According to the relevant standards, partial discharge quantity of more than 110KV of the transformer should be controlled below 500pC. But in the actual operation, even if the discharge quantity is about 5000pC ,transformer can also run as usual. Therefore, the on-line monitoring equipment, can effectively protect the transformer effect.
Because the electrical signal is obtained through the Rogowski coil coupling, the coil is only required to be connected to the ground terminal of the device. It does not affect the normal operation of the equipment and protection. In order to obtain the power frequency signal and the partial discharge high frequency signal at the same time, two kinds of materials are designed to detect the impedance.
In the transformer core terminal on Rogowski coil for partial discharge pulse signal which has many advantages. First of all, the core of the high and low voltage winding has a larger capacitance, therefore, whether the partial discharge signal is produced in high or low voltage winding, the response in the core sampling point is good. In addition, it is helpful to suppress the interference, because it is very similar to the signal waveform obtained from the ground wire of the transformer case and the neutral point of the high voltage winding. In the practical application, because the transformer box passes through the rails and many ground, signal obtained from a spot is weak, therefore, we may use the neutral point as the balance to match signal.
When the transformer iron core occurs insulation failure, firstly the partial discharge signal , and finally lead to two points to the ground. The formation of power frequency short-circuit current signal, the signal processing unit should be the separation of power frequency and high frequency signal. When the amplitude and time of the power frequency current signal reaches the set threshold, the test instrument automatically records the value and sends out the alarm signal. Similarly, when the amplitude of the high frequency signal and the number of periodic pulses reach the set threshold and the pulse waveform satisfies the pulse width and frequency conditions, the instrument automatically sends out the high frequency alarm signal.
On line measurement, the suppression of interference is one of the key issues in the actual measurement, the corona and carrier amplitude modulation interference will reach more than 10 thousand pC. Due to the impact of various interference, will greatly reduce the sensitivity of the measurement.
The spectral range of partial discharge signal is 20 ~300kHz, and the carrier amplitude interference is in the range of about - 300kHz, which can effectively restrain the interference of radio amplitude modulation if the 40-120kHz is used to measure the frequency band of. It can effectively restrain the interference of corona and other external disturbances by using the method of balance identification.
By balancing the identification, although it can play a role in some types of fixed interference, many random interference occurs during the operation of the transformer, the phase and amplitude is not stable, which can use to identify and judge the pulse waveform of time and frequency, rising edge parameters can be random pulse interference and internal fault of transformer the difference between the discharge pulse.
In the early stage of the fault discharge, the discharge is not stable, and when the line is measured, the system will switch into the measurement system.
By using the waveform characteristic parameters to judge and identify the pulse amplitude and continuity, it can distinguish and eliminate the interference caused by the random pulse and scr. Identification of alarm system should also set up the anti interference judgment and automatic reset device. when the system occasional interference, and the interference coincides with the internal discharge of similar characteristics, identification system is automatically reset to wait; if the signal again, meet the amplitude frequency characteristic, can send out sound and light alarm signal, alarm and automatic recording parameter. If it is accidental, which do not meet the frequency characteristics, it can be ruled out, no alarm.
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