Years ago, researchers at Arizona State University, under the direction of Prof. Ravi Gorur, tested porcelain cap & pin insulators removed after 25-30 years in the field with the goal of assessing the condition of those remaining in service. Units that failed the puncture test were then cut open and analyzed. Electric field due to the presence of microscopic defects in porcelain was modeled using a 3-D software package since localized field enhancement can be high enough to cause dielectric breakdown of porcelain. Electrical deterioration can be a slow process and the process of breakdown creates phase changes in the porcelain microstructure. This was confirmed using a scanning electron microscope.
A growing population of insulators still in service are either reaching or have already crossed the threshold of expected operational lifetime. Therefore, to maintain transmission system reliability, electric power utilities must determine if these insulators are at greater risk of failure and, if so, replace them in a timely manner. Moreover, to avoid penalties, maintenance must often be done without taking the line out of service. It is therefore critical to perform such assessment and any other related work on the transmission network under energized conditions.
For example, several U.S. utilities in the past identified punctured units near the line end of insulator strings at 230 kV and above. Mechanical failure of the string can occur during flashover when the arc transfers internally in such faulty units, thereby creating enough mechanical force to separate the porcelain from the hardware (e.g. see Fig. 1). Proper understanding of the mechanisms responsible for such puncture problems can not only help manufacturers develop higher quality insulators but also allow utilities to use such information to remove inferior units from their networks.
Insulators Evaluated
Suspension and dead-end strings from 5 different manufacturers and that had been in service for 25 to 30 years were removed during regular maintenance. They were standard profiles and the combined mechanical and electrical ratings ranged from 67 to 178 kN (i.e. 15,000-40,000 lbs). These strings had caused several flashovers and in one case a line drop due to the porcelain being severed from the hardware. Apparently, there was no specific lightning, switching or bird related activity that could be associated with the flashovers. Contamination was also ruled out as cause owing to the relatively clean locations. Mechanical (i.e. tensile failure load) and electrical (i.e. puncture test in oil) were performed on all these units. Several units from two of the manufacturers were punctured while there were no punctured units among the other three suppliers.
Results & Discussion
Electron Microscopy
Test specimens were obtained by chipping off porcelain from the skirts. Several samples were taken from insulators made by the different manufacturers. They were then coated with a 12 nm gold layer using a sputter coater. Fig. 2 shows the typical microstructure of the different porcelain materials obtained under the same experimental conditions of accelerating voltage and magnification. Presence of pores can be clearly seen.
Dimension of pores was taken in two perpendicular directions. Thirty pores were measured from each sample and the average of these measurements was used as a representative pore size for each sample, as shown in Table 1. It was interesting to note that microstructure did not reveal much difference in pore size. However, there was considerable difference in the electrical and mechanical strengths of the insulators. This suggested that other details such as quartz / alumina / mullite / corundum content, cement composition, porcelain-cement interface, assembly and manufacturing details play an important role in insulator performance.
Possible Failure Mechanisms
Thermally Induced Failure
Heat generation and its subsequent accumulation within ceramic bodies is responsible for increased dielectric losses and can cause failure. Physical characteristics such as specific heat, thermal conductivity and coefficient of thermal expansion of the ceramic material are all factors influencing the destructive effects of heating. Specific heat value determines temperature rise in a ceramic body for a given thermal conductivity and heat input. These values vary over a relatively narrow range for most ceramic bodies. Any condition favoring localized current flow in insulators increases temperature of the ceramic with a cyclic increase in localized body temperature, resulting in high thermal gradients. Unequal temperature rise in a ceramic body is accelerated by non-uniformities at the microscopic level due to lattice imperfections. Porosity also plays a role such that thermal conductivity of wet process porcelain decreases with increasing porosity. The failure process within porcelain can then be aided by the presence of localized high temperature regions, which accompany increased dielectric loss, decreased dielectric strength and unequal expansion within the body, causing development of internal tensile stresses.
To fully understand the failure process, it is also essential to acknowledge the effect of temperature on the dielectric constant of porcelain. The dielectric constant of most ceramic bodies increases with increasing temperature. As temperature rises, the ionization and electric discharge process in pores also increases due to higher electric fields as an effect of this increased dielectric constant of ceramic material. As with most insulating materials, the dielectric strength of porcelain reduces with increasing temperature. Porcelain, like other ceramic materials, exhibits constant breakdown strength until a critical value of temperature is reached. Breakdown strength decreases significantly at higher temperatures. This critical value is a material characteristic and varies from one ceramic to another. For electrical grade porcelain, it is estimated to be about 100°C.
