Research On Hybrid Electric Stress Evaluation Method For XLPE Insulation Life Index

With the rapid development of smart grid,HVDC transmission is widely used due to its advantages of long transmission distance and flexible power flow control.The cable network composed of cross-linked polyethylene(XLPE)cables is a key part of flexible direct current transmission.The insulation life index is an important tool for evaluating the electrical life of XLPE cables and their insulating materials,and has important theoretical research and engineering application value.The constant stress method is a common method to obtain the insulation life index in engineering,but this method has the disadvantage of long test time.Moreover,due to the difference in voltage withstand capability between the tested samples,there is currently no unified test standard to meet the test requirements of all samples.The evaluation of the XLPE insulation life index requires an efficient test device.However,the existing devices generally only work on a group of samples,and there are phenomena such as refusal to act,malfunction,and discharge when the sample breaks down.Aiming at the above problems,an XLPE accelerated life test device based on electrical stress was proposed in this paper,and hybrid electrical stress test method was also developed,in which voltage was applied to the sample in the order of "sequential advance-constant-sequential advance".The main conclusions of the paper were as follows:(1)An XLPE accelerated life test device based on electrical stress was developed.In the device,the multi-electrode pressurizing tank could control the distance between the multi-electrodes,preventing the occurrence of the discharge phenomenon between the electrodes when conducting multiple sets of breakdown tests.During the pressurization process,the precise adjustment of the AC/DC control circuit could prevent the device from refusing to operate and malfunctioning.After the sample is broken down,the breakdown timing counter could record and display multiple groups of breakdown time and an alarm signal would be send out at the same time.(2)A method to find the optimum voltage rise rate for sequential progressive stress accelerated life test was proposed.The equivalence of constant and sequential stress tests was established by the cumulative damage model and the inverse power model.When the insulation life index changes,according to the changing law of the movement trajectory of the sequential stress life curve under different voltage rise rates,the optimal voltage rise rate of the sequential stress accelerated life test was obtained.The effectiveness of the method was verified by the results of short-time breakdown test and sequential stress test.(3)A method for evaluating XLPE insulation life index using mixed electrical stress was proposed.In this method,the insulation life index was estimated by applying electrical stress to the sample in the order of "progressive-constant-progressive".First,according to the life curves under different electrical stresses,the applied voltage and pressing time in the constant stress test part were determined.Second,according to the inverse power model and the Weibull probability distribution model,the data processing of the hybrid electric stress test was carried out.Finally,a mixed electrical stress test and a constant stress control test were performed.The estimated insulation life index n was 13.80,and the total test time was 18172.5s.The deviation from the insulation life index estimated by the constant stress test was 8.49%,and the total time was reduced by 99.21%.It was proved that the method can effectively evaluate the XLPE insulation life index and overcome the defects in the constant stress test.