Are there any limitations for the distance between electronics unit installed in the substation control building, and a sensing optical element installed on the switchyard?For the fiber optical current transformer there is a certain restriction for said distance – maximum 1300 m.
The restriction can be explained with certain operation aspects of the fiber optical modulator, one of the key elements of a current instrument transformer. The modulator works on the principle of delayed phase modulation of the phase shift between the effective light waves of the optical interferometer. The practical implementation of this principle demands certain correlations between the operating frequency and the length of interconnecting line.
The modulator of a fiber optical current instrument transformer is located in the electronics unit. This location allows ensuring an almost ideal electrical insulation from a high potential and meeting one of important requirements placed on instrument transformers – ensuring maximum security of the operating personnel. There is no steel core and armour, and the fiber connecting cable is a great dielectric. Mechanical integrity of the cable is ensured by the fiber-glass bars.
For electronic voltage instrument transformers there are no crucial limitations for the distance between electronics unit and insulating column. However, in practical terms 1300 m is usually enough.