SELECTION OF INSULATORS FOR OVERHEAD LINE DESIGN
What are the guidelines to be considered in selecting insulators for overhead line design?
What are the guidelines to be considered in selecting insulators for overhead line design?
One of the most important and yet one of the most vulnerable links in transmission and distribution is insulators. Porcelain and toughened glass are the materials principally used for supporting conductors on overhead lines, and although these materials are relatively brittle and inelastic, they have proven service experience and are still widely used. The design of synthetic type insulators has improved both electrically and mechanically in recent times and they are being used in urban areas to minimise radio interference and in areas where gunshot or stone throwing is a problem. Insulator damage may occur due to such widely varying causes as lighting (puncture), power arcs, stone throwing, corrosion, gunshot and pollution.
The following points must be considered in the selection of the appropriate insulation of an overhead line:
• 50Hz performance (usually a pollution requirement)
• Impulse capabilities
• Switching capabilities
• 50Hz performance (usually a pollution requirement)
• Impulse capabilities
• Switching capabilities
Pin Insulators
This type was amongst the earliest designs, and although it has improved both electrically and mechanically, it has altered little in appearance. It provides the most economic, simple and efficient method of conductor support for voltage up to and including 33kV. Pin type insulators for the lower voltages are designed so that the puncture voltage is higher than the flashover voltage, however if the insulator glazing under the conductor is damaged (usually caused by vibration) the insulator may puncture.
This type was amongst the earliest designs, and although it has improved both electrically and mechanically, it has altered little in appearance. It provides the most economic, simple and efficient method of conductor support for voltage up to and including 33kV. Pin type insulators for the lower voltages are designed so that the puncture voltage is higher than the flashover voltage, however if the insulator glazing under the conductor is damaged (usually caused by vibration) the insulator may puncture.
Post Insulators
These insulators are of one piece porcelain construction and have a cemented on a galvanised malleable cast iron base provided with a taped hole for fixing stud. It will be apparent that this type of construction renders it almost non-puncturable and a further advantage is that if any expansion of the cemented base joint does occur the porcelain is put into compression. If this occurs with the cemented joint of the screwed lead thimble of the pin type insulator as discussed above, the porcelain is placed in tension, a type of load, which it has little ability to withstand, and the porcelain will fail. Where adequate additional insulation is not provided by the support (eg. Timber, fibreglass etc.) then the insulators should be of two piece or non-puncturable construction to minimise the risk of an electric hazard due to insulator failure.
These insulators are of one piece porcelain construction and have a cemented on a galvanised malleable cast iron base provided with a taped hole for fixing stud. It will be apparent that this type of construction renders it almost non-puncturable and a further advantage is that if any expansion of the cemented base joint does occur the porcelain is put into compression. If this occurs with the cemented joint of the screwed lead thimble of the pin type insulator as discussed above, the porcelain is placed in tension, a type of load, which it has little ability to withstand, and the porcelain will fail. Where adequate additional insulation is not provided by the support (eg. Timber, fibreglass etc.) then the insulators should be of two piece or non-puncturable construction to minimise the risk of an electric hazard due to insulator failure.
Stay Insulators
The stay insulator inserted in the stay wire is usually of porcelain and is so designed that in the event of failure of the stay insulator the stay wires will not fall to ground. All stays wires attached to wooden poles supporting active conductors should be fitted with stay insulators. The insulators should be mounted not less than 2.7 metres vertically above ground and have a wet power frequency flashover voltage not less than one and a half times the highest voltage conductor supported by the pole. The selection and placement of stay insulators should be in accordance with ESAA C (b) 1.
The stay insulator inserted in the stay wire is usually of porcelain and is so designed that in the event of failure of the stay insulator the stay wires will not fall to ground. All stays wires attached to wooden poles supporting active conductors should be fitted with stay insulators. The insulators should be mounted not less than 2.7 metres vertically above ground and have a wet power frequency flashover voltage not less than one and a half times the highest voltage conductor supported by the pole. The selection and placement of stay insulators should be in accordance with ESAA C (b) 1.
Cap and Pin Type Disc Insulators
These insulators are used at tension positions (ie. Termination and suspension) in high voltage lines and are available in 70kN and 160kV strengths to suit the various conductor loadings. The cap and pin design ensures that the porcelain or glass of the high insulator is always in compression. In areas of high pollution, particularly costal areas the pin of the insulator should be fitted with zinc collar.
These insulators are used at tension positions (ie. Termination and suspension) in high voltage lines and are available in 70kN and 160kV strengths to suit the various conductor loadings. The cap and pin design ensures that the porcelain or glass of the high insulator is always in compression. In areas of high pollution, particularly costal areas the pin of the insulator should be fitted with zinc collar.
Insulator Testing
All porcelain insulators taken out of service must be tested before re-erection. Toughened glass insulators however, need not be tested, since the smallest fault will cause disintegration of the insulator.
Information on selection of insulators is contained in the Design manual section on “Insulators”
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All porcelain insulators taken out of service must be tested before re-erection. Toughened glass insulators however, need not be tested, since the smallest fault will cause disintegration of the insulator.
Information on selection of insulators is contained in the Design manual section on “Insulators”
TO VIEW THE WHOLE ARTICLE!
This is a great help for insulators. Thanks for sharing!
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