CN-122026345-A - Temporary overvoltage suppression method for flexible low-frequency power transmission system

CN122026345ACN 122026345 ACN122026345 ACN 122026345ACN-122026345-A

Abstract

The invention relates to the technical field of flexible low-frequency power transmission, in particular to a temporary overvoltage suppression method of a flexible low-frequency power transmission system, which aims to solve the problems of poor temporary overvoltage suppression effect and insufficient system reliability and economy of the flexible low-frequency power transmission system under multi-mode faults. The method comprises the steps of collecting electric quantity signals in real time and calculating multi-mode fault characteristic quantity, intelligently identifying fault types by using a fault classification model based on a mixed kernel function Support Vector Machine (SVM), dynamically adjusting an additional control strategy of a modularized multi-level converter (M3C) according to identification results to perform preliminary inhibition, and triggering a controllable energy consumption voltage limiting device to perform accurate voltage clamping and energy dissipation if the preliminary inhibition effect is insufficient. By adopting the technical scheme, the temporary overvoltage suppression effect of the flexible low-frequency power transmission system can be remarkably improved, and the safety, stability, reliability and operation efficiency of the system are improved.

Inventors

XIANG NIANWEN
Jin Chuanshuang
CHEN ZHIWEI
SHAO BINGBING
ZHU ZHIQIAN
WANG CHENXING
PENG LONG
LI JIANCHAO
WANG SHULAI
WANG ZHITAO
LI PENGFEI
CHENG HAILONG

Assignees

合肥工业大学
国网冀北电力有限公司电力科学研究院
国家电网有限公司

Dates

Publication Date: 20260512
Application Date: 20260113

Claims (10)

