Voltage and potential 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 and potential 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.
Voltage and potential transformers can be used with voltmeters for voltage measurements, or with current transformers for wattmeter or watt-hour meter measurements. Voltage transformers and potential 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 or potential transformers generally require greater accuracy. For examples, products used by public utilities for determining electricity usage must be accurate since these voltage or potential transformers are used for billing customers.
Performance specifications for voltage and transformers include accuracy, operating temperature, primary voltage range, secondary voltage range, burden, and 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 of Voltage and Potential Transformers
There are many different types of voltage and potential 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.
Voltage and potential transformers include wound primary current transformers, devices with primary windings that consist typically of more than one turn. Bar transformers or bar current transformers have a bus bar that serves as the primary conductor. PC-mount transformers feature a small footprint for measuring sensor current or voltage on a board. Voltage and potential transformers include under-current sensing and over-current sensing devices, as well as Noncontact current transformers. This latter class of device accurately measures current waveforms without coming into electrical contact with the circuit.