Effect of Pores on Mechanical Strength
The mechanical fracture strength of porcelain is inversely proportional to pore size. The smaller the pores, crystals and residual particles, the higher the intrinsic strength of a porcelain body. Strength of a material is dependent on interatomic forces. The theoretical strength of a material is estimated to be approximately one-tenth of the elastic modulus of the material. However, imperfections at the microscopic level can cause the experimental strength value to be much lower than the theoretical value. Such reduction in mechanical strength is attributed to the phenomena of stress amplification in the areas surrounding the flaw. The magnitude of this rise depends mainly on orientation and geometry of the flaw. Insulators in service always operate under mechanical stress while supporting electrical conductors. As a result, mechanical stress intensification (as explained above) accelerates the failure process.
Edge Breakdown & Treeing
The presence of pores causes electric field enhancement within the pore. At the cement-porcelain interface, one can expect heterogeneities, simply due to the multiple materials, each of which contain grains of different sizes. There is a bituminous coating that is applied to the inside of the metal hardware and this works as a ‘stress reliever’. At the microscopic level, various interfaces create numerous sites for electric stress concentration. The electric stress is higher in the void than in the surrounding porcelain by a magnitude that is equal to the dielectric constant of porcelain. Consequently, the gaseous part breaks down at relatively smaller stresses.
It has been reported in the literature that the dielectric strength of porcelain is about 14 kVrms/mm. Normally, each unit of an insulator string is subjected to about 10 kVrms. Units near the line end are subjected to greater voltage, especially on longer strings (i.e. 230 kV and higher) due to non-uniformity in voltage distribution. Under lightning and switching operations on long strings, line end units can be subjected to several times this nominal value of 10 kV. This gives rise to localized electric stress in the 15 kV/mm value range – sufficient to initiate localized breakdown. With each surge, the breakdown channel elongates. Complete breakdown can occur with formation of many such breakdown channels in the solid dielectric. Such a breakdown path assumes a tree-like structure due to its stepwise progression through the insulating media.
Fig. 3 shows two porcelain insulators that have been cut in the head region and have two different puncture patterns. Distinct channels in the form of a tree and straight channels were examined in detail under a scanning electron microscope. Fig. 3a shows a magnified image of one of the channels. It is interesting to note the phase change in porcelain structure in the immediate vicinity of the channel. This region displays a glassy amorphous structure as opposed to the distinct crystalline regions further from the channel. High temperatures (above 1300°C) must have been created locally to melt the porcelain and create this glassy appearance. Fig. 3b shows a puncture that originated from the cement-porcelain interface. It appears that the bituminous layer has invaded the cement and subsequently created its own conduction path in the porcelain.
Electric Field Computation
Electric field was computed using the 3-D software package Coulomb® and a 3-D scale model of the insulator was created (see Fig. 4). A hairline crack was modeled as a cylinder of diameter 1 µm and height of 4 µm. The tip of the crack was simulated by a hemisphere of diameter 1 µm, as shown in Fig. 5. High voltage applied to the pin was varied to simulate unusual circuit conditions of faults and electrical transients. The highest electric field at the tip of the channel was computed for different voltages in the range of 10 to 25 kV applied between the cap and pin electrodes. Fig. 6 shows the results.
To capture the effect of increased dielectric constant, simulations were run with dielectric constants of value 6 (corresponding to room temperature) to a value of 20 (corresponding to a localized temperature of 400°C). It was noticed that with increasing dielectric constant, the highest electric field in porcelain also increased, further increasing risk of electrical breakdown.
Such high electric fields are sufficient to initiate breakdown inside pores and voids leading to electrical discharges. These repetitive electrical discharges over an extended period may cause enough breakdowns to form a tree-like path between the electrodes, causing permanent failure.
Conclusions
Porcelain insulators can develop internal conduction paths during service that will eventually cause puncture of the unit. Development of such channels depends on the materials, microstructure and interface details. It has been observed that there are porcelain insulators that did not display any internal degradation, even though they were in service for the same time as failed units. This demonstrates that the quality of porcelain insulators can vary widely among different manufacturers.
Electric field computations confirm the availability of localized high electrical stresses, especially during surges and for units near the line end of long insulator strings. This was shown to initiate localized degradation of material over a prolonged period. A tree-like structure of puncture paths confirmed a slow degradation mechanism. Scanning electron microscopy demonstrated the presence of high temperatures at the tip of the channels.
