Mechanism of second harmonic generation of photovoltaic grid-connected system faults and its impact on transformer protection

Abstract

The short-circuit transient characteristics of photovoltaic(PV) power system are different from traditional power sources. When the grid voltage drops, the output current of PV will contain a high proportion of second harmonics which cannot be ignored. This paper takes a single-phase turn-to-turn short-circuit fault inside the PV grid-connected transformer as an example. First, it analyzes the mechanism of single-frequency input and double-frequency output of a three-phase grid-connected inverter based on a phase-locked loop (PLL), and secondly, considering the comprehensive grid-connected control strategy, the principle of current second harmonic in the minor faults is analyzed and the analytical expression of second harmonics is derived. It is pointed out that the transient dc component will produce second harmonic component in the grid-connected current through control system of the inverter. This current induces a second harmonic voltage in the impedance, and as a disturbance term it generates a new harmonic current through the control link. Superimposed second harmonic currents in two scenarios may threaten the reliability of transformer differential protection. Finally, the correctness of the theoretical analysis is verified by simulation based on MATLAB/Simulink.

Keywords

• PV grid-connected system
• Transient characteristics
• Transformer protection
• Second harmonic
• Vector control