Highly Compact Transformerless Universal Power-Flow and Quality Control Circuit

This paper presents a novel, highly compact direct-injection modular universal power flow and quality control topology exclusively using lower power components. In addition to conventional high-voltage applications, it is particularly attractive for the distribution grid down to low voltage as it can exploit the latest developments in low-voltage high-current semiconductors. The distribution grid is under immense pressure due to rapid growth of high-power loads and sources, such as vehicle chargers and photo-voltaic installations, which can locally overload lines and generate both over-and under-voltage conditions in the same grid section so that conventional voltage adjustment through transformer tap changers fails. In contrast to conventional universal power flow and/or quality controllers (UPFC, UPQC) or soft-open point (SOP) circuits, the concept both eliminates any injection transformer, which is large, costly, and limited in dynamics and the need to convert the full grid power. The presented direct-injection circuit exploits the recent developments in high-current low-voltage transistors. The centre piece is a low-voltage module connected in series with each phase, which is floating with the grid voltage so that it only needs to deal with a small voltage difference between its outputs and can maintain that condition also under fault currents of several kiloampères, Due to the low-impedance of the distribution grid, only a few volts are sufficient to control several hundred amperes in active and reactive current in a loop. In contrast to commercially available transformer-injection-based SOP solutions with often considerable additional loss, the injection modules have almost negligible additional impedance
$(< \mathbf{4}\ \mathbf{m}\mathbf{\Omega})$
.