توپولوژی بهبود یافته برای مبدل های AC / DC چندگانه در سیستم های HVDC و VFD: عملیات در حالت های تخریب شده An Improved Topology for Multi-pulse AC/DC Converters within HVDC and VFD Systems: Operation in Degraded Modes

I. INTRODUCTION RELIABILITY, availability and maintainability (RAM) of high-voltage DC (HVDC) transmission and variable frequency drive (VFD) systems are the key criteria to effectively secure the global demand for the generation of electric power [1] – [3]. In these systems, the load commutated converter (LCC) technology for very high-power AC/DC and DC/AC conversion not only provides pulsating DC or AC power, but also an efficient and high-power quality interface to AC systems (e.g., grid, motors) [1] – [4], [3] – [6]. In addition, there is the need to study the reliability of multi-pulse LCC in the Zhundong-Sichuan HVDC project which was launched in China to better understand the power and investments involved in making multi-pulse LCC reliable (2015, ±1100 kV, 10 GW, 2600 km) [6]. Figure 1 is a summary of potential candidates for a multi-pulse AC/DC rectifier for high-power systems. a) Conventional multi-pulse AC/DC. This rectifier is the earliest and most widely known technology that is used within HVDC and VFD systems. It is efficient and gives a large amount of power, it is also reliable, and its architecture may allow it to continue to operate even in faulty conditions [5] – [7]. Fig. 1a is a 12-pulse rectifier-based thyristor which is also known as a 12-pulse LCR (load commutated rectifier). It is a current source converter that consists of a 12-pulse transformer (or phase-shifting transformer) and two series-connected 6- pulse rectifiers. The shifting angles ( and ) between each secondary winding and the primary winding of the transformer are key elements for obtaining pulsating DC power with reduced voltage ripples and current harmonics at the grid side. However, the structure of the 12-pulse transformer does not enable the rectifier to operate efficiently in a degraded mode. For example, a severe failure in any part of a transformer may result in a total system shut down if no redundant transformer is installed; additionally, including a redundant transformer will increase the size and cost of the rectifying system.