Will the measurement error of the current transformer change over time?

Classification of errors: The error of the current transformer is usually divided into ratio difference (ratio difference) and phase difference (angle difference). Ratio difference refers to the difference between the measured value of the secondary current multiplied by the rated current ratio and the actual primary current, as a percentage of the latter; angle difference refers to the angle between the secondary current phasor and the primary current phasor after rotating 180°, also known as angle error, and stipulates that when the secondary current phasor leads the primary current phasor, the error is positive, otherwise it is negative.
Factors causing the error to change over time:
I. Internal factors:
1. Changes in the magnetization characteristics of the core: The core of the current transformer may change its magnetization characteristics due to the effect of the magnetic field during long-term use. This will cause the magnetic permeability of the core to change, thereby affecting the transformation ratio and phase difference of the current transformer, causing the error to increase. For example, in some old current transformers, due to the long-term effect of large current on the core, the magnetization characteristics may change significantly, resulting in increased measurement errors.
2. Changes in winding parameters: The winding parameters of the current transformer, such as resistance and inductance, may also change over time. This may be caused by aging of the winding, degradation of insulation performance, etc. Changes in winding parameters will cause changes in the secondary output voltage and current of the current transformer, thereby affecting the measurement error. For example, when the resistance of the winding increases, the secondary output current will decrease, resulting in an increase in the ratio difference.
3. Degradation of insulation performance: The insulation performance of the current transformer is essential for its normal operation. If the insulation performance degrades, it may cause problems such as partial discharge inside the current transformer, thereby affecting the measurement accuracy. In addition, the degradation of insulation performance may also cause short-circuit failures of the current transformer, further affecting the measurement error. For example, in some humid environments, the insulation performance of the current transformer may be affected, resulting in increased measurement errors.
2. External factors:
1. The magnitude and change of the primary current: When the primary current is small, the error of the current transformer may be relatively large. This is because in the case of small currents, the magnetic flux density of the core is low, and the nonlinear part of the magnetization curve has a more significant impact on the error. In addition, changes in the primary current will also have an impact on the error. For example, when the primary current suddenly increases or decreases, the current transformer may not respond in time, resulting in an increase in the measurement error. In the power system, the magnitude and change of the primary current are inevitable, so this is an important factor that causes the current transformer measurement error to change over time.
2. Changes in secondary loads: The secondary loads of the current transformer include connecting cables, measuring instruments, relays, etc. If the secondary load changes, such as the increase in the length of the connecting cable, the change in the internal resistance of the measuring instrument, etc., the secondary output current and voltage will change, thus affecting the measurement error. In addition, the power factor of the secondary load will also affect the error. For example, when the power factor of the secondary load is low, the angular difference of the current transformer may increase.
3. The influence of ambient temperature: Changes in ambient temperature will affect the performance of the current transformer. For example, when the ambient temperature rises, the magnetic permeability of the core will decrease, resulting in changes in the transformation ratio and phase difference of the current transformer, thereby affecting the measurement error. In addition, changes in ambient temperature may also cause changes in the resistance and inductance of the winding, further affecting the measurement error. Under some extreme environmental conditions, such as high temperature, low temperature, humidity, etc., the measurement error of the current transformer may increase significantly.
4. Changes in system frequency: Although the system frequency changes very little in general and its impact on the error can be ignored, in some special cases, such as when the power system fails, the system frequency may change significantly. Frequency changes will affect the core loss, magnetic flux density and coil leakage reactance, and also affect the value of the secondary load reactance, which in turn affects the error of the current transformer.
3. Methods to reduce the change of error over time:
1. Select a suitable current transformer: When selecting a current transformer, it is necessary to select a suitable model and specification based on the actual measurement requirements and the use environment. For example, for occasions with high measurement accuracy requirements, a current transformer with a higher accuracy level can be selected; for large current measurements, a current transformer with a larger ratio can be selected. In addition, factors such as the core material, winding structure, and insulation performance of the current transformer need to be considered to ensure that it has good performance and stability.
2. Regular calibration and maintenance: In order to ensure the measurement accuracy and performance of the current transformer, it needs to be calibrated and maintained regularly. The calibration cycle is generally 1-3 years, and the specific calibration cycle can be determined based on factors such as the use environment and frequency of use. The verification content includes the detection of indicators such as ratio difference, angle difference, and saturation multiple. In terms of maintenance, it is necessary to keep the current transformer clean and dry to avoid interference from external magnetic fields. In addition, it is necessary to regularly check the connection of secondary loads such as connecting cables and measuring instruments to ensure good contact.
3. Take compensation measures: In order to reduce the measurement error of the current transformer, some compensation measures can be taken. For example, add appropriate magnetic materials to the core material to increase the magnetic permeability of the core and reduce the influence of the nonlinear part of the magnetization curve on the error; connect an appropriate capacitor in parallel on the secondary side to compensate for the inductance of the secondary load and reduce the angle difference. In addition, digital signal processing technology can be used to process and compensate the output signal of the current transformer to improve the measurement accuracy.
4. Optimize the use environment: In order to reduce the influence of ambient temperature on the current transformer, some measures can be taken to optimize the use environment. For example, when installing the current transformer, try to choose a place with less temperature change; in a high temperature environment, heat dissipation measures can be adopted, such as installing a radiator, ventilation equipment, etc.; in a low temperature environment, insulation measures can be adopted, such as wrapping insulation materials, etc. In addition, care should be taken to prevent the current transformer from being affected by moisture, corrosion and other factors.