Transient overvoltage type identification method and system based on CVT high-voltage capacitor current

Abstract

The application discloses a transient overvoltage type identification method and system based on a CVT high-voltage capacitor current, and aims at the problem of power grid side overvoltage type identification, and uses accurate real-time current signals to replace CVT secondary side distorted transient overvoltage signals or transient overvoltage measurement technology based on a voltage division principle to complete overvoltage type identification. Through analysis on the current signals flowing through the high-voltage capacitor under different types of overvoltage signals collected, the accuracy of the real-time identification system for overvoltage type identification is verified.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of overvoltage type identification, and particularly relates to a system for realizing real-time identification of grid-side transient overvoltage type according to current flowing through a CVT high-voltage capacitor. BACKGROUND

[0002] In recent years, with the increasing voltage level of power grids and the large number of access of power electronic devices, the structure of power systems is becoming more and more complex, and power devices are more seriously affected by different waveform and amplitude transient overvoltages. The transient overvoltage problem has new requirements and new characteristics different from traditional technical cognition. In order to ensure the stable operation state of the power system, it is particularly important to improve the real-time monitoring and identification ability of transient overvoltage.

[0003] The occurrence mechanism, development process and waveform characteristics of various overvoltages in power systems are different, and their harm to power systems is also different. Overvoltage is generally divided into lightning overvoltage and internal overvoltage according to its energy source. Different protection measures need to be taken for different overvoltage types, and accurate monitoring, identification and classification are the key. Compared with the electromagnetic voltage transformer (Potential Transformer, PT), the capacitor voltage transformer (Capacitor Voltage Transformer, CVT) has the advantages of low cost, high impact insulation strength, saving installation area and cost, etc., so the CVT has been widely used in the transmission lines of 110kV and above voltage level in China. Due to the existence of a large number of capacitors and inductors and other elements in the CVT, when the system is subjected to transient overvoltage impact, the output voltage of the secondary side of the CVT is seriously distorted, so that the transient overvoltage on the grid side cannot be correctly restored.

[0004] At present, the recognition of overvoltage type is based on the collected voltage signal, whether from the extraction of characteristic quantity or from the selection of classification decision algorithm. The extraction of characteristic quantity includes traditional Fourier transform, S transform, mathematical morphology and time domain analysis method; the classification decision algorithm includes direct criterion construction method, statistical based machine learning method and other methods, which are all based on the collected voltage signal. Since the CVTs produced by different manufacturers or even the CVTs produced by the same manufacturer are different in parameter design, if the type recognition is performed on the transient overvoltage signal of the secondary side distortion of the CVT, the uncertainty of the recognition criterion setting will be caused, and then the incorrect recognition result will be caused. However, if the currently recognized transient overvoltage signal collection technology based on the voltage division principle is used, the voltage divider method and the bushing end screen method will change the grounding mode of the primary equipment, thereby affecting the safe operation of the equipment, and the sensor is exposed outside the equipment, which is not suitable for long-term network operation; the series C3 method needs to perform switching operation to disconnect the bus during signal extraction, which is complex, and the impact voltage will be generated when the bus is cut off, so protective measures should be taken to ensure the safety of the measurement personnel, and the workload is large. Therefore, a new overvoltage type recognition method needs to be proposed to overcome the above problems. SUMMARY

[0005] The application provides a transient overvoltage type recognition method and system based on CVT high-voltage capacitor current, which uses the real-time current signal flowing through the CVT to replace the voltage signal to complete the overvoltage type recognition, further improves the real-time monitoring capability of the overvoltage, and thereby improves the stability of the power system operation.

[0006] To achieve the above purpose, the transient overvoltage type recognition method based on CVT high-voltage capacitor current provided by the application comprises the following steps:

[0007] S1, collecting the high-voltage capacitor current signal of the capacitor voltage transformer;

[0008] S2, extracting the characteristic quantity of the high-voltage capacitor current signal;

[0009] S3, recognizing the transient overvoltage type according to the characteristic quantity of the high-voltage capacitor current signal.

[0010] Further, in S1, the high-voltage capacitor current signal is collected by connecting the current sensor in series at the high-voltage capacitor of the capacitor voltage transformer.

[0011] Further, the current sensor is built in the secondary terminal box of the capacitor voltage transformer, and the input of the current sensor is connected with the output end of the capacitor voltage transformer.

[0012] Further, in S2, the characteristic quantity is extracted from the phase with rich transient quantity information.

[0013] Further, S2 comprises the following steps:

[0014] S2.1, the high-voltage capacitor current signal of the capacitive voltage transformer is processed by discrete Fourier transform and simplification to obtain a high-voltage capacitor current signal after fast Fourier transform;

[0015] S2.2, a set Fourier current ratio is calculated according to the high-voltage capacitor current signal after fast Fourier transform as a characteristic quantity.

