Wildfires continue to devastate portions of the western United States, Australia and other regions across the world. These disasters can be attributed to a variety of factors. Utility companies, in turn, are working to minimize the fire risk posed by overhead lines.
MOV, or metal-oxide varistor, surge arresters are widely applied to protect critical utility equipment and improve grid reliability. Surge arresters come in various designs to protect distribution, transmission and substation assets. The characteristics of the surge arrester are customized to meet the varying requirements of each application. IEEE and IEC standards govern the minimum requirements for all surge arresters, however other characteristics must be considered to meet Hubbell’s demanding quality standards.
There are numerous traits that ensure the quality and longevity of MOV surge arresters. Five key characteristics tend to overshadow others. These include:
A commonly overlooked consideration in arrester applications is the impact of lead length. The inductance of lead wires can produce an inductive voltage drop which in turn will lower system protection. This voltage drop only occurs if the lead carries surge current and is in parallel with the equipment being protected. This resultant voltage is added to the discharge voltage of the arrester during a surge event, thereby reducing the protective margin of the system. The lead wire’s inductance is not strongly influenced by conductor diameter, but rather of overall lead length. Therefore, care must be taken to keep the lead length to a minimum in both distribution and substation applications.
With the recent and upcoming revisions of IEEE C62.11 and IEC 60099-4 surge arrester standards, writing a technical specification for arresters can be challenging for both new and experienced engineers. Most arrester manufacturers have a dedicated team of engineers who are familiar with the evolving standards and can support the revision and/or creation of arrester specifications.
Polymer compounds suitable for electrical insulation can consist of 10 or more ingredients which can be broken down to three major categories. These include the base polymer, fillers which can make up nearly 50% of the total compound, and active additives. Compounding of an elastomer with fillers and additives to achieve the desired results for a given application is critical. The components are carefully selected to enhance field performance and ease of manufacture.
After defining the characteristics required of an ideal polymer housing material, the next step is to develop an appropriate test protocol. Good polymer compounds used for high voltage insulation should be tested for the ability to resist tracking, erosion, corona, and ultra-violet (UV) radiation exposure to ensure long term reliability. The section below provides a high-level overview of the key test procedures defined to achieve the previously mentioned characteristics. The testing regime, outlined in Table 1, allows various materials to be evaluated and led to the optimum material selection for electrical insulation applications.
It’s a commonly held belief that the single most important characteristic for insulating materials is hydrophobicity, the ability to shed water or cause water films to bead, breaking up the potential leakage current path. Because the polymer housing is the primary defense for system critical distribution equipment, there are several other important polymer characteristics worth taking into consideration.
Electric utility operating system reliability is an important factor of utility performance. As a common practice, distribution arresters are assembled with a ground lead disconnector (GLD) designed to respond to arrester fault current during a short by detonation of a cartridge inside of the disconnector housing.
There are three distribution arrester types commonly used to protect overhead distribution equipment from the damaging effects of overvoltage. IEEE C62.11 defines Normal Duty (ND) and Heavy Duty (HD) classes by their ability to withstand certain current impulse levels. The third, Heavy Duty Riser is a type, or variation, of the HD classification and utilizes a larger diameter Metal Oxide Varistor (MOV) disc.
When comparing different arrester designs, it is important to understand how the arrester was built to correctly evaluate the amount of protection it will provide. The IEEE C62.11 standard covers two types of Metal Oxide Varistor (MOV) distribution arresters that are available today, internally gapped and gapless. These arresters might look identical from the outside, but the different internal module design affects how the arrester protects voltage sensitive equipment.