Research On The Key Technologies Of PV Inverter Based On Micro Grid Operation

Renewable energy power (Such as solar power, wind power) becomes an important approach to get energy with the gradual reduction of traditional energy; it provides energy to the user by being connected in parallel with the power line. However, while large numbers of distributed power generations are connected in traditional radial distribution network, it will result in fundamental changes and cause the grid security risks, and still can not be accepted by Power Grid Company. Micro grid is a smart mini power system that consists of load and distributed power. It isn't inclined to change existing power distribution network, and the effect of micro power acting on grid will be eliminated by managing the relation of need and support power between load and power supply, meanwhile the potentiality of distributed power will be made good use of. In this paper, some key technologies, such as topological structure of micro-grid, distributed generation (DG) control strategy for running in connected grid mode and standalone mode, islanding detected strategy, integrated operation system of micro grid are studied deeply. The contributions of this dissertation are as follows:A novel micro grid topological structure based on distributed power was proposed, which has center controllers and load controllers; that DGs, load and load controller will be grouped, being connected in micro grid bus by automatically switch. Micro grid control strategy of distributed power was studied, and based on CAN-bus protocol, the communication protocol of micro grid was designed, which provides a low cost system scheme for micro grid. According to micro grid voltage characteristic being different when distributed power is located DG on different position, a proposal for located DG on better position is taken in the paper.On the basis of conventional connected grid PV inverter, a novel connected grid inverter controlled by center controller was designed, which supply power to load with current injection method during being connected in grid. When load power demanded is greater than micro grid power supplied, center controller makes PV inverter output maximum power with power point tracking control strategy. Otherwise, PV inverter outputs given power dictated by the center controller and insures power flow from grid.Combining passive islanding detection method (including over/under voltage method, over/under frequency method) and two proposed novel active islanding detection algorithms, an integrative islanding detection method was established, which narrows the non-detection zone during islanding. One of the active islanding detection methods is implemented by slight asymmetry modulation wave of PWM, which will induce the output current of the inverter to produce asymmetry and the output power to produce an orderly change between two consecutive half-cycle. Because of the change of output power, the frequency of PCC (the point of common coupling) will change when islanding scenario happens. As a rule, the change of frequency can be used to identify whether islanding happens. The asymmetry of modulation wave can be controlled easily by software to avoid to large current harmonic. The other of the active islanding detection method is implemented based on variance estimation. In order to implement islanding detection, a binary pseudo-random signal will added to the modulation waveform to make the voltage of PCC have different properties. When in grid-connected scenario, the voltage of PCC dominated by voltage of power grid, the variance has no remarkable peak; However, when the islanding event takes place, the variance of the voltage of PCC has a remarkable peak. Therefore the islanding status can be detected by monitoring the peak of the variance. Simulation and experimental results verify the accuracy and validity of two islanding detection strategies.According to traditional wireless droop control method, the parallel operation strategy of multi distributed generator in islanding scenario was studied, it controls regulate voltage and frequency of distributed generator and make power supply to output corresponding active and reactive power by regulates voltage and frequency of distributed generator. A single phase transient power measurement method is proposed, that it designs a close-loop control system which input is inverter output voltage/current signal. After getting virtual quadrature voltage/current signal component from the close-loop system output, then inverter output transient power will be obtained by processing these virtual quadrature voltage and current signal. These transient power obtained real time can verify the validity of wireless droop control method.Finally, according to the idea of micro-grid proposed in this dissertation, a simple experiment system has been constructed to validate the theory analysis and simulation result.