Overhead Line Insulators

 

4) Shackle type insulators [8].  These are mostly applied to support line strain (tension), such as at changes of transmission line direction [6].

 

5) Post type. These may have thicker insulation and more discs than pin types and can be mounted via clamp [9] or pin method. They may be applied as a pin or strain type insulator, but rarely as a suspension type [10].   Since post-type insulators may also also act as a cantilever to support line weight, post-type insulators normally have a Maximum Design Cantilever Load (MDCL) rating [10].  6) Hewlett type. A variation of the disc type, but can take more mechanical strain due to internally insulated steel bolt interlocks holding discs together instead of cement. On the other hand, the Hewlett type has higher internal electrical stress due to its internal steel bolts. 6) Pot type [13], which are usually pin mounted and often used with telephone lines.  Telephone utilities may use various types of insulators other than the above pot types .  However, most telephone line insulator design variations appear very similar to the pot type.

1.2             Design

Overhead line insulators are designed to have both electrical insulation and mechanical strength.  Highly insulative material is used (see section 1.3) and a recurring design theme are the “watershed” fins that discourage conductive water paths during rain and provides the required electrical leakage insulation distance [10]. 

Design and manufacturing care is taken to ensure smooth electrical stress loading and mechanical integrity of line insulators:

 

1) Insulation materials may only be drilled or cored parallel sided, and may only be hot-punched at forging temperatures [14].

 

2) Sharp radii of curvature shall be avoided to reduce electrical stress [14]. In practice, this also means that porcelain insulators will be glazed and free from rough particles and unevenness [15]. Glass insulators will be free from internal bubbles [15] that allow partial discharge and put the insulator at risk of explosion and failure.

3) Dimensions such as shed and creepage distances may be adjusted for service in high pollution environments (with or without rainwashing), areas of airborne sea salts, icing [10] and bird risk areas  [16]. Extra creepage distances are used to avoid inadvertent flashover in such highly ionised atmospheres or areas with large bird sizes (e.g. Sudan, North America).

4) Dimensions of insulator couplings are material strength dependent and guidelines are specified in standards such as AS2947.3 [17] and IEC 120.

5) Voltage and waveform tests, including parameters such as water and dampness are also specified in standards such as AS2947.4 [18].

 

6) For disc-type insulators, the cement adhesion strength should be strong enough to hold the contacted disc insulator material to the cap and pin even when the exposed disc perimeter is shattered or vandalised (Figure 23, section 2.9).

  

 

1.3             Materials

Overhead line insulators are mostly made of the following materials [3]:

1) Porcelain, which is widely used for all the abovementioned overhead line insulator types.

2) Glass, which may be used for disc and pin types. It’s thermal stability is consistent up to 538 degrees C [3].

 

3) Composite synthetics, which may be a combination of fibreglass, plastic and resin. These are sometimes used for the longrod and post type insulators and have been in service for more than 25 years [10]. When modern composite synthetics are used, often the insulative core consists of glass fibers in a resin-based matrix to achieve maximum tensile strength [19].

Figure 7

Synthetic composite insulator end-fitting, silicone rubber housing and FRP rod core.

(Source: Kobayashi et al, 2000)

The housing (Figure 7) that encloses a composite synthetic also forms the water-sheds [21] and may be hydrophobic (water repellent), which helps reduce leakage current. Some housings are designed to remain hydrophobic when polluted, giving composite synthetics a distinct advantage over porcelain types [23]. A rule of thumb operating temperature range spec for housing is -50 to 50 degrees Celsius [10].

 

4) Plasticised wood, also referred to as Polymer Concrete [20] has been used for post type insulators. Polymer Concrete has demonstrated thermal stability in excess of 300 degrees C.  Since both these designs utilise organic material, there have been concerns about material lifespan and lack of UV resistance [21].

 

5) Coupling fittings for overhead line insulators (i.e. the ball-socket and clevis-tongue interlocks) are normally galvanised cast iron and forged or mild steel. Clevis and pins may be specified with a coating of hot-dipped galvanised zinc to protect the base metal against severe corrosion [25] , [15].

6) For ease of load specification identification, each insulator is marked with its specified Electromechanical Failing Load and the name or trademark of the manufacturer in conformance with IEC 60383 [15].

