145 research outputs found
CONTROL STRATEGIES OF DC MICROGRID TO ENABLE A MORE WIDE-SCALE ADOPTION
Microgrids are gaining popularity in part for their ability to support increased penetration
of distributed renewable energy sources, aiming to meet energy demand and overcome global
warming concerns. DC microgrid, though appears promising, introduces many challenges in the
design of control systems in order to ensure a reliable, secure and economical operation. To enable
a wider adoption of DC microgrid, this dissertation examines to combine the characteristics and
advantages of model predictive control (MPC) and distributed droop control into a hierarchy and
fully autonomous control of the DC microgrid. In addition, new maximum power point tracking
technique (MPPT) for solar power and active power decoupling technique for the inverter are
presented to improve the efficiency and reliability of the DC microgrid.
With the purpose of eliminating the oscillation around the maximum power point (MPP),
an improved MPPT technique was proposed by adding a steady state MPP determination algorithm
after the adaptive perturb and observe method. This control method is proved independent with
the environmental conditions and has much smaller oscillations around the MPP compared to
existing ones. Therefore, it helps increase the energy harvest efficiency of the DC microgrid with
less continuous DC power ripple.
A novel hierarchy strategy consisting of two control loops is proposed to the DC microgrid
in study, which is composed of two PV boost converters, two battery bi-directional converters and
one multi-level packed-u-cell inverter with grid connected. The primary loop task is the control of
each energy unit in the DC microgrid based on model predictive current control. Compared with
traditional PI controllers, MPC speeds up the control loop since it predicts error before the
switching signal is applied to the converter. It is also free of tuning through the minimization of a
flexible user-defined cost function. Thus, the proposed primary loop enables the system to be
expandable by adding additional energy generation units without affecting the existing ones.
Moreover, the maximum power point tracking and battery energy management of each energy unit
are included in this loop. The proposed MPC also achieves unity power factor, low grid current
total harmonics distortion. The secondary loop based on the proposed autonomous droop control
identifies the operation modes for each converter: current source converter (CSC) or voltage source
converter (VSC). To reduce the dependence on the high bandwidth communication line, the DC
bus voltage is utilized as the trigger signal to the change of operation modes. With the sacrifice of
small variations of bus voltage, a fully autonomous control can be realized. The proposed
distributed droop control of different unit converters also eliminates the potential conflicts when
more than two converters compete for the VSC mode.
Single-phase inverter systems in the DC microgrid have low frequency power ripple, which
adversely affects the system reliability and performance. A power decoupling circuit based on the
proposed dual buck converters are proposed to address the challenges. The topology is free of
shoot-through and deadtime concern and the control is independent with that of the main power
stage circuit, which makes the design simpler and more reliable. Moreover, the design of both PI
and MPC controllers are discussed and compared. While, both methods present satisfied
decoupling performances on the system, the proposed MPC is simpler to be implemented.
In conclusion, the DC microgrid may be more widely adopted in the future with the
proposed control strategies to address the current challenges that hinder its further development
Design And Implementation Of Co-Operative Control Strategy For Hybrid AC/DC Microgrids
This thesis is mainly divided in two major sections: 1) Modelling and control of AC microgrid, DC microgrid, Hybrid AC/DC microgrid using distributed co-operative control, and 2) Development of a four bus laboratory prototype of an AC microgrid system. At first, a distributed cooperative control (DCC) for a DC microgrid considering the state-of-charge (SoC) of the batteries in a typical plug-in-electric-vehicle (PEV) is developed. In DC microgrids, this methodology is developed to assist the load sharing amongst the distributed generation units (DGs), according to their ratings with improved voltage regulation. Subsequently, a DCC based control algorithm for AC microgrid is also investigated to improve the performance of AC microgrid in terms of power sharing among the DGs, voltage regulation and frequency deviation. The results validate the advantages of the proposed methodology as compared to traditional droop control of AC microgrid. The DCC-based control methodology for AC microgrid and DC microgrid are further expanded to develop a DCC-based power management algorithm for hybrid AC/DC microgrid. The developed algorithm for hybrid microgrid controls the power flow through the interfacing converter (IC) between the AC and DC microgrids. This will facilitate the power sharing between the DGs according to their power ratings. Moreover, it enables the fixed scheduled power delivery at different operating conditions, while maintaining good voltage regulation and improved frequency profile.
The second section provides a detailed explanation and step-by-step design and development of an AC/DC microgrid testbed. Controllers for the three-phase inverters are designed and tested on different generation units along with their corresponding inductor-capacitor-inductor (LCL) filters to eliminate the switching frequency harmonics. Electric power distribution line models are developed to form the microgrid network topology. Voltage and current sensors are placed in the proper positions to achieve a full visibility over the microgrid. A running average filter (RAF) based enhanced phase-locked-loop (EPLL) is designed and implemented to extract frequency and phase angle information. A PLL-based synchronizing scheme is also developed to synchronize the DGs to the microgrid. The developed laboratory prototype runs on dSpace platform for real time data acquisition, communication and controller implementation
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