###### Abstract  
In modern controller design, various solutions for controlling power converter systems can be found depending on their applications, speed of working, pulse width modulation (PWM) techniques, switching frequencies--\(f_{c}\), and different load types. The need to manipulate control parameters can be often observed in classical structures, e.g., well-known PID, repetitive, or deadbeat control particularly sensitive to distinct parameters uncertainties. The purpose of this paper is to present an improved version of controllers designated for a UPS that will be considerably resistant to model changes. Proposed control techniques are independent of unexpected output filter changes: \(L_{f}\)--filter coil inductance and \(C_{f}\)--filter capacitor conductance. The second aspect of this paper is to compare effectiveness of modified predictive MPC (model-predictive control) and feedback PBC (passivity-based control) controllers in reducing output voltage total harmonic distortion (THD) for various load values. The biggest distortions of output voltage were observed during experiments with nonlinear RC load. Both simulation and experimental verification of mismatching parameters were performed and examined. Thanks to the proposed solution, the output voltage \(THD_{v}\) quality factor was reduced below 8% in an efficient way for all the applied loads and stayed at the level of 1% when well-matched filter parameters were provided. DC/AC converter; PBC; MPC; robustness; uninterruptible power supply (UPS); power converters; inverters; control algorithms +
Footnote †: journal: Applied  
performed simultaneously. It must be also distinguished that the harmonics higher than \(f_{c}/2\) are compensated by the previously implemented \(LC\) output filter for the standalone inverter device [7; 8]. The designing process of the DC/AC converter should start with designing the \(LC\) filter (proper \(C_{f}\) and \(L_{f}\) material selection) and calculating its values [9]. This procedure has a significant influence on the quality of the output voltage \(v_{o}\)[9; 10]. Nonetheless, switching frequency \(f_{c}\) value also has a big impact on the control and system stability. Divergence from the previously calculated \(LC\) values may occur in the system due to many reasons: electronic component damage or poor quality of manufacturer's design. However, the designed controller should be able to decrease the values of the output distortion according to the applicable standards, e.g., IEC 61000-2-2, EN 62040-3, and IEEE 519-2014 [11; 12; 13]. Lower harmonics must be reduced by the feedback control loop or by prediction of the state variables. They are typically implemented in stationary \(\alpha\beta\) or \(dq\) rotating reference frame in accordance with the three-phase topology and potential necessity of decoupling application. The motivation behind this paper is to show advantages and disadvantages of the proposed improved controllers dedicated especially for the DC/AC standalone inverters in UPS application [4]. The robustness of the system with an output filter provided with mismatching parameters is highlighted in this study for two forms of MISO control: feedback tracking system with a modulator and state variable prediction without using a modulator. The goal is to discriminate between the two control systems based on two critical performance metrics and recommend the best operation using either passivity or predictive methods. In this context, especially in terms of controller design, it is preferable to choose a method that allows for a flexible design capable of addressing the issue of having fewer control inputs than control variables while maintaining system stability. Passivity-based control approaches [1; 2] provide for a methodical approach to controller design, with the system structure clearly defined. Interconnection and damping assignment (IDA) is one of these strategies. It entails selecting a desirable closed-loop energy function and ensuring that the control error converges to zero.