[0016] Further, S2.1 comprises the following steps:

[0017] S2.1.1, the acquired high-voltage capacitor current signal of the capacitive voltage transformer is processed by discrete Fourier transform to obtain a frequency spectrum after discrete Fourier transform;

[0018] S2.1.2, the frequency spectrum after discrete Fourier transform is simplified according to the periodicity and symmetry of the coefficients of the frequency spectrum after discrete Fourier transform to obtain a high-voltage capacitor current signal after fast Fourier transform.

[0019] Further, in S2.2, the Fourier current ratio calculation formula is:

[0020] r m =i m / i 50 , m = 1, 2, …, 10000;

[0021] Wherein, r m is the Fourier current ratio, i m is the current amplitude under the frequency of mHz of the high-voltage capacitor current signal after fast Fourier transform, and i 50 is the power frequency current amplitude of the high-voltage capacitor current signal after fast Fourier transform.

[0022] Further, S3 comprises the following steps:

[0023] According to the characteristic quantity of the high-voltage capacitor current signal and the closing capacitor bank overvoltage criterion, it is judged whether it is a closing capacitor bank overvoltage:

[0024] If yes, the recognition result is a closing capacitor bank overvoltage;

[0025] Otherwise, according to the current signal characteristic quantity and the closing no-load line overvoltage criterion, it is judged whether it is a closing no-load line overvoltage:

[0026] If yes, the recognition result is a closing no-load line overvoltage;

[0027] Otherwise, according to the current signal characteristic quantity and the closing no-load transformer overvoltage criterion, it is judged whether it is a closing no-load transformer overvoltage:

[0028] If yes, the recognition result is closing no-load transformer overvoltage;

[0029] Otherwise, according to the current signal feature quantity and power frequency overvoltage criterion, whether it is power frequency overvoltage is judged:

[0030] If yes, the recognition result is power frequency overvoltage;

[0031] Otherwise, the recognition result is lightning overvoltage.

[0032] A transient overvoltage type recognition system based on CVT high-voltage capacitor current, comprising an input module, a feature quantity extraction module and a recognition module:

[0033] The input module is used for collecting the CVT high-voltage capacitor current signal;

[0034] The feature quantity extraction module is used for extracting the feature quantity of the high-voltage capacitor current signal;

[0035] The recognition module is used for recognizing the transient overvoltage type according to the feature quantity of the high-voltage capacitor current signal.

[0036] Further, the recognition module comprises a first recognition module, a second recognition module, a third recognition module and a fourth recognition module connected in sequence;

[0037] The first recognition module is used for judging whether it is closing capacitor group overvoltage according to the feature quantity of the high-voltage capacitor current signal and the closing capacitor group overvoltage criterion: if yes, the recognition result is closing capacitor group overvoltage; otherwise, the second recognition module is started;

[0038] The second recognition module is used for judging whether it is closing no-load line overvoltage according to the feature quantity of the high-voltage capacitor current signal and the closing no-load line overvoltage criterion: if yes, the recognition result is closing no-load line overvoltage; otherwise, the third recognition module is started;

[0039] The third recognition module is used for judging whether it is closing no-load transformer overvoltage according to the feature quantity of the high-voltage capacitor current signal and the closing no-load transformer overvoltage criterion: if yes, the recognition result is closing no-load transformer overvoltage; otherwise, the third recognition module is started;

[0040] The fourth recognition module is used for judging whether it is power frequency overvoltage according to the current signal feature quantity and the power frequency overvoltage criterion: if yes, the recognition result is power frequency overvoltage; otherwise, the recognition result is lightning overvoltage.

[0041] Compared with the prior art, the present application has at least the following beneficial technical effects:

[0042] The recognition method provided by the application utilizes the current signal flowing through the CVT collected in real time to replace the voltage signal to recognize the transient overvoltage type in real time. Since the current transformer has a better effect on the transient signal transformation, the transient overvoltage type is recognized based on the current signal and the distorted transient overvoltage signal of the CVT secondary side, which has the advantages of simple and clear recognition criterion setting, high recognition accuracy and the like.

[0043] Further, compared with the transient overvoltage signal obtained based on the voltage division principle, the method only needs to embed the through-type current sensor in the CVT secondary terminal box, that is, the input terminal is directly connected with the CVT output terminal, the modification process is relatively simple, time-saving and labor-saving, the safety factor is high, and the long-term online monitoring and type recognition of the transient overvoltage can be realized.

[0044] Further, the high-voltage current signal after the fast Fourier transform is utilized to recognize the overvoltage type, so that the operation workload is greatly reduced, the recognition efficiency is improved, and the recognition result can be given faster.

