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Selecting a Voltage Transformer

  

Accuracy of Voltage Transformers | Important Warning for VT's


Defining the transformer ratio Up/Us

• Preferably select as primary voltage the nominal service voltage of the network.
• Select as secondary voltage the nominal input voltage of the equipment to be fed.
 

Defining the accuracy class against the application

Standard reference = Class 0,1 or 0,2
Tariff metering = Class 0,5 or 0,2
Divisional industrial metering = Class 1
Measure display = Class 1 or 3
Protection = Class 3P or 6P


Defining the voltage factor

• The voltage factor is determined by the maximum operating voltage, which depends on the neutral system and the voltage transformer primary winding earthing conditions.
• When the voltage transformer is connected between two phases, the voltage factor is 1.2Un continuous.
• When the voltage transformer is connected between phase and earth, the voltage factor depends on the neutral system:
    - 1.9 Un for 30 s, in a non-effectively earthed neutral system with automatic earth-fault tripping.
    - 1.9 Un for 8 h, in an isolated neutral system, or in a resonant earthed system, without earth-fault tripping.

© RS ISOLSEC - Instrument transformers
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