-phase alternating current signal is superimposed on the basis of the original DC voltage U dc output by the MMC, and the generated composite signal is used as the target voltage U goal of the MMC, and the composite process adopts 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 dc output by the MMC and each phase output voltage U abc , each phase output voltage U abc is converted to d-axis voltage U sd and q-axis voltage U sq through park coordinate transformation;

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

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

[0022] ; wherein 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;

[0023] The q-axis current instruction value I vq * is output by real-time calculation of the reactive power of the MMC and negative feedback adjustment of the reactive power by a PI controller;

[0024] ; wherein Qs * is a target value of the reactive power of the MMC, and Q s is a real-time calculation value of the reactive power of the MMC;

[0025] 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.

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

[0027] The target voltage U goal output by the MMC to the DC cable is detected in real time, and the phase currents I abc are compared to obtain the d-axis current I vd and the q-axis current I vq after Park coordinate transformation;

[0028] The d-axis current I vd and the q-axis current I vq are compared with the received d-axis current instruction value I vd * and the q-axis current instruction value I vq * respectively to calculate a current error;

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

[0030] ;

[0031] ; wherein U diffdq* U diffd * and U diffdq * , w is angular frequency, L ac is inductance of MMC sending end power AC cable, L0 is inductance of upper and lower bridge arms, K P1 is d-axis current proportional coefficient, K I1 is d-axis current integral coefficient, K P2 is q-axis current proportional coefficient, K I2 is q-axis current integral coefficient, s is Laplace integral term;

[0032] The MMC outputs target voltage U goal signal to DC cable by real-time detection of each phase internal loop current I cirabc , after Park coordinate transformation, d-axis and q-axis internal loop currents I cirdq are obtained, d-axis and q-axis internal loop target values I cirdq * are suppressed to 0 as control target, common-mode voltage U comdq * is generated;

[0033] ;

[0034] In the formula, U comdq * = U comd * and U comq * ; K P3 is d-axis inter-phase loop proportional coefficient, K I3 is d-axis inter-phase loop integral coefficient, K P4 is q-axis inter-phase loop proportional coefficient, K I4 is q-axis inter-phase loop integral coefficient, s is Laplace integral term;

[0035] The difference-mode voltage U diffdq * and common-mode voltage U comdq * are sent to the pulse generation module.

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

[0037] The received difference-mode voltage U diffdq * and common-mode voltage U comdq * are subjected to Park coordinate transformation, and the MMC outputs target voltage U goal signal to DC cable, wherein each phase difference-mode voltage Udiffj * and each phase common-mode voltage U comj * ;

[0038] based on each phase differential-mode voltage U diffj * and each phase common-mode voltage U comj * calculate the modulation wave U pj * of the upper bridge arm nj * ;

[0039] ;

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

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

[0042] Preferably, after the low-frequency three-phase alternating current signal injection is completed, the output voltage of the MMC DC side is measured by a voltage sensor, the leakage current I leak of the DC cable shield layer is measured by a current sensor; then the dielectric loss angle of the DC cable is calculated; the insulation state of the DC cable is evaluated based on the change of the dielectric loss angle; wherein as the degree of insulation deterioration of the DC cable deepens, the test dielectric loss angle presents a trend of increasing.

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

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

[0045] a2, extract the low-frequency fundamental component of voltage and current: a filter is used to extract the low-frequency fundamental signal of the output voltage of the MMC DC side and the low-frequency fundamental component of the leakage current I leak of the DC cable from the above obtained output voltage data and leakage current I leak data, suppress the frequency band signal and interference noise irrelevant to the injected low-frequency three-phase alternating current signal;

[0046] a3, low-frequency fundamental component data analysis and processing: the output voltage low-frequency fundamental signal and the DC cable leakage current I leak The low-frequency fundamental component is decomposed into a plurality of superimposed sinusoidal waves, and then the output voltage low-frequency fundamental signal, the DC cable leakage current I leak The phase information of the low-frequency fundamental component is obtained, and the low-frequency fundamental voltage phase φ of the MMC DC side is obtained U And the leakage current phase φ leak ; wherein the calculation process of the Fourier transform is:

