Methods for handling secondary open-circuit faults in AC current transformers CT

Thread Content

This post was last edited by jennifer12580 on 2016-9-12 17:09. Changhui Instruments, yunrun.com.cn: The primary winding of the current transformer CT is connected in series with the circuit being tested, and it has a small number of turns； The secondary winding is used in series with current meters, current transmitters, power meters, pointer current meters, relays, multi-functional power meters, and other current coils, and it has a large number of turns. When operating normally, the CT is in a near-short-circuit state. The primary current value of the current transformer CT determines the secondary current value, and the magnetic flux of the secondary current balances that of the primary current. If the CT secondary is open-circuited, both the CT secondary current and magnetic flux are zero, and the impedance is infinite; as a result, the entire primary current is used for excitation, causing severe saturation of the core. Magnetic saturation causes an increase in the iron loss of the CT, resulting in heating; this in turn leads to damage to the insulation of the CT coil, and the core’s residual magnetism increases the error of the transformer ； At the same time, magnetic saturation changes the alternating magnetic flux from a sine wave to a trapezoidal wave. At the moments when the flux changes rapidly, a high voltage of several thousand volts is induced in the secondary coil. If such high voltages act on the secondary circuit and coil instantly, it can cause serious damage to equipment and pose a threat to human safety. It is strictly prohibited for current transformers CT to operate with an open secondary circuit! Symptoms of an open circuit in the secondary side of a current transformer CT: 1. The reading on the circuit ammeter decreases or becomes zero, indicating an abnormal condition. An open circuit in the current circuit used by the measuring instruments can cause the three-phase ammeters to show inconsistent readings, the power meter to indicate a lower value, and the meter to rotate slowly or not at all. If the measuring gauge indicates intermittently, there may be a poor connection. 2. The wattmeter, relays, etc. may smoke and get damaged; while the damage to the reactive power meter, wattmeter, AC current transducer, and protective device relays can not only cause an open circuit in the CT secondary side but also a short circuit in the PT secondary side. 3. There are discharge and arcing phenomena at the wire ends and terminals of the secondary circuit components of the current transformer CT. 4. Malfunction or failure of the relay protection; this situation is usually detected only in cases of accidental tripping or over-level tripping. 5. The CT unit exhibits issues such as uneven vibration, excessive heating, noise, and smoking. It should be noted that the phenomenon before load hours is not easily detectable. The above are the common phenomena of CT secondary open circuit. Changhui Automation advises you that when the primary load is low, the secondary side is not in operation, and it is not an open circuit in the current circuit used for measurement, the fault of a CT secondary open circuit is not easy to detect; therefore, it is necessary to gain experience through actual work. Methods for dealing with open-circuit faults in the secondary side of current transformers CT: When dealing with such faults, technicians should try to reduce the primary load current in order to lower the voltage in the secondary circuit ； When operating, one should stand on an insulating mat, wear insulating gloves, and use tools with good insulation to ensure personal safety. 1. If a CT open circuit is detected during inspection, first determine which current circuit is faulty, which phase is open, and whether it has an impact on the protection systems; subsequently, report the fault to the dispatching team promptly and deactivate any protection systems that might malfunction as a result. 2. As soon as possible, use a good shorting wire to short-circuit the CT secondary according to the diagram at the nearest test terminal, and then check for and address any open circuits. 3. Minimize the inrush current as much as possible. If the CT is severely damaged, the load should be shifted and power should be cut off. 4. An effective short circuit on the secondary side will generate sparks; the fault point should be located in the circuit downstream of the short-circuit point. If there is no spark when short-circuiting, the short circuit may be ineffective; the fault could lie in the circuit prior to the short-circuit point. The short-circuit point should be moved forward step by step to narrow down the area for inspection. 5. Within the fault range, the terminals and components prone to failure should be checked. For the detected faults that can be resolved on one’s own, such as loose external components like terminal blocks or poor connections, they should be fixed immediately before re-enabling the protection that had been disabled. If the open circuit is at the terminal of the CT body, the power supply must be shut down. If it cannot be handled by oneself (such as inside the relay) or if the fault cannot be identified on one’s own, the CT secondary side should first be short-circuited before reporting to superiors.