The generalized electromagnetic interference includes not only the disturbances that enter the monitoring system through the current sensor together with the PD signal, but also the disturbances that affect the monitoring system itself, such as grounding, shielding, and interference caused by improper handling of the circuit. The latter can be improved. System design, reasonable selection of circuits and components, and improved system production levels are addressed. Site electromagnetic interference, especially the former, is the focus of research. It can be divided into continuous periodic interference, impulse interference and white noise. Periodic interference includes system harmonics, carrier communications, and radio communications. Pulse type interference is divided into periodic pulse type interference and random pulse type interference. Periodic pulse-type interference is mainly caused by high-frequency inrush current generated by the action of power electronic devices. Random pulse interference includes corona discharge on high-voltage lines, partial discharges from other electrical equipment, discharges from tap-changer actions, arc discharges from motor operation, and suspension potential discharges from poor contact. The white noise includes thermal noise of the coil, noise of the ground network, and various noise coupled into the relay protection signal line of the transformer.
Electromagnetic interference usually enters the measuring point through direct coupling of space and line conduction. With different measurement points, the interference coupling path will be different, and the impact on the measurement will also be different. With different measurement points, the type and intensity of interference will also be different.
The principle of selection of transformer partial discharge monitoring points is that the partial discharge signal intensity is large, the signal to noise ratio is high, and the measurement is simple. There are mainly grounding cables for the enclosure and grounding cables for the screen at the end of the casing. Some of them also select the neutral point grounding wire, the core grounding wire, and the high-voltage outlet end. Sometimes, in order to suppress interference, the reference interference signal is also measured from the transformer power supply line. Because it is inconvenient to install the sensor at the neutral point and the high voltage outlet end, and some transformer cores are grounded internally, the monitoring system selects the ground wire of the casing and the end of the casing as the measurement points.
Second, the commonly used suppression methodThe interference suppression is always considered from three sources: interference source, interference path, and signal post-processing. Finding the interference source to directly eliminate or cut off the corresponding interference path is the most effective and fundamental method to solve the interference, but it requires a detailed analysis of the interference source and the interference path, and generally does not allow to change the original transformer operation mode, so in these two aspects The measures that can be taken are always limited. For various interferences coupled into the monitoring system via the current sensor, various signal processing techniques are used to suppress it. The PD signals and interference signals are generally distinguished from the following aspects: power frequency phase, frequency spectrum, pulse amplitude and amplitude distribution, signal polarity, repetition rate and physical position, etc., and a large number of anti-interference techniques are proposed accordingly.
There are two different ideas in the anti-jamming technology: one is based on narrow-band (typically 10 kHz to 10 kHz) signals. It picks up signals through a narrow-band current sensor and a band-pass filter circuit in an appropriate frequency band, avoids various continuous periodic interferences, and improves the signal-to-noise ratio of the measurement signal. This method is only suitable for a specific substation and is inconvenient to use. In addition, since the partial discharge signal is a wide-band pulse, narrowband measurement will cause distortion of the signal waveform, which is disadvantageous to the subsequent digital processing. The other is a processing method based on a wide frequency band (typically 10 to 1000 kHz). The detection signal contains most of the energy of the partial discharge and a large amount of interference, but the signal to noise ratio is relatively low. The processing steps for these interferences are generally: a. suppress continuous-period interference; b. suppress periodic-type pulse interference; c. suppress random-type pulse interference. With the development of digital technology and the application of pattern recognition methods in partial discharge, this kind of processing method can often achieve better results.
According to the above two ideas, different signal to noise ratio detection signals can be obtained. In the post-processing, many processing methods are the same. It can be summarized as frequency domain processing and time domain processing methods. The frequency domain method uses the characteristics of periodic interference in the frequency domain to discriminate; and the time domain processing method is based on the characteristics of pulse-type interference in the time domain. There are two implementations of hardware and software. Introduced separately below.
Third, the periodic interference suppressionPeriodic interference is also called narrow-band interference, which occupies a large proportion in various types of interference. The suppression and elimination of interference should also start with this. Due to its high intensity and fixed phase distribution, most frequency domain methods are used. Mainly include FFT threshold filter, adaptive filter, fixed coefficient filter and ideal multi-pass digital filter (IMDF).
Narrowband interference suppression algorithms are more mature and mature. From the perspective of application effects, fixed-coefficient filters and ideal multi-band filters are ideal. Because IMDF needs to perform multiple FFTs and IFFTs when processing data, it will spend a lot of calculation time, which is not conducive to real-time processing. But according to the best monitoring frequency band that IMDF finds, the finite impulse response (FIR) digital filter, which can form a fixed coefficient, is directly processed in the time domain, simplifying the operation and speeding up the processing speed.
