[Colloquium EE 27 Jul 2023] Dual Mode Operation of Grid-tied Inverters: Modeling, Stability Analysis, and Islanding Detection

Dual Mode Operation of Grid-tied Inverters: Modeling, Stability Analysis, and Islanding Detection

Speaker: SUGOTO MAULIK . of Ph.D. (Engg) in Electrical Engineering
Date/Time: Jul 27 / 10:30:00 am
Location: MMCR EE, IISc
Research Supervisor: Vinod John
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Abstract:
Increased penetration of renewable energy sources like solar PVs and wind is fundamentally altering the power flow dynamics in distribution networks. These localized forms of generation add redundancy to the power system and increase its load-handling capacity. However, these advantages come at the cost of reduced stability and altered protection requirements. These distributed forms of generation (DGs) are interfaced with the power grid via power electronic converters operating at high bandwidths compared to conventional sources. While these offer higher performance, but consequently lower the stability margins. An analytical framework is thus necessary for modeling and stability analysis of such systems. The dynamics involved in modeling a grid-tied DG system span a wide spectrum of frequencies. While simplified modeling can lead to inaccuracies, an all-inclusive model leads to an over-complicated and unintuitive model. This work proposes a systematic approach to model the behavior of 3-phase grid-tied DG systems using dynamic phasors. Dynamic phasors allow for a state-space representation of the relevant dynamics. The developed state space model is then used for the following:
1. Islanding detection: Islands are formed in 3-phase distribution networks when an active distributed generation (DG) is disconnected from the grid. If undetected, the DG continues to energize its local loads leading to safety concerns. In this work, a state-feedback approach is developed for islanding detection, which places a system pole in the right half plane (RHP). This ensures the destabilization of the islanded network and a zero non-detection zone. The scheme is designed and implemented experimentally.
2. Transfer of Control: Post-island detection, the DG is required to disconnect from the grid while ensuring uninterrupted power flow to its local loads. A control scheme involving a voltage control loop and grid current feed-forward is developed to achieve a fast transfer from grid-following to grid-forming mode of operation. The introduced voltage control loop ensures that rated voltage is maintained across the loads, and the grid current feed-forward is used to minimize the transients during the transfer process. The method is designed and implemented in conjunction with the islanding detection scheme and verified experimentally with local loads.
3. Stability analysis of grid-tied DG systems: Owing to the formation of microgrids and weak grids in the distribution network, the stability assessment of such networks becomes essential. This assessment is performed by extending the dynamic phasor-based model for islanded systems to model grid-tied systems as well. The developed model includes the dynamics of the PLL, grid, DG current levels, and load. In addition to passive loads, considered in the relevant literature, the proposed model also incorporates the effect of constant power and constant current type power electronic 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.
All the proposed methods are validated on hardware prototypes that have been developed as a part of the work.

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