Protective current transformers are generally used in relay protection circuits where multiple busbars cross. Developed to detect short-circuit faults for the protection system, they come with different accuracy levels and accuracy limit factors, and can be extended to accommodate different hole sizes. They are widely used in low-voltage distribution protection systems. They can also be used to collect low-voltage overload and short-circuit signals and are used in conjunction with protective relays.
When measuring high currents of alternating current, current transformers (CTs) need to convert them into a relatively uniform current for the convenience of secondary instrument measurements (in China, the secondary rating of CTs is specified as 5A or 1A). Whether CTs can correctly transmit primary current during the short-circuit transition process plays a decisive role in the correct operation of relay protection. [1]
The protective current transformer (TA) primarily works in conjunction with relay protection devices. When faults such as short circuits occur in the line, it provides a signal to the relay device to cut off the faulty circuit, thereby protecting the safety of the power system. [2]
Protective current transformer: low voltage protective current transformer. Primary measurement range: 200-6300A, secondary output: 5A, 1A. Main accuracy levels include: 5P10, 10P10, 10P20, 5P20, etc. Collects low-voltage overload and short-circuit signals, used in conjunction with motor protection units. Main current ratios include 250A/50mA and 800A/100mA.
The working principle of a current transformer is illustrated in Figure 1. The primary winding of the current transformer is connected in series with the line under test. I1 represents the line current, which is also the primary current of the current transformer. N1 denotes the number of primary turns of the current transformer. I2 represents the secondary current of the current transformer (typically 5A or 1A), and N2 denotes the number of secondary turns of the current transformer. Z2e represents the impedance of the secondary circuit equipment and connecting wires. When the primary current flows into the current transformer from terminal P1 and out from terminal P2, and the secondary Z2e is connected, according to the principle of electromagnetic induction, a current I2 flows through the secondary winding of the current transformer from S1, through Z2e to S2, forming a closed loop. From this, it can be derived that under ideal conditions, I1×N1=I2×N2, thus I1/I2=N2/N1=K, where K is the transformation ratio of the current transformer.
