Instrument transformers first appeared in the late 19th century. With the development of the power industry, the voltage levels and accuracy of instrument transformers have greatly improved. Many special-purpose instrument transformers have also been developed, such as voltage-current composite transformers, DC current transformers, high-accuracy current and voltage ratio transformers, high-current laser current transformers, electronic circuit compensation transformers, photoelectric transformers in ultra-high voltage systems, and voltage and current transformers in SF6 fully enclosed switchgear (GIS). In the power industry, the development of power systems of a certain voltage level and scale necessitates the development of instrument transformers with corresponding voltage levels and accuracy to meet the needs of power system measurement, protection, and control.
With the continuous application of many new materials, many new types of instrument transformers have emerged. Currently, electromagnetic instrument transformers have been largely developed, among which iron-core current transformers, with various structures including dry-type, oil-immersed, and gas-insulated types, have adapted to the development needs of power construction. However, with the continuous growth of power transmission capacity, the increasing voltage levels of power grids, and the ever-improving protection requirements, the structure of conventional iron-core current transformers has gradually revealed its inadequacies. These inherent weaknesses, such as large size, magnetic saturation, ferroresonance, small dynamic range, and narrow operating bandwidth, make it difficult to meet the development needs of next-generation power system automation and digital power grids.
With the rapid development of optoelectronic technology, new types of electronic current transformers, developed using optical sensing technology and electronic methods, have gradually emerged. The International Electrotechnical Commission (IEC) has published standards for electronic current transformers. The term "electronic current transformer" encompasses not only optoelectronic transformers but also various other voltage and current sensors utilizing electronic testing principles.