Buffered-microgrid Structure for Future Power Networks; a Seamless Microgrid Control

This paper proposes a new structure and control scheme for future microgrid-based power system, which is designed to achieve a seamless operation in both islanded and grid-connected modes, while the load is appropriately shared by all units (i.e. renewable sources, energy storage systems and the grid). The proposed method, which involves physical separation of the microgrid from the grid by using AC/DC/AC converters, ensures safe, secure and seamless operation of both modes. Such a “buffered” structure enables reduction in the transmission losses by reducing the exchanged energy with the grid through using a dead-zone in the control of the buffering AC/DC/AC converter. An inverse-droop control technique has been implemented to control the voltage magnitude and frequency, using current control in the dq-frame. PSCAD/EMTDC software has been used to validate the proposed method through simulating different scenarios. The solution provides a simple, smooth, and communication-free decentralized control for multi-sources microgrids. Moreover, the proposed buffered structure separates the dynamics of the microgrid and the grid, which enables a faster microgrid voltage and frequency control and protects the grid and the microgrid from faults on the other side

Power-Based Droop Control in DC Microgrids Enabling Seamless Disconnection From Upstream Grids

This paper proposes a local power-based droop controller for distributed energy resource converters in dc microgrids that are connected to upstream grids by grid-interface converters. During normal operation, the grid-interface converter imposes the microgrid bus voltage, and the proposed controller allows power flow regulation at distributed energy resource converters\u2019 output. On the other hand, during abnormal operation of the grid-interface converter (e.g., due to faults in the upstream grid), the proposed controller allows bus voltage regulation by droop control. Notably, the controller can autonomously convert from power flow control to droop control, without any need of bus voltage variation detection schemes or communication with other microgrid components, which enables seamless transitions between these two modes of operation. Considering distributed energy resource converters employing the power-based droop control, the operation modes of a single converter and of the whole microgrid are defined and investigated herein. The controller design is also introduced. Furthermore, the power sharing performance of this control approach is analyzed and compared with that of classical droop control. The experimental results from a laboratory-scale dc microgrid prototype are reported to show the final performances of the proposed power-based droop control

Control strategies for seamless transfer between the grid-connected and islanded modes of a microgrid system

Design of control strategies for Distributed generation systems is very important to achieve smoother transition between the grid connected and islanding modes of operation. The transition between these two modes of operation should be seamless, without any severe transients during the changeover. In this paper, two different control strategies namely inverter output current control and indirect grid current control for the seamless transfer between the modes of operation has been explored for the suitability. The design and analysis of the cascaded control loops based on Proportional Integral (PI) controller has been dealt in detail for both inverter output current control and indirect grid current control strategy. Control parameters are designed using the control system toolbox in MATLAB. A 10kW grid connected microgrid system has been designed and simulated in MATLAB/Simulink and the results are presented under grid connected operation, islanding operation and the transition between the modes considering fault condition in the grid side. The simulation studies are carried out using both the control strategies and the results are presented to validate the design methodology

Microgrids of commercial buildings: strategies to manage mode transfer from grid connected to islanded mode

Microgrid systems located within commercial premises are becoming increasingly popular and their dynamic behavior is still uncharted territory in modern power networks. Improved understanding in design and operation is required for the electricity utility and building services design sectors. This paper evaluates the design requirements for a commercial building microgrid system to facilitate seamless mode transition considering an actual commercial building microgrid system. A dynamic simulation model of the proposed microgrid system is established (utilizing DIgSILENT Power Factory) to aid the development of planning and operational philosophy for the practical system. An economic operational criterion is developed for the microgrid to incorporate selective mode transition in different time intervals and demand scenarios. In addition, a multi-droop control strategy has been developed to mitigate voltage and frequency variations during mode transition. Different system conditions considering variability in load and generation are analyzed to examine the responses of associated microgrid network parameters (i.e., voltage and frequency) with the proposed mode transition strategy during planned and unplanned islanding conditions. It has been demonstrated that despite having a rigorous mode transition strategy, control of certain loads such as direct online (DOL) and variable-speed-drive (VSD) driven motor loads is vital for ensuring seamless mode-transition, in particular for unplanned islanding conditions

