Control Strategies of DC Microgrids Cluster:A Comprehensive Review

Multiple microgrids (MGs) close to each other can be interconnected to construct a cluster to enhance reliability and flexibility. This paper presents a comprehensive and comparative review of recent studies on DC MG clusters’ control strategies. Different schemes regarding the two significant control aspects of networked DC MGs, namely DC-link voltage control and power flow control between MGs, are investigated. A discussion about the architecture configuration of DC MG clusters is also provided. All advantages and limitations of various control strategies of recent studies are discussed in this paper. Furthermore, this paper discusses three types of consensus protocol with different time boundaries, including linear, finite, and fixed. Based on the main findings from the reviewed studies, future research recommendations are proposed

Microgrid Control and Protection: Stability and Security

When the microgrid disconnects from the main grid in response to, say, upstream disturbance or voltage fluctuation and goes to islanding mode, both voltage and frequency at all locations in the microgrid have to be regulated to nominal values in a short amount of time before the operation of protective relays. Motivated by this, we studied the application of intelligent pinning of distributed cooperative secondary control of distributed generators in islanded microgrid operation in a power system. In the first part, the problem of single and multi-pinning of distributed cooperative secondary control of DGs in a microgrid is formulated. It is shown that the intelligent selection of a pinning set based on the number of its connections and distance of leader DG/DGs from the rest of the network, i.e., degree of connectivity, strengthens microgrid voltage and frequency regulation performance both in transient and steady state. The proposed control strategy and algorithm are validated by simulation in MATLAB/SIMULINK using different microgrid topologies. It is shown that it is much easier to stabilize the microgrid voltage and frequency in islanding mode operation by specifically placing the pinning node on the DGs with high degrees of connectivity than by randomly placing pinning nodes into the network. In all of these research study cases, DGs are only required to communicate with their neighboring units which facilitates the distributed control strategy. Historically, the models for primary control are developed for power grids with centralized power generation, in which the transmission lines are assumed to be primarily inductive. However, for distributed power generation, this assumption does not hold since the network has significant resistive impedance as well. Hence, it is of utmost importance to generalize the droop equations, i.e., primary control, to arrive at a proper model for microgrid systems. Motivated by this, we proposed the secondary adaptive voltage and frequency control of distributed generators for low and medium voltage microgrid in autonomous mode to overcome the drawback of existing classical droop based control techniques. Our proposed secondary control strategy is adaptive with line parameters and can be applied to all types of microgrids to address the simultaneous impacts of active and reactive power on the microgrids voltage and frequency. Also, since the parameters in the network model are unknown or uncertain, the second part of our research studies adaptive distributed estimation/compensation. It is shown that this is an effective method to robustly regulate the microgrid variables to their desired values. The security of power systems against malicious cyberphysical data attacks is the third topic of this dissertation. The adversary always attempts to manipulate the information structure of the power system and inject malicious data to deviate state variables while evading the existing detection techniques based on residual test. The solutions proposed in the literature are capable of immunizing the power system against false data injection but they might be too costly and physically not practical in the expansive distribution network. To this end, we define an algebraic condition for trustworthy power system to evade malicious data injection. The proposed protection scheme secures the power system by deterministically reconfiguring the information structure and corresponding residual test. More importantly, it does not require any physical effort in either microgrid or network level. The identification scheme of finding meters being attacked is proposed as well. Eventually, a well-known IEEE 30-bus system is adopted to demonstrate the effectiveness of the proposed schemes

State-Space Modeling and Stability Analysis for Microgrids with Distributed Secondary Control

© 2018 IEEE. High penetration of renewable energies in power systems leads to the necessity of comprehensive modelling of a microgrid (MG) for its appropriate control. The distributed secondary control in the MG can be used for complementing the role of primary droop-based control. This paper presents a systematic way of developing a linearized small signal state space model with distributed secondary control as well as stability analysis of an islanded AC MG. The MG considered here, consists of three distributed generations (DGs) represented in the synchronous (DQ) reference frame. To show the effect of controller parameters on system stability, the eigenvalue analysis is presented here. The MATLAB/Simulink model of islanded MG with both primary and secondary control strategies is also developed to verify the outcomes of small-signal analysis. The simulation results show that the voltage controller simultaneously achieves the critical voltage restoration and accurate reactive power sharing

Secondary control in islanded smart distribution systems considering renewable resources and energy storage: A centralized approach based on convex optimization

This thesis proposes a receding horizon strategy for the secondary control of islanded microgrids. The proposed control takes into account the action of the primary control as well as the references given by the tertiary control. A convex optimization model is solved in each time step, based on a linear approximation of the frequency-dependent power flow equations. The main objective of the control is to carry out frequency and voltages to suitable values, taking into account capacity limits of renewable sources and energy-storage devices..

Optimal Power and Frequency Control of Microgrid Cluster with Mixed Loads

In recent years, voltage and frequency regulation issues have been extensively discussed for the microgrid clusters (MGCs), as the high penetration of renewable energy resources (RES) might affect the continuous operation of the microgrid (MG). Furthermore, to enhance the MGC’s operation reliability, stability concerns need to be addressed. In this study, a residential MGC connected to a commercial MGC has been considered. A novel control scheme that combines both droop control and virtual inertia control is proposed. This control strategy relies on online measurements, and it can be adapted to different situations. At each iteration, the damping coefficient and droop coefficient are calculated this allows the system to switch as needed between the droop and the virtual inertia controller. This dynamic coefficient calculation allows plug and play capability, which provides the MGC with major flexibility in terms of the MGs operation and flexibility