Chendan Li

Voltage scheduling droop control for State-of-Charge balance of distributed energy storage in DC microgrids

Due to higher power quality, lower conversion loss, and more DC loads, there has been an increasing awareness on DC microgrid. Previous emphasis has been on equal power sharing among different units in the DC microgrid, while overlooking the coordination of the energy storage units to maintain the State-of-Charge balance. In this paper, a new droop method based on voltage scheduling for State-of-Charge balance is proposed to keep the SoC balance for the energy storage units. The proposed method has the advantage of avoiding the stability problem existed in traditional methods based on droop gain scheduling. Simulation experiment is taken in Matlab on a DC microgrid with two distributed energy storage units. The simulation results show that the proposed method has successfully achieved SoC balance during the load changes while maintaining the DC bus voltage within the allowable range.

Multiagent based distributed control for state-of-charge balance of distributed energy storage in DC microgrids

In this paper, a distributed multiagent based algorithm is proposed to achieve SoC balance for DES in the DC microgrid by means of voltage scheduling. Reference voltage given is adjusted instead of droop gain. Dynamic average consensus algorithm is explored in each agent to get the required information for scheduling voltage autonomously. State-space analysis on a single energy storage unit and simulation verification shows that the proposed method has two advantages. Firstly, modifying the reference voltage given has less impact on system stability compared to gain scheduling. Secondly, by adopting multiagent methodology, the proposed distributed control has less communication dependence and more reliable during communication topology changes.

Power flow analysis for DC voltage droop controlled DC microgrids

This paper proposes a new algorithm for power flow analysis in droop controlled DC microgrids. By considering the droop control in the power flow analysis for the DC microgrid, when compared with traditional methods, more accurate analysis results can be obtained. The algorithm verification is carried out by comparing the calculation results with detailed time domain simulation results. With the droop parameters as variables in the power flow analysis, their effects on power sharing and secondary voltage regulation can now be analytically studied, and specialized optimization in the upper level control can also be made accordingly. Case studies on power sharing and secondary voltage regulation are carried out using proposed power flow analysis.

Power Flow Analysis for Low-Voltage AC and DC Microgrids Considering Droop Control and Virtual Impedance

In the low-voltage (LV) ac microgrids (MGs), with a relatively high R/X ratio, virtual impedance is usually adopted to improve the performance of droop control applied to distributed generators (DGs). At the same time, LV dc MG using virtual impedance as droop control is emerging without adequate power flow studies. In this paper, power flow analyses for both ac and dc MGs are formulated and implemented. The mathematical models for both types of MGs considering the concept of virtual impedance are used to be in conformity with the practical control of the DGs. As a result, calculation accuracy is improved for both ac and dc MG power flow analyses, comparing with previous methods without considering virtual impedance. Case studies are conducted to verify the proposed power flow analyses in terms of convergence and accuracy. Investigation of the impact to the system of internal control parameters adopted by DGs is also conducted by using proposed method.

Power flow analysis for droop controlled LV hybrid AC-DC microgrids with virtual impedance

The AC-DC hybrid microgrid is an effective form of utilizing different energy resources and the analysis of this system requires a proper power flow algorithm. This paper proposes a suitable power flow algorithm for LV hybrid AC-DC microgrid based on droop control and virtual impedance. Droop and virtual impedance concepts for AC network, DC network and interlinking converter are reviewed so as to model it in the power flow analysis. The validation of the algorithm is verified by comparing it with steady state results from detailed time domain simulation. The effectiveness of the proposed algorithm makes it a potential method for planning, dispatching and operation of droop controlled LV hybrid AC-DC.

Economic Dispatch for Operating Cost Minimization Under Real-Time Pricing in Droop-Controlled DC Microgrid

In this paper, an economic dispatch problem for total operation cost minimization in dc microgrids is formulated. An operating cost is associated with each generator in the microgrid, including the utility grid, combining the cost-efficiency of the system with demand response requirements of the utility. The power flow model is included in the optimization problem, thus the transmission losses can be considered for generation dispatch. By considering the primary (local) control of the grid-forming converters of a microgrid, optimal parameters can be directly applied to this control level, thus achieving higher control accuracy and faster response. The optimization problem is solved in a heuristic method. In order to test the proposed algorithm, a six-bus droop-controlled dc microgrid is used in the case studies. Simulation results show that under variable renewable energy generation, load consumption, and electricity prices, the proposed method can successfully reduce the operating cost by dispatching economically the resources in the microgrid.

Multi-agent-based distributed state of charge balancing control for distributed energy storage units in AC microgrids

In this paper, a multiagent based distributed control algorithm has been proposed to achieve state of charge (SoC) balance of distributed energy storage (DES) units in an AC microgrid. The proposal uses frequency scheduling instead of adaptive droop gain. Each DES unit is taken as an agent and they schedule their own frequency reference given of the real power droop controller according to the SoC values of the other DES units. Further, to obtain the average SoC value of DES, dynamic average consensus algorithm is adapted by each agent. A small-signal model of the system is developed in order to verify the stability of the control system and control parameters design. Simulation results demonstrate the effectiveness of the control strategy and also show the robustness against communication topology changes.

Optimal power flow based on glow worm-swarm optimization for three-phase islanded microgrids

This paper presents an application of the Glowworm Swarm Optimization method (GSO) to solve the optimal power flow problem in three-phase islanded microgrids equipped with power electronics dc-ac inverter interfaced distributed generation units. In this system, the power injected by the distributed generation units and the droop control parameters are considered as variables to be adjusted by a superior level control. Two case studies with different optimized parameters have been carried out on a 6-bus test system. The obtained results showed the effectiveness of the proposed approach and overcomes the problem of OPF in islanded microgrids showing loads unbalance.

Convergence analysis of distributed control for operation cost minimization of droop controlled DC microgrid based on multiagent

In this paper we present a distributed control method for minimizing the operation cost in DC microgrid based on multiagent system. Each agent is autonomous and controls the local converter in a hierarchical way through droop control, voltage scheduling and collective decision making. The collective decision for the whole system is made by proposed incremental cost consensus, and only nearest-neighbor communication is needed. The convergence characteristics of the consensus algorithm are analyzed considering different communication topologies and control parameters. Case studies verified the proposed method by comparing it without traditional methods. The robustness of system is tested under different communication latency and plug and play operation.

Multiagent based distributed control for operation cost minimization of droop controlled AC microgrid using incremental cost consensus

Microgrid, as a promising technology to integrate renewable energy resources in the distribution system, is gaining increasing research interests recently. Although many previous works have been done based on the droop control in a microgrid, they mainly focus on achieving proportional power sharing based on the power rating. With various types of distributed generator (DG) units in the system, factors that closely related to the operation cost, such as fuel cost and efficiencies of the generator should be taken into account in order to improve the efficiency of the whole system. In this paper, a multiagent based distributed method is proposed to minimize operation cost of the AC microgrid. Each DG is acting as an agent which regulates the power individually using proposed frequency scheduling method. Optimal power command is obtained through carefully designed consensus algorithm with only light communication between neighboring agents. Case studies verified that the proposed control strategy can effectively reduce the operation cost.