A Communication-Less Pre-Synchronization Strategy for Microgrids Based on Temporary Master-Slave Scheme

Parallel operation of inverter modules is the solution to increase the reliability, efficiency, and redundancy of inverters in microgrids. Load sharing among inverters in distributed generators (DGs) is a key issue. This study investigates the feasibility of power-sharing among parallel DGs using a dual control strategy in islanded mode of a microgrid. PQ control and droop control techniques are established to control the microgrid operation. P-f and Q-E droop control is used to attain real and reactive power sharing. The frequency variation caused by load change is an issue in droop control strategy whereas the tracking error of inverter power in PQ control is also a challenge. To address these issues, two DGs are interfaced with two parallel inverters in an islanded AC microgrid. PQ control is investigated for controlling the output real and reactive power of the DGs by assigning their references. The inverter under enhanced droop control implements power reallocation to restore the frequency among the distributed generators with predefined droop characteristics. A dual control strategy is proposed for the AC microgrid under islanded operation without communication link. Simulation studies are carried out using MATLAB/SIMULINK and the results show the validity and effective power-sharing performance of the system while maintaining a stable operation when the microgrid is in islanding mode.

The distributed generation (DG) should supply continuous power to local critical load, even in the condition of utility outage. This paper proposes a novel seamless transfer control strategy, which combines indirect current control and droop control. A microgrid structure of multi-masters and multi-slaves is also proposed, which based on the master-slave control and droop control. The master DGs are controlled by the novel control strategy and the slave DGs are controlled as conventional current sources based on PQ control. The advantages of this proposed strategy and microgrid structure are as follows. Firstly, the quality of critical load voltage can be maintained when utility outage occurs, Secondly, the reliability of microgrid in islanding mode can be improved because of the existence of multiple master DGs. The simulation results are provided to verify the effectiveness of this proposed control method.

In the case of sensitive and critical load, the distributed generation (DG) should supply continuous power, even in the condition of utility outage. This paper proposes a novel seamless transfer control strategy, which based on special master and slave DGs. The master DG unit is controlled by a novel control strategy, which combines indirect current control and droop control. A part of slave DGs are controlled as conventional current sources based on PQ control, which are named as Slave-Type1. Another part of slave DGs are controlled based on PQ control and droop control, which are named as Slave-Type2. When the islanding is confirmed, the control block of Slave-Type2 DGs will transfer from PQ control to droop control. The advantages of this proposed strategy and microgrid structure are as follows. Firstly, the quality of critical load voltage can be maintained when utility outage occurs, Secondly, the reliability of microgrid in islanding mode can be improved because of the existence of multiple master DGs. The simulation results are provided to verify the effectiveness of this proposed control method.

Microgrid (MG) control in off-grid mode of operation is one of the main challenges in MGs. Under different loading conditions, voltage and frequency deviate from their nominal limits. The droop control method that applied with the current control and voltage control loops can restore the MG voltage and frequency to their nominal limits. However, it can’t regulate these parameters to their specified values. Additionally, the droop control method can’t do well when the system supplies either nonlinear or unbalanced loads. This paper proposes a control technique for adjusting the voltage and frequency of an MG in islanded mode under different loading conditions. The proposed method is based on applying the H-infinity ($$H\infty$$) robust control as a secondary controller with the droop control method to improve its performance. The proposed method is composed of four stages including; droop control loop, voltage control loop, current control loop and control loop for inductance–capacitance–inductance (LCL) filter and coupling circuit. $$H\infty$$ control has an internal model that can control the frequency and improve the system power quality by decreasing the disturbance frequencies. Moreover, it has two weighted parameters $$\xi$$ and $$\mu$$ that must be correctly adjusted to improve the stability of the system. For a proper adjusting of these parameters, the Harmony Search (HS) optimization method is applied. To prove the effectiveness and superiority of the proposed method, it is applied on a test system with three different loading scenarios using MATLAB/Simulink program. The results are compared with those of droop controller and they confirm that the proposed control method is more effective in controlling the microgrids under islanded mode operation.

