I. Introduction
New energy is playing more and more important role in world energy strategy [1]–[2]. Nowadays research on wind power control theory is much more than PV system [3]. Research on PV system is mostly based on PV inverter control theory or modeling of PV cells. Low-frequency oscillation suppression has been researched for a short term. Most control strategies are based on power system stabilizer (PSS) or static var generator (SVG), which has an important effect in system low-frequency oscillation [4]–[5]. PV system will have a higher penetration in the future power system. If PV system can use its spare capacity to suppress power oscillation, it's very useful for its application [5].