[0045] Further, the recognition mode of layer-by-layer classification is adopted, and each time of recognition only recognizes a type of transient overvoltage, so that the problem of too complex selection of characteristic quantities and judgment basis in the overall recognition is avoided, and thus the system recognition speed and accuracy are improved, the protection of the power system overvoltage is laid a foundation, and the operation stability of the power system is improved. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 is a specific classification diagram of the transient overvoltage type to be recognized by the application;

[0047] Figure 2 is a CVT equivalent model diagram of the application;

[0048] Figure 3 is a CVT high-voltage capacitor current signal acquisition schematic diagram, CT1 and CT2 are micro-current sensors;

[0049] Figure 4 is a flow chart of the overvoltage type recognition module of the application;

[0050] Figure 5 is a flow chart of the overvoltage type recognition system of the application;

[0051] Figure 6 is a flow chart of the overvoltage type recognition system of the application;

[0052] Figure 7 is a frequency domain signal obtained by FFT transformation of signal T1;

[0053] Figure 8 is a frequency domain signal obtained by FFT transformation of signal T2;

[0054] Figure 9 T3 is the frequency domain signal of the signal of the application after FFT transform;

[0055] Figure 10 T4 is the frequency domain signal of the signal of the application after FFT transform.

[0056] Wherein, R1 is the equivalent loss resistance of the high-voltage capacitor, R2 is the equivalent loss resistance of the medium-voltage capacitor, L1 is the residual inductance of the high-voltage capacitor, L2 is the residual inductance of the medium-voltage capacitor, C s C is the equivalent stray capacitance of the compensating reactor, T1 C is the stray capacitance of the primary winding of the intermediate transformer to ground, T2 C is the stray capacitance of the secondary winding of the intermediate transformer to ground, T12 C is the coupling capacitance between the primary winding and the secondary winding. DETAILED DESCRIPTION

[0057] In order to make the purpose and technical scheme of the application more clear and convenient to understand. The following will be further described in detail in combination with the drawings and examples, the specific examples described herein are only used to explain the application, and are not used to limit the application.

[0058] In the description of the application, it should be understood that the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore it cannot be understood as a limitation on the application. In addition, the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features limited by "first", "second" can be explicitly or implicitly included one or more of the features. In the description of the application, unless otherwise specified, the meaning of "a plurality of" is two or more. In the description of the application, it should be noted that, unless otherwise specified and limited, the terms "mounting", "connecting", "connecting" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium, or it can be the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the application can be understood according to the specific circumstances.

[0059] Considering that the micro-current sensor is easy to obtain the current flowing through the inside of the CVT, and has good transient response characteristics, if the real-time collected accurate current signal flowing through the inside of the CVT can be used to complete the type identification of the transient overvoltage signal of the power grid side, it undoubtedly has high engineering practical value. The current signal mentioned in the application refers to the current flowing through the high-voltage capacitor of the CVT.

[0060] The technical scheme adopted by the application is: realizing the type identification of the transient overvoltage of the power grid side based on the current signal flowing through the high-voltage capacitor of the CVT, mainly including obtaining the current signal flowing through the high-voltage capacitor of the CVT under the action of different types of overvoltage to be identified, adopting a suitable analysis method to extract characteristic quantities according to the characteristics of the current signal, selecting a corresponding classification decision algorithm, and finally constructing a complete overvoltage type identification system to identify the type of overvoltage, thereby laying a foundation for the protection of the power system overvoltage.

[0061] Embodiment 1

[0062] The transient overvoltage type real-time identification method based on the high-voltage capacitor current of the CVT includes the following steps:

[0063] Step 1: determining the classification of the overvoltage to be identified according to the actual operating conditions of the CVT, and classifying it according to the occurrence mechanism, development process, etc.

[0064] The transient overvoltage signal in the power grid can be subdivided into multiple types, but due to the large difference in the occurrence probability of various types of transient overvoltage in the power grid, and considering the design parameters and application scenarios of the CVT, for example, arc grounding overvoltage only occurs in a neutral point ungrounded system, and the CVT is mainly used for voltage monitoring in a 110kV and above voltage level system; at the same time, in a 110kV or 220kV system, the ferroresonance overvoltage is caused by the resonance of the inductance coupling of the high-voltage circuit breaker capacitor and the inductance of the electromagnetic voltage transformer, the linear resonance is mainly caused by the inductance element without a core and the capacitor element in the system, and the parameter resonance is caused by the inductance element with periodic change of inductance parameters and the capacitor element in the system, that is, the CVT is not prone to resonance overvoltage. Therefore, in order to make the transient overvoltage type real-time identification method based on the high-voltage capacitor current of the CVT more practical in engineering, the characteristics of the following several typical overvoltage types are analyzed, and the identification of the type of overvoltage is realized.

