Non-destructive tests on insulators

Non-destructive tests check the quality and condition of insulation at voltages near or below the working voltage, without damaging it. They find moisture, contamination, voids and ageing early, so faults can be corrected before a destructive failure. The main tests are measurement of resistance, dielectric constant and loss ($\tan\delta$), and partial discharges.

1. Measurement of insulation (DC) resistance and resistivity

• A DC voltage (500 V – 10 kV, from a megger or DC source) is applied and the leakage current is measured with a sensitive galvanometer or electrometer.
• A three-terminal (guard ring) electrode system separates volume current from surface leakage, giving volume resistivity $\rho_v$ and surface resistivity $\rho_s$.
• Polarization index $PI = R_{10\,min}/R_{1\,min}$ shows moisture and dirt: $PI > 2$ is good, $PI < 1$ is dangerous.
• Used on transformers, machines, cables and insulator units.

2. Measurement of dielectric constant and loss factor ($\tan\delta$)

            HV
          /    \
     Cx,Rx      Cs (standard)
        |        |
        A--[D]---B
        |        |
       R3     R4 || C4
          \    /
          earth

• The high-voltage Schering bridge is balanced; then

$$C_x = C_s\frac{R_4}{R_3}, \qquad \tan\delta = \omega C_4 R_4$$

• $\varepsilon_r = C_x/C_0$ ($C_0$ = capacitance with vacuum/air). A rise in $\tan\delta$ with voltage ("tip-up") or with time indicates moisture, voids and ageing.
• The transformer ratio-arm bridge is used for higher accuracy and for earthed specimens.
• Used on bushings, cables, CTs, capacitors and machine windings.

3. Partial discharge (PD) measurement

• Partial discharges in voids, cracks or at sharp edges produce small current pulses. They are detected with a coupling capacitor and a detection impedance in parallel with the test object (straight detection or balanced bridge circuit), and displayed on a PD detector/oscilloscope.
• The result is the apparent charge in picocoulombs (pC), the inception and extinction voltages, and the pulse pattern (phase-resolved).
• Calibrated by injecting a known charge $q = V_0 C_0$.
• Acoustic (ultrasonic) and UHF sensors locate PD in transformers and GIS.

4. Other non-destructive tests

• DC leakage current / step voltage test on cables and machines.
• Dissolved gas analysis and oil BDV tests for oil-filled equipment.
• Visual, ultrasonic and infrared (thermography) inspection for cracks and hot spots in insulators.
• Leakage current monitoring of surge arresters (third-harmonic resistive current).

Tests by type of insulation