Principle of current transformer

1.   Its working principle is the same as that of a transformer. and its basic structure is also an iron core and primary and secondary windings. The characteristic is that the capacity is small and relatively constant, and it is close to the no-load state during normal operation.
2.   The impedance of the current  transformer itself is very small. Once a short circuit occurs on the secondary side, the current will increase sharply and burn the coil. For this reason, the primary side of the current transformer is connect with a fuse, and the secondary side is reliably ground to prevent personal and equipment accidents cause by high potential to ground on the secondary side when the insulation between the primary and secondary sides is damage.
3.   Current transformers for measurement are generally make of a single-phase double coil structure. The primary voltage is the measur voltage (such as the line voltage of the power system). It can be use in a single phase, or two units can be connect in a V-V shape to form a three-phase use. current transformers use in laboratories often have multiple taps on the primary side to meet the needs of measuring different voltages. The current transformer use for protective grounding also has a third coil, which is call a three-coil current transformer. The third coil of the three phases is connect to form an open triangle, and the two terminals of the open triangle are connect to the voltage coil of the ground protection relay.
4.   During normal operation, the three-phase voltage of the power system is symmetrical, and the sum of the three-phase induced electromotive force on the third coil is zero. Once a single-phase grounding occurs and the neutral point is displac, a zero-sequence voltage will appear between the terminals of the open triangle to cause the relay to operate, thereby protecting the power system.
5.   When a zero-sequence voltage appears in the coil, a zero-sequence magnetic flux will appear in the corresponding core. For this reason, this three-phase voltage transformer uses a side-yoke core (10KV and below) or three single-phase current transformers. For this kind of transformer, the accuracy of the third coil is not high, but it is required to have certain overexcitation characteristics (that is, when the primary voltage increases, the magnetic flux density in the core also increases by a corresponding multiple without being damaged).
6.   Current transformer is an indispensable electrical appliance in power transmission and power supply systems such as power plants and substations. Precision current transformer is an instrument use in electrical testing laboratories to expand the measurement limit and measure voltage, power and electrical energy. Current transformers are very similar to transformers and are use to transform the voltage on the line.
7.   Why does voltage need to be converte on the line? This is because the voltage on the lines varies greatly depending on the conditions of power generation, transmission and consumption. Some are low voltage 220V and 380V, and some are high voltage tens of thousands or even hundreds of thousands of volts. To directly measure these low-voltage and high-voltage voltages, it is necessary to make corresponding low-voltage and high-voltage voltmeters and other instruments and relays according to the size of the line voltage. This will not only bring great difficulties to the production of instruments, but more importantly, it is impossible and absolutely not allowed to directly make high-voltage instruments and measure voltage directly on high-voltage lines.
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