Anti-interference measures in partial discharge testing1. Source of interference
In a broad sense, electromagnetic interference includes not only the interference that enters the monitoring system through the current sensor together with the partial discharge signal, but also the interference that affects the monitoring system itself, such as the interference caused by grounding, shielding, and improper handling of circuits. On-site electromagnetic interference refers specifically to the former, which can be divided into continuous periodic interference, pulsed interference and white noise. Periodic interference includes system higher harmonics, carrier communication and radio communication. Pulse-type interference is divided into periodic pulse-type interference and random pulse-type interference. Periodic pulse-type disturbances are mainly caused by high-frequency inrush currents generated by the action of power electronic devices. Random pulse-type interference includes corona discharge on high-voltage lines, partial discharge from other electrical equipment, discharge from tap changer action, arc discharge from motor operation, and floating potential discharge from poor contact. White noise includes thermal noise of coils, noise of ground network, power supply lines, and various noises coupled into relay protection signal lines of transformers.
Electromagnetic interference generally enters the measurement point through direct space coupling and line conduction. Different measurement points have different interference coupling paths and different influences on the measurement; different measurement points have different types and intensity of interference.
2. Commonly used interference suppression methods
Interference suppression is always considered from three aspects: interference source, interference path and signal post-processing. Finding the interference source and directly eliminating or cutting off the corresponding interference path is the most effective and fundamental method to solve the interference, but requires detailed analysis of the interference source and interference path, and generally does not allow to change the original transformer operation mode, so in these two aspects What can be done is always limited. Various signal processing techniques are adopted to suppress the various disturbances coupled into the monitoring system via the current sensor.
Generally, PD signals and interference signals are distinguished from the following aspects: power frequency phase, frequency spectrum, pulse amplitude and amplitude distribution, signal polarity, repetition rate and physical location, etc.
There are two different ideas in anti-jamming technology:
One is based on narrowband (usually 10kHz to several 10kHz) signals. It picks up the signal through a narrow-band current sensor with a suitable frequency band and a band-pass filter circuit, 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, the narrow-band measurement will cause distortion of the signal waveform, which is not conducive to the subsequent digital processing.
One is a processing method based on broadband (typically 10 to 1000 kHz frequency band) signals. The detection signal contains most of the PD energy and a lot of interference, but the signal-to-noise ratio is low. The processing steps for these disturbances are generally:
a. Suppress continuous periodic interference;
b. Suppress periodic pulse interference;
c. Suppress random pulse interference. With the development of digital technology and the application of pattern recognition methods in partial discharge, this processing method can often achieve better results. In the post-processing, many processing methods are consistent. It can be summarized into frequency domain processing and time domain processing methods. The frequency domain method uses the discrete characteristics of periodic interference in the frequency domain to process it; while the time domain processing method is based on the discrete characteristics of the pulsed interference in the time domain.
Since the partial discharge pulse signal is a very weak signal, the on-site electromagnetic interference will produce large errors in the measurement results, so it is difficult to make accurate measurements. In order to improve the measurement accuracy, in addition to the anti-interference measures described above, the following measures should also be taken during the measurement:
The equipment used in the test should be used as far as possible without halo equipment, especially the test transformer and coupling capacitor Ck
The performance of the filter is better, and the high frequency isolation of the power supply and the measurement loop should be achieved.
The test time should be selected as far as possible in a time period with less interference, such as at night.
The parameter matching of the measurement loop should be appropriate, and the coupling capacitor should be as small as possible as small as the test capacitor Cx, so that the charge can be quickly converted between Cx and Ck during partial discharge.