Abstract on Defects, Faults, Causes and Countermeasures of Porcelain & Glass Insulators for Overhead Lines

Abstract on Defects, Faults, Causes and Countermeasures of Porcelain & Glass insulators for Overhead Lines

Porcelain and glass disc insulators account for approximately two-thirds of the total usage in overhead transmission lines. During long-term operation, various defects and faults frequently occur, including zero-value failure and string breakage of porcelain insulators, as well as excessive self-explosion rate and corrosion of glass insulators. Once these problems occur, they will seriously threaten the safe and stable operation of power transmission lines. This paper focuses on exploring the intrinsic and external causes of the aforementioned defects and faults, and puts forward targeted countermeasures to provide technical reference for the safe operation and maintenance of overhead transmission lines.

1. Zero-value and String Breakage of Porcelain Insulators

1.1 Mechanism of Porcelain Insulator String Breakage

For porcelain insulators, if there are defects such as micro-cracks in the porcelain part at the insulator head, moisture will intrude during operation, resulting in a decrease in the insulation resistance of the insulator head and forming a zero-value insulator.

When the insulator string suffers flashover caused by lightning strike or other factors, the resistance of the insulator head is extremely low, while the resistance along the shed disc is relatively high due to the longer discharge distance. Consequently, most of the energy is released through the discharge channel at the insulator head. The heat generated by the electric arc passing through the head will burst the porcelain insulator and lead to string breakage.

1.2 Causes of Zero-value Defects

① Poor Insulator Quality

Poor assembly technology at the cemented interface and inferior porcelain material of porcelain insulators will lead to the formation of zero-value insulators.

The cemented interface of porcelain insulators consists of materials with different thermal expansion coefficients, such as porcelain, cement and steel. Normally, asphalt and other materials are coated as a buffer layer. If the buffer layer is too thin or poorly compatible with porcelain, cement and other materials, temperature changes in the external environment will cause interface cracking of the insulator.

Shrinkage cavities in the porcelain body or excessively large grain size of porcelain will induce microcracks inside the porcelain insulator during long-term operation. When the cracks expand to a certain extent, low-value and zero-value insulators will be formed.

② Repeated Lightning Strikes

When a tower is struck by lightning, the insulator may withstand impulse voltage with high steepness and amplitude. At this time, the porcelain part (especially the insulator head) bears the highest voltage, and partial discharge damage may occur at relatively weak insulation positions. After enduring multiple steep wave impacts, the damage at the head of the porcelain insulator gradually expands, eventually resulting in the formation of low-value insulators.

③ Excessive Mechanical Load

Severe ice coating and strong wind exceeding the designed wind speed impose excessive mechanical stress on insulators, which may cause cracking at the steel pin position and further produce low-value and zero-value insulators.

In the early stage, most zero-value faults of porcelain insulators were caused by inherent material defects. With the improvement of porcelain insulator manufacturing technology, such faults have decreased relatively. However, as the service life of porcelain insulators increases, zero-value defects induced by repeated lightning strikes deserve high attention.

1.3 Countermeasures

① Strengthen Zero-value Detection

Zero-value detection entails heavy workloads, and its efficiency is closely correlated with detection methods. Live-line measurement on transmission towers is difficult to implement and remains an unresolved technical challenge. Under current technical conditions, the following approaches can be adopted to improve the detection efficiency of zero-value insulators:

A. Conduct random sampling of porcelain insulators in lightning-prone areas. Select a portion of insulators and send them to the laboratory for sampling inspection. Perform insulation resistance tests under high ambient humidity, as some low-value insulators cannot be detected effectively under dry conditions. Meanwhile, carry out steep-front impulse tests to inspect damage on the head of porcelain parts.

B. Increase the applied voltage for insulation resistance testing.

② Adopt Diversion Lightning Protection Measures

Install diversion lightning protection devices such as parallel gaps and surge arresters to prevent lightning current from passing through insulators, and thereby reduce the risk of insulator damage.

③ Improve the Lightning Withstand Level of Transmission Lines

Optimize grounding configurations and strengthen insulation design to limit the amplitude of impulse voltages borne by insulators after lightning strikes.

