Statistical variation of the partial discharge pulse height distribution during a 100-second acquisition period (pattern display)

Calculated number of PD pulses vs. acquisition time and average PD amplitude distribution during the total acquisition time of 100s.

For DC Testing, a so-called count level (a value in pC) can be defined so that all partial discharges exceeding this level are accumulated and are displayed on-line. This helps the operator to decide whether he should continue or stop the test. Also, short testing times as low as 10 ms and a temporal resolution of 40 µs are possible.

Calculated number of PD pulses vs. acquisition time and average PD amplitude distribution during the total acquisition time of 400ms

Noise Rejection The ICMsystem can be used in noisy environment. The instrument contains a software-controlled noise gate. This noise gate is a second analog input that can be connected to an antenna or a current transformer to receive noise impulses. If the noise pulse exceeds a pre-set trigger level, the PD amplifier and/or the analog to digital converter are locked, and the noise pulse is removed from the fingerprint. This noise removal procedure is completely asynchronous and is active only when noise is present. Thus, the gate can not only handle phase stable pulses (such as thyristor pulses) but also external corona or brush noise from DC machines.

Test of a power transformer bushing without noise gating (disturbances created by electric cars and external corona of test transformer)

Pulse noise eliminated by the noise gate (the noise was received by a wire antenna), leaving only true internal discharges with their typical phase distribution visible.

Superimposed PD from all three phases of a hydrogenerator (on-line) and the brush noise of the exciter machine. The three phases are superimposed because a CT clamped to the generator neutral picks up the PD signal.

Pulse noise of the brushes eliminated by the noise gate. (The noise was received by a wire antenna mounted close to the exciter brushes.)

Frequency Domain Signal Conditioning In the presence of continuous radio frequency noise, the noise gate is not applicable, and a frequency range of enhanced signal-to-noise ratio must be found. This frequency range is often higher than the highest frequency provided by the ICMsystem. By a procedure of mixing down a frequency band at a high-center frequency into the acquisition bandwidth of the ICMsystem, signals extracted at center frequencies even in the GHz range can still be used to acquire a partial discharge pattern. Some spectrum analyzers, which are used as mixers and demodulators, provide the highest flexibility (PD Systems can assist in choosing an appropriate device). Optimized signal conditioning modules are available for some of the applications listed below (RPA2 & RPA3). In case of other well-defined frequency bands PD Systems will provide/develop dedicated and tailor-made preamplifier/mixer systems. This enormous extension of the frequency range offers in addition to a noise reduction also an enhanced flexibility and some new applications: On-line GIS Testing, >100 MHz, connection of the ICM to GIS sensors or field plates Enhanced on-line testing of cable joints On-line generator and transformer testing Overhead lines and medium-voltage systems: antenna measurements when no conventional coupling is possible Fine tuning to pickup circuits In some cases the frequency band matches system resonances of the test object. This means that tuning of the center frequency can be used to Localize PD sources (qualitatively) Separate superimposed PD patterns originating from different locations Enhance sensitivity The choice of high center frequencies may, however, also result in loss of data and overall calibration as shown in the following examples. (The examples are done with the ICM unipolar acquisition mode, since the pulse polarity is lost by the mixing procedure.)

Superimposed PD from all three phases of a hydrogenerator (on-line) and the brush noise of the exciter machine. Center frequency 3MHz, bandwidth 300kHz

PD of a hydrogenerator (on-line) and the brush noise of the exciter machine. PD of one phase only is visible, due to too high center frequency of 9MHz, bandwidth 300kHz

Test Voltage Measurement and Synchronization

The test voltage is digitized and displayed in the partial discharge pattern. This illustrates both the sensitivity of the PD pattern's appearance to the voltage shape, and the transformer overload and resulting voltage distortion.

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