Study On Battery Energy Storage System Performance Based On Modern Multilevel Cascaded PWM Converter

In recent years,fossil fuel depletion and environmental pollution are among the global issues that have stimulated interest in renewable energy resources such as wind,solar,ocean energy,geothermal,biomass,and hydrogen energy.As a result,dependence on renewable energy sources has become more significant than fossil fuel.Thus,there is a need for all types of energy storage systems(ESSs)because renewable energy sources provide intermittent supplies that contend with demand.Therefore,it will be necessary to continue to develop ESSs in the future at a global level.Battery energy storage system(BESS)is the technology of storing electrical energy using advanced batteries specially made and with high technology for this purpose,where the stored energy form grid or renewable energy sources can be used by releasing it to the network in times of need,which increase in the stability,reliability,and optimal operation of the electrical supply.The traditional BESS topologies based on the multi-pulse converter uses complex multi-winding transformers.At the same time,the multi-winding transformer is costly and complicated,in addition to being prone to failure,malfunctions,and losses.Therefore,the modern multilevel converters such as a flying capacitor converter,diode-clamped converter,and cascade H-bridge converter preferred to the traditional multi-pulse converter.Among the different multilevel topologies for converters,cascaded H-bridge multilevel topology has been the right solution for high-power medium-voltage applications because of its modularity structure,voltage balancing,separated DC sources,harmonics reduction,reliability and lower stresses on switching devices.As a result,these unique features make it suitable for battery power storage systems without using multi-winding transformers.The idea of this research arose as a result of the growth and increasing needs of Electrical Energy Storage(EES)as a critical technology for the use of more renewable energy and less fossil fuel,and the future smart grid,to achieve carbon dioxide reduction.Thus,EES techniques have shown high capabilities in dealing with some essential electrical properties,for example,the changes in demand and price around the day.Consequently,by storing energy from clean and less expensive energy sources,and then it can be released to the grid during peak periods,this leads to the elimination of high-cost power generation.Accordingly,the research aims to study BESSs performance based on a multilevel cascaded H-bridge PWM converter.Therefore,the main contributions and work of this thesis shown in the following points:Firstly,the imbalance of power and energy in the cells of the multilevel cascaded H-bridge converter leads to an increase in the switching losses,which affects the imbalance of charging and discharging conditions of battery energy storage systems and thus reduces their efficiency and the deterioration of the health status of the batteries.Therefore,the research presents a comparison of power and energy distribution(inter-phase and inter-bridge)of three-phase cascaded H-bridge multilevel inverter using different PWM techniques such as the phase-shifted and the level-shifted carrier PWM(phase disposition).The research also discusses the total harmonic distortion(THD)in the output voltages for both cases using a filter and without the filter.By the comparative analysis of THD for two techniques modulation methods,it observed that among all the control schemes,the THD is low.However,the PS-PWM process shows better harmonic performance after using the filter.Consequently,for the comparison,we note that the PS-PWM preferred as an option of distributed balance power,energy,and reducing harmonics in CHBC.Secondly,the research focuses on the SOC balancing control strategy for the battery energy storage system based on the CHBM converter.Hence,the investigation of SOC balancing is essential and indispensable for practical applications.Therefore,the SOC-unbalance may occur between battery units in the BESS based on 10kV/2MW/1000Ah three-phase 31 level CHBM-PWM converters,due to unequal switching losses and conduction time of converter,asymmetric battery units,and so on.Therefore,this may reduce the total capacity available for battery units,and may also increase the charging/discharging of a particular battery unit.This research adopted two types of control the first one,the active power control based on a non-ideal PR controller to provide sufficient gain for eliminating the current tracking error.While the second one,the phase-phase SOC-balancing control based on injecting zero-sequence voltage to realized SOC balance between phases and battery unit SOC-balancing control based on superimposing the corresponding AC voltage to achieved SOC balance between battery units.The research also describes a 10 kV,2MW,1000Ah system with forty-five Li(Lithium)-ion battery units to analyze the active power for the BESS.The method based on a three-phase 31 levels CHBM-PWM converter with star configuration focusing on the active power balancing of individual converter cells.The whole control system consists of active power control and state of charge(SOC)balancing control(inter-phase SOC balancing control and phase-phase SOC balancing control),the first one required to produce the three-phase balanced line to line voltage.In contrast,the second one is indispensable for the effective utilization of the battery energy to manage the SOC value for each battery to avoid the over-charge/over-discharge.Thirdly,in a cascaded H-bridge(CHBC)multilevel converter based battery energy storage system,the power processing capabilities of battery units may vary due to operating conditions,manufacturing tolerance,and when a new one replaces one or more battery units.For maximum utilization of battery energy,it would then be necessary to operate one or more battery units at lower or higher power levels.The research presented and verifies the active-power control of individual converter cells make it possible to charge and discharge the battery units at different power levels while producing a three-phase balanced line-to-line voltage.Therefore,the whole system designed was studied under normal and abnormal operating conditions under the influence of load,and the results show the control strategies' effectiveness.Therefore,the system rapidly demonstrated a response,which confirms the suitability of the system for practical application.