Study of Space Charge Behavior under AC-DC Compound Electric Field

The AC-DC superimposed electric field is one of the common types of electric fields endured in the operation of actual power equipment. For the purpose of analyzing the effect of space charge under AC electric field with DC bias field inside the dielectric, the dynamic response of space charge was tested under 5-20 kV/mm frequency sinusoidal electric field and 4-12 kV/mm DC electric field on standard sample PMMA recommended by IEC equipment calibration specification. The results show that the space charge that accumulated on the surface side and inside of PMMA specimen has the same polarity of the upper electrode polarity under sinusoidal electric field of setting working frequency, and the cumulative amount of charge increases markedly as the amplitude of the AC electric field increases. This may be related to the easier injection of charge into the semiconducting material. Under the electric field polarization reversal point where the zero-point is, there is no space charge hysteresis caused by positive and negative charge residues inside the specimen. This may be on account of the balance of injection and extraction of space charge in positive and negative half-cycle. Under the electric field that AC-DC superimposed, the polarity of charge accumulated near the surface side and inside the specimen is still the same as that of the upper electrode, moreover the phenomenon of charge hysteresis is obvious at the zero-point of AC-DC superimposed field where the polarization of superimposed electric field reverses. The phenomenon of charge hysteresis is more evident with the amplitude of superimposed DC electric field increasing. It may be associated with the imbalance between injection and extraction charge due to different effective action time of positive half-cycle and negative half-cycle of superimposed AC-DC electric field.