**1. Introduction**

Solar power still has huge potential and the rate of installation has been growing drastically. A report by the European Photovoltaic Industry Association (EPIA) revealed that the European cumulative photovoltaic (PV) capacity, had increase to more than 120 GW from around 29 GW in 2010 [1]. The report suggests that the PV installation rates will continue to grow through the next decade. A small-scale PV system for residential use is connected to the distribution network, and acts to reduce the electric fee by supplying local demand. An energy managemen<sup>t</sup> system (EMS) including a building energy managemen<sup>t</sup> system (BEMS) could be implemented for these kinds of sources, although grid operators concentrate mainly on farm-scale generation systems. Currently, PV plants with a capacity of over 1 GW have been connected worldwide as described in [2], and a number of megawatt (MW)-scaled PV arrays have been configured in those farm networks. These trends will continue to increase, and the production managemen<sup>t</sup> which covers their stochastic characteristics will pose a challenge in the power system industry.

To stably and e fficiently harness energy from various renewable resources, clustered distribution farms have pursued integrating compensation devices for supporting grid operation with enhanced

controllability [3]. These circumstances have been presented by the reported variation resulting from unpredictable natural resources, which is a major issue in renewable energies [4]. Although a clustered farm exhibits better power profile with the in-built smoothing e ffect, a classic power system still requires advanced solutions by concentrating on predictable usage of storage devices. Currently, most storage applications are oriented to time shifting of real power supply rather than support the connected grid [5]. However, as the requirement in terms of response between cluster and operator expands, further concretion on the controller is required in the power system industry [6].

Recently, the renewable-storage application has been increased due to the increasing large-scale wind farm, something that could significantly a ffect an integrated power grid, [7]. As a fastidious requirement from grid operator is expected in future power grid, the authors in [8] highlight the importance of both real and reactive power compensation according to the power extracting condition. The reactive power compensation generally focuses on the voltage at local power system; it relies on installed auxiliary devices. On the other hand, an energy storage system (ESS) such as a battery storage could act as real power compensation at the distribution generator side. Both compensation options are based on power-conversion system (PCS) that normally follow an optimized signal generated based on the reference by the main system [9–11]. To achieve a close cooperation with distinct distribution units, conceptual approaches that integrate ESS at the direct current (DC) network have been considered through full converter-based wind turbines in Ref. [12]. Recently, typical DC-based sources including PV and tidal have been extended as an ESS clustered form with other resources as described in Ref. [13]. The main objective of those integration forms is to collaborate among the connected devices, which can enhance the controllability based on the imposed system order [14]. The PV system that is able to compose a hybrid network along with ESS at the DC section is usually composed of a number of solar panels for generating usable input voltage and reasonable power extraction. As described in Refs. [15,16], these specifications generally allow not only electric loss in a cable but also voltage drop below the standard testing condition. Therefore, a direct application of ESS in large-scale PV-based DC clusters is classified as being supported by current analysis at each point. In industrial application forms which are based on inbuilt complementary modes to respond to grid requirements, both limitation and compensation options have to match the harsh power fluctuation of renewable sources [17]. In order to continuously utilize these hybrid features, power flow analysis-based approaches and appropriate ESS design are required.

Meanwhile, the increased utilization of storage in the power system industry has resulted in demands of enhanced storage feature, since power swapping requires fast response capability and suitable energy/power capacity [18]. In the case of conventional ESS applications, utilization of a number of control schemes including hierarchical control were tried as described in Ref. [19]. The described works sugges<sup>t</sup> that we can utilize an ESS configuration to achieve power balance in a power grid where there is presence of several renewable energy and variable loads. However, in previous research, the ESSs were separately composed of the renewable network, and the classified power conversion devices had to respond to the order from the operator based on their own topology. To perform a detailed control within a combined DC circuit that integrates ESS, an operation signal for charging/discharge must be generated reflecting an algorithm based on the expectation of power flow. In case the ESS solution is to be implemented in a PV-integrated DC system, it is deemed that a fast voltage analysis method should support the order decision process in order to take instantaneous voltage fluctuation into the controller [20]. In particular, given that renewable generators based on DC could impose unpredictable profiles on ESS operating processes, specific support strategies are required to manage a transient situation to cover these issues.

In this paper, the main objective is to come up with an ESS compensation scheme that considers exact voltage level so as to implement a power-managemen<sup>t</sup> plan for a DC-combined system by focusing on the demand of the power system. What distinguishes it from previous research on the renewable-ESS integrated networks is that it focused on a method to minimize errors based on detailed circuit analysis rather than systemic utilization. The main analysis in this document focuses on the small

power generated by the modules rather than simply focusing on the power extracted from each unit. The entire managemen<sup>t</sup> process is formed based on power flow analysis for DC systems, which deals with controllable elements that can be reflected on connected equipment. A simulation for verification is designed using the electro-magnetic transient DC (EMTDC) tool. In order to cause a considerable voltage fluctuation according to the output of the connected distributed resources, and to carry out an experiment, a distribution network with a scale of 0.5 km was constructed considering the real radial system. The ESS model configurations, including control topologies, are utilized to implement the case studies. The case studies focus on the operational accuracy of the proposed control scheme in regards to imposed order from operator.