[0065] 1) Operating overvoltage, the operating overvoltage includes the following three types:

[0066] (1)Switching capacitor bank overvoltage. When the capacitor bank is switched on, the arc burning of the circuit breaker will cause high-frequency electromagnetic vibration on the line, and when the ground capacitance is small, it will cause low-frequency oscillation of 3-4 power frequency cycles. At the same time, the continuous burning of the arc will also cause the generation of overvoltage in the system, and its amplitude generally will not exceed 2 times of the rated condition.

[0067] (2)Switching overvoltage of no-load line. During the switching process of the line, the capacitance and inductance parameters in the system will change, thereby causing the redistribution of electromagnetic energy. At the same time, arc burning, three-phase waveform high-frequency oscillation, and further formation of overvoltage with high amplitude are caused. But affected by line loss, switching phase and some limiting measures, its amplitude generally will not exceed 3 times of the rated condition.

[0068] (3)Switching overvoltage of no-load transformer. When the transformer is switched on, the transformer excitation winding will appear inrush current, thereby generating switching overvoltage.

[0069] 2)Power frequency overvoltage. The overvoltage generated due to asymmetric short-circuit fault, line no-load or load shedding in the system, and its waveform characteristics are mainly that the frequency is power frequency, and the voltage amplitude exceeds the maximum working phase voltage.

[0070] It should be noted here that since the frequency band range of lightning overvoltage is wide, and generally the high-voltage capacitance and medium-voltage capacitance values of the CVT capacitor unit are large, when lightning overvoltage occurs at the power grid side, the current flowing through the CVT high-voltage capacitor will be severely distorted and thus the characteristics thereof cannot be accurately extracted. In the overvoltage type recognition based on the CVT current signal, the lightning overvoltage is not directly recognized, but the internal overvoltage type is first recognized, and the lightning overvoltage is classified into other overvoltage types by chance, and the overvoltage type to be recognized of the present application is as shown in Figure 1

[0071] Step 2, acquiring real-time current signals flowing through the CVT high-voltage capacitor under different types of overvoltage;

[0072] Step 2.1, building an equivalent model reflecting the full operating condition of the CVT;

[0073] The CVT is composed of a capacitance divider and an electromagnetic unit. In actual application, the CVT structure of 35kV-1000kV voltage level is completely the same, and the difference lies in that the capacitance of the capacitor unit is different, and thus the parameters of each element in the electromagnetic unit will also change accordingly.

[0074] ​The capacitor divider of CVT mainly includes high voltage capacitor C1 and medium voltage capacitor C2. The rated capacitance value of high voltage capacitor is determined according to the rated voltage of primary system, and the rated capacitance value of medium voltage capacitor is determined according to the intermediate voltage of CVT. Since the active power loss exists in the non-ideal capacitor, the equivalent model of the capacitor can be represented by RC series structure, and the resistance value of the equivalent loss resistor can be obtained by the dielectric loss factor. The insulation structure, temperature and voltage of the capacitor are important factors affecting the size of the dielectric loss factor, and the dielectric loss factor tanδ is

[0075]

[0076] In formula (1), P a is the active power of the capacitor; P r is the reactive power of the capacitor.

[0077] The calculation formula of the equivalent loss resistor R1 of high voltage capacitor and R2 of medium voltage capacitor is as follows:

[0078]

[0079] In formula (2), X c1 is the capacitive reactance value of high voltage capacitor, X c2 is the capacitive reactance value of medium voltage capacitor; ω is the angular frequency; f is the power frequency of 50 Hz; C1 is the capacitance value of high voltage capacitor, and C2 is the capacitance value of medium voltage capacitor.

[0080] The electromagnetic unit is composed of compensation reactor, intermediate transformer T and damper. Under the condition of power frequency, the inductance of compensation reactor should be adjusted so that it can be in series resonance with the equivalent capacitance C1+C2 of capacitor divider, so as to eliminate the voltage loss caused by the reactance of capacitor divider, that is, the inductance value L s of compensation reactor can be obtained by the following formula:

[0081]

[0082] In formula (3), L T1 is the leakage inductance of primary winding of intermediate transformer; L' T2 is the leakage inductance of secondary winding of intermediate transformer, which is converted to the value on the primary side.

[0083] The compensation reactor also has active power loss, and the active power loss resistor R s of the compensation reactor is

[0084]

[0085] In formula (3), X L is the inductive reactance value of compensation reactor; Q is the quality factor of compensation reactor.

[0086] The intermediate transformer mainly functions to further reduce the voltage after the voltage is divided by the capacitor unit to the standard value of the CVT rated secondary voltage according to the transformation ratio. Meanwhile, since the impedance value of the damper is very large in the normal working state of the CVT, it can be approximately equivalent to an open circuit, and will not affect the normal operation of the monitoring equipment connected to the secondary winding of the CVT, so the damper can be ignored when the CVT model is built.