[0047] ; wherein,

[0048] 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 n-th cosine component amplitude, b n is the n-th sine component amplitude; t is the time variable;

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

[0050] ; wherein,

[0051] φ n is the n-th harmonic phase, A n is the n-th harmonic amplitude;

[0052] The calculation process of the extraction is:

[0053] ;

[0054] According to the above process, the output voltage low-frequency fundamental signal, the DC cable leakage current I leak The phase information of the low-frequency fundamental component;

[0055] a4, calculate the phase angle difference θ by the following formula:

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

[0057] a5, calculate the dielectric loss angle of the DC cable: using the following relationship between the dielectric loss angle δ and the phase angle difference θ, the dielectric loss angle of the DC cable is calculated:

[0058] .

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

[0060] The MMC controller comprises 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 connection with the outer loop controller module, and sends a target voltage U of the MMC to the outer loop controller module; goal signal;

[0062] The outer loop controller module is in communication connection with the inner loop controller module, and sends a d-axis current instruction value I and a q-axis current instruction value I to the inner loop controller module; vd * vq * ;

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

[0064] 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 outputting a target voltage U signal to the DC cable based on the PWM pulse control signal; goal ;

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

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

[0067] A voltage sensor: installed between the positive and negative DC bus bars of the MMC DC side, for measuring the output voltage of the MMC DC side;

[0068] A current sensor: installed at the ground of the DC cable shielding layer, for measuring the leakage current I of the DC cable shielding layer; leak ;

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

[0070] Insulation state evaluation module: according to the low-frequency fundamental signal of the output voltage of MMC DC side and the leakage current I of DC cable extracted by the filter leak Low-frequency fundamental component, estimate the dielectric loss angle of DC cable, and evaluate the insulation state of DC cable online according to its trend.

[0071] The beneficial effects of the present application mainly lie in: the present application realizes the active injection of low-frequency AC signal with specific frequency and amplitude by setting specific MMC control strategy, without modifying the hardware structure of MMC system main loop, which is safe, simple, flexible; after completing the active injection of low-frequency AC signal with specific amplitude and frequency, the insulation characteristics parameters of DC cable are accurately calculated online: dielectric loss angle, and then the online quantitative evaluation of the insulation state of DC cable is realized, which is high in monitoring process precision, simple and efficient. BRIEF DESCRIPTION OF DRAWINGS

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

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

[0074] Figure 3 is the structure schematic diagram of MMC rectifier in the specific embodiment of the present application;

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

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

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

[0078] Figure 7 is the dielectric loss angle calculation flowchart of DC cable in the specific embodiment of the present application;

[0079] Figure 8 is the result comparison of the theoretical value and the simulation test value of the dielectric loss angle of DC cable in the specific embodiment of the present application.​ Detailed Implementation

[0080] Please refer to the above. Figure 1 , Figure 2 and Figure 3 As shown, this embodiment proposes an online monitoring system for the condition of high-voltage DC cables based on MMC low-frequency injection. The DC cable (i.e.,...) Figure 1 The "Cable Under Test" marked as such serves as the output cable on the DC side of the MMC, including the DC cable shielding layer; the MMC includes an MMC controller, which is connected to both the sending and receiving power supplies; 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, each bridge arm containing a submodule SM (see details...). Figure 3 The SM1 to SM1 shown are shown. n It consists of ) and each submodule SM is composed of switching devices (such as Figure 3 The V shown T1 V T2 V D1 V D2 ) and capacitors (such as Figure 3 The C0 shown constitutes the three-phase AC signal U of the power supply at the sending end. sj (including such as) Figure 3 The U shown sa U sb U sc The AC input is connected to the AC input side of the MMC rectifier via an AC cable.

[0081] Please see further. Figure 2 As shown, in this embodiment, the MMC controller includes a target voltage generation module (i.e., Figure 2 The marked "low-frequency injection target voltage generation module" and outer loop controller module (i.e. Figure 2 The marked "outer loop controller") and inner loop controller module (i.e. Figure 2 The marked "inner loop controller" and pulse generation module (i.e. Figure 2 (The marked "pulse generation");

[0082] 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. goal Signal (corresponding) Figure 1 (The marked "injected low-frequency AC signal").

