© Electrical Research and Development Association (ERDA) 2023.
All rights reserved.
Designed and Developed by Trizone India
ERDA News, Volume: 41 No: 2, April-June 2024
Root Cause Analysis (RCA) of Electrical Component Failure
Root Cause Analysis of Failed Porcelain Disc Insulator of 400 kV Overhead Transmission Line
The porcelain disc insulator of 400 kV overhead line was found punctured during service at the installation site. The porcelain disc insulator was found failed after 10 years in service. The detailed analysis of failed and unfailed porcelain discs was carried out to investigate the root cause of failure. Various analysis techniques were used to assess the mechanical and electrical integrity of insulator disc. The porcelain material characterization was carried out using optical microscope, scanning electron microscope (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD) analysis.
Visual examination of failed porcelain discs indicated the fracture of porcelain and removal of cap and pin from disc. The cap and pin were in good condition without any corrosion deposits. Dimensions analysis of porcelain disc insulators showed no deviations. Non-destructive testing of failed insulator show presence of internal cracks near the pin-cushion-cement interface and near the interface of cement and upper steel cap region. Failed insulator discs exhibited better mechanical failing load capacity, no porosity in porcelain material and good galvanizing condition. However, failed insulator disc #2 did not conform the other mechanical tests such as temperature cycle test and electromechanical test. Radial cracks on porcelain were formed after temperature cycle test. Fractography examination of broken disc showed radial crack propagation initiating from the interface of cement and porcelain to outside. Many voids were observed on fractured surface mostly formed near the SiO2 particles. Microstructural analysis confirmed the presence of SiO2 particles (10-20 um size and 69.8 wt %) dispersed in mullite matrix (30.1 wt %). SiO2 particles observed in two different crystal form i.e. quartz and cristobalite. The additional quartz form cristobalite should not be present in the porcelain insulators. Difference in thermal coefficient in two phases may result in formation of micro-cracks.
In comparison to failed insulator discs, un failed disc and unused new disc did not fail in thermal cycle test and no internal electrical puncture observed during electrical test. Also, microstructure of both unfailed disc showed only quartz and mullite phase. No cristobalite phase was observed in the good insulators.
It was inferred from the detailed analysis that the failure of porcelain insulator occurred due to fracture of porcelain disc with radial crack formation from the cement-porcelain interface. The susceptibility of failed insulator discs to radial crack initiation at the cement-porcelain interface and presence of high temperature stable cristobalite phase in porcelain material resulted in failure of the insulators.
As the failure of porcelain insulator has occurred due to fracture of porcelain discs due to electrical puncture from internal cracks and radial cracks formed at the porcelain-cement interface, it is recommended to replace the old porcelain discs as it is possibly the whole batch quality issue.
Root Cause Analysis of Burnt High Voltage Double Break Isolator
Root cause analysis of failed high voltage double break isolator was carried out for one of the electric power utility. The isolator was installed in 220 kV switch yard of one of the power generating stations in India. The isolator failed after 14 years of service during operation. The failure of isolator led to significant melting of both isolator tube and copper contact at one end of isolator during operation. Other end of failed isolator was not affected. Chemical composition of isolator tube and contact material matches with aluminium alloy 1100 grade and 99.9% pure copper respectively. No abnormality observed in material.
Visual examination of failed tube shows significant melting and accumulation of white colour loose deposits at externa surface of tube below the copper contact region near the failure. The outer periphery of tube observed uniform which indicates that the isolator tube did not fail by overheating or creep during operation. Fractography analysis carried out using optical and SEM microscopy showed the presence of smooth surface with lot of porosity/ voids formed on failed surface, confirming melting of isolator tube and contact.
EDS analysis of loose deposits showed presence of Al2O3 along with concentration of Na, Mg, Si, S, Cl, Ca, P and K which indicate the possibility of corrosion of isolator tube below copper contact region. Copper contact had uneven thickness resulting in gap between the copper contact and isolator tube might have led to corrosion of tube due to ingress of water and arcing events during the operation. In case of unfailed isolator tube, corrosion deposits were not observed on isolator tube and copper contact and the cross section thickness profile was uniform without any gap between copper contact and isolator tube.
It was inferred from the detailed analysis that the failure of isolator might have occurred due to arcing at the region between copper contact bottom surface and isolator tube. The gap developed between the copper contact and isolator tube led to arcing and caused the melting of isolator tube and bottom surface of copper contact (which is in contact with isolator tube). Such gap might have formed due to improper fixing of copper contact during installation which gradually increased during routine operation. The significant corrosion of external surface of isolator tube below the contact resulted due to such gap, leading to increased resistance and arcing. FEM analysis of isolator at maximum operating current during failure and for overcurrent condition shows predicted maximum temperature of isolator far below the melting point of aluminium (~660 oC). It indicated that overheating due to maximum operating current may not cause the melting failure of isolator tube and supports the failure due to arcing event during operation.
It was recommended to ensure that the copper contacts are properly fixed with the isolator pipe and there is no visible gap. If dust/impurities are observed below the copper contacts, proper cleaning of the contacts should be carried out.
Root Cause Analysis of Failed Cable Joint
A 33 kV, 3 core straight through cable joint failed after a service life of approximately 30 years. The cable joint was dissected for evaluation of internal damage and to check for construction of the joint. The steel armour in the failed cable joint was found corroded indicating moisture entry in the joint. It is possible that the sealing may have undergone degradation due to ageing resulting in ingress of moisture inside the cable joint. The insulation inside the cable joint was damaged due to the fault and degradation of insulation was spread over a large area inside the failed cable joint. The three cable phases of the joint were separated and dissected to understand the cable joint construction. Analysis of all the three joint did not indicate workmanship related error while preparing the cable joint: the semi-conducting layer was properly removed, XLPE insulation was smooth and uniform gaps were observed between connector and XLPE insulation. Among the three cable joints, insulation of two were damaged to a large extent. The final fault resulted in puncture in the connector of one of the joint. In the punctured connector the flow of aluminium was found localised indicating the overheating was momentary. This indicated that the puncture was not the result of the localised resistance in the punctured connector, rather it was the outcome of the ground fault.
Based on the analysis of the joint connectors and visual observations, aging related degradation of cable insulation may have resulted in the joint failure. Partial discharge (PD) initiated in insulation over the joint leading to progressive degradation of insulation and loss of dielectric property. The degraded insulation may not have withstood the potential difference between the conductor (connector) and earth wire resulting in ground fault. It is recommended to increase frequency of hot spot monitoring and PD monitoring.