[0087] In addition, in the normal operation state of the CVT, stray capacitance and residual inductance will be generated between the two devices and the conductor due to electromagnetic induction. Although the stray capacitance value and the residual inductance value are very small, under the overvoltage condition, they jointly act with the inductance and capacitance parameters of the compensation reactor and the intermediate transformer to generate various series and parallel resonance circuits, thereby destroying the original resonance condition and causing the sensor transfer characteristic to be seriously disturbed. Therefore, in order to fully consider the influence of the stray capacitance and the residual inductance on the CVT transfer characteristic, so as to make it closer to the operating condition of the CVT in the engineering practice, the stray capacitance and the residual inductance need to be equivalent when the CVT model is built. Since the stray capacitance and the residual inductance value cannot be obtained by experiment in actual engineering, the experience value is adopted in the present application, specifically including: the residual inductance L1 of the high-voltage capacitor is 0.9 μH, the residual inductance L2 of the medium-voltage capacitor is 0.3 μH; the equivalent stray capacitance C s of the compensation reactor is 10 pF, the stray capacitance C T1 of the primary winding of the intermediate transformer to ground is 20 pF, the stray capacitance C T2 of the secondary winding of the intermediate transformer to ground is 20 pF, and the coupling capacitance C T12 between the primary winding and the secondary winding of the intermediate transformer is 10 pF.

[0088] Finally, the equivalent model of the CVT is built as shown in Figure 2 .

[0089] Step 2.2, obtaining the current signal.

[0090] As shown in Figure 3 , based on the CVT model established in step 2.1, different types of transient overvoltage signals in step 1 are applied to the primary side of the CVT, and micro-current sensors are connected in series at the high-voltage capacitor and the medium-voltage capacitor to collect the high-voltage capacitor current signal in real time.

[0091] According to the engineering practice, the current signal can be collected by using a through-type micro-current sensor, and the current sensor is built into the CVT secondary terminal box, and the input terminal is directly connected with the output terminal of the CVT, so that the CVT itself has the current signal output capability.

[0092] Step 3, based on the three-phase current signals under each type of overvoltage measured in step 2, the waveform of one phase or any phase with rich transient quantity information is selected for time-frequency domain feature analysis and feature extraction.

[0093] The current signal T1 under the closing capacitor bank overvoltage, the current signal T2 under the closing no-load line overvoltage and the current signal T3 under the closing no-load transformer overvoltage are processed as follows:

[0094] The first-phase closing time is obtained

[0095]

[0096] In formula (4), respectively, the closing time of the three-phase A, B and C; min(·) represents the minimum value.

[0097] The current signal of one cycle and the current signal T4 of one cycle under the power frequency overvoltage of the first-phase closing time after one cycle of the phase with the most transient information in each of the above three types are selected as the research object, and fast Fourier transform (FFT) is used for analysis, which mainly includes two steps:

[0098] First, for the limited sampling data sequence length N, in the range of 0≤k≤N-1, the current signals under each type of overvoltage, i.e. the current signals T1, T2, T3 and T4 under overvoltage, are respectively subjected to discrete Fourier transform (DFT):

[0099]

[0100] In formula (5), X(k) is the frequency spectrum of the current signal after discrete Fourier transform; x(n) is the current signal under each type of overvoltage, n is the sampling point number; e is the natural logarithm; W N is the matrix coefficient,

[0101] On this basis, in order to improve the processing speed of digital signals, FFT is used to decompose the frequency spectrum of each current signal, and the basic principle is:

[0102] For formula (5), x(n) is divided into two groups according to odd and even, and r represents a natural number:

[0103]

[0104] At the same time, the periodicity and symmetry of the rotation factor are used to simplify the operation, i.e.

[0105]

[0106]

[0107] Thus, the simplified current signal spectrum expression is obtained:

[0108]

[0109] As can be seen from the above formula, for N-point DFT operation, the first half of the points X(k) can be represented by array A(k) and array B(k), both of which are N / 2-point DFTs, and the operation of the second half of the points X(k+N / 2) can be represented by the already calculated A(k) and B(k), and here the operation amount is truly reduced. According to the same step, continue to perform odd-even grouping on A(k) and B(k), and each time grouping is performed, the operation amount of nearly half of the original sequence can be reduced, until the decomposition into two-point DFT operation.

[0110] Fourier current ratio r m is defined as follows:

[0111] r m = i m / i 50 , m = 1, 2, …, 10000 (10)

[0112] In formula (9), i m is the current amplitude of the high-voltage capacitor current signal T1, T2, T3, T4 at m Hz after fast Fourier transform, and i 50 is the power frequency current amplitude of the high-voltage capacitor current signal T1, T2, T3, T4 after fast Fourier transform.