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

[0084] Voltage sensor: Installed between the positive and negative DC buses on the DC side of the MMC, used to measure the output voltage on the DC side of the MMC. Please refer to [link / reference]. Figure 1 As shown, the output voltage includes the original DC voltage U. dc and the injected low-frequency three-phase AC signal U sin (Corresponding to the "output voltage U of each phase" mentioned in this article) abc "),correspond Figure 1 The label reads "DC voltage online monitoring";

[0085] Current sensor: Installed at the grounding point of the DC cable shield, used to measure the leakage current I of the DC cable shield. leak ,correspond Figure 1 The marked "Leakage Current Online Monitoring";

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

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

[0088] Please see also Figure 1 , Figure 2 and Figure 3 As shown in the figure, this embodiment proposes the online monitoring method used in the online monitoring system for high-voltage DC cables based on MMC low-frequency injection, including: the MMC controller injects low-frequency three-phase AC signals (including such as...) into the DC cable based on the MMC low-frequency injection control strategy.Figure 3 The U sa , U sb , U sc ); wherein the MMC low-frequency injection control strategy comprises:

[0089] 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 the common mode voltage U comdq * are calculated through the inner current loop, so that the MMC outputs the target voltage U goal signal;

[0090] Preferably, in the present embodiment, 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 tracks the target voltage U goal signal of the MMC, and after calculating the voltage error, 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 tracks the d-axis current command value I vd * and the q-axis current command value I vq * , and after calculating the current error, the differential mode voltage U diffdq * and the common mode voltage U comdq * are generated;

[0091] Further preferably, in the present embodiment, 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 adopts the following formula:

[0092] ; in the formula, U dc represents the original DC voltage output by the MMC, k is a proportional constant, k ≦ 1%, and t is a time variable;

[0093] After the MMC is started to reach a stable DC output, the low-frequency three-phase alternating current signal U sin is superimposed on the basis of the original DC voltage U dc output by the MMC, and the generated combined signal is used as the target voltage U goal of the MMC, and the combination process adopts the following formula:

[0094] ;

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

[0096] Real-time detection of the raw DC voltage Uout of the MMC dc and the phase output voltage U abc , the phase output voltage U abc is converted into the d-axis voltage U sd and the q-axis voltage U sq through Park coordinate transformation;

[0097] Comparison of the real-time detection value of the MMC DC side output voltage with the received target voltage U goal signal, to calculate the voltage error;

[0098] Adjustment of the voltage error through a PI controller to output the d-axis current command value I 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, and is a constant;

[0100] Real-time calculation of the reactive power of the MMC and negative feedback adjustment of the reactive power through a PI controller to output the q-axis current command value I vq * ;

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

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

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

[0104] Real-time detection of the phase current I goal of the MMC output target voltage U abc signal, to obtain the d-axis current I vd and the q-axis current I vq after Park coordinate transformation;

[0105] 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 * , and the current error is calculated;

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

[0107] ;

[0108] 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 supply 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, and K I2 is the q-axis current integral coefficient. s is the Laplace integral term, that is, the transformation parameter of the Laplace integral, and is a constant;

[0109] The internal circulating current I goal of each phase of the MMC output target voltage U cirabc signal is detected in real time, and the d-axis and q-axis internal circulating currents I cirdq are obtained after park coordinate transformation. The d-axis and q-axis internal circulating current target values I cirdq * are suppressed to 0 as the control target (i.e. “I cirdq * = 0” shown in the figure), and the common mode voltage U comdq * is generated; Figure 2

[0110] ;

[0111] In the formula, U comdq * is composed of U comd * and U comq * ; K P3 is the d-axis inter-phase circulating current proportional coefficient, and K 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;

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

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

[0114] 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 * ;

[0115] 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 * (”);

[0116] ;

[0117] 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;

[0118] 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 ;

[0119] It should be noted that, Figure 2 The "T 3S-dq ", "T dq-3s " in the above embodiments represent the Park coordinate transformation.

