Operational Principles And Applications Of Global Synchronous Pulse Width Modulation

In order to protect the environment and reduce the consumption of fossil energy,many countries have focused on the development of renewable energy.In the future,many governments will promote the further development of renewable energy.The renewable energy based generation,such as the grid-connected photovoltaics and the grid-connected wind power,are the best ways to use the renewable energy.On the other hand,the energy storage system will be widely used because of the power fluctuation caused by renewable energy.The grid-connected inverter is one of the most important equipment in the grid-connected renewable energy system and the energy storage system.Its efficiency and cost will greatly affect the economics of the whole system.Compared with the traditional generator,the inverters contain power devices,output filters.So,the efficiency of inverter is lower while the cost is much higher.At the same time,the switching characteristic of power devices leads to the high order harmonics which can effectively deteriorate the power quality.Many standards have been developed to limit the harmonics injected into the grid.In order to meet the standards,improving the topology,increasing the switching frequency or employing larger filter can be the solutions.But all of these methods will inevitably increase the cost or reduce the efficiency.So,how to reduce the cost,increase the efficiency while meet the standards are the topical issues in the past decades.After the deep researching in the past few years,it is much harder to further reduce the cost and increase the efficiency because of the limitation of power devices and materials.The wide-band gap device can work with much higher switching frequency and much lower loss.But it is hard to be widely employed in grid-connected inverters because of its high price.So,the further improvement of inverter has been blocked.Compared with the traditional generator,the capacity of one renewable energy system or energy storage system is much lower.That means,there are many inverters in the renewable energy plants or the energy storage systems.Many inverters will be connected to the grid trough the point of common coupling(PCC).So,it can be a new solution to reduce the cost,increase the efficiency with utilizing the coordination potential.With the development of master-slave coordination method,droop control,virtual synchronous generator,the multi-inverters have the coordination ability.But the existing methods are only beneficial to the grid operation,which means it cannot help to reduce the cost and increase the efficiency of the grid-connected inverters.The reason is that pulse width modulation(PWM)among multi-inverters have not been coordinated controlled.The PWM coordination has been widely employed in high-voltage and high-power applications.Phase shift PWM method is very popular in multi-leg inverter.This method can significantly reduce the switching frequency and filter size,which make it to be the best solution to reduce the cost and increase the efficiency.So,it is worthy to research how to extend the PWM coordination method into multi-inverters.After summarizing the PWM coordination methods,its most popular application is the multi-leg inverters.One of the characteristics of multi-leg inverter is that both the DC side and AC side are connected in parallel.The other characteristic is that the multi-leg inverter is controlled by a central controller.Compared with the multi-leg inverter,the multi-inverters are distributed in a large area and each inverter is controlled by its own controller.On the one hand,the PWM are not synchronized among inverters.On the other hand,the parameters of inverters are different,which can challenge the calculation of phase shift angles.This thesis innovatively researches the PWM coordination among multi-inverters,which can effectively help to reduce the cost and increase the efficiency.The topic of this thesis is "Operational principles and applications of Global Synchronous Pulse Width Modulation(GSPWM)".And the details of this thesis are listed in the following:(1)This thesis proposes the basic principles of GSPWM.Firstly,this thesis proposes the GSPWM whose main purpose is to eliminate the total current harmonics injected into the grid.The goal is to minimize the total harmonics distortion(THD)of total current.In the GSPWM system,the Global Synchronization Unit(GSU)will send the low frequency synchronization signals to synchronize the PWM.Then,with fully considering the different parameters of multi-inverters,this thesis proposes the general expression of total current harmonics.Particle Swarm Optimization(PSO)method is employed to solve the optimal phase shift angles and the frequency of synchronization signals.With these methods,the applicability of GSPWM can be improved.Finally,the GSPWM method is verified by the prototype with 4 parallel-connected inverters.(2)This thesis proposes the fast calculation method of GSPWM.Firstly,this thesis proposes an allocation method,which can allocate a huge number of inverters into several groups.This method can reduce the dimension of the GSPWM model,which can help to relieve the difficulty of calculation.Then,this thesis proposes the method to choose the initial position,which can replace the totally random initial position.The fast calculation method of GSPWM can reduce the calculation burden.So,the cost of calculation unit can be reduced.Finally,the proposed GSPWM fast calculation methods are verified by the prototype and the simulation(3)This thesis proposes the carrier synchronization method of GSPWM.Firstly,this thesis proposes the compensation method and the self-synchronization method.With employing the measurement of time delay,the phase distortion caused by the delay can be compensated.With analyzing the historical synchronization signals,the synchronization constant can be well calculated.Then,this thesis proposes the Phase Locked Loop based Carrier Synchronization(PLL-CS).The PLL-CS method can synchronize the carrier by employing the output of phase locked loop.So,it do not need the additional hardware,which means it can be easily embedded into the inverters.The applicability of GSPWM is improved.Finally,the proposed carrier synchronization method is verified by the experimental prototype.(4)This thesis proposes an offline redesign of multi-inverters using GSPWM.Firstly,based on the basic principles of GSPWM and PLL-CS,this thesis deeply analyzes the operation modes of GSPWM.After that,the worst operation conditions are calculated.Then,based on the worst operation conditions,this thesis proposes the method that can calculate the boundaries "Redesign Coefficients" for switching frequencies and output filters.Thirdly,this thesis proposes the method that can finally obtain the optimal solutions of "Redesign Coefficients".The goal in this thesis is to maximum the efficiency of inverters.Finally,the proposed method is verified by the multi-inverter prototype.(5)This thesis proposes the Two-layer GSPWM method employed in photovoltaic(PV)station.Firstly,the GSPWM method that can eliminate the circulating leakage current method is proposed.With this method,the topologies which can eliminate the common mode voltage are unnecessary.So,the cost of topology can be reduced.Then,the GSPWM method that can reduce the total current harmonics is employed among several groups.So,the two-layer GSPWM method can reduce the circulating leakage current while reduce the total current harmonics.Finally,the proposed two-layer GSPWM is verified with the prototype which contains step-up transformers.In summary,this thesis focuses on how to further reduce the cost and increase the efficiency.The basic idea is to coordinate the PWM sequences among multi-inverters.This thesis research the main issues of GSPWM deeply and comprehensively.