High voltage substation arresters play a vital role protecting electrical equipment from surge-induced overvoltages in both seismically active and non-seismic zones. However, unlike other critical substation components such as transformers and circuit breakers, they are not essential to the immediate restoration of power after an earthquake.
This edited contribution to INMR by James Taylor of Hitachi Energy Sweden proposes a practical approach for defining seismic performance requirements for HV substation surge arresters in AIS applications. This approach considers that, while seismic resilience is important, the stringency of these requirements can be moderated compared to other equipment given the non-critical nature of surge arresters in post-seismic power restoration. At the same time, it ensures that their sufficient mechanical and electrical integrity remains both during and after seismic events.
Both international and country-specific standards are used for seismic qualification of HV substation equipment. IEEE 693, for example, is one most widely referenced to define seismic requirements, offering guidance on seismic qualification for all types of substation equipment, including surge arresters. However, surge arresters are generically grouped with other equipment that have higher operational criticality. While it is important that there is confidence that structural integrity of surge arresters can be maintained during earthquakes, their function is not critical to immediate restoration of power following seismic events. This allows for a more moderated, yet still effective, approach to defining their seismic performance requirements.
While surge arresters should be expected to withstand seismic loads, HV AIS surge arresters may not need to be required to uphold the same stringent seismic performance requirements as other key substation components. IEEE 693 requirements are specifically referenced, but the same principles and objectives could equally be applied to demands for qualification methodology for any seismic specification.
Key objectives are:
1. To demonstrate that the operational priority of surge arresters in post-earthquake scenarios can be set lower than that of other substation components;
2. To propose a more tailored seismic performance requirement for surge arresters that balances cost, practicality, and structural resilience;
3. To provide recommendations for practical seismic qualification methods, including appropriate testing, for surge arresters that will ensure adequate, but not unrealistically stringent, seismic performance.
Function
Surge arresters are selected primarily based on their electrical characteristics to protect against harmful overvoltages with which the electrical system is not economically designed to cope. Hence, optimization of surge arrester design, their connections to and placement in the electrical network for daily use over many years is to the clear advantage of system reliability and for users drawing power from it.
Slim, compact surge arresters placed as close as possible to the protected object (e.g. on a transformer tank) and connected by light, slack, near-vertical droppers are usually the optimal style and arrangement of choice in surge protection. However, they may be deemed too weak and flexible for seismic zones, especially if the structure supporting the surge arrester (e.g. transformer tank, bracket, pedestal, etc.) is considered to significantly amplify the ground acceleration load transferred to the arrester.
Given this, another arrester style with greater mechanical withstand strength is chosen, even though it may be less cost-effective and/or less technically optimized. It may even be necessary to physically place the surge arrester away from the transformer on a standalone purpose-designed pedestal to reduce the seismic amplification factor. This could mean diminished overvoltage protection due to separation effects, added cost and a potentially larger substation footprint.
Presuming that a surge arrester has a nominal design life of 30 years, surges may need to be handled effectively on any given day and many times over this lifetime. Conversely, a less-effective design may end up being chosen predominantly based on handling a low-likelihood significant seismic event that may not even occur within the service life of the arrester. Hazard versus risk, in combination with appropriate consideration of consequences, should ideally form part of the seismic evaluation criteria for surge arresters.
Hazard Versus Risk
A commonly used approach to define seismic hazard is to describe the probability of ground motions that are significant enough to cause major harm or damage. This is characterized by, for example, peak ground acceleration (PGA), i.e. maximum acceleration of the ground during an earthquake. Seismic risk is then the probability of occurrence (over some time period) of adverse consequences due to the seismic hazard. Overall risk is a combination of the seismic hazard occurrence and a given structure’s fragility to being damaged by the hazard.
Consequences of damage are often expressed in terms of: 1) Deaths and casualties; 2) Damage in terms of cost due to failure; and 3) Downtime measured by length of HV substation power interruption.