[0120] Further preferably, in the present embodiment, after the low-frequency three-phase alternating current signal injection is completed, the output voltage of the MMC DC side is measured by the voltage sensor, the leakage current I leak of the DC cable shielding layer is measured by the current sensor, the dielectric loss angle of the DC cable is calculated, and the insulation state of the DC cable is evaluated based on the change of the dielectric loss angle; as the insulation of the DC cable deteriorates, the test dielectric loss angle tends to increase; specifically preferably, as shown in Figure 1 , a DC cable insulation aging threshold (labeled as "threshold" in Figure 1 ) can be set, and the insulation state 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, it is determined that 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] As shown in Figure 7 , further preferably, in the present 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 "A / D sampling of sensor signals" in Figure 7 ): A / D conversion is performed on the collected data of the voltage sensor and the collected data of the current sensor to obtain output voltage data and leakage current I leak data;

[0123] a2, extracting low-frequency fundamental components of voltage and current (corresponding to "filtering and extracting low-frequency fundamental components of voltage and current" in Figure 7 ): a filter is used to extract the output voltage low-frequency fundamental signal of the MMC DC side and the DC cable leakage current I leak low-frequency fundamental component from the above obtained output voltage data and leakage current I leak data, respectively, to suppress signals and interference noise unrelated to the injected low-frequency three-phase alternating current signal;

[0124] a3, low-frequency fundamental component data analysis and processing (corresponding to "low-frequency fundamental component data analysis and processing" in Figure 7 ): the output voltage low-frequency fundamental signal and the DC cable leakage current I leak low-frequency fundamental component are both decomposed into the superposition of multiple sine waves through Fourier transform, and then the output voltage low-frequency fundamental signal and the DC cable leakage current I leakThe phase information of the low-frequency fundamental component, obtaining the low-frequency fundamental voltage phase φ of the MMC DC side U And the leakage current phase φ leak ; wherein, the calculation process of the Fourier transform is:

[0125] ; in the formula,

[0126] f(t) is a 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 n-th cosine component amplitude, b n is the n-th sine component amplitude; t is the time variable;

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

[0128] ; in the formula,

[0129] φ n is the n-th harmonic phase, A n is the n-th harmonic amplitude;

[0130] The calculation process of the extraction is:

[0131] ;

[0132] According to the above process, the low-frequency fundamental signal of the output voltage, the DC cable leakage current I leak The phase information of the low-frequency fundamental component;

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

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

[0135] a5, calculate the dielectric loss angle of the DC cable (corresponding to Figure 7 "Calculate the dielectric loss angle of the cable" in the

[0136] .

[0137] In order for those skilled in the art to better understand the technical solutions in the present application, on the basis of the above embodiments, the following specific embodiments will be presented 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, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should belong to the scope of protection of the present application.

[0138] In the following specific embodiments, a ±30 kV MMC flexible HVDC power transmission system is taken as an example for simulation analysis, and the system Matlab simulation model is shown in Figure 4 , that is, the receiving end power source carried by the present specific embodiment is shown in the simulation model shown in Figure 4 ; during the simulation test, after outputting a stable DC voltage at the DC side of the MMC, a low-frequency AC signal U dc is superimposed on the basis of the original DC voltage U sin , to realize the injection of a low-frequency AC signal at the DC side of the MMC; a voltage sensor is installed between the positive and negative DC buses at the DC side of the MMC, a current sensor is installed at the ground of the shielding layer of the DC cable, and the measurement data is saved to the working area for subsequent data analysis.

[0139] In order to avoid the decrease of power transmission quality caused by the excessively large 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 of the on-line monitoring of the dielectric loss angle, and compared with the power frequency, the dielectric loss angle is increased by 10 times, so that the detection accuracy is improved.

[0140] In the MMC starting stage, the original DC voltage output by the MMC is set to the rated output DC voltage 60 kV of the system; after the MMC outputs a stable DC voltage, a 600 V, 5 Hz sinusoidal AC signal (i.e. the injected low-frequency three-phase AC signal U sin ) is superimposed on the basis of the original DC voltage; the output voltage waveform at the DC side of the MMC (i.e. the "MMC DC side output voltage" marked in Figure 5 ) is shown in Figure 5 , and it can be seen from Figure 5 that: the original DC voltage U dc waveform in the first 10 seconds is a steady-state output, and from the 10th second, the original DC voltage U dc waveform superimposes a low-frequency three-phase AC signal to form a target voltage U goal signal; correspondingly, the leakage current I leak of the shielding layer of the DC cable (i.e. the "cable shielding layer leakage current" marked in Figure 6 ) waveform is shown in Figure 6 .

