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Implementation of Unbalance Compensation using Grid-Supporting Converters

Boek - Dissertatie

The number of grid-coupled converters in the low-voltage electrical grid has been steadily increasing for the past few years, among others due to the drive for sustainability. More and more applications such as distributed generation, electric vehicles, storage etc. are connected each day using converters. This amplifies some challenges for the control of the grid such as voltage and frequency stability, fault behavior and harmonics.Especially the increase in single-phase distributed generation poses a particular challenge: interphase unbalance. While single-phase loads inherently cause unbalance in the three-phase grid, single-phase distributed generation causes the power flows to reverse, amplifying the problems and additionally, in three-phase four-wire grids, increasing the current through the neutral conductor.However, the increase in grid-coupled converters also offers control possibilities, because the behavior of (some) converters is programmable. As such, in the first part of this thesis, these control opportunities are explored. An industry-standard controller is proposed and then expanded with grid-supporting features such as voltage and frequency droop control, unbalance compensation, fault ride-through and harmonic compensation.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. The grid-state estimator is benchmarked and experimentally verified on a rapid prototyping system for power electronic control.The second part of this thesis then extensively researches unbalance, focusing on two aspects: current redistribution and the addition of a neutral connection to converters. Current redistribution is a novel means to compensate unbalance with converters, which ideally then have a neutral connection. The results concerning the neutral connection show that it is beneficial to include this connection for a number of reasons. It can be both advantageous to the grid as to the converter owner. However, the current going through the neutral connection is significantly higher than the current through the phase connections. This current is quantified based on three criteria: simultaneity, reactive power and the expected amount of distributed generation.Therefore, one cannot expect that each converter has enough capabilities to always compensate unbalance perfectly. Hence, current redistribution is expanded with an optimization algorithm that allows to compensate unbalance to the best of the abilities of the converter. This optimal approach guarantees that the primary goal of the converter is not hindered: exchanging balanced (re)active power with the grid. Additionally, the hardware constraints are pro-actively taken into account. The approach is experimentally verified by implementation on the rapid-prototyping system.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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