Common faults of current transformers

Current transformers are used in various industries and fields. Once a fault occurs, it will cause serious losses. The most likely fault in current sensors is a short circuit of the current transformer. We know that the primary winding of the current transformer, or CT, has a small number of turns. When in use, the primary winding is connected in series in the measured circuit. The secondary winding has a large number of turns and is used in series with current coils such as measuring instruments and relays. The impedance of the current coils of measuring instruments and relays is very small, so the CT is close to a short circuit state during normal operation.

The magnitude of the secondary current of CT is determined by the primary current. The magnetic potential generated by the secondary current is to balance the magnetic potential of the primary current. If the secondary circuit is open, its impedance is infinite, the secondary current is equal to zero, and its magnetic potential is also equal to zero. It cannot balance the magnetic potential generated by the primary current, and then the primary current will act entirely on the excitation, causing the iron core to be severely saturated. Magnetic saturation increases iron loss, CT heats up, and the insulation of the CT coil will also be burned due to overheating. Residual magnetism will also be generated on the iron core, increasing the mutual inductor error. The most serious is that due to magnetic saturation, the sinusoidal wave of the alternating magnetic flux becomes a trapezoidal wave. At the moment when the magnetic flux changes rapidly, a very high voltage will be induced on the secondary coil, and its peak value can reach several thousand volts. Such a high voltage acts on the secondary coil and the secondary circuit, posing a serious threat to both personnel and equipment. Therefore, CT is not allowed to operate with the secondary side open at any time.

So how do we find the CT secondary open circuit fault? Generally, we can check and judge from the following phenomena:
(1) The loop instrument indication is abnormal, usually reduced or zero. The current loop used to measure the meter is open, which will cause the three-phase ammeter indication to be inconsistent, the power meter indication to be reduced, and the meter speed to be slow or not rotating. If the meter indication is sometimes there and sometimes not, it may be in a semi-open circuit state (poor contact).
(2) Whether the CT body has noise, uneven vibration, severe heating, smoke, etc., of course, these phenomena are not obvious when the load is small.
(3) There is discharge and sparking at the CT secondary circuit terminals and component wire ends.
(4) The relay protection has malfunction or refusal to operate. This situation can be discovered and handled when it is tripped or tripped by mistake.
(5) The watt-hour meter, relay, etc. smokes and burns out. If there are reactive power meters and watt-hour meters, transmitters of remote control devices, and relays of protection devices burn out, it will not only cause the CT secondary circuit to be open, but also cause the PT secondary circuit to be short-circuited.
The above are just some basic clues for checking CT secondary open circuit. In fact, in normal operation, when the primary load is not large, the secondary circuit is not working, and the current circuit for measurement is not open, the secondary open circuit fault of CT is not easy to find. We need to explore and accumulate experience in actual work.
When checking and handling CT secondary open circuit fault, we should try to reduce the primary load current as much as possible to reduce the voltage of the secondary circuit. Pay attention to safety during operation, stand on insulating mats, wear insulating gloves, and use well-insulated tools.
(1) When the CT secondary open circuit is found, we should first distinguish which group of current circuits are faulty, the phase difference of the open circuit, and whether it affects the protection, report to the dispatcher, and release the protection that may be malfunctioning.
(2) Try to reduce the primary load current. If the CT is seriously damaged, the load should be transferred and the power should be cut off.
(3) Try to use a good short-circuit wire to short-circuit the CT secondary at the nearest test terminal as soon as possible according to the drawing, and then check and handle the open circuit point.
(4) In the fault range, the terminals and components that are prone to failure should be checked. For the faults found, if they can be handled by themselves, such as loose external components such as wiring terminals, poor contact, etc., they should be handled immediately and the protection that has been withdrawn should be put into operation. If the open circuit point is on the wiring terminal of the CT body, the power should be turned off for processing. If they cannot be handled by themselves (such as inside the relay) or the fault cannot be found by themselves, the CT secondary should be short-circuited first and then reported to the superior.
(5) If sparks are found during short-circuiting, the short-circuiting should be effective, and the fault point should be in the circuit below the short-circuiting point, which can be further searched. If there is no spark during short-circuiting, the short-circuiting may be ineffective, and the fault point may be in the circuit before the short-circuiting point. The short-circuiting points can be changed point by point to narrow the range of inspection.