Ⅰ、The function of the transformer
The current transformer converts the current in the high-voltage system or the large current in the low-voltage system into the standard small current (5A or 1A) of the low-voltage system; A voltage transformer converts the high voltage of the system into a standard low voltage (100V or 100/√ 3V). The function of a transformer is to:
1. Cooperate with electrical instruments to measure the current, voltage, and energy of circuits and equipment;
2. Cooperate with relay protection and automatic devices to protect the system and equipment from overcurrent, overvoltage, overload, and single-phase grounding;
3. Electrically isolate the secondary equipment and personnel from the primary high voltage, and have a grounding point on the secondary side to ensure the safety of the equipment and personnel;
4. Transforming current and voltage into unified standard values to facilitate standardization of instruments and relays;
5. Connect and control the secondary equipment with low voltage and low current for centralized control;
6. Make the secondary circuit not limited by the primary system, thus simplifying the wiring.
Ⅱ、Working principle of transformer
⒈current transformer
Current transformers operate according to the principle of electromagnetic induction, with very few turns in the primary winding and are connected in series in the tested circuit. The secondary winding has a large number of turns and is connected in series with the current coils of measuring instruments and relays. The current in the primary winding during operation depends on the load current of the line and is independent of the secondary load. Due to the small impedance of the current coils of the measuring instruments and relays connected to the secondary winding, the current transformer approaches a short-circuit state during normal operation, equivalent to a transformer operating in a short-circuit.
⒉voltage transformer
Voltage transformers work using the principle of electromagnetic induction, with a large number of turns in the primary winding and a small number of turns in the secondary winding. When used, the primary winding is connected in parallel with the measured circuit, and the secondary winding is connected in parallel with voltage coils such as measuring instruments or relays. Due to the high impedance of voltage coils such as measuring instruments and relays, a voltage transformer is equivalent to a no-load step-down transformer in normal operation. Its secondary voltage is basically equal to the secondary electromotive force value and depends on a constant primary voltage value. Therefore, the voltage transformer can accurately measure the primary voltage within the allowable load range of accuracy.