1. A method for temporary overvoltage suppression in a flexible low frequency power transmission system, comprising the steps of: s1, monitoring key nodes of a flexible low-frequency power transmission system in real time, collecting electric quantity signals of the key nodes, wherein the electric quantity signals comprise instantaneous three-phase voltages Ua, ub and Uc and instantaneous three-phase currents Ia, ib and Ic, and calculating zero sequence voltage V0, positive sequence voltage V1 and negative sequence voltage V2 based on the instantaneous three-phase voltages Ua, ub and Uc; S2, calculating a multi-mode fault characteristic quantity based on the collected electric quantity signals, wherein the multi-mode fault characteristic quantity comprises a zero sequence voltage mutation index Kzero, a negative sequence dominant factor Fneg, a current change rate di/dt and a total harmonic distortion rate THD; S3, constructing a four-dimensional fault feature vector, wherein the four-dimensional fault feature vector is formed by combining the zero sequence voltage mutation index Kzero, the negative sequence dominant factor Fneg, the current change rate di/dt of time and the total harmonic distortion rate THD, and performing min-max normalization processing on the four-dimensional fault feature vector to obtain a normalized feature vector; S4, inputting the normalized feature vector into a pre-trained fault classification model, wherein the fault classification model is realized by adopting a classifier based on a Support Vector Machine (SVM), a decision function is constructed by a mixed kernel function of a fused radial basis kernel function and a polynomial kernel function, and the fault type of the system is judged according to the output result of the decision function; S5, calculating the confidence coefficient of the fault judgment, if the confidence coefficient is lower than a preset first confidence coefficient threshold value, starting a redundancy judgment mechanism based on a multi-feature threshold value rule, and further judging the system fault type according to the multi-feature threshold value rule; S6, dynamically adjusting an additional control strategy of the modularized multi-level converter (M3C) according to the determined fault type, wherein the M3C performs preliminary suppression on temporary overvoltage by introducing the zero sequence voltage amplitude and the negative sequence voltage amplitude as feedback quantities for suppressing overvoltage, and a new d-axis low-frequency outer ring voltage reference value is obtained; S7, judging the primary inhibition effect of the M3C, and immediately sending a trigger signal to a controllable energy-consumption voltage limiting device if the system voltage fails to drop below a preset first safety voltage level within a preset first time window; S8, after the controllable energy-consumption voltage limiting device receives the trigger signal, a thyristor valve group in the controllable energy-consumption voltage limiting device is conducted, a resistor disc group of a controllable part of the controllable energy-consumption voltage limiting device is short-circuited, the equivalent conducting voltage of the controllable energy-consumption voltage limiting device is reduced, overvoltage energy is dissipated, and the system voltage is clamped below the preset first safety voltage level; S9, after the system backup protection action clears the fault, the controllable energy consumption voltage limiting device is automatically turned off because the internal current of the controllable energy consumption voltage limiting device is reduced below the preset thyristor maintaining current, the operation is stopped, the M3C releases the additional overvoltage suppression control mode, and the normal operation control strategy is restored.
2. The method for temporary overvoltage suppression of a flexible low frequency power transmission system according to claim 1, wherein the step S1 specifically includes: Collecting the instantaneous three-phase voltages Ua, ub, uc and the instantaneous three-phase currents Ia, ib, ic by means of voltage transformers and current transformers arranged at the key nodes; The voltage transformer and the current transformer adopt electronic transformers with the precision of 0.2S level and have frequency response ranges covering direct current to thousands of hertz; All acquired instantaneous electric quantity signals are synchronously sampled and digitized through a high-speed data acquisition unit, wherein the high-speed data acquisition unit comprises a multichannel synchronous sampling analog-to-digital converter, and each channel has at least 16-bit quantization precision and sampling frequency not lower than 5 kHz; the digitized data is transmitted to a central control unit through an optical fiber communication link for real-time processing; And a digital signal processor in the central control unit performs symmetrical component transformation on the collected three-phase voltage data to calculate the zero sequence voltage V0, the positive sequence voltage V1 and the negative sequence voltage V2 in real time.
3. The method for temporary overvoltage suppression of a flexible low frequency power transmission system according to claim 1, wherein in the step S2, the calculation method of the zero sequence voltage mutation index Kzero and the negative sequence dominant factor Fneg specifically includes: the zero sequence voltage mutation index Kzero is used for quantitatively representing the instantaneous change degree of the zero sequence voltage when the asymmetric ground fault occurs in the system, and the calculation method comprises the following steps: ; Wherein the said Representing the maximum absolute value of the zero sequence voltage V0 within a preset 50ms period, the 、 And said Representing the maximum absolute values of the three-phase voltages Ua, ub and Uc, respectively, over the 50ms time period; the negative sequence dominant factor Fneg is used for quantifying the degree of asymmetric faults of the system, and the calculation method comprises the following steps: ; wherein t0 represents a fault starting time, fneg represents a ratio of negative sequence voltage energy to positive sequence voltage energy in a preset second time window after the fault occurs, and the preset second time window is set to be 20ms.
4. The method for suppressing temporary overvoltage of flexible low-frequency power transmission system according to claim 1, wherein in the step S2, the method for calculating the rate of change di/dt of the current with respect to time and the total harmonic distortion THD is specifically as follows: the change rate di/dt of the current to time is obtained by calculating the instantaneous derivatives of the three-phase currents Ia, ib and Ic through a digital differential module, the digital differential module is realized by adopting a three-point center differential algorithm, and the sampling frequency of the digital differential module is consistent with the data acquisition frequency; The total harmonic distortion ratio THD calculates the total harmonic distortion ratio of the three-phase voltages Ua, ub and Uc or the three-phase currents Ia, ib and Ic through a harmonic analysis module, and the harmonic analysis module performs spectrum analysis on the collected electric quantity signals by adopting a Fast Fourier Transform (FFT) algorithm and calculates the THD according to a standard, and takes into consideration harmonic components up to 50 times.
