Coordination of Damping Control in Transmission Networks with HVDC links

Abstract

The primary goal of this thesis is to investigate the small-signal stability of transmission networks with wind generation and high voltage direct current (HVDC) transmission integrated in them. The topic reflects a number of concerns of transmission network operators in the U.K. and beyond over how stability is maintained as networks evolve. Damping low frequency power oscillations using wide-area signals are illustrated for both line commutated based current source converter (interchangeably referred as LCC or CSC) and self commutated voltage sourced converter (VSC) based HVDC links integrated within host AC networks. With VSC HVDC offering flexibility greater than LCC HVDC in terms of modulating both active and reactive power, it was shown that optimally allocating/sharing the control duty among the multiple control options that exist within a VSC HVDC link, ensures that the overall control duty is reduced, and hence, the dynamic ratings of the expensive converters is minimised. An important consideration in the design of power oscillation damping (POD) controllers is to ensure that the controller is robust across a range of practical operating conditions. To achieve this, an analytical control design technique is introduced in which all the control loops for a VSC HVDC link, are designed simultaneously in a multi-variable framework. The method results in a set of decentralized, robust single-input-singleoutput (SISO) controllers at the two ends of a VSC HVDC link, which ensures coordinated control action and an acceptable performance level in the event of loss of a remote feedback signal. Damping contribution from remote offshore wind farms connected via VSC HVDC is an important consideration for systems with high penetration of wind energy. The effectiveness of coordinating the supplementary control of the wind farm and the onshore HVDC converter is shown in terms of dynamic variations in wind farmreal power output, DC link voltage and turbine speed. Having examined the damping contribution, frequency support from offshore wind farms is also explored. A method using appropriate droop control on the offshore and onshore HVDC converters is proposed which enables offshore wind farms connected through VSC HVDC link to contribute to system inertia and primary frequency control without having to rely on remote communications. The effectiveness of the proposed approaches is illustrated through detailed frequency domain analysis and extensive time-domain simulation results in DIgSILENT PowerFactory on two test systems.