What is the voltage transformer?
A voltage transformer (PT or VT) is a specialized transformer that converts dangerously high voltages in power systems into a lower, standardized, safer voltage for measurement and protection purposes.
1. Core Functions and Purposes
A voltage transformer has two main purposes:
Safety Isolation: Electrically isolates secondary equipment such as instruments and relays from dangerous high-voltage circuits to ensure the safety of personnel and equipment.
Scaling: Accurately scales down high voltages (e.g., 10kV, 35kV, 110kV) to a standardized lower voltage (typically 100V or 100/√3V) for measurement using a standard-range voltmeter.
2. Key Features
Transformation Ratio: Similar to the transformation ratio of a current transformer, a voltage transformer also has a rated voltage ratio, such as 10,000V/100V. This means that when the primary voltage is 10,000V, the secondary output will be 100V.
High Precision: The design requires high measurement accuracy and minimal error to ensure accurate metering and protection.
Low Capacity: Due to its high load impedance, the power transmitted is very low, typically only tens to hundreds of volt-amperes.
The secondary side must be reliably grounded for safety. If high voltage from the primary side accidentally leaks into the secondary side, earthing can divert the high voltage to the ground, protecting equipment and personnel.
3. Operating Principle
The operating principle of a voltage transformer is identical to that of a conventional power transformer, based on the law of electromagnetic induction.
It also has two windings: a primary (high-voltage side) and a secondary (low-voltage side).
The primary winding is connected in parallel to the high-voltage line to be measured.
The alternating magnetic field in the core induces a proportionally reduced voltage in the secondary winding.
Summary
| Characteristic | Description |
|---|---|
| Essence | A special transformer operating in a near no-load state |
| Function | Steps down high voltage proportionally to a standard low voltage (typically 100V) |
| Core Principle | Electromagnetic induction |
| Key Requirement | The secondary side must be grounded and must never be short-circuited during operation. |
| Main Types | Electromagnetic Type (for medium/low voltage), Capacitive Type (for high voltage and above) |
| Applications | Voltage measurement, metering, protection, and control in electrical power |