Broadband harmonic online monitoring method for flexible direct current system

Abstract

The invention discloses a broadband harmonic online monitoring method for a flexible direct current system, and the method comprises the following steps: obtaining a topological structure and a modulation mode of a converter of the flexible direct current system, determining the switching frequency of the system, and dividing the types of characteristic harmonics of the system; proper sampling frequency and sampling interval are selected to carry out online monitoring and data analysis on the current of the flexible direct current system, whether the condition that the harmonic amplitude exceeds the alarm value exists in the current or not is judged, and if the harmonic amplitude does not exceed the alarm value, the online monitoring state is maintained; if the alarm value is exceeded, entering the next step; and judging the harmonic wave type exceeding the alarm value in the flexible direct current system, and performing specific processing according to the harmonic wave type. According to the invention, rapid and accurate analysis of broadband harmonic wave types and frequency spectrums is realized, and rapid judgment of whether harmonic wave amplitude exceeds the standard in the flexible direct current system is realized. According to the invention, the communication and storage resource occupation of the monitoring data is reduced, the load rate of the CPU of the control protection device is reduced, and the operation is stable and safe.

Description

Technical Field

[0001] The invention belongs to the technical field of flexible DC power transmission, and particularly relates to a method for on-line monitoring of broadband harmonics in a flexible DC system. Background Technique

[0002] Flexible DC power transmission technology is a new type of power transmission technology based on turn-off power electronic devices and voltage source converters, and has many technical advantages such as the ability to supply power to passive systems, dynamic reactive power support ability, and no need for AC filtering devices. Therefore, flexible DC power transmission technology has broad engineering application prospects, and flexible DC power transmission systems with different application prospects can adopt different topological structures and different modulation strategies - common topologies include two-level / three-level converters and modular multilevel converters. The two-level / three-level converter usually adopts pulse width modulation (PWM), while the modular multilevel converter usually adopts nearest level modulation (NLM). The use of PWM or NLM modulation in flexible DC power transmission systems is the main reason for the appearance of complex frequency harmonics and interharmonics in the power grid, which further makes the waveform distortion and oscillation at abnormal frequencies prone to occur in the power grid. In the high-frequency oscillation events that have occurred in flexible DC power transmission systems, the oscillation frequencies are mostly near the switching frequencies of the converters, usually high frequency and ultra-high frequency. For example, the 1270 Hz oscillation that occurred in the Luxi back-to-back DC project, the 1810 Hz oscillation that occurred in the Yu'e DC back-to-back project, and the 1550 Hz oscillation that occurred in the Zhangbei flexible DC project, etc. The voltage / current waveform distortion and oscillation problems caused by high-frequency harmonics and interharmonics will seriously affect the power quality and operation stability of the power system. Therefore, harmonic-related protection is also set in flexible DC systems. Therefore, in order to ensure the safe and stable operation of flexible DC systems, it is usually necessary to on-line monitor the voltage / current of flexible DC systems, so as to quickly judge whether there are harmonic over-limit in flexible DC systems, as well as the frequencies and amplitudes of the over-limit harmonics.

[0003] At present, the harmonic analysis of flexible DC systems generally uses the Fast Fourier Transform (FFT) to extract each harmonic, and judges whether the harmonic amplitude exceeds the standard according to the extracted value of each harmonic. Although this method can correctly reflect each harmonic in the system, in engineering applications, the method based on Fourier transform has a very large computational amount, slow detection of ultra-high frequency harmonics, difficult detection of low-frequency inter-harmonics, and insensitive detection means. At the same time, it will also cause a large amount of power quality monitoring data to occupy a large amount of communication and storage resources. The load rate of the CPU of the control and protection device is high, which is not conducive to the long-term, safe and stable operation of the control and protection device. Therefore, for the online monitoring of high-frequency harmonics in flexible DC systems, it is necessary to consider both the rapid judgment of the frequency and amplitude of ultra-high frequency harmonics and the problem of occupying communication and storage resources. Summary of the Invention

[0004] The present invention is proposed to solve the problems existing in the prior art, and its purpose is to provide a method for online monitoring of broadband harmonics in a flexible DC system.

