The primary difference between a Current Transformer (CT) and a Voltage Transformer (VT) lies in their function, construction, and how they are connected in a power system. CTs measure current, while VTs measure voltage.
Key Differences Between CTs and VTs
Here's a breakdown of the differences in a table format for easy comparison:
| Feature | Current Transformer (CT) | Voltage Transformer (VT) |
|---|---|---|
| Function | Measures high currents by stepping them down to measurable levels. | Measures high voltages by stepping them down to measurable levels. |
| Primary Winding | Few turns, connected in series with the circuit. | Many turns, connected in parallel with the circuit. |
| Secondary Winding | Many turns, connected to measuring instruments. | Few turns, connected to measuring instruments. |
| Secondary Load | Approximates a short circuit. | Approximates an open circuit. |
| Operation | Operates with primary current independent of secondary load. | Operates with primary voltage independent of secondary load. |
| Secondary Open-Circuit | Dangerous, can result in high voltage buildup. | Generally not dangerous. |
| Application | Metering, protection, and control of high current circuits. | Metering, protection, and control of high voltage circuits. |
Detailed Explanation
Current Transformer (CT)
- Function: A CT reduces high alternating currents to a lower value, typically 1 or 5 amps, which can be safely measured by ammeters and protective relays.
- Connection: The primary winding of a CT is connected in series with the conductor carrying the current being measured. All the current flows through the CT primary.
- Turns Ratio: CTs have a high turns ratio (secondary turns > primary turns).
- Safety: Never open-circuit the secondary of a CT while the primary is energized. This can lead to dangerously high voltages across the secondary terminals due to the core's attempt to maintain the ampere-turns balance, potentially damaging the CT and endangering personnel. The secondary should always be shorted or connected to a burden (meter or relay).
- Example: A CT with a ratio of 1000:5 would reduce a primary current of 1000 amps to a secondary current of 5 amps.
Voltage Transformer (VT)
- Function: A VT (also known as a Potential Transformer or PT) reduces high voltages to a lower value, typically 100-120 volts, which can be safely measured by voltmeters and protective relays.
- Connection: The primary winding of a VT is connected in parallel with the circuit whose voltage is being measured. The VT sees the full voltage of the line.
- Turns Ratio: VTs have a low turns ratio (primary turns > secondary turns).
- Safety: While open-circuiting the secondary of a VT is not ideal, it is generally safer than open-circuiting a CT secondary.
- Example: A VT with a ratio of 10000:100 would reduce a primary voltage of 10000 volts to a secondary voltage of 100 volts.
In summary, CTs and VTs are essential components in power systems, enabling the safe and accurate measurement of high currents and voltages for metering, protection, and control purposes. Their fundamental difference lies in their intended function and how they are integrated into the electrical circuit.
