Research On Energy Management And Stability Of Distributed Generation System With EDLC As Energy Storage

As a promising energy storage element, Electric Double Layer Capacitor (EDLC) has thevirtues of both high energy density of battery and high power density of electrolytic capacitor.Moreover, it has long cycle life, high storage efficiency, sound temperature adaptability andenvironmental character. EDLC storage system and EDLC/Battery hybrid storage system andtheir application in Distributed Generation (DG) systems were studied in the paper with stand-alone Photovoltaic (PV) system as an example. Small signal stability improvement ofDistributed Power System (DPS) with EDLC was explored in the paper.The character of solar cell and PV system were introduced. An effective MaximumPower Point Tracking (MPPT) method ( dP dD)and its theoretic basis were presented. Anengineering-oriented simplified equivalent model of EDLC was given and then the equivalentmodel of stand-alone PV system with EDLC as energy storage was set up. The control loopand energy management process were designed. Simulation and experiment results show thatthe charging/discharging cycle efficiency of EDLC is as high as 92.5% and PV array canoperate on the maximum power point quickly and precisely. System is very stable when thegenerating power of PV array or the load power fluctuating.The equivalent model of directly paralleled EDLC/Battery hybrid was setup and theperformance and its influence factors were analyzed quantificationally with pulsant load as anexample. Three hybrid structures including passive and actively controlled hybrids wereexplored including theoretic analysis, simulation, experiment, and character comparison. As tothe actively controlled hybrid, a method was presented which can make the discharge currentof battery nearly constant and its output power equal to the average power of the pulsant load.The discharge process of battery is optimized markedly.The idea of EDLC/Battery hybrid as the storage of stand-alone PV system was advancedin the paper to optimize the operating process of battery and then lengthen its lifetime. Theperformance of the hybrid was analyzed quantificationally taking PV array and its chargecontroller as pulsant power supply in an uttermost situation. A passive hybrid structure wasbrought forward originally which can optimize the charge/discharge process of battery. Anactively controlled hybrid structure was also given which can make the energy managementflexible through the control of parallel controller. As to the actively controlled structure,battery can be charged with optimized current under a special method advanced in the paper.The economic performance of the EDLC/Battery hybrid was analyzed. When drivingpulsant load, the battery capacitance and cost can be reduced greatly since battery just affordthe average power of the load. The charging and discharging cycles can be reduced thanks tothe long cycle life of EDLC and the power converter between EDLC and battery. As a result,the life of battery is prolonged and then the operating cost is lessened.The small signal stability of DC DPS was explored. Two factors influencing systemstability including the negative impedance of constant power converter and the interactionsamong converters were analyzed. The method for checking small signal stability wasintroduced including forbidden region, impedance specification and stability marginmeasurement. The idea was brought forward that the small signal stability of DC DPS can beimproved with EDLC paralleled connected to the DC BUS. Due to the small equivalentimpedance, the output source impedance is reduced and the Nyquist curve of the system loopgain is far from the forbidden area. It was proved proper in a PV system.