2 String Breakage of Glass Insulators

2.1 String Breakage Mechanism

For disc suspension insulators, string breakage mainly occurs in porcelain insulators while being extremely rare in glass insulators, though a few relevant cases have been reported.

Once internal defects exist inside a glass insulator, self-explosion will occur, leaving residual insulator fragments. When flashover is triggered by lightning strikes or other factors, the insulation distance on the surface of residual fragments is extremely short. Most of the lightning energy is dissipated along the surface of the fragments; hence, string breakage of glass insulators does not generally occur.

However, when the head of a glass insulator becomes damp, its insulation resistance drops significantly. This may increase the discharge energy passing through the insulator head. If the short-circuit discharge current of the line is relatively large, the energy injected into the head may cause explosion and string breakage of the glass insulator.

2.2 Countermeasures

① When "zero-value self-explosion" occurs to glass insulators, the self-exploded insulators shall be replaced in a timely manner to reduce the risk of string dropping. ② Adopt diversion-type lightning protection measures, such as parallel gaps, new-type parallel gaps with zinc oxide valve plates, and line arresters, so that the discharge arc will not contact the insulators when short circuits are caused by lightning strikes or other reasons.

③ Improve the lightning withstand level of transmission lines and reduce the probability of insulator strings bearing short-circuit current. This is relatively easy to achieve for low-voltage grade lines, for example, reducing grounding resistance and increasing the protection angle within a certain range.

3 Excessively High Self-explosion Rate of Glass Insulators

3.1 Self-explosion Mechanism of Tempered Glass Insulators

The glass body of a glass insulator is made of tempered glass, which is characterized by compressive stress on the surface and tensile stress inside the material.

The stress inside the glass body originates from temperature variation during processing. When the glass body is heated to its softening temperature of 760～780 ℃ and then rapidly cooled, the surface layer contracts sharply due to rapid chilling, while the interior remains at a relatively high temperature and stays in an expanded state. This restricts the contraction of the surface layer and leaves compressive stress in the surface layer.

Subsequently, the internal temperature drops and begins to contract. However, the surface layer has already hardened at this time, restraining internal contraction and generating tensile stress inside. After complete cooling and full disappearance of the temperature gradient, these two types of stress are uniformly distributed within the glass body as permanent residual stress.

Once the balance between compressive stress and tensile stress in the glass body of a glass insulator is broken, cracks will rapidly initiate under stress, further causing the glass body to shatter, which is defined as self-explosion.

3.2 Causes and Characteristics of Self-explosion

The self-explosion causes of glass insulators can be divided into two categories: inherent product quality factors and external operating environment factors. In actual engineering cases, both factors usually exist simultaneously.

① Product Quality Factors

The main cause is impurity particles inside the glass body of glass insulators, among which nickel sulfide (NiS) particles are the most common.

During the melting and annealing process of the glass body, the phase transition of NiS particles is incomplete. After the insulator is put into operation, NiS undergoes slow phase transition and volume expansion, inducing internal cracks in the glass.

If the diameter of impurity particles is smaller than a certain critical value, they cannot be eliminated by thermal shock screening, resulting in an excessively high self-explosion rate of insulators in service.

When impurity particles are located in the internal tensile stress layer of the glass body, the probability of self-explosion is much higher. Glass is a typical brittle material that resists compression well but is weak against tension, so most glass fractures are triggered by tensile stress.

Characteristics

a. Self-explosions induced by internal impurity particles occur frequently within the first 3 years of operation, and then gradually decline. This is an important rule for judging the cause of self-explosion.

b. The self-explosion probability is basically the same at different positions of the insulator string.

② External Environmental Factors

The main influencing factors are pollution deposition and temperature difference variation.

Under the combined action of pollution accumulation, moisture ingress and electric field, the surface leakage current of the insulator increases sharply, forming partial dry bands. When air breakdown occurs at the dry band, the generated electric arc ablates the glass sheds. Severe ablation depth will directly lead to self-explosion.

If the insulator is struck by lightning during this process, the self-explosion probability of arc-eroded glass insulators will increase significantly.

Excessive pollution deposition is the key factor, which may be caused by excessively high salt density of contamination or excessive metal powder particles in pollutants.

