I. INTRODUCTIONHOTOVOLTAIC (PV) generation has been one of the most popular renewable energy technologies in the world. Conventionally, dozens of panels are series-connected to increase the output voltage, and then feeds to a grid-tied inverter. The output currents of all panels are equal due to the series connection. However, the current generation of PV panel is dependent on solar intensity and its condition. If some panels generate less current than the string current due to partial shading, the unshaded panels have to lower their output current to match with the shaded ones. Consequently, the unshaded panels will not work at their maximum power point (MPP), which leads to a waste of power [1]. Such problem can also be Manuscript received February 4, 2018; revised April 24, 2018; accepted May 4, 2018. This work is sponsored by the National Nature Science Foundation of China under Grants 51577170, National Key Technologies R&D Program of China No. 2017YFE0112400, and the European Union's Horizon 2020 research and innovation programme under grant agreement No. 734796.Y. Zhu, J. Wu, R. Wang and X. He are with the College of Electrical Engineering, Zhejiang University, Hangzhou; 310027, China. (e-mail: y_zhu@foxmail.com; eewjd@zju.edu.cn; rui_chi@163.com;hxn@zju.edu.cn).Z. Lin is with the Department of Electrical, Electronic and Power Engineering, Aston University, Birmingham, B4 7ET, U.K. (email: z.lin@ieee.org).induced by manufacture difference, nonuniform aging, etc.To mitigate the aforementioned problem, dc-dc power optimizer (DCPO), which is a dc-dc converter with input connected to the PV panel and output connected to other DCPOs in series, has been proposed and widely studied [1]- [9]. By equipping each PV panel with a DCPO, the panel can work independently at its MPP, thus distributed maximum power point tracking can be achieved.In [1]-[4], different topologies of DCPO have been compared and analyzed. Buck, boost, buck-boost, etc., are candidate topologies for DCPO. Buck and boost converters are the most efficient and low-cost topologies [2][3], while buck is only suited for long string and boost is for short strings. Buck-boost, cuk and sepic converters are flexible in voltage ranges but always at an efficiency and alternatively cost shortcoming [1][2] [4]. In [5]- [7], modeling of a single DCPO and of a PV string equipped with DCPOs have been studied in detail, which is very helpful for system designing. In [8], a novel maximum power point tracking (MPPT) technique based on DCPO is proposed, which shows the effectiveness of DCPO in panel MPPT. [9] proposes an intelligent PV module concept based on DCPO and PLC, and it shows great potentials of DCPO in intelligent PV system development.In a PV system equipped with DCPOs, PV status monitoring, intelligent control algorithm, and outage management can be implemented only if communication links among DCPOs are provided. Due to the cost-sensitive characteristic of PV system, low-cost communication method is preferred. For conventional PV status monitoring s...