Control Strategy in Parallel AC-DC Re-configurable Links System

Abstract

Recent studies have shown that energy distribution systems based on hybrid (or parallel)point-to-point AC and DC links are interesting solutions for capacity enhancement andon-line improvement of energy efficiency under specific operating conditions, particularlyfor high power, medium to high voltage levels and long-enough distances, e.g.>5km.Additionally, due to the power controlability provided by the power electronic circuitswithin the DC links the parallel distribution system can be re-configured through GISdisconnectors operating with improved switching performance. In this work this featureis explored and strategies for the system re-configurations are proposed. These aim toprovide a nearly zero current switching for the GIS in order to prevent its deteriorationand to extend the number of maneuvers for the typical GIS lifetime.
In fact, the proposed re-configuration strategies rely on the DC bus voltage and powercontrols of a Back-to-Back (B2B) power electronic system constructed with Voltage SourceConverters (VSC). Herein, three-wire three-phase two-level VSCs with thrid-order AC har-monic filters (or LCL filter) are adopted in the B2B system. Moreover, Grid-side CurrentControls (GCC) for the inner/fast control loops of the front- and back-end circuits are im-plemented in order to enhance the system performance against grid disturbances. Herein,a notch filter-based GCC scheme with a harmonic rejection control is proposed. This isable to deliver attenuation to the LCL filter resonances while suppressing the harmonicsin the grid-side currents originated when the AC voltages are distorted. Interestingly,the parallel AC and DC links provides a low impedance path for the zero-sequence com-ponents which can be created by the power electronics, thus the implementation of aZero-sequence Circulating Current (ZSCC) controller becomes necessary for the properoperation of the system. Hence, ZSCC controllers for the VSCs are used, while bothfront- and back-end circuits are modulated with constant switching frequency using theconventional sinusoidal PWM strategy.
All in all, results are obtained in both, computational simulations carried out in MAT-LAB/SIMULINK and laboratory experiments performed in a5kVA VSC or a B2B circuit,are used to verify the study and to prove the superior performance of the investigatedand proposed technical concepts. Herein, more specifically for the re-configuration studyof the parallel AC and DC links, the standalone mode control of the receiving-end VSCin the full DC link configuration is investigated and the smooth transition between grid-connected and standalone modes are realized in both simulation and experimental tests.