[0141] The phase angle difference θ is calculated through the calculation process provided by the 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 of the dielectric loss angle δ of the DC cable is the core early warning signal of the deterioration of the insulation state, and its essence is the increase of the conductance loss, polarization loss or discharge loss, which corresponds to the aging, dampness, discharge and other defect states of the insulation material.

[0144] On the basis of the above implementation, the insulation resistance R in the equivalent model of the DC cable (the insulation resistance R is arranged in the DC cable) is changed to simulate different insulation states of the DC cable in the simulation model, and the corresponding dielectric loss angle δ test results under different insulation states of the DC cable are shown in Table 1 below:

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

[0146] Insulation resistance (MΩ / km) Tested dielectric loss angle δ (°) 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 deterioration degree of the DC cable deepens, the test dielectric loss angle δ shows a trend of increasing, which conforms to the cable insulation aging law;

[0148] Further reference can be made to the comparison results of the theoretical value and the simulation test value of the dielectric loss angle of the DC cable shown in Figure 8 ; wherein, Figure 8 The dashed curve in Figure 8 is the relationship curve of the theoretical dielectric loss angle and the insulation resistance of the DC cable (i.e. the "theoretical dielectric loss angle" marked in Figure 8 ); the scattered points are the test results of the dielectric loss angle under different insulation states of the DC cable (i.e. the "simulation dielectric loss angle" marked in

[0149] As can be seen from Figure 8 , the test result of the dielectric loss angle is close to its theoretical dielectric loss angle, which verifies the feasibility and effectiveness of the application of the MMC low-frequency injection for online monitoring of the insulation state of the high-voltage DC cable.

[0150] ​It will be obvious to a person skilled in the art that the application is not limited to the details of the foregoing exemplary embodiments and can be implemented in other concrete forms without departing from the spirit or essential characteristics of the application. The 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 reference signs in the claims should be considered as limiting the scope of the claims to the identity of the reference signs therein.

[0151] Furthermore, it should be understood that although the description is made on the basis of the embodiments, not every embodiment contains only one independent technical solution, and the description of the specification is only for the sake of clarity, and those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that those skilled in the art can understand.

Claims

1. A method for on-line monitoring of the state of a high-voltage DC cable based on low-frequency injection of MMC, characterized by, The DC cable, as an output cable of the MMC DC side, comprises a DC cable shielding layer; the MMC comprises an MMC controller and is provided with an MMC rectifier as an output of the DC side; The online monitoring method comprises: the MMC controller injects a low-frequency three-phase alternating current signal into the DC cable based on an MMC low-frequency injection control strategy; wherein the MMC low-frequency injection control strategy comprises: 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 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; 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.

2. The method for on-line monitoring of HVDC cable condition based on MMC low frequency injection according to claim 1, characterized in that, The control process of the target voltage generation module includes that the frequency of the low-frequency three-phase alternating current signal is f, the frequency f is in the range of ≦50 Hz, 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, k is a proportional constant, k < 1%, and t is the time variable. 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 HVDC cable condition based on MMC low frequency injection according to claim 1, characterized in that, 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 sabc , each phase output voltage U sabc 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 Kp is a proportional coefficient of the PI controller, K1 is an integral coefficient of the PI controller, and s is a Laplace integral term. p where Kp is a proportional coefficient of the PI controller, K1 is an integral coefficient of the PI controller, and s is a Laplace integral term. I where Kp is a proportional coefficient of the PI controller, K1 is an integral coefficient of the PI controller, and s is a Laplace 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 HVDC cable condition based on MMC low frequency injection of claim 1, wherein, 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 * are compared respectively, and a current error is calculated. The current error is adjusted by a PI controller to generate a differential mode voltage U diffdq * ; ； wherein 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 , after park coordinate transformation, the internal circulating current I of d-axis and q-axis cirdq the target value I of the internal circulating current of d-axis and q-axis cirdq * suppressing 0 as a control target, generating common-mode voltage U 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 HVDC cable condition based on MMC low frequency injection of claim 1, wherein, 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 HVDC cable condition based on MMC low frequency injection of claim 1, wherein, After the low-frequency three-phase alternating current 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 shielding layer is measured by a current sensor leak ; then the dielectric loss angle of the DC cable is calculated; the insulation state of the DC cable is evaluated based on the change of the dielectric loss angle; wherein as the degree of insulation deterioration of the DC cable deepens, the test dielectric loss angle presents a trend of increasing.

