(1) The primary coil of a current transformer is connected in series within the circuit, and it typically has a small number of turns. As a result, the current flowing through the primary coil is directly determined by the load current of the circuit being measured, and it is not affected by the secondary current. This means that the primary current is entirely dependent on the actual current in the system under test.
(2) The secondary coil of the current transformer is connected to the current coils of measuring instruments and relays, which have very low impedance. Therefore, under normal operating conditions, the current transformer functions almost like a short-circuited device. This ensures that the secondary current remains proportional to the primary current, without causing excessive voltage buildup.
The ratio between the rated primary current and the rated secondary current of a current transformer is referred to as the rated transformation ratio, denoted as kn = I1n / I2n.
Since the rated primary current I1n is standardized, the secondary current I2n is usually set to a fixed value of 5 A, or sometimes 1 A or 0.5 A. This standardization makes the rated transformation ratio of the current transformer consistent across different applications. In practice, the rated transformation ratio can also be approximated as the turns ratio of the primary and secondary windings, expressed as kn ≈ kN = N1 / N2, where N1 and N2 represent the number of turns in the primary and secondary coils, respectively.
Understanding these principles is essential for correctly selecting and using current transformers in electrical systems, ensuring accurate measurements and safe operation. Proper design and installation help maintain the performance and reliability of the entire measurement and protection system.
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