# AC Fault Ride-Through Strategy of a Transformerless Soft Normally Open Point
## 5 Optimization Method
### Control Strategy Optimization  
From the theoretical deduction above, it can be seen that the fluctuation of DC voltage in the power frequency is caused by the non-zero value of the \"o\" point voltage \(u_{o}\). In order to suppress the transfer of the zero-sequence component from one side to another through the SNOP and reduce the effect range of the fault, it is necessary to restrict \(u_{o}\) to zero by means of corresponding control strategies. It can be seen from Equation (7) and Figure 3 that only if \(e^{0}\) is controlled as the following equation, can the purpose of DC voltage fluctuation suppression be achieved effectively.  
\[e^{0}=u^{0}-\left(R\dot{t}^{0}+L\frac{d\dot{t}^{0}}{dt}\right) \tag{18}\]  
On the other hand, the zero-sequence current will flow through the bridge arm, and cause the overheating of power electronic devices, especially in the case of large zero-sequence currents. Therefore, the zero-sequence current control needs to be added.  
The zero-sequence current is in AC form. Based on the existing proportional resonant (PR) controller [19; 20] which can realize the non-error adjustment of alternating quantities, this paper designs the zero-sequence control loop as shown in Figure 6, according to the mathematical relationship among the variables in Equation (18). Where \(K_{\text{P}}\) is the proportional coefficient, \(K_{\text{R}}\) is the resonance coefficient; \(\omega_{0}\) is the angular frequency used to control the AC signal, \(\omega_{\text{c}}\) is the cut-off angular frequency. In order to eliminate the zero-sequence current component, the zero-sequence current reference value of the PR controller is set to zero. Meanwhile, in order to suppress the zero-sequence voltage caused by the voltage fluctuation of \"o\" point, the zero-sequence component \(u^{0}\) of the AC voltage is used as the feed-forward value.  
In Figure 7a, with the increase in the \(K_{\text{P}}\) value, the amplitude margin at the resonant frequency does not change significantly, while the gain in other frequency bands increases, indicating that too large a \(K_{\text{P}}\) value will weaken the comparative advantage of the resonant link; that is, indirectly affect the bandwidth and stability of the controller. Different \(\omega_{\text{c}}\) values in Figure 7b have little effect on the gain of resonant frequency point, but the control system bandwidth is positively correlated with it, thus enhancing the controller's ability to quickly track the sinusoidal alternating quantity and improving the system's robustness to grid frequency fluctuations. In Figure 7c, with the increase in \(K_{\text{R}}\) value, the gain at the resonant frequency of the controller increases, and the effect of eliminating the steady-state error is achieved. Therefore, the PR controller parameter design needs to take into account  
Figure 6: Optimized control strategy of MMC.  
the interaction between the dynamic and static performance of the system. The regulation of its parameters is: Adjust the proportional coefficient \(K_{\text{p}}\) and the resonant coefficient \(K_{\text{R}}\) to meet the system stability, dynamic and static performance; Adjust the cut-off angular frequency \(\omega_{\text{c}}\) to suppress the disturbance of frequency fluctuation to the converter.