Title:Learn about Current Transformer Test and Its Importance

Transformers are widely used in power systems and play a crucial role in measuring electric current. To ensure accuracy in current measurement, transformers are categorized into different accuracy grades. Typically, the accuracy grade refers to the maximum electric current error when the primary current is equal to the rated value within a specified range of secondary load variation. 

Special transformers (designated as S) typically require a high level of precision within the 1% - 120% load range. These transformers are tested at five different load points and the error must be within the specified range. In contrast, transformers without an S designation are tested at four load points.

0.2 and 0.2S grade transformers are designed for current measurement, with the main difference being the higher precision of the 0.2S grade transformer at low loads. These transformers are ideal when the load variation range is large and at times, nearly empty. When the actual load current is less than 30% of the rated current, the comprehensive error of the 0.2S grade transformer is smaller than that of the 0.2 grade transformer.

Protective transformers are classified into steady-state protective transformers (designated as P) and transient protective transformers (designated as TP). One of the most commonly used accuracy grades for protective transformers is the 5P and 10P. Due to the complex relationship between I1 and I2 during short-circuits, the accuracy of protective transformers is based on the error value under the rated accuracy limit current. For example, 5P20 denotes that the error of the protective transformer should be below ±5% when the current of the primary coil is less than 20 times the rated efficiency value.

TP transformers are differentiated into TPX, TPY, and TPZ. TPX transformers consist of an iron core without any gaps and the current error should not exceed ±5% and the phase difference should not be more than ±30 degrees at rated current and load. During the entire short-circuit process, the maximum instantaneous current error should not exceed 5% of the peak value of the secondary symmetric short-circuit current. The phase difference should not be more than 3 degrees when the current is zero.

TPY transformers have small gaps in the iron core, with the gap length being approximately 0.05% of the average magnetic path length. The gap makes it easier for the core to reach saturation, which makes it easier for the direct current component to decay quickly. The current error should not exceed ±1%, and the phase difference should be only 1 degree at rated current and load. During the entire short-circuit process, the maximum instantaneous current error should not exceed 7.5% of the peak value of the secondary symmetric short-circuit current. The phase difference should not be more than 4.5 degrees when the current is zero.

TPZ transformers have larger gaps in the iron core, with the gap length being approximately 0.1% of the average magnetic path length. The core can easily reach saturation, and the linearity of the magnetization curve is good, making the transformer suitable for rapid reclosing. However, the gap in the core allows the residual magnetism to be ignored, and the transformer has a poor ability to transform direct current components.

For testing current transformers, it is necessary to use transient-type current transformers when the electric system has a high capacity and a long time constant, or when the system must have a quick and accurate protection response time. The transient-type transformer is necessary to prevent the core from reaching saturation during the temporary current, which could affect the accuracy of current information conveyed by the transformer.

In conclusion, the accuracy grade selection for transformers is an essential consideration for ensuring precision in electrical measurement. To ensure the most accurate results, protective transformers must undergo testing under rated conditions, particularly if the electric system has high capacity and a long time constant.