Behavioral Modeling Paradigm For Nanogrid DC Distributed Energy Systems

The rising global concerns regarding extinction of fossil fuels,carbon emissions,and climate change are the driving forces behind the trends towards renewable energy sources(RES).In addition,development of power electronic converters(PEC),storage devices,and robust control systems aim to facilitate this transition.At the same time,the remarkable boost in the consumption of electrical energy and growing demand for its reliable supply has led to significant developments in the field of distributed energy systems(DES).The DES is composed of conventional and renewable energy sources with the capability of bidirectional power flow control among different electrical loads and sources.The existing ac based power systems would require a lot of efforts like time reliability,synchronization,latency,criticality of data delivery and support for multicast to build up the advanced ac smart grids and find a feasible solution for these challenges offered.Compared to the traditional ac systems,the dc system offers several advantages,i.e.no need for frequency synchronizing,lower power losses in high voltage dc(HVdc)transmission,lower cable capacitance and dodging of skin effects and a higher degree of suitability for the integration of conventional and renewable energy sources and storage elements.Particularly,the dc nanogrid based DES has attracted the interest of many researchers as it enables a variety of power systems to work either in the standalone mode or integrated into other DES.This directly enhances the overall performance by reducing size,weight,and manufacturing cost.Since dc nanogrid based DES embody different power sources and loads,power electronic converters are employed to serve as an effective interface for integrating different converters,providing control functionality as well as dynamic independence.The dc nanogrid based DES consists of conventional and renewable energy sources(RES)with different loads,all interconnected through a central HVdc bus.It results in the constitution of a new paradigm in dc nanogrid based DES due to the high variability of the operating conditions,mainly due to the intermittent behavior of the RES and energy consumption by the customers.It exacerbates the dynamic analysis of the overall dc nanogrid based DES using traditional modeling techniques.The reason is that modern DES mostly employ the “ready to use” power converters available as modules.As these modules are provided by different vendors with less or no design information that is required for modeling,therefore,newer techniques are required to estimate the dynamics of these systems.The focus of this thesis is to model the dynamic behavior of power electronic converters to be deployed in dc nanogrid based DES without incorporating the core information about the power converter components,called behavioral modeling.It provides the design engineer a necessary tool to analyze the dc nanogrid based DES.The proposed modeling technique relies solely upon terminal parameters i.e.-parameters which can be obtained using frequency or transient response based measurements at the external terminals of the system.Transfer functions are obtained from the measured data,serving as dynamic models representing the input-output behavior of the system.In this work,for power electronic converters whose behavior exhibits negligible change over the entire operating range,linear behavioral modeling is employed,otherwise non-linear behavioral modeling methodology has been adopted.The polytopic structure based non-linear behavioral model is constructed by performing measurements at a number of operating points and then combining these linear models to form a polytopic structure model.The main contribution of this thesis is the proposal of behavioral modeling technique for nanogrid dc distributed energy system.The nanogrid consists of conventional and renewable energy sources with different loads,all interconnected through a central 380 V dc bus.This interconnection often brings a change in the behavior of the converter’s operation compared to the case,when it operates independently.To address this,the un-terminated behavioral modeling approach is implemented,which results in the removal of the effect of the interconnection.The terminal port nature of behavioral models has this distinct feature that it can be easily interconnected.The results validate that the proposed modeling technique for nanogrid dc-based DES offers an effective way to model the system.The concept of behavioral modeling has also been applied to three-phase systems.The modeling of three-phase systems is more challenging due to ac quantities.Besides,the number of -parameters required to be measured also increases,which further adds to the complexity of the modeling technique.For three-phase ac-ac network,a practical impedance measurement methodology is proposed based upon a set of independent perturbations.Finally,the two-port network-based behavioral modeling technique is also extended for a three-phase PWM dc-ac converter.