| Interconnection of distributed energy resources helps to improve the efficiency,reliability and safety of the power grid.To realize the grid's sustainable development,renewable and clean energy resources are exploited in a high priority.Thus,the installation capacity of largescale distributed photovoltaic(PV)generation systems is growing boomingly.Nowadays,the power injected to the grid is required to be in a high quality by the standards.And the efficiency of the PV system is required to be with a high efficiency,which is born with the market competition.Thanks to the guidance of the grid's standards and the desire to receive higher efficiencies,the photovoltaic techniques have got rapidly developed.Cascaded multilevel converters(CMC)can interact with the medium/high voltage distributed grid without bulky fundamental transformer.Thus,the high power density can be easily achieved.On the other hand,the more the modules are,the higher the equivalent switching frequency will be.Thus,the high quality of the grid current can be achieved at the cost of limited module's switching frequency.Moreover,the high power injection is shared by a number of modules,and the high voltage regulation is also shared by all the modules.Thus,the selection of module's size can be simplified.The manufacturing cost of every single module can be lowered.Besides,CMC is featured by its modularity,scalability,and distributed maximum power point tracking(MPPT)capability,etc.Nevertheless,module-mismatch will cause over-modulation of phase voltage.Concurrently,the dc-link overvoltage will happen since the solar power harvested cannot be transferred to the grid in time.As an easy solution,the system can be over-designed,i.e.adding energy storages or power converters,increasing the power and voltage ratings,disabling the maximum power point tracking(MPPT)function etc.However,such solutions have disadvantages like insufficient usage of the added hardware parts,cost increase,volume increase,more complicated design and lower efficiency etc.Alternatively,a few researchers proposed modulation based strategies in order to realize a higher efficiency.By these strategies,the PWM pulse on each module can be adjusted according to the solar power distribution.However,such strategies is not able to manage severe module-mismatch issues.Further,some researchers proposed reactive power compensation based solutions.However,the reactive power injection or absorption is restricted by the grid,and will increase the system burden as well.Therefore,in this paper,we will try to find some solutions that are more efficient and are suitable to more severe module-mismatch issues.The main work of this paper is:1 Analysis of module mismatch issues Cascaded multilevel converter is born with module-mismatch issues,more or less.Once the system is suffering from severe module-mismatch,a number of issues like overvoltage/undervoltage of the dc-link voltage,distortion of the grid current etc.will arise.If keep running,the system might be unstable.Existing solutions are not as efficient,not sufficient for the system to reach a wide operating range as well.Thus,this paper presented the relationship between the power distribution and the voltage distribution.Then,this paper analyzed the impact mechanism of the module-mismatch.Finally,this paper reviewed most of the convertional solutions and summarized their disadvantages.2 An improved carrier phase-shifted PWM(LSPWM)modulation strategyIn power electronics systems,modulation strategies can solve specific issues that cannot be addressed by conventional controls.In this paper,LSPWM's characteristic of tolerating module-mismatch is analyzed.Then,the switching states of cascaded H-bridge converter(CHB)are listed.The states' features are also refined.Moreover,the states are arranged in an optimal way.Finally,a switching mode that has less switching events and is able to adjust the dc-link voltage utilization ratios comes into being.Such strategy is also an improved LSPWM,by which,not only the outputs of cascaded PV system can satisfy the requirements of related standards,but also the rating of the system is sufficiently utilized.3 A compensation strategy based on improved LSPWM and reactive power injectionTo further extend the system operating range,this paper proposed a compensation strategy that is based on the improved LSPWM and reactive power injection.This paper analyzed the reactive power compensation concept.Then,combined with the improved LSPWM,the reactive power compensation is optimized.In contract to the conventional strategies,the proposed one can save a lot reactive power,can reduce the switching events,and can distribute the reactive power in an autonomous and optimal way.Thus,the PV system is able to run in an extended operating range,efficiently.4 A coordinated solution for both the module-mismatch issue and the three-phase power imbalance issueIn this paper,another bottleneck issue of the cascaded PV system,i.e.the three-phase power imbalance issue,and its zero-sequence injection based solution is analyzed.Specifically,this paper studied the impact of the three-phase power balanced control on the module-mismatch.Based on that,a coordinated solution for both the module-mismatch issue and the three-phase power imbalance issue is proposed. |
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