Voltage measurement can come from inductive VTs, capacitive or resistive sensors, and low-power instrument transformers. Each technology has different burden, interface, failure and test requirements.

Learning objectives

Select a measurement technology, design safe secondary circuits and preserve accuracy from primary voltage to relay input.

Core engineering principles

Inductive VTs provide conventional secondary power

Ratio, accuracy class, burden and winding connection must match protection and metering. Primary or secondary fuses and MCBs change failure detection and maintenance.

Ferroresonance is a system interaction

Inductive VTs connected to lightly loaded networks can interact with system capacitance. Damping, connection and switching studies may be needed.

Low-power sensors use defined interfaces

Resistive, capacitive and optical devices provide low-energy analogue or digital output. Cable, connector, calibration and merging-unit compatibility are part of the measurement chain.

Earthing and open-delta circuits require control

Secondary neutral earthing, residual-voltage windings and broken-delta damping must follow the scheme. Unintended multiple earths can create circulating current or unsafe isolation.

VT failure affects protection logic

Fuse failure, MCB trip or sensor-channel loss can corrupt directional, distance, voltage and synchronism functions. Quality supervision and fallback behaviour must be designed.

Engineering application method

  1. Step 1: List voltage quantities and accuracy needed by each function.
  2. Step 2: Select VT or LPIT technology and ratio/interface.
  3. Step 3: Calculate burden or verify sensor input compatibility.
  4. Step 4: Design fusing, MCBs, earthing and test isolation.
  5. Step 5: Test ratio, polarity, phase rotation and channel-failure logic.

Practical example

A numerical relay compatible with a specific low-power sensor input cannot be connected to a conventional 100 V secondary without the correct interface. Both are voltage measurements, but their electrical and calibration domains differ.

Common mistakes

  • Selecting a VT only by ratio.
  • Ignoring ferroresonance and residual circuits.
  • Applying conventional burden calculations to LPITs.
  • Creating multiple secondary neutral earths.
  • Failing to supervise one lost voltage channel.

Design and review checklist

  • Which functions consume each voltage?
  • What accuracy and burden apply?
  • Is the interface electrically compatible?
  • Are fusing and earthing documented?
  • How does protection respond to channel failure?

Standards basis and official sources

Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.

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