How does a small meter measure a line current of thousands of amperes? And how does an electronic relay monitor a busbar voltage of hundreds of kilovolts? The answer: neither does directly — instead, each reads a safe, scaled-down copy produced for it by the current and voltage transformers, the substation's eyes upon itself.
The Shared Function
Instrument transformers do not feed loads; their function is to convert high values into small, standard, safe, and isolated values suitable for meters, protection relays, and SCADA systems:
| Current Transformer (CT) | Voltage Transformer (PT/VT) | |
|---|---|---|
| Converts | Large line 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 network |
| Feeds | Ammeters, energy meters, current relays | Voltmeters, voltage relays, synchronization, SCADA |
| Distinctive hazard | Opening the secondary under load | Short-circuiting the secondary |
Its Role in the Protection System
The entire protection chain begins here: measurement (CT/PT) → decision (the relay) → execution (the circuit breaker). The accuracy of their ratios and the integrity of their connections are a condition for the validity of the entire protection scheme — an error in CT ratio or polarity can blind the relay or cause it to trip falsely.
The Golden Rule: Never Open a CT Secondary
The CT primary current is imposed by the line and is indifferent to the secondary's condition. With the secondary open, all of the primary current goes into magnetizing the core, which saturates and generates a lethal high voltage across the open terminals — endangering personnel and damaging insulation. Before any work: short-circuit the secondary using the dedicated shorting terminals/blocks.
In AIS and GIS
- In air-insulated substations you will see them as standalone equipment on insulated bases next to the circuit breakers.
- In GIS substations they are integrated within the gas-insulated enclosures — present even though you don't see them.
- For more detail on their relationship to transformer protection, see CT and VT in the Transformers section.
Sample answer: Their function is to convert the network's high currents and voltages into small, safe, isolated standard values (5A/1A for current and 110V for voltage) that feed meters, protection relays, and SCADA — they are the measurement source for the entire protection chain. Opening a CT secondary under load is prohibited because the primary current is imposed by the line; when the secondary opens, all of it goes into magnetizing the core, which saturates and generates a lethal high voltage across the open terminals — the secondary must be short-circuited via shorting terminals before any work.
Using a Metering-class CT to feed protection relays. The metering class deliberately saturates early to protect meters from fault currents, while protection requires a class that remains accurate at large currents — the two classes are not interchangeable.
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