Types of HV tests; sphere gap measurement

Types of tests on high voltage equipment

• By purpose: type tests (on one sample of a design), routine tests (on every unit), sample/acceptance tests, and special tests.
• By voltage: power-frequency withstand and flashover tests (dry and wet), DC tests, high-frequency tests, and impulse tests (lightning 1.2/50 μs and switching 250/2500 μs); plus non-destructive tests such as $\tan\delta$ and partial discharge.

Measurement of high voltage using sphere gaps

A sphere gap consists of two identical polished metal spheres separated by an air gap. The uniform field between spheres gives a breakdown voltage that is accurately known (IEC 60052 / IEEE Std 4 tables) for a given sphere diameter $D$ and spacing $S$. By noting the spacing at which flashover occurs, the peak value of AC, DC or impulse voltage is found.

  HV test     R (series
  source ----/\/\/\----+
  (AC/DC/              |
  impulse)          ( HV )  upper sphere
     |               |  S   gap spacing
     |              ( E  )  lower sphere
     |               |      (earthed)
     +-------earth---+

Construction and arrangement

• Two equal spheres of copper, brass or aluminium, of standard diameters (2, 5, 6.25, 10, 12.5, 15, 25, 50, 75, 100, 150, 200 cm).
• Small spheres are mounted horizontally; large spheres (about 50 cm and above) vertically with the lower sphere earthed.
• Shafts, supports and nearby objects must keep the clearances given in the standard so that the field is not disturbed.
• A series resistor limits the current at flashover, protects the sphere surfaces from pitting and damps oscillations: about 100 kΩ to 1 MΩ for AC and DC; a low, non-inductive value (below about 500 Ω) for impulses so the wave is not distorted.

Procedure

1. Set the spacing $S$ larger than expected; apply the voltage.
2. For AC/DC, raise the voltage slowly (or reduce the gap) until flashover occurs; repeat several times and take the mean.
3. For impulses, find the 50% flashover voltage: the spacing at which half of the applied impulses cause flashover.
4. Read the standard breakdown voltage $V_s$ (kV peak) for that $D$ and $S$ from the table.

Correction for air density

Tables are for 20 °C and 760 mm Hg. Relative air density

$$\delta = \frac{b}{760}\times\frac{293}{273 + t} = \frac{0.386\,b}{273 + t}$$

($b$ in mm Hg, $t$ in °C). The actual breakdown voltage is $V = kV_s$, where $k$ is a correction factor; for $0.95 < \delta < 1.05$, $k \approx \delta$.

Factors affecting accuracy

• Spacing should not exceed $0.5D$ for ±3% accuracy (up to $0.75D$ with lower accuracy).
• Nearby earthed objects and the shaft affect the field.
• Dust, fibres and humidity cause erratic flashovers; spheres must be clean and smooth.
• For gaps below about 1 cm (and impulse tests) the gap is irradiated with UV light or a radioactive source to provide initiating electrons and reduce scatter.

Advantages: measures peak of AC, DC and impulse voltages; simple, rugged; used to calibrate other instruments. Limitations: the measuring process causes flashover, so it cannot monitor voltage continuously, and corrections are needed.