Megger Calibrator CAL1 Manual De Usuario página 7

Area of application
Technical background
Technical description
Measuring partial discharge (PD) according to IEC 60270 is a proven method for early
detection of impending damage to the insulation of cables, transformers and other
electrical equipment and thus minimising the likelihood of failure. However, a
PD measurement system connected to the end of a cable does not record the actual
charge transfer but only an apparent charge. Accordingly, in order to be able to analyse
the origin and amplitude of charge pulses, correct calibration of the measurement circuit
before starting measurement is indispensable.
The PD Calibrator CAL1 performs the function of a pulse generator which continuously
injects calibration pulses of known charge into the circuit being tested. The pulses
reflected back at the end of the cable and subsequently received by the PD
measurement system enable the user to calibrate the measurement system and ensure
comparability of the charge pulses recorded during the subsequent measurement.
The calibration sequence described in IEC 60270 is based on a simulation of the charge
transfer within a tested circuit to its connection points.
As the cause of a partial discharge is not accessible, the actual charge transfer of a
partial discharge cannot be captured by measurement technology. Instead, the
subsequent charge flow from the coupling capacitor of the measurement system (called
the apparent charge) serves as the measure for quantifying the partial discharge. The
signal for that apparent charge is fed to the measuring instrument via the coupler unit of
the measuring system.
In order to determine the ratio between the measured apparent charge and an actual
charge pulse occurring in the tested circuit, the CAL1 calibrator connected as shown in
the circuit diagram below feeds equally spaced charge pulses of known charge (e.g.
1 nC) into the circuit being tested.
PD measuring system
Z
C
K
MI CD
C
C
C
CD
MI
7
C
0
C
Coupling capacitor
K
Calibration capacitor
0
Tested circuit
a
Coupler unit
Measuring instrument
English
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