[0005] The technical solution of the present invention is: a method for online monitoring of broadband harmonics in a flexible DC system, including the following steps:

[0006] A. Obtain the topological structure and modulation method of the converter in the flexible DC system, determine the system switching frequency, and divide the system characteristic harmonic types;

[0007] B. Select a suitable sampling frequency and sampling interval to conduct online monitoring and data analysis on the current of the flexible DC system, and judge whether there is a situation where the harmonic amplitude exceeds the alarm value in the current. If it does not exceed the alarm value, maintain the online monitoring state; if it exceeds the alarm value, proceed to the next step;

[0008] C. Judge the harmonic type of the flexible DC system that exceeds the alarm value, and perform specific processing according to the harmonic type.

[0009] Furthermore, in step A, obtaining the topological structure and modulation method of the converter in the flexible DC system, determining the system switching frequency, and dividing the system characteristic harmonic types, the specific process is as follows:

[0010] First, obtain the topological structure and modulation method of the converter in the flexible DC system involved;

[0011] Then, determine the system switching frequency f s ;

[0012] Next, obtain the fundamental frequency f0 of the AC power grid of the flexible DC system;

[0013] Finally, according to the harmonic spectrum characteristics, divide the harmonic types of the flexible DC system into four categories.

[0014] Furthermore, the four categories include the first category of low-frequency harmonics, the second category of low-frequency interharmonics, the third category of high-frequency harmonics, and the fourth category of high-frequency interharmonics.

[0015] Furthermore, in step B, a suitable sampling frequency and sampling interval are selected to perform on-line monitoring and data analysis on the current of the flexible DC system, and it is judged whether there is a situation where the harmonic amplitude exceeds the warning value. The specific process is as follows:

[0016] First, on-line monitoring and data acquisition are performed on the output current of the flexible DC system;

[0017] Then, the data collected by on-line monitoring of the current of the flexible DC system is filtered and noise-reduced;

[0018] After that, a sliding window fast Fourier transform is performed, and the window width and sliding width are set according to the adopted data, so as to obtain the amplitudes of the harmonic currents of each order of the flexible DC system;

[0019] After that, the obtained harmonic amplitudes are compared with their limits to judge whether there is a situation where the harmonic / interharmonic current amplitude exceeds the warning value.

[0020] Furthermore, in step C, the harmonic type in the flexible DC system that exceeds the warning value is judged, and specific processing is performed according to the harmonic type. The specific process is as follows:

[0021] First, the harmonic frequency in the flexible DC system whose amplitude exceeds the warning value is obtained;

[0022] Then, combined with the harmonic type classification in step A, it is judged which type of harmonic has an amplitude exceeding the warning value;

[0023] Finally, specific processing is performed according to the type of the harmonic exceeding the warning value.

[0024] Furthermore, the characteristic spectrum of the first category of low-frequency harmonics is (6k±1)f0.

[0025] Furthermore, the characteristic frequency f of the second category of low-frequency interharmonics ih The value range is 0 < f ih < f0 or f0 < f ih < 2f0.

[0026] Furthermore, the characteristic spectrum of the third category of high-frequency harmonics is mf s ±nf0, where when m = 1, 3, 5, …, n = 0, 2, 4, …; when m = 2, 4, 6, …, n = 1, 3, 5, ….

[0027] Furthermore, the characteristic spectrum of the fourth category of high-frequency interharmonics is mf s ±nf ih, where when m = 1, 3, 5, …, n = 0, 2, 4, …; when m = 2, 4, 6, …, n = 1, 3, 5, ….

[0028] The beneficial effects of the present invention are as follows:

[0029] The present invention provides a method for online monitoring of broadband harmonics in a flexible DC system, which can quickly and accurately analyze the types and spectra of broadband harmonics, and quickly judge whether there is an excessive harmonic amplitude in the flexible DC system.

