PhD Defense Jeroen Stuyts - Implementation of Unbalance Compensation using Grid-supporting Converters
- Prof. dr. ir. Johan Driesen (promotor)
- Prof. dr. ir. Willy Sansen (chairman)
- Prof. dr. Jef Beerten (secretary)
- Prof. dr. ir. Patrick Reynaert
- Prof. dr. ir. Jozef Poortmans
- Dr. Reinhilde D'Hulst , VITO
- Prof. dr. Frede Blaabjerg , Aalborg University
- Dr.Ir. Sven De Breucker , Dynniq
Converters are increasingly appearing in the low-voltage electrical grid. Converters are most commonly used to convert alternating current in direct current and vice versa. Use cases are then the grid-coupling of solar panels or batteries, charging of electric vehicles etc.
The increase of grid-coupled converters has amplified a number of challenges in the control of the electrical grid such as voltage and frequency stability, fault behavior etc. Especially the increase in single-phase converter connections to three-phase grid poses a particular challenge: interphase unbalance. Such unbalances appear when, in a three-phase grid, the currents and/or voltages in/over the different wires are different. Unbalance appears almost all the time, but it has gotten worse due to these single-phase connections of converter, typically for the grid-coupling of solar panels.
However, the increase in grid-coupled converters also offers control possibilities, because the behavior of (some) converters is programmatically controlled. As such, in the first part of this thesis, these control opportunities are explored. A basic controller is proposed and then expanded with grid-supporting features. These grid-supporting features aid the grid in maintaining its designed state.
A common controller aspect is the need for extended and accurate information about the grid-state. Therefore, a grid-state estimator is proposed that estimates all information required for grid-support. This grid-state estimator is implemented and tested. It is proven to be faster and more accurate than the state-of-the-art.
The second part of this thesis then extensively researches unbalance. A novel unbalance compensation technique is proposed: current redistribution. The current in the grid is actively redistributed in the grid by injecting compensating currents with the converter. In that way, an unbalanced current of e.g. 1A in phase 1 and 2, and 4A in phase 3 is redistributed by the converter to become balanced, i.e. 2A per phase. Current redistribution can be executed while the converter is still executing its main task, i.e. transferring power between the alternating and direct current side. This method is implemented and tested. It is proven to be very effective, eliminating unbalance, when the converter can inject the required amount of compensating current.
However, when that is not the case, an enhancement is proposed by means of optimal current redistribution. An optimal solution is calculated that takes into account the hardware constraints of the converter. The solution represents then the best way that this converter can compensate unbalance by redistributing current. Also this optimal approach has been implemented and tested.
In future work, this optimal approach can be expanded to incorporate the other grid-supporting features and therefore optimally distribute the effort. This would allow any controllable converter to actively and optimally support the grid.
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