Research On Modeling And Coordinated Control Strategy For Multi-Element Composite Energy Storages In Microgrid

Microgrid is commonly defined as a new type of integrated power system, consisting of distributed generators (DGs), energy storages (ESs) and interconnected loads and control devices, which can operate in grid-connected mode or in intentional island mode. Where the DGs and ESs are usually set together to avoid the threat caused by the power fluctuation produced by intermitted and random renewable sources (e.g., wind, solar). Futhtmore, the ESs can provide stabile power to meet the microgird’s demand to maintain its voltage and frequency levels. In a word, the ESs paly an important role in microgrid and this paper will focus on the microgrid with multi-element composite energy storages, and the optimal power allocation and operation strategy of them are detailed as following:Fistly, a typical energy storage system is introduced through two aspects:1) the basic source unit; 2) the power convertion systems (PCS). For the former, the classical equivalent models of lithium battery, lead-acid battery, liquid flow battery, super capacitor and flywheel energy storage are established to analyse their characteristics. For the latter, the general topologies, mathematical model and basic control methods (i.e., PQ control, V/f control and droop control, inverse droop control) of the PCS are introduced in detail. Inherently, a comprehensive control model integration of these four kinds of control methods is put forward to realize the optimal power allocation and coordination of the multiple composite energy storages.Next, ESs with different characteristics (e.g., battery, super capacitor) are connected to the grid through a common PCS to work as a composite energy storage, and the power allocation algorithm is the key point. In this paper, a power allocation algorithm based on SOC adaptive feedback is put forward to optimize the operation of composite energy storages. Compared with the traditional low-pass filtering algorithm and preset SOC feedback, the adaptive SOC feedback control module can regulate the SOC of battery or super capacitor to be the preset value finally, so that the composite energy storage system can keep the output ability to play effectively in the next.Besides, this paper discusses the operation strategy of multi-element composite energy storage system in master-slave controled microgrid or peer-to-peer controlled microgrid. Considering that the ESs have different types, performances, topologies and capcities, this paper takes a practical microgrid engeering for example to design the coordinated operation strategy of three different ESs in microgrid with traditional control scheme, and achieves perfect performance through certain sumilation work.Finally, an autonomous control strategy is proposed and investigated on the coordination of multi-elememt composite energy storages. Every ES can autonomously and independently regulates its control strategies according to the system state. Furthermore, a competitive mechanism derived from the carrier sense multiple accesses with collision detection (CSMA/CD) is proposed to decide wether the ES need to change its control strategy, which define the regulation priority of ES and then ESs can work orderly. Inherently, the proposed control strategy overcomes the drawbacks of dependence on communication networks in master-slave control scheme, and also avoids the current circulation caused by the differences of magnitude and phase of output voltage and frequency of multiple paralleled ESs in peer-to-peer control scheme. The simulation results shows that ESs work orderly or cooperatively without any communication, and autonomously keep system stable and safe.