327 / 2018-10-01 18:49:15

Study of DC Breakdown in Multilayered Insulation Systems

Breakdown, multilayer, space charge, structure

In recent years, the rapid development of high voltage direct current (HVDC) technologies greatly benefits the capability and reliability of modern power networks. The development of novel and high performance dielectric materials has attracted abundant research interests, which enables the new generation of HVDC power equipment with higher operating voltage, larger capacity and reasonably low costs [1]. On the other hand, except the insulation properties, the structure of insulation system plays a vital role in the performance of the insulation system [2].

Multilayered insulation system have been widely applied for many decades in HV power equipment, such as power transformers and power cables, for various purposes. The optimized insulation structure could improve the mechanical and thermal properties without losing, sometimes even enhancing, electrical properties of the whole insulation system. However, for the applications for HVDC technologies, the breakdown and failure mechanism could be different from those for HVAC technologies, for example, the presence and accumulation of space charges under HVDC electric fields have been a major concern for the early breakdown of dielectric materials and insulation systems [3-4]. Therefore, in order to achieve the optimized insulation structure for HVDC applications, it is very important to investigate the relationships between multilayered insulation structures and their electrical properties.

As the initial of the research work, in this study, the relationship between multilayered insulation systems and their DC breakdown strengths have been investigated. Polyethylene terephthalate (PET) with different thicknesses were used in this study. Five multilayered structures were investigated in this study, including: a. a layer of 100 μm thick PET film; b. two layers of 50 μm thick PET films; c. a layer of 25 μm thick PET film + a layer of 75 μm thick PET film; d. two layers of 25 μm thick PET films + a layer of 50 μm thick PET film; e. four layers of 25 μm thick PET films. The total thickness of all the samples were fixed to 100 μm, in order to eliminate the influence of sample thickness on the breakdown strength. The DC breakdown strength of each structure was measured by a standard method with a plate-plate electrode system under a ramping voltage. The ramp rate of the applied voltage was 100 V/s. For each sample, more than 20 breakdowns were measured. The samples and the electrode system were immersed in silicon oil to avoid the flashover on the surface of the samples.

The initial breakdown results indicates two clear trends that a) for the same total thickness, the more layers of films brought better performance of tolerating high electric field which means have higher electric breakdown strength; b) the difference of thickness combination would also affect the breakdown strength under same layer number and total thickness. The order of the electric breakdown strength are : 100 μm ≈ 75 μm +25 μm < 50 μm +50 μm <25 μm+50 μm+25 μm< 25 μm+25 μm+25 μm+25 μm.

The obtained initial trends could contribute to the design of the practical insulation system by increasing the breakdown strength without changing materials. However, to further understand the mechanism behind this, space charge measurements for these multilayer insulation systems could be applied, which potentially could indicate the influences of interfaces and space charge characteristics on the electric field distribution within these different structures.