[0113] Among them, the high-voltage capacitor current signal T1, T2, T3 under overvoltage after fast Fourier transform has a large number of high-frequency components in addition to the power frequency component, and in general, when the frequency exceeds 10 kHz, the energy of each frequency band is very small and can be ignored; and the spectrum of T4 theoretically contains only the power frequency component. Therefore, in order to realize the identification of the high-voltage capacitor current signal T1, T2, T3, T4 under overvoltage, the current amplitude at the intermediate frequency band 5000 Hz of the operating overvoltage overall frequency band is selected for Fourier current ratio calculation, that is, the characteristic quantity is r 5000 ; At the same time, in order to more accurately distinguish the three types of high-voltage capacitor current signals T1, T2, T3 under overvoltage, the characteristic quantity r 650 should also be added when identifying this type.

[0114] Step 4, establish an identification module and build a transient overvoltage type real-time identification system.

[0115] Based on the current signal characteristic quantity under different types of overvoltage extracted in step 3, a direct criterion method is used to select appropriate numerical values as the judgment basis of the current signal of this type.

[0116] The identification rule of T1 is set as: 6.4 < r 650 <6.8, 0.1 < r 5000 <0.5;

[0117] The identification rule of T2 is set as: 0.8 < r 650 <1.2, 0.1 < r 5000 <0.5;

[0118] The identification rule of T3 is set as: 0.1 < r 650 <0.5, 0.6 < r 5000 <1;

[0119] The identification rule of T4 is set as: r 5000 <0.1.

[0120] On this basis, the identification modules of different overvoltage types are separately constructed, which mainly include the extraction of signal characteristic quantity and the decision of current type, and the identification flowchart is as shown in Figure 4 According to the current signal characteristic quantity and the identification rule of high-voltage capacitor current signal T1 under overvoltage, it is judged whether it is closing capacitor bank overvoltage:

[0121] If yes, the recognition result is closing capacitor bank overvoltage;

[0122] Otherwise, according to the current signal characteristic quantity and the identification rule of high-voltage capacitor current signal T2 under overvoltage, it is judged whether it is closing no-load line overvoltage:

[0123] If yes, the recognition result is closing no-load line overvoltage;

[0124] Otherwise, according to the current signal characteristic quantity and the identification rule of high-voltage capacitor current signal T3 under overvoltage, it is judged whether it is closing no-load transformer overvoltage:

[0125] If yes, the recognition result is closing no-load transformer overvoltage;

[0126] Otherwise, according to the current signal characteristic quantity and the identification rule of high-voltage capacitor current signal T4 under overvoltage, it is judged whether it is power frequency overvoltage:

[0127] If yes, the recognition result is power frequency overvoltage;

[0128] Otherwise, the recognition result is lightning overvoltage.

[0129] Taking the T1 identification module as an example, the specific logic of the identification module is as follows: when the to-be-identified overvoltage signal is applied to the CVT primary side, the current signal flowing through the high-voltage capacitor enters the T1 identification module, and the current signal feature quantity r 650 and r 5000 are extracted, which are input to the classification decision maker as input signals to realize the discrimination of the criterion of the identification module, that is, 6.4 < r 650 <6.8, 0.1 < r 5000 <0.5. If the identification rule is met, the output of the classification decision maker is 1, which proves that the current signal belongs to T1, that is, the to-be-identified overvoltage is the overvoltage of this type; otherwise, the next identification module is entered for identification.

[0130] On the basis of the establishment of each identification module, a transient overvoltage type real-time identification system is constructed. The identification system adopts a layer-by-layer classification identification mode, and each identification module is independent of each other. The identification system is composed of the identification modules in the order of the high-voltage capacitor current signals T1, T2, T3 and T4 under overvoltage, and the identification flowchart is as shown in Figure 5 If the current signal flowing through the CVT high-voltage capacitor under a to-be-identified overvoltage type is not identified by all the identification modules, since the occurrence probability of other internal overvoltages of the CVT is almost 0, the overvoltage type is classified as lightning overvoltage.

[0131] Embodiment 2

[0132] Referring to Figure 6 , a transient overvoltage type real-time identification system based on a CVT high-voltage capacitor current includes an input module, a feature quantity extraction module, an identification module and an output module. The identification module includes a first identification module, a second identification module, a third identification module and a fourth identification module.

[0133] The input module is connected with the input end of the feature quantity extraction module. The input end of the feature quantity extraction module is connected with the input end of the first identification module. The output end of the first identification module is connected with the second identification module and the output module. The output end of the second identification module is connected with the third identification module and the output module. The output end of the third identification module is connected with the fourth identification module and the output module. The output end of the fourth identification module is connected with the output module.