  

1.1             Testing

1.4.1   Porcelain and glass insulators

In countries having IEC-based test standards, two types of tests [14] are required for porcelain and glass insulators: type tests and batch tests.  For type tests, AS1154.1 requires three representative samples from each factory production specification run tested^1.  Type tests for porcelain and glass insulators are mainly voltage withstand tests [26].

For batch tests, AS1154.3 requires both voltage and mechanical loading tests, based on the amount of units per batch of customer orders.

^{Note: 1. A single factory production specification run is a factory production run using a consistent set of manufacturing specifications.}

1.4.2   Composite synthetic insulators

Due to greater manufacturing variability in electrical and mechanical properties of composite synthetic insulators and difficulty in predicting polymer insulation performance [10], two additional tests are required to ensure design viability and quality consistency: (1) design tests and (2) routine tests [19].

The design, type, batch and routine tests for composite synthetic insulators specified in AS4435.1 appear more comprehensive and stringent than the tests specified for porcelain and glass in AS1154.3.  Tests detailed in AS4435.1 are:

1)         Design tests, which include sudden release loading, thermal-mechanical loading, water immersion, mechanical loading and dye penetration tests to detect material permeability.

2)         Type tests, which include wet power testing and mechanical loading.

3)         Batch (sample) tests, which include dimension, locking system and mechanical load testing.

4)         Routine tests, which include visual inspection and mechanical testing.

The mechanical loading test consistently recurs at each stage of testing, underlining the importance of insulator mechanical integrity under both static and dynamic loading caused by wind, ice sloughing and fault current [27].   

1.4.3   Design tests
Design test standards vary from manufacturer to manufacturer.  In general, design tests will indicate specifications for holding tension¹, failing load², and nominated conductor tension³ for each insulator design.  For composite synthetic insulators, holding tensions (MDWL) are typically 80-90% of failing load (Ultimate Strength), while nominated conductor tension (SML) are usually specified at 50% of holding tension [28].

^{Notes: 1.  Also referred to as Maximum Design Withstand Load (MDWL) [10]}

                ^{2.    Also referred to as Ultimate Strength, which Is usually twice the MDWL  [10]}

^{3.    Also referred to as Specified Mechanical Load (SML) [10]}

Minimum design tests for composite synthetic insulators described in IEC 61109 clauses 5.1-5.4 are as follows [29]:

1)                 Tests on Interfaces and Connections of Metal Fittings, which include the following:

-     Dry power frequency voltage test

-     Sudden load release test

-     Thermal-mechanical test

-     Water immersion test

-     Steep-front impulse voltage test

-     Repeated dry power frequency test after the initial five tests

 

2)                 Assembled Core Load-Time Test, which consist of:

-     Determination of the average failing load of the core of the assembled

      insulator

-     Control of the slope of the strength-time curve of the insulator

 

3)                 Test of Housing: Tracking and Erosion Test

 

4)                 Tests for Core Material, which consists of a:

-     Dye penetration test

-     Water diffusion test

 

Depending on application requirements, a more rigourous set of composite synthetic insulator design test standards may be specified as follows [22]:

 

1)         Overall performance:

-     UV durability test (ASTM G53)

-     Ozone durability test (JIS K 6301)

-     Durability of end-fitting interfaces (IEC 61109)

-     Core load-time test (IEC 61109)

-     Housing tracking and erosion test (IEC 61109)

-     Core material test (IEC 61109)

-     Flammability test to (IEC 60707)

 

2)                 Electrical performance of insulator:

-     Power-frequency wet withstand voltage (IEC 60383)

-     Lightning impulse wet withstand voltage (IEC 60383)

-     Switching impulse wet withstand voltage (IEC 60383)

-     Maximum withstand voltage of pollution to (JEC 170)

-     Arc-withstand characteristics (IEC SC36B (Secretariat) 116)

-     Corona characteristics (IEC 60437)

-     TV interface test (for V string insulators only)

 

3)                 Mechanical performance of insulator:

-          Tensile breakdown strength (IEC 61109, JIS C 3801)

-          Tensile withstand load (IEC 61109)

-          Bending characteristics (JIS C 3801)

-          Bending breakdown strength (JIS C 3801)

 

The IEC 61109 durability of end-fittings test is particularly salient because end fittings are particularly prone to water ingress due to electromechanical defects.  The insulator must endure 1000kV/microsecond steep-front voltage tests in positive and negative polarity, 25 times each without puncture of the end-fitting, housing or core material.