When evaluating the seismic hazard, an input ground motion should be selected based on level of acceptable risk. A common approach used in seismic design is to choose the ground shaking that, on average, will be reached or exceeded in 475 years. This average return period has an annual exceedance probability of 1/475 = 0.21%, and the chance the ground acceleration will be met or exceeded in an exposure period of the nominal design life of a surge arrester, i.e. 30 years, is: (1-1/475)30 = ~94% chance it will not occur, ~6% chance it will occur.
Alternatively, users may decide to take a highly conservative approach towards assigning risk, and design for earthquake accelerations that have a much longer mean recurrence interval over a longer exposure period, for example using a 2475-year return period.
It must be acknowledged that probabilistic ground motion with a given return period is not known with certainty. Return period is simply a way to express an annual and long-term average probability. Hence, there is no absolute validity for the return period defining the timing of an occurrence, nor on the other hand that the occurrence is certain to happen during the lifetime of the surge arrester. To be more pragmatic, multiple seismic hazard levels could be assigned with differing return periods, and each with corresponding alternative withstand performance expectations based on the associated risk of occurrence.
Assigned seismic hazard and risk is usually generically applied to all equipment in a specific substation. Thereafter, seismic standards judge all equipment equally vulnerable and have equal consequences for
1), 2), 3) listed above. What is typically not taken account of, is that these consequences are individual and specific to each piece of apparatus, and how possible mitigations can differ.
Applicable Seismic Standards
To properly evaluate valid seismic hazard and risk, it is preferable to use a site-specific response spectrum that accounts for soil type and other geological conditions at the location. However, for simplicity, generic design rules and guidelines are often specified, including reference to building codes.
Building codes cover the design of buildings and other structures to be used in seismic regions where risk to life and/or risk of structural damage must be reduced. General rules apply for earthquake-resistant design of buildings and elements constructed with various structural materials.
Clearly, building code and structural design criteria have no relevance to surge arresters. As stated in IEEE 693, “The mechanical properties and characteristics of insulators, which have good insulating/dielectric properties, are different from materials typically used as load-resisting structures. Codes and standards for seismic design (e.g. ACI 318, AISC 341, ASCE 7, ADM) were developed to address commonly used structural materials (e.g. concrete, steel, aluminum) and do not address the design of insulator materials.”
Application of building code methodologies to substation equipment and structures, including those building codes that have sections specifically for non-building structures, can be problematic and cannot be used to appropriately evaluate a surge arrester. Nonetheless, building codes can prove useful for providing insight into expected level of seismicity of a location and selecting appropriate apparatus qualification according to appropriate seismic guidelines.
Neither earthquake intensity nor earthquake magnitude present any information about the frequency content of the earthquake. For engineering/design purposes, other representations of earthquakes are used, e.g. recorded ground acceleration or response spectrum (see Fig. 1).
Three seismic qualification levels are specified: high, moderate, and low, which are defined by the zero-period acceleration, ZPA, (peak ground acceleration) and corresponding response spectra for respective percentage modal damping.
Performance Level qualification is a response spectrum that directly reflects the accelerations associated with meeting the seismic qualification objective at the specified qualification level. Design Level qualification uses a spectrum defined by halving the Performance Level accelerations, while requiring a safety factor of at least 2 to the assigned seismic strength of the arrester, i.e. < 50% of mechanical strength is utilized.
“Seismic qualification objective” in this context is to secure that surge arresters will not sustain damage of such a nature or to such a significant extent that they a) cease to perform their electrical function, or b) are at risk of imminent collapse at nominal electric operating conditions. While complete and total fracture may be considered significant damage, it is important to understand that yielding is permitted provided that it does not impair electrical function or pose risk of imminent collapse. Since damage can be tolerated (damage not deemed to be significant), it can be equally acceptable for equipment such as surge arresters to be repaired or replaced after an earthquake.
