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How to select the transformation ratio and accuracy of key performance parameters of current transformer

How to select the transformation ratio and accuracy of key performance parameters of current transformer


Current transformer is a unique transformer. Its function is to change large current into standard small current, and cooperate with measuring instruments, metrological verification instruments, automobile relays and other machinery and equipment. In this way, it can expand the detection range of the instrument and improve the credibility and safety factor of the power supply circuit. The wiring circuit schematic diagram of the current transformer is shown in the figure. The primary solenoid coil of the current transformer is connected in series in the primary main power supply circuit, and the secondary connected instrument and the solenoid coil of the automobile relay are also connected in series.

The transformation ratio and accuracy are two key performance parameters when using current transformer.

1. Transformation ratio

The rated voltage at the secondary side of the current transformer is 5A or 1a, and 5A is generally selected. Common transformation ratios for accurate measurement of current transformer include 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. How to properly select the transformation ratio of current transformer?

The code for design of electrical measuring instruments of electric power installations requires that "the selection of the detection range of pointer measuring instruments should ensure that the rated current of power engineering is marked at 2 / 3 of the instrument scale." According to this standard, people can use the following formula to calculate and select the transformation ratio n of current transformer.

In the calculation of this formula, I is the large load current of the control circuit, 0.7 means that when the load current is large, the mark refers to 70% of the instrument panel, and 5 is the secondary rated value of the current transformer

electric current. Then, the transformation ratio of relative current transformer is selected according to the calculated transformation ratio. For example, I is 50a, n = 14.29 can be obtained according to the calculation method, so the current transformer with 75 / 5 transformation ratio is selected.

2. Accuracy

The accuracy of current transformer is also called precision, because the ratio deviation and angle deviation of current transformer are difficult to avoid. The accuracy of current transformer is equivalent to the percentage of deviation limit of this ratio in scale value. For example, the larger ratio deviation of a current transformer of class 0.5 is ± 0.5%. The current transformers for accurate measurement are 0.1, 0.2, 0.5, 1, 3 and 5 levels, and the current transformers for maintenance are 5p and 10p levels. In general, level 0.2 is selected for metrological verification, level 0.5 for accurate measurement and level 1 for general monitoring instruments.

In addition, when applying current transformer, people must pay attention to: the secondary side of current transformer must be grounded to avoid high voltage from the primary side to the secondary side, which will endanger the safety of machinery and equipment and life road; Pay attention to the optical rotation of the first and second sides of the current transformer. If the optical rotation is inserted incorrectly, the instrument will be ablated; The secondary circuit of current transformer shall not be led. It is forbidden to install circuit breaker at the secondary side to avoid high working voltage induced by the magnetic induction of the secondary side, which will endanger the road safety.

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