Jacobs Engineering India Pvt. Ltd., Vadodara.
Often the values of current and voltage in a power system are too high to permit direct connection of measuring instruments or relays. The coupling is made through transformers, which produce a scaled down version of the input quantity to the accuracy expected for the particular measurement. Voltage and current transformers for low primary voltage or current ratings are very similar. For higher ratings, dissimilarities of construction are usual. The main difference is in the way it is connected to the power circuit. Current transformers have their primary windings connected in series with the power circuit. The voltage transformers are connected in parallel to the power circuit. The performance of CTs and PTs during and following large instantaneous changes in the input quantity are important, as the input quantity may depart from the sinusoidal waveform. The deviation may consist of a step change in magnitude or a transient component that persists for an appreciable period or both. Many protection schemes are required to operate during the transient phase. The error in CT output may abnormally delay the tripping operation or cause unnecessary operations. This paper discusses the Optical instrument transformers after discussing the basics of Electromagnetic types.