5. The method for suppressing temporary overvoltage of flexible low-frequency power transmission system according to claim 1, wherein in the step S3, the four-dimensional fault feature vector x is defined as: ; The formula of the min-max normalization processing is as follows: ; Wherein the said For normalized feature vectors, the And said Respectively obtaining the minimum value and the maximum value of each characteristic quantity under normal working condition or according to historical fault data statistics, wherein And said All are stored in the parameter library of the fault classification model and loaded when the system is initialized.
6. The method for suppressing temporary overvoltage of flexible low-frequency power transmission system according to claim 1, wherein in said step S4, a mixed kernel function of said fault classification model The specific scheme of (a) is as follows: ; Wherein the said For the ith support vector, the Is an input feature vector; The parameter values in the mixed kernel function are optimized by system simulation and historical data, the method comprises the following steps of Has a value of 0.7, said Has a value of 0.3, said Has a value of 0.02, said The value of (2); The decision function The calculation formula of (2) is as follows: ; Wherein the said To support the vector weight coefficients, the To support vector labels, the The number of the support vectors is preferably 120, and the value of the bias term b is 0.38; The fault classification model outputs the decision function And determining the fault type in combination with a preset threshold range, wherein the determination rule comprises: if Kzero is greater than 35% and Fneg is less than 0.3, then a single-phase earth fault is determined, at which point the decision function The output result of (2) is greater than 1.5; if Fneg is greater than 0.6 and Kzero is less than 10% and di/dt is greater than 5kA/ms, then a two-phase short circuit fault is determined, at which time the decision function The output result of (2) is greater than 0.2 and less than or equal to 1.5; If Fneg is greater than 0.6, kzero is greater than 20%, and THD is less than 15%, then a two-phase ground fault is determined, at which point the decision function The output result of (2) is greater than or equal to-0.8 and less than or equal to 0.2.
7. The method for suppressing temporary overvoltage of flexible low-frequency power transmission system according to claim 1, wherein in the step S5, the confidence coefficient is calculated by the formula: ; The preset first confidence threshold value is 85%; the rules of the redundancy decision mechanism include: if said Kzero is greater than 35% and said Fneg is less than 0.3 and said di/dt is less than 5kA/ms, then determining a single phase earth fault; if said Fneg is greater than 0.6 and said Kzero is less than 10% and said di/dt is greater than 8kA/ms, then determining a two-phase short circuit fault; if Fneg is greater than 0.6 and Kzero is greater than 20% and THD is less than 15%, then a two-phase ground fault is determined.
8. The temporary overvoltage suppression method for a flexible low frequency power transmission system according to claim 1, wherein: in the step S6, the low frequency side of the M3C adopts a V/F control mode; The M3C additional control strategy extracts and calculates the zero sequence voltage amplitude in real time through a digital filter and a symmetrical component extraction module on the basis of the original low-frequency side outer ring control And the negative sequence voltage amplitude ; The new d-axis low-frequency outer ring voltage reference value The calculation formula of (2) is as follows: ; Wherein the said The reference value is the original d-axis low-frequency outer ring voltage reference value of M3C under the normal operation condition, and k is a temporary overvoltage control coefficient; The value of the temporary overvoltage control coefficient k is adaptively adjusted according to the determined fault type: when a single-phase earth fault occurs in the system, k is adjusted to be k1, and the value of k is 0.38; when a two-phase ground fault occurs to the system, k is adjusted to be k2, and the value of k is 0.72; when a two-phase short circuit fault occurs in the system, k is adjusted to be k3, and the value of k is 0.60; the M3C initially clamps the system voltage below a preset first safe voltage level, which is 1.3p.u., by the additional control strategy.
9. The temporary overvoltage suppression method for a flexible low frequency power transmission system according to claim 1, wherein: in the step S7, the preset first time window is set to 5ms; the real-time monitoring of the system voltage is performed through a voltage sensor arranged at the key node (11), and the system voltage monitoring module (50) comprises a high-speed comparator, so that any phase voltage peak value or instantaneous value of the monitored system can be continuously compared with the first safety voltage level threshold value at microsecond response speed; When the central control unit judges that the primary inhibition effect of the M3C is insufficient, a level trigger signal is generated, and the trigger signal is sent to a gate control module of the controllable energy consumption voltage limiting device through a high-speed optocoupler isolation circuit and a driving circuit; In the step S8, the controllable energy-consuming voltage limiting device is formed by connecting a fixed part and a controllable part in parallel, the fixed part is a conventional Metal Oxide (MOV) valve block group, and the controllable part is formed by connecting the thyristor valve block and another group of metal oxide resistor blocks in series and is connected with the fixed part in parallel; The thyristor valve group is formed by connecting a plurality of high-voltage thyristors in series, and after the controllable energy-consumption voltage limiting device receives the trigger signal, the thyristor valve group is rapidly conducted and presents an extremely low impedance state to short-circuit the metal oxide resistor disc group connected in series in the controllable part.
10. The temporary overvoltage suppression method for a flexible low frequency power transmission system according to claim 1, wherein: In the step S9, when the main loop fault is detected and isolated by the system backup protection device, and the current flowing through the thyristor valve group is lower than the preset maintenance current threshold value, the thyristor valve group is automatically turned off, so that the controllable energy-consumption voltage limiting device completely exits from running; And the M3C releases the additional overvoltage suppression control mode, the low frequency side of the M3C is restored to the original V/F control mode, and the zero sequence voltage amplitude and the negative sequence voltage amplitude are not introduced any more as feedback quantities.

