CN110361627A - A kind of single-ended traveling wave fault location method based on MMC-HVDC - Google Patents

Abstract

The single-ended traveling wave fault location method based on MMC-HVDC that the present invention relates to a kind of, belongs to Relay Protection Technology in Power System field.When the HVDC transmission system misoperation containing modularization multi-level converter, phase-model transformation is carried out to positive and negative anodes DC voltage and obtains line mode voltage；The voltage traveling wave after phase moding is analyzed using empirical mode decomposition-Hilbert-Huang transform and fault point reflection wave head is demarcated；Ranging is carried out to failure according to the wave head of calibration.Empirical mode decomposition-Hilbert-Huang transform (HHT) is applied to the HVDC transmission system containing modularization multi-level converter by the present invention, carries out single-ended fault location.

Description

A single-ended traveling wave fault location method based on MMC-HVDC

technical field

The invention relates to a single-end traveling wave fault distance measurement method based on MMC-HVDC, which belongs to the technical field of electric power system relay protection.

Background technique

In recent years, due to the good waveform quality of modular multilevel converters, no need to configure filters, low switching losses, simple device voltage equalization, modular design, easy expansion, and power supply to passive systems, etc. However, the research on flexible direct current transmission with modular multilevel converters (MMC-HVDC) is still in its infancy. Since my country's research on DC side line faults of HVDC transmission systems based on modular multilevel converters is only based on simple simulation analysis of fault characteristics, after a permanent unipolar ground fault occurs in the system, in order to reduce the line inspection time , it is necessary to quickly measure the distance of the fault location, find the fault location, and then repair the faulty line in time, so as to restore the normal operation of the system. At present, there are relatively few studies on unipolar ground fault location for modular multilevel flexible DC transmission.

Contents of the invention

The technical problem to be solved by the present invention is to provide a single-end traveling wave fault location method based on MMC-HVDC for the existing technical problem of fault protection of HVDC transmission system based on modular multilevel converter.

The technical solution of the present invention is: a single-ended traveling wave fault location method based on MMC-HVDC, when the high-voltage direct current transmission system containing the modular multilevel converter operates abnormally, the positive and negative direct current voltages are phased Transform to obtain the line mode voltage; use the empirical mode decomposition-Hilbert Huang transform (HHT) to analyze the voltage traveling wave after the phase mode change, and calibrate the reflected wave head at the fault point to build the MMC-HVDC system model; according to the calibration The wave head measures the distance of the fault.

The specific steps are:

Step1: When the HVDC transmission system with modular multilevel converters is operating abnormally, extract the positive and negative DC voltages after the grid fault, and use the Karrenbauer phase-mode transformation matrix to obtain the line-mode voltage U ₁ (k ), the conversion formula is as follows:

In the formula, U ₊ (k), U _- (k) are the positive DC voltage and the negative DC voltage of the fault line respectively, k=1, 2, 3, 4...N, and N is the length of the sampling sequence;

Step2: For the line-mode voltage U ₁ (k) obtained in Step1, the steps of Empirical Mode Decomposition (EMD) are as follows:

(1) extracting all local maximum and minimum points of the original signal U ₁ (k);

(2) Calculate the upper and lower envelopes of U ₁ (k) with spline function, and calculate the average value m(k);

(3) Calculate the error h(k)=x(k)-m(k) between the original signal U ₁ (k) and the average value m(k) at each moment;

(4) Determine whether h(k) satisfies the conditions of the Intrinsic Mode Function (IMF):

1) The number of maximum and minimum points contained in the signal and the number of zero-crossing points of the signal (the values of adjacent points at both ends of this point have different signs) are not greater than 1;

2) The upper envelope formed by local maximum points of the signal and the lower envelope formed by local minimum points have an average value of 0, that is, the upper and lower envelopes are symmetrical with respect to the time axis.

