How to select the transformation ratio and accuracy of main technical parameters of current transformerCurrent transformer is a special transformer. Its function is to change large current into standard small current and work with measuring instruments, measuring instruments, relays and other equipment. This can expand the measuring range of the instrument and improve the reliability and safety of the circuit. The wiring schematic diagram of current transformer is shown in the figure below. The primary coil of current transformer is connected in series in the primary main circuit, and the coils of secondary connected instruments and relays are also connected in series.The selection of current transformer parameters is mainly based on the following six points:
1. Rated primary voltage - determined by the nominal voltage of the system. High voltage current transformer can be used in low voltage system, such as 10kV current transformer in 6kV system.2. Rated primary current - selected according to the rated current level specified in gb1208. If the primary current cannot be selected according to the specified levels, the following methods can be used: When the transformation ratio requirements of protection circuit and measurement circuit are different, the secondary winding current transformer with tap can be used. The current transformer with primary tap can also be changed. Generally, it can be connected in series and parallel, and the current transformer with multiple transformation ratio or half transformation ratio can be obtained. The current transformer for measuring circuit is of class s for special purposes, and it shall maintain the accuracy requirements within the range of 10 110% of the rated current.
3. Rated secondary current - 1A and 5A.Selection principle: for all conditions of new power plant and substation, 1A should be selected. If the original TA is 5A in favor of transformer installation or expansion project, and in some cases to reduce the secondary open circuit voltage of TA, the rated secondary current can be 5A. For one plant, the rated secondary current in the station can be 1a and 5A at the same time.4. The accuracy level and transient characteristics are described in the following topics.
5. Number of iron cores - there are two types of current transformer cores: one is the single core type in which a current transformer has only one primary winding and secondary winding, and most low-voltage current transformers are this type; One is the multi core type with one primary winding and two or more secondary windings. Each secondary winding is configured according to different purposes. On the premise of meeting the accuracy level, the electric energy metering instrument and measuring meter can share a secondary winding.6. According to the structure, it can be divided into oil immersed, resin cast and SF6 Current transformers.At ordinary times, we mainly consider the transformation ratio and accuracy when selecting current transformer.
1. Transformation ratioThe rated current at the secondary side of the current transformer is 5A or 1a, and 5A is generally selected. The transformation ratios commonly used for measuring 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 correctly select the transformation ratio of current transformers?Code for design of electrical measuring instruments for power installations stipulates that "the selection of measuring range of pointer measuring instruments should ensure that the rated value of power equipment is indicated at 2 / 3 of the instrument scale." according to this specification, we can use the following formula to select the transformation ratio n of current transformer.
In this formula, I is the maximum load current of the circuit, 0.7 means that the indication at the maximum load current refers to 70% of the instrument panel, and 5 is the secondary rated current of the current transformerCurrent value. Then the transformation ratio of the corresponding current transformer is selected according to the calculated transformation ratio. For example, I is 50a, and N = 14.29 can be obtained according to the calculation formula, so the current transformer with 75 / 5 transformation ratio is selected.2. Accuracy
The accuracy of current transformer is also called accuracy. Due to the ratio error and angle error of current transformer, it is inevitable. The accuracy of current transformer is numerically equal to the percentage of the error limit of this ratio. For example, the maximum ratio error of current transformer of class 0.5 is 0.5%. The measuring current transformer has grades 0.1, 0.2, 0.5, 1, 3 and 5, and the current transformer for protection has grades 5p and 10p. Generally, level 0.2 is selected for metering, level 0.5 for measurement and level 1 for general monitoring instruments.
At the same time, we need to pay attention when using current transformer: the secondary side of current transformer must be grounded to prevent high voltage from channeling from the primary side to the secondary side, which will endanger the safety of equipment and life; Pay attention to the polarity of the first and second sides of the current transformer, and burn the instrument in case of wrong polarity; The secondary of current transformer shall not be open circuit. It is strictly prohibited to install fuse on the secondary side to prevent high voltage induced by open circuit on the secondary side, which will endanger life safety.
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