China Education Equipment Purchasing Network: As a large number of computer-based control equipment and highly automated electrical equipment have been put into use one after another, industrial users have increasingly higher requirements for power quality, even a fraction of a second is abnormal It can cause huge losses.
According to statistics, industrial users with a high degree of automation generally suffer from 10 to 50 disturbances related to power quality problems each year, of which the number of accidents caused by dynamic voltage quality problems including voltage fluctuations and flickers accounts for approximately 83% [1]. Voltage fluctuation and flicker have become one of the main threats to the reliability of power supply for many important users, and must be effectively monitored and suppressed.
The voltage fluctuation and flicker in the power system are mainly caused by the load with impact power [2], such as frequency conversion speed regulation device, steel-making electric arc furnace, electrified railway and steel rolling mill. These non-linear, unbalanced impact loads have large and small changes in active and reactive power randomly or periodically during the production process. When their fluctuating current flows through the impedance of the power supply line, a variable voltage drop occurs, causing other users on the same grid The voltage fluctuates at the same frequency. This voltage amplitude varies regularly or randomly within a certain range (usually 90% to 110% of the rated value), which is called voltage fluctuation. Voltage fluctuations usually cause many electrical equipment to not work properly, such as affecting the quality of TV pictures, pulsating the speed of motors, malfunctioning electronic instruments, causing flickering of incandescent lights, and so on. Since the sensitivity of general electrical equipment to voltage fluctuations is much lower than that of incandescent lamps, for this reason, people choose subjective perception of incandescent lamp illumination fluctuations, that is, "flicker", as an evaluation indicator to measure the degree of voltage fluctuation damage.
1 Detection of voltage fluctuation and flicker
1.1 AM wave detection
To effectively suppress voltage fluctuations and flickers, the first task is to accurately extract the fluctuation signal. Usually, the fluctuation voltage is regarded as the carrier frequency rated voltage as the carrier, and the amplitude of the voltage is subject to the frequency range of 0.05 ~ 35Hz The amplitude modulation wave of the voltage fluctuation component is modulated. Therefore, the detection method of the voltage fluctuation component can use the high power carrier modulation signal demodulation method in communication theory, multiply the modulated signal by the periodic signal in the same frequency and phase as the carrier signal, and separate the voltage fluctuation component from the power frequency carrier voltage. The wave component is obtained through a band-pass filter.
Considering the voltage fluctuation component, a series of amplitude modulation waves are superimposed on the fundamental voltage. In order to simplify the analysis without loss of generality, the detection method of voltage fluctuations can analyze the modulation of a single frequency amplitude modulation wave on the power frequency carrier. The analytical formula of the instantaneous value of the power frequency voltage u (t) is written as:
In the formula: A is the amplitude of the power frequency carrier voltage, ω0 is the angular frequency of the power frequency carrier voltage, m is the amplitude of the amplitude modulation wave voltage, and mcos (Ωt) is the fluctuating voltage.
At present, there are three commonly used detection methods for fluctuating voltage: square demodulation detection method, full-wave rectification detection method and half-wave effective value detection method. Figure 1 shows a block diagram of the principle structure of the three methods.
(1) Square demodulation detection method
The International Electrotechnical Commission (IEC) recommends the square demodulation detection method, that is, u (t) squared, and then uses the demodulation band-pass filter to detect the amplitude modulated wave. Through the band pass filter of 0.05 ～ 30HZ, the DC component and the double power frequency component can be filtered out, so that the amplitude modulation wave of mA2cos (Ωt), that is, the voltage fluctuation component, can be detected. This method is more suitable for digital signal processing.
(2) Full-wave rectification demodulation detection method
The basic principle of the full-wave rectification detection method is to rectify the input AC voltage u (t) full-wave rectification, that is, to obtain the ripple signal after the absolute value calculation and then pass the demodulation band-pass filter. Suppose u (t) 's rectified voltage is g (t), then g (t) can be regarded as the product of u (t) and the square wave whose amplitude is ± 1 and the frequency is the power frequency. Can be detected by the bandpass filter of 0.05 ～ 30HZ
The amplitude modulation wave is the voltage fluctuation component.
This method is more suitable for analog circuits to be implemented, and the British ERA and French EDF and other flicker meters use this scheme. Like the square detection method, the amplitude-modulated wave is retained through a band-pass filter, but there is a detection error, and the size of the error depends on the spectrum structure of the fluctuating signal.
(3) Half-wave effective value detection method
The half-wave effective value method is to use the RMS / DC converter to convert the fluctuating input AC voltage into a pulsating DC voltage, and then obtain the fluctuating signal after demodulating the band-pass filter. The DC voltage output by the RMS / DC converter is the root mean square value of the input AC voltage, and its pulsating component reflects the change of the root mean square value of the input voltage. According to half wave
In this method, as far as the actual line is concerned, it is quite difficult to accurately set the calculation time of the rms value in a half power frequency period, and the setting of its component parameters is more difficult. In addition, this method can remove the DC component and the double power frequency component, etc., and only retain the amplitude-modulated wave, but there will not be no DC component at all, and DC blocking and filtering are still required. The Swiss MEFP type flicker meter, the domestically produced VFF-1 type voltage fluctuation flicker analyzer and the Japanese â–³ V10 measuring instrument all use an effective value for each cycle.
(4) Wavelet multi-resolution signal decomposition synchronous detection method and other methods
In recent years, some new theories and principles have been applied to amplitude modulation wave detection. For example, document [3] proposes a new method of voltage flicker signal detection using wavelet multi-resolution signal decomposition and synchronous detection. This method replaces the demodulation bandpass in the synchronous detector with a wavelet multi-resolution signal decomposition filter The filter can detect the sudden time of the voltage flicker signal, each frequency component and its amplitude in the envelope signal. However, this method has the characteristics of more sampling data required for the signal, a large amount of calculation, and a larger delay in detecting the sudden fault signal. Therefore, when using wavelet multi-resolution signal decomposition, it is necessary to seek fast wavelet functions and their corresponding Wavelet transform.
In addition, the random voltage flicker power flow method based on random theory and admittance matrix can calculate the maximum voltage fluctuation value and flicker value of each bus, and can also detect the impact of the flicker source on the system voltage. But this method has great difficulty in practical application.
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