Series impedance voltmeter and its drawbacks

A series impedance voltmeter measures a high AC voltage by passing a small current through a known high impedance (resistance or capacitance) in series with a low-range ammeter. The ammeter is at earth potential, and the voltage is calculated from the current.

Construction

  HV (V) ---[ Z: high R or C ]---+
                                  |
                              (  A  )  microammeter
                                  |    with protective
                             gap / zener across it
                                  |
  earth --------------------------+

• Series impedance $Z$: either
  • a very high resistance (wire-wound or carbon/metal-film resistors in series, housed in an insulating tube filled with oil or SF₆, with corona-free end caps), or
  • a capacitor (compressed-gas standard capacitor) for AC.
• Ammeter: a moving-coil microammeter (with rectifier for AC) or a sensitive AC ammeter at the earthed end.
• Protection: a spark gap, neon glow tube or zener diodes across the ammeter so that a flashover of $Z$ does not put full voltage across the meter.

Working principle

(a) Series resistance: current $I = V/R$, so

$$V = I\,R$$

With residual inductance $L$ the impedance is $Z = R + j\omega L$ and $V = I\sqrt{R^2 + \omega^2L^2}$; $R$ is chosen so that $\omega L \ll R$.

(b) Series capacitance: current $I = j\omega C V$, so

$$V = \frac{I}{\omega C}$$

In the Chubb–Fortescue method, two diodes are connected in anti-parallel in the capacitor arm, and a moving-coil meter is placed in one diode branch, so it carries the charging current of one half-cycle only. The charge per cycle is $C\cdot 2V_m$, so the meter reads $I_{av} = 2fCV_m$, and the peak voltage is $V_m = I_{av}/(2fC)$. (If a full-wave bridge rectifier is used instead, $I_{av} = 4fCV_m$.) The scale is calibrated directly in kV.

Typical currents are a few μA to about 1 mA, so the impedance is very high, e.g. about $10^{9}$–$10^{11}$ Ω for resistive arms at hundreds of kV.

Drawbacks

1. Stray capacitance and inductance: the long resistor has distributed capacitance to earth and residual inductance, so its impedance changes with frequency; errors increase for harmonics and high frequency.
2. Heating and temperature coefficient: power loss $I^2R$ heats the resistor; resistance changes with temperature, causing drift.
3. Corona and leakage: corona at terminals and leakage over the insulating surface add to the measured current, giving wrong readings at very high voltages.
4. Harmonics (capacitive type): current in a capacitor is proportional to frequency, so harmonics in the waveform cause large errors; also depends on exact supply frequency.
5. Waveform dependence: reads rms or mean, not peak (except the rectifier version), while breakdown depends on peak.
6. Bulky and costly for voltages above a few hundred kV; risk of damage to the meter on flashover without protection.