# AC Fault Ride-Through Strategy of a Transformerless Soft Normally Open Point
## 3 Fault Response Characteristics under Traditional Positive and Negative Sequence Control  
In order to realize the AC fault ride-through, the voltage-current double closed loop control strategies for the positive and negative sequence have been matured [11; 18]. Whether the power or the current is controlled in the positive or negative sequence, the final instruction generated by the control loop is executed by adjusting \(u_{ip}\) and \(u_{jn}\) as shown in Figure 2. However, if the controller only has the positive- and negative-sequence control function, \(u_{ip}\) and \(u_{jn}\) will only contain positive- and negative-sequence components. So \(e_{j}\) in Equation (4) only contains positive- and negative-sequence components, without the zero-sequence component, that is, \(e^{0}=0\). Then, under the traditional positive- and negative-sequence control strategy, Equation (7) can be rewritten as  
\[R!^{0}+L\frac{di^{0}}{dt}=u^{0}-u_{o} \tag{8}\]  
When symmetrical or asymmetrical grounding faults occur in the 10 kV distribution system with arc suppression coil grounding, the fault current is small, and \(i^{0}\approx 0\). Therefore, Equation (8) can be simplified as follows.  
\[u_{o}\approx u^{0} \tag{9}\]  
It can be seen from Equation (9) that the zero-sequence component caused by the AC asymmetrical fault leads to the fluctuation of \(u_{o}\). Since \(U_{\text{dp}}\) and \(U_{\text{dn}}\) are \((0.5U_{\text{d}}+u_{o})\) and \((u_{o}-0.5U_{\text{d}})\), respectively, the non-zero state of \(u_{o}\) leads to the fluctuation of the positive DC voltage and negative DC voltage, but the pole-to-pole DC voltage keeps constant.  
Since the MMC1 and MMC2 shown in Figure 1 are connected back to back, the \"\(\circ\)\" point can be regarded as a common virtual DC neutral point. It is known from the reverse calculation of Equation (8) that if MMC2 is only configured with the positive- and negative-sequence controller, the AC voltage of MMC2 will result in the zero-sequence component when the asymmetrical grounding fault occurs on the AC side of MMC1 which leads to the \"\(\circ\)\" point voltage offset. Furthermore, the amplitude of the zero-sequence component is almost the same as the MMC1 side if the zero-sequence impedance is neglected. The introduction of zero-sequence components leads to fluctuation in the AC voltage on the MMC2 side. In this way, the asymmetric grounding fault on the MMC1 side is transferred to the supply area where MMC2 is located, resulting in the expansion of the fault.  
The zero-sequence component has two paths as shown in Figure 4. Path 1 is the loop from the fault point to the neutral point of supply area 1, similar to the traditional AC grounding fault. In path 2, the zero-sequence current flows through the fault point, the neutral point of supply area 2 and SNOP.  
In the 10 kV distribution system with arc suppression coil grounding, it is allowed to run temporarily for two hours under the single-phase grounding fault, to improve the reliability of the power supply. In this case, if SNOP is not blocked, the two supply areas connected to MMC1 and MMC2 will experience the grounding fault together for two hours. The safety and stability of the whole distribution system will be greatly impacted.  
Figure 3: Zero-sequence model of MMC.