Power Sharing And Control In Standalone Hybrid Microgrids

The concept of distributed generation(DG)has facilitated the development of microgrids which can be regarded as small power systems designed to power local community.It involves linking together similar sources to minimize power conversion losses and increase system reliability,efficiency,flexibility and redundancy.Ac microgrids have been widely adopted due to their inherent capability of operating with the existing power system but dc microgrids continue to gain attention due to developments in semiconductor industry and widespread use of dc loads.To utilize the advantages of both ac and dc systems,hybrid microgrids have been proposed where ac and dc grids are tied by interlinking converters(ILC).The ILC is an important system component as it manages bidirectional power flow while regulating dc voltage and ac frequency.The control schemes of ILC can broadly be categorized as autonomous control and communication-based control.Communication-based schemes involve exchange of information via physical channels which can increase total system cost along with inherent security issues.Droop control is considered as an alternate where local information is used for system management.It has been widely deployed in electric power systems due to its lower cost,increased reliability and plug-and-play feature.Various droop schemes have been proposed in literature which involve dc grid voltage and ac grid frequency to manage bidirectional power.The majority of these schemes involve switching between control modes which is not feasible due to increased power losses.The control complexity is another issue which leads to electrical isolation problems.As the power grid continues to evolve,the hybrid microgrids will be coupled together to form a meshed network and this system requires a simplified ILC scheme which can incoporate individual grid capacities for autonomous power flow.The system operational cost should also be minimized through an economic power exchange.In addition,different aspects of hybrid microgrid design should be studied in detail to define a stable system operation.The research work described in this thesis aims to fill these gaps as:(i)investigate exisiting ILC controls schemes in hybrid microgrid while highlighting their pros and cons,(ii)propose an improved converter control which can consider ac and dc grid capacities along with tie-line power,(iii)increase understanding of the dynamic behavior of hybrid microgrids while focusing on ILC and ac and dc grid dynamics,(iv)propose cost-effective strategy for ILC to transfer power economically in interconnected hybrid microgrids.The main contributions of this work can be grouped into three areas,namely improved ILC control,hybrid microgrid dynamic analysis and economical ILC scheme for interconnected hybrid grids.In the area of improved ILC control,a generalized droop scheme is presented which defines voltage and frequency droops at dc and ac ends of the converter to manage bidirectional power.The control couples ac grid voltage,dc grid voltage and ILC active power via three scaling factors which can be used to adjust droop coefficients.The proposed scheme considers the capacities of individual grids along with the maximum power flow through the tie-line linking ac and dc grids.In the area of dynamic analysis,small signal analysis is employed to analyze the stability from three different aspects.First,the generalized ILC control is adopted and the impact of droop gains is studied to define a nominal operational range.Second,the ac and dc microgrids are represented as second-order synchronous generator and PV system,respectively,to study the grid dynamics.Third,the hybrid microgrid stability is discussed from the aspect of short-ciruit ratio(SCR)which depicts the ac grid strength.Both inverter and rectifier mode of ILC are considered for a comprehensive analysis.In the area of economical interconnected hybrid microgrids,a cost-based ILC droop scheme is introduced which aims to minimize the total generation cost on equal incremental cost principle.At the basic level,a voltage-frequency droop scheme is proposed for the converter which can regulate frequency and voltage while managing autonomous power flow from underloaded to overloaded grid.Secondary control is introduced to eliminate voltage and frequency deviations and the equal incremental cost principle is embedded into the voltage-frequency droop to transfer power economically among multiple interconnected hybrid microgrids.