Voltage sharing control strategy of half-bridge grid-connected inverter
In the grid-connected situation, considering the line impedance and the inductive and resistive impedance existing in the actual power grid, Zload is equivalent to resistive-inductive. Therefore, the model established above and the voltage sharing control strategy discussed are also applicable to the half-bridge grid-connected inverter, the only difference being that Zload is a relatively small resistive and inductive load. Both class A and class B voltage equalization control strategies can be applied to grid-connected inverter occasions.
The waveform of the Class A voltage equalization control strategy is shown in Figure 1. It can be seen from the figure that when there is a capacitance deviation and a capacitor voltage difference in the DC side capacitor, the capacitor voltage can be quickly equalized, and the voltage equalization effect is good.
The simulation waveform of the B-type equalizing control strategy is shown in Figure 2, where Gc(s) is the QR regulator and the PR regulator, respectively.
As can be seen from Figure 2, when the DC side capacitor has capacitance deviation and capacitance voltage difference, under the action of the QR and PR regulators, the capacitor voltage can be quickly equalized, which verifies the correctness of the Class B voltage equalization control idea. However, when the QR regulator is used, the capacitor voltage has low-frequency oscillation, which has a certain impact on the grid current. In addition, in order to ensure the stability margin of the system, the gain of the system at the fundamental wave is low, which makes the tracking effect of the fundamental wave of the incoming current not ideal. When the PR regulator is used, the tracking effect of the incoming current is better.
By comparison, it can be seen that in the grid-connected inverter occasion, the class A voltage-sharing control strategy with capacitor differential feedforward and the class B voltage-sharing control strategy using PR regulators have better performance.
According to the deduced equivalent circuit of the 3L-NPC half-bridge inverter, a method for judging whether the modulation strategy helps the capacitor voltage balance is obtained from the frequency domain analysis of capacitor voltage self-balance mechanism. The relationship between the capacitor voltage difference and the capacitor current is analyzed from the perspective of the time domain, and it is concluded that 3L-NPC has self-balancing characteristics, that is, when there is a capacitor voltage difference, the capacitor current will generate a DC current that is opposite to the capacitor voltage difference. This current will eliminate the capacitor voltage difference, and its self-balancing current mainly comes from the DC component of the bridge arm output current and the even-order harmonic component near the switching frequency. According to the effect of closed-loop control on the self-balancing current of the 3L-NPC topology derived from the DC component of the load current, two types of voltage sharing control strategies are summarized: ① Directly or indirectly close-loop control of the capacitor voltage difference to eliminate the factors that cause the capacitor voltage unbalance; ②Appropriate regulator or control structure is used to ensure that 3L-NPC still has self-balancing characteristics under the action of closed-loop control, thereby balancing the capacitor voltage. Among the several voltage-sharing control schemes listed according to the A-type and B-type voltage-balancing control ideas, the B-type voltage-balancing control strategy does not need to detect the capacitor voltage, which can simplify the system control and reduce the system cost.