If h(k) satisfies the conditions of IMF, then it is IMF;

If; h(k) does not satisfy the condition of IMF, just set it as original data, repeat steps (1)-(2), until h(k) satisfies the condition of IMF component;

(5) Let c=h(k), record the obtained first IMF as c ₁ (k), subtract the first IMF component c ₁ (k) from the original signal, and obtain the remaining part r ₁ (k )=U ₁ (k)-c ₁ (k), and then take the remaining part as a new original signal, follow the steps of (1)-(4) to obtain IMF, and sequentially obtain the nth order IMF component c _n (k );

During the decomposition process, the termination condition is set as:

For h(k), judge whether the number of extremum points and the number of zero points of h(k) are equal or differ by at most 1;

For r(k), judge whether the number of extreme points of r(k) is less than 2;

After the EMD decomposition is completed, the criteria for detecting the mutation point are as follows:

1) First-order difference is performed on the decomposed high-frequency IMF components to determine the position and direction of the largest signal change, which can be regarded as the moment and polarity of the traveling wave head reaching the measurement end;

2) Hilbert transform is performed on the decomposed high-frequency IMF components, and then the instantaneous frequency, instantaneous phase and instantaneous amplitude of each order IMF component are obtained;

3) Calculate the extreme point of the decomposed high-frequency IMF component, calculate the absolute value of the amplitude difference between the adjacent maximum value point and the minimum value point, and the interval between the adjacent maximum value point and the minimum value point, The position where the absolute value of the extreme value difference is the largest and the interval is the smallest is the position of the signal mutation point;

Step3: According to the results of step 2, the reliable identification of reflected wave identification and time calibration t2 or t3 are obtained, and the final fault point distance is obtained as:

In the formula: l ₁ is the fault distance, L is the total length of the line, v is the wave velocity of the downgoing wave with a frequency of 2πf, t ₁ is the time when the initial wave head of the fault reaches the busbar at the measurement end, and _t2 is the time at which the reflected wave at the fault point reaches the busbar at the measurement end Time, t ₃ is the time when the reflected wave from the bus at the opposite end reaches the bus at the measuring end.

The beneficial effects of the present invention are: the present invention applies the empirical mode decomposition-Hilbert-Huang transform (HHT) to the high-voltage direct current transmission system containing the modularized multilevel converter to locate single-end faults.

Description of drawings

Fig. 1 is a flow chart of the present invention;

Fig. 2 is the MMC-HVDC system structural diagram that the present invention is used for emulation;

Fig. 3 is the line-mode voltage waveform on the rectification side of Embodiment 1 of the present invention;

Fig. 4 is the embodiment of the present invention 1 IMF1 high-frequency component;

Fig. 5 is the detection result of HHT wave head of embodiment 1 of the present invention;

Fig. 6 is the line-mode voltage waveform on the rectification side of Embodiment 2 of the present invention;

Fig. 7 is the embodiment of the present invention 2IMF1 high-frequency component;

Fig. 8 is the detection result of the HHT wave head in Example 2 of the present invention.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

A single-ended traveling-wave fault location method based on MMC-HVDC. When the high-voltage direct current transmission system with modular multilevel converters operates abnormally, the positive and negative direct current voltages are phase-mode transformed to obtain line-mode voltages; using Empirical mode decomposition-Hilbert-Huang transform analyzes the voltage traveling wave after the phase mode changes, and calibrates the reflected wave head of the fault point; the fault distance is measured according to the calibrated wave head.

The specific steps are:

Step1: When the HVDC transmission system with modular multilevel converters is operating abnormally, extract the positive and negative DC voltages after the grid fault, and use the Karrenbauer phase-mode transformation matrix to obtain the line-mode voltage U ₁ (k ), the conversion formula is as follows:

In the formula, U ₊ (k), U _- (k) are the positive DC voltage and the negative DC voltage of the fault line respectively, k=1, 2, 3, 4...N, and N is the length of the sampling sequence;

Step2: For the line-mode voltage U ₁ (k) obtained in Step1, the steps of empirical mode decomposition are as follows:

(1) extracting all local maximum and minimum points of the original signal U ₁ (k);

(2) Calculate the upper and lower envelopes of U ₁ (k) with spline function, and calculate the average value m(k);

(3) Calculate the error h(k)=x(k)-m(k) between the original signal U ₁ (k) and the average value m(k) at each moment;

(4) Judging whether h(k) satisfies the condition of the intrinsic mode function:

1) The number of maximum and minimum points contained in the signal and the number of zero-crossing points of the signal (the values of adjacent points at both ends of this point have different signs) are not greater than 1;

2) The upper envelope formed by local maximum points of the signal and the lower envelope formed by local minimum points have an average value of 0, that is, the upper and lower envelopes are symmetrical with respect to the time axis.