[0030] The present invention solves the problems of slow detection of ultra-high frequency harmonics, difficult detection of low-frequency inter-harmonics, and insensitive detection means, reduces the communication and storage resource occupancy of monitoring data, reduces the load rate of the CPU of the control and protection device, and operates stably and safely. Description of the Drawings

[0031] Figure 1 is the flowchart of the method of the present invention;

[0032] Figure 2 is the MMC topology diagram of the present invention;

[0033] Figure 3 is the waveform diagram of the output voltage of the MMC arm of the present invention. Detailed Embodiments

[0034] Hereinafter, the present invention will be described in detail with reference to the drawings and embodiments:

[0035] As Figures 1 to 3 shown, a method for online monitoring of broadband harmonics in a flexible DC system includes the following steps:

[0036] A. Obtain the topology structure and modulation mode of the converter in the flexible DC system, determine the system switching frequency, and divide the system characteristic harmonic types;

[0037] B. Select an appropriate sampling frequency and sampling interval to perform online monitoring and data analysis on the current in the flexible DC system, and judge whether there is a situation where the harmonic amplitude exceeds the warning value in the current. If it does not exceed the warning value, maintain the online monitoring state; if it exceeds the warning value, proceed to the next step;

[0038] C. Judge the harmonic type with an excessive warning value in the flexible DC system, and perform specific processing according to the harmonic type.

[0039] The process of step A for obtaining the topology structure and modulation mode of the converter in the flexible DC system, determining the system switching frequency, and dividing the system characteristic harmonic types is as follows:

[0040] First, obtain the topology structure and modulation mode of the converter in the flexible DC system involved;

[0041] Then, determine the system switching frequency f s ;

[0042] After that, obtain the fundamental frequency f0 of the AC power grid of the flexible DC system;

[0043] Finally, according to the harmonic spectrum characteristics, divide the harmonic types of the flexible DC system into four categories.

[0044] The four categories include the first category of low-frequency harmonics, the second category of low-frequency interharmonics, the third category of high-frequency harmonics, and the fourth category of high-frequency interharmonics.

[0045] In step B, select appropriate sampling frequency and sampling interval to conduct on-line monitoring and data analysis of the current of the flexible DC system, and judge whether there is a situation where the harmonic amplitude exceeds the alarm value. The specific process is as follows:

[0046] First, conduct on-line monitoring and data acquisition of the output current of the flexible DC system;

[0047] Then, filter and denoise the data collected by on-line monitoring of the current of the flexible DC system;

[0048] After that, perform a sliding window fast Fourier transform, and set the window width and sliding width according to the adopted data, so as to obtain the amplitudes of the harmonic currents of each order of the flexible DC system;

[0049] After that, compare the obtained harmonic amplitude with its limit value to judge whether there is a situation where the harmonic / interharmonic current amplitude exceeds the alarm value.

[0050] In step C, judge the harmonic type in the flexible DC system that exceeds the alarm value, and conduct specific processing according to the harmonic type. The specific process is as follows:

[0051] First, obtain the harmonic frequency in the flexible DC system whose amplitude exceeds the alarm value;

[0052] Then, combined with the harmonic type classification in step A, judge which type of harmonic has an amplitude exceeding the alarm value;

[0053] Finally, conduct specific processing according to the type of the harmonic exceeding the alarm value.

[0054] The characteristic spectrum of the first category of low-frequency harmonics is (6k±1)f0.

[0055] The characteristic frequency f of the second category of low-frequency interharmonics ih The value range is 0 < f ih < f0 or f0 < f ih < 2f0.

[0056] The characteristic spectrum of the third category of high-frequency harmonics is mfs ±nf0, where when m = 1, 3, 5, …, n = 0, 2, 4, …; when m = 2, 4, 6, …, n = 1, 3, 5, ….

[0057] The characteristic spectrum of the fourth type of high-frequency inter-harmonics is mf s ±nf ih , where when m = 1, 3, 5, …, n = 0, 2, 4, …; when m = 2, 4, 6, …, n = 1, 3, 5, ….