 

1.4.4   Type tests

Minimum type tests are covered in detail in IEC 61109 clauses 6.1-6.4 are [29]:

 1)                 Dry Lightning Impulse Withstand Voltage Test

 2)                 Wet Power Frequency Test

 3)                 Wet Switching Impulse Test

 4)                 Mechanical Load-Time Test

 As in design testing, type testing may also be expanded and/or made more rigourous, depending on customer requirements. 

 

1.4.5   Tests to simulate lifespan and wearing

For synthetic composite insulators, electrical aging simulation tests may be carried out according to IEC 61109 Annex C, which involves accelerated electrical stressing under a natural environment and measuring cumulative insulator charge, leakage current, hydrophobicity and surface conditions using scanning electron microscopy (SEM) and photoelectron spectrometry (XPS) [21].

In Japan, it was found that overhead line insulator failures increased during typhoons.  Hence there have been studies on the links between increased leakage currents during typhoons and electrical aging due to “accelerated pollution” during the typhoon [21].

IEEE Std 987 provides helpful guidelines in designing tests that simulate insulator aging, including load-time tests.

 

 

1.5             Other relevant standards

Apart from the AS, JIS and IEC design, manufacturing and test standards referred to in sections 1.2-1.5, relevant BS, IEC and IEEE standards for line insulators and associated fittings commonly used are:

 

BS

BS 3288-1 :1997      -           Insulator and conductor fittings for overhead power lines.

BS 3288-2 :1990      -           Specification for a range of fittings

BS 3288-3 :1989      -           Dimension of ball and socket coupling of string insulator unit

BS 3288-4 :1989      -           Locking devices for Dimension of ball and socket coupling of string insulator

                                           unit: dimensions and test

IEC

 

IEC 60060 – 1 (1989-11)    ‑    High Voltage Test Techniques: General definitions and test requirements

 

IEC 60060 – 1 (1989-11)    ‑     High Voltage Test Techniques: Measuring system

 

IEC 60120     (1984-01)       ‑     Dimensions of ball and socket coupling string insulator units

 

IEC 60305 (1995-12)           ‑     Characteristics of string insulators of the cap and pin type

 

IEC 60372 (1984-01)           ‑     Locking devices for ball and socket couplings of string  insulator units:

                                               dimensions and tests

 

IEC 60383-1(1993-04)        ‑     Insulators for overhead lines with a nominal voltage above 1000V: Ceramic or

                                             Glass Insulator units for A.C system

 

IEC 60383-2(1993-04)        -     Insulators for overhead lines with a nominal voltage above 1000V: Insulator

                                             String and Insulator for A.C System

                                          

IEC CISPR 18-2                 ‑     Radio Interference characteristics of overhead power lines and high voltage

                                               equipment

IEEE

 

IEEE 987 (2001)                   -    IEEE Guide for Application of Composite Insulators

                                                (includes sample and routine tests for tension loading)

 

IEEE C135.61 (1997)           -      IEE Standard for the Testing of Overhead Transmission and

 Distribution Line Hardware

 (includes batch testing guidelines)

 

 

2.0       Customer requirements, selection, installation, wearing

Line insulator manufacturers supply their insulators to customers such as utilities or utility contractors based on customer and application requirements, which in good practice should exceed application requirements.  Prior to placing an order for line insulators, the customer, contractor or design consultant would specify line insulator designs based on load bearing requirement calculations, electrical withstand requirements and creepage distances (which are typically based on IEC 815 pollution indices).

Longrod and disc types insulators are widey applied as either strain (tension) type insulators or suspension type insulators (Figures 3,4) [6]. 

 

Figure 3

Line insulator assembly for strain (tension) applications.

(Source: Yusof, 2006a)

 

 

Figure 4

Line insulator assembly for suspension applications.

(Source: Yusof, 2006a)

Strain type insulator assemblies are commonly applied at long river and road crossings and changes of transmission line directions (Figure 5).

