Design and operation of Low Frequency AC (LFAC) transmission system for connecting offshore wind farms

Student thesis: Doctoral Thesis

Abstract

The offshore power transmission market continuously grows as government incentives for Renewable Energy, and technological advancements lead to the exploitation of offshore wind for electricity generation due to its high capacity factors and the abundant offshore space. Currently, the High Voltage AC (HVAC) and High Voltage DC (HVDC) transmission systems (TS) are the commercial options for integrating the wind power to the onshore grid. The technical and economic efficiency of each approach is evaluated on a case-by-case basis and is mainly dependent on the Offshore Wind Power Plant (OWPP) capacity and its distance from shore, with HVAC TS being the most common solution for distances lower than 100kmThe Low Frequency AC (LFAC) technology can provide an attractive transmission solution for exporting power from vast and remote OWPPs within a distance range of 80-300km. The basic idea of the LFAC system is to use lower AC frequency to reduce the effective electrical length of the equivalent HVAC subsea cable, so that its transmission capacity is increased, and also to eliminate the need for bulk offshore HVDC platforms, so that the installation, operation and maintenance costs are decreased. Despite the theoretical advantages of LFAC, the related research and its previous application in the rail and oil industries, there is no prior operational experience in similar type and scale networks, increasing the uncertainty for investors regarding the LFAC compliance capability and technology readiness level for offshore transmission purposes.The aim of this industrial PhD Thesis is to result in robust and feasible LFAC TS options that can be realised by common industry practices and comply with typical Transmission System Operator'.¬ôs (TSO) grid code requirements. The LFAC TS is designed as a combined offshore HVAC system utilising an onshore Back-to-Back Modular Multilevel Converter (BtB MMC) scheme for frequency conversion. Point-to-Point (PtP) and Multi-Terminal (MT) LFAC topologies are examined for various cable lengths to export the nominal capacity of the OWPPs or interconnect different OWFs and different grids through single-cable connections to minimise costs. The technical feasibility of the LFAC TS is assessed through comprehensive steady-state, frequency and time-domain studies to optimise the system performance, its transmission capability and identify probable limitations.Initially, the main transmission equipment specifications are appropriately adapted and rated for LFAC operation. Frequency scan studies are performed to investigate the low-order resonances of the LFAC TS, and suitable C-Type harmonic filter combinations are introduced to avoid potential harmonic stability issues. Power-flow cases are studied to calibrate the operating conditions of the system (e.g. reactive power compensation, control strategies, etc.) and optimise the schemes in terms of power losses and shunt cable compensation, placed only at the onshore side. It is yielded that by utilising the AC voltage control strategy of the OWPP inverters together with the reactive power capability of the BtB MMCs can maximise the power transfer through the cables.Finally, detailed EMT models of the optimised PtP and MT-LFAC TS are simulated to evaluate its dynamic performance, stability and FRT capability against faults in the offshore LFAC and AC sides. It is concluded that if the LFAC grid-forming MMC employs a typical islanded control scheme without Inner Current Controllers (ICCs), it should always use harmonic filtering to avoid low order harmonic stability issues caused by the system resonance. Even then, its FRT response against an LFAC side fault would be unacceptable, due to its unconstrained LFAC fault current. However, by implementing high-bandwidth ICCs with appropriate current saturation and integrator anti-windup mechanisms to the islanded-control
Date of Award28 Jul 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SupervisorLie Xu (Supervisor) & Grain Philip Adam (Supervisor)

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