What is the effect of current transformer in open circuit condition

May 23 , 2020

The current transformer, CT, has fewer primary winding turns. When using, the primary winding is connected in series in the circuit under test, and the secondary winding has more turns. It is used in series with the current coils of measuring instruments and relays. ,So the CT is close to short circuit during normal operation. The magnitude of the CT secondary current is determined by the primary current. The magnetic potential generated by the secondary current balances 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 is impossible to balance the magnetic potential generated by the primary current, then the primary current will all act on the excitation and make the iron core severely saturated. The magnetic saturation increases the iron loss, the CT generates heat, and the insulation of the CT coil is also burned due to overheating. It will also generate residual magnetism on the iron core, increasing the transformer error. The most serious is that due to magnetic saturation, the sine wave of the alternating magnetic flux becomes a trapezoidal wave. At the moment when the magnetic flux changes rapidly, a high voltage will be induced on the secondary coil, with a peak value of several thousand volts, so high The voltage on the secondary coil and the secondary circuit poses a serious threat to both people and equipment.

Therefore, CT is not allowed open circuit on the secondary side at any time.

So how do we find the CT open circuit failure, generally can be checked and judged from the following phenomena:

(1) The indication of the loop instrument is abnormal, which is generally reduced or zero. Opening the current loop for the meter will make the three-phase ammeter indication inconsistent, the power meter indication lower, and the meter speed slow or not. If the meter indicates that there is sometimes no, it may be in a semi-open state (poor contact).

(2) The CT body is free of noise, uneven vibration, severe fever, smoke, etc. Of course, these phenomena are not obvious when the load is small.

(3) CT secondary circuit terminals and component wire ends are discharged and sparked.

(4) Misoperation or refusal occurred in the relay, this situation can be found and dealt with when it is accidentally tripped or overstepped.

(5) The watt-hour meter, relay and other smoke are burnt out. The burnout of the reactive power meter and watt-hour meter, the transmitter of the remote control device, and the protection device will not only make the CT open circuit twice, but also make the PT secondary short circuit.

The above are just some basic clues to check the CT secondary open circuit. In fact, in normal operation, the primary load is not large, the secondary is not working, and it is not when the measurement current circuit is open, the secondary open circuit failure of the CT is not easy to find We need to explore and accumulate experience in our actual work.

Check and deal with the CT secondary open circuit fault, and 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 the insulation pad, wear insulated gloves, and use well-insulated tools.

(1) If the CT is opened twice, it is necessary to distinguish which group of current loop failures, the difference between the open circuits and whether they have an impact on the protection, report the dispatch, and release the protection that may be erroneously operated.

(2) Reduce the primary load current as much as possible. If the CT is seriously damaged, the load should be shifted and power off.

(3) As soon as possible, try to short-circuit the CT twice according to the drawing with good short wiring on the nearest test terminal, and then check the open point.

(4) 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 the short circuit, the short circuit may be invalid. The fault point may be in the circuit before the short circuit. You can change the short circuit forward point by point to narrow the scope of inspection.

(5) Within the scope of the failure, the terminals and components prone to failure should be checked. For the detected faults, those that can be dealt with by themselves, such as loose external terminals such as wiring terminals and poor contact, etc., are immediately treated and put into the protection of withdrawal. If the open point is on the terminal of the CT body, the power supply shall be cut off. If it cannot be handled by itself (such as inside the relay) or if the failure cannot be identified by itself, the CT should be short-circuited first and then reported to the superior.

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