Help: Transformer detection of low frequency or DC residual current

viseng

Help: Transformer detection of low frequency or DC residual current [Copy link]

The following is a sampling circuit of a residual current detector for an IT power distribution system. It was copied from someone else's board. The circuit and device parameters in Figure 2 have been verified by replacing them with a small board made by myself. The current transformer uses a 1:1000 current transformer (non-Hall type current transformer).
1. The input of Figure 1 is a square wave signal from 10Hz to 500Hz output by the MCU. There are about 65 step
outputs added to the common end of the transformer
. 2. The input CHN1_CT1 of Figure 2 is the current transformer input. CHN1_CTL is a 100Hz PWM output by the MCU. The pulse width ratio seems to be fine-tuned in the process.
3. The changes of the two waveform signals CHN1_CTL and CT_PWR_CTL are only during the startup initialization process. After the initialization is completed, CT_PWR_CTL is a 500Hz PWM signal with a 50% ratio, CHN1_CTL is a 100Hz PWM, the pulse width ratio is also fixed, and CHN1_ADC_IN (i.e. the acquisition circuit output) stays at the 2.5V level.
4. When working, a 1mA square wave current signal with a frequency of 0.5Hz is superimposed on the primary bus of the current transformer. However, when the above three steps are not completed in the startup process, this signal is not added. This signal is triggered to superimpose this signal as long as it receives other alarm signals.
5. I personally feel that the above two PWM signals are only used to eliminate the background noise that may exist in the power grid during the initialization process. That is, when the residual current signal of point 4 is not added, an output balance point is reached.
6. After the current signal at point 4 is triggered, there will be a square wave signal on CHN1_ADC_IN that is synchronized with it and has a bad waveform, with a bias of 2.5V.
7. The current transformer cannot be used to detect signals below 20Hz, so the PWM signal also acts as an excitation to make the signal reach the secondary of the transformer.
8. The Hall current transformer can detect DC signals, but the zero point drift is relatively large. Since the signal superimposed on the primary of the current transformer is too small, it is not suitable to use the Hall current transformer.

The control frequency of the circuit in Figure 1 increases slowly from 10Hz to 500Hz, with about 65 times of gradual increase in the middle. The
frequency of the CHN1_CTL signal remains unchanged, only the duty cycle is adjusted. Figure 3 is the waveform I captured using logic. These are just the frequency surges after power-on. At this time, no current signal is added to the primary end of the transformer.

alan000345

Experts please come and answer this.