When measuring the large current of alternating current, the current transformer needs to be converted into a relatively uniform current for the convenience of secondary instrument measurement (the secondary rating of current transformer is 5A in China, and the secondary rating of current transformer is 5A in China). In addition, the voltage on the line is relatively high, so it is very dangerous to measure directly. Current transformer plays the role of current conversion and electrical isolation It is a sensor for measuring instruments, relay protection and other secondary equipment in the power system to obtain the current information of the electrical primary circuit. The current transformer converts the high current into the low current in proportion. The primary side of the current transformer is connected to the primary system, and the secondary side is connected with the measuring instrument and relay protection. There are only 1 to several turns on the primary side, and the cross-sectional area of the wire is large, and it is connected into the circuit under test in series. The secondary side turns are large and the wire is thin, which forms a closed circuit with the instrument with small impedance (current coil of Ammeter / power meter). The operation condition of current transformer is equivalent to that of transformer with short circuit at secondary side. Generally, low magnetic density (0.08-0.1t) is selected and excitation current is ignored, then I1 / I2 = N2 / N1 = K. The current ratio of primary winding current I1 to secondary winding I2 of current transformer is called actual current ratio. Excitation current is the main source of error. 0.2/0.5/1/3, 1 means the error of transformation ratio is less than 1%.
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