Due to the increased environmental and economic challenges, in recent years, renewable based distribution generation has been developed. More penetrations from the side of consumers caused a new concept called microgrids which are able to stand with or without connection to the bulk power system. Control of microgrids in islanded mode is very crucial for decreasing the amplitude of frequency deviations as well as damping speed. This chapter aims to propose an optimal combination of FOPD and fuzzy pre-compensated FOPI approach for load-frequency control of microgrids in islanded mode. The optimization parameter of the control scheme is designed by the differential evolution (DE) algorithm which has been improved by a fuzzy approach. In the optimization, control effort is considered as a constraint. Due to the robustness and flexibility of the proposed method, the simulation results have been improved substantially. Robust performance of the proposed control method is examined through sensitivity analysis.

As a kind of generation and distribution system with autonomous control, AC–DC hybrid microgrid plays a pivotal role in the transformation and development of modern power grids. To address the problems of microgrid system instability that occur in islanding mode, the study proposes a coordinated control strategy for hybrid AC/DC microgrid in islanding mode. First, the bidirectional AC/DC interconnection converter is modelled and parametrically designed; second, the control strategy of the interconnection converter in islanding mode is raised, including both active power control with double-loop control and seamless switching of operation modes with pre-synchronous control, and in the end, the effectiveness of the strategy is examined. The data display that under this strategy, the real and reactive power output gaps between the micro sources in the hybrid microgrid do not exceed 0.01 kW and 0.03 kVAR, respectively, and the current during islanding switching is more stable up to 32 A. This shows that the strategy effectively improves the shortcomings of the traditional regulation method, realizes the accurate distribution of real and reactive power, improves the power sharing between DC and AC subgrids, strengthens the stability and reliability of the hybrid microgrid, and helps to promote the technological progress of the hybrid microgrid and escorts the efficient and green development of the smart grid.

Coordinated Control Strategy for Microgrid in Grid-Connected and Islanded Operation

This paper presents a seamless transition method for a droop-controlled inverter. The droop control is suitable to make the inverter work as a voltage source in both islanded and grid-connected modes, however, the transfer between theses modes can result in a big inrush current that may damage the system. The proposed method allows the droop-controlled inverter to improve the transient response when transferring between modes, by detecting the inrush current, activating a current control loop during transients, and then transferring back to droop-controlled mode smoothly by using a virtual inductance loop. In addition, a local phase-locked-loop (PLL) is proposed to align the inverter voltage with the grid in order to reduce the transient current during the transition. Therefore, the droop-controlled inverter is able to operate in both grid-connected and islanded modes, providing as well a smooth transition between them, requiring neither synchronization signals nor grid-side information. The control algorithm and design procedure are presented. Experimental results from a laboratory prototype validate the effectiveness of the proposed method.

A hierarchical power management scheme is proposed in this paper for a typical DC Microgrid. Different from other Microgrids, the DC Microgrid can interface to the distribution system by Solid-State transformer (SST). The hierarchical power management strategy includes three control levels: 1) primary control for DC Microgrid to implement distributed operation 2) secondary control for the DC Microgrid bus voltage recovery to achieve seamless mode switch 3) tertiary control to manage the battery charge and discharge. The DC Microgrid can operate in islanding mode, including the individual control for distributed renewable energy source (DRER) and distributed energy storage device (DESD). In addition, the DC microgrid can operate in SST-enabled mode to interface to the distribution system. The DC Micorgrid can seamlessly switch between islanding mode and SST-enable mode. The consideration of state of charge (SOC) for battery is also involved into the tertiary control. To this end, a lab test-bed is constructed to verify the system performance. Lastly, several typical case studies are carried out and the experimental results verify the proposed power management strategy.

