# Performance and Control Strategy of Real-Time Simulation of a Three-Phase Solid-State Transformer
## 5 Real-Time Operation  
One of the main differences with offline simulations is that the operation should be treated as a physical system to avoid possible overruns; hence a soft-start strategy is needed in order to avoid inrush voltages and currents. The operation begins with the \(VSC_{i}\) converter operating as a three-phase diode rectifier, with a voltage in C\({}_{1}\), the DAB converter operates with an angle \(\theta=0\), and \(VSC_{o}\) is needed to transfer 0 W.  
Simulation Results  
In order to corroborate the operation of the implemented topology, the control is asked to follow a reference from 0 W to 500 kW. The results are shown below, in all the cases currents have an output gain \(K_{o}=\frac{1}{10}\), and the power signals are escalated by a factor \(K_{f}=\frac{1}{1000}\). Figure 9 shows tracking of the control scheme for the active power reference \(P_{o}^{*}\), with a overshoot corresponding to the 12%, this value can be reduced by the use of trajectory planning if higher regulation is needed for the application on critical loads; and the reactive power is compensated at the input and output PCC, as shown achieving a power factor \(P_{F}=1\). Figure 8: Proposed PI controller scheme. Figure 7: Proposed DAB modulation scheme. In Figure 10 the transient behavior of the input currents at the \(PCC_{i}\) and the voltage \(V_{\rm C1}\) is shown. Having a smooth evolution on the current dynamics is desired for a physical application, and for the \(V_{\rm C1}\) exists a 20% of drop in the DC value during the transition. Figure 11 presents the current signals between the converter \(VSC_{i}\) and the LCL filter, with the currents without the mitigation effect of the LCL filter, showing the capability of the RTS to recreate physical signals with a high fidelity in comparison with fundamental DAM models which the signal will be the same escalated by the filter gain without any ripple, and in order to recreate switching effects there is the need to create more complex models to emulate the switching frequency impact. Figure 11: \(VSC_{i}\) input currents and \(V_{\rm C1}\). Figure 9: Total active and reactive power. In Figure 12 the input voltages at the \(PCC_{i}\) are shown, the signal \(VSC_{i}\) is used to synchronize the voltage signals with the active power step, not finding any disturbance during the transient condition. In Figure 13 the \(VSC_{i}\) and \(VSC_{o}\) control signals during the transient are shown, as seen all the references reach the steady-state without saturation, with a smooth behavior. Figure 14 shows the DAB control signal \(\theta\) also with a transition without saturation, and its effect over the voltage \(V_{C2}\) as shown in the inferior trace. Figure 14: DAB control signal (\(\theta\)) and \(V_{C2}\). Figure 12: \(PCC_{i}\) voltages and \(V_{C1}\). Figure 13: \(VSC_{i}\) and \(VSC_{o}\) control signals. The currents at the \(PCC_{o}\) are shown in Figure 15, with a desirable behavior and an output total harmonic distortion \(THD=0.98\%\) in steady state, which is adequate to achieve the interconnection with the grid. Figure 16 shows the output currents of the \(VSC_{o}\) converter, in this case is noticeable the mitigation effect of the LCL filter, in comparison with the currents after the LCL filter in Figure 15.  
Figure 17 shows the voltages at the \(PCC_{o}\), as in \(PCC_{i}\) the signal \(V_{\text{C2}}\) is used to synchronize the voltage signals with the power transient, and in the same way have a behaviour without finding any disturbance. Figure 17: \(PCC_{o}\) voltages and \(V_{\text{C2}}\). Figure 15: \(PCC_{o}\) currents and \(V_{\text{C2}}\). In Figure 18 the control is needed to follow beforehand a reference \(P_{o}^{*}=700\) kW, and suddenly this reference is changed to \(P_{o}^{*}=-700\) kW, showing how the SST is capable of inverting the power flow among the three-phase AC sources.