[0058] Specifically, in step A, according to the voltage level and capacity of the point of common coupling where the flexible DC system is connected to the power grid, and in combination with standards such as GB / T 14549-1993 "Power Quality - Harmonics in Public Power Grids" and GB / T 24337-2009 "Power Quality - Inter-harmonics in Public Power Grids", the alarm values of the amplitudes of the four types of harmonics of the flexible DC system and the alarm value of the total harmonic distortion rate of the current are determined as the preliminary basis for judging whether the harmonics exceed the standard. The alarm value of the harmonic amplitude is less than the maximum allowable value in the national standard.

[0059] Specifically, in step B, a suitable sampling frequency and sampling interval are selected to conduct on-line monitoring and data analysis of the current of the flexible DC system, as follows:

[0060] Analyze the on-line monitoring requirements for the frequencies of the four types of harmonics of the flexible DC system. At the same time, considering the requirements of on-line monitoring data of high-frequency / ultra-high-frequency harmonics for communication and storage resources, reasonably select appropriate data such as the sampling frequency, sampling interval, and the number of sampling points in the power frequency cycle.

[0061] Then, conduct on-line monitoring and data acquisition of the output current of the flexible DC system. Usually, in order to meet the detection of the switching frequency and its sideband harmonics, the sampling frequency of the on-line monitoring of the flexible DC system current is at least twice the switching frequency. Considering the requirements of on-line monitoring of ultra-high-frequency harmonics for communication and storage resources, in some working conditions, the resolution of the sampling frequency for ultra-high-frequency harmonics and ultra-low-frequency inter-harmonics may not be high enough, and in some working conditions, spectrum aliasing may occur for ultra-high-frequency harmonics. Therefore, it is necessary to re-conduct data acquisition and data analysis.

[0062] Specifically, in step C, specific processing is carried out according to the type of harmonics. The specific process is as follows:

[0063] c1. If the amplitude of the first type of medium-frequency harmonics exceeds the alarm value

[0064] There is no need to make further judgments, and directly output the harmonic frequency / spectrum and its corresponding amplitude that exceed the alarm value.

[0065] c2. If the amplitude of the second type of low-frequency inter-harmonics exceeds the alarm value

[0066] c21. It is necessary to judge whether there are aliasing phenomena, leakage phenomena, fence phenomena, etc. based on the fast Fourier transform results. If not, the results are output; if so, it is also necessary to re-select the window function to perform the fast Fourier transform on the current acquisition data.

[0067] c22. Then compare the newly obtained current harmonic spectrum and amplitude with the previously obtained ones to judge whether there are still aliasing phenomena, leakage phenomena, fence phenomena, etc. If not, the results are output; if still present, it is also necessary to re-select the window function to perform the fast Fourier transform on the collected current data until no aliasing phenomenon occurs.

[0068] c23. Finally, obtain the accurate spectrum of the low-frequency interharmonics of the flexible DC system and its corresponding amplitude, and output the interharmonic frequency / spectrum exceeding the alarm value and its corresponding amplitude.

[0069] c3. If the amplitude of the third type of high-frequency harmonics exceeds the alarm value

[0070] c31. It is necessary to re-determine the sampling frequency, shorten the sampling interval, and perform online monitoring and data acquisition on the current of the flexible DC system again.

[0071] c32. Then perform the sliding window fast Fourier transform on the collected data again to obtain the accurate spectrum of the high-frequency harmonic current in the flexible DC system and its corresponding amplitude.

[0072] c33. Output the high-frequency harmonic frequency / spectrum exceeding the alarm value and its corresponding amplitude.

[0073] c4. If the amplitude of the fourth type of high-frequency interharmonics exceeds the alarm value

[0074] c41. It is necessary to re-determine the sampling frequency, shorten the sampling interval, and then perform online monitoring and data acquisition on the current of the flexible DC system again.

[0075] c42. Re-select the window function band and perform the sliding window fast Fourier transform on the collected data again to obtain the accurate spectrum of the high-frequency interharmonic current in the flexible DC system and its corresponding amplitude.

[0076] c43. Output the high-frequency interharmonic frequency / spectrum exceeding the alarm value and its corresponding amplitude.

[0077] The present invention proposes a wide-band harmonic online monitoring method for a flexible DC system, which realizes the fast and accurate analysis of the wide-band harmonic type and spectrum, and realizes the fast judgment of whether there is an excessive harmonic amplitude in the flexible DC system.

