Study On Frequency Conversion And High Voltage Transmission Based On Photovoltaic Water Pump System

In recent years,there has been a gradual increase in the number of projects that rely on solar-powered water pumps to lift water for agricultural irrigation and supply water for human and animal consumption,and the application of photovoltaic water pumps in the Yunnan region has also made positive progress.Photovoltaic water pumps use solar power systems to convert solar energy directly into electricity,which in turn drives an electric motor to drive the pump to carry water from deep wells,rivers,lakes,ponds and other water sources.However,given the specificity of the area where the PV water pumping system is applied,and taking into account factors such as topography when installing the PV water pumping system,very often the PV arrays will be far away from the pumping station,and the line loss generated by conventional power transmission will be a problem that affects the economy of the system,at which point it will be necessary to use variable frequency high voltage transmission to reduce the transmission line loss.Although this method adds a section of high voltage transmission line,the investment is significantly lower compared to low voltage transmission technology solutions,which is very beneficial to the long term operation of the PV pumping system.Therefore,it is of great importance to study the variable frequency and high voltage transmission of photovoltaic water pumping systems.（1）Based on the mathematical model of the solar cell,a simulation model was built using Matlab/Simulink to obtain the I-V and P-V output characteristics of the solar cell under standard test conditions（STC,1000 W/m²,25°C）.An engineering model of the solar cell was constructed based on the"five parameter method"(I_sc,V_oc,I_m,V_m,P_m)to simulate the output characteristics of the PV module under different solar irradiance（400 W/m²,600 W/m²,800 W/m²,1000 W/m²）and different temperatures（15°C,25°C,35°C,45°C）.（2）The AC-DC-AC frequency converter is divided into voltage-type frequency converter and current-type frequency converter according to the difference of the energy storage components of the intermediate DC circuit.Matlab/Simulink is used to build simulation models for the rectifier part of the voltage-type frequency converter and current-type frequency converter.Including capacitor filter three-phase uncontrollable rectifier circuit（inductance and no inductance on the DC side）,three-phase bridge fully controlled rectifier circuit（resistance and resistive-inductive load）;Classification modeling and simulation of AC-DC-AC inverter circuit,including three-phase voltage inverter circuit and three-phase current inverter circuit,through the simulation of different circuit characteristics;The basic principle of PWM technology is analyzed,which provides a theoretical basis for the research of the following chapters.（3）A simulation model of the AC-DC-DC frequency converter circuit was built using Matlab/Simulink to simulate the waveform conditions of the output-side voltage and current of the AC-DC-DC frequency converter.The modulation ratio was set to 0.8and the harmonics of the output-side voltage and current of the inverter were studied at different carrier frequencies（1 k Hz,1.5 k Hz,2 k Hz,2.5 k Hz,3 k Hz and 5 k Hz）;the carrier frequency was set to 2 k Hz and the harmonics of the output-side voltage and current of the inverter were studied at different modulation ratios（0.1-0.9）.According to the characteristics of different filters,LC filter（star connection method）is selected as the filter type in this paper,the designed filter is verified using Matlab/Simulink,and the voltage and current harmonics on the output side of the AC-DC-DC frequency converter after filtering are analysed,and the THD_u and THD_i after filtering are 4.07%and 0.42%respectively,which achieves a better filtering effect.（4）The transient process of no-load closing of a three-phase transformer is analysed from the theoretical aspect;a simulation model of no-load closing of the transformer excitation inrush is built using Matlab/Simulink to simulate the three-phase transformer excitation inrush waveform;a simulation circuit is built to analyse the load operation and no-load operation of the transformer,and the voltage and current of the transformer under load operation and no-load operation conditions are derived respectively.Finally,the economy of variable frequency high voltage transmission system is analyzed.