Power Converter Resonant Control for an Unbalanced and Non-Constant Frequency Supply

Abstract

Distorted voltage supplied as unbalanced and/or non-constant frequency can be found in weak grids, such as microgrids, or systems working in islanding mode. These kinds of systems are more sensitive under load changes. Particularly, an unbalanced voltage supply may be produced for large, single-phase loads. On the other hand, the connection/disconnection of high current loads may lead to important frequency variation, especially in weak grids where the short circuit current capacity is reduced. These conditions make the control of the power converter a more difficult task, because of the variations in the frequency and unbalancing. To address these issues, this paper proposes a resonant control algorithm to deal with variations in the voltage amplitude as well as grid frequency when a distorted power supply is considered. The frequency variation is an important drawback for resonant control because the resonance must be tuned at the grid frequency. This issue is overcome by using a variable sampling frequency in order to avoid re-tuning the controller parameters. On the other hand, under unbalanced conditions, the proposed method relaxes the phase with lower voltage amplitude by taking more power from the other phases in order to help the stability of the grid supply. To corroborate the mathematical analysis and the proposed control, a stability study is performed, including experimental and simulated results.

Improved Feedback Quantizer with Discrete Space Vector

“…From (3), if H(z) = 1, we have perfect quantized voltage tracking, but this is not possible due to causality; therefore, the optimal H(z) to use will be z −1 , which has several advantages, such as ease of system analysis, the natural behavior of the system for lowfrequency harmonic mitigation, and easy implementation of notch filters [16,17]. Therefore, the equation of the system is as shown in Equation ( 4) and replacing H(z) = z −1 in (4), we obtain Equation (5).…”

“…From ( 5), we can obtain the transfer function of the system sensitivity S(z) (6), and using the optimal H(z), we have (5):…”

“…From the magnitude obtained in (8) and Figure 1c, we can observe the natural behavior of the FBQ system, where it tends to mitigate the low-frequency harmonics, and, in addition, good tracking of the voltage reference at the fundamental frequency (50 Hz) is obtained. Then, it can be observed from (5), replacing H(z) with z −1 , that by having a zero at z = 1 the system will not have DC components in the voltages produced by the converter.…”

“…AC/DC or DC/AC power conversion is now essential because of the new changes in the energy matrix and the global context of generating clean energy. Thanks to technological advances, researchers have created various converter topologies and control schemes to meet the needs of each application [ 1 , 2 , 3 , 4 , 5 ]. These applications include AC motor drives [ 6 ], renewable energies [ 7 ], HVDC systems [ 8 ], electromobility [ 9 ], and microgrids [ 10 ].…”