The Effects of Carbon and Material Differences of Bushing Connections on Power Transformers

Abstract

Power transformer is one of the critical equipment in transmission system. In order to maintain the reliability of power transformers, a regular test of the power transformer is very important. Dissolved Gas Analysis (DGA) is one of the diagnostic tools which is widely used for interpreting condition and faults diagnostic of power transformer, that depend on concentrations and composition of gases in power transformer's oils. In PT. PLN (Persero) Unit Induk Transmisi Jawa Bagian tengah (UITJBT), DGA testing is conducted every 6 months, so that deterioration that occurs in power transformer can be known early. If increased proportions of hydrocarbon gases are found in the oil, the fault must be founded and fixed as soon as possible. In this contribution, a case study of power transformer 150/70 kV 100 MVA at Banjar Substation which have high gas rates and D1. Low Energy Discharge in Duval Triangle assessment result, is presented. In order to find out the source for high gas rates, further tests are performed. The result of winding resistance measurement, transformer turn ratio, dissipation factor (tan delta) of low voltage to ground (CLG) is not according to standards, with a suspected problem in the winding structure, connection or short inter turn. Further corrective action taken by on site repair. The major findings in this case is carbon which attaches to the active parts and broken insulation in the lead bushing insulation due to material difference. This paper highlights that non-standard test results do not always indicate the power transformer is damaged, it may only require repairs to make power transformer operating normally again.