DD IEC TS 61934:2011 pdf – Electrical insulating materialsand systems — Electrical measurement of partial discharges (PD) under short rise time and repetitive voltage impulses.
4.5.2 Effect of environmental factors
In general, PD-associated quantities may be affected by the following factors:
— temperature:
— atmospheric pressure;
— type of environment gas:
– degree of contamination of the test object
NOTE PD ph.nom.na may chang. w,th Iong.r nsa tim. m the cc.. o high athIud.
4.5.3 effect of testing conditions and ageing
PD-associated quantities may be affected by
— voltage distribution,
— position of PD occurrence.
— previous voltage applications as well as the time between voltage applications.
— operation time or time under stress of the test object.
In addition, they may vary as ageing of the electrical insulation occurs, that Is. during
operation of the EIS.
5 PD detection methods
5.1 General
Any PD pulse detection system where the test object is excited by voltage impulses requires strong suppression of the residual voltage impulse, measured by the PD detection circuit, and negligible suppression of the PD pulse. The PD pulse shall have a magnitude after processing by the detection system that Is greater than the residual transmitted voltage impulse. The amount of impulse voltage suppression required will be dependent on the test voltage and the rise time ot the impulse.
As the impulse voltage increases in amplitude, greater suppression is required in order to ensure that important PD pulse magnitudes are higher than the residual transmitted voltage impulse on the output of the detector. Similarly, as the rise time of the applied impulse voltage becomes shorter, the suppression shall be greater, due to the Increased overlap of frequency spectra of supply Impulse and PD pulse (see Annex A). PD pulse coupling devices shall be designed to ensure that Important PD pulse magnitudes are higher than the residual transmitted voltage Impulse on the output of the detector, or the residual be clearly distinguishable from the PD pulses.
Annex A provides indications of the voltage impulse suppression action required by the coupling device. Suggestions for the amount of supply voltage impulse suppression needed as a function of impulse magnitude and rise time are given.
Examples of PD pulses extracted from a supply voltage impulse through filtering techniques are reported in Annex B.
5.2 PD pulse coupling and detection devices
5.2.1 Introductory remarks
PD current or voltage pulses in a test object can be detected either by means of high-voltage capacitors, high-frequency current transformers (HFCT) or electromagnetic couplers (e.g. antennae). The detectors, in conjunction with the rest of the measuring system, shall be able
to suppress the impulse voltage to a magnitude less than that expected from the PD pulse (using e.g. appropriate filters).
Short low-inductance connections between the supply, the test object and the PD detector are required, since the voltage impulses and PD pulses contain high-frequency components. The impulse supply shall be as physically close to the test object as possible, in order to prevent attenuation and dispersion of the applied impulse due to the equivalent transmission parameters of the connecting leads. Since the PD is measured with a UWB detection system, earthing of the test object shall be made directly to the impulse voltage supply, with leads as short as possible and with low inductance. It is recommended that lead lengths should not exceed I m,
The following circuits are applicable for PD pulse detection.
5.2.2 Coupling capacitor with multipole filter
A coupling capacitor with a voltage rating exceeding that of the expected applied impulse voltage together with a filter that strongly attenuates the test voltage impulses can be used. The filter shall have at least three poles and special measures to inhibit cross coupling of the input signal to the output. The filter can be designed using passive or active filtering technology. The coupling capacitor is connected to the test object high-voltage terminal (Figure 1). Annex A shows a schematic example of filter behaviour. Figure 2 reports an example of frequency spectra of PD pulse and impulse voltage before and alter filtering for an 8’ order filter.