Voltage transformers

A Voltage Transformer (VT), also known as a Potential Transformer (PT), is a type of instrument transformer that is designed to step down the system voltage to a safer and measurable value. They are used in power systems to measure high alternating currents and are designed to present a negligible load to the supply being measured.

Voltage transformers have an accurate voltage ratio and phase relationship, which enables accurate secondary connected metering. They are primarily used for protection and measurement purposes in a power system.


The primary winding of a VT consists of a single ‘turn’, which can be a heavy copper bar to carry current through the core. The secondary winding may have a large number of coil turns wound on a laminated core of low-loss magnetic material. This arrangement allows the VT to maintain an accurate ratio between the currents in its primary and secondary circuits over a defined range.

There are three primary types of potential transformers (PT): electromagnetic, capacitor, and optical. The electromagnetic potential transformer is a wire-wound transformer. The capacitor voltage transformer (CVT) uses a capacitance potential divider and is used at higher voltages due to a lower cost than an electromagnetic PT. An optical voltage transformer exploits the Faraday effect, rotating polarized light, in optical materials.

In essence, voltage transformers play a crucial role in maintaining the health and efficiency of power systems by providing a convenient way of safely monitoring the actual electrical current flowing in an AC transmission line.



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Why to test Voltage Transformers?

Testing voltage transformers is essential for several reasons:

  1. Accuracy: Regular testing ensures that the voltage transformers are performing accurately. This is crucial as they are used for metering purposes and must have a high degree of accuracy to ensure precise billing.

  2. Protection: Voltage transformers used for protection must react quickly and correctly in the event of a fault. Testing helps verify this.

  3. Early Fault Detection: Electrical changes in a voltage transformer, caused for example by aging insulation, can be identified at an early stage through testing.

  4. Preventive Maintenance: Regular testing can identify minor issues before they become major ones, saving time and money in the long run. It also extends the lifespan of transformer units.

  5. Safety: Testing reduces risks such as confusing instrument transformers for metering and protection, or mixing up connections.

  6. Cost-Effective: Instrument transformer-based transformers are less costly, and damaged parts can be easily replaced.

  7. Higher Output Voltage: The higher output voltage can be achieved with smaller and cheaper units.

  8. Lower Insulation Stress: Lower insulation stress on each unit due to lower individual voltages.

  9. Higher Efficiency and Lower Losses: Higher efficiency and lower losses due to lower currents.

In summary, regular testing of voltage transformers is crucial for maintaining the health and efficiency of power systems.


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