How to choose the ratio and accuracy of main technical parameters of current transformer
Current transformer is a kind of special transformer. Its function is to change large current into standard small current and cooperate with measuring instruments, measuring instruments, relays and other equipment. In this way, the measuring range of the instrument can be expanded and the reliability and safety of the circuit can be improved. The wiring schematic diagram of the current transformer is shown in the following figure. The primary coil of the current transformer is connected in series in the primary main circuit, and the coils of the secondary instruments and relays are all connected in series.
The selection of current transformer parameters is mainly based on the following six points:
- 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.
- Rated primary current: selected according to the rated current level specified in gb1208. If the primary current cannot be selected according to these grades, the following methods can be used:
① When the ratio requirements of the protection circuit and the measuring circuit are different, the secondary winding with tap current transformer can be used. It can also change the current transformer of primary tap, generally divided into series connection and parallel connection, and can obtain multiple ratio or half ratio current transformer. ② If the current transformer for measuring circuit has special purpose of S-class, it shall maintain the accuracy requirements within the range of 10-110% of rated current.
- Rated secondary current: 1A and 5A.
Selection principle: ① 1A should be selected for all conditions of new power plant and substation. ② If the original TA of transformer installation or expansion project is 5a, and in some cases to reduce the secondary open circuit voltage of TA, the rated secondary current can be 5A. ③ In one plant, 1a and 5A can be selected for the rated secondary current in the station at the same time.
- The accuracy level and transient characteristics are described in the following topics.
- Number of cores – there are two types of cores of current transformer: one is single core type with only one primary winding and secondary winding for a current transformer, and most of low-voltage current transformers are of this type; the other is multi core type with two or more secondary windings for a primary winding, and each secondary winding is configured according to different purposes. Under the premise of meeting the accuracy level, the electric energy measuring instrument and the measuring meter can share a secondary winding.
- According to the structure, it can be divided into oil immersed, resin cast and SF6 Current transformers.
At ordinary times, we mainly consider the two technical parameters of transformation ratio and accuracy when selecting current transformer.
1, variable ratio
The rated current of secondary side of current transformer is 5A or 1a, and 5A is generally selected. The commonly used transformation ratios 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 choose the transformation ratio of current transformers correctly?
According to the code for design of electric measuring instrument device of electric power device, “the selection of measuring range of pointer type measuring instrument should ensure that the rated value of electric 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 when the maximum load current is indicated at 70% of the instrument panel, 5 is the secondary rating of the current transformer
Current value. Then, the transformation ratio of the corresponding current transformer is selected according to the calculated transformation ratio. For example, if I is 50a, n = 14.29 can be obtained according to the calculation formula, so 75 / 5 ratio current transformer is selected.
The accuracy of current transformer is also called accuracy, because the ratio error and angle error of current transformer are inevitable. The accuracy of current transformer is equal to the percentage of error limit of this ratio. For example, the maximum ratio error of 0.5 class current transformer is ± 0.5%. The current transformer for measurement has classes of 0.1, 0.2, 0.5, 1, 3 and 5, and the current transformer for protection has classes of 5P and 10p. Generally, level 0.2 is selected for measurement, level 0.5 is selected for measurement, and level 1 is selected for general monitoring instrument.
At the same time, when using current transformer, we need to pay attention to: the secondary side of current transformer must be grounded to prevent the primary side from high-voltage channeling into the secondary side, which may endanger the safety of equipment and life; pay attention to the polarity of the primary and secondary sides of current transformer, and burn the instrument in case of serious polarity connection error; the secondary side of current transformer cannot be opened, and it is strictly prohibited to install fuse on the secondary side to prevent the secondary side from open circuit induction high voltage, crisis life safety.