Abstract: With the development of industrial technology, a large number of high-power variable loads are entering the power grid. In particular, some nonlinear loads cause rapid changes in power demand, which in turn cause voltage fluctuations. This severely affects various electrical equipment operating in the power grid, such as flickering lighting equipment, malfunctioning control equipment, and fluctuations in motor speed. This paper details the detection method of voltage fluctuations and designs a voltage fluctuation alarm system to ensure that information is obtained as soon as possible when voltage fluctuations occur.
0 Introduction
Voltage deviations and fluctuations both affect power quality, impacting the normal and safe use of electricity. Excessive actual voltage can cause overvoltage in equipment, threatening insulation and reducing its lifespan; insufficient actual voltage will disrupt normal operation for users, causing equipment and appliances to malfunction or stop operating. Voltage fluctuations and flicker are irregular and rapid changes in voltage that can cause light flickering and visual discomfort. Even some voltage fluctuations, though within normal voltage variation limits, can still interfere with the normal operation of voltage-sensitive electronic devices and instruments such as computers due to their rapid frequency.
Therefore, studying voltage fluctuations and flicker, acquiring voltage fluctuation information in real time, and taking proactive measures to ensure power quality have become important tasks for power supply and consumption.
1. Detection and Acquisition of Fluctuating Voltage
To effectively suppress voltage fluctuations and flicker, the first task is to accurately extract the fluctuation signal. Typically, the fluctuating voltage is considered as an amplitude-modulated wave with the rated power frequency voltage as the carrier, and its amplitude modulated by a voltage fluctuation component with a frequency range of 0.05~35Hz. Therefore, the detection method for the voltage fluctuation component can employ the high-power carrier modulation signal demodulation method from communication theory. This involves multiplying the modulated signal by a periodic signal with the same frequency and phase as the carrier signal, separating the voltage fluctuation component from the power frequency carrier voltage, and then obtaining the fluctuation component through a bandpass filter.
Considering voltage fluctuation components, which involve a series of amplitude-modulated waves superimposed on the fundamental voltage, to simplify the analysis without losing generality, the study of voltage fluctuation detection methods can analyze the modulation of a single-frequency amplitude-modulated wave on the power frequency carrier. The instantaneous value of the power frequency voltage u(t) can be expressed analytically as follows:
Currently, there are three commonly used methods for detecting voltage fluctuations: square demodulation detection, full-wave rectification detection, and half-wave RMS detection.
1.1 Squared demodulation detection method
The square detection method involves squaring the amplitude-modulated voltage signal and then passing it through a demodulation filter to obtain the fluctuating voltage signal. Its principle is shown in Figure 1.
Figure 1. Basic principle diagram of square demodulation detection method
Based on the principle of the square demodulation detection method, the derivation process of the detection method is as follows:
(2)
In the formula, the first term is the DC component; the second term is the fluctuation component; the third term is the harmonic component of the amplitude-modulated signal; and the following terms are high-frequency components. After passing through a bandpass filter of 0.5 to 35 Hz to remove the DC and power frequency and higher frequency components, the amplitude-modulated wave signal can be obtained.
The advantages of the square detection method are its simplicity and ease of digital implementation; its disadvantages are that it contains harmonic components of the modulation component and does not contain an attenuation coefficient. If the demodulation result is processed as a fluctuating voltage signal, there will be errors.
1.2 Full-wave rectification and detection method
The basic principle of the rectification and detection method is to rectify the amplitude-modulated voltage and then demodulate and filter it to detect the fluctuating signal. The principle is shown in Figure 2.
This method is simple and convenient, and suitable for implementation in analog circuits; however, like the square detection method, it is affected by the spectral structure of the wave signal and has detection errors. It is also not suitable for digital implementation and does not contain an attenuation coefficient.
1.3 Half-wave RMS detection method
The half-wave effective value detection method uses an RMS/DC converter to convert the fluctuating input AC voltage into a pulsating DC voltage, and then obtains the fluctuating signal through a demodulation bandpass filter. Its principle is shown in Figure 3.
Figure 3. Basic principle diagram of half-wave effective value detection method
Its specific implementation is as follows:
Square the amplitude-modulated voltage, subtract the DC component, and then integrate (assuming the starting value for RMS calculation is at any time):
(4)
In the formula, T is the power frequency period.
Neglecting quantities with frequencies greater than twice the carrier frequency in the integral, we get:
The advantage of the RMS detection method is that it includes an attenuation coefficient, making the results more reflective of voltage fluctuations. Its disadvantage is that deviations in the RMS voltage or changes in the grid frequency can introduce errors into the detection results.
The above three methods are mainly suitable for detecting voltage fluctuation components with a single frequency, and none of them are suitable for detecting flicker signals with multiple frequencies. Half-wave RMS detection is an averaging effect and is easily affected by the fundamental voltage and fundamental frequency. For square detection and rectification detection methods, after the grid voltage passes through a squarer or rectifier, and then through a filter to remove the DC component and twice the power frequency component, the modulated wave can be extracted. Therefore, this design selects the full-wave rectification detection method to obtain the fluctuating voltage u0(t).
2. Design of Voltage Fluctuation Alarm System
This detector can trigger an alarm response to fluctuations in the power supply voltage of the circuit system under test. These subtle fluctuations are often unacceptable for some systems, as they can cause logic errors or significant damage.
The voltage fluctuation alarm system circuit is shown in Figure 4. The circuit mainly consists of a 5G7556 dual timer. The left half of the circuit forms a power supply drop wave detection circuit, and the right half forms a power supply rise wave detection circuit. The working power supply is directly taken from the circuit system under test, i.e., the voltage u0(t) obtained in the previous section. This power supply is also the object of detection.
Under normal conditions, the center potential of W2 is adjusted to be slightly less than two-thirds of the voltage (VDD-VSS), and the center potential of W1 is adjusted to be slightly higher than one-third of the voltage (VDD-VSS). Pressing switch AN1 once sets F1 to a high level, and LED1 will not light up. Pressing switch AN2 once sets F2 to a low level, and LED2 will not light up. C1 ensures that the potential of TH1 is unaffected by power supply voltage fluctuations; similarly, C2 ensures that the potential of TH2 is unaffected by power supply voltage fluctuations.
When the power supply voltage drops and fluctuates, the reset threshold voltage of 7556 also drops. When this threshold is lower than the voltage stored in C1, F1 is reset and outputs a low level, and LED1 lights up to indicate this.
When the power supply voltage fluctuates, the trigger set voltage of the 7556 also rises. When the trigger voltage rises above the voltage value stored on C2, F2 is set to output a high level, and LED2 illuminates to indicate an alarm. Once LED1 and LED2 have illuminated to trigger the alarm, they can only be turned off by pressing switches AN1 and AN2.
Figure 4 Circuit diagram of voltage fluctuation alarm system
3. Conclusion
In power transmission and distribution systems, voltage fluctuations and flicker have become significant factors affecting power quality. Voltage fluctuations caused by impulsive power loads are transmitted through the point of common coupling to other feeder lines in the power grid, damaging electrical equipment of other users and severely polluting the power quality of the distribution system. Therefore, it is necessary to strengthen the monitoring and control of voltage fluctuations and flicker. The primary prerequisite is to detect voltage fluctuations in the power grid and issue alarms and reminders in real time, so as to take more rapid and effective measures to control and eliminate voltage fluctuations and minimize their harm. This paper discusses commonly used detection methods for voltage fluctuations and flicker and designs a voltage fluctuation alarm system. These studies have reference value for the development of flicker detection instruments.
References
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