Information

Voltagel transformers are used to measure voltage (potential). The secondary voltage is substantially proportional to the primary voltage and differs from it in phase by an angle that is approximately zero. Voltage transformers that are designed for monitoring single-phase and three-phase line voltages in power-metering applications are used mainly as step-down devices. They are designed for connecting line-to-line or line-to-neutral in the same way as ordinary voltmeters. The secondary voltage has a fixed relationship to the primary voltage so that a change in potential within the primary circuit is monitored accurately by meters connected across the secondary terminals.

Applications

Voltage transformers can be used with voltmeters for voltage measurements, or with current transformers for wattmeter or watt-hour meter measurements. Voltage transformers are also used to operate protective relays and devices, and in many other applications. Because they are used mainly in a monitoring capacity, however, voltage transformers generally require greater accuracy. For examples, products used by public 

utilities for determining electricity usage must be accurate since these voltage transformers are used for billing customers.

Specifications

Performance specifications for voltage transformers include:

accuracy

operating temperature

primary voltage range

secondary voltage range

burden

insulation voltage

Accuracy is the degree of uncertainty to which the measured current (secondary) value agrees with an ideal value. Burden is the maximum load that a voltage transformer or potential transformer can support while operating within its accuracy rating. Burden is expressed in volt-amperes (VA), the product of the root mean square (RMS) voltage applied to a circuit and the RMS current, in amperes, flowing through it.

Types 

There are many different types of voltage transformers. Commercial devices are suitable for most low-power current-monitoring applications. ANSI metering class transformers are designed especially for power-monitoring applications where high accuracy and minimum phase angle error are required. Multi-ratio transformers have several outputs. Three-phase devices are also available. Split-core voltage and potential transformers feature a hinge and locking snap mechanism that allows attachment without interrupting the current-carrying wire. Toroidal or donut-shaped transformers do not have an internal primary winding.