Buy Seamless Transition Of Microgrids

Buy Seamless Transition Of Microgrids
Abstract—One of the main features of Microgrids is the ability to operate in both grid-connected mode and islanding mode. In each mode of operation, distributed energy resources (DERs) can be operated under grid-forming or grid-following control strategies. In grid-connected mode, DERs usually work under grid-following control strategy, while at least one of the DERs must operate in grid-forming strategy in islanding mode. A microgrid may experience remarkable fluctuations in voltage and current due to an unintentional islanding event. To achieve a smooth transition to islanding mode and mitigate disturbance effect, this paper proposes a control strategy includes a) a linear voltage controller with capacitor current feedback as an input to the voltage controller and output current feedforward as an input to current controller, and b) modified droop control to emulate the inertia response of a synchronous generator. The proposed controller can suppress voltage, current and frequency fluctuations and also guarantee a smooth transition. A small signal analysis of the proposed control strategy is developed to design its coefficients as well as the destabilizing effect of constant power load (CPL). Experimental results are provided to verify the effectiveness of the proposed control strategy.
Index Terms—Grid-connected, islanding mode, microgrids, modified droop control, smooth transition.
I. INTRODUCTION
Buy Seamless Transition Of Microgrids
ICROGRID, as a small-scale power system, can work in both grid connected (GC) and islanding (IS) modes. In each mode of operation, distributed energy resources (DER) in microgrids (MGs) can be controlled using different strategies. DERs based on power electronic converters are usually the dominant part of a MG. DERs can operate in two different modes, 1) current source with grid-following control strategy and 2) voltage source with grid-forming control strategy [1].
Manuscript received February 12, 2019; revised July 13, 2019 and September 7, 2019; accepted October 10, 2019. Date of publication October 15, 2019; date of current version April 21, 2020. This work was supported by the Babol Noshirvani University of Technology under Grant BNUT/370445/98. Paper no. TSG-00235-2019. (Corresponding author: M. Shahabi.)
M. Ganjian-Aboukheili and M. Shahabi are with the Department of Electrical and Computer Engineering, Babol Noshirvani University of Technology, Babol 4714871167, Iran (e-mail: [email protected]; [email protected]).
Q. Shafiee is with the Department of Electrical Engineering, University of Kurdistan, Sanandaj 66177-15177, Iran (e-mail: [email protected]).
J. M. Guerrero is with the Institute of Energy Technology, Aalborg University, 9220 Aalborg, Denmark (e-mail: [email protected]).
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Digital Object Identifier 10.1109/TSG.2019.2947651
The former is useful for converters that only inject a specific current to the MG, e.g., converter used for the renewable energy source (RES), while the latter can be employed in both modes of operation. In GC mode, the voltage and frequency of the MG are dictated by upstream grid, thus DERs tend to operate in grid-following strategy. In islanding mode, however, it is crucial to have some of DERs operating in grid-forming strategy to regulate the voltage and frequency of the MG.
The stability and robustness of a MG depends on the performance of the DERs. Number of control strategies have been introduced for DERs in the literature which can be used in both GC and IS modes of operation [2]. These control strategies can be categorized into two types [3]: 1) control strategies for both modes of operation with a single control scheme (usually based on voltage control) which remain in service to provide further capabilities [4]–[9], 2) control strategies with two different control schemes where each mode is activated according to the pre-assigned control objective [10]–[15].
The majority of the first types of controllers are based on nonlinear control theory, e.g., Lyapunov-based method [5], [8], model predictive control [4], [9], which usually need an accurate model of the system and DER dynamic behavior. However, these controllers not only have a complex structure with a high computational burden but also their realization is very difficult. Furthermore, these types of control techniques are not easily implementable in practice. In contrary, linear control strategies provide a simple structure, low computational burden, and they are very convenient in design and implementation [9]. Due to using feedback or feedforward of the physical variables, linear control strategies give a better sense to the controller performance. Cascade control strategies have already been introduced in control design and implementation [16], [17].
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These types of controllers must be able to not only operate in both modes of operation, but also provide a seamless transition between them. This transition should occur smoothly while eliminating the disturbances or at least staying within a reasonable limit. During transition, the following issues may exist: 1) frequency fluctuation because of transition from a grid-following to a grid-forming strategy which leads to a disturbance on the power-angle of DERs and even threats the MG stability, 2) voltage and current deviation in DERs output due to switching between the modes.