Characteristics

a. Self-explosion is not obvious in the early years of operation, but occurs intensively after several years of service, usually caused by drastic changes in local pollution sources leading to heavy pollution accumulation.

b. The self-explosion probability at the high-voltage end and low-voltage end of the insulator string is higher than that in the middle section. The electric field intensity is stronger at both ends; under heavy pollution, partial creeping discharge firstly occurs near the steel pin position.

c. For non-self-exploded insulators on the same tower, damage can be found at the steel pin, and microcracks exist inside the shed surface. This is caused by local electric arcs induced by heavy pollution, which damage the glass body around the steel pin.

3.3 Residual Hammer Analysis

After the self-explosion of tempered glass insulators, the shed glass fragments fall off and form a residual hammer. The morphological characteristics of the glass on the residual hammer can provide a basis for analyzing the cause of self-explosion.

Types of glass morphology of residual hammers:

① Radial Pattern

Self-explosion caused by a single defect can be traced backward along cracks to locate the initiation point.

When the glass debris on the residual hammer presents a radial shape, the crack initiation point, namely the self-explosion starting position, is located at the head of the glass body. Self-explosion under such circumstances is caused by inherent quality problems of the glass body, such as defects in batching, melting and other production processes.

② Fish-scale Pattern

If the glass fragments on the residual hammer present a fish-scale shape, and the self-explosion initiation point is located near the bottom of the glass body close to the metal cap, there are two possible causes for such self-explosion. It may result from inherent defects of the product, or glass fracture induced by external force.

The external force can be either mechanical stress or electrical stress, such as continuous electric spark impact, power-frequency large current, and glass damage caused by uneven leakage current.

③ Mixed Pattern

If both fish-scale and radial shapes appear simultaneously on the glass fragments of the residual hammer, the self-explosion initiation point is located on the shed of the glass body. In this case, the self-explosion may be caused by both internal quality factors and external environmental factors.

3.4 Countermeasures

① Grid access quality control

Adopt sampling inspection on mechanical failure performance and steep impulse withstand performance to control the quality of glass insulators put into power grid operation.

② Application of composite insulators in heavy pollution areas

For sections suffering concentrated self-explosion caused by severe pollution deposition, replace glass insulators with composite insulators.

③ Strengthen patrol and inspection

Carry out special patrol inspection on transmission lines in a timely manner after severe weather such as lightning strikes.

④ Standardize transportation protection

Protective measures shall be adopted during the transportation of tempered glass insulators for infrastructure construction and emergency maintenance, so as to avoid mechanical damage.

At present, major domestic manufacturers have achieved sound quality control for glass insulators. The previous requirement of storing glass insulators for half a year before putting into service is no longer necessary in engineering practice.

4 Hardware Corrosion of Porcelain and Glass Insulators

4.1 Test and Judgment

(1) Hardware Corrosion of DC Insulators

Hardware corrosion including steel cap and steel pin corrosion has occurred on disc suspension insulators used in domestic UHV DC and ±500 kV DC transmission lines.

Corrosion of steel caps is mainly concentrated at the cap mouth, while corrosion of steel pins mostly occurs at the contact position between steel pins and cement.

Corrosion products of steel caps often adhere to the insulator sheds along with rainwater, increasing the equivalent salt deposit density on the shed surface and bringing the risk of pollution flashover.

Steel pin corrosion will reduce the diameter of steel pins and lead to a decline in mechanical failure load. In addition, the corroded steel pins inside the cemented assembly will expand in volume, resulting in cementation failure.

Corrosion of steel pins and steel caps of DC insulators originates from anodic corrosion caused by electrolytic corrosion. Under the electromotive force of the DC power supply, the metal electrode connected to the positive terminal loses electrons, detaches from the material surface in the form of cations, and consequently forms anodic corrosion.

When the insulator surface is wet, the steel pin and steel cap act as a pair of electrodes, and the water film on the shed surface serves as the electrolyte. The flowing leakage current continuously sustains the electrolytic process.

When the conductor is of positive polarity, the potential on the steel pin side is relatively high, and corrosion occurs at the junction between the steel pin and the cement surface, namely at the zinc sleeve, which is equivalent to the contact position between the positive electrode and the electrolyte.

