Research On Islanding Detection And LVRT Control Technology For Distributed Grid-connected Inverter-typed Power Sources

The energy shortage and environmental pollution problems become the key factor restricting the social economic development and human sustainable development with the sustained rapid development of world economy and the growing depletion of traditional energy.And meanwhile,the limitations of the traditional centralized power generation mode are gradually exposed.People hope that the development and use of plentiful and non-polluting renewable energy sources such as solar energy,wind energy,and nuclear to realize the improvement of the traditional energy structure and maintain the human sustainable development.Under the new background of the power electronics in the power system,power electronics technology has become the key technology to solve the scale grid-connected renewable energy and distributed grid-connected power generation.Most of renewable energies are connected to distribution network by means of distributed generation,and the inverter-typed power sources are in the majority.The inverter-typed power sources connected to grid changes the structure and operation mode of power system and improve the safety,reliability and flexibility of power system.However,the interaction between the inverter-typed power sources and the power grid is becoming more and more significant with the increasing of the scale and the permeability of the inverter-typed power sources.The performance of the voltage-source grid-connected inverter,as the interface equipment of the inverter-typed power sources and power grid,directly affects the security and reliability of the inverter-typed power sources.The grid-connected inverter should have the functions of the islanding detection and the low voltage ride-through(LVRT),especially when the power grid failure or transient voltage sags.This paper takes the three phase voltage-source grid-connected inverter of the inverter-typed power sources as the study object.Islanding detection and low voltage ride-through technology are intensively studied in order to improve the stability and security of the inverter-typed power sources aiming at the power grid failures or transient voltage sags.The inverter must possess the islanding detection function,so this paper putsforward further appropriate improvement measures which can improve detection speed and power quality based on the further study of the slide-mode frequency shift method,active current disturbances method and negative-sequence current perturbation method.As the LVRT is also a mandatory function of the grid-connected inverter,this paper proposed a new LVRT control strategy based on flexible control of positive and negative sequence can reduce the negative impact of active power oscillation.The main contents are as follows:(1)The mechanism of traditional slip-mode frequency shift method and the frequency deviation in grid disconnection are described,and the non-detection zones(NDZs)based onf 0Q ?f frame are analyzed.It is found that the islanding detection speed of the traditional SMS method reduces rapidly even failure within the prescribed time with the increase of load quality factor.In order to overcome the above defects of traditional SMS method,a novel improved SMS algorithm based on parabola phase shift curve is put forward.And the algorithm parameters are optimized.In addition to maintaining the advantages of traditional SMS method,the detection speed of the improved method which is not affected by the load quality factor has been greatly enhanced.And it can still quickly detect the islanding state even in the worst operating conditions.(2)The principles of the constant and the periodic active current disturbance methods are described,and the deficiencies in NDZs and influence on power quality are analyzed.The relationship between the minimum active disturbance current which can trigger islanding protection and the amplitude of PCC voltage is deduced.And then the islanding detection method based on adaptive active current disturbance is proposed that the required active disturbance current can be adaptively added to the inverter according to the amplitude of PCC voltage.The proposed method,which has no NDZs,can reduce the negative effects on power quality by means of reducing the active current disturbance.(3)The mechanism of the negative sequence current perturbation method with positive feedback is described.And the necessary condition that the islanding can be detected is given.It is found that the linear feedback method will affect the symmetry of grid current when the presence of grid voltage unbalance exists fluctuation.In order tomake the unbalance factor of grid current gets a quantitative restriction,a novel islanding detection method with nonlinear positive feedback which injects less negative-sequence perturbation current is put forward.The proposed method makes the unbalanced degree of grid current gets a quantitative restriction,and it also significantly reduces the influence of the injection of negative sequence current.(4)Three phase voltage vectors under grid voltage sags are described,and the grid voltage positive and negative sequence components detection method based on DSOGI-FLL is analyzed.A flexible control method which realizes output power and current quality coordinated control has been proposed based on the analysis of the power control structure,control targets and five kinds of reference currents of power control strategy under grid fault.The principle of supporting the point of common coupling(PCC)voltage and the traditional flexible positive and negative sequence voltage support control strategy are analyzed.Since the presence of output active power oscillation in the aforementioned methods,an improved method that can effectively reduce active power oscillation is put forward.Further more,the mechanism of grid current overflow phenomenon has been revealed and a current-limited strategy is adopted.