## 1 Introduction  
The involvement and incorporation of active components such as distributed generation (DCs) have transformed power systems from being passive ones (operated based on unidirectional power flow) to being non-passive (based on bidirectional power flow) or active distributed systems (ADS). ADS is a part of a smart grid (SM) whose operational characteristics are dependent on advanced controllers and smart communication infrastructures, including smart meters, to meet users' needs and utility preferences effectively. ADS is a relatively recent concept that has arisen in the context of the developments that have evolved over the past few decades in the area of electric power supply. It essentially relates to a salient shift in the manner in which electric power distribution systems are planned, designed, and operated [1]. The distribution network is a key component of the electric power supply system, creating the vital link between (large-scale, centralized) electric power generation and high-voltage transmission on the one side and end-users of the power supply on the other. Traditionally, it has fulfilled this role in a largely \"passive\" manner, having little to no real-time automation and power flow control. Distribution networks have traditionally been designed to passively cater to all possible load scenarios, as well as contingencies, without much reliance on active control. Moreover, with one-way (and largely predictable) power flow, protection coordination and response to abnormal system conditions have also been fairly manageable, with little need for sophisticated protection solutions. This approach to electrical distribution system design and operation has served the intended purpose quite well for a long time. New developments, however, notably the deregulation of the electric power supply industry (in most countries), as well as the growth of distributed generation, have necessitated a reconsideration of the adequacy of the traditional approach, leading to the concept of \"_active distribution systems_\". Grid-integrated distributed systems [2] in general have in the last few decades gone from being nearly non-existent to becoming a significant (and growing) component of all new generations in most parts of the world [3; 4]. In South Africa, the Renewable Energy Independent Power Producer Procurement Program (REIPPPP), launched in 2011, has since resulted in the contracting of nearly 12 GW of renewable energy, of which about 6.1 GW has thus far been integrated into the electric power grid [5]. Through the Integrated Resource Plan (IRP) 2010-2030, very ambitious targets have been set for the contribution of environmentally friendly generation to the meeting of total South African energy needs going forward, especially given the need to honor the carbon emissions reduction commitments made by the South African government [6] and the ongoing challenge of severe electric power supply shortages the country is facing. In the context of an ever-growing share of distributed (and especially variable renewable) generation in the power system, the efficient grid integration and management of these new forms of generation have been identified as being pivotal to ensuring the continued operation of a stable, efficient and more economical electric grid [7]. This, in turn, raises the need for efficient control and grid integration strategies that enable the active contribution of distributed energy resources (DER) to the realization of the power system's operational objectives [2; 8]. One strategy for the grid integration of DERs that has been widely studied is that of aggregating a variety of DER technologies and operating them in a coordinated manner to combine their synergistic characteristics and simultaneously enhance the exploitation of intermittent renewable generation. Microgrid and Virtual Power Plant (VPP) are two such aggregation approaches that continue to be active areas of extensive research [9].