Meiqin Mao

Multiagent-Based Hybrid Energy Management System for Microgrids

Energy management systems for microgrids (EMS-MG) play an important role in ensuring their stable and economic operation. This paper presents a multiagent-based hybrid EMS-MG (HEMS-MG) with both centralized and decentralized energy control functionalities. Based on this framework, three-level hierarchical energy management strategies are presented, in which a cooperation method with contract net protocol and multifactor evaluation mechanisms are applied. A coordinated energy management framework is realized by the combination of autonomous control of local distributed energy resources at the local level with coordinated energy control at the central level of the microgrid. A novel simulation platform for the HEMS-MG is designed in terms of the client-server frame and implemented under the C++ Builder environment. To prove the effectiveness and benefits of the proposed control system, an example of generation cooperation control of a laboratory microgrid is provided. The simulation results show that the proposed control system is an effective way to manage and optimize microgrid operation.

Autonomous controller based on synchronous generator dq0 model for micro grid inverters

The parallel operation of voltage source inverter (VSI) in microgrid could benefit from characteristics of synchronous generator (SG) such as the kinetic energy stored in rotor, the self-isochronous and the droop characteristics if inverter can behave like generator in power system. Therefore an autonomous controller based on dq0 model of SG for microgrid inverter is proposed to mimic the above those characteristic of SG. Moreover, the inverter is designed to serve as either UF node or PQ node in micro grid by integrating a virtual primary frequency regulator (VPFR) and a virtual excitation regulator (VER) in the proposed controller. The results of simulation show the excellent performances of the proposed autonomous controller on voltage regulation, power sharing and seamless mode transfer.

Economic Analysis and Optimal Design on Microgrids With SS-PVs for Industries

This paper proposes an optimal design model of microgrids with small-scale photovoltaic systems (SS-PVs) and storage batteries for industries along with three economic evaluation indicators. The proposed indicators are levelized energy cost, emission reduction benefits, and payback period. They are combined to comprehensively evaluate economic performances and investment risk of the microgrids. The optimal model considering local solar insolation level, environment benefits, different operation modes, and initial investment costs is formulated and applied to obtain the optimal configuration of the microgrids with SS-PVs by particle swarm optimization (PSO) method. The simulation results based on 4-month real operation data of a 500-kW PV microgrid for industries in Guangdong, China, have verified that the optimal design by the proposed method can meet the technical requirements as well as the economic performances, thus effectively optimizing the microgrid with SS-PVs for industries in both economic and environmental aspects. Furthermore, the sensitivity analysis of the impact of different parameters on the system is also performed. The results provide a reference for the application of similar projects.

Energy coordinated control of hybrid battery-supercapacitor storage system in a microgrid

For a microgrid consisting of photovoltaic generators and hybrid energy storage systems (HESS) with the battery and supercapacitor (SC) banks, this paper presents a real time energy coordinated control method for both grid-connected and stand alone operation modes. In the proposed method, the low-frequency and high-frequency power of fluctuant loads are decomposed by the algorithm of Fast Fourier Transfer (FFT), and are sent to battery and supercapacitor banks, respectively, thus extending the service life of battery. To achieve the control objectives and realize the smooth transfer of different operation modes for the microgrid, a unified control strategy is designed for the inverter of battery, which enables it to operate at both islanding and grid-connected mode without detecting the operation mode of microgrid in advance. Simulation results show that the power of loads absorbed by battery is smooth, and the supercapacitor can respond to rapid fluctuation of the power of loads. With the unified control strategy, the power flows to grid can be controlled when the microgrid operates at grid-connected mode while the voltage and frequency on AC bus can be stabilized by the inverter of battery at the islanding mode.

A new impedance-based approach for passive islanding detection scheme

This paper presents a new approach for islanding detection that is based on monitoring measures of impedance at the point of common coupling (PCC). A frequency dependent model that characterizes the change in circuit topology when an island occurs is developed as a first step in systematically selecting impedance metrics and thresholds for islanding detection. To implement passive islanding detection, the monitoring operation exploits the presence of harmonic distortion in the current and voltage at the PCC as a basis for computing measures of impedance at frequencies where there is sufficient harmonic content. Unlike signals based schemes that rely on heuristics, the proposed approach is based on an analytic model that reflects the interconnection topology. The main contribution of the work is the development of an analytic model that reflects the interconnection topology of distributed generators (DGs) with the electric power system (EPS) for DGs that include power converters and DGs that do not include power converters.

