Dual-Mode Operation of Grid-tied Inverters: Modeling, Islanding Detection and Transfer of Control

Abstract

Increased penetration of renewable energy sources like solar and wind is fundamentally altering the power flow dynamics in modern distribution networks. These distributed generations (DG) boost system reliability by enabling dual-mode operation via operating in grid-following mode in the presence of the grid and grid-forming mode in its absence. The use of high-bandwidth power electronic converters as a DG’s grid interface unlocks these benefits. These converters, however, alter system stability and protection specifications. A modeling approach is required to tread the fine line between an inaccurate simplified model and a complex higher-order model. This thesis proposes a systematic approach to identify and model the relevant dynamics of 3-phase grid-tied DG systems using dynamic phasors, which enable a state-space representation of the relevant dynamics of both grid-tied and islanded DGs. The developed model is employed for the following:

1. Stability analysis of grid-tied and islanded DG systems: DGs often operate as microgrids or in weak distribution grids. The stability assessment of such networks is performed via eigenvalue analysis of the developed state-space model. Conventional analytic formulations consider the dynamics of the PLL, grid, and DG. However, the proposed framework incorporates load dynamics into the fold. In addition to conventionally considered passive loads, the proposed model also incorporates the effect of constant power and constant current type loads. It is demonstrated, analytically and experimentally, that the presence of local loads has a stabilizing impact on the synchronization stability of a DG. Additionally, an upper limit on the bandwidth of power-electronic type constant power loads is derived, affirming the observation that high bandwidth loads lead to reduced system stability.

2. Islanding detection: Islands are formed in distribution networks due to the disconnection of a DG from the grid. If undetected, the DG continues to energize its local loads, jeopardizing the system’s safety and stability. In this thesis, a state-feedback is applied to place a pole of the islanded system in the right half plane (RHP). This ensures the destabilization of the islanded network and leads to island detection with a zero non-detection zone.

3. Transfer of Control: Upon islanding, a DG is switched to a grid-forming mode to ensure continuous power supply to local loads. A transfer of control logic involving a grid current feed-forward is proposed to achieve a fast mode transfer. The transfer logic, amalgamated with the islanding detection scheme, forms a unified scheme that ensures rated voltage is maintained across loads and minimizes the transients during the transfer process.

All the proposed models and developed islanding detection and transfer schemes are validated on hardware prototypes built to IEEE 1547 specifications.