Current transformer principle, type, accuracy and transformer type selection

Current transformer is an important secondary device in the power system, widely used in metering, measurement, relay protection and other secondary circuits, in the high current or high voltage occasions we can not directly use the ammeter to measure the current size of the circuit, only through the secondary side of the current transformer to measure, so that it will be safe, then for the parameters of the current transformer, the 0.5 level, 1.0 level, as well as 10P20, 5P20 what does it mean? How do we choose to use it? The following will be carefully explained:

　　First, the principle of the current transformer

　　Current transformer and transformer is based on the principle of electromagnetic induction will be the primary side of the large current into the secondary side of the small current to measure the instrument. Current transformer is composed of a closed core and winding. The primary winding has a small number of turns and is strung in the line of the current to be measured. The secondary side of the winding turns is more, series in the measuring instrument and protection circuit, the current transformer in operation, its secondary side of the circuit is always closed, so the measuring instrument and protection circuit series coil impedance is very small, the current transformer's operating condition is close to a short circuit.

　　Types of current transformers

　　According to the use can be divided into:

　　Measuring current transformer: settlement of electricity costs

　　Measurement current transformer: measuring current, calculate the degree of electricity, etc., the accuracy is generally lower than the machine with two transformers, not as a settlement.

　　Protection current transformer: current speed protection, overcurrent protection, overload protection, etc.;

　　1. calibration current transformer accuracy: 0.1S level. Tolerance of 0.1%, commonly used in calibration measurement level current transformer accuracy. 

　　2. measurement of current transformer accuracy: 0.2S 0.5 level. Tolerance of 0.2% and 0.5%, used for electricity billing basis, some occasions will also use the 0.5-level

　　3. Measuring current transformers: 0.5, 1.0, 2.0, etc., generally used for ammeters.

　　4. Protection of current transformer accuracy: 10P10, 10P20, 5P10, 5P20, etc., the meaning of accuracy: 10P10, for example, that is, the current flowing through the current transformer, is within 10 times its rated current, the inductor error within ± 10%.

　　5. In some special occasions, there are more accurate current transformers, there are 0.005 level, 0.05 level, etc., the use of occasions less.

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