# Droop Control Strategy of Utility-Scale Photovoltaic Systems Using Adaptive Dead Band
## 1 Introduction  
Recently, a wide range of sustainable energy policies have been implemented in several countries to increase the share of electricity generation from renewable energy sources (RESs). Broad-based policies, such as tradable energy certificates, are more likely to induce innovation in RES' technologies that are close to competitive with conventional generators. In particular, carbon emission issues can be resolved using RESs, such as photovoltaic (PV) and wind power, and an increase in the number of RESs shifts the system topology from large-scale generations to distributed generations (DGs) [1]. However, the rapid increase in DGs can cause various power system stability issues [2], including system inertia reduction [3], transmission congestion [4], substation overload [5], and voltage fluctuation [6]. In particular, the voltage problem caused by randomly generated output power from climate-affected DGs is the main concern for system operators. The integration of a DG with high variability in a conventional network may create a voltage problem [7, 8]. Regarding voltage regulation in transmission systems, several coordination strategies have been proposed so far, which can be classified into two groups: (1) coordinated control between DGs and reactive power compensation facilities, such as a flexible AC transmission system (FACTS) [9, 10], and (2) supplementary control functions, which have been implemented for DGs to use the advanced reactive power-voltage control method [11, 12, 13]. To achieve advanced control ability, previous studies have focused on decentralized and centralized control strategies [14, 15, 16, 17]. The decentralized control method acquires network data (e.g., bus voltage magnitude) at the point of interconnection (POI), and if the measuring voltage does not meet the reliability criteria, the individual parameters of DGs are set to compensate the voltage error and adjust the voltage set point within an acceptable range. The basic approach to implementing decentralized control of DGs is to use the sensitivity method [18,19]. Sensitivity cannot be calculated online without the communication infrastructure, such as the phasor monitoring unit, and the method used to calculate sensitivity using the Jacobian matrix incurs large calculation time. Thus, the authors in [20] presented improved decentralized control with a surface fitting technique in offline studies, where multiple DGs were coordinated by the time delay setting technique to perform accurate voltage control. However, these methods are less accurate because the two control strategies cannot consider the sudden network topology change caused by a rapid change in the RES output power. Moreover, the primary control objectives in the no-feedback control and grid-connected modes of operation are conflicting. The decentralized droop controllers force only the bus voltages to deviate from their nominal values in the primary control, because the current standard employs proportional control loops locally at each inverter. This causes voltage fluctuations, especially under an asymmetrical fault. Furthermore, the system voltage is adjusted by adding a control function that matches the allowable voltage to individual DGs, rather than through interaction with other DGs or facilities in the system. In the centralized method, the control signal is transmitted to DGs and other reactive power resources by the central controller through a communication system, and is optimized for better voltage regulation. For example, model predictive control was used to reduce the modeling inaccuracies and measurement noise of facilities in a distribution system [21], which can be applied in transmission systems. In that study, a cooperative control for DGs and the load tap changer (LTC) was proposed to regulate the voltage to be within the allowable range. However, the LTC action takes a few seconds per tap to move, and thus, is not reasonable for controlling the power electronics-interfaced DGs, which can rapidly change the output power.