Considerations Specific to Surge Arresters
Surge arresters are not system critical to restore power after an earthquake. As such, alternative considerations should apply compared to more critical equipment. Otherwise, with due consideration to the expected cost and functionality specific to arresters, the requirement for disproportionate strength safety margins could result in a less than optimal design and additional project cost. Therefore, seismic impact of surge arrester performance should be assessed in the context of the following:
Surge Arrester Design
Seismic specifications typically focus on defining seismic strength of apparatus insulators based on some form of generic mechanical testing criteria considered applicable to all apparatus. These are typically not synchronized with mechanical testing according to surge arrester standards IEC 60099-4 or IEEE C62.11 used for the respective designs. Nor are they easily specified by some separate product-specific requirement.
There are 3 fundamental gapless metal-oxide surge arrester designs used in traditional HV AIS applications: a) porcelain-housed hollow-core, b) polymer-housed hollow-core, c) polymer-housed solid-core (see Fig. 2)
Below is a review of considerations regarding defining seismic strength for each design.
Porcelain-Housed
IEEE 693 Annex X expects 10 samples of porcelain-insulators to be tested to breakage to accept a 100% (safety factor 1.0) loading of the 2 standard deviation value to the arithmetic mean (MBL – 2σ) for Performance Level qualification. With a test of 3 samples, the limit is set to 80% (safety factor 1.25).
For porcelain-housed surge arrester designs mechanically tested according to surge arrester standard IEC 60099-4, 3 samples are tested for MBL and SSL. MBL is to be at least 120% of SSL and SLL is defined to be safely set at 40% of SSL. These respected loads are defined as follows:
• SLL: Specified Long-term Load
Force perpendicular to the longitudinal axis of an arrester, allowed to be continuously applied during service without causing any mechanical damage to the arrester.
• SSL: Specified Short-term Load
Greatest force perpendicular to the longitudinal axis of an arrester, allowed to be applied during service for short periods and for relatively rare events (for example, short-circuit current loads and extreme wind gusts) without causing any mechanical damage to the arrester.
• MBL: Mean Breaking Load
The average breaking load for porcelain or cast resin-housed arresters determined from tests.
For example, assume a porcelain surge arrester insulator has been mechanically tested on 3 samples, and has a verified SSL = 7.5 kNm, SLL = 3 kNm, MBL = 12 kNm and MBL-2σ = 9 kNm. According to IEEE 693, since only 3 samples were tested, 80% of 9 kNm = 7.2 kNm is the assigned limit for seismic strength. However, this value is considered unnecessarily low since it is below the guaranteed SSL withstand value, and a higher value than SSL can be permitted for extremely rare seismic loads. The value proposed in IEEE 693 is especially conservative in the case where all results are greater than MBL-2σ.
This methodology presumes that either all samples have consistent break values or that there is a scattering of results above and below a close statistical average. It is reasonable to expect that there will be a spread of break values when a large population of porcelain insulators are tested, but there is no account taken for the case where minimum value is always well above the required load. For example, assuming a certain arrester insulator style has been mechanically tested on 10 samples, and 9 out of 10 have a break load in the range 10 – 13 kNm, but one sample broke at the much higher value of 17 kNm. Further, assume the verified MBL from the samples is determined to be 12 kNm, noting that no sample broke under 10 kNm. Presuming that the seismic load required to be withstood in a specific case is 9 kNm, then the results show that all samples are above this with good margin.
However, because 1 out of the 10 samples broke at a much higher value, this disproportionately skews the standard deviation, such that MBL-2 = 8 kNm, i.e. below the required 9 kNm despite all samples actually being well above (as depicted in Fig. 3). Increasing sample size may not necessarily improve the situation since just one further sample with a much higher break load could result in the MBL-2 value being even lower; still without any results having fallen below the minimum needed.
For hollow-core porcelain insulators produced to IEC 62155, a routine bending test is to be performed on every supplied insulator. The standard requirement is for a value of 50% of withstand bending moment. However, an option to increase this to 70% of the specified mechanical failing load is also given. Such tests should be conducted in 4 mutually perpendicular directions.