The above methods can be implemented by software or hardware lines. Although the hardware filter adjustment is not flexible, after selecting the best frequency band in the field test, narrowband interference can be effectively suppressed. Although the software method is more flexible, it has the disadvantage of slow real-time computing speed.
Fourth, the suppression of periodic pulse interferenceWhen the signal removes periodic interference, other interference rises as a major contradiction. For the suppression of periodic pulse interference, there are mainly two types of processing methods: analog methods and digital methods. The simulation methods include differential balance method, directional coupling method, and reference signal method; the first two methods are also applicable to suppression of random pulse interference, which will be described later. Select a distribution line measurement pulse interference signal that contains only impulse interference but not discharge pulses. Use the measured interference pulse as a control signal. Stop the analog-to-digital converter (ADC) when the signal level exceeds a set threshold and determines interference. Work to eliminate interference pulses from distribution lines.
The principle of the digital method is to use the characteristics of different phase distributions of the interference and PD signals. For example, KONIG.G. and KOPF.U. propose a method that first records the signals of a plurality of cycles and then averages the data in the same phase of each cycle to form a template subtracted from the original signal, thereby eliminating the period Type of interference signal. When the authorities put less signals on this type of method and the distribution characteristics are relatively clear, the effect of removing interference is better. When the authorities release signals that are strong and powerful, they do not work well.
India's V. Nagesh and BIGururaj proposed a method that draws on some of the results of bio-signal processing. The basic principle is that starting from the PD signal and the periodical interference signal have different shapes, the data segmentation is first performed, and the pulse is Waveform signals are separated out to form a single pulse sequence. The FFT algorithm is used to perform cross-correlation calculations on each pulse in the frequency domain. The similarity is determined and grouped according to a certain standard. According to these group pulses, a class signal template is obtained, and then each One type of signal is synthesized in the time domain. The analysis found that the PD signals are more dispersed and the interference is more concentrated. Using this feature to eliminate periodic pulse jamming signals and reconstructing the remaining signals, the signal after removing the periodic pulse jamming can be obtained.
It can be seen that it is feasible to use the PD and periodic pulse interference to suppress the interference in the waveform and phase. This method can also be used for positioning, which is identified by analyzing the characteristics of the pulse waveform caused by different discharge points. The disadvantage of this method is: When the authorities put a high repetition rate, it is possible to consider two adjacent pulses as one, affecting the effect of recognition. In addition, when the pulse waveform is large, the operating speed has an effect, but with the microcomputer operation With a substantial increase in capabilities, this effect will be increasingly ignored.
V. Suppression of random impulse interferenceThis type of interference is the most difficult to remove. Since the characteristics of interference and PD signals in the frequency domain are similar, many existing methods are considered from the time domain. Common methods are hardware circuit method, software waveform recognition method and artificial intelligence method.
1. Hardware circuit method
Its basic idea is to take advantage of the fact that the external pulses in the output signals of the two measurement points interfere in the same direction, while the internal discharge pulses have the opposite direction, removing the pulse interference. The specific implementation is a hardware circuit. Commonly used circuits include a differential balancing method, a pulse polarity identification method, and a directional coupling method.
In practical applications, the effects of the first two are not ideal. This is because for the differential balance method, due to the different propagation paths, the two signals that make up the differential often do not correspond well, so the differential effect is not good. The concept of differential "balanced pair" was proposed to improve this, which can eliminate the interference and obtain the amplitude and pulse number of the PD. The limitation of pulse polarity discrimination is that because the analog delay and polarity discriminators are influenced by external factors, they can cause the electronic gate to malfunction and reduce the accuracy of polarity discrimination.
The directional coupling method was introduced by Borsi H in Germany in 1987. The schematic is shown in Figure 1. It uses a specially wound Rogowski coil to couple the partial discharge signal near the flange at the bottom of the high-voltage bushing and determine whether the partial discharge signal or the external electromagnetic interference depends on the voltage across the coil. This method connects the middle tap of the Rogowski coil to the measuring terminal of the transformer bushing. At this time, the terminal screen measuring terminal string is connected to a small resistance ground, which can be seen as the low voltage arm of the capacitive voltage divider formed by the capacitances of the last screen and the last screen. After the low resistance is grounded, a high-pass filter is formed, and only the high frequency signal can pass through. . The Rogowski coil is connected with the measuring terminal of the high-voltage bushing to form a directional coupling circuit.
When the current I is as shown, U (1) = U c U 1, U (2) = Uc - U 2 = Uc - U1. U (1)†U (2) at this time; if current I is reversed, U (1)
In practical applications, people have improved this, using two Rogowski coils instead of the original measuring coils and using frequency-selective methods to improve the signal-to-noise ratio of the measurement signal. According to the paper, good results have been obtained.
