A Highly Compact Transformerless Universal Power-Flow and Quality Control as well as Soft Open Point Circuit
The electricity system, particularly the low-voltage grid is under immense pressure due to rapid growth of high-power loads and sources, such as vehicle chargers and photovoltaic 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 tap changers fails. Soft-open points (SOP) with either full ac-ac conversion or conventional series voltage injection through transformers with universal power flow and/or quality controllers (UPFC, UPQC) are studied and promoted but are large due to either the transformer or the full-power electronics, in case of a transformer injection also limited in dynamics, and costly. We present a direct-injection circuit for soft open points, universal power flow and quality control, which exploits the recent developments in high-current low-voltage transistors. The centre piece is a low-voltage (e.g., 48 V) 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 between its outputs and can maintain that condition also under fault currents of several kiloampères and worst-case enforce 0 V. Due to the low-impedance of the distribution grid, only a few volts are sufficient to control several hundred ampères in active and reactive current in a loop. Thus, the circuit only needs to control a fraction of the controlled power in contrast to full ac-ac concepts. The direct injection eliminates the bulky transformer with its dynamic limitations. In contrast to commercially available transformer-injection-based SOP solutions with often considerable additional loss, the injection modules have almost negligible additional impedance (< 4 mΩ), which is particularly important in the low-voltage grid.
