A Study of Physical Limitation of Reactive Power Output Capability of Modern PV Inverters

With the increased penetration of photovoltaic (PV) systems, PV inverters with smart features are proposed to provide grid support functionalities similar to conventional power plants. One smart feature is to use PV inverters to inject static and dynamic reactive power to support the grid voltage. Various reactive power control strategies have been studied and very fast and precise control has been demonstrated. However, the physical limitation for reactive power output capability has not been comprehensively discussed. Due to the difference in semiconductor operation when injecting real or reactive power, the output capability for reactive power is also different from active power's. In this paper, the maximum reactive power capability of three popular PV inverter topologies, i.e. 2-level full bridge, 3-level Neutral Point Clamped (NPC) and T-type Neutral Point Clamped (TNPC) are studied and compared. It is found that the reactive power capability of PV inverters is typically higher than the real power capability. This feature is very useful for gird voltage support functionality of smart PV inverters. Based on the results, a new apparent power constraint model is proposed to aid the reactive power dispatch controller.