On-Site AC Withstand Test of 200kV XLPE Crosslinked Cable System

HIMALAYAL - SHANGHAI - CHINA

Abstract: This article raised existing questions in current domestic XLPE crosslinked cable commissioning test, discussed feasibility of 110kV, 220kV HV power cable on-site AC withstand with variable frequency resonance instrument, demonstrated effect and feasibility of variable frequency resonance transformer in XLPE crosslinked cable on-site AC withstand with practical examples in engineering practice applying variable frequency resonance instrument.
Key words: cable, on-site, AC withstand, variable frequency resonance

1.       Problem introduction
1.1   Problems and status
More and more XLPE crosslinked polyethylene insulating power cable are applied as substitute to the original oil charged butter paper insulating power cable both inland and overseas, yet due to factors of large power capacity and limited test equipments, crosslinked cable commission test continues to adopt DC withstand test method. Multiple research results from research institutes inland and overseas indicate that DC test causes damage of different degrees on XLPE crosslinked polyethylene cable. Some researches point out that XLPE structure stores accumulated unipolar residual charge adding AC voltage peak value may only cause cable breakdown. Some inland research institutes consider that during crosslinked polyethylene cable test, due to space charge effect, actual insulated electric field strength can be 11 times higher than working electric field strength. Even though crosslinked polyethylene insulating cable passes DC test without breakdown, it will cause severe damage to the insulator. Moreover, by reason that imposed DC voltage field strength distribution is different from operated AC voltage field strength distribution, DC test is incapable of simulating real overvoltage borne by cable during operation nor efficiently detect flaws on cable, cable joint and construction craft. Therefore, application of non-DC method for crosslinked cable withstand test is receiving more and more attention. Our peers overseas are currently using VLF(Very Low Frequency) power source on medium and low voltage cables for withstand test. Due to urban power network construction and rehabilitation in recent years, 110kV and above XLPE crosslinked cable or cable joint breakdown occurred occasionally. The solution to efficient commissioning test for 110kV and above XLPE crosslinked cable is becoming most concerned.

1.2   Standard problem
On account of problems on instrument capacity and volume, there is, for the time being no national standard regarding on-site AC withstand test of high voltage cable after installation, yet there’ve been doubts against DC withstand test standard for the reasons mentioned. DC test method is applied by CIGRI (Conference International des Grands Reseaux Electriques) 21^st workgroup squadron in <High voltage cable with extruded insulation acceptance test recommendation guide>. In standard IEC60840, 45-160kV installed cable test standard added 1.7U05 minute or 1U024 hour AC test standard besides the original DC test standard, with only requirements on AC test, namely 20-300HZ1.4U0.60 minutes. In order to conduct efficient commissioning test on installed crosslinked cable, Shanghai Himalayal Corporation recently added standard on crosslinked cable AC withstand test to the item of cable main insulating withstand test into the revised <Power equipment commissioning and preventive test procedures>.