A voltage transformer is a transformer with an iron core. It is mainly composed of primary and secondary coils, iron cores and insulation. When a voltage U1 is applied to the primary winding, a magnetic flux φ is generated in the iron core. According to the law of electromagnetic induction, a secondary voltage U2 is generated in the secondary winding. By changing the number of turns of the primary or secondary winding, different ratios of primary voltage and secondary voltage can be generated, which can form voltage transformers with different ratios. The voltage transformer converts high voltage into low voltage proportionally, that is, 100V. The primary side of the voltage transformer is connected to the primary system, and the secondary side is connected to measuring instruments, relay protection, etc.; it is mainly electromagnetic (capacitive voltage transformer application Wide range), and other non-electromagnetic, such as electronic, photoelectric.
The principle of current transformer is based on the principle of electromagnetic induction. A current transformer consists of a closed iron core and windings. Its primary winding has a small number of turns, and it is connected in series in the line of the current that needs to be measured, so it often has all the current of the line flowing through it. When the transformer is working, its secondary loop is always closed, so the impedance of the series coil of the measuring instrument and the protection loop is very small, and the working state of the current transformer is close to a short circuit.
The main difference is that the working state is very different during normal operation, which is expressed as:
1) The secondary current transformer can be short-circuited, but not open; the secondary voltage transformer can be open, but not short-circuited;
2) Compared with the load on the secondary side, the primary internal impedance of the voltage transformer is so small that it can be ignored, and it can be considered that the voltage transformer is a voltage source; while the primary internal resistance of the current transformer is so large that it can be considered that It is a current source with infinite internal resistance.
3) The magnetic flux density of the voltage transformer is close to the saturation value when it is working normally, and the magnetic flux density decreases when the fault occurs; the magnetic flux density of the current transformer is very low when it is working normally, and the short-circuit current on the primary side becomes very large during the short circuit, which makes the magnetic flux density decrease. The flux density is greatly increased, sometimes far exceeding the saturation value.