Polypropylene Insulation for Cables

Polypropylene Insulation for Cables

DOI: 10.4018/978-1-5225-2309-3.ch004
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Abstract

Cable insulation made of polypropylene is of great prospective since it is recyclable to some degree. There are many problem the cable insulation need to deal with in HV system. The tree properties of polypropylene with repetitive pulse voltage at low temperature is discussed firstly. At different temperature, the pattern and length of electrical tree are shown in our results. This research analyzed the influence of temperature on the grow speed of electrical tree. The effects of mechanical stretching on space charge behaviors of pp/poe blend is also included in this chapter. This part presents the influence of stretching rate on electric properties. Finally, this chapter also provided some research of PP/POE/ZnO nanocomposites for HVDC cables. This part includes the space charge behavior analysis considering nano fillers.
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Introduction

HVDC transmission develops in a fast speed due to its advantages of large electric energy conveying with the minimum loss. Therefore it is urgent to research and develop HVDC plastic cables (Murata et al., 2006; Chen et al., 2015). The operating impulse voltage is a kind of overvoltage that often occurs in HVDC converter transformers because of the on and off states of thyristors and polarity reversal (Gao et al., 2013; He et al., 2013). The repeated impact of pulse voltage on HVDC cable insulation could not be ignored. Considering the lowest temperatures can reach -52.3 and -80.6 °C in the northeast China and Antarctic, HVDC cable may work under low temperature (Du et al., 2014). Over years, many researchers have investigated the tree growth mechanism in dielectric polymers. It was found that many factors such as the voltage, temperature, frequency and so on affected the treeing process. Chen et al have examined the effect of frequency with the applied AC voltage on electrical tree characteristics in the XLPE and found that higher frequency can accelerate the insulating breakdown, while the fractal dimension is mainly affected by the magnitude of AC voltage (Chen et al., 2009). Stone et al investigated the epoxy resin to understand the aging phenomenon with positive and negative pulse voltage. The epoxy resin lifetime showed a decrease tendency with the increase of repetition surge voltage (Stone et al., 1992). Holto et al investigated the electrical tree characteristics in extruded syndiotactic polypropylene (s-PP) at room temperature and found two types of electrical trees. One type was the simple straight tree - once a single branch reached the ground electrode and the breakdown occurred. Another type was the multiple branches tree - breakdown occurred after several branches reached the ground electrode (Holto et al., 2010; Holto et al., 2012). In our previous researches, the influence of high temperature up to 90 °C and low temperature down to -196 °C on the tree structure in silicon rubber has been concluded (Du et al., 2011; Du et al., 2015; Du et al., 2014). However, few researches are done about the low temperature effect on the treeing process in the PP with repetitive pulse voltage. This chapter compares the treeing process in the PP with that in the XLPE under low temperature and investigates the treeing process of the PP with different pulse frequencies and amplitudes under low temperature. The test temperature was set to three gradient, which was -30, -90 and -196 °C. It indicates that the tree is more difficult to be generated from the needle tip in the PP than in the XLPE. With the same treeing time, the tree length and fractal dimension in the PP are smaller than these in the XLPE. Also, the temperature, pulse frequency and amplitude have great effects on the tree characteristics.

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