# A Short-Circuit Current Calculation Model for Renewable Power Plants Considering Internal Topology
## 2 Classification and Control Strategies of N-SMSs  
### Control Strategies of N-SMSs  
The SCC calculation method proposed in this paper is closely related to the classification and control strategies of N-SMSs. Therefore, the control strategies for N-SMSs are introduced first. The typical structure of a non-synchronous power source is shown in Figure 1[18]. In Figure 1, \(U_{\text{dc}}\) represents the DC-side voltage of the voltage source converter (VSC), and \(u_{\text{v}}\) represents the AC bus voltage on the grid side of the VSC. At a macro level, N-SMSs can be categorized into two grid-connection modes: grid-following and grid-forming. The primary distinction between grid-following and grid-forming N-SMSs lies in how they maintain synchronization with the grid. Grid-following sources use a phase-locked loop (PLL) to synchronize with the grid, which requires the grid-following N-SMSs to be connected to an active grid for operation. Grid-following N-SMSs can be further classified into two types based on their ability to support AC bus voltage: PV grid-following and PQ grid-following sources. Both types aim to maintain the DC-side voltage \(U_{\text{dc}}\) at a setpoint \(U_{\text{dc}}\), or the active power \(P_{\text{s}}\) at a setpoint \(P_{\text{s}}\). The key difference between them is that the PV grid-following source aims to maintain the VSC grid-side bus voltage amplitude \(U_{\text{sm}}\) at a setpoint, \(U_{\text{sm}}\), analogous to a PV node in power flow analysis. In contrast, the PQ grid-following source aims to maintain the reactive power output \(Q_{\text{s}}\) of the VSC at a setpoint, \(Q_{\text{s}}\), which corresponds to a PQ node in power flow analysis. On the other hand, grid-forming sources maintain synchronization with the grid via a power synchronization loop (PSL). Based on the adjustable range of active power, grid-forming N-SMSs can be further divided into two types: V0 grid-forming and PV grid-forming sources. The difference between the two is that the PV grid-forming source can only operate when connected to an active grid. The four types of N-SMSs mentioned above all utilize a three-loop controller structure, with the outermost loop tailored to their respective control objectives. The inner dual-loop controllers, however, are conventional inner-outer loop controllers, a control strategy that is well-established and widely adopted. These controllers typically adopt proportional-integral (PI) control. Taking the PQ grid-following non-synchronous power sources as an example, their control principle is illustrated in Figure 2, where the control equations for the dual-loop system are presented. \[\left\{ \begin{array}{l}u_{\text{d}}\ =\ (k_{\text{P}}\ +\ \frac{k_{\text{d}}}{s})(i_{ \text{d}}\,^{*}-i_{\text{d}})\ +\ Ri_{\text{gd}}-\omega Li_{\text{q}}\ +\ u_{\text{gd}}\\ u_{\text{q}}\ =\ (k_{\text{P}}\ +\ \frac{k_{\text{q}}}{s})(i_{\text{q}}\,^{*}-i_{ \text{q}})\ +\ Ri_{\text{gq}}\ +\ \omega Li_{\text{d}}\ +\ u_{\text{gq}}\\ i_{\text{q0}}\ ^{*}\ =\ (k_{\text{vP}}\ +\ \frac{k_{\text{d}}}{s})(u_{\text{dc}}\,^{*}-u _{\text{dc}})\\ i_{\text{q0}}\,^{*}\ =\ 0\end{array} \right.