Current Transformer Secondary Winding Always Closed
The question asks which transformer type requires its secondary winding to be always kept closed.
Understanding Current Transformers (CTs)
- A Current Transformer (CT) is used to measure high AC currents.
- It works by stepping down the high primary current to a lower, measurable secondary current.
- Crucially, the CT is connected in series with the circuit through which the current is being measured.
Reasoning for Closed Secondary
The secondary winding of a CT must always be connected to a closed circuit (a burden, like an ammeter or relay coil, or simply short-circuited) for the following reasons:
- Safety: If the secondary circuit is opened while the primary circuit has current flowing, the primary winding's magnetic field collapses rapidly. This induces a very high, potentially lethal voltage across the open secondary terminals.
- Operation: A CT is designed to operate with its secondary terminals having a very low impedance. Keeping the secondary closed maintains this low impedance, ensuring the secondary current is proportional to the primary current according to the transformer's turns ratio ( \(I_s \approx (N_p / N_s) \times I_p\) ). An open circuit presents a high impedance, disrupting this relationship and potentially damaging the transformer core due to over-excitation.
Comparison with Other Transformers
- Potential Transformers (PTs): Used for measuring voltage, connected in parallel. Their secondaries are typically connected to high-impedance voltmeters and can be left open without significant danger.
- Power/Distribution Transformers: Used for stepping voltage levels for power transfer. Their secondaries supply power to loads and are not required to be continuously closed in the same manner as CTs for operational safety.
Conclusion
Based on its operational principle and safety requirements, the Current transformer is the type where the secondary winding must always be kept closed.
Similar Questions
Connection of voltage transformer and current transformer across line are respectively.
Important Questions from Instrument Transformers
A 100 : 5 current transformer is used in conjunction with a 5 A ammeter. If the later reads 3.5 A, find the line current.
Up to a voltage of __________, Potential Transformers (PTs) are usually of dry type.
A potential transformer of 12000/112 V and current transformer of 80/6 A are used to measure voltage and current in a transmission line. If the voltmeter indicates 120 V and current reads 5 A, what is the voltage and current in this line?
The table lists two instrument transformers and their features:
Instrument Transformers Features P) Primary is connected in parallel to the grid X) Current Transformer (CT) Q) Open circuited secondary is not desirable Y) Potential Transformer (PT) R) Primary current is the line current S) Secondary burden affects the primary current The correct matching of the two columns is
In which of the following transformers, is the secondary winding always kept closed?
