An MVDC to MVAC converter for cold ironing on an offshore mooring and power platform

Abstract

In this dissertation, in accordance with the DISTRICT project by the department of electrical engineering, a power supply connection compatible with Onshore Power Supply (OPS) standards was designed to supply ships berthed at an offshore platform. This would allow ships waiting to enter port or bunkering in Maltese territorial waters to be supplied with clean energy. In order to supply large ships with energy, the power supply connection had to be capable of outputting Medium Voltage AC (MVAC). Thus, a Multi-Level Converter (MLC) topology was required. From the existing topologies, a Modular Multilevel Converter (MMC) topology was selected. The converter was set operated with Phase-Shift Carrier (PSC) Pulse-Width Modulation (PWM), with the submodule (SM) capacitors and arm inductors sized accordingly to reduce SM capacitor voltage ripple. To reduce harmonic emissions, the output filter was designed as an LCL filter with the load-side inductance representing the leakage inductance of a dual-winding transformer. The converter control consisted of both positive sequence and negative sequence control in order to produce balanced AC voltages. An outer droop controller provided voltage magnitude and frequency references to cascaded voltage and current controllers for each sequence. Two case studies were performed to analyse the performance of the designed power supply system at the Point Of Connection (POC) with the ship. This was conducted using data obtained from an OPS facility in the Grand Harbour, Valletta. In the first case study, voltage and current data from the OPS facility was used to generate per-phase active and reactive load power profiles, to which the designed converter model was subjected to. It was found that the per-phase load voltages and operating frequency were compliant with the IEC 80005-1-2012 standard for OPS installations. In the second case study, the power profiles obtained in the first case study were altered so as to create an artificial load power unbalance between the phases. Under these unbalanced conditions, the designed converter remained compliant with the IEC 8005- 1-2012 standard. Furthermore, the voltage unbalance at the POC was shown to remain negligible, even under conditions of significant unbalance.