ارزیابی سیستم کابل زمینی AC با ولتاژ بالا مبتنی بر نظارت زمان واقعی و محاسبه نظری جریان های زمینی Evaluation of high-voltage AC cable grounding systems based on the real-time monitoring and theoretical calculation of grounding currents

1 Introduction For high-voltage (HV) AC cable transmission systems, three separate single-core cables are frequently used instead of threecore cables. In practise, to suppress the induced voltages in sheaths, metal sheaths of HV cables are cross-bonded at each end of the cable section. The cross-bonding introduces return paths for the grounding currents [1, 2]. It is expected that, as a result of crossbonding, the voltages induced in the metal sheaths will not exceed the limit specified in GB50217-2007 [3], and the grounding currents will be minimised, leading to less heat generation in the metal sheaths [4, 5]. However, the effectiveness of the grounding may be reduced due to water seepage into cross-connected boxes, failure of outer sheath insulation, or damages by termites, causing more heat emission from the metal sheaths and reduction of the current carrying capacity of HV cables [6, 7]. Marzinotto and Mazzanti [8] reported the feasibility of cable sheath fault detection by monitoring grounding currents at the ends of cross-bonding sections. Dong et al. [9, 10] established a numerical model to simulate the currents in cross-bonded cable sheaths and then analysed the sheaths currents under typical fault conditions, including an open-circuit fault in a metal sheath, short-circuit fault in cross-bonded boxes and short-circuit faults due to the breakdown between metal sheaths in joints. The status of HV cable grounding systems evaluated by grounding currents is, therefore, an important consideration for HV cables [11–13]. Status evaluation standards for cable grounding systems in Beijing have been established based on data obtained from real-time monitoring of grounding currents over the period 2010–2012, in accordance with the relevant standards Q/GDW 456-2010 established by State Grid Corporation of China (SGCC) [4]. However, some erroneous judgments resulted from the application of these standards, which should, therefore, be modified. In this paper, we present the status evaluations of 93 cable grounding systems in Beijing, obtained using the standards established by SGCC. Thirteen of these systems were classified as abnormal or defective. A theoretical calculation of grounding currents was proposed. These 13 systems are re-evaluated by comparing the calculated and measured values of certain parameters. Based on this work, we propose a modification of SGCC evaluation standards.