7. The method for on-line monitoring of HVDC cable condition based on MMC low frequency injection according to claim 6, characterized in that, The calculation process of the dielectric loss angle of the DC cable is as follows: a1, sensor signal A / D sampling: A / D conversion is performed on the collected data of the voltage sensor and the collected data of the current sensor to obtain output voltage data, leakage current I leak data; a2, extract low-frequency fundamental components of voltage and current: use filters to extract the output voltage data, leakage current I leak The low-frequency fundamental component of the output voltage of the MMC DC side and the DC cable leakage current I leak Low-frequency fundamental component, suppress the frequency band signal and interference noise irrelevant to the injection of low-frequency three-phase alternating current signal; a3, low-frequency fundamental component data analysis and processing: the output voltage low-frequency fundamental signal and the DC cable leakage current I leak The low-frequency fundamental component is decomposed into a superposition of multiple sinusoidal waves, and then the output voltage low-frequency fundamental signal, the DC cable leakage current I leak The phase information of the low-frequency fundamental component is obtained, and the low-frequency fundamental voltage phase φ of the MMC DC side is obtained U And the leakage current phase φ leak ; wherein the calculation process of the Fourier transform is: ; in which formula, f(t) is the fractional decomposition function, a0 is the DC component of the fractional decomposition function, n is the harmonic number, w is the angular frequency, a n is the amplitude of the n-th cosine component, b n is the amplitude of the n-th sine component; t is the time variable; Then, the same frequency items are combined according to the following formula: ; in which formula, φ n is the n-th harmonic phase, A n is the n-th harmonic amplitude; The calculation process of the extraction is as follows: ； According to the above process, the output voltage low-frequency fundamental signal and the DC cable leakage current I leak the phase information of the low-frequency fundamental component; a4, the phase angle difference θ is calculated according to the following formula: ; where φ leak is the leakage current phase, φ U is the low-frequency fundamental voltage phase of the MMC DC side; a5, the dielectric loss angle of the DC cable is calculated: the dielectric loss angle δ and the phase angle difference θ are related, and the dielectric loss angle of the DC cable is calculated according to the following formula: 。 8. A system for on-line monitoring of the state of a high-voltage DC cable based on low-frequency injection of MMCs, characterized by The DC cable, as an output cable of the MMC DC side, comprises a DC cable shielding layer; the MMC comprises an MMC controller and is provided with an MMC rectifier as an output of the DC side, and the MMC rectifier is composed of an upper bridge arm and a lower bridge arm; wherein 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 diffdq * and 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, and 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 1-7.

9. The system for on-line monitoring of the state of a high-voltage DC cable based on low-frequency injection of MMC according to claim 8, characterized in that, Further comprising: A voltage sensor: installed between the positive and negative DC buses of the MMC DC side, used for measuring the output voltage of the MMC DC side; Current sensor: installed at the grounding of the DC cable shield for measuring the leakage current I of the DC cable shield leak ; A filter: used for filtering the fluctuations caused by the high-frequency switching of the MMC submodule IGBT and the load end of the power grid ​ The generated harmonic extracts 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; The insulation state evaluation module: according to the low frequency fundamental wave signal of the output voltage of the MMC DC side extracted by the filter and the DC cable leakage current I leak The low frequency fundamental wave component, estimate the dielectric loss angle of the DC cable, and evaluate the insulation state of the DC cable online according to the change trend.

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