IEEE C57.113-2010 pdf download IEEE Recommended Practice for Partial Discharge Measurement in Liquid-Filled Power Transformers and Shunt Reactors
4. Specification of PD measuring circuits
4.1 General
To measure the apparent charge, the following major circuit components are required!
Coupling unit, which captures the PD signal from the terminals of the test objectMeasuring instrument, which processes the captured PD pulses and evaluates the apparent charge leveAssociated high-voltage (HVand measuring cables, which connect the individuacomponents
Generally the coupling unit contains a couplingwhich is connected in series with measuring impedancecabacitor2m. lf the test object is equipped with capacitive graded bushings, the capacitance between HV conductor and bushingtap, C1, may substitute the coupling capacitor.Ck, as illustrated in Figure 1
The HV connection leads between the test object and coupling capacitor should be PD-free up to the highestapplied ac test voltage level. The ground connection leads should be kept as short as possible in order to reduce theinductance and thus to minimize the impact of clectromagnetic interferences disturbing sensitive PD) measurementsOptional HV and LV filters may also be utilized to reduce the influence of environmental disturbances.
4.2 Coupling capacitor
The coupling capacitor, Cy, is intended for the decoupling of the high-frequency PD signal from the terminals of thctest object at low attenuation, due to the high-pass filter characteristics of this unit. Additionally the ac test voltagelevel appears extremely reduced at the output of Ck. This response is also achieved, if instead of the couplingcapacitor, Ck, the capacitance bctwcen HV conductor and bushing tap, C, is utilized. To minimize the impact otray capacitances on PD test results, the capacitance of Ck and C should exceed 300 pF. Moreover, these unitsshould be PD-free up to the maximum applied ac test voltage level.
4.3 Measuring impedance
The measuring impedance.is intended for the conversion of PD current pulses into equivalent voltage pulses00Using the classical counling mode by means of a separate coupling capacitor, the measuring impedance, Zm. isgenerally formed by a parallel connection of a resistor, Rm with an inductor, Lm: see Annex A
if the bushing tap coupling mode according to Figure 1 is used, the measuring impedance, 2: consists of the parallelconnection of a resistor, Rm: and an inductor, Lm. Both elements are additionally shunted by the capacitancebetween bushing tap and grounded bushing flange. C2: see Annex A.
Moreover, passive and active eleents could be utilized for PD signal filtering and overvoltage protection. All thescclements are usually integrated in a terminating box, referred to in IEC 60270 as a coupling device. D
Due to the high-pass filter characteristics of the series connection of either Ck or Ci with the measuring impedancem: care should be taken that the specified lower limit frequency, f, of the complete PD measuring circuit is notsubstantially affected by the parameters of the PD coupling unit; see Annex A