Research On Key Issues Of Electric Springs

The traditional operating mode of power systems can be changed greatly by electric springs(ESs)that can match the load demand with power generation dynamically.With the help of ESs,the bottleneck of power generation from renewable energy sources(RESs)will be solved and qualitative leap will be seen in the high penetration of such power generation from RESs,especially with larger scale and high proportion.In this thesis,ESs,including topology study and improvement,operating modes,system modeling,control strategies and also possible applications,are mainly investigated.Some control strategies based on the phase control algorithm are proposed.Besides,some new topologies together with their control methods are also proposed.Both the simulation and experimental results are provided to verify the theoretical analysis.The fruits of this thesis are as follows:1.The topology and principle of ESs are introduced and its mathematical model is given.The simulation platform is established based on MATLAB/Simulink.laying a foundation for simulation research on ESs.2.A new phase control algorithm is proposed for the existing ES topology.Two control objectives can be achieved by the phase control and namely,1)critical load(CL)voltage,2)operation modes of ES.Several controllers are proposed based on the phase control,of which one contains a proportional resonant(PR)controller for CL voltage as the outer loop and a proportional(P)controller for inductor-current as the inner loop,one contains a PR controller with grid voltage feed-forward,and another one is based on Bode diagram optimization.Both the simulation and experimental results prove the validity of the proposed controllers,showing that similar effectiveness can be achieved by these three controllers except the one based on Bode diagram optimization has better performances on harmonic suppression.To make the phase control be more practical,some improvements have also been studied.3.A new type of ES topology is proposed for single-phase ESs.The balanced points of the ESs are modified compared with existing functions.With the proposed topology and circuit parameters,voltage of non-critical load(NCL)can be controlled to follow the grid voltage.4.The new concept of integrating three-port converters(TPC)and ESs is proposed to offer an active solution for energy management in AC microgrids.With an ES locating at the port where storage devices previously were,the so-called critical flux is regulated to a constant value within the core of the transformer.The voltage on each winding is then clamped so that CL voltage is regulated to a predefined value.The integration of a TPC and an ES can ensure a safer environment for the ES utilization.Thus,the power generated from RESs can be safely used at residential locations,with no need to worry about voltage fluctuations across CLs.Moreover,users can sell electricity to the power companies considered as CLs when the electricity generation exceeds the personal consumption.5.A novel control strategy of direct current control(DCC)and harmonic suppression function similar as active power filter(APF)is proposed using a new type of ESs with current-source inverters(CSIs)to improve the performances of ESs.Total harmonic distortion(THD)can be reduced greatly by changing voltage-source inverters(VSIs)to CSIs and also by changing voltage control to DCC.System performance can also be improved further by the additional harmonic suppression function.6.A control scheme is proposed for the three-phase electric springs(TPESs),based on the phase control.The scheme utilizes three independent phase controls,one for each phase,to stabilize the voltages across the CLs while handing over imbalance and distortion of the microgrid voltages,in addition to their fluctuations,to the NCLs.The scheme is also able to stabilize the CL voltages under imbalanced three-phase CLs and NCLs.Simulation results obtained under various conditions of imbalance of the microgrid voltages and/or the loads,and of distortion of the microgrid voltages,taken individually and together,show the effectiveness of the proposed phase control scheme of TPESs.7.A different structure of DCES is proposed in this paper to achieve a new type of DCES only using DC/DC converters.The idea and process are well explained and four opearting modes are set and anlyzed.An additional mechanism for fast charging or fast discharging is also designed to secure normal operation of batteries.The proposed DCES is suitable for the applicaion of roof photovoltaic(PV)systems.8.Two platforms of the ESs with 220V votlage rating are built in the lab,of which one is based on DSP platform to validate the proposed topologies and control strategies and the other one is based on dSPACE platform for demo.The experimental platforms have laid a foundation for producibility of the ESs and also for the integration of ESs and microgrids.