Description

Temporary overvoltage suppression method for flexible low-frequency power transmission system Technical Field The invention relates to the technical field of flexible low-frequency power transmission, in particular to a temporary overvoltage suppression method of a flexible low-frequency power transmission system. Background The flexible low-frequency alternating current transmission technology has the technical advantages of the traditional high-voltage alternating current transmission and the flexible direct current transmission by reducing the transmission frequency below the power frequency, and has unique values in the aspects of improving the current carrying capacity of a line, reducing the capacitance effect, engineering transformation economy and flexible controllability of a system. Similar to the power frequency and flexible direct current system, the flexible low-frequency alternating current transmission system can be divided into temporary overvoltage and transient overvoltage according to time after the fault and subsequent overvoltage, wherein when the main protection of the system fails, the system is switched to the backup protection by hundreds of milliseconds, the duration of the fault transient process is longer, and the stable operation of the system is greatly damaged. If the temporary overvoltage cannot be suppressed below 1.3p.u., the insulation tolerance threshold of the converter valve is exceeded, so that breakdown of the converter valve, aging of insulation of the transformer are accelerated, and even cascading failure of the system is caused. Therefore, the temporary overvoltage suppression method for the flexible low-frequency power transmission system based on the cooperation of multi-mode fault identification and grading is provided, and has important significance for safe and stable operation of the flexible low-frequency alternating-current power transmission system. The conventional flexible low-frequency alternating current transmission system has single temporary overvoltage suppression means, only reduces the temporary overvoltage at the low frequency side of the system through a M3C line voltage control strategy, suppresses the temporary overvoltage generated by the power frequency side faults and the valve internal faults only by configuring a conventional arrester, has the defects of poor adaptability, incapability of distinguishing fault types by the conventional method, difficulty in optimizing the suppression strategy according to different fault characteristics, poor economy, continuous operation of the conventional arrester at high potential, short service life and high maintenance cost. Therefore, the conventional single temporary overvoltage suppression means have the problems of poor suppression effect and small application range, and a suppression method combining fault identification, hierarchical control and device cooperation is needed to ensure safe and stable operation of the flexible low-frequency power transmission system. Disclosure of Invention The invention discloses a temporary overvoltage suppression method of a flexible low-frequency power transmission system, which aims to solve the problems of principle disjointing which is difficult to reconcile between a generalized and single suppression strategy and complex and diversified fault characteristics in the prior art, and the limitations of short service life, high maintenance cost and lack of initiative and adaptability of the traditional lightning arrester, so that the transient overvoltage suppression effect of the flexible low-frequency power transmission system under multi-mode faults and the reliability and economy of system operation are improved. According to the invention, through introducing the real-time monitoring and accurate calculation of the multi-mode fault characteristic quantity, the intelligent recognition of the fault type is carried out by combining the mixed kernel function support vector machine model, and on the basis, a hierarchical collaborative and dynamic response inhibition strategy is constructed. According to the strategy, the additional control of the modularized multi-level converter (M3C) is utilized to carry out preliminary self-adaptive suppression on overvoltage, and then, according to the preliminary suppression effect, a controllable energy-consumption voltage limiting device is triggered to carry out accurate voltage clamping and energy dissipation when necessary, so that the system can effectively limit the temporary overvoltage amplitude below a safety threshold under various complex fault conditions, and the safety stability and the operation efficiency of the flexible low-frequency power transmission system are comprehensively improved. Specifically, the temporary overvoltage suppression method for the flexible low-frequency power transmission system provided by the invention comprises the following technical steps: The method comprises the steps of