LEAKAGE CURRENT MEASUREMENT AS A DIAGNOSTIC TOOL FOR MVDC CABLE SYSTEMS CASE STUDY: INDONESIAN 12/20 KV CABLE SYSTEMS

• Main Author: Wulung, Ario
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/81720
• Institution: Institut Teknologi Bandung

The power sector has experienced a rise in renewable energy production due to goal of zero carbon emission and the continuously rising energy demand. Advancements in direct current (DC) transmission systems and projects are developed to fulfill these requirements. DC transmission is offering advantages such as increased transmission capacity, enhanced power quality compared to conventional AC grids, and efficient integration with renewable energy sources. In addition to the developments in the high-voltage direct current (HVDC) transmission, this advancement will extend to medium-voltage (MV) levels in future. However, DC cables exhibit phenomena absent in AC cables, such as field inversion, space charge accumulation, and thermal runaway, which are partly linked to insulation material degradation. Assessing cable degradation involves evaluating the DC conductivity through leakage current (LC) measurements. LC is the flow of electric current penetrating from the conductor through the insulation material of the cable to the cable shield. For example, under AC stress, a higher value of LC indicates a high severity of aging, implying that an increasing trend in LC will elevate the potential risk of insulation failure. This research enables initial observation, contributing to the understanding and application of LC measurement as a diagnostic tool. Leakage current can be measured using both offline and online methods. Offline measurement, such as performing tests prior to cable usage, offers an initial evaluation of the insulation properties. Therefore, conducting advanced LC measurements on the cable post- operation and comparing them to prior conditions can yield insights about the cable's degree of aging and losses intensity. Online measurement can be carried out continuously from the initial operation of the cable, allowing for constant diagnosis of the cable system's present condition. This allows for the prompt identification of any irregularities to mitigate potential equipment harm. Therefore, this contribution presents a basic concept for LC measurement as a diagnostic tool for MVDC cable systems in operation. To investigate the application of leakage current measurement as a diagnostic tool for MVDC cable system, leakage current measurements are performed at Graz ii University of Technology on a 12/20 kV cross-linked polyethylene (XLPE) cable system, which was already qualified for a nominal DC voltage of UDC = ±55 kV DC. The cable used refers to the standard specifications for medium-voltage cable in Indonesia, with a cross section of 150 mm2 and an insulation material thickness of 5.5 mm, as well as an outer and inner insulation radius of 13.4 mm and 7.9 mm respectively. The measurements included assessing two cable systems, one cable system includes a joint, while the other does not. Both cable systems are 18 m in length and are equipped with two terminations. Cable jointing were made using the AC cold-shrink joint with a joint body which is made of silicone. The test object is exposed to a test voltage of UT = ±80 kV and a conductor temperature of ? = 90 °C, equivalent to a heating current of Ieff ? 500 A. The investigations are performed during 6/6 h load cycles, according to optimized PQ test and type test for MVDC cables. The main goal is to assess the influence of joints and terminations on the LC. This examination is crucial due to the possibility of the presence of several joints in the actual operation of MVDC cable system. This study offers additional investigations about the behavior of the apparent DC conductivity related to the impact of joints and terminations by using DC voltage. Additionally, supplementary tests are conducted to assess how current magnitude disparities, voltage polarity differences, various voltage levels, and different load patterns affect the DC conductivity of the cable insulation. Real-time LC measurements conducted under various scenarios provide valuable insights into insulation properties. The findings of this research hold significant implications for the formulation of appropriate maintenance strategies in DC systems. By identifying the optimal timing for maintenance, the risk of cable failure can be prevented. Furthermore, the application of MVDC cable systems, as investigated in this study, holds particular relevance for Indonesia. Considering the continuously increasing energy demand and the expansion of medium-voltage electrical networks throughout the archipelago, the adoption of advanced diagnostic techniques for MVDC cable systems can provide significant benefits for Indonesia's energy infrastructure in the future.