Hollow Insulator

Hollow Insulators are insulators which have a hollow core.  These insulators are used in substations, in circuit breakers, station posts, etc.  They are used to enclose other components such as lightning arrestors, potential transformers etc.  

They are usually made of composite materials.  However porcelain insulators are also available.  They are also lighter and have greatly reduced chances of catastrophic failure.  Hollow Insulator also have excellent seismic resistance. 

The hollow tube in these insulators can be made of FRP.  Fibre reinforced Plastic has excellent resistance to bending and can be used in cantilever-type applications. 

These insulators can be manufactured in a number of shapes such as conical, cylindrical and straight inner or outer appearance.

Arcing Grounds

Arcing Grounds is a phenomenon which is observed in ungrounded three phase systems.  In ungrounded three phase systems operating in a healthy balanced conditions, capacitances are formed between the conductors and ground.  The voltage across these capacitances is the phase voltage. 

Now, in the event of a ground fault, the voltage across the faulty conductor becomes zero while the voltages across the healthy conductors increase by a factor of 1.732. 

The arc caused between the faulty conductor and the ground gets extinguished and restarts many times, this repeated initiation and extinction of the arc across the fault produces severe voltage oscillations of the order of nearly three to four times the nominal voltage. 

This repeated arcing across the fault due to the capacitances between the conductors and the ground is known as arcing grounds.  Arcing grounds can be eliminated by the use of Peterson Coils (see Article) and Arc Suppression Coils

Impact of Pollution on Insulators

Insulators on transmission lines are subject to the deposition of pollutants from the environment.  The nature of the pollutants depends on the nature of the environment. 

In Coastal areas, the pollutants is usually sodium chloride which is deposited onto the the insulator surface.  If the humidity increases or if rainfall occurs, the sodium chloride gets moist and provides a conduction path which can result in a flash over. 

In industrial areas, the chemical salts deposited depend on the nature of the industry.  The moisture absorption by these deposits depend on the relative humidity and climatic conditions.  Some of these deposits can become acidic and can corrode the insulator surface.

The deposits on the insulator surface become conductive when they become and provide a path for the leakage current across the insulator.  When the temperature rises, dry bands form on the insulator surface.(See article on dry band formation). the voltage gradient across these bands increases until arcing occurs across the bands.  These arcs can develop into a flash over.

Post Type Insulators

Post Type insulators are usually used in substations.  These insulators are constructed in the form of a cylinder.  The circular surface of the cylinder is corrugated to increase the leakage distance.  They are mounted on pedestals.  These insulators exhibit high resistance to puncture. 

Post insulators are used in areas with heavy pollution and risk of vandalism.  Slightly damaged post insulators will still have structural integrity and good excellent properties.  Hence, there is no interruption in power.  The damaged insulator can be replaced at the next opportunity. 

Long Rod Insulators

Long Rod insulators are used in applications in place of conventional string insulators.  For HV applications, conductors are usually supported on the transmission towers by means of disc insulators in the form of strings.  The string of disc insulators provides a maximum leakage distance and prevents flashovers across the insulator.

With the use of composite material for the manufacture of insulators, the string insulators are being increasingly replaced with long rod insulators.  Long rod insulators appear similar to the string insulators.  However, they are manufactured in a single piece.  The insulator consists of a "long rod" usually of Fibre reinforced Plastic (FRP) to bear the insulator load.  This rod is designed to have high tensile strength.

The housing of the insulator is usually made of silicone rubber or similar material.  The end fittings of the insulator are directly crimped on to the FRP rod.  Long rod insulators are puncture proof and have high arc resistibility. 

Long Rod insulators are lighter than strings of disc insulators of a similar voltage rating. Long Rod Insulators are used in both HVAC and HVDC applications.

Dry Band formation and Flash overs in Insulators

When an insulator gets wet a thin film of water forms on the surface and a small leakage current starts to flow.  When this film of water evaporates due to rise in atmospheric temperature a "dry band" forms on the surface.  This dry band is formed around the insulator.  When this dry band is formed, the current flow is interrupted and a voltage gradient appears across the dry band. 

This voltage gradient exerts electrostatic stress across the surface and causes further evaporation and an increase in the width of the dry band.  This increase in width of the band causes a higher voltage gradient which causes minor arcing and can lead to flash over.

Dry band formation can be prevented by designing the insulators with  semiconductor glazing the insulator surface.  and by designing the insulator to have increased leakage distance.

Polymeric Insulators - an Overview

Since the 1960s, polymeric insulators have been used in transmission equipment in addition to conventional ceramic or glass insulators.  Polymeric insulators were originally used in areas with high pollution, high risk of vandalism and in urban areas.  Today though, their use has greatly expanded and they are used at almost all voltage levels. They are also known as composite insulators. 

Some of the advantages of polymeric insulators are

• They are hydrophobic and do not allow the accumulation of water on their surface, thus preventing surface currents and flashovers. 
• Their light weight enables easy fitment with smaller cranes and increased clearance distance between conductor and the ground.
• Their higher mechanical strength enables transmission towers to be placed at longer spans. 
• They are resistant to pollution. 
• Composite insulators do not allow the accumulation of dirt.  Hence, the cleaning and maintenance costs of the insulators are reduced. 
 

The disadvantages of polymeric insulators are that damages in these insulators are difficult to detect. These insulators are also vulnerable to erosion and tracking on the surface.Their life expectancy is not predictable

Controlled discharging of Inductive HV Equipment

HV equipment need to be discharged prior to any maintenance work.  The discharge is usually done with an earthing rod after verifying that there is no voltage present on them with an non-contact voltage detector.

In the case of inductive equipment such as transformers and motors with high inductance value in their windings, a controlled discharge needs to be carried out.  Inductive components such as windings in transformers and motors have high inductance.  Sudden discharge of these windings will create a high discharge current and a rapid change in the flux which will result in a high voltage pulse (according to Lenz' Law). This can damage the winding insulation.

The controlled discharge is carried out with a discharge rod with a resistor in series.  The resistor used is a special non linear resistor which has a reverse temperature coefficient which means that as the temperature increases the resistance falls.  When the discharge is started, the resistance is high. As current flows through the resistor, the temperature of the resistor rises and its resistance falls.  Thus the current flow is increased.

The resistor ensures that the discharge is gradual.