| Power cable is an important basic equipment in the power system,which bears the important responsibility of power transportation and power distribution.Once the power cable fails,it will seriously affect the reliability of power supply for enterprises and residents.Inspection can prevent property damage due to cable failure.The withstand voltage test can quickly expose the existing serious defects in the cable insulation and eliminate the potential safety hazard of the cable.The 0.1Hz VLF cosine withstand voltage test equipment has the advantages of small size,small capacity,good waveform and power frequency equivalence,and is receiving more and more attention.At present,the 0.1Hz VLF cosine withstand voltage test equipment exists on the market mainly for 10 k V power cables,with low test voltage and small capacity.However,the domestic technology of VLF cosine withstand voltage equipment for35 k V cables is immature and mainly relies on imports.Therefore,it is necessary to study the VLF cosine withstand voltage test equipment of high voltage level.In this thesis,a main circuit of an VLF cosine square wave generator is proposed.The circuit mainly includes two parts: a charging circuit and a polarity conversion circuit.The charging circuit adopts a half-wave rectification scheme and uses two transformers,which are respectively responsible for charging the positive polarity of the test product and Negative polarity charging.The polarity conversion adopts the LC oscillation principle,and realizes the polarity reversal through the reactor.On this basis,the main circuit parameters are set and calculated,including the resistance value of the protection resistor,the parameters of the silicon stack,the quality factor of the reactor,the capacity of the transformer and the voltage regulator,etc.The simulation model is built with Matlab/Simlink software,and the rationality and correctness of the design are verified.According to the working characteristics of the system polarity changeover switch,a pneumatic high-voltage switch is designed,and the switch,silicon stack and resistor divider are combined into a combined component to meet the miniaturization requirements of the system.Finally,the insulation margin of the combined component is verified.Based on the main circuit,this paper designs the measurement and control circuit of the VLF withstand voltage test system.The voltage transmitter and current transmitter are used to collect the output voltage and output current of the voltage regulator,and the resistance voltage divider is used to collect the voltage of the test product..The single-chip microcomputer controls the on and off of the transistor and the relay through the optocoupler isolation chip,and realizes the control of the voltage regulator,solid state relay,solenoid valve and other components in the main circuit.In terms of programs,the upper computer is the master and the lower computer is the slave to control the pressure-resistant system.The upper computer is responsible for issuing commands,and the lower computer executes and uploads the collected data after receiving the commands.The upper computer is designed with Labview graphical programming software,and the front panel has functions such as voltage waveform display,overcurrent protection,and data storage.Finally,a test and control circuit verification platform is built to verify the correctness of the test and control circuit hardware and software programming. |
