Study On Space Charge Distribution Detection Technique In Polythelene Applied Alternating Electric Field

Space charge accumulation under alternating electric field is a reason for the electrical tree phenomenon in polymer materials.There is apparent significance in the detection and research of space charge distribution in polymer nanocomposite under alternating electric field.However,the equipment for such application is not well established.In this paper,an ac space charge measurement system is designed and space charge distribution in LDPE sample is measured.Pulse generator is the key equipment in the space charge measurement system whose parametershave a direct affect on the properties of the measurement system.According to the wave processing,narrow pulse can be generated using a single transmission line combining with switch technology.In this paper,a high-frequency,high-voltage pulse generator is design based on high voltage transistor switch.This pulse generator is able to output high voltage pulses with a basis frequency up to 3kHz,the half-height peak width is adjustable from 5ns to 25 ns and the amplitude of the pulse is up to 2.5kV.Moreover,a burst of 2~100 pulses can be issued on the basis frequency and the minimum pulse interval can be as low as 1μs.By improving the data acquisition module,high voltage source,control device on the base of the original dc PEA space charge measurement system,a new space charge measurement system for ac detection is built using the high-frequency,high-voltage pulse generator.Measurement software and data processing software are compiled to make the system work.The outcome of the test shows that the new PEA system can detect the space charge distribution in up to 60 phase points in one single 50 Hz ac cycle.20~100 signals are averaged in one phase point to get a signals with high S/N(signal to noise).The pulse trainapplied on the sample is short enough to get a signal of high precision on phase.Due to the complexity of data detection,acquisition and storage caused by the changing periodic ac electric field,proper method for measurement is a must.Three kinds of sampling modes are designed and compared to make sure the system can work smoothly.Space charge measurement under both dc and ac electric field in LDPE film is finished to verify the performance of the system.In ac measurement,pulse trains containing100 pulses are applied to the sample associating with 50 Hz ac voltage.Fourty phase points in one single ac cycle are detected and each point contains 100 pieces of space charge signals.After averaging process,space charge signals with high signal to noise ratio is obtained.The outcome of the measurement shows: it is more difficult for space charge to be detected in the bulk of LDPE film,but space charge do accumulate in LDPE.Applied dc electric field,the space charge accumulation in LDPE can be observed when the field intensity is 15kV/mm,the space charge mainly accumulated besides the electrodes and show opposite polarity to the electrodes.Applied ac electric field,the space charge accumulation in LDPE occurrs when the field intensity is 30kV/mm,the space charge mainly accumulated in the centre area of the bulk,the quantity of space charge accumulated is less than that under dc electric field.The outcome of the measurement has verified the properties of the measurement system.