When the conductor is of negative polarity, the potential on the metal cap side is relatively high, and corrosion occurs at the mouth of the cap, which corresponds to the contact position between the positive electrode and the electrolyte.

Steel pins of DC insulators are fitted with zinc sleeves, and steel caps are equipped with zinc rings. Since zinc is more chemically active than steel, the zinc sleeves and zinc rings will be corroded preferentially. Once the zinc sleeves or zinc rings are missing, or completely consumed on one side, the steel pins and steel caps will suffer rapid corrosion.

(2) Hardware Corrosion of AC Insulators

The electrolytic corrosion effect on the surface of insulators for AC lines is relatively weak. Rusting of steel pins and steel caps is generally caused by overall oxidation. After the surface galvanized layer is depleted during long-term operation, the internal steel substrate begins to corrode, usually presenting overall discoloration of steel pins and steel caps.

Corrosion of steel caps will affect the contamination degree on the shed surface, while corrosion of steel pins will degrade mechanical strength. The hazard caused by steel pin corrosion is more severe.

4.2 Countermeasures

Maintain an obvious temperature difference. For DC insulators, it shall be ensured that the steel pins are equipped with zinc sleeves and the steel caps are fitted with zinc rings.

In the early stage, no zinc rings were installed on the steel caps of DC insulators. Corrosion can be prevented by retrofitting zinc rings. According to the above analysis of corrosion locations, zinc rings shall be mounted at the cap mouth where the steel cap contacts the insulating shed.

The national standard GB/T 19443—2004 stipulates that zinc sleeves shall be manufactured with zinc of a purity of no less than 99.8%. The fusion area shall be not less than 80% of the connecting section of the cap ring; the weight of the zinc sleeve shall be at least 5 g, and the exposed dimension shall be approximately 50% of the total length.

There is currently no issued national standard for zinc rings. However, relevant provisions have been specified in the technical specifications for ±1100 kV UHV DC insulators, which generally require a maximum thickness of no less than 6 mm and a mass of at least 150 g.

For AC insulators, if steel cap corrosion and damage to the galvanized layer are detected, the steel pins shall be inspected accordingly. Where steel pin corrosion has occurred, the insulator shall be replaced as soon as possible. If only the steel cap is corroded, the insulator may remain in service, but enhanced monitoring shall be implemented to prevent surface pollution flashover.

5 Conclusions

① The key cause of string breakage of porcelain insulators is the occurrence of zero-value defects. The main reasons for low and zero-value faults of porcelain insulators include poor product quality, repeated lightning overvoltage impacts, and excessive external mechanical stress.

② Targeted countermeasures for low and zero-value problems of porcelain insulators include strengthening low/zero-value detection, adopting diversion lightning protection measures, and improving the lightning withstand level of transmission lines.

③ Self-explosion of glass insulators is caused by the breakdown of the balance between compressive stress and tensile stress inside the glass body. Residual impurity particles inside the glass and severe external contamination are the common causes of excessively high self-explosion rate.

④ Self-explosions induced by internal impurity particles occur frequently within the first three years of operation and then gradually decline, with an equal self-explosion probability at different positions of the insulator string. If the glass fragments of the residual hammer present a radial shape, the self-explosion can be determined to be caused by internal impurities. For self-explosions triggered by severe contamination, the probability at the high-voltage and low-voltage ends of the string is higher than that in the middle part, and the occurrence of self-explosion is correlated with changes in pollution sources.

⑤ Under extreme conditions, self-explosion of glass insulators can also lead to string breakage. Self-exploded insulators in lightning-prone areas shall be replaced as soon as possible.

⑥ Corrosion of steel pins and steel caps of DC insulators stems from anodic corrosion induced by electrolytic corrosion, and the wet film on the shed surface acts as the electrolyte. Steel cap corrosion is concentrated at the cap mouth, while steel pin corrosion mainly occurs at the contact interface between steel pin and cement. Rusting of steel pins and steel caps of AC insulators is generally caused by oxidation after long-term operation.

⑦ DC insulators shall be equipped with zinc sleeves on steel pins and zinc rings on steel caps. For AC insulators, steel pins shall be replaced promptly once corrosion occurs.