Grid-Connected Control of Single-Phase Inverters and Active Frequency Drift Islanding Detection

Research on Grid-Connected Control and Islanding Detection Technologies for Single-Phase Inverters

Single-phase inverters play a vital role in distributed generation systems, with their core function being the conversion of DC power to AC power and synchronization with the grid. The key to grid-connected control lies in maintaining strict synchronization between the inverter output and the grid voltage in terms of frequency, phase, and amplitude. Typically, Phase-Locked Loop (PLL) algorithms are employed to track grid voltage phase information. Through a dual-loop control strategy comprising an inner current loop and an outer voltage loop, the inverter dynamically adjusts output current to ensure high-quality power injection into the grid while suppressing harmonic interference. In code implementation, the PLL module often uses trigonometric calculations (e.g., dq-transform) for phase detection, while PI controllers regulate current/voltage references.

Islanding effect represents a significant safety hazard for grid-tied inverters. When the grid power is interrupted but the inverter continues supplying local loads, it poses electrocution risks to maintenance personnel. Active Frequency Drift (AFD) method serves as an effective active islanding detection technique, operating on the principle of injecting slight frequency perturbations into the output current. Under normal grid operation, the grid's strong voltage characteristics counteract these perturbations. During islanding conditions, lacking grid clamping, frequency disturbances accumulate rapidly, causing system frequency to exceed normal ranges and triggering protection mechanisms. This method achieves a good balance between detection speed and reliability while minimally impacting power quality. Algorithm implementation typically involves programming frequency drift patterns (e.g., chopping fraction calculation) in the inverter's DSP controller.

Notably, islanding detection requires careful trade-offs between detection sensitivity and anti-interference capability to avoid false tripping during normal grid fluctuations. Modern grid-connected inverters commonly integrate AFD with passive detection methods (e.g., voltage/frequency monitoring) to form multi-layer protection systems. With smart grid development, communication-based cooperative islanding detection technologies are emerging as research hotspots, potentially involving SCADA system integration and protocol-based distributed triggering mechanisms.