When the current transformer is finalized and its internal parameters are determined, the magnitude of the error will be affected by the secondary current (or primary current), secondary load, power factor, and frequency. These factors are called external factors, and the error of the current transformer in operation is mainly affected by these four factors.
(1) The influence of the change of current frequency on the error is complex. In general, the frequency variation of the system is small and the effect is negligible. If the frequency changes too much, such as using a current transformer rated at 50Hz in a 60Hz system, the influence of frequency should be considered, because the frequency change not only affects the core loss, magnetic flux density and coil leakage reactance, but also affects the magnitude of the reactance value of the secondary side load.
(2) When the primary current decreases, the magnetic flux density decreases proportionally, but at low magnetic flux density, the decrease in excitation amperage is slower than the decrease in magnetic flux density, so the absolute value of the difference and the angular difference increase relatively.
(3) The error of the current transformer has the following characteristics: when the primary current changes within the specified range, the secondary current changes proportionally, and when the secondary load impedance changes within the specified range, the secondary current is not affected. Therefore, when the secondary load is reduced to the rated range, the magnetic flux density also decreases. Since the secondary current is constant, the excitation current decreases and the error is also reduced. The factory instructions of current transformers generally indicate the rated secondary load impedance value. The error should be checked at runtime according to the maximum secondary load impedance value for a given wiring scheme.
(4) The power factor of the secondary load increases, that is, Rfh increases, Xfh decreases, the angular difference increases, and the ratio decreases. For the saturation factor, the saturation factor indicated in the transformer manufacturer's instructions refers to the saturation factor when the power factor is 0.8. This value corresponds to the "minimum" of the saturation factor, so the power factor increases or decreases, and the saturation factor increases.
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