[0078] The present invention solves the problems of slow detection of ultra-high frequency harmonics, difficult detection of low-frequency inter-harmonics, and insensitive detection means, reduces the communication and storage resource occupancy of monitoring data, reduces the load rate of the CPU of the control and protection device, and operates stably and safely.

Claims

1. A method for online monitoring of broadband harmonics in a flexible DC system, characterized by: The following steps are involved: A. Obtain the topology and modulation mode of the flexible DC system converter, determine the system switching frequency, and classify the system characteristic harmonic types; B. Select appropriate sampling frequency and sampling interval to conduct online monitoring and data analysis on the current of the flexible DC system, and determine whether the harmonic amplitude in the current exceeds the alarm value. If it does not exceed the alarm value, maintain the online monitoring state; if it exceeds the alarm value, proceed to the next step; C. Determine the type of harmonics that exceed the alarm value in the flexible DC system and perform specific processing based on the harmonic type.

2. The method for online monitoring of broadband harmonics in a flexible DC system according to claim 1, characterized in that: Step A obtains the topology and modulation mode of the flexible DC system converter, determines the system switching frequency, and classifies the system characteristic harmonic types. The specific process is as follows: Firstly, the topological structure and modulation mode of the flexible DC system converter involved are obtained; Then, determine the system switching frequency f s ; Then, the AC grid fundamental frequency f0 of the flexible DC system is obtained; Finally, according to the harmonic spectrum characteristics, the harmonic types of the flexible DC system are divided into four categories.

3. The method for online monitoring of broadband harmonics in a flexible DC system according to claim 1, characterized in that: The four major categories include the first category of low-frequency harmonics, the second category of low-frequency interharmonics, the third category of high-frequency harmonics, and the fourth category of high-frequency interharmonics.

4. The method for online monitoring of broadband harmonics in a flexible DC system according to claim 1, characterized in that: Step B selects appropriate sampling frequency and sampling interval to perform online monitoring and data analysis on the current of the flexible DC system, and determines whether the harmonic amplitude in the current exceeds the alarm value. The specific process is as follows: First, the output current of the flexible DC system is monitored online and data is collected; Then, the data collected by online monitoring of the current of the flexible DC system is filtered and denoised; Then, a sliding window fast Fourier transform is performed, and the window width and sliding width are set according to the adopted data, so as to obtain the amplitude of each harmonic current of the flexible DC system; Then, the obtained harmonic amplitude is compared with its limit value to determine whether the harmonic / interharmonic current amplitude exceeds the alarm value.

5. The method for online monitoring of broadband harmonics in a flexible DC system according to claim 1, characterized in that: Step C determines the type of harmonics exceeding the alarm value in the flexible DC system, and performs specific processing according to the harmonic type. The specific process is as follows: Firstly, the harmonic frequency with amplitude exceeding the alarm value in the flexible DC system is obtained; Then, based on the harmonic type classification in step A, determine which type of harmonic has an amplitude exceeding the alarm value; Finally, specific processing is performed according to the type of harmonic that exceeds the alarm value.

6. The method for online monitoring of broadband harmonics in a flexible DC system according to claim 3, characterized in that: The characteristic spectrum of the first type of low-frequency harmonics is (6k±1)f0.

7. The method for online monitoring of broadband harmonics in a flexible DC system according to claim 3, characterized in that: The characteristic frequency f of the second type of low-frequency interharmonics ih has a value range of 0 < f ih < f0 or f0 < f ih < 2f0.

8. The method for online monitoring of broadband harmonics in a flexible DC system according to claim 3, characterized in that: The characteristic spectrum of the third type of high frequency harmonics is mf s ±nf0, where, when m=1,3,5,…, n=0,2,4,…; when m=2,4,6,…, n=1,3,5,….

9. The method for online monitoring of broadband harmonics in a flexible DC system according to claim 3, characterized in that: The characteristic spectrum of the fourth type of high frequency interharmonic is mf s ±nf ih , where, when m=1,3,5,…, n=0,2,4,…; when m=2,4,6,…, n=1,3,5,….

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