Active Power Modulation Assisting Controller Scheme Implemented on a VSC-HVDC Link Establishing Effective Damping of Low Frequency Power Oscillations Melios Hadjikypris #1 , Vladimir Terzija #2 # Department of Electrical Energy and Power Systems, The University of Manchester, School of Electrical and Electronic Engineering, Ferranti Building, Sackville Street, Manchester, M13 9PL, United Kingdom 1 Melios.Hadjikypris@manchester.ac.uk 2 Vladimir.Terzija@manchester.ac.uk Abstract—Inter-area power oscillations damping is of fundamental importance in today’s era of sophisticated and highly complex smart grids. The present paper demonstrates a practical solution obtaining satisfactory performance in damping power system oscillations utilizing the flexible VSC-HVDC transmission technology in an optimal combination with a power oscillation damping (POD) controller, utilizing active power modulation technique. Primary focus of the control scheme is to improve damping of lightly damped or even unstable modes of the interconnected AC/DC power system through modal analysis technique and a residue based approach. The proposed control scheme guarantees stability margins through eigenvalue analysis and non-linear time domain simulations, which are performed on a four-machine two-area system. The software platform under which the various simulation scenarios were implemented was DigSILENT PowerFactory. Keywords: Low Frequency Power Oscillations, Inter-area oscillatory modes, Power Oscillation Damping, POD controller, VSC-HVDC, Eigenvalue analysis, Residues, DigSILENT PowerFactory. I. INTRODUCTION In today’s era of advanced technological intelligence, Cybernet and advanced automations, electrical power networks play a key role towards a smarter and greener sustainable future of the modern world [1]-[3]. A future smart grid must comply specific requirements associated with reliability, robustness and protection. A viable solution to this challenge is found in the recent advancements in high speed power electronics which soon enough gave rise to flexible AC transmission system (FACTS) devices [4]-[5]. A powerful tool that accomplish enhanced static and dynamic performance of the power system is the use of HVDC transmission technology, in fact Voltage Sourced Converter (VSC) based [6]-[7]. The fast power electronic switches of the Insulated Gated Bipolar Transistors (IGBTs), (constituting the core mechanism of VSCs along with pulse width modulation PWM technique), enables the self-commutated power converters to promptly and independently control the active and reactive power flows in the power system. This in turn boosts system’s power transfer capability and stability performance during transient periods. The potentials provided by VSC-HVDC technology were very soon adopted by ABB and Siemens, under the code names HVDC Light ® and HVDC PLUS [8], [9]. However, through the years of power systems evolution, a persistent challenge for system operators is associated with low frequency power oscillations developed in power systems [10]. The nature of these oscillations arises from lightly damped or even unstable local or inter-area oscillatory modes, under the case when the system experience small or large signal disturbances. Power oscillations appear as a result of inadequate damping torque of generators’ rotors. The oscillating rotors in turn, develop the oscillations of other system’s variables, such as active and reactive power flows, current flows, bus voltage magnitudes, phase angles, and electrical frequencies. The low frequency electromechanical oscillations arise in a power system, are usually found in the range between 0.1-2Hz, depending on the number of contributing generators [11]. Local oscillations, which consist of oscillations of a single generator or a group of generators against the rest of the system, are usually found in the higher end of that range. Inter-area oscillations on the other hand, which occupy the lower end of frequencies (0.1-0.8 Hz) [12], are the oscillations associated between a group of generators in one area swinging against a group of generators in another area of the system. In large interconnected power systems, poorly damped or even unstable inter-area oscillations usually occur between grids which are weakly connected. The weak connection refers to adjacent power grids connected over high impedance transmission lines, or transmission corridors subject to sudden and substantial change in power flow [13]. Over the years of innovation, substantial research has been undertaken in limiting the phenomena of inter-area oscillations, which span from instabilities, cascading events, or ultimate system blackouts (August/1996 western US/Canada interconnected system total blackout [13]). The mitigation actions developed involve the applications of traditional power system stabilizers (PSSs) to counteract