Optimal Voltage Waveform for Better Utilization of Existing AC Cable Systems

Abstract

The electricity demand is increasing with the development of industry. Nowadays, sustainable resources used for electricity generation are playing a more and more important role in the power system due to serious energy problems. Higher capability of power transmission and distribution thus becomes significantly necessary to keep the stability and reliability of the whole system. Instead of investing in new infrastructure, this thesis concentrates on existing HVAC power cables using XLPE insulation, trying to find out a way to improve their capability of power transmission without reducing their expected lifetime.
For the purpose of utilization improvement, two models based on a realistic 132kV-XLPE power cable are developed first and then analyzed for AC, DC and combined voltages. Inspirations for creating voltage waveforms at an increased level, and thus improved capability of transmission are obtained from different field distributions in AC and DC voltage situations. The process of space charge development and the formation of the corresponding charge induced field is used in this work.
Secondly, two kinds of hybrid voltage waveforms are created: trapezoidal and ones in shapes of sinusoidal and DC waves. Those types of waveforms are able to force the maximum electric field stress in XLPE to move within a small range, resulting in a higher utilization compared to the AC nominal voltage application. The optimal voltage waveform for the 132kV HVAC power cable is determined based on the highest rms value of voltage.
Thirdly, for the suggested voltage waveform, sensitivities are evaluated for changing load levels, XLPE conductivity and the thermal conductivity of soil which depends on weather conditions. It turns out that the load level has highest sensitivity, followed by XLPE conductivity. Once the ampacity of the power cable is determined, the soil property needs little consideration. These important factors must be taken into account when designing the combined voltage waveform.
Finally, cable terminations are modeled to check the feasibility to cope with the new waveform. The main conclusion is that: it is possible to apply the optimal waveform to existing high-voltage cables using SCT field grading technique in their terminals, while those terminations with stress cones may be endangered if higher voltages than the designed value are applied.

Keywords: cable insulation, space charges, electric field distribution, capacity improvement.