- 2073 Bhadra · 8 marks
Current transformer vs conventional; CT open
Short answer
A CT has primary in series with line (few turns) and secondary connected to ammeter (many turns), providing electrical isolation. If secondary opens under primary current, demagnetizing mmf vanishes, core saturates, flux spikes induce extreme secondary voltage (several kV) causing insulation failure and operator hazard.
How current transformer is different from conventional transformer. Explain how CT is used to measure high currents. Also explain, what happens if the secondary of CT is open when there is high current flowing in primary side.
Current transformer (CT) versus a conventional (power) transformer
| Basis | Current transformer | Power transformer |
|---|---|---|
| Purpose | Measure/step down large alternating currents for instruments and relays | Transfer power at a changed voltage |
| Primary | A few turns (or a single bar) in series with the line | Many turns, connected across the supply |
| Secondary | Many turns, connected to an ammeter / relay coil, nearly short-circuited | Connected to the load |
| Primary current | Fixed by the line (load), independent of secondary | Depends on the secondary load |
| Burden | Very low (a few VA) | Rated power (kVA) |
| Voltage across secondary | Very low in service | Rated voltage |
| Flux in core | Low in service; very high if secondary is open | Nearly constant |
Use of a CT to measure high currents
line ──[ primary: Np (1-2 turns) ]── load
core
[ secondary: Ns turns ]──(A)──
- The primary is placed in series with the high-current line. The secondary has many turns and is connected to a standard 5 A (or 1 A) ammeter, which acts nearly as a short circuit.
- Ip Np ≈ Is Ns, so Is = Ip (Np/Ns). Line current = Is × (Ns/Np). E.g. a 1000/5 A CT with 5 A on the meter indicates 1000 A in the line.
- The meter and the line are separated electrically, so the instrument can be safely used for high-voltage lines.
Secondary open with high primary current
- In normal service, the secondary mmf almost cancels the primary mmf, so the net mmf (and flux) is small.
- If the secondary is opened, Is = 0 and nothing cancels the primary mmf. The whole primary ampere-turns Ip Np acts as magnetising mmf, and the flux in the core rises to very high values.
- Core saturates heavily and the core loss gives severe heating of the core.
- The emf in the secondary (many turns, high rate of change of flux) becomes extremely large. Peaks of several kV can appear, endangering the insulation and the operator.
- Therefore the CT secondary should never be opened under load. It must be short-circuited before disconnecting an instrument.
✓ Answer checked by Claude Opus 5.5 (high effort)
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