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CABLES AND CABLE ACCESSORIES


The introduction of digital partial discharge measuring systems has led to improved measurements, far exceeding the capabilities of older measuring systems, says Omicron’sDavid Brazier.


T


he MPD600 applies multi-channel, synchronous processing of PD sig-


nals that make efficient statistical analysis, not only for discrimination between PD and interference, but also for fault-type identification and location.


The MPD600 system is designed to per- form continuous and synchronous multi- channel PD measurements in real time.


This system is modular in design, consist- ing of one or more acquisition units, which can be placed in close proximity to the asset(s) being monitored, and connected to a PC via a fibre optic connection, allow- ing for greater distances between the PC and the acquisition unit(s).


The PD signals are filtered, amplified, and digitised in real-time. The centre frequency can be freely chosen in the range for dc up to 20 MHz, while the bandwidth is selecta- ble from 9kHz up to 3MHz. This allows the operator to ‘tune out’ disturbances and to reach an optimal signal-to-noise ratio, even under noisy on site conditions. For ac phase synchronisation, as well as for ac waveform display and amplitude reading, a second A/D-converter digitises the volt- age curve.


All data acquisition and pre-processing are performed in the acquisition unit, close to where the signal was measured, to guar- antee optimal performance in speed and signal quality. A four parameter vector for each single PD pulse is identified.


In parallel, the instantaneous value of the ac voltage is sampled. All parameters can be stored for off-line processing, re- played or exported to an application such as MatLab for post-processing. An oscil- loscope mode and a FFT (Fast Fourier Transforms)-based spectrum viewer are provided to analyse input signals with full bandwidth, and the ability to locate PD by TDR (time-domain-reflectometry) are also included. Each acquisition unit contains an internal test generator to enable closed- loop self-testing.


The 3-phase amplitude relation diagram (3PARD) was originally designed for eval- uation of three-phase PD measurements. 3PARD application requires three inde- pendent PD monitoring channels such as the capacitive taps of three-phase trans-


former bushings, or coupling capacitors connected to the terminals of a genera- tor. In the case of measurements on long three-phase single-line cable systems, cross-bonding links enable distributed inductive three-phase PD detection for online measurements. 3PARD visualizes the relationships between the amplitudes of a single PD pulse in one phase and its cross-talk generated signals in the other two phases.


By repetition of this procedure for a large number of PD pulses, PD sources within the test object, as well as external noise, appear as a clearly distinguishable concen- tration of dots (a cluster) in a 3PARD.


The 3-channel frequency relation diagram (3CFRD) correlates synchronous multi- spectral measurement to separate PD from


3PARD cluster


interference. In contrast to 3PARD meth- ods, 3CFRD evaluation does not necessar- ily require three independent acquisition units and thus, can be used with a single MPD600 acquisition unit.


Multi-spectral PD measurements are based on pulse spectra correlation by ap- plying three different band-pass filters si- multaneously. The three band-pass filters can be tuned to areas with low interfer- ence level. In general, the first band-pass filter should be tuned to a low centre fre- quency (fc1) to enable conformity to IEC or IEEE standards. The second (fc2) and third (fc3) band-pass filters are tuned to higher centre frequencies, where pulse propagation effects caused differences in the spectral responses of the PD signal. By proper choice of the band pass center fre- quencies, it becomes possible to perform PD measurements according to the stand- ards while, at the same time, removing practically all superimposed interference. 3CFRD correlates the output of the three band-pass filters in a similar fashion to the 3PARD.


Construction of 3CFRD


By using the Omicron MPD600 with syn- chronous multi-channel measurements in real-time it is now possible to measure partial discharges in the field and discrim- inate it out from other sources of noise within that environment. Secondly, by us- ing evaluation methods such as 3PARD, and 3CFRD, further analysis and location of partial discharges are now possible.


FOR MORE INFORMATION


David Brazier, sales manager T: 01785 251 000 E: david.brazier@omicron.at W: www.omicron.at


rail technology magazine Feb/Mar 11 | 81


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