What is the current transformer?

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Option A: Transformer used with an A.C. ammeter

Explanation & Concepts:
A Current Transformer (CT) is an instrument transformer used to step down high currents to a standardized, measurable level for A.C. ammeters. It isolates the measuring instrument from high-voltage circuits and ensures safety during monitoring.

Working Principle: A CT operates on the principle of electromagnetic induction, similar to a power transformer but designed to maintain a nearly constant ratio between primary and secondary currents. Since it is connected in series with the line, the primary winding carries the full load current, while the secondary winding delivers a proportional current (typically 1A or 5A) to the ammeter.

Key Formulas: $I_s = I_p 	imes \frac{N_p}{N_s}$ — where $N_p, N_s$ are number of turns and $I_p, I_s$ are primary/secondary currents; $CT \ Ratio = \frac{I_{primary(rated)}}{I_{secondary(rated)}}$ — standard definition of transformer ratio

Key Points: CT secondary must never be kept open-circuited while primary is energized to prevent high-voltage core saturation.; The secondary current is typically standardized to 5A or 1A.; Used extensively in power grids for protection relays and metering.; The primary winding consists of very few turns, often just a single bar.

Advantages: Isolation of measuring instruments from high-voltage primary circuits.; Allows the use of low-current standard ammeters for high-current measurement.

Disadvantages: Secondary open-circuit leads to dangerously high induced voltages causing core damage.; Phase shift between primary and secondary current causes minor measurement errors.

Applications: Electrical energy metering in substations.; Overcurrent protection systems for transmission lines and motors.

Additional Info: Options B and D are incorrect because transformers work on the principle of mutual induction, which requires a time-varying magnetic flux produced by A.C., making them incompatible with pure D.C. circuits.. Option C is incorrect as potential transformers (PT) are used for voltage measurement, not current transformers.

Answer: A is correct — A current transformer is specifically designed to step down high primary A.C. currents for measurement by standard low-range A.C. ammeters.

Exam Tip: Always remember: Current Transformers are connected in series with the load, while Potential Transformers are connected in parallel.

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Option B: Transformer used with a D.C. ammeter

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Option C: Transformer used with an A.C. voltmeter

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Option D: Transformer used with a D.C. voltmeter

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