This could be considered sufficient validation of the required mechanical strength in many cases in lieu of a once-off round of break tests, i.e. when the routine mechanical tested value is at least equal to the seismic loading, and the safety factor to a proven withstand test value then only needs to be >1.0. In case of need, order-bound insulators could possibly be tested at a somewhat higher load value to validate the seismic load withstand, while not being so high that they are at risk of being damaged.
For example, in the above case with MBL = 12 kNm, testing at 9 kNm would equate to a load ratio of 9/12 = 75%. This may be acceptable after agreement between porcelain manufacturer and arrester manufacturer.
Polymer-Housed
Polymer-housed surge arrester designs mechanically tested according to IEC 60099-4, have similar definitions for SLL and SSL, except that there is no fixed relationship between them (manufacturer defined). Moreover, according to this standard, SLL is to be applied in a cyclical manner over 1000 cycles to capture any potential fatigue effects due to the flexible nature of polymer-housings that could affect SSL and ultimately the break load. MBL does not strictly have to be investigated, though it could easily be included in the testing sequence if deemed advantageous to do so by the arrester manufacturer.
A. Hollow-Core Polymer-Housed (Composite Insulator)
IEEE 693 Annex X composite insulator acceptance criteria for Performance Level qualifications is that the load upon the insulator does not exceed 100% of SML. Furthermore, SML is referenced to MML by the factor 2.5 x MML. These loads are defined according to IEC 61462 as:
• SML: Specified Mechanical Load
Load specified by the manufacturer that is used in the mechanical tests.
• MML: Maximum Mechanical Load
Highest mechanical load that is expected to be applied to the hollow insulator in service and in the equipment in which it is used.
SML, in brief, is a load that the insulator can maintain for a period of 1 minute. It is a “rated” load, which is confirmed by test to be achievable, but it is a subjective rather than an objective quantity. Further, although it may place the insulator in an irreversible plastic phase, it is still below the actual failure bending load. By the nature of their production, composite insulators do not tend to have the same potentially wide statistical spread of mechanical strength as for brittle porcelain insulators. Hence only one sample is required to be tested in 3, or possibly 4, steps of increasing applied load: a) MML, b) 1.5x MML, c) 2.5x MML = SML, d) to failure (optional). Nonetheless, an arrester manufacturer may specify that several samples be tested to provide a more statistically robust result and may also set other criteria as deemed appropriate.
IEC 61462 is a generic standard for composite hollow-core insulators and not specific to surge arresters. Moreover, surge arrester standards make no reference to MML or SML as part of their mechanical testing requirements. It is, however, reasonable to presume that the requirements placed on the supplier for production of the composite insulators could be made according to IEC 61462. It is therefore justifiable to set the defining seismic strength equal to SML and to validate it by appropriate mechanical type testing.
A routine mechanical test is also to be performed on every produced composite insulator. Similarly, as discussed, this test could be considered sufficient to verify the seismic strength of the order-bound insulators against that to be applied during an earthquake, and so avoid the need for repeatedly performing a test of SML (in cases when a user may require re-validation).
B. Solid Polymer-Housed
Polymer solid-core surge arresters are addressed in a slightly different manner from the requirements of hollow-core housings, as SML is not explicitly defined for such designs. Instead, the manufacturer may define a corresponding value for SML based on appropriate mechanical testing.
The ultimate cantilever load experienced during shake-table testing should not exceed 100% of the manufacturer-defined SML for solid-core polymer arresters. Notwithstanding, it should also be considered that, even in a rare case of a surge arrester failing during seismic activity, the direct impact on the grid’s ability to recover and restore power immediately is likely minimal with this type.
A fact often overlooked in the case of solid polymer designs is that, while a physical mechanical breakdown beyond the deformation limit may affect its ultimate load carrying capacity, in contrast to hollow core designs, it need not necessarily affect its inherent electrical function. Obviously, individual designs need to be evaluated on their merits, taking account of manufacturer-assigned values.
The effect of breakage on electrical performance has been investigated on a Hitachi Energy solid-design type PEXLIM surge arrester by the following test steps and results:
• Electrical characteristics measured: Reference voltage, Partial discharge, Power losses.
