Application limitation of electronic current and Voltage Transformers in digital substations

Electronic voltage transformer (EVT) and electronic current transformer (ECT) are important equipments in smart grid. In order to meet the requirements of power reliability in the grid, the annual failure rate of EVT and ECT should be less than 0.2% during normal operation. Power transformer is the electrical device, its annual failure rate is only 0.01%, and the comprehensive reliability of secondary relay protection devices can be improved to more than 99.8% by redundancy design. EVT and ECT´s reliability can be hardly improved to 99.8% due to the limit of failure relevance, unless designed and manufactured according to aerospace standards. EVT and ECT´s utilization is limited in the digital substation considering of reliability and economy. In the digital substation, Non-traditional transformer is an economical and rational design, because the reliability of 99.8% can be guaranteed by sending analog small-signals from non-traditional transformers to the relay unit. Non-traditional transformer´s analog output is sent to the merged cell of transformer, and a digital signal is converted into the data bus serving for substation monitoring and control systems, so the reliability can reach 99%. Thus non-traditional transformer´s analog output and merged cell´s output can respectively meet the reliability requirements of relay protection and monitoring system´s informatization in the digital substation.