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 on the original board. The current transformer used is 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, with about 65 steps

Output is added to the common end of the transformer

2. The input CHN1_CT1 in Figure 2 is the current transformer input, and CHN1_CTL is a 100Hz PWM output by the MCU. The pulse width ratio seems to be fine-tuned during the process.

3. The changes of the two waveform signals CHN1_CTL and CT_PWR_CTL only occur 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, and the pulse width ratio is also fixed. CHN1_ADC_IN (i.e., the acquisition circuit output) stays at the 2.5V level.

4. When working, a 1mA, 0.5Hz square wave current signal is superimposed on the primary bus of the current transformer. However, if the above three steps are not completed during the startup process, this signal will not be added. This signal will be triggered and superimposed as long as it receives other alarm signals.

5. I personally feel that the two PWM signals above 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 at 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 synchronous square wave signal with a poor waveform on CHN1_ADC_IN, with a bias of 2.5V

7. Current transformers cannot be used to detect signals below 20 Hz, so the PWM signal also acts as an excitation to make the signal reach the secondary of the transformer.

8. Hall-type current transformers can detect DC signals, but the zero-point drift is relatively large. Since the signal superimposed on the primary side of the current transformer is too small, it is not suitable to use Hall-type current transformers.

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. The figure below is the waveform I captured using logic analysis. This is just the frequency surge after power-on. At this time, no current signal is added to the primary end of the transformer.

Figure 1

Figure 2

Figure 3

maychang

The most important thing is: how the current transformer is connected is not explained in detail in the diagram or in the text. Even the "one end of the current transformer is connected to the common end of the six transformers" is not clearly shown or described whether it is the primary winding or the secondary winding of the current transformer.

maychang

It is speculated that this circuit uses "residual current" to modulate the square wave signal generated by the MCU.

It says earlier that "CHN1_CTL is a 100Hz PWM output by the MCU, and the pulse width ratio seems to be fine-tuned during the process", and it says later that "CHN1_CTL is a 100Hz PWM, and the pulse width ratio is also fixed". The two are contradictory.