If h(k) satisfies the conditions of IMF, then it is IMF;

If; h(k) does not satisfy the condition of IMF, just set it as original data, repeat steps (1)-(2), until h(k) satisfies the condition of IMF component;

(5) Let c=h(k), record the obtained first IMF as c ₁ (k), subtract the first IMF component c ₁ (k) from the original signal, and obtain the remaining part r ₁ (k )=U ₁ (k)-c ₁ (k), and then take the remaining part as a new original signal, follow the steps of (1)-(4) to obtain IMF, and sequentially obtain the nth order IMF component c _n (k );

During the decomposition process, the termination condition is set as:

For h(k), judge whether the number of extremum points and the number of zero points of h(k) are equal or differ by at most 1;

For r(k), judge whether the number of extreme points of r(k) is less than 2;

After decomposition, it can be seen that the original signal consists of several IMF components and a residual component:

In the formula, U ₁ (k) is the original signal, ci ( _k ) is the IMF component of each order, and r _n (k) is the residual component, which is a monotone function.

After the EMD decomposition is completed, the criteria for detecting the mutation point are as follows:

1) First-order difference is performed on the decomposed high-frequency IMF components to determine the position and direction of the largest signal change, which can be regarded as the moment and polarity of the traveling wave head reaching the measurement end;

2) Hilbert transform is performed on the decomposed high-frequency IMF components, and then the instantaneous frequency, instantaneous phase and instantaneous amplitude of each order IMF component are obtained;

3) Calculate the extreme point of the decomposed high-frequency IMF component, calculate the absolute value of the amplitude difference between the adjacent maximum value point and the minimum value point, and the interval between the adjacent maximum value point and the minimum value point, The position where the absolute value of the extreme value difference is the largest and the interval is the smallest is the position of the signal mutation point;

Step3: According to the results of step 2, the reliable identification of reflected wave identification and time calibration t2 or t3 are obtained, and the final fault point distance is obtained as:

In the formula: l ₁ is the fault distance, L is the total length of the line, v is the wave velocity of the downgoing wave with a frequency of 2πf, t ₁ is the time when the initial wave head of the fault reaches the busbar at the measurement end, and _t2 is the time at which the reflected wave at the fault point reaches the busbar at the measurement end Time, t ₃ is the time when the reflected wave from the bus at the opposite end reaches the bus at the measuring end.

Embodiment 1: A model of a modular multi-level direct current transmission system at both ends of 77 levels is built in the simulation software. The capacitance value of the sub-module C=2800uF, the rated capacitance voltage of the sub-module Uc=8.5KV, the reactance of the bridge arm is 50mH, the DC voltage U _dc =±300KV, and the length of the overhead line at the DC side is 400km.

Assuming that a ground fault occurs on the positive line 100km away from the rectifier side, the transition resistance is 0Ω, and the sampling rate is set to 100kHz, the fault line-mode voltage waveform obtained at the measurement end of the rectifier side is shown in Figure 3, and the line-mode voltage traveling wave The high-frequency components of IMF1 are shown in Figure 4, and the HHT wave head detection results of the line-mode voltage traveling wave are shown in Figure 5.

In Fig. 5, a is the initial traveling wave of the fault, b is the reflected wave at the fault point, and c is the reflected wave at the inverter side of the opposite end. In the figure, t ₀ =0.75ms, t ₁ =1.42ms, t ₂ =2.76ms, According to formula (4), taking the wave velocity v as 2.985×102km/ms, the calculated fault distance x=99.9975km, that is, the distance between the fault location and the rectifier side is 99.9975km, which is 2.5m different from the actual fault distance.

Example 2: Suppose a ground fault occurs on the positive line at 350km away from the rectifier side, the transition resistance is 0Ω, and the sampling rate is set to 100kHz. The IMF1 high-frequency component of the voltage traveling wave is shown in Figure 7, and the HHT wave head detection results of the line-mode voltage traveling wave are shown in Figure 8.

In Figure 8, a is the initial traveling wave of the fault, b is the reflected wave at the fault point, and c is the reflected wave at the inverter side of the opposite end. In the figure, t ₀ =1.63ms, t ₁ =3.98ms, t ₂ =1.96ms, according to Formula (3), taking the wave velocity v as 2.985×102km/ms, can calculate x=350.7475km, that is, the distance from the fault location to the rectifier side is 350.7475km, which is 747.5m away from the actual fault distance.

