The safe operation and regulation of electrical grids requires a large number of grid sensors to monitor voltage and current at key grid nodes. High voltage and current are sensed through instrument transformers, high-accuracy electrical devices, which scale the grid voltage and current to fit the input levels of measuring instruments and secondary control circuitry - with the additional advantage of ensuring their galvanic isolation. The primary winding of the transformer is connected to the high-voltage or high-current circuit, and the measuring instrument is connected to the secondary circuit. Most instrument transformers are inductive, wire-wound transformers using an electromagnetic principle, but capacitive voltage transformers use a capacitor potential divider for operation at higher voltages.
Traditionally, traceability for instrument transformers is established for pure sine waves and at 50 Hz only. The presence of harmonics can impact the measurement accuracy of an instrument transformer, but this effect is not taken into account by the traditional calibration carried out at power frequency only. The limited measurement bandwidth and the lack of traceability for frequencies other than 50 Hz limits the possibility to analysing harmonic signals and interferences. The limited bandwidth also impacts the possibility to measure transient over-voltages. Instrument transformers could also be used in association with the injection of traveling waves for fault location or protection or in conjunction with phasor measurement units (PMUs).
Other wideband low-power-output passive or electronic instrument transformers, which operate on different principles such as Rogowski coils and dividers, are now commercially available. Instrument transformers based on magneto-optical or electro-optical effects of an optical fibre offer significant advantages over inductive and capacitor instrument transformers. Their bandwidth is much larger and their size is considerably smaller. Since the optical fibre is an electrical insulator, they are inherently decoupled from the power lines and almost immune to electromagnetic interference. They could support the instrumentation of high-voltage power lines for the application of dynamic line rating, monitoring, and modelling. Electronic instrument transformers permit the recording of a large number of dynamic parameters and they are usually fitted with digital outputs compliant with IEC 61850 and other standards, making them compatible with digital substations. However, the metrology infrastructure for calibration and evaluation of (optical) electronic instrument transformers is currently incomplete.