• The arrester was mechanically loaded until the point of physical breakage – determined by a dip in the force vs deflection curve, and visibly observed by crack in a fiberglass loop, split in polymer housing, small gap between surge arrester module and metal base. The break occurred at a value well above the assigned seismic strength for the arrester (150%). Despite having exceeded the damage limit, the assembly remained generally intact afterwards, with only a minor remnant deflection from the vertical.
• After breakage, electrical characteristic measurements were repeated: No perceivable change in reference voltage, partial discharge or power losses were noted.
The test showed that a PEXLIM surge arrester can still function (at least in the short term) even after mechanical damage – physically intact and with electrical contact maintained, indicating that the arrester would still be capable of remaining connected to the network and provide surge overvoltage protection even when broken. Moreover, the mold-bonding production process used on PEXLIM ensures sealing along the entire length, meaning that a split in the housing would not immediately lead to moisture ingress propagating further to cause a short-circuit of the internal elements.
Inspection of the entire plant is always warranted after an earthquake regardless. Replacement of an individual arrester at a later date may be needed, but the arrester could remain in service for a period of time in the damaged state until after the crisis has passed.
Surge Arrester Mechanical Limits
As per IEC 60099-5 cl 5.2.2.7.12, SSL as a withstand value verified in bending moment tests is intended to cater for relatively rare events, such as short-circuit loads or high wind gusts. However, earthquakes are more considered to be extreme and rare. In this same clause it states “It is important to note that SSL does not cover any seismic loads, which may require much higher SSL values than usually applied for normal service conditions, and which therefore needs special consideration…”
Hence it is understood that the load limit for which seismic loads are to be evaluated is greater than the standard defined SSL. How much greater may be design-dependent, as discussed above, but the security of the results can be assured by making appropriate specification of mechanical testing relevant to surge arresters.
For example, testing SLL on a polymer-housed arrester over 1000 cycles already accounts for potential fatigue effects during normal operation, which may be considered sufficient. A shorter duration vibrational test at higher load (and frequency) could easily be adopted if it is deemed necessary to assign a “specified seismic load”. Conversely, in those cases where the seismic loading requirement is evaluated to be lower than the standard SLL or SSL, respectively, the user can have high confidence that the arrester is not at risk of being mechanically damaged and hence a safety factor of >1.0 would be deemed sufficient.
A degree of responsibility for the overall seismic security of the substation should also be taken by the system operator and not simply place all demands solely on the surge arrester manufacturer. In the event of high mechanical demands, or remnant concern that a damaged arrester could lead to consequential damage of other primary plant, then additional recommendations can always be incorporated by the user themselves in their own procedures and policies, e.g. install polymer housings in preference to heavy brittle porcelain, arresters to be shunt-connected and not used as mechanical support, pedestal redesign, etc.
Surge Arrester Housing Type
Prior to advent of polymeric housings on surge arresters in the mid 1980s, the only choice was to use porcelain housings and their shortcomings in seismic performance had to be accepted, i.e. notably heavy mass, brittle fracture mode and statistical spread in break load. Polymeric housings inherently resolve these negative issues thanks to their relative light weight, flexibility, and different production methods. It has been the conservative nature of the electrical power industry that has slowed wholesale transition to polymeric housings. However, the specific use of silicone for the housings (rather than generically “polymer”) around the world has over time negated past concerns since it has proven to be UV stable (even at high altitude), pollution resistant (even under extreme conditions) and able to handle installation under a variety of physical and environmental conditions. Silicone as the housing on surge arresters has been used for almost 40 years (60 years on other apparatus) and is well-proven. In many regions with known high seismic activity, the preference is to use only silicone (polymer) housings on all electrical substation apparatus. There should no longer be any technical justification for users to exclude use of silicone-housed surge arresters in preference to porcelain-housed and silicone housings should therefore also be the preferential design type of choice in areas with high seismicity.
