How can differential mode filtering in an electrical force measurement switch (PMS) not affect measurement accuracy?

Adding a differential-mode filter (X capacitor and differential-mode inductor) to the PMS power input introduces additional phase shift and amplitude attenuation. Improper design may affect the sampling accuracy of the voltage and current waveforms by the power metering chip, leading to errors in active power calculation. The key is to select components with low loss and good phase characteristics and control the filter cutoff frequency. The X capacitor should be a polypropylene film capacitor (CBB), which has a small dielectric loss tangent (tanδ), low temperature drift, and stable capacitance. The differential-mode inductor PBZ3216E120Z0T should use a high-permeability, low-coercivity core to reduce hysteresis loss.

The filter cutoff frequency fc needs to be much higher than the power frequency (50Hz) but lower than the main interference frequency (e.g., 150kHz), typically set between 1-10kHz. For example, selecting a 0.22μF X capacitor and a 12μH differential-mode inductor results in fc of approximately 3kHz, attenuating the 50Hz signal by less than 0.1% and reducing the phase shift by less than 0.1°. In addition, a small capacitor (e.g., 100pF) of the same material as the X capacitor can be connected in parallel before the voltage sampling divider network of the metering chip to compensate for the phase. Through meticulous design, the additional error introduced by differential-mode filtering can be controlled within 0.1%, meeting the accuracy requirements of a Class 1 energy meter.