Dynamic Model of Back-to-Back Converter for System-Level Phasor Simulation

TL;DR

The paper addresses the need for scalable system-level modeling of back-to-back (BTB) converters in microgrid-integrated power systems by developing a phasor-domain BTB converter model implemented in C++ and integrated into GridLAB-D. It derives precise phasor-domain equations for both grid-side and microgrid-side converters, including DC-link dynamics and PLL-aligned frames, and implements these using a three-object (GSC, MSC, DC-link) Norton-equivalent structure with predictor-corrector Euler integration. The model is validated against detailed EMT simulations in MATLAB/Simulink under grid-to-microgrid and bi-directional power-flow scenarios, demonstrating accurate representation of DC-link voltage, currents, and converter powers with significantly reduced computation time. This approach enables large-scale, system-level studies of networked microgrids and BTB interfaces, facilitating rapid assessment and planning of future grids with high penetration of power-electronic interfaces.

Abstract

The power system is expected to evolve rapidly with the increasing deployment of power electronic interface and conditioning systems, microgrids, and hybrid AC/DC grids. Among power electronic systems, back-to-back (BTB) converters can be a powerful interface to integrate microgrids and networked microgrids. To study the integration of such devices into large power systems, a balance between power electronics model fidelity and system-level computational efficiency is critical. In system-level simulations of bulk power systems dominated by synchronous generators, detailed electromagnetic models of back-to-back converters may be unnecessary and also computationally inefficient. This paper focuses on developing a simple phasor model for back-to-back converters that can be easily integrated into powerflow solvers to facilitate large-scale power system simulations. The model is implemented using C$^{++}$ language and integrated into GridLAB-D, an open source software for distribution systems studies, as a potential new capability. The GridLAB-D phasor domain model is validated against the electromagnetic transient (EMT) simulation of the detailed switching model. Simulation results show that the phasor model successfully captures the dominant dynamics of the converter with significantly shorter simulation elapsed time.

Figures (6)

• Figure 1: Back-to-back converter connected to a grid and a microgrid network.
• Figure 2: Network and converter frames, where the dq frame represents the converter side, while the Re/Im frame represent the network side. (a) A general case; (b) the case when the dq frame is aligned with the network terminal voltage using a PLL, i.e. $V_g=V_{gd}$ and $V_q=0$.
• Figure 3: Thevenin and Norton representation of the GSC and the grid networks. (a) Thevenin equivalent; (b) Norton equivalent
• Figure 4: Three-object structure implementation and data exchange of the back-to-back converter in GridLAB-D.
• Figure 5: Model performance in response to variations in the power imported by the microgrid. (a) DC-link voltage, (b) DC-link current flowing into the MSC ($I_{dc-m}$), and (c) DC-link current flowing into the GSC ($I_{dc-g}$).