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How to choose the key performance parameter transformation ratio and accuracy of the current transformer

by:Transmart     2022-05-06

The current transformer is a unique transformer, its function is to change the large current into the standard small current, and cooperate with the measuring instrument, the measuring instrument and the automobile relay and other machinery and equipment. This can expand the detection range of the instrument and improve the reliability and safety factor of the power circuit. The schematic diagram of the wiring circuit of the current transformer is shown in the figure. The primary electromagnetic coil of the current transformer is connected in series in the primary main power circuit, and the secondary connected instrument and the electromagnetic coil of the automotive relay are also connected in series. Usually, when people use current transformers, the two performance parameters of transformation ratio and accuracy are mainly considered.

1. The rated voltage value of the secondary side of the variable ratio current transformer is 5A or 1A, and 5A is selected under normal conditions. Common transformation ratios for accurate measurement of current transformers are 5/5, 10/5, 15/5, 20/5, 25/5, 30/5, 40/5, 50/5, 75/5, 100/5, 150/5, 200/5, 250/5, 300/5, 400/5, 500/5, 600/5, 750/5, 800/5, etc., then how to properly choose the current transformer ratio? 'Code for Design of Electrical Measuring Instruments for Electric Power Installations' requires that 'the selection of the detection range of pointer measuring instruments should ensure that the rated current of the power engineering is marked at 2/3 of the scale of the instrument.' According to this standard, people can use the following The formula calculates the transformation ratio N of the selected current transformer. In this formula calculation, I is the larger load current of the control loop, 0.7 means that the larger load current is indicated at 70% of the instrument panel, and 5 is the secondary rated current of the current transformer. Then select the relative current transformer transformation ratio according to the calculated transformation ratio value. For example, I is 50A, and Nu003d14.29 can be obtained according to the calculation method, so a current transformer with a ratio of 75/5 is selected.

2. Accuracy The accuracy of the current transformer is also called the precision, because the ratio deviation and angular deviation of the current transformer are unavoidable. The accuracy of the current transformer is scaled as a percentage of the deviation limit of this ratio. For example, the maximum ratio deviation of a class 0.5 current transformer is ±0.5%. There are 0.1, 0.2, 0.5, 1, 3, and 5 grades for accurate measurement of current transformers, and 5P and 10P grades for maintenance current transformers. Under normal circumstances, choose class 0.2 for metrological verification, class 0.5 for precise measurement, and class 1 for general monitoring instruments. In addition, people must pay attention to the use of current transformers: the secondary side of the current transformer must be grounded to avoid the high voltage on the primary side and the secondary side, which will endanger the safety of machinery and equipment and life and roads; pay attention to the first and second sides of the current transformer The optical rotation of the instrument is relatively serious, and the optical rotation is incorrectly inserted into the instrument under serious conditions; the secondary current transformer cannot lead the way, and it is forbidden to install a circuit breaker on the secondary side, so as to avoid the magnetic induction of high working voltage on the secondary side, which will endanger the safety of life and road.
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