Low-Frequency Small-Signal Modeling of Interconnected AC Microgrids

Abstract

As a solution for optimized, reliable and flexible operation of distribution systems, multiple AC microgrids can be interconnected. In this paper, a low-order low-frequency small-signal model is proposed for large-scale interconnected AC microgrids in order to analyze stability and dynamics as well as synthesize high-level controllers, e.g. inter-microgrid power flow controller. A sensitivity analysis-based technique is introduced to find significant modules of fully inverter-based AC microgrids for preserving the dominant low-frequency modes. The low-order model of AC microgrids including both droop-based and PQcontrolled distributed generation units is obtained by removing insignificant modules of the detailed model and reconfiguring the significant modules. The concept of virtual swing equation and the aggregation modeling method are employed to achieve a single-order model from the low-order model for each AC microgrid with any number of sources. The analysis and synthesis of the large-scale interconnected microgrids can easily be done using the proposed single-order model. The frequency analysis and control of three interconnected AC microgrids are presented as a case study, which leads to introducing the inter-microgrid oscillatory modes.

Comprehensive Small-Signal Modeling and Prony Analysis-Based Validation of Synchronous Interconnected Microgrids

“…Note that in order to simplify the model and use in stability analysis, one can reduce the main model order to these lowfrequency modes (Naderi et al, 2020a). As it can be seen in the first two rows of ∆f MG2 2 and ∆δ MG1 2 in Table 3, these low frequency modes are validated with an appropriate MVE shown in Fig.…”

H∞$H_{\infty }$ optimal frequency control in islanded AC microgrids: A zero‐sum dynamic game approach

“…The power electronic devices are placed between the distributed resources and the AC bus to convert AC to DC in charging mode or vise versa in discharging modes. Each unit in this system is modeled via the following transfer function [31,32]:…”

“…The time-constant values of the DGs are 𝜏 1 = 1.5, 𝜏 2 = 2, 𝜏 3 = 4, 𝜏 4 = 2, 𝜏 5 = 0.1, and 𝜏 6 = 0.1 [31]. Furthermore, the microgrid is modeled by the following transfer function [31,32]:…”

“…The power electronic devices are placed between the distributed resources and the AC bus to convert AC to DC in charging mode or vise versa in discharging modes. Each unit in this system is modeled via the following transfer function [31, 32]: $$$$\begin{align} G_{\mathrm{i}}(s)=\frac{1}{\tau _{\mathrm{i}}s+1}\nobreakspace (i=1,\ldots,6). \end{align}$$$$…”

Detailed analysis of grid connected and islanded operation modes based on P/U and Q/f droop characteristics

“…Different controllers have been proposed in this context where [19] introduces a virtual resistance so that the system acts in a resistive manner where and might be regulated by respectively drooping and . [20] and [21] have introduced a virtual reactance and a virtual PQ method, respectively to mimic the inductive system by increasing the / ratio. The − & − droop scheme has the disadvantage of exhibiting power quality problems in terms of frequency and voltage deviations.…”