Performance Analysis and Control of HVDC Links In Multi-Machine Systems with Wind Farms

Abstract

A global trend in the growth of power systems is to build interconnections with the aim of achieving technical, economical and environmental benefits.The interconnections facilitate exchange of power between different regions or countries improving the utilization, flexibility, power quality, efficiency of transmission and emergency support. High Voltage Direct Current (HVDC) systems emerge as a compelling and efficient solution for long-distance power transmission, asynchronous system interconnection, and renewable energy integration. As offshore wind generation is exploited at increasing distances from shore, HVDC stands out as the preferred transmission method. Offshore wind farms equipped with Direct Drive Permanent Magnet Synchronous Generators (DD-PMSG) are drawing increased attention due to their advantage over other variable speed technologies. Voltage Source Converter based High Voltage Direct Current (VSC-HVDC) links are considered the most suitable option for transferring power to the onshore system. The rapid development of VSC-HVDC based offshore wind farms have highlighted the need for these stations to synchronise with the grid independently and provide inertial support, especially as grid strength declines, rendering phase-locked loops (PLLs) ineffective. In this context, the research work deals with the systematic analysis of Line Commutated Converter based High Voltage Direct Current (LCCHVDC) links, Voltage Source Converter based High Voltage Direct Current (VSC-HVDC) links and integration of VSC-HVDC based offshore wind farms into multi-machine systems. Specifically, the study presents a performance analysis of synchronous and asynchronous multi-machine systems using tie-lines as LCC-HVDC links. Detailed inferences are drawn for the systems subjected to various dynamic events in comparison to systems with Thyristor Controlled Series Capacitor (TCSC) lines. The effect of variation of synchronous tie-line power levels on the modal behaviour is investigated. The asynchronous system modes are analysed with reference to the modes of its synchronous counterparts. The impact of VSC-HVDC active and reactive power controls on frequency controllers is assessed. The effect of feedforward and feedback active power loops are examined through bandwidth analysis. The influence of reactive power controllerson the performance of frequency controllers is also studied. Additionally, a simplified model of VSC-HVDC link is proposed that can be efficiently used for analysis of links embedded in large AC systems. This model offers lesser modelling complexity and computation time, enhancing efficiency in analysis. Following the above analysis, the integration of VSC-HVDC connected DDPMSG based offshore wind farms into multi-machine systems is explored. A novel approach for power flow and initial condition calculations is proposed to facilitate dynamic analysis of the system. For three cases of the most commonly specified quantities of the wind farm, efficient methods have been described. This approach enables the user to build the model in any basic graphical dynamic modeller and numerical computational software without requiring power system toolboxes or electromagnetic transient packages. Case studies and simulations are conducted to verify the proposed technique. The work further presents a novel approach for grid synchronisation and inertial support from VSC-HVDC based offshore wind farms. The DC capacitor dynamics are utilised to imitate the rotor dynamics of a synchronous generator for synchronisation, while inertia support is derived from the DC capacitor energy and the hidden inertia of the offshore wind farm. A time constant based approach employing two distinct DC voltage tolerance bands for the control is proposed. This leads to improved inertial response and a very easy selectionof controller parameters. The effectiveness of the strategy has been proved in high impedance and low inertia systems. MATLAB/SIMULINK has been used for the study.