Improved fast short-term wind power prediction model based on superposition of predicted error

Accurate prediction of short-term wind power is an effective way to rationally adjust the scheduling strategies and to improve the operation stability and economy of microgrid with wind turbines. A new improved prediction method which does not rely on any basic prediction methods is proposed based on analysis of a traditional wind power prediction procedure and in terms of strategy how to use a basic prediction method in wind power prediction procedure. By the proposed method, an additional error prediction model is built to predict the error of the predicted results by the traditional prediction method. And the predicted error value is added back to the predicted results mentioned above to reach the final predicted results. Taking Back propagation (BP) neural network as a basic prediction method, the proposed prediction method is validated by output power prediction of a real wind farm. Support vector machine (SVM) is chosen as another basic prediction method to test the versatility of the proposed improved prediction method. The simulation results show that the proposed wind power prediction method can improve the prediction accuracy by about eight percent. The proposed method does not involve the internal characteristics of any basic prediction methods or require any auxiliary methods, which is quite different from the traditional improved methods available and thus is more universal.

Real-time energy coordinated and balance control strategies for microgrid with photovoltaic generators

A typical microgrid system comprises photovoltaic generator which usually operates at both grid-connected and intentionally islanding operation modes. In terms of such operation characteristics of the microgrid, this paper proposes an active power control strategy dynamically to balance the energy output and consumption in the microgrid. By the control strategy, the photovoltaic generators may actively curtail their output energy instead of maximum output power by MPPT when the total microgrid system has surplus energy specifically at intentionally islanding operation mode. In order to ensure the stability of the voltage amplitude and frequency at PCC and achieve energy real-time coordinated and balance control, a method by real-time calculating the change of load power according to P-f and Q-V drooping relationship, then assigning to each micro-source by energy dispatching layer is proposed. The simulation results prove that proposed active power control strategy and coordinated method operates effectively to dynamically manage energy of the microgrid.

Optimal allocation and economic evaluation for industrial PV microgrid

An economic evaluation method and optimal allocation model of the industrial PV microgrids are proposed in this paper. The economic indexes include levelized energy cost, emission reduction benefits and payback period. The optimal model considering various relevant issues such as local insolation level and detailed investment costs is established and applied to improve the configuration of the PV microgrids for four different operation strategies. The optimization problem is solved by Particle Swarm Optimization to obtain the minimum annual energy cost of the system and bring more economic and emission reduction benefits for industrial users. The simulation results based on three months running data of a 500kW industrial PV microgrid in Dongguan City, China, verify that the optimal allocations by the proposed method can meet the technical indicators as well as the economic optimum, thus effectively optimizing industrial PV microgrid in both economic and environmental way. In addition, the added batteries can not only be used to maximize the utilization of renewable energy, but also enhance the systems ability of peak load shifting. Besides, the sensitivity of the impact of different parameters on the system is also performed. The economic evaluation and optimal allocation of the industrial PV microgrids in this paper provide a reference for the application of similar projects.

Application of IEC61850 in energy management system for microgrids

In order to achieve the interoperability among different devices made from different manufacturers, IEC61850 protocol is applied to the energy management system of microgrids (MEMS) in this paper to make the standardization of data communication among the microgrid. The communication system model of three layers for the microgrid is proposed and applied into microgrids. And the IEC61850-based data model for a microgrid is established based on the characteristics of IEC61850 standard modeling, the real-time data in MEMS and objective-oriented concept. In the proposed data model, the whole distributed generation (DG) unit or a load unit is considered as a logical physical device with different data nodes. A wind generating unit is taken as an example to illustrate the proposed modeling process in detail. The proposedIEC61850 Client and Server based communication system and corresponding testbed are also developed in this paper. The test results verify that the proposed method provides an effective technical way for the equipment integration, operation analysis and intelligent control in microgrids.

Performance evaluation for grid impedance based islanding detection method

This paper develops an approach for islanding detection based on measurements of the frequency dependent impedance at the point of common coupling (PCC) that exploits the presence of harmonics introduced by the electric power system (EPS) and harmonics introduced by the distributed generator (DG). The approach is new: i) an analytic model for the frequency dependent impedance is derived from the interconnection topology and this may be used as a basis for selecting features of the impedance that change when islanding occurs, and ii) the variation of a frequency dependent feature is used to characterize a non detection zone (NDZ) and iii) the load parameter space is used to compare the NDZ of the grid impedance based scheme with under/over frequency schemes. Although the focus is passive islanding detection, it also encompasses active schemes where certain harmonics are intentionally injected rather than inherent.