IEEE C57 113-2010 pdf free download.IEEERecommended Practice for Partial Discharge Measurement in L iquid-Filled Power Transformers and Shunt Reactors .
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4.2 Coupling capacitor The coupling capacitor, C k , is intended for the decoupling of the high-frequency PD signal from the terminals of the test object at low attenuation, due to the high-pass filter characteristics of this unit. Additionally the ac test voltage level appears extremely reduced at the output of C k . This response is also achieved, if instead of the coupling capacitor, C k , the capacitance between HV conductor and bushing tap, C 1 , is utilized. To minimize the impact of stray capacitances on PD test results, the capacitance of C k and C 1 should exceed 300 pF. Moreover, these units should be PD-free up to the maximum applied ac test voltage level. 4.3 Measuring impedance The measuring impedance, Z m , is intended for the conversion of PD current pulses into equivalent voltage pulses. Using the classical coupling mode by means of a separate coupling capacitor, the measuring impedance, Z m , is generally formed by a parallel connection of a resistor, R m , with an inductor, L m ; see Annex A. If the bushing tap coupling mode according to Figure 1 is used, the measuring impedance, Z m , consists of the parallel connection of a resistor, R m , and an inductor, L m . Both elements are additionally shunted by the capacitance between bushing tap and grounded bushing flange, C 2 ; see Annex A. Moreover, passive and active elements could be utilized for PD signal filtering and overvoltage protection. All these elements are usually integrated in a terminating box, referred to in IEC 60270 as a coupling device, D c . Due to the high-pass filter characteristics of the series connection of either C k or C 1 with the measuring impedance, Z m , care should be taken that the specified lower limit frequency, f 1 , of the complete PD measuring circuit is not substantially affected by the parameters of the PD coupling unit; see Annex A.
NOTE 2—To eliminate stochastically appearing noise pulses at comparatively low repetition rate, for instance one pulse per cycle of the applied ac test voltage, some PD detectors are equipped with special features for noise suppression which may reject pulses having a repetition rate below 100 Hz. Care should be taken when using this instrumentation because PD pulses of high magnitude may not be recognized if they do not ignite in each half-cycle of the applied ac test voltage. To avoid such erroneous measurements, a visualization of the phase-resolved PD pulses is strongly recommended using a suitable display unit, such as a scope or a computer. NOTE 3—The specified pulse train response is appropriate only for ac test voltages where the frequency may range between 40 Hz and 400 Hz. For dc test voltages or test voltages composed by ac and dc voltages, it is recommended to evaluate the number versus the magnitude of PD pulses. 4.4.3 Display unit In addition to the measurement of the apparent charge level by means of analog or digital meters it is strongly recommended to display the phase-resolved PD patterns by means of a suitable display unit, such as an oscilloscope or a computer. This may assist not only the identification and classification of harmful PD defects but also the discrimination of disturbing electromagnetic interferences, which are often not phase-correlated. 4.4.4 Basic sensitivity The basic sensitivity should be determined by means of calibrating pulses specified in 4.6, which are injected into the input of the measuring impedance connected to the PD measuring instrument via the associated measuring cable. A calibrating charge of 50 pC should cause a minimum deflection of 50% of the full reading of the indicating instrument or of the optional display unit.