Advanced control in smart microgrids

University of Technology, Sydney. Faculty of Engineering and Information Technology.This thesis presents various advanced control strategies in smart microgrid applications. In recent years, due to the rapid depletion of fossil fuels, increasing demand of electricity, and more strict compulsory government policies on reduction of greenhouse gas emissions, renewable energy technologies are attracting more and more attentions and various types of distributed generation (DG) sources, such as wind turbine generators and solar photovoltaic (PV) panels, are being connected to low-voltage distribution networks. Because of the intermittent nature of the renewable energy sources, it would be a good idea to connect these DG units together with energy storage units and loads to form a local micro power system, known as microgrid. This PhD thesis project aims to develop new and competitive control methods for microgrid applications. Based on a review of the state of the art of the wind power techniques, a new predictive direct control strategy of doubly fed induction generator is proposed. This method can achieve fast and smooth grid synchronization, and after grid connection, the active and reactive power can be regulated flexibly, which enables the wind power systems contributing to the grid voltage support and power quality improvement. The proposed strategy is simple and reliable, and presents excellent steady-state and dynamic performance. A new control approach using the model predictive scheme is developed for a PV system in microgrid applications. In the islanded operation, the inverter output voltage is controlled stably for the local loads. A simple synchronization scheme is introduced to achieve seamless transfer, and after being connected to the utility grid, the PV system can inject both active and reactive power into the grid flexibly within its capacity. As the capacity of DGs getting larger, the power conversion efficiency becomes more important. In order to reduce the switching loss, a multi-objective model-predictive control strategy is proposed for the control of high power converters. By revising the cost function properly, the switching frequency can be reduced considerably without deteriorating the system performance. The control strategy is simplified using a graphical algorithm to reduce the computational burden, which is very useful in practical digital implementation where high sampling frequency is required. The proposed method is very flexible and can be employed in both AC/DC and DC/AC energy conversions in microgrids. For a microgrid consisting of several DG units, various system level control methods are studied. A novel flux droop control approach is developed for parallel-connected DGs by drooping the inverter flux instead of drooping the inverter output voltage. The proposed method can achieve autonomous active and reactive power sharing with much lower frequency deviation and better transient performance than the conventional voltage droop method. Besides, it includes a direct flux control (DFC) algorithm, which avoids the use of proportional-integral (PI) controllers and PWM modulators. For a microgrid system consisting of a 20 kW PV array and a 30 kW gas microturbine, a coordinated control scheme is developed for both islanded and grid-connected operations. The experimental results from a renewable energy integration facility (REIF) laboratory confirmed the feasibility of the control strategy. The response of this microgrid under the condition of grid faults is investigated and the relevant protection mechanism is proposed. Given the intermittent nature of the renewable energy sources, and the fluctuated load profile, an appropriate solution is to use energy storage systems (ESS) to absorb the surplus energy in the periods when the power production is higher than the consumption and deliver it back in the opposite situation. In order to optimize the power flow, a model predictive control (MPC) strategy for microgrids is proposed. This method can flexibly include different constraints in the cost function, so as to smooth the gap between the power generation and consumption, and provide voltage support by compensating reactive power during grid faults

Plug and Play DC-DC Converters for Smart DC Nanogrids with Advanced Control Ancillary Services

This paper gives a general view of the control possibilities for dc-dc converters in dc nanogrids. A widely adopted control method is the droop control, which is able to achieve proportional load sharing among multiple sources and to stabilize the voltage of the dc distribution bus. Based on the droop control, several advanced control functions can be implemented. For example, power-based droop controllers allow dc-dc converters to operate with power flow control or droop control, whether the hosting nanogrid is operating connected to a strong upstream grid or it is operating autonomously (i.e., islanded). Converters can also be equipped with various supporting functions. Functions that are expected to play a crucial role in nanogrids that fully embrace the plug-and-play paradigm are those aiming at the monitoring and tuning of the key performance indices of the control loops. On-line stability monitoring tools respond to this need, by continuously providing estimates of the stability margins of the loops of interest; self- tuning can be eventually achieved on the basis of the obtained estimates. These control solutions can significantly enhance the operation and the plug-and-play feature of dc nanogrids, even with a variable number of hosted converters. Experimental results are reported to show the performance of the control approaches

