What Is the Transformation Ratio and Accuracy Level of Current Transformer
The secondary parameters of current transformer include transformation ratio and accuracy level.For the current transformer for protection, the accuracy level shall be nominal as the percentage of the maximum allowable composite error of the accuracy level under the rated accuracy limit primary current, followed by "P" for protection.Rated accuracy limit primary current refers to the maximum current that can ensure that the composite error does not exceed the allowable value of the accuracy level indicated by the current transformer when leaving the factory. It is generally indicated by the accuracy limit coefficient, and the rated accuracy limit coefficient is generally indicated after its accuracy level; The so-called rated accuracy limit coefficient refers to the ratio of rated accuracy limit primary current to rated primary current.
Transformation ratio: it refers to the ratio of primary current to secondary side current. It is an important parameter for relay protection setting calculation and measurement.
The selection of transformation ratio shall first consider the indication accuracy of the measuring instrument under rated working conditions and meet the requirements of rated input current and working accuracy of the protection device. For example, when the rated input current of the protection device is 5a, under normal working conditions, the secondary output current of the current transformer of the measurement stage should be between 1 4.5a. If it is too small, (such as less than 0.5A), it is unreasonable. For the protection level current transformer, the transformation ratio is generally greater than that of the measurement level current transformer because it is necessary to ensure that it is unsaturated in case of system fault.
Note that the capacity of the primary winding of the current transformer with constant series ratio is doubled; The parallel transformation ratio is doubled and the capacity remains unchanged. For secondary winding, the series transformation ratio remains unchanged and the capacity is doubled; The parallel transformation ratio is reduced by half and the capacity remains unchanged.Accuracy level: at present, there are six accuracy levels of current transformers used in China: 0.1, 0.2, 0.5, 1, 3 and 5. According to the discussion of measurement, measurement and protection, the measurement needs to be measured accurately during operation to meet the measurement requirements under normal load, and the protection needs to be protected in fault state to meet the requirements under limit conditions.Measurement accuracy level: 0.1, 0.2, 0.5, etc. For example, level 0.5 means that the transmission error of current transformer shall not be greater than 0.5% under rated working conditions.
Protection accuracy level: generally, P level is adopted, for example, 5p20, which means that the error is 5% under 20 times of rated current. Therefore, although the accuracy of protection level is lower than that of measurement level, it has strong anti saturation ability.Therefore, CT winding cannot be used incorrectly, otherwise it is prone to saturation, and there will be misoperation or refusal of operation for relay protection (longitudinal differential protection is easy to misoperate because the detection difference flow is large. Backup protection will refuse to operate because the collected value is too small).
