Method for checking and processing open circuit short-circuit fault on secondary side of current transformer

- Jul 08, 2020-

Since the secondary circuit of the current transformer is only allowed to have a small impedance, it is close to a short circuit state under normal working conditions, and the sound is extremely small, generally considered silent. The failure of current transformers is often accompanied by sound or other phenomena. If the iron core through screw is not tight, the silicon steel sheet will loosen and the alternating magnetic flux in the iron core will change. With the change of the alternating magnetic flux in the iron core, the vibration amplitude of the silicon steel sheet increases and a buzzing sound is produced. This sound does not change with the load and will be maintained for a long time. At light load or no load, some silicon steel sheets leaving the stack oscillate at the end, causing a certain amount of hum. This sound is sometimes absent and disappears as the line load increases.

When the open circuit current of the secondary circuit is 0, the impedance is infinite, the secondary winding generates a high electromotive force, and its peak value can reach several thousand volts. Because the secondary circuit is in a closed circuit state during normal operation of the current transformer, the secondary circuit The magnetic flux generated by the side magnetic potential demagnetizes the magnetic flux generated at the primary side. When the secondary side is open, the demagnetized magnetic flux disappears, causing the magnetic flux inside the iron core to increase and become in a severely saturated state. This is the change of magnetic flux with time, and the waveform is a flat-topped wave. Since the induced electromotive force of the secondary winding is proportional to the change of magnetic flux, it is obvious that it may cause the core to overheat and burn out the current transformer. Due to the increase of the magnetic flux density and the non-sinusoidal nature of the magnetic flux, the silicon steel sheet oscillates unevenly, which makes a large noise.

When the secondary side of the current transformer is open, the on-duty personnel should wear insulated shoes and gloves to short-circuit the test terminals of the secondary circuit of the accident current transformer on the distribution cabinet for inspection and treatment. If the above measures are invalid, it is considered that there may be a fault inside the current transformer, and it should be stopped. If the current transformer may cause the protection device to operate, the relevant protection device should be disabled.

When the secondary winding or circuit of the current transformer is short-circuited, it can make the indication of the energy meter and the power meter zero or reduce, and may also cause the relay protection device to malfunction or not operate. If the on-duty personnel do not find it in time and still load according to the normal situation, it will cause the equipment to be overloaded and damaged. After such a fault occurs, the load should be kept unchanged, the protection device that may malfunction may be disabled, and the maintenance personnel shall be notified to eliminate it quickly.

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