energies Article Aggregation of Radial Distribution System Bus with Volt-Var Control Hiroshi Kikusato * , Taha Selim Ustun , Dai Orihara , Jun Hashimoto and Kenji Otani   Citation: Kikusato, H.; Ustun, T.S.; Orihara, D.; Hashimoto, J.; Otani, K. Aggregation of Radial Distribution System Bus with Volt-Var Control. Energies 2021, 14, 5390. https:// doi.org/10.3390/en14175390 Academic Editors: Pavlos S. Georgilakis and Marco Pau Received: 9 June 2021 Accepted: 27 August 2021 Published: 30 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Fukushima Renewable Energy Institute, AIST (FREA), 2-2-9 Machiikedai, Koriyama 963-0298, Japan; selim.ustun@aist.go.jp (T.S.U.); orihara.dai@aist.go.jp (D.O.); j.hashimoto@aist.go.jp (J.H.); k.otani@aist.go.jp (K.O.) * Correspondence: hiroshi-kikusato@aist.go.jp; Tel.: +81-29-861-8168 Abstract: The high penetration of the distributed energy resources (DERs) encourages themselves to implement grid-supporting functions, such as volt-var control. The quasi-static time-series (QSTS) simulation is an essential technique to evaluate the impact of active DERs on the grid. Meanwhile, the increase of complexity on the circuit model causes a heavy computational burden of QSTS simulation. Although circuit reduction methods have been proposed, there have been few methods that can appropriately handle the distribution system (DS) with multiple voltage control devices, such as DERs implementing volt-var control. To address the remaining issues, this paper proposes an offline bus aggregation method for DS with volt-var control. The method determines the volt-var curve for the aggregated bus on the basis of historical data to reduce error in the aggregated model, and its offline process solves the computational convergence issue concerned in the online one. The effectiveness of the proposed method is validated in the simulation using a Japanese low-voltage DS model. The simulation results show that the proposed method can reduce the voltage error and computational time. Furthermore, the versatility of the proposed method is verified to show the performance does not heavily depend on how to select historical data for model-building. Keywords: advanced inverter; distribution system; IEC 61850-90-7; IEEE 1547; photovoltaic; power system modeling; power system analysis; reactive power control; voltage regulation 1. Introduction Continued cost reduction and policy support are driving the sustained uptake of wind power and solar photovoltaic (PV) across the world [1]. With the increased integration of the distributed energy resources (DERs) to power systems, the required capabilities of them have also changed. Earlier, it was required that DERs did not actively regulate voltage and shall trip on abnormal voltage/frequency [2]. Recently, requirements were revised so that they are capable of actively regulating voltage/frequency and shall ride through abnormal volt-age/frequency [3]. Such migration in the power system makes it challenging and important to understand the interaction between the power system and DERs. In the DSs, many solutions have been proposed to mitigate the voltage rises, as well as to maximize the captured energy by the DERs. As solutions on the supply side, on- load tap changers (OLTCs) and their control schemes have been improved [4,5]. While on the demand side, various DER management methods are proposed [6,7]. Voltage- reactive power (volt-var) control by inverters of DERs is a reasonable solution to which DERs themselves, the cause of the issue, can contribute [8,9]. Since these methods will be eventually integrated to the same DS, coordinating them must be studied [10,11]. Quasi-static time-series (QSTS) analysis is essential to analyze the DSs with high penetrations of DERs for developing the above-mentioned solutions [12]. This solves a series of sequential steady-state power flow. Each time-step of the calculation is solved to rely on the information from the previous time-step solution [13]. Due to the features of QSTS analysis, as the number of buses, components, and a calculation time-step of the Energies 2021, 14, 5390. https://doi.org/10.3390/en14175390 https://www.mdpi.com/journal/energies