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The basic PD(Partial Discharge) test circuit
Electrical PD detection methods are based on the appearance of a 'PD (current or voltage) pulse‘ at the terminals of a test object, which may be either a simple dielectric test specimen for fundamental investigations or even a largeh.v. apparatus which has to undergo a PD test. For the evaluation of the fundamental quantities related to a PD pulse we simulate the test object, as usual, by the simple capacitor arrangement as shown in Fig.5.9(a), comprising solid orfluid dielectric materials between the two electrodes or terminals A and B, and a gas-filled cavity. The electric field distribution within this test object is here simulated by some partial capacitances, which is possible as long as no space charges disturb this distribution. discharge current which cannot be measured, would have a shape as governed by the gas discharge process and would in general be similar to a Dirac function, i.e. this discharge current is generally a very short pulse in the nanosecond range. Let us now assume that the sample was charged to the voltage Va but the terminals A, B are no longer connected to a voltage source.
If the switch Sis closed and Cc becomes completely discharged, the current i releases charge υqc D Ccυ Vc from Cc, a charge which is lost in the whole system asassumed for simulation. By comparing the charges within the system before and after this discharge, we receive the voltage drop across the terminal υVa as This voltage drop contains no information about the charge υqc, but it is proportional to Cbυ Vc, a magnitude vaguely related to this charge, as Cbwill increase with the geometric dimensions of the cavity. Va is clearly a quantity which could be measured.
It is a negative voltage step with a rise time depending upon the duration of ic. The magnitude of the voltage step, however, is quite small, although υVc is in a range of some102 to 103 V; but the ratio Cb/Ca will always be very small and unknown according to eqn. Thus a direct detection of this voltage step by measurement of the whole input voltage would be a tedious task. The detection circuits are therefore based upon another quantity, which can immediately be derived from a nearly complete circuit shown in Fig. 5.10.
The test object, Fig. 5.9 (a), is now connected to a voltage source V, in general an a.c. power supply. An impedance Z, comprising either only the natural impedance of the lead between voltage source and the parallel arrangement of CK and enlarged by a PD-free inductance or filter, may disconnect the 'coupling capacitor‘ CK and the test specimen Ct from the voltage source during the short duration PD phenomena only. Then CK is a storage capacitoror quite a stable voltage source during the short period of the partial discharge. It releases a charging current or the actual 'PD current pulse‘ i between CK and Ct and tries to cancel the voltage drop υVa across υVa is completely compensated and the charge transfer provided by the current pulse i is given by
and is the so-called apparent charge of a PD pulse, which is the most fundamental quantity of all PD measurements. The word ‗apparent‘ was introduced because this charge again is not equal to the amount of charge locally involved at the site of the discharge or cavity Cc. This PD quantity is much more realistic than Va in eqn as the capacitance Ca of the test object, which is its main part of Ct, has no influence on it. And even the amount of charge as locally involved during a discharge process is of minor interest, as only the number and magnitude of their dipole moments and their interaction with the electrodes or terminals determine the magnitude of the PD current pulse. The condition CK × Ca_.DCt is, however, not always applicable in practice, as either Ct is quite large, or the loading of an a.c. power supply becomes high and the cost of building such a large capacitor, which must be free of anyPD, is not economical.
For a finite value of CK the charge q or the current is reduced, as the voltage across CK will also drop during the charge transfer. Designating this voltage drop by υVŁa, we may compute this value by assuming that the same charge Cb, Vc has to be transferred in the circuits of Figs 5.9(b) and 5.10 . Therefore The relationship qm/q indicates the difficulties arising in PD measurements for test objects of large capacitance values Ct. Although CK and Ct may be known, the ability to detect small values of q will decrease as all instruments capable of integrating the currents i will have a lower limit for quantifying.
Equation therefore sets limits for the recording of ‗pi co coulombs‘ in large test objects.
During actual measurements, however, a calibration procedure is needed during which artificial apparent charge q of well-known magnitude is injected to the test object, critical note is made with reference to the definition of the apparent charge q as given in the new IEC Standard 60270.31 The original text of this definition is: apparent charge q of a PD pulse is that unipolar. charge which, if injected within a very short time between the terminals of the test object in a specified test circuit, would give the same reading on the measuring instrument as the PD current pulse itself. The apparent charge is usually expressed inpicocoulombs.
This definition ends with:
NOTE – The apparent charge is not equal to the amount of charge locallyinvolved at the site of the discharge and which cannot be measured directly. This definition is an indication of the difficulties in understanding the physical phenomena related to a PD event. As one of the authors of this book has been chairman of the International Working Group responsible for setting up this new standard, he is familiar with these difficulties and can confirm that the definition is clearly a compromise which could be accepted by the international members of the relevant Technical Committee of IEC. The definition is correct. It relates to a calibration procedure of a PD test and measuring circuit, as already mentioned above. The ‗NOTE‘, however, is still supporting the basically wrong assumption that a certain amount or number of charges at the site of the discharge should be measured. As already mentioned: it is not the number of charges producing the PD currents, but the number of induced dipole moments which produce a sudden increase in the capacitance of the test object.
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