Analysis Of DC-side Fault Response of MMCs with Controlled Fault Blocking Capability for Different Transmission Line Types

MMCs with controlled fault blocking capability retain control of their currents during a dc-side fault, thereby reducing the required interruption capabilities for switchgear. To design the dc-side control to achieve this capability, it is important to take into account the interaction between the converter control and the transmission line during a short-circuit on the transmission line. This paper uses a dc-side equivalent model to assess the interactions of the converter control for two types of converters, i.e., full-bridge and hybrid, with two main types of transmission lines, i.e., cable and overhead line, during dc-side faults. In general, the dc-side voltages and currents for a full-bridge MMC connected to an overhead line show more oscillatory behavior compared to an MMC connected to a cable. Furthermore, a hybrid MMC connected to an overhead line may provide a more damped dc-side fault response due to its limited negative voltage capability.

Introduction

DC systems are increasingly used within the power system, with applications in High-, Medium- and Low-Voltage dc (HVDC, MVDC and LVDC) [1]. In these systems, converters with fault blocking capability are considered attractive given that they can eliminate the need for dc-side switching equipment with fault interruption capability, as e.g., described in [2], [3] and used in a protection strategy as discussed in [4], [5]. In HVDC, modular multilevel converters (MMCs) with controlled fault blocking capability are planned for first installation in a German project connecting the north of the country to the south [6]. The increased use of these converters calls for a unified approach towards modeling the dc-side fault behavior, as detailed converter models are computationally inefficient and control details may not be available to the users of such converters or to suppliers of other equipment connected to these converters. In this paper, we analyze the influence of controller gains as well as transmission line type on the dc-side fault behavior of converters with controlled fault blocking capability using the modeling approach introduced in [7].