Abstract
Nowadays, the increasing number of nonlinear loads and renewable energy resources pose new challenges for the standard electrical grid. Conventional solutions cannot handle most of them. The weakest component in the whole system is a conventional distribution (converting medium to low AC voltage) transformer. It should not operate with unbalanced, heavily distorted voltage and cannot control power flow or compensate current harmonics. One of the promising solutions to replace the conventional transformer and thus minimize power flow and grid distortions is a power electronics device called a solid state transformer (SST). Depending on the SST topology, it can have different functionalities, and, with the proper control algorithm, it is able to compensate any power imbalances in both low voltage (LV) and medium voltage (MV) grid sides. In the case of a three energy conversion stage SST, the LV and the MV stages can be treated separately. This paper focuses on the MV-AC to the MV-DC stage only based on a star-connected cascaded H-bridge converter. In this paper, a simple control solution for such a converter enabling different current control strategies to distribute power among the phases in an MV grid in the case of voltage imbalances is proposed. Simulation and experimental results proved good performance and verified the validity of the proposed control algorithm.
Highlights
Many non-linear loads connected to the grid, such as switchedmode power supplies, air conditioners, or fluorescent lighting, might cause disturbances that conventional distribution medium to low AC voltage (MV/low voltage (LV)) transformers cannot compensate, which may result in improper work of the whole system [1]
The paper presents a simple control solution for medium voltage (MV)-AC to MV-DC stages based on a cascaded H-bridge (CHB) converter enabling different current control strategies to distribute power among the phases in an MV grid in case of voltage imbalances depending on what is needed from the MV grid perspective such as constant MV grid active power, symmetrical line currents, or the faulted phase being unloaded
A simulation model to prove the performance of the proposed control was created in PLECS software with the algorithm written in C language
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The paper presents a simple control solution for MV-AC to MV-DC stages based on a cascaded H-bridge (CHB) converter enabling different current control strategies to distribute power among the phases in an MV grid in case of voltage imbalances depending on what is needed from the MV grid perspective such as constant MV grid active power, symmetrical line currents, or the faulted phase being unloaded. There are methods to control the star-connected CHB converter presented in the literature [11,12,13,14,15], such current control strategies with additional input parallel output parallel dual active bridge converters (IPOP DAB) reference current calculations were not reported In such scenarios, the IPOP DAB (interfacing MV-AC to MV-DC and LV-DC to LV-AC conversion stages) works as an active load with non-equal current distribution for the SMs of the star-connected CHB converter.
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