[0134] The first identification module is used for judging whether it is a closing capacitor bank overvoltage according to the feature quantity of the high-voltage capacitor current signal and the closing capacitor bank overvoltage criterion. If yes, the identification result is output as the closing capacitor bank overvoltage; otherwise, the second identification module is started.

[0135] The second identification module is configured to determine whether it is the closing no-load line overvoltage according to the high-voltage capacitor current signal characteristic quantity and the closing no-load line overvoltage criterion; if yes, the identification result is the closing no-load line overvoltage; otherwise, the third identification module is started;

[0136] The third identification module is configured to determine whether it is the closing no-load transformer overvoltage according to the high-voltage capacitor current signal characteristic quantity and the closing no-load transformer overvoltage criterion; if yes, the identification result is the closing no-load transformer overvoltage; otherwise, the third identification module is started;

[0137] The fourth identification module is configured to determine whether it is the power frequency overvoltage according to the current signal characteristic quantity and the power frequency overvoltage criterion; if yes, the identification result is the power frequency overvoltage; otherwise, the identification result is the lightning overvoltage.

[0138] Simulation verification

[0139] In order to verify the identification accuracy of the transient overvoltage type real-time identification system based on the CVT high-voltage capacitor current, the application obtains one group of current signal samples under each type of overvoltage for verification.

[0140] The FFT decomposition is performed on each current signal according to the method described in step 3 to obtain the frequency domain signal of each current signal. Figure 7 is the frequency domain signal of the high-voltage capacitor current signal T1 under the overvoltage after FFT decomposition, Figure 8 is the frequency domain signal of the high-voltage capacitor current signal T2 under the overvoltage after FFT decomposition, Figure 9 is the frequency domain signal of the high-voltage capacitor current signal T3 under the overvoltage after FFT decomposition, Figure 10 is the frequency domain signal of the high-voltage capacitor current signal T4 under the overvoltage after FFT decomposition.

[0141] Based on the judgment basis in the classification decision maker of each identification module, the current amplitude of the corresponding frequency is extracted and the Fourier voltage ratio is calculated, and the results are shown in Table 1.

[0142] Table 1: Current amplitude and Fourier voltage ratio calculation results

[0143]

[0144] The identification effect of each type of overvoltage is shown in Table 2.

[0145] Table 2: Overvoltage type identification effect

[0146]

[0147] It can be seen from the above table that the Fourier voltage ratio calculated under each type of overvoltage is within the set identification range. In summary, the transient overvoltage type real-time identification system based on the CVT high-voltage capacitor current can realize real-time and accurate identification of the overvoltage type.

[0148] The above is only to illustrate the technical idea of the present application, and cannot limit the protection scope of the present application. Any modification made according to the technical idea of the present application on the basis of the technical scheme falls within the protection scope of the claims of the present application.

Claims

1. A method for identifying the type of transient overvoltage based on CVT high-voltage capacitor current, characterized in that, The method comprises the following steps: S1, collecting the high-voltage capacitor current signal of the capacitive voltage transformer; S2, extracting the characteristic quantity of the high-voltage capacitor current signal; The S2 comprises the following steps: S2.1, the high-voltage capacitor current signal of the capacitive voltage transformer is simplified through discrete Fourier transform, and a high-voltage capacitor current signal after fast Fourier transform is obtained; S2.2, a set Fourier current ratio is calculated according to the high-voltage capacitor current signal after fast Fourier transform as the characteristic quantity; In the S2.2, the Fourier current ratio calculation formula is: ； wherein is the Fourier current ratio, is the frequency of the high voltage capacitor current signal after fast Fourier transformation, m is the current amplitude at 50 Hz, is the power frequency current amplitude of the high voltage capacitor current signal after fast Fourier transformation. To realize the recognition of high-voltage capacitor current signals T1, T2, T3 and T4 under overvoltage, the current amplitude under the intermediate frequency band 5000 Hz of the operating overvoltage overall frequency band is selected to perform Fourier current ratio calculation, that is, the characteristic quantity is ; meanwhile, to more accurately distinguish the three types of high-voltage capacitor current signals T1, T2 and T3 under overvoltage, and add the characteristic quantity when performing the type recognition; the T1 is a current signal under the closing capacitor bank overvoltage, the T2 is a current signal under the closing no-load line overvoltage, the T3 is a current signal under the closing no-load transformer overvoltage, and the T4 is a current signal under the overvoltage of the first phase closing time and the power frequency. The identification rule of T1 is set as: , ; The identification rule of T2 is set as: ;​ The identification rule of T3 is set as: , ; The recognition rule of T4 is set as: ; S3, identifying the transient overvoltage type according to the characteristic quantity of the high-voltage capacitor current signal; The S3 comprises the following steps: According to the characteristic quantity of the high-voltage capacitor current signal and the closing capacitor bank overvoltage criterion, it is judged whether it is the closing capacitor bank overvoltage: If yes, the identification result is the closing capacitor bank overvoltage; Otherwise, according to the current signal characteristic quantity and the closing empty line overvoltage criterion, it is judged whether it is the closing empty line overvoltage: If yes, the identification result is the closing empty line overvoltage; Otherwise, according to the current signal characteristic quantity and the closing empty transformer overvoltage criterion, it is judged whether it is the closing empty transformer overvoltage: If yes, the identification result is the closing empty transformer overvoltage; Otherwise, according to the current signal characteristic quantity and the power frequency overvoltage criterion, it is judged whether it is the power frequency overvoltage: If yes, the identification result is the power frequency overvoltage; Otherwise, the identification result is the lightning overvoltage.

