Capacitor voltage balancing based on self balancing characteristics
According to the equalization control idea of class B, the equalization control method based on the self-balancing characteristic is discussed below.
According to the effect of closed-loop control on the self-balancing characteristics of 3L-NPC, in order to satisfy the idea of B-type equalizing control, the closed-loop control needs to meet two conditions under the action of the regulator Gc(s), namely: ①When there is a capacitor voltage difference, the modulating wave has no DC component or the response to the DC component is small; ②When the capacitor voltage is balanced, the modulating wave has no DC component. In addition, in order to obtain a better tracking effect of the load fundamental current, the open-loop gain of the system at the fundamental frequency is required to be large enough, that is, the gain of Gc(s) at the fundamental frequency is required to be large enough.
(1) Scheme B.1: It can be seen from the effect of the aforementioned closed-loop control on the self-balancing characteristics of 3L-NPC that the regulator Gc(s) adopts a PR regulator or a QR regulator, which can realize capacitor voltage balance control.
Features: No need to detect capacitor voltage, which reduces system cost and simplifies system control. Among them, the OR regulator only affects the open-loop amplitude and phase characteristics at ωr, and has no effect on the open-loop gain at other frequencies, so it is not easy to adjust the open-loop cutoff frequency of the system, so that the dynamic response characteristics of the system cannot be optimized. Furthermore, the regulator form of this scheme is not limited to PR or QR regulators.
(2) Scheme B.2: Dual-loop control strategy with OR regulator
Principle: On the basis of the single closed-loop control with QR regulator, in order to improve the dynamic performance of the system, a double-loop control with self-balancing characteristics is used, as shown in Figure 1. The voltage outer loop adopts a QR regulator, and the current inner loop adopts a proportional regulator kp.
Features: There is no need to detect capacitor voltage, which reduces system cost and simplifies system control; it is suitable for independent inverter occasions.
The waveform of capacitor voltage equalization is shown in Fig. 2(a), and the simulation waveform when the load suddenly changes is shown in Fig. 2(b). It can be seen from Figure 2(a) that when there is an initial capacitor voltage difference on the DC side capacitor, the capacitor voltage can gradually become balanced. Compared with the simulation waveform diagram of the scheme A.2 of the closed-loop control of capacitor voltage deviation, the equalization speed of the system is slower, but the response speed when the load changes suddenly is improved, but it is not significant. In addition, this strategy utilizes the self-balancing characteristics of capacitor voltage to achieve capacitor voltage equalization without capacitor voltage detection.
According to the voltage equalization effect of the voltage equalization control strategy by adding the capacitor voltage difference feedforward and adopting the PR or QR regulator and its influence on the load current, it can be known that both the A and B class voltage equalization control strategies have good characteristics in the independent inverter occasion.