Inter-Microgrid Operation: Power Sharing, Frequency Restoration, Seamless Reconnection and Stability Analysis

Electrification in the rural areas sometimes become very challenging due to area accessibility and economic concern. Standalone Microgrids (MGs) play a very crucial role in these kinds of a rural area where a large power grid is not available. The intermittent nature of distributed energy sources and the load uncertainties can create a power mismatch and can lead to frequency and voltage drop in rural isolated community MG. In order to avoid this, various intelligent load shedding techniques, installation of micro storage systems and coupling of neighbouring MGs can be adopted. Among these, the coupling of neighbouring MGs is the most feasible in the rural area where large grid power is not available. The interconnection of neighbouring MGs has raised concerns about the safety of operation, protection of critical infrastructure, the efficiency of power-sharing and most importantly, stable mode of operation. Many advanced control techniques have been proposed to enhance the load sharing and stability of the microgrid. Droop control is the most commonly used control technique for parallel operation of converters in order to share the load among the MGs. But most of them are in the presence of large grid power, where system voltage and frequency are controlled by the stiff grid. In a rural area, where grid power is not available, the frequency and voltage control become a fundamental issue to be addressed. Moreover, for accurate load sharing a high value of droop gain should be chosen as the R/X ratio of the rural network is very high, which makes the system unstable. Therefore, the choice of droop gains is often a trade-off between power-sharing and stability. In the context, the main focus of this PhD thesis is the fundamental investigations into control techniques of inverter-based standalone neighbouring microgrids for available power sharing. It aims to develop new and improved control techniques to enhance performance and power-sharing reliability of remote standalone Microgrids. In this thesis, a power management-based droop control is proposed for accurate power sharing according to the power availability in a particular MG. Inverters can have different power setpoints during the grid-connected mode, but in the standalone mode, they all need their power setpoints to be adjusted according to their power ratings. On the basis of this, a power management-based droop control strategy is developed to achieve the power-sharing among the neighbouring microgrids. The proposed method helps the MG inverters to share the power according to its ratings and availability, which does not restrict the inverters for equal power-sharing. The paralleled inverters in coupled MGs need to work in both interconnected mode and standalone mode and should be able to transfer between modes seamlessly. An enhanced droop control is proposed to maintain the frequency and voltage of the MGs to their nominal value, which also helps the neighbouring MGs for seamless (de)coupling. This thesis also presents a mathematical model of the interconnected neighbouring microgrid for stability and robustness analysis. Finally, a laboratory prototype model of two MGs is developed to test the effectiveness of the proposed control strategies

Control Strategy for Uninterrupted Microgrid Mode Transfer during Unintentional Islanding Scenarios

This paper presents a microgrid control strategy to unify the control topology for energy storage systems (ESS) and renewable energy sources (RES) inverters in an AC microgrid and to protect the microgrid reliability from unintentional islanding instability using control loops which use the DC link voltage as a feedback. This bounds the DC link voltage and provides reliable operation in the microgrid. Simulation validates the proposed control strategy, and experiment results extol the concept

A review of microgrid development in the United States – A decade of progress on policies, demonstrations, controls, and software tools

Microgrids have become increasingly popular in the United States. Supported by favorable federal and local policies, microgrid projects can provide greater energy stability and resilience within a project site or community. This paper reviews major federal, state, and utility-level policies driving microgrid development in the United States. Representative U.S. demonstration projects are selected and their technical characteristics and non-technical features are introduced. The paper discusses trends in the technology development of microgrid systems as well as microgrid control methods and interactions within the electricity market. Software tools for microgrid design, planning, and performance analysis are illustrated with each tool's core capability. Finally, the paper summarizes the successes and lessons learned during the recent expansion of the U.S. microgrid industry that may serve as a reference for other countries developing their own microgrid industries