What are the classification levels of current transformers? Let's hear it from Nanjing Zhenhengtong Current Transformer Manufacturer.
The accuracy level is specified for the transformation ratio error generated by a current transformer under rated operating conditions. The accuracy level refers to the percentage value of the maximum current error when the primary current is at its rated value within the specified range of secondary load variation. The accuracy levels of domestic current transformers include: 0.01; 0.02； 0.05； 0.1； 0.2； 0.5； 1；3； Level 10. According to the standard "Current Transformers" GB1208-75, the error limit for current transformers used in power systems.
Current transformers marked with "S" are special ones, requiring high accuracy within the load range of 1% to 120%. Generally, their errors are measured at five load points to ensure they fall within the specified range. Current transformers of Class 0.1 or above are primarily used for precise measurements in laboratories or as standards to verify lower-grade transformers. They can also be paired with standard instruments to calibrate them. Therefore, they are called standard current transformers. In industry, Class 0.2 and 0.5 transformers are used to connect electrical measuring instruments, requiring high accuracy within the load range of 20% to 120%. Generally, their errors are measured at four load points to ensure they fall within the specified range (errors include ratio error and angle error, as current is a vector, requiring both magnitude and phase angle differences to be considered). Transformers of Class 3.0 and below are primarily used to connect certain relay protection devices and control equipment. For example, Class 5P and 10P current transformers are generally used for relay protection, requiring composite errors to be less than a certain value under short-circuit current conditions, specifically less than 5% for Class 5P and less than 10% for Class 10P. Current transformers marked with Class B (or D) are used to connect differential protection and distance protection devices. Therefore, current transformers are specified with different accuracies based on their purposes, which correspond to error precisions within different current ranges.
Protective current transformers are classified according to their functional characteristics as follows:
Protective current transformers are divided into steady-state protection (P) and transient protection (TP) types according to their purposes
Level P: The accuracy limit is specified as the composite error under steady-state symmetrical primary current, with no remanence limit. 5P20 indicates that the error is less than or equal to 5% when subjected to 20 times the rated current
The accuracy levels of current transformers for transient protection are divided into three categories: TPX, TPY, and TPZ.
TPS level: Low magnetic flux leakage current transformer, with its performance specified by the secondary excitation characteristics and the turn ratio error limit. No remanence limit.
TPX level: The accuracy limit is specified as the peak transient error within a specified transient duty cycle. There is no remanence limit. TPX level current transformers have no air gap in their toroidal cores, and their current error is not greater than ±0.5% under rated current and load conditions
TPY level: The accuracy limit is specified as the peak transient error within the designated transient duty cycle. The remanence does not exceed 10% of the saturation flux. The core of the current transformer at this level features a small air gap, with an air gap length of approximately 0.05% of the average length of the magnetic circuit. The air gap makes the core less prone to saturation, facilitating the rapid decay of the DC component. Under rated load, a large current error of ±1% is allowed.
TPZ level: The accuracy limit specifies the error of the peak instantaneous AC component during a single energization with a large DC offset under a specified secondary circuit time constant. There is no requirement for DC component error limits, and the residual magnetism can be practically ignored. TPZ level current transformer cores have a relatively large air gap, with an air gap length of approximately 0.1% of the average length of the magnetic circuit. Due to the large air gap in the core, it is generally not prone to saturation, making it particularly suitable for use on lines with fast reclosing (with no current time gap greater than 0.3s).
