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Explain in detail how to choose the key performance parameter transformation ratio and accuracy of the current transformer
The current transformer is a unique transformer. Its function is to change the large current volume into a standardized small current volume, and cooperate with measuring instruments, metrological verification instruments and automobile relays in the work of machinery and equipment. That can expand the detection range of the instrument and improve the credibility and safety factor of the power supply 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 automobile relay are all connected in series.
Usually, when using current transformers, the two performance parameters of transformation ratio and accuracy are the key considerations.
1. Turn ratio
The rated voltage of the secondary side of the current transformer is 5A or 1A, and 5A is generally selected. The 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., so how to properly choose the ratio of the current transformer?
The "Code for Design of Electrical Measuring Instruments for Power Installations" requires that "the selection of the detection range of pointer-type measuring instruments should ensure that the rated current of the power project is marked at 2/3 of the scale of the meter." According to this standard, people can use the following The formula calculates the transformation ratio N of the current transformer.
In the calculation of this formula, I is the larger load current of the control loop, 0.7 means that when the larger load current is labeled 70% of the dashboard, 5 is the secondary rated current of the current transformer. Then select the relative current transformer's transformation ratio according to the measured transformation ratio value. For example, I is 50A, and N=14.29 can be obtained according to the calculation method, so a current transformer with a conversion ratio of 75/5 is selected.
The accuracy of the current transformer is also called precision, because the ratio deviation and angular deviation of the current transformer are unavoidable. The accuracy of the current transformer is equivalent to the percentage of the ratio deviation limit on the scale. For example, the larger ratio deviation of a 0.5-level current transformer is ±0.5%. The current transformer for accurate measurement has 0.1, 0.2, 0.5, 1, 3, and 5 levels, and the maintenance current transformer has 5P and 10P levels. Under normal circumstances, select level 0.2 for metrological verification, level 0.5 for accurate measurement, and level 1 for general monitoring instruments.
In addition, people must pay attention to the application of current transformers: the secondary side of the current transformer must be grounded to prevent the primary side from pressing on the secondary side, which will threaten the safety of machinery and equipment and the road of life; pay attention to the first and second sides of the current transformer The instrument can be ablated under serious conditions of optical rotation and mis-insertion; the current transformer cannot lead the way at the second time. It is forbidden to install circuit breakers on the secondary side to avoid the high working voltage induced by the magnetic induction of the secondary side lead circuit, which will endanger the road safety of life.
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