2. The method according to claim 1, wherein, In the S1, the high-voltage capacitor current signal is collected by connecting the current sensor in series at the high-voltage capacitor of the capacitive voltage transformer.

3. The method according to claim 2, wherein, The current sensor is built in the secondary terminal box of the capacitive voltage transformer, and the input of the current sensor is connected with the output end of the capacitive voltage transformer.

4. The method of claim 1, wherein the method is characterized by, In the S2, one phase with rich transient quantity information is selected to extract the characteristic quantity.

5. The method of claim 1, wherein the method is characterized by, The S2.1 comprises the following steps: S2.1.1, the acquired high-voltage capacitor current signal of the capacitive voltage transformer is subjected to discrete Fourier transform to obtain a frequency spectrum after discrete Fourier transform; S2.1.2, according to the periodicity and symmetry of the coefficients of the frequency spectrum after discrete Fourier transform, the frequency spectrum after discrete Fourier transform is simplified to obtain the high-voltage capacitor current signal after fast Fourier transform.

6. A system for identifying the type of transient overvoltage based on CVT high voltage capacitor current, characterized by, The method comprises an input module, a characteristic quantity extraction module and an identification module: The input module is used for collecting the high-voltage capacitor current signal of the capacitive voltage transformer; The characteristic quantity extraction module is used for extracting the characteristic quantity of the high-voltage capacitor current signal; the characteristic quantity of the high-voltage capacitor current signal is extracted by the following method: S2.1, the high-voltage capacitor current signal of the capacitive voltage transformer is simplified through discrete Fourier transform, and a high-voltage capacitor current signal after fast Fourier transform is obtained; S2.2, a set Fourier current ratio is calculated according to the high-voltage capacitor current signal after fast Fourier transform as the characteristic quantity; in the S2.2, the Fourier current ratio calculation formula is: ； wherein is the Fourier current ratio, is the frequency of the high voltage capacitor current signal after fast Fourier transformation, m is the current amplitude at 50 Hz, is the power frequency current amplitude of the high voltage capacitor current signal after fast Fourier transformation. The identification module is used for identifying the transient overvoltage type according to the characteristic quantity of the high-voltage capacitor current signal, and the identification of the transient overvoltage type according to the characteristic quantity of the high-voltage capacitor current signal comprises: To realize the recognition of high-voltage capacitor current signals T1, T2, T3 and T4 under overvoltage, the current amplitude under the intermediate frequency band 5000 Hz of the operating overvoltage overall frequency band is selected to perform Fourier current ratio calculation, that is, the characteristic quantity is ; meanwhile, to more accurately distinguish the three types of high-voltage capacitor current signals T1, T2 and T3 under overvoltage, and add the characteristic quantity when performing the type recognition; the T1 is a current signal under the closing capacitor bank overvoltage, the T2 is a current signal under the closing no-load line overvoltage, the T3 is a current signal under the closing no-load transformer overvoltage, and the T4 is a current signal under the first-phase closing time and a current signal under the power frequency overvoltage. The identification rule of T1 is set as: , ; The identification rule of T2 is set as: , ; The identification rule of T3 is set as: , ; The recognition rule of T4 is set as: ; According to the characteristic quantity of the high-voltage capacitor current signal and the closing capacitor bank overvoltage criterion, whether it is the closing capacitor bank overvoltage is determined: If yes, the recognition result is the closing capacitor bank overvoltage; Otherwise, according to the current signal characteristic quantity and the closing no-load line overvoltage criterion, whether it is the closing no-load line overvoltage is determined: If yes, the recognition result is the closing no-load line overvoltage; Otherwise, according to the current signal characteristic quantity and the closing no-load transformer overvoltage criterion, whether it is the closing no-load transformer overvoltage is determined: If yes, the recognition result is the closing no-load transformer overvoltage; Otherwise, according to the current signal characteristic quantity and the power frequency overvoltage criterion, whether it is the power frequency overvoltage is determined: If yes, the recognition result is the power frequency overvoltage; Otherwise, the recognition result is the lightning overvoltage.

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