Control of VSC-HVDC with Electromechanical Characteristics and Unified Primary Strategy Weiyi Zhang 1 , Kumars Rouzbehi 1 , J. Ignacio Candela 1 1 Department of Electrical Engineering Technical University of Catalonia Barcelona, Spain, 08222 Email: weiyi.zhang@estudiant.upc.edu Alvaro Luna 1 , Pedro Rodriguez 1,2 2 Abengoa Research Abengoa Sevilla, Spain, 41014 Abstract—High voltage dc (HVDC) systems act as the prevailed solution for transmitting offshore wind energy to onshore main grids. Control of the voltage source converters (VSC) in HVDC systems is decisive for the performance. This paper proposes the control of VSC-HVDC with electromechanical characteristics and unified primary strategy, as a reaction to the updated requirements of the ac grid transmission system operators. As two important aspects of VSC-HVDC control, converter control and primary control are both designed in detail. Electromechanical characteristics make the VSC capable of providing inertia to the ac networks as well as simplicity in island operation. Besides, unified primary control is given as a universal primary strategy for VSC stations, and especially takes into account frequency support and control mode transition. The proposed converter control is validated in scaled-down 10 kW laboratory setups, while the proposed primary control is endorsed by the simulation tests on a CIGRE multi-terminal HVDC model. I. INTRODUCTION For the past decade, high voltage dc (HVDC) transmission systems based on voltage source converters (VSC) have emerged as a promising technology due to its technical and economic advantages in different applications compared with the traditional systems based on line commuted converters (LCC) [1]. In HVDC systems, offshore VSCs normally perform power/frequency control, while onshore VSCs control the dc voltage. In this manner the balance of energy is achieved and the power is transmitted from the offshore to the ac network following the wind status. However, with expected increasing integration of HVDC systems, their impact on the power system stability, especially the frequency stability, will be more visible. Therefore, frequency support and inertia emulation should be considered in the control design of future VSC-HVDC. VSC-HVDC control with inertia emulation has been being discussed in recent years. The authors in [2] define the power- frequency swing based on inertia, and link the offshore frequency, dc voltage and onshore frequency by droop control. In this way the offshore grids can provide the specified amount of inertia to stabilize the frequency perturbations in the onshore grids. In [3], inertia emulation control of VSC-HVDC is also designed, in which the dc voltage is regulated following the ac frequency in a specified trajectory based on the inertia characteristics, and the energy stored on the dc bus acts as the inertia reserve. Besides, VSC control design with inertia emulation and virtual admittance is seen in [4] for supporting low-inertia island grid. Regarding primary control of VSC-HVDC, voltage droop control is frequently used. The operation of HVDC systems with voltage droop characteristics is critical for the system stability. The droop slope can be specified considering the steady-state voltage range of the dc grid and the power processing constraint that is applied to each VSC [5]. Since the HVDC systems are interfaced with ac systems, the requirements of the ac transmission system operators (TSO) should also be met. Frequency support is hence necessary for control design of future onshore VSCs. A secondary control structure for multi-terminal HVDC (MTDC) grids with load frequency control is proposed in [6], thus the power order of VSC can hence be set taking into account the ac grid frequency. The frequency support can also be achieved by configuring the primary control as shown in [7]. Conventionally, the VSC stations in HVDC systems are devoted to the stabilization of the dc systems, and simply act as feeders to the ac systems. In contrast, this paper addresses the control of VSC-HVDC considering the ac TSO requirements as well as the stabilization of dc systems. The proposal can be divided into two aspects, the converter control and primary control. Regarding the converter control, the VSCs are equipped with electromechanical characteristics (the emulation of inertia, damping and output impedance of synchronous machines). For primary control, a universal structure is designed with multiple modes of operation (including frequency droop) and availability of mode transition for flexible and autonomous operation. The proposed control is validated by simulation and scaled-down experimental results. II. PROPOSED CONTROL FOR VSC-HVDC A VSC based on the proposed control strategy is shown in Fig. 1. Electromechanical block and unified primary controller are the two main features of the proposed control. A. Converter control The inertia emulation block is designed in detail in section III. It is able to generate a virtual synchronous frequency Ȧ,