# Droop Control Strategy of Utility-Scale Photovoltaic Systems Using Adaptive Dead Band
## 1 Introduction
In [22,23], multiple devices were used to counteract the uncertain voltage fluctuations using the multi-timescale method. For example, capacitor banks and on-load tap changers with slow and discrete responses were dispatched every hour without information on stochastic renewable output and load variation. The inverter interfaced with the DGs adjusted the reactive power output by checking the uncertainty every 15 min. However, depending only on probability distribution in the dispatch algorithm, an inaccurate control may occur in the sudden network topology change. In the centralized control method with a transmission system operator (TSO) [24], a voltage fluctuation was detected by an accurate state estimation scheme within the control center of the TSO. TSOs contain information about the network state, and thus, can check for voltage limit violation for changes in load or RES output and take appropriate action. Another approach that uses a multiagent system (MAS) for intelligent volt/var control in power systems was proposed in [25, 26, 27]. MAS enables autonomous control of local distribution energy resources through a centralized energy management system (EMS) [5]. In addition, using this system, it is possible to not only maintain the voltage profile but also minimize the system loss and switching times of shunt capacitors [25]. Although the method using MAS with EMS suggests an optimal use of system devices, all reactive power resources must communicate with all other resources or a central controller, thus requiring network observability and a dense communication architecture. In addition, these MAS approaches require the parameters and operation points to be updated every time the network state changes; moreover, the complex nonlinearities of power and voltage in the optimal power flow model make the problem-solving time-consuming. Thus, the present paper proposes an adaptive dead band strategy, which is a simpler and more intuitive control strategy than the previously proposed ones. Most of the previously proposed reactive power-voltage control strategies for transmission systems have many drawbacks compared to the proposed method. Decentralized coordination control methods [21,23], which perform voltage regulation in power systems, are less accurate than the proposed method. To increase the accuracy of the sensitivity method [18, 19, 20], historical information regarding DG output and load variation is utilized to accurately obtain the value of sensitivity in the system and estimate the required reactive power. A centralized MAS [25; 26; 27] requires network observability and a dense communication architecture. The proposed method can solve the voltage problem against RES output variability by using information about the daily load curve, RES output, and system topology. The dead band of the reactive power-voltage control of each RES calculated through the proposed algorithm is transmitted to the plant controller through a TSO. Therefore, this paper proposes a centralized control law for controllable DGs and plant controllers using an adaptive dead band. The key contributions of this paper are as follows. * Estimation of required reactive power for each bus in the transmission system through the calculation of voltage sensitivity in an offline study. * Achievement of more accurate voltage regulation between the DGs and plant controller by using the adaptive dead band strategy. * Provision of a suitable solution to the voltage problem by injecting an accurate reactive power into each POI. * Achievement of flexibility and redundancy using the architecture, even with unforeseen network topology changes. The remainder of this paper is organized as follows. Section 2 presents the conventional and proposed schemes for the reactive power-voltage droop control method. Section 3 presents a case study on the modified IEEE 39 bus system to validate the proposed control scheme. Section 4 concludes this paper.