Around every power transformer lives a family of small transformers: current and voltage transformers. They don't feed loads, but they are the eyes of the measurement and protection system — without them, relays become blind and meters become deaf.
The Shared Function
Grid values (thousands of volts and amperes) cannot be fed directly into meters and relays. Instrument transformers convert them into small, safe, isolated standard values:
| Current Transformer (CT) | Voltage Transformer (VT/PT) | |
|---|---|---|
| Converts | Large current to a standard 5A or 1A | High voltage to a standard 110V or 100V |
| Connection | Primary connected in series with the line | Primary connected in parallel with the grid |
| Example | A 2000/5 CT on the secondary of a 1000 kVA transformer | A 13800/110 VT to measure the busbar voltage |
| Feeds | Ammeters, energy meters, protection relays | Voltmeters, voltage relays, synchronization systems |
Their Role Around the Power Transformer
- Differential protection relies entirely on the CTs on both sides, and REF on the neutral and phase CTs.
- Overcurrent protections, measurements, and metering for billing all go through CTs and VTs.
- The accuracy of all protection depends on the accuracy of these instrument transformers and the correctness of their ratios and connections — a CT ratio error can blind the protection or cause it to trip falsely.
Never open a CT secondary circuit while the primary is carrying current. Without a load on the secondary, all of the primary current becomes magnetizing current, causing the open secondary's voltage to rise to lethal levels and potentially damaging the CT. Short-circuit the secondary first, using the dedicated shorting terminals, before doing any work on it.
Sample answer: Because the primary current of a CT is imposed by the line and is unaffected by the secondary's condition. When the secondary is opened, the opposing current that was balancing the magnetization is lost, so all of the primary current converts to magnetizing current, driving the core into deep saturation, and a very high voltage develops across the open secondary terminals — high enough to kill a worker and damage the CT's insulation. Therefore, the secondary must be short-circuited via the shorting terminals before disconnecting any device.
Treating a CT like any "harmless" small transformer. The usual transformer rule is reversed here: a voltage transformer fears a short circuit on its secondary, while a current transformer fears an open circuit on its secondary — confusing the two can be a fatal mistake.
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