Surge Arrester Connection
Surge arresters are an active protective device, which may acceptably overload as part of its normal operating duty, for example due to surge energy injection beyond its rated handling capability. To mitigate consequential effects, arresters are inherently not intended to be permanently mechanically loaded in service to any significant extent.
Irrespective of whether the phase conductor is a long horizontal dropper or a heavy bus construction, the use of an arrester to support the conductor means it is acting principally as a support insulator. Surge arresters are not intended to be used as mechanical supports and are not designed nor tested for this kind of heavy physical duty. They are typically expected to be connected by use of a light, slack, and near-vertical dropper in shunt-connection rather than having the arrester directly in series with the phase conductor. This reasoning is exemplified in the standardized short-circuit (pressure relief) type test for surge arresters wherein connections are specified to be made using a conductor that is slack, vertical, and flexible, i.e. as per the recommended installation method.
A similar configuration is proposed for “seismically suitable” connections in IEEE 1527. It has been established that flexible conductors configured with appropriate length will provide the desired flexibility and conductor movement without applying excessive force to the connected equipment terminations. Although vertical drops have not yet been specifically studied, it is nonetheless recommended that these should be designed with sufficient slack to allow for the expected relative displacement between their interconnection points. At present, conductor interaction effects need only be considered for equipment that are primarily interconnected horizontally, and so conductor loading on arresters connected by vertical droppers – as they are intended to be – could in fact be neglected altogether.
Notwithstanding that surge arresters do have rated mechanical loadings, as discussed, the actual load applied in service should always be limited by installing them using good engineering practice. When connected appropriately, normal operational (continuous) loading is negligible. Further, by making connections in the proper way, the arrester will also not be loaded significantly at the line terminal by dynamic loads from external factors, e.g. short-circuit, conductor movement, etc. Hence, large forces from conductor interaction should not need to be applied to a surge arrester even during an earthquake, and proposed concurrent line pull loads could justifiably be removed (reduced significantly); perhaps with the added requirement that the installation is made by the user with “seismically suitable” connections.
Surge Arrester Mounting,
For the sake of seismic qualification, equipment compliance depends largely on the amplification factor of the support structure on which the arresters are mounted. When the expectation is to use a generic structure factor, as much as 250% amplification is applied to the required response spectra (RRS) [1]. In the words of one seismic lab: High Performance Level ZPA = 1 g x 2.5 = 2.5 g isn’t an earthquake, it’s an “apocalypse”! Since HV AIS surge arresters typically have an eigen frequency in the region of 1 – 8 Hz (when seismic dampers are not used), this puts the greatest load on them from the peak of spectra. For High Performance Level qualification, this may result in evaluation/test at 3.24 g x 2.5 = 8.1 g, which is extreme and arguably unjustified compared to what really occurs. Further, this amplification is applied to the entire RRS, when any potential amplification would in fact occur at a single frequency and not over the whole frequency range. Additionally, the assumption is that the structure and arrester go into resonance at the exact same frequency and by the same mode, which is extremely unlikely.
A user in a seismic zone would surely be equally concerned about the seismic performance of the support structure itself, as the effect on the equipment mounted on it. It can therefore be argued that a 250% amplification of the ground acceleration is unrealistic for surge arrester support structures used in practice. Seismically suitable structures can be designed that do not amplify ground acceleration by a significant amount. From experience, the amplification factor of supports of typical switchyard equipment is known and significantly lower (often in the order of magnitude 1.0 – 1.5).
Studies have shown that simple and symmetrical equipment mounted on typical steel supports is subject to support amplification that closely resembles that of a single degree of freedom (SDOF) oscillator mounted in series with a secondary spring element. Hence, typical substation steel supports act as simple spring elements, introducing flexibility and reducing eigen frequencies. For equipment with eigen frequencies below approximately 10 Hz, which essentially applies to all main substation circuit apparatus including surge arresters, a generic support amplification factor of 1.5 will suffice; for all but perhaps the most extreme of seismically-poor designs.
