What Is a Voltage Transformer and What Is Its Purpose?

9 May 2023

A voltage transformer, also known as a potential transformer or VT, is a type of transformer that reduces the voltage on the primary side and transfers it to the secondary side of the circuit. It is used when the voltage on the primary side, which needs to be measured, exceeds a safe operating range for measurement instruments like wattmeters, meters, and voltmeters.

The primary functions of a voltage transformer include:

1. Reducing Voltage: The voltage transformer lowers the voltage of the primary circuit to a safer and more manageable level for measurement and instrumentation.
2. Electrical Isolation: Voltage transformers electrically isolate the primary circuit from the secondary circuit, allowing safe measurement without direct connection.
3. Measurement Accuracy: They expand the measurement range of alternating current (AC) instruments, enabling precise voltage measurements.
4. Safety: Voltage transformers are crucial for safety in high-voltage environments, ensuring that measurement instruments are not exposed to excessive voltage levels.

Features of Voltage Transformers:

Voltage transformers have specific characteristics that make them suitable for their applications:

1. Low Capacity: They typically have low power ratings, commonly ranging from 15 to 600 volts.
2. Multiple Instruments: A single voltage transformer can be used with several measuring instruments.
3. Thin Windings: Both the primary and secondary windings are relatively thin, with a significantly larger number of turns in the primary winding.
4. Standard Secondary Voltage: The secondary voltage is standardized, often set at 100-120 volts regardless of the magnitude of the primary voltage.
5. Open-Circuit Secondary: Voltage transformer secondaries operate as open circuits, meaning they do not provide short-circuit capability like current transformers.
6. Ratio Conversion: The readings from instruments connected to the secondary terminals should be multiplied by the transformation ratio (n = U1/U2) to obtain the actual values.
7. Grounding: To protect against high-voltage faults, the secondary terminals are grounded.
8. Protection: To prevent short circuits, a fuse is often installed in one of the secondary terminals.

Voltage Transformer Selection Criteria:

When selecting a voltage transformer, consider the following factors:

1. Operating Temperature
2. Insulation Voltage
3. Primary Voltage Range
4. Accuracy
5. Capacity in VA (Volt-Amps)
6. Secondary Voltage Range

Types of Voltage Transformers:

Voltage transformers are categorized based on their applications into two main types:

1. Phase-to-Phase Voltage Transformers: These transformers are connected between two phases in the network. They are commonly used in balanced medium-voltage networks, where they reduce the voltage for measurement purposes.
2. Phase-to-Ground Voltage Transformers: These transformers have only one insulator and connect the primary to the ground and the secondary to a phase conductor. They are used in unbalanced networks or for specific measurements.

In summary, voltage transformers play a crucial role in reducing and isolating high voltages for safe and accurate measurement purposes in various electrical applications. They help protect instruments and ensure measurement accuracy in electrical systems.