Current Transformers (CT)

Identify the primary winding, iron core, and secondary winding, and explain how the primary carries load or fault currents and the core uses laminations, with the burden on the secondary.

Explore the technical specifications of current transformers, including rated voltage, primary and secondary currents, turns ratio, and accuracy class, and how magnetizing current affects protection and measurement devices.

read and understand the nameplate of an 11 kV current transformer, including 12 kV max continuous, 28 kV power-frequency withstand, 75 kV lightning withstand, and dual primary and secondary windings.

Explore the various symbols used for current transformers in electrical drawings, identify primary and secondary windings, cores, and winding counts, and distinguish symbols with shared versus separate cores.

Explore the three main current transformer types: window or ring type, bar type, and wound type, including their configurations, primary and secondary windings, and turns-ratio options for measurement and protection.

Connect measuring and protective devices to the secondary terminals of current transformers at 220 and 66 kilovolt, using 600 to 5 amperes turns and burdens of 30 and 45 volt-amperes.

Explain how 1 ampere and 5 ampere secondary currents in current transformers affect voltage drop in the secondary circuit wires, especially when distances exceed 30 m between transformers and devices.

Explains current transformer saturation, core limits on secondary current, the knee point voltage VK, the saturation curve, and how burden and accuracy limit factors govern safe operation.

Learn why an open secondary winding circuit may explode a current transformer and how the secondary voltage can rise dramatically. Short-circuit the secondary before removing protective devices to prevent damage.

Compare current transformers, voltage transformers, and power transformers as instrument transformers; CTs and VTs use variable flux, while power transformers use fixed flux to keep secondary voltage stable.

Learn how to conduct CT tests by recording nameplate data—rated voltage, primary and secondary currents, and turns ratio tests—and perform winding resistance, insulation, polarity, and saturation tests.

Conduct winding resistance tests on current transformers with a mega device to measure primary and secondary resistance in ohms, ensuring coil continuity and detecting secondary short circuits.

Learn how to perform an insulation test on a current transformer using a mega device, including disconnecting devices, shorting windings, and measuring insulation resistance between primary, secondary, and earth.

Test current transformer polarity by applying a dc source to the primary and using a dc voltmeter on the secondary; a positive movement confirms correct polarity.

Learn how to perform the turns ratio test on a current transformer by injecting the primary winding with rated current and measuring the secondary current to determine the turns ratio.

Perform saturation test on the current transformer by open-circuit primary, applying AC source with variac, and measuring with voltmeter and ammeter to locate NE voltage and plot current versus voltage.

Analyze how to select a suitable current transformer for measuring or protective devices. Calculate the secondary current and voltage under short-circuit conditions and compare to knee point and saturation voltages.

Explore protective devices on the secondary of current transformers, including overcurrent, distance, differential, and directional relays, and learn how distance protection uses current and voltage signals to locate faults.

Describe how current transformers feed the secondary-side measuring devices, including ammeters, watt meters, volt-ampere meters, power factor meters, and energy meters, using current, voltage, and phi signals.

Explore the critical side problems of current transformers and how to avoid them by ensuring measuring and protective devices match the CT’s turns ratio, ensuring accurate readings.