The zero sequence current transformer is a line fault current monitor. Generally there is only one iron core and secondary winding. When in use, pass the primary three-core cable through the iron core window of the transformer, and connect the secondary lead to a dedicated relay, and then connect the output terminal of the relay to the signal device or alarm system. Under normal circumstances, the three-phase current in the primary circuit is basically balanced, and the resulting composite magnetic flux is also close to zero. No current is induced in the secondary winding of the transformer. When a single-phase ground fault occurs in the primary circuit, an unbalanced current (meaning zero-sequence current) is generated in the primary circuit, and a tiny current is induced in the secondary winding. The relay is activated and a signal is generated. The current that makes the relay operate is very small (mA level), which is called the sensitivity of the secondary current or zero-sequence current transformer (it can also be expressed by a small operating current), which is the main operation index.
The current transformer located inside the switch in the 10kV feeder switch cabinet is generally divided into two-phase or three-phase depending on the wiring method. The current transformer is made of primary windings (L1, L2), secondary windings, iron cores and casted with silicon rubber.
Current transformer is to convert the large current of the primary equipment into the small current used by the secondary equipment. Its working principle is equivalent to a transformer with very small impedance. The primary winding is connected in series with the primary main circuit, and the secondary winding is connected to the load. The transformation ratio of the current transformer is generally X/5A or X/1A (X is not less than the long-term load current that may occur in the device), so that the secondary side current of the current transformer is not greater than 5A or 1A.
In power plants and substations, if the high-voltage power distribution device is far away from the control room, in order to increase the secondary allowable load of the current transformer, reduce the wire interface of the connecting cable and improve the accuracy level, use a current transformer with a secondary rated current of 1A. . Correspondingly, the microcomputer protection device should also select products with an AC current input of 1A.
In substations, current transformers are used in three types of circuits: microcomputer protection, measurement and metering, and these three types of circuits have different requirements for the accuracy of current transformers. According to the different accuracy levels, the windings of the current transformer can be divided into 10P10 (protection), 0.5 (measurement) and 0.2S (measurement). The windings used for measurement and metering focus on accuracy, and the windings used for protection focus on capacity to avoid iron core saturation from affecting the actual transformation ratio.
There are three commonly used connections for the secondary windings of current transformers, complete star connection, incomplete star connection and delta connection.https://www.ctsensorducer.com/