Abstract
Microgrids are about to change the architecture and the operation principles of the future power systems towards smartness and resiliency. Power electronics technologies are key enablers for novel solutions. In this paper we analyze the benefits of a “microgrid by design” architecture (MDA), using a solid-state transformer (SST) as a low-voltage grid-former and inverter-based generation only. In this context, the microgrid stability is maintained with the help of “electrostatic energy inertia” that can be provided by the capacitor connected to the DC busbar behind the SST inverter topology. This happens in a natural way, alike the mechanical inertia in power systems with synchronous machines, however without depending on frequency and without the need of a rotational inertia. This type of microgrid always operates (both fully connected to the main grid or in islanding mode) with all the necessary mechanisms needed to maintain the microgrid stable—no matter of the perturbations in the upstream of the point of common coupling (PCC). In the case of microgrids with inverter-based generation only (including the energy storage systems), there is no mechanical inertia and different stability mechanisms need to be applied compared to the stability principle of the classical power systems. Our proposed mechanism differentiates from the recently proposed stability assessments of microgrids based on virtual synchronous generators from the control theory perspective. This paper is a continuation of our previous work where the MDA was first introduced. The use-cases and scenarios are based on realistic and yet reasonable complexities, by coupling the disturbance magnitude with the voltage stability limit in power grids. The paper finds meaningful disturbances to test the electrostatic energy inertia at the boundaries of grid stability, as guidance to understand the range of voltage variation for extreme conditions. The results show that in microgrids with inverter-based generation only and passive loads (RLC type) the operation is no longer frequency dependent. The energy of the DC busbar capacitor as electrostatic energy inertia of the MDA has a role similar to that of the rotational machines in classical grids in terms of maintaining dynamic stability, however impacting two different types of stability.
Highlights
Microgrids are electrical grid entities with clear delimitations from the main grid
The research directions are focusing on two major approaches: (a) from a control theory perspective, dominant approaches are small-signal stability assessments, normally considering very basic microgrid architectures [8,9,10] complemented by methods for model order reduction to further asses the high nonlinearity of a closed-loop model [11,12]; (b) while from a power system perspective, the common approaches in the literature are looking into the dynamics of the inverter using frequency droop control methods [13,14,15,16,17]
In this work we aim to show that a similar inertial role is played by the electrostatic energy stored in DC capacitors, that are behind the inverters, under the form of a so-called “electrostatic energy inertia” (ELEI), which limits the microgrid voltage to fall too deep and provides the microgrid by design” architecture (MDA) with a ride-through during the microgrid disturbance
Summary
Microgrids are electrical grid entities with clear delimitations from the main grid. They are normally connected to the main grid via one point only [1,2], and they are using their own (intelligent). Our direction of analysis follows the microgrid architecture solutions, integrating inverter-based energy resources only by employing a solid-state transformer (SST) in order to asynchronously interface the microgrid with the main grid, assuming uninterrupted operation between the two By assuming this microgrid-by-design-architecture, the islanded type operation strategy can be implemented into the microgrid, while the main grid can support the microgrid by providing the necessary active power. The findings from the simulations support and consolidate the concept that a different stability principle needs to be applied in AC microgrids consisting of inverter-based energy resources only, where the electrostatic energy inertia provided by the capacitors from the DC busbar behind the inverters play a very important role This is a refinement and generalization of previous works [1,28,29], which brings a systematization of such microgrids for the inertia (as a natural reaction to disturbances) and primary control (as man-made automated response to disturbances).
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