Abstract
Distributed generation (DG) allows the production of renewable energy where it is consumed, avoiding transport losses. It is envisioned that future DG units will become more intelligent, not just injecting power into the grid but also actively improving the power quality by means of active power filtering techniques. In this manner, voltage and current harmonics, voltage unbalance or over-voltages can be mitigated. To achieve such a smart DG unit, an appropriate multi-functional converter topology is required, with full control over the currents exchanged with the grid, including the neutral-wire current. For this purpose, this article studies the three-phase four-wire split-link converter. A known problem of the split-link converter is voltage unbalance of the bus capacitors. This mid-point can be balanced either by injecting additional zero-sequence currents into the grid, which return through the neutral wire, or by injecting a compensating current into the mid-point with an additional half-bridge chopper. For both methods, this article presents a discrete time domain model to allow controller design and implementation in digital control. Both techniques are validated and compared by means of simulation results and experiments on a test setup.
Highlights
Compared with the PI controller of the Zero-Sequence Current Injection (ZSCI) method given in Section 3.3, the bandwidth can be significantly higher because the Half-Bridge Chopper (HBC) method does not use a Low-Pass Filter (LPF) filter
The three-phase four-wire split-link converter is proposed as a multifunctional converter topology for smart distributed generation (DG) units
By implementing an active power filtering (APF) technique in the converter control, the power quality can be improved in a distributed manner
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Several APF techniques use the neutral-wire current to compensate for voltage unbalance, over-voltage or harmonics, e.g., the harmonic current compensation technique [5], the damping-based droop control technique [6] or resistive harmonic voltage filtering [7] To deploy these active power filtering techniques in three-phase DG units, the use of a four-wire converter is essential. An appropriate multi-functional converter topology is required to achieve a smart DG unit This topology has full control over the currents exchanged with the grid, including the neutral-wire current. The challenge is to keep the dc bus voltage shared between both capacitors, i.e., to balance this mid-point This balancing can be achieved by means of an additional control loop based on the injection of zero-sequence currents [15,16], or by means of an additional active balancing circuit [10,17]. Both techniques are validated and compared, both in simulation and on an experimental setup
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