# Enhanced Control Strategy for Three-Level T-Type Converters in Hybrid Power-to-X Systems
## 2 Literature Survey  
For the hybrid system to receive instantaneous power, sinusoidal current and voltage need to be injected, and the controller must track or compensate for these sinusoidal signals [20; 21; 22]. Traditionally, proportional-integral (PI) control in the synchronous reference frame has been used to achieve zero steady-state tracking error. However, this approach often exhibits steady-state errors in both amplitude and phase when managing AC signals [1; 23; 24]. These issues have been mitigated with stationary frame resonant compensators [25; 26; 27; 28]. These compensators, single-resonant or multi-resonant, provide exceptionally high gain at specific non-zero frequencies, distinguishing them from classical stationary frame PI controllers and effectively minimizing steady-state errors to near-zero levels [29; 30; 31; 32]. This paper proposes a novel dual-loop control configuration for three-level  
Figure 1: Block diagram of the proposed hybrid system. three-phase T-type converters. Existing dual-loop control frameworks typically incorporate an inner current loop and an outer voltage loop, but their ability to handle disturbances efficiently varies significantly. Traditional implementations rely heavily on PI-based controllers, which struggle to maintain robust performance under dynamic grid conditions. Our proposed control strategy builds upon these frameworks by incorporating a novel feed-forward (FF) action that enhances disturbance rejection, improves transient response, and ensures more stable voltage regulation compared to conventional methods. The FF action is implemented within the inner loop to preemptively cancel disturbances before they propagate through the system. Unlike traditional feedback-based disturbance rejection, which reacts only after a disturbance has impacted system performance, the FF action proactively mitigates its effects by adjusting control inputs in real time. This approach significantly improves system robustness, particularly under fluctuating grid conditions and load variations [33; 34]. The proposed dual-loop control configuration in this paper includes an inner loop containing a two-degrees-of-freedom control structure, i.e., (a) inner loop of FF action to cancel disturbances and (b) an outer loop of tracking controller for inverter output current which injected into the grid or to an AC load through an \(LC\) or \(LCL\) filter, and the outer loop of the dual-loop control structure contains tracking controller for the capacitor voltage (output voltage in case of an \(LC\) filtered inverter). Since the disturbances are cancelled by the FF action, the control problem of both outer loops reduces to imposing tracking behavior. Hence, the control strategy of the tracking controllers is based on the time-domain design of a single-resonant controller [35; 36]. In addition, the proposed design considers actuator delay present in any practical system and the bandwidth limitation resulting from the two-degree-of-freedom inner loop by considering the closed loop transfer function of the inner loop when designing the controller of the outer loop. Another critical aspect is the balancing of DC voltage capacitors, a task that becomes increasingly complex in three-level T-Type converters due to their inherent topology [37; 38]. Unlike two-level converters, whose DC link is usually implemented using a single capacitor, three-level converters employ split DC links; thus, their control structures must contain two DC link voltages in order to regulate the sum and the difference of partial DC link voltages. This paper introduces a control strategy to address the imbalance problem alongside the implementation of the dual-loop control setup. This paper is structured as follows: In Section 2, three-phase three-level bidirectional AC-to-DC and DC-to-AC converter structures are introduced, as well as their role in P2X systems. Section 3 presents the proposed control structure with detailed analysis, including the DC link capacitors' voltage balancing.