This invention relates to a device for the online measurement of partial discharge in an electric winding, such as, a transformer winding.
A partial discharge (PD) measurement on power transformers is a quality control measure in the factory. A partial discharge test is a non-destructive dielectric test that is performed at normal stresses. Two types of discharge detectors exist. They art of the broadband and narrow band type. The bandwidth of a broadband detector is about 400-600 kHz while that of a narrow band detector is about 10 kHz. Narrow band detectors have oscillatory responses while those of broadband detectors are more unidirectional. In the factory the test is performed with the help of a coupling capacitor and measuring impedance. When used as condition monitoring technique at site, the voltage tap of a capacitive bushing on a high frequency current sensor is needed.
A problem with existing measurements at she is that it is very difficult to distinguish external noise from partial discharges within the winding. Similarly, switching activities in the network result in high frequency spikes that ace picked up by conventional PD measuring equipment. A further difficulty is that strong radio stations in the vicinity can give rise to PD like pulses. Measuring the maximum PD within an interval of time can thus result in erratic measurements making on-line assessment difficult.
The invention proposed herein is adaptive in nature and avoids the drawbacks mentioned above. It is dependent on the characteristic frequency response of the winding being monitored It works in the region where the signal to noise ratio (SNR) of the winding response is high and avoids effects of switching events. Because it is adaptive, radio interference can be suppressed.

The device for online measurement of partial discharge in an electric winding, according to this invention, comprising a digital phase sensitive detector connected to the said winding; and a low pass filter famishing the values of the said measurement
The invention will now be described with reference to the accompanying drawings which illustrate by way of example, and not by way of example, the operation of die invention proposed herein,
Fig. 1 illustrating a parameter model of the winding. Ls indicating the self-inductance of section, Cs indicating series capacitance, Cg indicating ground capacitance, Rs indicating series resistance. Fig. 2 illustrating spectrum analyzer response of partial discharge at different locations.
A physical model of a winding with 10 nodes is shown in Fig. 1. This is a standard high frequency model, hi order to study the behavior due to a partial discharge, the winding is energized by a pulse source across nodes 1-2, 2-3, _ etc. The response is measured across a current measuring resistor. Fig. 2 shows the responses in the frequency domain corresponding to such energisation. From Fig. 2 we see that specific resonance frequencies show higher responses, in principle, the method proposed here captures these responses.
We note the principle resonant frequencies of the transformer marked as F1, F2, F3 etc. We use a digital signal processing based lock-in technique to compute the product between the winding current response and certain reference frequencies. The winding current response is obtained either from a capacitive tap or a high frequency current sensor.

Partial discharges in power transformers are identified by using a lock-in-amplifier (LIA) technique. It employs digital multiplier, demodulator (also known as phase sensitive detector PSD), followed by low pass filter (LPF). PSD multiplies the signal with one of the principle resonant frequency of the transformer marked F1, F2 and F3.. .etc. The output of the PSD contains many signals. Most of the output signals have frequency, which arc cither the sum or difference between an input signal and the reference frequency. If there is no relative phase-shift between the signal and reference phases, the demodulator output takes the form of a sinusoid at twice the reference frequency, but with a mean or average value. This mean value is proportional to the product of the signal and reference frequency amplitudes and it is (secondly) related to the phase angle between the signal and reference.
if i(t) is the response due to partial discharge as measured on a sensor and F1 is the first resonant frequency then the output of first PSD Y1(t) = i(t)) * sin (2πF1t) and second PSD Y2t) = i(t) * sin (2ΠF1t + 90). The DC component of the PSD output is to be isolated by using a low-pass filter. But in real applications the signal will be accompanied by noise. Noise components at frequencies very close to that of the reference do result in demodulator outputs at very low frequencies, but by setting die low-pass filter to a sufficiently low cut-off frequency these can be rejected. Hence the combination of a demodulator and low-pass output filter allows signals to be measured even when accompanied by significant noise. Then output Y(t) = ÖY1(t)2 + Y2(t)2. An advantage using dual-phase unit is that if the signal channel phase changes then the output from one detector will decrease and other will increases. However, the vector magnitude remains the same.

The said instrument employs digital multiplier then input signal is multiplied by a digital representation of a sine wave at the reference frequency. A digital signal processor (DSP) is used for tins task and the output is therefore no longer an analog voltage but rather a series of digital values. This technique offers the advantages of a perfect multiplication with no inherent errors and minimizes the DC coupling and thereby reducing output drift.
Hence PSD along with the LPF detects signal whose frequency is very close to the lock in range. Noise signal at frequency far from the reference frequency are attenuated at file PSD output by the LFF. Hence LPF provides an output, which is an indication of FD.
The winding response is initially filtered with a high pass filter whose cutoff is just below the first anti-resonant frequency. The lock in amplifier uses (he winding resonance frequencies as references. The time constant of the DSP fitter is less than 10 µm and adjusted to maximize the sensitivity of the response.