HVIC Introduction
Since the introduction of insulators for high voltage electrical transmission, engineers have constantly battled against the effects of pollution on the insulator and the potential catastrophic effects the pollution can bring to an entire network.
Both Glass and Porcelain insulators offer the electrical design engineer a proven and long term method of providing insulation for the OHL. To provide reliability within their transmission system, or in substations, it is necessary to either clean the insulators periodically or to provide some form of pollution control. The traditional methods in use are Water Washing, Increased Insulation level/creepage or application of Silicone Grease.
- Water washing is a repetitive operation (from every 2 months to every 2 years depending on severity of pollution) providing only a short term solution. Although it can be done energised there are obvious dangers to this and sometimes flashover will result during energised washing, in addition to the safety implications this method will require dedicated equipment and crews. Most common method of water washing is to shut down the line and perform "cold" washing; this has hidden costs due to interruption of supply. The lifetime costs of water washing are high.
- Increasing the insulation level or creepage distance realizes a longer surface path over the insulator, the main reason being to limit leakage current. Typically this will extend the time between washing or cleaning, however due to long periods of "No Rain" and high humidity, flashovers may still occur. Additionally, this solution for pollution control may require replacement of the existing insulation. Principally this method of pollution mitigation is a short term fix.
- An older and now somewhat outdated method of pollution mitigation is to use Silicon Grease. Although this does offer water repellent properties the grease encapsulates pollutants and eventually needs replacing when saturated. Disposing of the waste silicone grease is another consideration for the utility as more local governments impose stricter controls on waste disposal.
The superior electrical properties of today's HVIC product offers the user enhanced hydrophobicity, much faster hydrophobic recovery after water immersion (such as after heavy rainfall) and far greater leakage current suppression and hence flashover mitigation.
The attributes of today's HVIC products are based on two decades of R&D by the leading manufacturers in the field of RTV technology. HVIC derives its characteristics from the special process employed during the manufacturing of the silicone polymer.
Typical industrial pollution affecting HV transmission lines and sub stations
Cost Comparison: Water Washing vs HVIC
Water washing is, as stated earlier, an ongoing and necessary maintenance routine, meaning costs year on year will generally increase. When the constant loss due to leakage current is added to water washing costs, the annual cost to the utility is quite substantial. Despite routine water washing there is still a constant risk to the network from flashovers.
With the ever increasing need for more power capacity, we are seeing even greater numbers of 400 kV over head lines, in some countries 500 kV lines are commonplace. These 400 kV+ lines require specialised equipment in order to perform the necessary cleaning. The cost of such cleaning equipment is generally more expensive than coating the insulators with HVIC during the installation phase.
Based on all statistical evidence, the application of HVIC will give the utility an immediate cost saving during installation of new 400kV+ lines, and a return on investment within 1 to 2.5 years for other lines and substations.
Summary:
Glass or porcelain insulators coated with Silicone RTV High Voltage Insulator Coating provide excellent service in even the most severe pollution environments. The hydrophobic quality of the coating does not deteriorate with age. The coating can be used in very high voltage applications without problems. No water washing is required during the lifetime of the coating, therefore the only ongoing cost for the utility is routine and normal inspection.
For long term reliability with minimum maintenance the choice must be Glass or Porcelain with High Voltage Insulator Coating for your installation.
How RTV Silicone HVIC Functions
The purpose of an RTV coating for high voltage insulators is to suppress the onset of leakage current. Leakage current develops along the surface of 'dirty' insulators. As the contaminant (typically carbon deposits, desert sand, industrial pollution or, salt deposits from coastal exposure) settle on the insulator surface and combines with environmental moisture (rain, fog or dew), to form an electrically conductive solution. It is very common for insulators to become contaminated in the manner described above. When this happens, small amounts of electricity leak along the body of the insulator. This is what is called leakage current. If leakage current becomes too high, there is a short circuit as electricity flows directly from the conductor and straight to ground. What ensues is a power outage and extensive equipment damage. The development of leakage current is greatly reduced by coating the insulator. In the case of HVIC, leakage current is virtually eliminated. Once the insulator has been coated and the RTV silicone has fully cured, there remains within the body of the coating small chain, low molecular weight silicone (known as "cyclics") polymer that is free to move within the body of the coating. Different manufacturers will have varying amounts of this "cyclics" in the coating. It is the cyclics in the coating that plays an essential role in the function of the coating. Two key properties of cyclics are its very high dielectric strength (i.e. high electrical insulation properties) and very low surface free energy. It is a common phenomenon that microscopic matter existing in a high concentration in one area but in a low concentration in another area will migrate from the area of higher concentration to that of lower concentration. This is termed diffusion. Because the concentration of cyclics is greater within the body of the coating than at the surface of the coating, there is a natural tendency for the microscopic cyclics to diffuse to the surface of the coating. But, since the surface free energy of cyclics is so low, it exists only in a monolayer on the coating surface. That is to say, when the cyclics migrates to the coating surface, it stops migrating when the surface is covered in a one molecule thick layer of cyclics.
When an insulator has been coated with RTV silicone, contaminants from the atmosphere deposit directly onto the surface of the coating resting on the monolayer of cyclics. Again, because of its very low surface free energy, the cyclics begins to creep over the contaminant particle until it is fully encapsulated in a monolayer of cyclics.
At this point, when moisture from the environment collects on the surface of the contaminated insulator, the contamination particles are isolated from the water droplets. This fact combined with the very high dielectric strength of cyclics means an electrically conductive solution cannot form along the surface of the insulator.
If rainfall is heavy, the contaminants will be washed away. In some instances, the monolayer of cyclics will also be washed away. In this situation, the cyclics from within the body of the HVIC layer again diffuses to the surface to regenerate the monolayer.
Clean uncoated porcelain insulator showing surface wetting. Water films over and when combined with pollution deposits creates an electrical path to ground and high risk of flashover.
Dirty coated insulator (10 year service) showing surface hydrophobicity. Water does not film over on the surface, the pollution deposits having been rendered hydrophobic therefore no electrical path to ground and no risk of flashover.