Current transformers (BI, CT, TI) or current transformers are electrical devices used to convert high-value and high-voltage currents to currents with standard values of 5A or 1A, safe voltage providing for measurement, control and protection circuits.
The main working mode of the current transformer is the short circuit mode (the secondary winding is always connected to an ammeter or extremely small impedance). In the open-circuit secondary winding field, the entire primary current is responsible for magnetizing the steel core, the inductance Bm increases suddenly, causing saturation of the magnetic circuit. When the primary current passes zero, the induced emf in the secondary coil of the current transformer will have a sharp peak with a very large amplitude. The voltage on the two ends of the open-circuit secondary winding will increase very high, causing damage to equipment and danger to people.
To further understand and clarify the danger of secondary open circuit during equipment operation and to improve knowledge for employees. ACEE’s Technical Department organized an experiment to measure voltage on the secondary winding of current transformers when open circuit for two types of current transformers 35kV with the ratio 200-400-600/1 A and 24kV billion current transformers. number 1000-1500-2500/1 A at the Company’s Laboratory.
Through the above test, the staff of the ACEE Technical Department has summarized and made the following assessment and lessons learned:
– For current transformers with larger current-transform ratio, the secondary winding open-circuit voltage is larger with the same primary current value.
– With the same primary current value, the open-circuit voltage on the protective coil will be much larger than that of the measuring coil.
– The primary current value only needs to reach 50% of the rated current for the measuring coil or 25-30% of the rated current for the protective coil, the peak voltage on the secondary winding has reached 4 ,5kV.
– When the secondary winding is open-circuited for current transformers with a large ratio like 2500/1 at the primary current value equal to the rated current, it will be extremely dangerous, there is a discharge between the two ends of the secondary winding or discharge with the current transformer case and cause the current transformer to be damaged.