 

  

Figure 5

Line insulators taking strain (tension) at change of transmission line direction

(Source: Yusof, 2006a)

 

2.1             Load bearing requirements

To meet mechanical service requirements, line insulator applications are normally ordered and supplied based on specifications for holding tension¹, failing load² and nominated conductor tension³ [31] based on load-bearing calculations.  An example of a line insulator load bearing calculation is given Appendix A.  The transmission line designer must be aware of such load specifications, dynamic tension and safety margins to apply the insulator correctly.  In practice, overhead line system designers will limit working loads for insulators to 50% of the manufacturer’s nominated conductor tension (SML) [10]. 

Sometimes, porcelain post-type insulators tested to ANSI standards may only have a single mechanical rating given as an average of failing load (Ultimate Strength) test results.  In such circumstances, IEEE Std 987 recommends taking the MDWL as 40% of failing load [10].  It is important for the line designer or drafter to be aware of the insulator rating systems being used.

To prevent galvanic corrosion, electrical contact (mating) surfaces of the insulator are normally specified to be of similar material to the adjacent connections [15].

^{Notes: 1.  Also referred to as Maximum Design Withstand Load (MDWL) [10]}

            ^{2.    Also referred to as Ultimate Strength, which Is usually twice the MDWL [10]}

            ^{3.    Also referred to as Specified Mechanical Load (SML) [10]}

 

2.2             Electrical withstand requirements

The transmission line designer must also specify the desired electrical withstand of the insulators, including switching impulse voltage magnitudes [29].  For voltages less then 220kV lightning impulses may have more effect on voltage transients than switching, so the designer may decide that insulator lightning withstand spec is the critical spec for insulators applied at less then 220kV.

 

2.3             Creepage distance requirements

Creepage distance calculations are based on pollution indices at the area of installation.  An sample creepage distance calculation is given in Appendix A and is based on IEC 815 pollution level classifications.  Minimum distances for porcelain and composite synthetic insulators may also be specified (typically 20-25 mm/kV).

 

2.4             Grading requirements and string efficiency

The transmission line designer must be aware that voltage stress falls off linearly from the line end disc with number of insulator discs used.  There is also capacitance coupling of each disc with the pylon and neighbouring structures.  This has an effect on string efficiency, defined as {total voltage insulated} divided by {number of discs x voltage on line end unit} [24].

Grading devices (Figure 12, section 1.1, Figure 28, section 2.10) are often used to even out voltage stress along the string by spreading the voltage stress densities away from line-end units, increasing string efficiency, [24].  Grading devices also channel external flashovers away from insulator surfaces, preventing surface damage.  They are specified for insulators 230kV and above in  [10]. 

Capacitance grading may also be used to increase string efficiency but is rarely implemented because discs of different capacitance are tedious to stock and replace [32].

Typically, 132kV lines have 10-14 11kV insulator discs and 275kV lines 20-25 11kV discs, depending on grading ring design and pylon material [32].

 

2.5             Handling requirements

Synthetic composite insulators are light and can cut and scratch easily.  The following handling practice is recommended in IEEE Std 987:

 

An inspection of type test certificates and data sheets supplied with the insulators is usually done by nominated utility inspectors prior to actual installation.  On-site certificate checking prior to installation may include checks of sample and routine test certificates.

 

2.6             Installation practice

A summary of insulator and line installation is as follows.

First, electrical pylons are designed and constructed according to function: suspension, strain or dead-end.  Line insulator specification selection is also made and is thus part of the pylon’s design and function [33].  For construction efficiency, line stringers (travelers) and line insulator assemblies are mounted while the pylon is assembled on the ground [34].

However, if pylon assembly is done from the air via the “pole stack” method, line insulator mounting is only done after the entire pole stack is completed to prevent “impact loading” of the line insulators. 

IEEE Std 1025 emphasises grounding of structures under erection when erecting in the vicinity of energized lines.

After complete assembly and erection of pylons, all pylon bolts should be securely tightened.  Line insulator bolts and nuts must be torqued to design drawing specifications.

Overhead lines are then strung along travellers at 5-8 km/h, avoiding unnecessary torsion.  Cable static seating for more than 24 hours is avoided to prevent cable creepage  [34].  

Figure 6

Overhead lines strung along travellers after pole erection [38].

After cables are tensioned and sagged to design drawings, suspension insulator connection locations are then attached (“clipped”) to their appropriate locations on the lines [35].  All lines are grounded during clipping.

