arXiv subjects

Alexander Koehler

Publications and source records attributed to Alexander Koehler.

3 recordsLinked to original sources

arXiv2025

Entirely Transformerless Universal Direct-Injection Power-Flow Controller

An increasing penetration of renewable energy resources, electric vehicle chargers, and energy storage systems into low-voltage power grids causes several power management and stability problems, such as reverse power flow, (local) overload lines, and over- / under-voltage. Previous power-flow and soft-open-point solutions are bulky and expensive. They need transformers and large magnetics, some on grid frequency, others more compact at high frequency. Even suggested circuits with high-frequency transformers still struggle with cost and size. We present a compact partial power-conversion high-current full-power-flow control circuit without a single transformer. We combine silicon and silicon-carbide, each with their specific advantages for current-dense direct injection. The circuit further needs fewer semiconductors than previous concepts. The circuit links a shunt converter through a non-isolated inverter bidirectionally with low-voltage series modules that practically float with their respective phases can serve between different feeders in low-voltage power grids. We analyze the circuit mathematically and evaluate the operation in simulation and experimental results.

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arXiv2025

Partial-Power Flow Controller, Voltage Regulator, and Energy Router for Hybrid AC-DC Grids

The share of electronically converted power from renewable sources, loads, and storage is continuously growing in the low- and medium-voltage grids. These sources and loads typically rectify the grid AC to DC, e.g., for a DC link, so that a DC grid could eliminate hardware and losses of these conversion stages. However, extended DC grids lack the stabilizing nature of AC impedances so that the voltage is more fragile and power flows may need active control, particularly if redundancy as known from AC, such as rings and meshing, is desired. Furthermore, a DC infrastructure will not replace but will need to interface with the existing AC grid. This paper presents a partial-power energy router architecture that can interface multiple AC and DC lines to enable precise control of voltages and both active as well as reactive power flows. The proposed system uses modular low-voltage high-current series modules supplied through dual active bridges. These modules only need to process a small share of the voltage to control large power flows. The topology reduces component size, cost, energy losses, and reliability more than three times compared to conventional technology. The optional integration of battery energy storage can furthermore eliminate the need for the sum of the power flows of all inputs to be zero at all times. Through dynamic voltage injection relative to the line voltage, the modules effectively balance feeder currents, regulate reactive power, and improve the power factor in AC grids. Real-time hardware-in-the-loop and prototype measurements validate the proposed energy router's performance under diverse operating conditions. Experimental results confirm the series module's functionality in both AC and DC grids as an effective solution for controlling extended grids, including power sharing, voltage, and power quality.

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arXiv2025

A Highly Compact Direct-Injection Power-Flow Controller and Line-Voltage Regulator with Shared Magnetics and Partial-Power Conversion for Full-Power Control

An increasing integration of photovoltaic units, electric vehicle chargers, heat pumps, and energy storage systems challenges low-voltage power grids and can cause voltage range violation, loss of stability, (local) overload of lines, and power management problems. Research suggested universal power-flow control (UPFC) to solve power management problems. In contrast to bulky, slow, and costly conventional UPFCs with their shunt and series transformers, this paper presents a highly compact and current-dense power-flow controller, which can serve between different feeders in the low-voltage power grids. The enabler is a systematic combination of silicon car-bide (SiC) with silicon (Si) transistors and a strict partial-power topology built around a multi-active bridge. The circuit links an active-front-end converter as a shunt stage through a multi-active-bridge converter bidirectionally with low-voltage series-injection modules floating with their respective phases. The topology can use small power to control high currents through the low-voltage series-injection modules. The multi-active bridge serves as a multi-input-output power router that exchanges energy between all elements. We assess the design as well as the implementation considerations of the proposed power-flow controller mathematically and verify its performance in simulation and real systems.

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