Unlike other substation equipment, the mounting structure is not normally within the surge arrester manufacturer’s scope of supply. Notwithstanding that it could be, there are typically too many permutations and combinations across multiple users to make it practical or economic to do so. Hence, it is left to the supplier of the support structure to ensure that an appropriate structure is used, with the necessary structure amplification factor to uphold the designated safety factor at the arrester base.
Surge arresters can on occasion be mounted on the transformer tank to uphold the shortest possible protective distance to the transformer bushings as well as to save space in the substation. The installation arrangement may use an outrigger platform/bracket, which has been argued to potentially transfer a significantly higher seismic load onto the arresters compared with on a ground-mounted pedestal. Each case would need to be evaluated on its merits, but it has been shown possible with due engineering diligence to design seismically-stiff platforms even for such applications – which would be preferable. Otherwise, the arresters can either be fitted with seismic dampers or be mounted elsewhere – which may have other negative consequences, e.g. added cost, clearance issues, separation effects, station layout and size, etc.
Use of lightweight polymer-housed designs can, nonetheless, permit innovative alternative solutions, e.g. suspended mounting with fixing arrangements that do not transfer load into the arrester component body in any direction or from any source – the arrester could then be considered “inherently seismically acceptable” (intrinsically robust to seismic events when properly anchored).
When the surge arresters are to be mounted on user-supplied support pedestals, this may involve different pedestals for use in separate locations. Rather than the arrester manufacturer having to review and re-review the structure amplification factor for each separate project, it should be defined by the user which is their “most seismically vulnerable” structure. This means that the user is expected to have made evaluations on the entire population of their arrester structures to calculate the support pedestal loadings, and so be able to define the actual structure factor for the most seismically vulnerable case to the arrester manufacturer, and not vice-versa. Regrettably, users do not always do this, and instead prefer to define the support structure as “unknown” and so expect the maximum prescribed structure factor be applied, despite it being understood it will be much lower.
With consideration to the above, the definition of “support parameters are known” could be taken to mean any kind of traditional steel support pedestal on the ground as is commonly and historically used in substations, e.g. pipe, box, lattice construction. Surge arresters mounted on this type of support structure could then be qualified without specific and separate evaluation of the pedestal using a structure factor in the range 1.0 – 1.5. This would apply to equipment that is to be qualified by analysis as well as by testing.
Thereafter, the definition of “support parameters are not known” would be applicable to use a higher generic structure factor only for other non-traditional support structures e.g. made of material other than steel, such as concrete, or on a particularly unusual style or design of support bracket.
Surge Arrester Replacement & Spares
Since surge arresters are not crucial for immediate post-earthquake recovery, there is an opportunity to implement post-event inspection protocols. After an earthquake, surge arresters can readily and easily be inspected, fixing hardware retightened and other minor remedial work made as required or, if necessary, completely replaced quickly and with minimal downtime – surge arresters do not take a long time to install, nor do they require commissioning tests before taking into service.
By comparison with most other HV substation equipment, surge arresters are relatively inexpensive and compact. Consequently, keeping spare surge arresters on hand would not require a large capital investment nor take up significant space in storage. It is therefore reasonable to expect it to be possible to hold spare surge arresters on site, at least at key substations in seismically active regions. Hence, the most stringent of criteria for seismic qualification should not need to apply where replacement equipment is readily available and planning an outage for their installation is manageable.
Surge Arrester Seismic Qualification Method
The method to be used for seismic qualification of surge arresters is commonly defined by either system voltage (Us), arrester rated voltage (Ur) or arrester continuous operating voltage (Uc or MCOV). See Table 1 for example.
There is a significant difference in the work and cost involved between evaluating a surge arrester that is inherently acceptable (no detailed evaluation needed), through performing analyses (requiring expertise and specialized programs), to undertaking full-scale testing (time-consuming and costly at a qualified lab with shake-table facilities).