After clipping, line conductors are lifted, stringers (travelers) removed, and suspension clamps are placed on conductors for permanent line tension.

2.7             Voltage stress effects

For line insulators in general, changes in surface resistance due to chemical changes and variations on the surface or pollutive films covering the surface have an effect on surface resistance, leakage currents and withstand voltage of the insulator [10].

Hence voltage discharge external to the insulator may occur when the insulation material is too polluted, wet, and has a reasonably low resistance path allowing for the discharge during lightning, switching or transient overvoltages.  Every time a discharge (external flashover) occurs, the insulator is at risk of “tracking”, a phenomenon where a physical indentation or scar appears as a semiconductive “track” caused by an electrical arc over the insulator surface [22].

Over time, with more and more discharges along the surface, the track may worsen and weaken the insulator further.  Arcing horns [36] installed on line insulators may reduce the risk of surface tracking by providing a discharge path further away from the insulator material.

For the specific case of synthetic composites, studies have shown that the surfaces  synthetic composite polymer housings are relatively mobile compared to porcelain and glass, and “have much greater freedom for rearranging in the bulk or at the surface”  [10].  Polymer surfaces also have the interesting ability to interact with pollutants to reduce the conductance of the pollution layer, thereby improving insulator performance  [10].

 

2.8             Mechanical stress effects

Mechanical stresses are caused by tension, bending, compression or torsion loads, which may be static or dynamic [10].  Repeated mechanical stresses can result in a unique creep¹ phenomenon for composite synthetic insulators, where the residual strength of the composite material remains very high until the instant of failure [10]. Mechanical stress effects are minimised by stringent mechanical testing of insulators.

Long term mechanical and insulative performance of composite synthetic insulators are critically dependent on the continued protection provided by the housing [10].  Housing deterioration or aging must never result in exposure of the rod to the environment because this will rapidly change the mechanical characteristics of the rod [10].

 

2.9             Environmental effects 

Environmental effects on line insulators such as end-fitting moisture ingress, surface area contamination and extreme wind-loading (such as in typhoons) contribute to insulator failure [37], [21].  Saltwater ingress can also corrode disc insulator pin material and weaken its strain (tension) rating (Figure 7).

 

Figure 7

Disc insulator pin corroded by airborne saltwater.

 (Source: Yusof, 2006a)

 

Periodic testing of insulators using the “Hi-test insulator tester” or “Buzzer” method (section 3.1) minimises risk of insulator failure due to moisture ingress [37], [15].

Insulators installed in areas with heavy airborne pollutants such as phosphate, cement, pulp and lime processing plants have experienced external flashovers due severe insulator pollution that sometimes engulfs the entire insulator.  In South Africa, line insulator reliability was found to suffer due to bird droppings and bird electrocutions.  In South Africa, birds perched above line insulators that do not bridge an electrical circuit still get electrocuted when they excrete continuous “streamer” droppings or urine that bridge a circuit across the below insulators [39].

Periodic insulator washing (section 3.2 and Figure 9) reduces the risk of insulator failure due to surface area contamination.  Maintenance techniques to prevent bird perching are discussed in section 3.3.

Bird electrocution occurs when a bird’s wings or “other appendages” complete an electrical circuit by bridging the gap between two live wires, or a live wire and a grounded wire or structure [40].  In addition to insulator failure, line outages and endangered bird deaths, hazardous bushfires and wildfires sometimes start when the electrocuted bird catches fire and falls to the ground.  During electrocution, the insulator is shorted out, the bird is killed or severely burned, and power outages occur. 

Studies by The German Society for Nature Conservation [41] have shown that:

 

A bird-safe arcing horn has been developed in Japan by IERE [43].  In effect, the device utilises a subhorn and “mini-insulator” that prevents  bird electrocution through the main horns under normal operating conditions.  Abnormal operating currents from lightning strikes and voltage surges are passed through the mini-insulator to the main horns.  The subhorn picks up flashovers due to surface area contamination.

Although vultures in South Africa have been known to eat fibre-optic cable insulation [42], incidents of synthetic composite line insulator housing being eaten by vultures or other wildlife are rare.

Crows’ nests that contain abandoned wiring material have also been known to short out insulators when they span a circuit bridge [39].  Maintenance techniques to reduce such risks are discussed in section 3.3.

