| In consideration of the limitations of the existing theory about insulation design,the erosion of harsh natural environment,the long-term effect of working voltage and the possible short-term impact of overvoltage,whether the insulation performance of power cables can operate safely and reliably must be verified by strict insulation tests.Due to the large dielectric capacitance of power cables,large capacitive current is needed in the power frequency withstand voltage test of AC cables,which will lead to the increase of the capacity,volume and weight of the withstand voltage test equipment,making it difficult to be widely used in field tests.Therefore,DC withstand voltage test is generally used to replace power frequency withstand voltage test.The test will lose the equivalence in the DC withstand voltage test,because the field strength distribution is different when different dielectrics of composite insulator work under constant electric field and alternate electric field;In addition,DC high voltage will cause the charge accumulation inside the power cables,and the superposition of the charge accumulation and rated voltage will cause potential risks to the cables insulation and accelerate the aging of insulators.Aiming to solve the above problems and realize the equivalence of test methods,the research and development of ultra-low frequency voltage withstand test system for power cables were carried out in this paper and the main work of the study was as follows:（1）In this study,a voltage withstand test system for power cables of 10kV and below was designed.The system can output 0～30kV ultra-low frequency cosine square wave voltage,and can carry out withstand voltage test for power cables with dielectric capacitance of 4.5μF and below.The test system was with the functions of voltage regulation control,test start/stop control,discharge and control for test items after the test,protection for test items with breakdown,etc.The system used a single chip microcomputer as the centers of control,measurement and protection,and the main circuit consisted of a high-voltage DC power circuit and a polarity conversion circuit.（2）The circuit of the test system was designed in this study.Based on the pre-stage of high voltage DC power circuit,the single cycle control chip IR1155S was used to insure the stable output of 220V power voltage from AC to DC and factor correction.The boosting unit adopted a high-frequency transformer and a voltage quadrupling rectifier circuit to realize two-stage boosting,so the volume of the test system was reduced;In order to feed back the control voltage,the sampling circuit was arranged at the rear end of the voltage-doubling rectifying circuit;And the polarity conversion circuit was designed that the transition process adopted LC oscillation,which not only achieved smooth transition,but also made the polarity conversion time in the range of 2～6ms.（3）The running program of the test system was designed,including comprised a main program,a measurement and display program and a control and protection program,which can ensure that the test system carry out the voltage withstand test of the power cables safely and reliably under the national safety standard,and reflect the insulation condition of the power cables.（4）In order to test the validity of the system,the Boost-APFC circuit based on one-cycle control was tested in this study.The results showed that the circuit had fast response speed,low waveform distortion and can effectively achieve the function of power factor correction,so it was qualified for the test system.By use of Simulink simulation software,the test system designed in this paper was studied by simulation.It was proved by the simulation results that the test system can be used to carry out the withstand voltage test of power cables of 10kV and below under the national safety standard. |
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