In an exclusive contribution to INMR, insulator expert, the late Richard Martin, explained why safety is increasingly the driver for changeover from porcelain to hollow core composite housings.
The composite apparatus housing is now being viewed more and more as a natural replacement for porcelain across a range of applications and voltage levels. This differs from past market behaviour, whereby composite hollow core insulators were most popular and cost-effective only either at very high voltages or for unique applications such as in seismic service environments. These were the segments where porcelain was expensive and also typically required comparatively long production lead-time. Another rationale that has driven the changeover in technology away from porcelain came from the possibility to reduce creepage, especially when designing insulators for demanding applications such as UHV or heavy pollution. Because composite housings are made with hydrophobic HTV or liquid silicone rubber sheds, they offer superior performance under contamination with little or no need for maintenance. Indeed, many power utilities that operate EHV and UHV networks now accept reduced creepage for composite housings in comparison to porcelain counterparts. Current practice in many large markets is to reduce creepage calculated according to IEC 60815 Part 3 by as much as 25% when using silicone rubber with a demonstrated high rate of hydrophobicity transfer and recovery.
Still, in recent years the driving motivation in specifying composite housings in place of porcelain has been their inherent safety. Each year there are numerous cases of costly material damage and even injury resulting from catastrophic equipment failures such as internal arcing, explosion, short circuit displacement of bus supports or seismic and other unexpected mechanical stresses. Vandalism and contamination flashovers only contribute further to the risks from sudden power arcs and failures of porcelain at a substation.
In fact, one of the first market segments for composite hollow core insulator technology involved so-called ‘explosion proof’ apparatus offered by a German supplier of instrument transformers. Their reference list for current and voltage transformers employing composite housings became significant in the mid 1980s when use of porcelain for such equipment had become problematic due to an inconsistent supply of high quality insulators.
The requirements necessary to achieve fully explosion-proof composite apparatus housings basically fall onto the FRP tube and end fittings assembly. Present day standards for pressurized housings describe severe tests such as pressure tests combined with partial discharge and routine tests at 10 bars. In the case of gas-insulated CTs and VTs, there is no need to have an inner liner since there is no arcing in SF6 that can produce aggressive by-products to attack a tube’s glass fibres. The burden of assuring an explosion proof housing then resides on the tube’s special construction, i.e. its winding angle, thickness, resin-to-glass ratio and bonding method to end fittings.
Insofar as seismic events, composite apparatus housings offer a level of performance that simply cannot be matched by porcelain. While it is true that certain complex designs involving porcelain could survive earthquakes, these would typically be unusually heavy due to greater wall thickness and this implies a high cost.
At the same time, it should be emphasized that superior mechanical and electrical withstand of composite hollow core insulators during a seismic event is not necessarily generic. Rather, it requires knowledge by the manufacturer of such factors as amplitude in 3 axes, duration, cycles per second, natural frequency of the affected apparatus, generated bending moments and allowed deflection. (IEEE document 693 provides excellent guidelines and testing techniques to evaluate apparatus destined for service in seismic prone areas). Indeed, the housing has to be specifically designed for the application. For example, the seal and tightness system must assure no leaks of oil or gas after shake table tests intended to simulate a seismic event.
The type of apparatus for which a composite insulator housing is intended places additional burdens onto their supplier in regard to maximum safety. In the case of high voltage disconnectors, such as vertical and horizontal break devices, the tube manufacturer also has to take into account the limited allowed displacement on the line side. There should therefore be an increased rigidity but not at the expense of greater bending moments at the base fitting or large increase in weight.
To assure true safety for application on circuit breakers, composite hollow core insulators must be designed to withstand possible arcing inside the SF6 insulated chamber as well as high temperatures of the end flanges and conductor. In the case of arcing in line tank breakers, designers must include a non-glass fibre liner to seal the vulnerable glass-resin matrix against any SF6 by-products. At the same time, the composite tubes used for such breakers (i.e. both support and chamber housings) need greater rigidity in order to maintain the alignment of electrodes in the arc chamber as well as external clearances.
Dead-tank breakers have still more parameters which must be considered to assure the promise of complete safety offered by composite housings. The high current electrode can generate enough heat as to elevate the temperature of the flange-tube assembly beyond 100°C. Moreover, the internal gas pressure generated is sufficient to cause failure of this important bond.
Fortunately, there are tests in the standards to verify the suitability of any composite housing design against combined heat and pressure cycles. Critical factors in this regard include the glass transition temperature of the tube, its burst pressure and the mechanical strength of the flange-tube bond under high temperature.
In conclusion, it is certainly accurate to state that application of composite hollow core insulators greatly increase the promise of safety at a substation. However, the end user should be aware that there is no generalized solution that will always work insofar as tube design, bonding of fittings or choice of materials. The particular insulator design ultimately selected requires the apparatus manufacturer to know the service environment, the mechanical and thermal stresses involved and any field experience of the selected design under relevant operating conditions.