Accuracy level of current transformer: Current transformer can be divided into different accuracy levels according to the size of measurement error. The accuracy level refers to the maximum current error when the primary current is the rated value within the specified secondary load change range.

With S (special) special current transformer, the accuracy is required to be high enough within the load range of 1%-120%. Generally, 5 load points are used for measurement and the error is less than the specified range. Without S, 4 load points are used for measurement. The error is less than the specified range.

Class 0.2 and class 0.2S are for measuring current transformers. The biggest difference is that when the load is small, class 0.2S has higher measurement accuracy than class 0.2; it is mainly used for a larger range of load fluctuations, and some The occasion is almost empty. When the actual load current is less than 30% of the rated current, the overall error of the 0.2S class is significantly smaller than that of the 0.2 class current transformer.

Protection type accuracy level: The current transformer for protection is divided into two types for steady-state protection (P stands for protection) and transient protection (TP) according to its purpose.

5P and 10P are commonly used for the accuracy level of current transformer for protection. Due to the complicated relationship between I1 and I2 during the short-circuit process, the accuracy level of the protection level is nominally based on the error under the rated accuracy limit of the primary current. The so-called rated accuracy limit primary current means that the primary current is a multiple of the rated primary current.

The meaning of 5P20 is: when the current flowing through the protection CT at a time is less than 20 times of its rated current, the error of this CT should be less than ±5%

TPX: There is no air gap in the toroidal core of the current transformer. Under the rated current and load, the current error is not more than ±5%, and the phase difference is not more than ±30 degrees. During the whole process of short circuit, the current transformer is rated accurately. Within the level range, the instantaneous maximum current error does not exceed 5% of the peak value of the rated secondary symmetrical short-circuit current, and the phase difference when the current crosses zero is not more than 3 degrees.
TPY: There is a small air gap in the toroidal core of the current transformer. The length of the air gap is about 0.05% of the average length of the magnetic circuit. The air gap makes the core not easy to saturate, which is conducive to the rapid attenuation of the DC component. Under the rated current and load, the current error is not more than ±1%, and the phase difference is 1 degree. During the whole process of short-circuit, within the rated accuracy level of the current transformer, the instantaneous maximum current error does not exceed the rated secondary symmetrical short circuit. 7.5% of the peak current, the phase difference when the current crosses zero is not more than 4.5 degrees.
TPZ: The ring core of the current transformer has a large air gap. The length of the air gap is about 0.1% of the average length of the magnetic circuit. Due to the air gap, the core is not easy to saturate, which is especially suitable for rapid reclosing. The gap is large, the remanence can be ignored, the iron core magnetization curve has good linearity, the time constant of the secondary circuit is small, and the transmission performance of the AC component is good, but the ability to transfer the DC component is poor.
Transformers for 500KV line protection generally use TPY-level transient transformers.

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