## 1 Introduction  
The global energy and environmental targets have directed most countries worldwide towards a greener energy generation portfolio. A large portion of renewable generation capacity is likely to be in form of distributed generations (DGs) directly connected to distribution (medium and low voltage) networks [1]. The increasing number of DGs, such as wind turbines and solar photovoltaic (PV) generators operating with an internal DC link has brought forward the concept of DC collector grids [2]. This concept is not limited to DGs integration as it can also be used as an interface for integrating DC loads such as electric vehicle's (EV) charging facilities, battery energy storage systems (BESS), LED lighting systems, DC data centers, and variable-speed machines. Furthermore, the DC system operates at a higher efficiency due to a reactive power that is not present, therefore reducing conduction losses. It is likely that the future medium voltage (MV) distribution network will consist of a hybrid AC/DC network or at least some parts of it. In recent years, there have been several demonstration projects of deployment of medium voltage DC (MVDC) networks. A campus project was developed by RWTH Aachen University to investigate the benefits of MVDC grids [3; 4; 5]. In the context of the UK, the first trial project of a MVDC link called \"Angle-DC\" will be installed by transforming the existing 33-kV AC distribution circuits between the Llanfair PG substation on Anglesey and the Bangor substation at North Wales [6] into DC operation. The voltage source converter (VSC) is the key element in enabling the deployment of MVDC infrastructure in existing distribution networks. A back-to-back (BTB) configuration of VSCs has been introduced in [7] by replacing the normally open mechanical switches at the end of feeders, called soft-open point (SOP), to facilitate DG penetration in distribution networks. In a later study, [8] advocated the benefit of SOPs in terms of fault isolation and supply restoration. In [9], the SOP was suggested to replace all remotely controlled switches which include both tie switches and sectionalising switches for a rapid network reconfiguration. The capability of SOP for fast voltage regulation was investigated in [10]. Since the application of SOPs provides operational flexibility to distribution networks, it has been considered as one of the factors to determine an optimal placement for distributed energy storage in [11]. A dynamic energy management framework considering renewable generations, energy storage systems, flexible demands, and the SOP application was proposed in [12]. In a recent study, [13] introduced a decentralised control of VSC stations to address the issue of communication failure. Like the demonstration project in [6], these studies [7; 8; 9; 10; 11; 12; 13] only considered the interconnection between any two feeders which would limit control (e.g., load transfer capability) to the two feeders. In recent years, the multi-terminal high voltage direct current (HVDC) configurations have been investigated extensively at the transmission level [14; 15; 16; 17; 18; 19]. A similar concept is plausible and may be applied at the MV level to achieve higher levels of operational flexibility by connecting all end feeders together. This paper proposes a flexible hybrid AC/DC MV distribution grid concept, which at its core takes advantage of the higher levels of operational flexibility by using an integrated MVDC interface that consists of a DC collection grid containing multiple VSCs and connects all end feeders together. Consequently, all DGs with internal DC links (wind and solar generators) can be connected directly to the MVDC system. The MVDC interface may also be used to achieve power transfer between the feeders, improving voltage control (e.g., at end feeders) and improving the control of the power flows. This expands the application of soft-open points (SOP) to all feeders.