Coordinated power management for islanded microgrids

• Main Author: Lin, Pengfeng
• Other Authors: Wang Peng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Electrical and electronic engineering::Electric power
• Online Access: https://hdl.handle.net/10356/136787
• Institution: Nanyang Technological University

This thesis enriches the power sharing concepts in four aspects, i.e., transient power sharing, stable power sharing, bidirectional interlinking converters (BICs) distributed power sharing and economic power sharing. These newly extended power sharing schemes are applied to different system applications and have inherent logical relations, which can be detailed as follows. To mitigate the generation intermittency of RES and load uncertainties, hybrid energy storage systems (HESSs) are constructed. In a particular HESS, those storages with high ramp rate but limited energy capacities are denoted as ESHs, whereas the storages with restrained dynamic responses are referred as ESLs. To coordinate ESHs and ESLs, this thesis proposes an integral droop (ID), inspired by the electrical characteristics of capacitor charging/discharging process, and apply the droop to ESHs. ESLs are with ordinary V-P droops. Through the coordination of the ID and conventional V-P droop, the transient power allocation in HESSs can be autonomously realized in a decentralized manner. The high frequency components of power demand can be compensated by ESHs, whereas ESLs respond to the smooth change of load power, thus achieving transient power sharing in energy storages. Additionally, by means of small signal analyses, it is revealed that the proposed ID can shape the output impedance of the HESS and stabilize the entire HESS system. The effectiveness of ID is verified by both simulations and experimental results. In a DC microgrid (MG), apart from HESSs, there also exist other power sources such as fuel cells and DC generators. The complex interactions between plenty of sources, HESSs, resistive loads and electronic loads, would unavoidably cause instabilities. For all that the proposed ID exhibits stabilizing effects to HESSs, however, small signal analyses are only tenable at certain equilibriums, whichwould not hold in full operation range. To guarantee HESS correct functions and overall DC MG stability, a decentralized composite controller (DCC) is proposed to attain global system large signal stabilization. The main idea of the DCC is to partition the DC MG into dispatchable unit (DU) subsystems and the lumped load. A well-devised high gain observer is adopted to estimate the electrical coupling of a particular DU subsystem with other DUs and the load. Then, rather than coping with the MG as a whole, the DCC is locally designed to offset the estimated coupling and stabilize the internal states of the DU subsystem. By doing so, each subsystem can be virtually decoupled from the MG and functions in an isolated fashion. When DU subsystems are interlinked, large signal stabilization of the entireMGwould be obtained by only guaranteeing the localized stability of individual subsystems. This stabilizing strategy has not been reported in the literature so far. Relevant theoretical analyses are rigorously conducted by employing Lyapunov stability theorem. The feasibilities of the DCC and the intended stable power sharing among DUs, have been verified by both simulations and experimental tests. Notice that both the transient power sharing and stable power sharing are addressed in DC MGs. Although many DC compatible sources and loads are emerging in modern power system, AC conversion techniques portray predominant roles in power generation, transmission and distribution networks. To merge the advantages of AC and DC systems, hybrid AC/DC MGs should be built up where AC and DC subgrids are bridged by a bidirectional interlinking converter (BIC). The increasing number of sources installed into the hybrid system would definitely result in the capacity increase of either AC or DC subgrid. This would bring about larger power interactions between the two subgrids, and more power may flow through the BIC. To meet the requirements of growing subgrid capacities, instead of frequently upgrading a single BIC, a more likely way is to deploy multiple BICs connected in parallel to stack up the power exchange capacity between AC and DC subgrids. In this context, this thesis proposes a distributed power management strategy (DPMS) for multi-paralleled BICs to avoid the overstress of a single BIC. Each BIC is assigned with a localized distributed controller (LDC) which generates the respective power reference for the BIC. By using the LDC, BICs are allowed to exchange information with one another in the distributed communication graph. The power interactions between AC and DC subgrids can be proportionally allocated to BICs based on their different power ratings in a fully distributed manner, which gives BICs distributed power sharing. The validity of DPMS is then verified by a RT-LAB hardware-in-loop (HIL) experimental platform. For a hybridAC/DCMGrunning for the long term, economic operations would e critically significant. As a continuous work of BICs distributed power sharing, the thesis also proposes a distributed control architecture for realizing economic power sharing among distributed generators (DGs). It is widely accepted that the entire power system operating cost can be minimized when DGs have the same incremental costs (ICs). The proposed architecture consists of two levels. In the first level, the AC frequency-IC ( fac-IC) droop and the DC bus voltage- IC (Vdc-IC) droop are employed in the AC and DC subgrids respectively. With the synchronization of fac and Vdc, DG ICs in each subgrid will be equalized. However, the droops will inevitably cause deviations of fac and Vdc. Then a distributed control canonical form (DCCF), which provides a generalized method for fac and Vdc recoveries, is proposed in the second level. The DCCF allows DGs to communicate only with their neighbors, thus alleviating the communication burdens and enhancing the system scalability. Due to the presence of DCCF, fac and Vdc fluctuations, which naturally indicate subgrid loading conditions, are invisible. An original relative loading index (RLI) is proposed to extract the hidden loading condition of each subgrid even though fac and Vdc are clamped as constants. By using RLI, the power reference of the BIC can be easily defined. All DG ICs the hybrid MG converge to the same value in the steady state, therefore obtaining the wanted economic power sharing. The viabilities of the proposed control architecture and the economic power sharing are verified by simulations and HIL tests.