What factors cause the error of the current transformer?

1. Core-related factors
Core material characteristics
Nonlinear change of magnetic permeability: The core material of the current transformer (such as silicon steel sheet or Permalloy) has a magnetic permeability that is not an absolute constant. The magnetic permeability changes under different magnetic field strengths. When the primary current is large, resulting in an increase in the magnetic field strength in the core, the magnetic permeability may decrease. This nonlinear change will cause the flux density of the transformer to no longer maintain a linear relationship with the primary current, thereby introducing a ratio error (ratio error) and an angle error (phase error). For example, under high current conditions, the magnetic saturation phenomenon of the core material will cause the magnetic permeability to decrease, so that the secondary current cannot accurately change in proportion to the primary current according to the transformation ratio.
The influence of hysteresis loss: The core will produce hysteresis loss under the action of an alternating magnetic field. Hysteresis loss will cause the core to heat up and cause a lag relationship between the magnetic field strength and the magnetic flux density. This will affect the phase relationship between the secondary current and the primary current, resulting in an angle difference. At the same time, the presence of hysteresis loss will also consume energy to a certain extent, affecting the accuracy of the transformer.
Core structure and manufacturing process
The influence of core shape and size: The shape (such as ring, rectangle, etc.) and size of the core have an important influence on the performance of the transformer. If the magnetic circuit design of the core is unreasonable, such as the magnetic circuit length is too long or the cross-sectional area is too small, the magnetic resistance will increase, resulting in uneven magnetic flux density, which will cause errors. For the ring core, although the magnetic circuit is closed and the magnetic resistance is small, if the ratio of the inner and outer diameters of the core is not appropriate, it may also affect the magnetic field distribution, thereby causing errors.
Defects in the core manufacturing process: During the manufacturing process of the core, such as poor lamination of silicon steel sheets, there may be air gaps inside the core. The air gap will greatly increase the magnetic resistance, increase the magnetic flux leakage, and reduce the accuracy of the transformer. In addition, if the core is affected by mechanical stress during the processing, its magnetic properties will also change, resulting in errors.
2. Winding factors
Winding turn error: The number of turns of the primary winding and the secondary winding is determined according to the transformation ratio. During the manufacturing process, if there is an error in the number of turns, it will directly affect the transformation ratio accuracy. For example, if the number of turns of the secondary winding is more than the design value, the secondary current will become smaller, resulting in a ratio difference; conversely, if the number of turns is less than the design value, the secondary current will become larger, which will also result in a ratio difference. Moreover, the accuracy of the number of turns of the winding is also affected by the winding process. For example, if there is an error in the number of turns count during the winding process, or if there is a local short circuit in the winding, it will cause a turn error.
Winding resistance and leakage reactance: The resistance of the winding will produce a voltage drop. When the secondary current passes through, the resistance voltage drop will reduce the voltage at the secondary winding end, thereby affecting the magnitude of the secondary current and producing a ratio difference. At the same time, the leakage reactance of the winding will cause a phase difference between the secondary current and the primary current, causing an angle difference. Especially in high-frequency conditions, the effect of leakage reactance will be more significant, because the leakage reactance is proportional to the frequency. The increase in frequency will increase the leakage reactance, thereby increasing the angle difference.
Winding distributed capacitance: There is distributed capacitance between the windings and between the windings and the iron core. When a high-frequency current passes through, the distributed capacitance will generate a capacitive current, which will be superimposed on the inductive current of the secondary winding, changing the magnitude and phase of the secondary current, thereby introducing an error. Moreover, the size of the distributed capacitance is also related to factors such as the geometry of the winding and the dielectric constant of the insulating material.
3. Factors related to working conditions
Primary current size and waveform
The influence of the primary current size: When the primary current is too small or too large, it will affect the accuracy of the transformer. When the primary current is too small, the magnetic flux density in the core is low, the magnetic permeability is unstable, and a large ratio difference and angle difference will be generated. When the primary current is too large, close to or exceeding the rated saturation current of the transformer, the core will be saturated, resulting in the secondary current not being able to accurately follow the primary current changes, resulting in serious errors.
The influence of the primary current waveform: If the primary current is not a standard sine wave, for example, it contains harmonic components, the core and winding of the transformer will respond differently to different frequency components, which will cause measurement errors. For currents containing high-order harmonics, the loss of the core will increase, and the leakage reactance and distributed capacitance of the winding will have different effects on different frequencies, thereby changing the amplitude and phase of the secondary current and introducing errors.
The influence of secondary load: The secondary load includes the equivalent impedance of devices such as ammeters, relays, and energy meters connected to the secondary winding. When the secondary load impedance is too large, according to Ohm's law, the secondary current will decrease, resulting in a ratio error. At the same time, the properties of the secondary load (such as inductive, capacitive or resistive) will also affect the phase of the secondary current and produce an angle error. For example, when the secondary load is an inductive load, the secondary current will lag behind the ideal phase, increasing the angle error.