The importance of semiconductive layers in medium- and high-voltage (MV/HV) cables can't be overstated. Whereas conductors inherently create higher stress points on top of the strands, semiconductive compounds evenly distribute voltage stresses, smoothing the peaks for the electrical field and extending cable life.
Because these compounds play a critical role in how well MV cables function, it's important to examine their properties at the material level and ensure the materials are designed and mixed properly, making design for manufacturability an important consideration.
Material Level Properties
At the material level, semiconductor compounds have several important properties that must be examined to ensure consistent physical and electrical performance. Examples include the material's tensile and elongation attributes, cold brittleness temperature and resistivity at both room temperature and the emergency operating temperature.
In addition, the semiconductor compound's material ingredients must be carefully selected and monitored. For example:
We use thermo-gravimetric analysis and differential scanning calorimetry (DSC) to ensure the polymer's consistency with the initial design. We also check the Vicat softening point.
We examine the carbon black's properties, including the surface area, structure, absence of metal impurities, low transition metal ions, moisture and pH.
We check the material's other additives using tools like DSC and Fourier transform infrared (FTIR) spectroscopy.
Design for Manufacturability in MV Cable Manufacturing
In addition to examining the material properties, the semiconductor layers must be designed and mixed properly. They must also be processed correctly in the MV cable factory.
We tailor our ESCONTEK semiconductive compounds for processability. Any bump in the layer surface becomes a high stress point and therefore a potential failure point. For this reason, we check the semiconductive compounds for uniform distribution of the carbon black fillers. We do this by extruding a tape of the semiconductive compound and then checking the tape for smoothness and the absence of bumps — also called PIPs.
MV cable manufacturers want to process the cable core as quickly as possible while maintaining all the required properties, making Rheological (flow) properties and Curing properties very important. Low viscosities allow for higher processing speeds through the extruders; we monitor each batch for Mooney viscosity at 100°C.
Cable manufacturers also want to avoid any premature crosslinking — or scorching — in the extruders, making typical residence times in the extruders less than 10 minutes. We monitor our ESCONTEK compounds for Mooney scorch at 120° or 125°C, with normal processing at 115°C to 120°C. We also monitor T5 — or the time it takes for the compound to increase Mooney viscosity by 5 units from the minimum — in the laboratory.
ESCONTEK compounds show T5 times of over 35 minutes at 120° or 125°C, with typical times over 40 minutes. Figure 1 shows even more stringent tests at 135°C. At this temperature, the scorch time is still 33 minutes.
In short, using ESCONTEK compounds, processors won’t experience any scorching under the normal CV extrusion processing.
Finally, semiconductive compounds should be completely crosslinked at the operating speeds. We monitor ESCONTEK compounds using a cure curve (Figure 2) on a Rheometer. We do the cure test at 180°C, which is the temperature the compound is expected to reach in the heating section of the CV tubes. We also monitor for T50 and T90, which refer to the amount of time it takes to reach 50% and 90% of the complete crosslinking.
Our T50 numbers are typically under 2.5 minutes and T90 numbers are below 5.5 minutes. The formulations are therefore designed for rapid curing at the expected run speeds.
To learn more, please visit: alphagary.com/semi-conductive-compounds.