Instrument transformers (ITs) are designed to transform voltage or current from the high values in the transmission and distribution systems to the low values that can be utilized by low voltage metering devices. There are three primary applications for which ITs are used: metering (for energy billing and transaction purposes); protection control (for system protection and protective relaying purposes); and load survey (for economic management of industrial loads).
Depending on the requirements for those applications, the IT design and construction can be quite different. Generally, the metering ITs require high accuracy in the range of normal operating voltage and current. Protection ITs require linearity in a wide range of voltages and currents. During a disturbance, such as system fault or overvoltage transients, the output of the IT is used by a protective relay to initiate an appropriate action (open or close a breaker, reconfigure the system, etc.) to mitigate the disturbance and protect the rest of the power system. Instrument transformers are the most common and economic way to detect a disturbance. Typical output levels of instrument transformers are 1-5 amperes and 115-120 volts for CTs and VTs, respectively. There are several classes of accuracy for instrument transformers defined by the IEEE, CSA, IEC, and ANSI standards.
Low power instrument transformers (LPITs) or Non-conventional instrument transformers (NCITs)
LPITs or NCITs, or similarly sensors, are near process devices based on alternative principles, and have been introduced as successors to conventional instrument transformers with iron-core and in connection with microprocessor-based intelligent electronic devices in power distribution applications. They could use optical techniques to measure. These sensors enable benefits, such as compact size and optimized design (weighing a few hundred grams), and is part of the concept of digital switchgear.
Some of the NCITs is converting the primary current to a data stream output known as sampled values and based on the standard IEC 61850-9.2, either direct from the device or through a merging unit which is a separate unit that converts an analogue value to the standardized data stream.