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Microgrid, which can be considered as an integration of various dispersed resources (DRs), is characterized by number of DRs interfaced through the power electronics converters. The microgrid comprising these DRs is often operated in an islanded mode. To minimize the cost, reduce complexity and increase reliability, it is preferred to avoid any communication channel between them. Consequently, the droop control method is traditionally adopted to distribute active and reactive power among the DRs operating in parallel. However, the accuracy of distribution of active and reactive power among the DRs controlled by the conventional droop control approach is highly dependent on the value of line impedance, R/X i.e., resistance to reactance ratio of the line, voltage setting of inverters etc. The limitations of the conventional droop control approach are demonstrated and a modified droop control approach to reduce the effect of impedance mis-match and improve the time response is proposed. The error in reactive power sharing is minimized by inserting virtual impedance in line with the inverters to remove the mis-match in impedance. The improved time response is achieved by modifying the real-power frequency droop using arctan function. Simulations results are presented to validate the effectiveness of the control approach.

Integration of sub-gradient based coordinate for multiple renewable generators in microgrid

Microgrids (MGs) are power systems consisting of an electrical network composed by distributed loads and generation units that may include a communication network for improved operation. The considered MG in islanded mode is driven by voltage source inverters implementing decentralized droop control for active power sharing together with a communication-based consensus algorithm for frequency regulation. This paper analyzes the MG performance subject to network failures that provoke network partitions. It is considered that the electrical partition leads to several sub-MGs working in parallel where the power demand can be always guaranteed by the generation units, and the communication partition leads to several consensus algorithms also working in parallel. The double partitioning is analyzed through a closed-loop system model derived using the power flow equations that includes the electrical and communication connectivity. Analytical expressions for the steady-state values for both frequency and active power depending on the partitioning are derived. Selected experimental results on a low-scale laboratory MG illustrate the (undesirable) impact that unexpected partitions have in system performance.

In this paper a novel concept of improving power quality (PQ) in low voltage networks with distributed generation (DG) is proposed. In contrast to traditional methods which require the application of dedicated resources to compensate for power quality disturbances, the proposed idea is simply to form a microgrid (MG) capable to isolate from the utility grid and operate islanded when a disturbance occur. In the paper conditions for island creation are discussed and a control strategy is presented for a small-scale active network with microturbine, renewable energy sources, loads and storage system. The strategy assumes using energy storage unit as a grid forming unit for islanded operation and a central controller for supervising the network operation in grid-connected and islanded modes. The part of strategy is islanding condition detection method which is based on immunity curve defined for the considered network. Performance of the proposed strategy is presented as a reaction of the network on voltage dips, however the idea is applicable to any PQ disturbance of unacceptable level occurring in the utility grid. For the considered network a simulation model has been developed using PSCAD/EMTDC platform. Simulation results are presented in the paper which illustrate the operation of the control system and show the microgrid behavior after transition from grid-connected to islanded operation mode.

Due to the increasing energy demands in microgrids (MG), the need for parallel-connected distributed generations (DG) to supply the load required by customers has been increased. An interfacing inverter connects the DGs to the microgrid. The operating performance of a microgrid is largely depends up on the interfaced inverter. For a stable and reliable operation of the microgrid, the inverters of the DGs have to be controlled for proper power-sharing. In this paper, active power-frequency(P-F)/ reactive power-voltage amplitude (Q-V) droop control is used to control the three parallel-connected DGs in a microgrid operated in islanded mode. To confirm the droop control effectiveness, three distinct types of load are examined. Such as balanced, unbalanced and non linear load. When operating in an islanded mode, the inverter droop control should maintain the frequency and voltage within the permissible ranges. Furthermore, the findings show that droop control has a substantial influence on balancing the voltage magnitude, frequency and active power-sharing. The three parallel-connected inverters share equal active power among the DGs for all types of loads. Matlab/Simulink is used to model the droop control for parallel-connected DGs and verify the effectiveness of the droop controller.