Question

Define a voltage transformer (potential transformer) and its voltage scaling function.

Answer 1

A voltage transformer, also known as a potential transformer (PT), is a type of instrument transformer used to step down high voltage levels to a lower, more manageable voltage suitable for measurement and protection purposes. It is commonly used in power systems to provide accurate and isolated voltage measurements to instruments and protective relays.

The primary function of a voltage transformer is to maintain a fixed turns ratio between its primary winding (high-voltage side) and secondary winding (low-voltage side). The turns ratio is usually defined by the manufacturer and remains constant under normal operating conditions. This turns ratio is typically represented as

=

primary

secondary
N=
V
secondary
    ​

V
primary
    ​

    ​

, where

primary
V
primary
    ​

 is the voltage on the high-voltage side and

secondary
V
secondary
    ​

 is the voltage on the low-voltage side.

The voltage scaling function of a potential transformer can be explained as follows:

Voltage Step-Down: The primary purpose of a voltage transformer is to step down the voltage from the high-voltage side to the low-voltage side. The turns ratio

N determines the magnitude of voltage reduction. For example, if the turns ratio is 100:1, a 100,000-volt primary voltage will be scaled down to 1,000 volts on the secondary side.

Isolation: Voltage transformers provide electrical isolation between the high-voltage system and the low-voltage system. This isolation ensures that any faults or abnormalities on the high-voltage side do not directly affect the low-voltage measurement or protection equipment.

Accuracy: Voltage transformers are designed to be highly accurate, providing precise voltage measurements on the secondary side. This accuracy is crucial for various applications, such as metering, control, and protective relay operation.

Burden Compensation: Voltage transformers are designed to minimize the effect of the connected load (burden) on the primary side. The burden compensation ensures that the voltage drop across the primary winding remains negligible and does not impact the accuracy of voltage measurement.

Phasor Relationship: The voltage transformer maintains the same phase relationship between the primary and secondary voltages. This phasor relationship is essential for accurate synchronization and coordination in power systems.

In summary, a voltage transformer (potential transformer) is a vital component in power systems, responsible for stepping down high voltages to a lower, standardized level suitable for various applications, while providing accurate voltage measurements and electrical isolation.