MEASUREMENTS OF DC RESISTIVITY

The shape of specimen and the arrangement of electrode should be such that the resistivity can be easily calculated .For a solid specimen, a flat plate with plane and parallel surfaces usually circular are preferred. The specimens are normally in the form of disc of 5 to 10 cm diameter and 3 to 12 mm thickness .

A three -electrode arrangement as shown is used for measurement of resistance

The electrode which covers the surface of the specimen is called the ‘unguarded’ electrode and is connected to the high-voltage terminal.

➢ The third electrode surrounding other measuring electrode is connected to a suitable terminal of the measuring circuit.

➢ The width of this guard electrode must be at least twice the thickness of the specimen, and the unguarded electrode must extend to the outer edge of the guard electrode.

The gap between the guarded and guard electrodes should be as small as possible. The effective diameter of the guarded electrode is greater than the actual diameter and is given as follows.

𝑟=𝑟1+𝑔2−𝛿

Where

r1, r2, and r – radii of the guarded electrode , guard electrode including the gap ,and the effective radius of the guarded electrode.

g – gap width

t – specimen thickness. And

𝛿𝑡=2𝜋𝑙𝑛 cosh (𝜋𝑔4𝑡)

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LOSS OF CHARGE METHOD

Current-time characteristic of the discharge current is obtained by placing the switch B in position 2. The slope of these current time characteristic gives the time constant τ = CR, where C is the capacitance of the specimen. The volume resistance of the specimen R can thus be obtained from the relationship by measuring τ and C. Alternatively, a capacitance previously charged (C1) may be discharged through the specimen by inserting it in place of the battery E. By knowing the value of the capacitance C1 which is very large compared to the specimen capacitance (≈10 μF) and knowing the current-time characteristic of the discharge current, its time constant can be calculated. Hence, the resistance of the specimen is obtained from the relationship 𝑅=𝜏/𝐶1 where C1 – capacitance used τ – time constant. If initial voltage V0 on the capacitor is measured and the voltage V across it at any time τ Then the unknown resistance R can be calcul...

PARTIAL DISCHARGE REPRESENTATION:

The weak points in an insulation like voids, cracks, and other imperfections lead to internal or discharges in the insulation and as they are small, were not revealed in capacitance measurements but revealed as power loss components, contributing for an increase in the dissipation factor ‘PARTIAL DISCHARGES’ in a course of time reduces the strength of insulation leading to a total or partial failure or breakdown of the insulation Consider Ca as capacitor with a void inside the insulation. Capacitance of the void is represented by a capacitor in series with the rest of the insulation capacitance (Cb). The remaining void-free material is represented by the capacitance Cc When the voltage across the capacitor is raised, a critical value is reached acr...

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