Similarly, in the case of different fault locations and different transition resistances, the ranging results shown in Table 1 can be obtained.

Table 1: Single-ended ranging results under different fault conditions

Analysis of Table 1 shows that the results of fault location are not affected by transition resistance, and the error of fault location is within 2km, which meets the requirements of fault location.

The specific embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the above embodiments. Variations.

Claims (2)

1. a kind of single-ended traveling wave fault location method based on MMC-HVDC, it is characterised in that: changed when containing modular multilevel When flowing the HVDC transmission system misoperation of device, phase-model transformation is carried out to positive and negative anodes DC voltage and obtains line mode voltage；Benefit The voltage traveling wave after phase moding is analyzed with empirical mode decomposition-Hilbert-Huang transform and fault point back wave Head is demarcated；Ranging is carried out to failure according to the wave head of calibration.

2. the single-ended traveling wave fault location method according to claim 1 based on MMC-HVDC, it is characterised in that specific step Suddenly are as follows:

Step1: when the HVDC transmission system misoperation containing modularization multi-level converter, after extracting electric network fault Positive and negative anodes DC voltage, line mode voltage U is found out with Karrenbauer phase mode transformation matrix to the two poles of the earth DC voltage₁(k), become It is as follows to change formula:

In formula, U₊(k), U- (k) is respectively faulty line positive DC voltage and negative DC voltage, and k=1,2,3,4 ... N, N are Sampling sequence length；

Step2: line mode voltage U is obtained to acquired in Step1₁(k), the step of empirical mode decomposition is as follows:

(1) original signal U is extracted₁(k) all Local modulus maximas and minimum point；

(2) U is found out with spline function₁(k) upper and lower envelope, and calculate average value m (k)；

(3) each moment original signal U is calculated₁(k) with the error h (k) of average value m (k)=x (k)-m (k)；

(4) judge whether h (k) meets the condition of intrinsic mode function:

1) number of signal is included maximum and minimum point number and signal zero-crossing is not more than 1；

2) the lower envelope line that the coenvelope line and local minizing point that the Local modulus maxima of signal is constituted are constituted, their mean value It is 0；

If h (k) meets the condition of IMF, it is exactly IMF；

If；H (k) is unsatisfactory for the condition of IMF, it is just set as initial data, repeats step (1)-(2), until h (k) meets Until the condition of IMF component；

(5) c=h (k) is enabled, first IMF found out is denoted as c₁(k), first IMF component c is subtracted with original signal₁(k), Obtain remainder r₁(k)=U₁(k)-c₁(k), it is then sought using remainder as new original signal according to (1)-(4) The step of IMF, successively seeks n-th order IMF component c_n(k)；

In decomposable process, the setting of termination condition are as follows:

For h (k), judge whether the extreme point number of h (k) and zero number are equal or at most poor 1；

For r (k), judge the extreme point number of r (k) whether less than 2；

After the completion of EMD is decomposed, the criterion for carrying out detection detection catastrophe point is as follows:

1) first-order difference is carried out to the high-frequency I MF component after decomposition, it can be determined that the maximum position and direction of signal intensity, it can be with Regard as wavefront reach measuring end at the time of and polarity；

2) Hilbert transformation is carried out to the high-frequency I MF component after decomposition, and then obtains the instantaneous frequency, instantaneous of each rank IMF component Phase and instantaneous amplitude；

3) extreme point is asked to the high-frequency I MF component after decomposition, calculates the absolute of the difference in magnitude of adjacent maximum point and minimum point The interval of value and adjacent maximum point and minimum point, extreme value absolute value of the difference is maximum and is spaced at minimum as sign mutation Point position；

Step3: obtaining reflected wave identification reliable recognition and time calibrating t2 or t3 according to step 2 result, obtain final fault point away from From are as follows:

In formula: l₁For fault distance, L is total track length, and v is the velocity of wave that frequency is 2 π f down going waves, t₁It is arrived for fault initial wave head Up to measuring end bus moment, t₂The time of measuring end bus, t are reached for fault point back wave₃It reaches and surveys for opposite end bus reflected wave Measure the time of end bus.