Fault Detection and Handling Methods for Open-circuit and Short-circuit on the Secondary Side of Current Transformers

Due to the small impedance allowed in the secondary circuit of the current transformers during normal operation, it approaches a short circuit state with very little sound, leading to the common misconception of it being silent.

Faults in the current transformers often come with sound or other phenomena. If the screws that hold the silicon steel sheets in place in the iron core are not tight enough, the silicon steel sheets will loosen and the alternating magnetic flux in the iron core will change.

As the alternating magnetic flux in the iron core changes, the amplitude of the vibration of the silicon steel sheets increases, resulting in a buzzing sound. This sound does not change with load and will persist for a long time. At light or no load, the end of some separated laminated silicon steel sheets vibrate, forming a certain buzzing sound. This sound sometimes exists and sometimes doesn't, and disappears as the load on the circuit increases.

When the secondary circuit of the current transformers is open-circuited and the current is 0, the impedance is infinitely high and the secondary winding produces a high electromotive force, with a peak value of several thousand volts. As the secondary circuit is in a closed state during normal operation of the current transformers, the magnetic flux generated by the secondary side magnetic flux acts as a demagnetizing effect on the primary side magnetic flux.
When the secondary side of the CT current transformer is open-circuited, the demagnetizing magnetic flux disappears, and the magnetic flux inside the iron core increases, reaching a state of severe saturation. This generates a waveform with a flat-topped wave as the magnetic flux changes over time. As the induced electromotive force of the secondary winding is proportional to the magnetic flux change, it is apparent that the iron core may become overheated and burn out the CT current transformer. Due to the increase in magnetic flux density and the non-sinusoidal nature of the magnetic flux, the silicon steel sheets oscillate unevenly, generating a loud noise.
When the secondary side of the CT current transformer is open-circuited, the operator should wear insulated shoes and insulated gloves, short-circuit the experimental terminal of the secondary circuit of the faulty CT current transformer on the distribution cabinet, and stop checking and handling the issue. If the above measures are ineffective, it is considered that the current transformer may have an internal fault and should be discontinued from use. If the current transformer may cause maintenance and installation actions, relevant maintenance and installation should be stopped.
When the secondary winding or circuit of the CT current transformer shorts, it can cause the electric energy meter, power meter and other indicators to show zero or reduced values, while also causing relay protection to malfunction or not operate. If the operator does not detect this fault in time and continues to load under normal conditions, it will cause equipment damage due to overload. After such a fault occurs, the load should be kept unchanged and maintenance and installation that may cause malfunctions should be stopped, and repair personnel should be notified to eliminate the issue promptly.

If smoke or fire is detected inside the current transformers, it should be cut off with a circuit breaker and extinguished with sand or a fire extinguisher.

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