Explain in detail how to select the key performance parameters of current transformers, the transformation ratio and accuracy -

The current transformer is a special transformer. Its function is to change the large current into a standard small current, and cooperate with measuring instruments, measuring instruments and relays and other equipment to work. This can play a role in expanding the detection range of the meter and improving the reliability and safety of the 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 supply circuit, and the secondary connected instrument and the electromagnetic coil of the automobile relay are also connected in series. Generally, when people use current transformers, they mainly consider the two performance parameters of transformation ratio and accuracy. 1. The rated voltage value of the secondary side of the variable ratio current transformer is 5A or 1A, and 5A is generally selected. The common transformation ratios of current transformers for accurate measurement 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 choose the transformation ratio of the current transformer properly? 'Specifications for the 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 electric power engineering is marked at 2/3 of the scale of the instrument.' According to this standard, people can use the following The formula is calculated to select the transformation ratio N of the current transformer. In the calculation of this formula, I is the maximum load current of the control circuit, 0.7 means that when the maximum load current is marked at 70% of the instrument panel, 5 is the secondary rated current of the current transformer. Then select the transformation ratio of the relative current transformer 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 choose a current transformer with a transformation ratio of 75/5. 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 the current transformer is equivalent to a percentage of the deviation limit of this ratio on the nominal value. For example, the maximum ratio deviation of a current transformer of class 0.5 is ±0.5%. There are 0.1, 0.2, 0.5, 1, 3, and 5 grades of accurate measurement current transformers, and 5P and 10P grades of current transformers for maintenance. Under normal circumstances, level 0.2 is selected for metrological verification, level 0.5 is selected for precision measurement, and level 1 is selected for general monitoring instruments. At the same time, we must pay attention to the use of current transformers: the secondary side of the current transformer must be grounded to prevent the high voltage on the primary side from entering the secondary side, which will affect the safety of equipment and life; pay attention to the first and second sides of the current transformer The optical activity of the instrument is ablated under serious conditions of optical rotation and mis-insertion; the secondary current transformer cannot lead the circuit, and it is forbidden to install a circuit breaker on the secondary side to avoid high voltage induced by the secondary side leading circuit, which endangers the safety of human life.