  

2.10         Vandalism

Glass and porcelain insulators are susceptible to shattering by thrown or shot projectiles (Figure 8).  This is a major reason for recent trends to replace older glass and porcelain insulators with composite synthetics.

Figure 8

A vandalised disc insulator suspension string. 

Note the cement integrity and adhesion to remaining glass despite disc destruction.

Note also the grading device rings below the suspension string to help distribute voltage stress.

(Source: Yusof, 2006a)

      1)                 Radio frequency interference (RFI)

2)                 Blinking lights

3)                 Nuisance overcurrent relay and ground fault tripping

4)                 Blown fuses

5)                 Pole top fires

Non-visible insulator failure includes non-visible moisture ingress inside an insulation string.  The moisture may get vapourised during a lightning strike or switching surge, resulting in sudden internal volume expansion and the insulator blown apart [37].

Condition monitoring of line insulators allows operation and maintenance crews to detect and pinpoint insulator failure.  This may be done in real time via leakage current monitoring, RFI troubleshooting, during periodic inspections or as a safety procedure prior to live line work [37].

 

3.1       Condition monitoring techniques

A power utility that receives customer complaints about non-visible insulator failure symptoms will usually investigate sources of RFI interference along nearby overhead line insulators.  Such investigations may utilise Radio Directional Finding (RDF) techniques.

Alternatively, or in conjunction with RDF, infrared or UV photography may be applied to further pinpoint the insulator responsible for the RFI noise (Figures 32, 33).  An advantage of UV over IR photography is the UV camera’s ability to capture electrical corona discharges on cracked or punctured insulators at up to 150m range [44].

Since non-visible insulator failure may also pose a safety threat to line crews, standard practice for condition monitoring prior to live-line work involves line testing using a the “Hi-test insulator tester” [37] or “Buzzer” [15] method.  This method has also been used to detect non-visible defects on insulators within hotstick range.

The “Hi-test insulator tester” or “Buzzer” method involves fitting a 10kVDC self-contained insulator tester at the end of a hotstick and placing the tester’s two probes such that the line insulator is in between them.  The condition of the insulator is then indicated via a LED display or audible warning buzzer [37].

 

3.2       Maintenance

One way of reducing or eliminating radio frequency interference (RFI) caused by insulator failure or defects is to de-energise the line and replace the insulator.  A temporary method for porcelain or glass insulators is to repair or patch the the insulator's arc voids using grease.  Greasing is not recommended for synthetic composite [10], because the grease may worsen tracking along the composite’s surface.

Preventive maintenance of line insulators involves ensuring they are debris, salt and pollution-free.  There have been studies and proposed techniques for assessing line insulator pollution rates by monitoring insulator leakage currents [45] but such real-time pollution monitoring techniques are uncommon. 

Scheduled washing of line insulators are a more practical and economical practice for preventive maintenance.  One method of washing is by helicopter, however, washing via live line approaches are also practiced.

IEEE Std 987 recommends consulting with insulator manufacturers prior to high pressure washing (3-7 MPa), because not all composite synthetic insulator designs can withstand such forces.

 

3.3       Replacement

Damaged and defective insulators usually require replacement.  If a porcelain or glass insulator is to be replaced with its synthetic composite equivalent, care must be taken in selection of the new insulator because insulators with similar dry arc ratings may have different electrical characteristics due to the difference in end-fitting lengths.  However, if grading rings are used, the dry arc distance is still the distance between rings [10].

Insulator replacement (Figure 9) is typically done via live-line methods (KTPower, 2006 and Smith & Mailey, 2003) to prevent grid power interruption.

        

Figure 9

Live line insulator replacement [46].

3.4       Reducing bird risk

A guide to reducing risks of bird electrocutions by making electrical pylon and design and insulator location more bird-friendly is provided in a copyrighted brochure by The German Society for Nature Conservation.

Current practice in South Africa and North America is to place safe perches and bird guard structures on pylons (Figure 10).

 

Figure 10

Application of safe bird perches and bird guards to protect line insulators [47].

To reduce the risk of birds nesting over power lines and their urine or dropping shorting out line insulators, bird nesting platforms are sometimes constructed in pylons safely below the level of the line insulators.