Root Cause Analysis of Converter Transformer Bushing Failure
Root Cause Analysis of Converter Transformer Bushing Failure

Blast furnace is one of the major processes of BF-BOF (Blast Furnace - Basic Oxygen Furnace) route of steel manufacturing in an integrated steel plant. In one of the Steel plants, four (4) winding converter transformers of capacity 42/21/21/17.8 MVA, 132/5.45/5.45/11 KV were used. These transformers are connected to 132 KV Gas Insulated Switchgear (GIS) by cables. The transformers feed inverter (load commutated inverter) driven blower motors (Synchronous motors) each of rating 29 MW, 5.45 KV. Converter transformer winding connected to harmonic filter system is rated at 11 KV, 17.8 MVA. Resin impregnated paper insulated (RIP) bushings were used in 132 kV oil-filled cable box of the transformers. Since blast furnace is a continuous process, blower motor has to run continuously. Power interruption to blower motor affects steel plant production causing enormous financial loss.

  • While in operation for some time, 132 KV bushings of 2 converter transformers failed one after another within a time span of about 24 hours. One bushing in each of the two transformers got physically damaged with clear markings of insulation failure and other two bushings in each of two transformers witnessed abnormally high capacitance and tan delta value. Owner of the steel plant entrusted Tata Consulting Engineers (TCE) to carry out a thorough investigation to ascertain the root cause of bushing failures and submit a report.

    • TCE carried out a methodical failure analysis considering the following aspects:
    • Review of SOE (sequence of events) and trip records downloaded from SCADA Questionnaires were prepared for collection of data and discussion with the manufacturer of the transformer and the Customer. Data collection from site, site test report on transformer/bushing, data sheet of transformer/bushing were reviewed Review of transformer manufacturer’s preliminary investigation report Insulation co-ordination & switching surge studies were carried out specific for the 132kV connection of converter transformers. Harmonic currents were measured at site for converter transformer which was healthy and in operation Analysis of published research papers relevant to the subject Review of earth mat design Review of 132 KV GIS layout and recommended earth points by OEM Review of GIS floor RCC construction details
    • A team of TCE power system experts carried out the study and investigated the following shortlisted probable causes of failure thoroughly :
    • Possibility of moisture ingress due to long term storage of bushings before usage Transient over voltage during switching in absence of SA/LA nearby bushing terminals Power frequency over voltages from downstream filter units Very fast transient over voltage (VFTO) which may be generated either due to restrike across breaker contacts or due to very fast operation of disconnectors

    After analysis, TCE submitted a report on root cause of the failure of transformer bushings. The report reveals that failure of bushings is not attributable to switching or lightning surges, as the maximum voltage surge which can appear at the bushing is well within its designed insulation level. Based on circumstantial evidences available at site, the report pointed out that the probable cause of bushing failures was due to permanent degradation of insulation due to moisture ingress over a long period of storage of bushings at site. The report has been accepted by the customer. The system has been in operation successfully for quite a long period after necessary rectification as suggested in the root cause analysis (RCA) report.

    TATA Consulting Engineers (TCE) is into the business of regularly carrying out similar studies with in-house power system experts and software tools. TCE have executed a number of root cause analysis studies on various equipment and system faults.