Detailed explanation of how to select the key performance parameters of current transformer: ratio and accuracy

The current transformer is a special transformer. Its function is to convert large current into a small standard current, and it works in conjunction with equipment such as measuring instruments, calibration instruments, and relays. This can have the effect of expanding the detection range of the instrument and improving the reliability and safety of the power circuit. The schematic diagram of the wiring circuit of the current transformer is as shown in the figure. The primary electromagnetic coil of the current transformer is connected in series in the primary main power circuit, and the electromagnetic coils of its secondary connected instruments and automobile relays are also connected in series.

Usually when people use current transformers, they mainly consider the two performance parameters of transformation ratio and accuracy.

1. Transformation ratio

The rated voltage value of the secondary side of the current transformer is 5A or 1A. Under normal circumstances, 5A is selected. 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 appropriately choose the transformation ratio of the current transformer?

The "Design Specifications for Electrical Measuring Instruments in Electrical Power Installations" requires that "the selection of the detection range of pointer measuring instruments should ensure that the rated current of the power project is marked at 2/3 of the instrument scale." According to this standard, people can use the following The formula is used to calculate the transformation ratio N of the selected current transformer.

In this formula calculation, I is the maximum load current of the control loop, 0.7 means that the maximum load current is indicated at 70% of the instrument panel, and 5 is the secondary rating of the current transformer.

current. Then select the transformation ratio of the relative current transformer based on the measured transformation ratio value. For example, if I is 50A, N=14.29 can be obtained according to the calculation method, so a current transformer with a 75/5 ratio is selected.

2. Accuracy

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 a current transformer is calibrated 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 levels of current transformers for accurate measurement, and 5P and 10P level two current transformers for maintenance. Under normal circumstances, choose level 0.2 for metrological verification, choose level 0.5 for precision measurement, and choose level 1 for general monitoring instruments.

In addition, people must pay attention when using current transformers: the secondary side of the current transformer must be grounded to prevent high voltage from the primary side from flowing into the secondary side, which may endanger the safety of equipment and life; pay attention to the first and second sides of the current transformer Optical rotation, optical rotation will ablate the instrument in serious cases; the secondary side of the current transformer cannot be leaded, and it is forbidden to install a circuit breaker on the secondary side to prevent the secondary side lead from magnetic induction of high voltage, endangering life and road safety.