Back to High Voltage Engineering

Study notes

LearnExams

Qwestrum Engineering360 · Electrical & Electronics · High Voltage Engineering

Measurement of High Voltage

High voltages are measured with dividers that scale the voltage down to a safe level — resistive for DC, capacitive for AC and impulse — with the sphere gap serving as the fundamental peak-voltage standard.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

  • Generating voltmeter and electrostatic voltmeter for AC
  • Rogowski coil for impulse current measurement
  • Digital techniques with fibre-optic isolation

Topic details

Introduction

A capacitive divider gives V_HV = V_LV × (C₁ + C₂)/C₁, where C₁ is the high-voltage arm and C₂ the low-voltage (measuring) arm. It is preferred for AC and impulse because it draws negligible power and has a fast response.

Scope in B.Tech and GATE syllabus

Resistive dividers suit DC but must handle the full voltage and dissipate power. The sphere gap is a calibrated standard: the breakdown voltage between two spheres of known diameter and spacing is tabulated, giving a direct peak-voltage reference.

Key relations & formulas

Potentialdivider:VHV=VLV×(C1+C2)C1Potential divider: V_{HV} = V_{LV} \times \frac{(C_{1} + C_{2})}{C_{1}}
(capacitive)

Formulas (Indian textbook notation)

  • Spheregap:standardforpeakvoltagecalibrationSphere gap: standard for peak voltage calibration

Formulas (Indian textbook notation)

  • ResistancedividerforDCHVmeasurementResistance divider for DC HV measurement

Notation and sign conventions

Relation 1 —
Potentialdivider:VHV=VLV×Potential divider: V_{HV} = V_{LV} \times
Potentialdivider:VHV=VLV×(C1+C2)C1Potential divider: V_{HV} = V_{LV} \times \frac{(C_{1} + C_{2})}{C_{1}}
(capacitive)
Write this relation with symbols exactly as in High Voltage Engineering — Rakesh Das before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
Spheregap:standardforpeakvoltagecalibrationSphere gap: standard for peak voltage calibration

Formulas (Indian textbook notation)

  • Spheregap:standardforpeakvoltagecalibrationSphere gap: standard for peak voltage calibration
Write this relation with symbols exactly as in High Voltage Engineering — Rakesh Das before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
ResistancedividerforDCHVmeasurementResistance divider for DC HV measurement

Formulas (Indian textbook notation)

  • ResistancedividerforDCHVmeasurementResistance divider for DC HV measurement
Write this relation with symbols exactly as in High Voltage Engineering — Rakesh Das before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Fundamentals and definitions

Divider ratio errors arise from stray capacitance to ground and from loading by the measuring instrument; a well-designed divider keeps the low-voltage arm impedance low relative to strays.

Governing relations in practice

The electrostatic voltmeter measures rms voltage directly from the electrostatic force between electrodes with no current drawn. The generating voltmeter measures DC by a rotating capacitor producing a current proportional to voltage.

Design and analysis considerations

For impulse measurement, the divider must reproduce the fast wavefront faithfully; a damped capacitive (mixed R–C) divider gives good high-frequency response without oscillation.

Assumptions and validity limits

State assumptions explicitly before using any relation for measurement of high voltage — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In High Voltage Engineering viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in High Voltage Engineering papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to measurement of high voltage.
4. Use equation 1:
Potentialdivider:VHV=VLV×Potential divider: V_{HV} = V_{LV} \times
.
5. Use equation 2:
Spheregap:standardforpeakvoltagecalibrationSphere gap: standard for peak voltage calibration
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Measurement of High Voltage appears in substations and cable systems. In Indian electrical curricula this topic is tested because it connects theory to insulation, breakdown, and testing.
GATE and semester exams often combine measurement of high voltage with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use measurement of high voltage?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

• Inverting the capacitive divider ratio (C₁ is the HV arm)
• Ignoring stray capacitance and loading errors
• Using a resistive divider for fast impulses (too slow / self-heating)
• Forgetting sphere-gap tables give peak, not rms, voltage

Quick revision checklist

Before attempting measurement of high voltage problems, confirm you can:
1. Generating voltmeter and electrostatic voltmeter for AC
2. Rogowski coil for impulse current measurement
3. Digital techniques with fibre-optic isolation
Revise the solved examples in High Voltage Engineering — Rakesh Das and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

Capacitive divider ratio

Problem

A capacitive divider has a high-voltage capacitor C₁ = 100 pF and a low-voltage capacitor C₂ = 100 nF. If 50 V is measured across C₂, find the high voltage.

Solution

V_HV = V_LV × (C₁ + C₂)/C₁.
C₁ = 100 pF = 0.1 nF; C₂ = 100 nF.
(C₁ + C₂)/C₁ = (0.1 + 100)/0.1 = 100.1/0.1 = 1001.
V_HV = 50 × 1001 = 50,050 V ≈ 50 kV.

Conceptual check — Measurement of High Voltage

Problem

In a High Voltage Engineering semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of measurement of high voltage." What should a complete answer include?

Exams & GATE

Rakesh Das — divider ratio and loading effect.

📖 Standard books (India)

  • High Voltage Engineering — Rakesh Das

    Read: Syllabus unit

    Breakdown, insulation, and testing

Explore related topics

See real electrical & electronics careers

After exams and interviews, see how engineers actually built careers — milestones and decisions from people in the field.

Browse Electrical & Electronics journeysPlan your path