2. Software waveform recognition method
With the development of computer technology and digital signal processing technology, logic judgment using pulse signal characteristics can also suppress interference. Its premise is pulse identification, that is, the determination of whether the pulse exists, the pulse duration and the corresponding start and end points, in order to more accurately determine the discharge phase and sonic delay.
At present, the threshold recognition method is mostly used for pulse recognition. The pulse measured in the field is mostly attenuating the oscillating wave. This method is easy to misjudge and cannot determine the pulse duration. A method of identifying the oscillating pulses based on the pulse amplitude threshold and waveform characteristics is proposed. In practice, good results are obtained.
3. Application of pattern recognition
The essence of this method is still to use the phase characteristics of the signal to distinguish. Although the amplitudes of the PD signals vary greatly, their phases are concentrated around 45° and 225°, respectively. For example, arc discharges can be easily discerned by an experienced expert because of the difference in phase and partial discharge due to arc discharge, small amplitude variations, and slightly different pulse shapes. The pattern recognition method is the software implementation of expert experience. It has been confirmed in the CIGER report, and some corresponding software has appeared. Common methods include fuzzy logic, kohonen network classification, KLT transform, and artificial neural network based on minimum distance. Overall, the difficulty of pattern recognition methods lies in the need to accumulate a large amount of a priori knowledge and to be able to identify the specific differences between interference and partial discharge. On-line measurement, it is difficult to find these differences in strong interference signals. Here are some of the methods.
(1) Karhunen-Loeve-Transform method
It is found that when the input vector dimension for pattern recognition is high, the classification is difficult and the effect is not good; after reducing the dimension, the classification effect can be improved. In other words, in order to increase the recognition rate and highlight the characteristics of the signal, it is first necessary to remove the interference or noise information in the signal. The principle of the KLT transformation is shown in Figure 2. As can be seen from the figure, if the x1-x2 coordinate system is used, the x1 and x2 coordinates must be used simultaneously for classification; if the orthogonal transformation is performed on this, it is transferred to the w1-w2 coordinate system. Only w 2 coordinates are needed for classification. It can be seen that the interference can be removed by the KLT transformation.
(2) Pulse sequence analysis - Kohonen network
This algorithm is an unsupervised algorithm (as shown in Figure 3). Its principle is to find the node with the shortest Euclidean distance from the input vector to the output layer, and use this as the output. Through the self-organizing algorithm algorithm, it can be adaptively classified to distinguish the partial discharge signal and the interference signal, so as to achieve the purpose of interference elimination and suppression. .
(3) Pulse sequence analysis
It is reported that the method is simple and effective and has a high recognition rate: it consists of the discharge voltage difference or phase difference between partial discharges to form an analysis sequence. These characteristics distinguish different discharge modes and interferences to achieve the purpose of interference suppression; Can locate the fault point.
Sixth, summaryA large number of research results show that with the improvement of A/D conversion rate and the development of computer technology, the use of broadband (10k-1000kHz) sensors combined with high-speed sampling transformers has become the mainstream of development. Signal processing has evolved from traditional spectral analysis to time-domain analysis of PD waveforms.
Some results in the field of digital processing technology and artificial intelligence have been widely used for interference suppression in online monitoring and are expected to achieve breakthrough results.
In order to further improve the effectiveness of anti-jamming measures, research on the propagation law of disturbances and impulses should be strengthened. This includes the research on substation transmission and internal transmission of transformers. It may be found that they are characteristic in terms of waveform, phase, and direction. difference.
Multi-core cpu is more and more popular, people are used to take dual core processors laptop before. However, Quad Core Laptop is becoming selling like a hot cake nowadays. You can find Quad Core Processor Laptop at our store, like I3 Quad Core Laptop,I7 Quad Core Laptop, I5 Quad Core Laptop, intel celeron J4125 or N5095 15.6 Inch Laptop, even N4120 quad core 14 Inch Laptop, etc. Therefore, you can just share the parameters prefer, like size, cpu, ram, rom, gpu, application scenarios, thus save much time to get a win-win solution.
Do you know the reason why more people choose quad core device? The core reason is that heavier tasks people need to finish at a higher speed than before. Nowadays quick rhythm is becoming the main style in city even everyone is eager to downshifting. So more powerful laptop, computer, mobile phone is a trend, though most functions never are used in lifetime.
Other 15.6 inch Gaming Laptop or 14 Inch Gaming Laptop is becoming the most popular level at the market.
Quad Core Laptop,I3 Quad Core Laptop,I7 Quad Core Laptop,I5 Quad Core Laptop,Quad Core Processor Laptop
Henan Shuyi Electronics Co., Ltd. , https://www.shuyiminipc.com