General HV tests; resistance divider with micro-ammeter

General tests on high voltage equipment

High voltage equipment is tested to prove that its insulation can withstand the stresses it will meet in service. By their nature, the general tests are:

1. Sustained low-frequency (power-frequency) tests – withstand and flashover tests at 50/60 Hz, dry and wet (rain), usually for 1 minute; done on insulators, bushings, transformers, switchgear, cables.
2. High-voltage DC tests – for cables, capacitors and rotating machines, where AC testing would need very large charging current; also leakage current measurement.
3. High-frequency tests – at kHz frequencies, for insulators used with carrier/radio frequencies and to check behaviour under high-frequency transients.
4. Surge or impulse tests – lightning impulse (1.2/50 μs), switching impulse (250/2500 μs) and chopped wave tests, to prove the basic insulation level (BIL/BSL).

By purpose, tests are grouped as type tests (on one design sample), routine tests (on every unit), sample/acceptance tests, and non-destructive tests (insulation resistance, $\tan\delta$, capacitance, partial discharge measurement) that check condition without damaging the insulation.

Measurement using series resistance micro-ammeter

A very high resistance $R$ is connected in series with a micro-ammeter between the high voltage terminal and earth. The current is proportional to the voltage:

$$V = I\,R$$

  HV (V)
    |
   [R]  very high R, in oil/SF6 tube
    |
  ( uA )----+
    |       |  protective gap
    |      ===  or zener
    |       |
  earth ----+

• $R$ is built from many wire-wound or metal-film resistors in series, inside an insulating tube filled with oil or SF₆, with corona-free end caps. It is chosen to pass a small current (a few μA to about 1 mA at full voltage).
• The micro-ammeter is at earth potential, so it can be read safely; it is calibrated directly in kV.
• A protective device (paper spark gap, neon tube or zener diode) across the meter bypasses the current if $R$ flashes over.
• Used mainly for DC; also for AC when the resistor's stray capacitance is small.

Measurement using resistance potential divider

  HV (V) ---+
            |
           [R1]   high-voltage arm
            |
            +-------> V2 (to electrostatic
            |              voltmeter / CRO)
           [R2]   low-voltage arm
            |
  earth ----+

$$V = V_2\,\frac{R_1 + R_2}{R_2}$$

The low-voltage output $V_2$ is read by a high-impedance meter (electrostatic voltmeter, digital voltmeter or CRO). For impulses the divider is connected by a matched coaxial cable to an oscilloscope.

Sources of error and remedies

Accuracy of about ±0.2% to ±2% is obtained with careful construction.