## 1 Introduction  
Governments of all countries in the world, including Taiwan, are intensively promoting renewable energy to reduce the dependence on petrochemical energy and the impact on the global environment. Taiwan has planned to increase the share of renewable energy generation capacity up to 20% by the year 2025, in which the solar photovoltaic (PV) installed capacity is planned to reach 20 GW. Specifically, the rooftop and ground-mounted systems will account for 3 GW and 17 GW, respectively. Energy Bureau of the Ministry of Economic Affairs in Taiwan also announced the Solar PV Two-Year Promotion Project [1] to increase 1.52 GW of installed capacity between July 2016 and June 2018. To install a PV system in Taipower distribution systems, the voltage variation percentage at the point of common coupling (PCC) should not exceed 2.5% [2]. This voltage limitation would impose an upper limit on the injection power from the PV system and the hosting capacity for renewable generation (RG). To progress towards the development of sustainable energy, a review on the current status, applications, developmental barriers, and prospects of solar photovoltaic systems in India was briefedin [3]. The study also mentioned the renewable energy policies and some recommendations. To speed up renewable energy development, policy experience in many countries has shown that mandatory buy-back policies with feed-in tariffs (FITs) that specify a favorable purchase price for renewable energy are one of the most effective incentives [4]. In Taiwan, the FIT policy is mainly applied. For successful operation of a grid-tied renewable energy system, lots of works on power inverters have been proposed and investigated in the literature. A survey of power-electronic systems for the grid integration of renewable energy sources was presented in [5] and a comparison with different technologies and wide range of power ratings for PV systems was found in [6]. To avoid the voltage violation problem caused by large intermittent injection of RGs, many invert-based techniques [7; 8; 9] allowing the distributed generations to absorb reactive power from the power system were proposed. In addition, to mitigate the overvoltage problem and enhance the PV penetration in distribution system, a distribution static compensator (DSTATCOM) application was proposed in [10] and a PV inverter control scheme with battery energy storage systems (BESS) was presented in [11]. As shown in [12], the smart PV inverter can enhance the maximum power generation limit, provide power output with fixed power factor, intelligently control the voltage variation, and conduct real power adjustment and reactive power compensation. Recently, a smart PV inverter with Volt-Var droop control method [13] for distributed voltage regulation was tested and analyzed under Maui Advanced Solar Initiative Project in Hawaii. However, the Vol-Var droop control curve is piecewise linear and the optimal real power output cannot be guaranteed. Recently, a coordination method of smart PV inverters and the novel Watt-Var control scheme with three operation phases [14] has been proposed to mitigate voltage violation for load transfer between distribution feeders with high penetration of PV installation under National Energy (NEP) II Project in Taiwan. The novel three-operation-phase Watt-Var control scheme can fully utilize PV's capacity to keep PV's real power output as high as possible. Thus, this paper will then present the novel Watt-Var control scheme to improve the PV penetration in the distribution system. To provide more information for investors and governments to make better decisions or policies for the PV industry, the economic feasibility of residential energy storage combined with PV panels in Italy [15] considering subsidies, and an economic evaluation of the hybrid enhancing scheme with DSTATCOM and active power curtailment for PV penetration in Taipower distribution systems [16] considering FITs, were presented.