# What are the active inhibitory DC current components?

1. Parallel transformer detection method
The detection circuit of the parallel transformer detection method consists of a small -power industrial frequency transformers with a Tan ratio of 1: 1, and the sub -filter circuit of the sub -edge of the sub -edge of the small power. The DC voltage component in the power grid is mainly generated on the equivalent resistance of the grid. Because the turbizer ratio is 1: 1, when the first -order RC circuit reaches a steady state, the capacitor voltage is close to the DC voltage component. The voltage constitutes a PI closed -loop feedback, and its output volume acts on the reference current to inhibit the DC component injected into the grid. The disadvantage of this method is that only the transformer’s leakage flux is sufficient for hours can the capacitor voltage not have no industrial frequency alternating volume in the capacitor voltage, otherwise it will introduce excessive traffic volume in the current ring reference value, resulting in the connection of the grid current.

2. Two -level RC filter detection method
Figure 6.35 (A) is a single -phase bridge inverter circuit based on the LCL filter. The DC component detection circuit consists of two exactly the same RC filter circuit [135]. If VF represents the voltage on the filtering induction and its parasitic resistance, V0 represents the capacitor voltage of the second -stage RC filter circuit, V1 represents the capacitor voltage of the first -stage RC filter circuit, then the second -level RC circuit voltage equation is:

Two -level RC circuit node current equation is:

From the formula (6.5), formula (6.6), you can get:

From the formula (6.7), it can be seen that the AC component represented by the microcontroller and the DC voltage component represented by V0 are in a steady -state condition. The DC voltage component on the inductor parasitic resistance is used to compensate the DC current component by using the closed -loop PI control method shown in Figure 6.35 (b).
Due to the small parasitic resistance of the filter inductance, the DC voltage component is not easy to detect. Therefore, the effectiveness of the detection method is closely related to the selection of the RC parameter. If the RFCF is selected too small, the current check -by -road current is large, affecting the current waveform quality of the grid into the grid, and reducing the system’s conversion efficiency; Large, the current check -by -the -road current is small, the accuracy of the detection is reduced, and the compensation effect decreases [125]. Therefore, it is necessary to determine the appropriate RFCF value through the actual situation of the system and the experimental method.

3. Critical saturated electric anti -resistor detection method
The detection circuit of the critical saturation electric resistor is shown in Figure 6.36. The detection circuit composed of the LC filter and the critical saturation electrical resistor is connected to the output terminal of the full bridge inverter circuit. The voltage is proportional. Therefore, the LC filter filtering the high frequency component of the output voltage of the inverter, and the remaining low -frequency signal below acts on the critical saturation electrical resistor. The critical saturation electrical resistor is composed of the original single -wound and the ring magnetic core. When the rated voltage is rated, the magnetic core works in the critical magnetic saturation state. Once the inverter output voltage has a DC voltage component, the current and magnetic saturated electrical resistor’s current and magnetic magnetic resistor There will be distortions in the whole [136]. The definition of the positive saturation index SIP and the negative saturation index SIN respectively indicate the points level of the critical saturated electrical resistor current IM near the zero point, which is represented by the formula (6.8) and the formula (6.9). The difference between the difference between the positive and negative saturation index is adjusted, and its output is the compensation of the DC current component.

The advantages and disadvantages of critical saturation electrical resistances are the key to the success or failure of the detection method. The degree of magnetic saturation is too high or too low, which will affect the effectiveness of the detection.

4. Self -correctional Dental Division current Reconstruction Method
The self -reliance The DC current reconstruction method uses the DC parent line interval sensor to sample the arm of the bridge arm and the current value of the current to reconstruct the inverter output current and the current sensor DC bias current. Figure 6.37 (A) is aimed at the H bridge inverter, placing a current sensor on its positive or negative DC bus [126], Figure 6.37 (b) A current sensor is placed on each DC bus [137]. When the bridge arm is in a divestable state, the sampling value of the DC bus current sensor is equal to the output current value of the inverter; the DC bus current sensor has no current flow during the continuation state. Therefore The current sampling value is not zero, and the sampling current is the DC bias current of the current sensor. The current value sampled by the sample of the bridge arm is reduced when the current value sampled is reduced, that is, the sampling current value after correction.

This method can omit the inverter output current sensor, and the above reconstruction method can be used to obtain the required inverter output current. This method is only suitable for single polarity PWM modulation, and it is not suitable for bipolar PWM. At the same time, when the PWM pulse is too narrow, the sampling current value is largely difficult, which limits the accuracy of the method. This method can only correct the DC bias of the current sensor, and cannot compensate the DC current component problems generated by other reasons.
5. Point action detection method
Integral detection method includes theoretical points and virtual capacitors. The theoretical points method refers to if the grid current contains the base wave, harmonic and DC components, only the DC current score is retained after the average points of the power grid cycle [138]. Feedback control can get better compensation effects.

The main circuit of the virtual capacitance separation method is shown in Figure 6.38 (a). If the AC capacitor C is added to the L filter circuit, the inverter circuit control model is shown in Figure 6.38 (b). According to the “transformation in the control theory The principle of transmitting functions in the front and rear circuits remain unchanged. “Figure 6.38 (b) can be converted into Figure 6.38 (c). From Figure 6.38 (C), it can be seen that the sample -connected grid current has the effect of integrating the integral current, and the difference in the difference between the PWM modulation signal and the results after the integral effect can be performed. The capacitance does not exist objectively, so it is called a virtual capacitor.

From the above analysis, it can be seen that the essence of virtual capacitance is also the method of integration. The longer the points action time, the slower the system’s dynamic response speed. At the same time, because DC current components are usually less than 0.5%of the rated current of the system, the detection accuracy and system sampling accuracy of the current sensor are particularly important.

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