Instrument Transformer
Current transformers (CTs) and voltage transformers (VTs) that scale high-voltage/high-current signals down to measurable levels for protection relays and metering — the analogue sensors that merging units digitise.
Also: CT, VT, current transformer, voltage transformer, instrument transformers
An instrument transformer is a precision device that produces a scaled-down replica of a primary power-system quantity — current or voltage — so that protection relays, meters, and control systems can work with it safely.
Current Transformers (CTs)
A CT wraps around (or is threaded by) a primary conductor carrying hundreds or thousands of amps and produces a proportional secondary current — typically 1 A or 5 A — that protection IEDs and meters can measure. The ratio is fixed by design (e.g., 2000:1). CTs are wired in series with the power circuit and must never be open-circuited on the secondary side while the primary is energised — doing so produces dangerously high voltages.
Voltage Transformers (VTs)
A VT (also called a potential transformer, PT) steps the primary voltage — which may be 132 kV, 275 kV, or 400 kV — down to a standard secondary voltage, typically 110 V or 63.5 V. VTs are connected in parallel with the power circuit. They provide the voltage reference that protection algorithms need alongside the CT current measurement.
Why they matter for digital substations
Instrument transformers are the analogue-to-digital boundary of the substation. In a conventional installation, CT and VT secondary outputs are carried over copper trunks — dedicated multi-core cables, one pair per signal — from the switchyard to the protection panels in the control building. A single bay can have dozens of individual copper connections.
The stand-alone merging unit (SAMU) specified in IEC 61869-13 sits at the CT and VT terminals, digitises the analogue waveforms at 4 kHz (80 samples per cycle on a 50 Hz grid), and publishes them as IEC 61850-9-2 Sampled Values over a fibre-optic process bus. The instrument transformers themselves do not need replacing — only the secondary wiring changes. This is what makes the merging-unit retrofit path viable without touching the primary plant.
IEC 61869-9 defines the digital interface standard for instrument transformers that have the digitisation built in (so-called “digital CTs and VTs”), but in practice the stand-alone merging unit bolted onto conventional CTs and VTs is the dominant retrofit approach in the UK transmission estate.