Control and operation of DC microgrids

• Main Author: Adhikari, Sujan
• Other Authors: Wang Peng
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Electrical and electronic engineering
• Online Access: http://hdl.handle.net/10356/74724
• Institution: Nanyang Technological University

The utilization of DC microgrids in power industry has increased rapidly with the expansion in use of renewable energy sources (RES), energy storages and DC inherent loads. DC microgrid reduces the power conversion stages, does not require frequency, phase and reactive power control in its operation making it advantageous over AC microgrids. However, the cost-effective solution to renewable intermittency, system topology and standards for reliable, stable and efficient power supply still needs examinations through research. Control strategies, for economic solution to mitigate renewable intermittency in the system with inclusion of hybrid energy storage system (HESS), interconnection of microgrids with tie-line for reliable power supply and bipolar-type DC microgrids for high quality and high efficiency power supply, is proposed, implemented and elaborated in this thesis. Energy storages are an option to solve renewable intermittency that can increase the utilization of RES into grid, changing the undispatchable generation to dispatchable. Energy storages can generate profit upon charging them when local electricity price lowers and can be discharged when the price is high. Choice of energy storage ranging from short term to long term is a key factor to address the compensation of load peaks, transients and to provide autonomy to the microgrids. Unfortunately, they cannot be achieved by a single energy storage, thus the need of hybridization of energy storages that have high specific energy and specific power is the solution for proper compensation of power fluctuation in autonomous microgrids. Upon review, it was also found that combination of lead-acid battery and supercapacitor compensate the gap of high energy density and high power density respectively and proved to be the best solution for hybrid energy storage applications and hence used as HESS in this research. The conventional droop based decentralized control achieves static power regulation however, lacks to address frequency based autonomous power management for dynamic power control. Hence, a frequency coordinating virtual impedance concept has been explored in this thesis for the co-ordination control of an HESS in DC microgrids. It investigates power decoupling methods to better attenuate the ripple power by filtration. This research uses lead-acid battery and supercapacitor to form HESS and are connected to a common DC bus through bi-directional DC-DC converters. Battery converter can absorb low frequency power variations while supercapacitor converter can absorb the high frequency power variations. Battery supplies the long-term power demand and supercapacitor responds to the short-term power fluctuations during transient process in this control approach. For the reduction of ripple power in battery, insertion of high order low pass filter for battery and high order high pass filter for supercapacitor converters instead of lower order low/high pass filters have been proposed. The effectiveness of the proposed concept is shown through simulation and experimental results. The interconnection of DC microgrids for providing reliable power supply is particularly important in rural areas where utility grid is not available. Although, various researches are conducted in single entity, the interconnection of DC microgrid cluster is still novice in research. The interconnection of identical DC buses through tie-line and formation of microgrid clusters improves reliability of the system. To achieve power flow control through the tie- line, decentralized control approach has been proposed where the bus voltage of each microgrid in the microgrid cluster is controlled to ensure the regulation of bus voltage deviations. The use of decentralized control approach mitigates the issue of communication stress when the microgrid control areas are geographically distantly located. The decentralized control is accompanied by mode change based operation so that the distributed units like solar photovoltaic (PV) and energy storage system (ESS) in microgrids can adaptively adjust their operation modes depending upon the designated voltage level. By doing so, the bus voltage regulation (BVR) and power flow control is adjusted making each microgrid in the microgrid cluster autonomous. The power is generated/injected from/into microgrid when there is power surplus/deficit caused by supply-demand mismatch in particular microgrid. Tie-line power flow takes place in a microgrid, from another microgrid, due to the bus voltage decrement beyond the designated level caused by the increment in local consumption which could not be satisfied by the local generation and storages. The effectiveness of proposed decentralized control has been verified experimentally in the cluster of two microgrids. Recently, bipolar-type DC microgrids have gained tremendous attention of researchers due to its advantages over conventional DC microgrids in terms of elevated level of quality, reliability and efficient power supply. Due to different loading in upper and lower terminals, voltage fluctuation from nominal value at respective terminals takes place and makes the system unbalanced. So, the control of parallel converters which interface distributed energy resources (DERs) and the topology of the converter along with voltage balancer play vital role in formation of efficient bipolar-type DC microgrid. The bipolar configuration of converters in a DC microgrid can be formed by utilizing a converter to boost the input voltage and adopting a three-wire system by the means of voltage balancing circuit. Although many control schemes have been proposed for bipolar-type DC microgrids, they mainly focus on coupled microgrids with a centralized voltage balancer. The centralized voltage balancing technique is prone to deteriorate the system functionality due to the failure in communication links. Replacement of a centralized proportional-integral controller with multiple decentralized voltage balancers in a microgrid enhances the system reliability by removal of the communication links. This thesis proposes a decentralized control for bipolar-type DC microgrids with a decentralized voltage balancing scheme for two battery energy storages. Droop control is implemented in this thesis for boost converters and voltage balancing circuits to realize decentralized power management for both load sharing and voltage balancing. The simulation results and experimental validations are provided to present the problem and associated solutions. The proposed control strategies for operation of DC microgrid incorporating rooftop PV and HESS, interconnection of two DC microgrids consisting of PV and energy storages and bipolar-type DC microgrid consisting of two energy storages have been verified with Piecewise linear electrical circuit simulation (PLECS) software. Experimental cases have been carried out to validate the simulation studies with laboratory scale DC microgrid(s) prototype(s) developed at Water and Energy Research Laboratory (WERL), School of Electrical and Electronic Engineering, Nanyang Technological University.