Asynchronous HVDC System -based on Three Level NPC Converter M. Flitti #1 , M. Khatir *2 , M.K. Fellah *2 , K. Mendez #1 Intelligent Control and Electrical Power System Laboratory 1 Djillali Liabes University of Sidi Bel-Abbes, Algeria 2 Belhadj Bouchaib Center University of Ain Témouchent , Algeria flitti_med@yahoo.fr Abstract— Voltage Source Converter High Voltage Direct Current (VSC-HVDC) systems have the ability to rapidly control the transmitted active power and independently exchange reactive power with transmissions systems. The present work investigates the modeling and control design of a high-voltage direct current (HVDC) transmission system based on three-level NPC multilevel converters with Subharmonic PWM modulation technique (SH-PWM), and a feed-forward decoupled current control strategy. By using this control strategy, not only the active and reactive power of HVDC can be controlled independently but also the dynamic responded time can be shortened. Simulation studies of a 200MW/ ±100 kV back to back VSC-HVDC system connecting two asynchronous ac networks are presented to confirm the satisfactory performance of the proposed system under active and reactive power variations from single phase to ground and three phases to ground fault conditions. Keywords— NPC multilevel converter, VSC based HVDC, SH- PWM I. INTRODUCTION Voltage Source Converter (VSC)-based High- Voltage Direct Current (HVDC) schemes using insulated- gate bipolar transistors (IGBTs) (known as VSC transmission) has attracted increasing attention. The main advantage of VSC power transmission is the high controllability, the ability to control independently active and reactive power at each terminal and the possibility for linking with dead networks. These characteristics make VSC transmission attractive in many applications like the emerging interconnection with renewable energy sources. The disadvantages are known as higher power losses and higher capital cost compared with conventional HVDC [1- 2]. In order to maximize the potential of VSC transmission systems, a number of technology breakthroughs are required. One requirement is the reduction of the power losses and the harmonic distortion generated by the converters. This will allow the reduction of cooling needs and space requirements as well as increasing the system’s operating efficiency and reliability. The other one is to ensure that the system operates satisfactorily during abnormal conditions, such as during severe network unbalances. Theoretically, one promising way forward could be the adoption of a multilevel converter as a building block for the system. There are a number of distinct multilevel converter topologies which have been used or proposed for the VSC transmission system, namely, the neutral-point clamped (NPC), the flying-capacitor (FC) converter, and the cascaded Converters. While these multilevel converters have their respective merits and shortcomings, the selection of the converter topology is a detailed engineering design exercise. It needs to take into account a number of parameters, including the system design and control, power loss, cost, etc… [3]. Due to these characteristics this paper present the element of an asynchronous back-to-back VSC- HVDC system which uses three level neutral point clamped inverter topology with (SH-PWM) technique and a current- control strategy in rotation frame that the ac current is feedforward decoupled made the active and reactive power exchange controlled independently. The simulation results got from MATLAB software confirm that the control strategy provides satisfactory response and strong stability. II. INVERTER TOPOLOGY Fig 1 illustrates the fundamental building block of a diode -clamped inverter. In this circuit, the dc-bus voltage is split into three levels by two series-connected bulk capacitors, 1 C and 2 C . The middle point of the two capacitors n can be defined as the neutral point. The inverter in Fig. 1 provides a three-level output across a and n.        2 dc an V V , 0,        2 dc V For voltage level       2 dc V , switches 11 S and 21 S need to be turned ‘ON’. For        2 dc V switches ' 11 S and ' 21 S need to be turned ‘ON’; and for the ‘0’ level, either pair   ' 11 21 , S S needs to be turned ‘ON’. The same switching pattern applies across the phase ‘b’ leg (if ‘a’ is replaced by ‘b’) but phase shifted by 180° for