Moreover, there are noteworthy inconsistencies in the reasoning behind the methods and methodology for how to seismically qualify a surge arrester when based solely on voltage rating, including:
1. The voltage rating of a modern-day surge arrester is determined by the quantity of MO resistors inside. The mass of an arrester is largely determined by the housing style, size, and number of units, together with the type and quantity of MO resistors and metallic spacers, and so too are seismic parameters like center of gravity, stiffness, modal frequencies, etc.
Regardless of whether the defining criteria is Ur or Uc (MCOV), a surge arrester housing will not change significantly in the ultimate seismic performance because of a minor difference in kV rating. For example, an arrester with Ur 84 kV / Uc 67 kV is likely to be used on a Us 100 kV system and so is one with Ur 90 kV / Uc 72 kV. The former would be able to be qualified by analysis whereas the latter would require to be shake-table tested. This is despite that they are highly likely in the same physical housing and the difference in mass could be as little as ~0.5 kg – in fact, it could be lower for the higher Ur, depending on the mass of metallic spacers used compared with the MO resistors.
2. An arrester with Ur 90 kV / Uc 72 kV could also be used on a Us 123 kV system. Subject to design and creepage distance, this is likely assembled in a single-unit housing of relatively low mass and height. Modern-day arrester designs tested to current standards [2, 3] have high security in their mechanical load withstand, especially for single-unit designs. Even multi-unit designs – at least up to a limit of significant modal mass – can be considered to be at negligible risk of mechanical failure due to a seismic load when within the limits verified by test, e.g. not exceeding SSL or SML.
Single-unit arrester housings have become longer and can accommodate much higher ratings than in the past. In porcelain-housing, upwards of Ur 240 kV / Uc 190 kV are possible if the creepage distance demand is not high. In polymer-housings, the ratings can be even higher, and single units with upwards of Ur 264 kV / Uc 212 kV (and perhaps higher) are available. Even a multi-unit arrester today is typically much shorter and lighter than in the past and the seismic performance is easily able to be modeled correctly.
3. For specific applications there can be a divergence between “voltage rating” and “seismic size” (mass, height, etc.). For example, a neutral-point arrester protecting a transformer neutral winding may have Uc = 0 kV assigned, as it is not subjected to any continuous voltage between neutral-ground. At the same time, other demands may require this to be a relatively tall design. According to Table 1, the arrester could be defined as “inherently acceptable” since Uc < 29 kV, or else a qualification method used, e.g. based on Ur, that is different to a phase-arrester having a comparable mass, height, etc. with other requirements for its design.
This can be particularly troublesome for defining surge arresters in specific applications within
capacitor/filter banks, HVDC, FACTS, and series-compensation schemes. These do not always uphold a traditionally defined Us, Ur, or Uc, may have “non-significant” ratings assigned due to their connection into the system, or be a multi-column design of “low rating” but with significant mass.
If Table 1 were followed, reduced seismic qualification requirements could apply, despite these arresters (with non-existent/low ratings) having a greater seismic risk due to their large dimensions being more comparable with much higher rated arresters. It is equally not desirable to deem them to be “non-categorized equipment”, since this would restrict qualification methods to time history shake-table test or dynamic analysis only and could result in requiring much more arduous and costly work than a corresponding standard application type.
Alternative Seismic Qualification Method
With consideration to the above issues, alternative considerations are proposed to define an appropriate qualification method based more on a relationship to seismic size/vulnerability.
From experience, the first modal frequency for tall and slender symmetrically built equipment like a surge arrester is always dominant and contributes by far to the largest portion of the seismic loading. Any subsequent higher modes are typically of lesser interest and have minimal influence. For simplicity, surge arresters can typically be suitably evaluated for seismic performance by a static coefficient calculation that removes the need to consider the actual modal frequencies by assuming the arrester is subjected to the peak of the RRS. The static coefficient for surge arresters may be taken as 1.0 in seismic analysis.
Due to the nature of the arrester product, it has not been seen that all 3 earthquake directional components will cause maximum response at the same instant. Thus, a simple static analysis is sufficient by considering seismic loading applied solely from the first modal frequency in the separate orthogonal directions acting individually and alone without interaction. The resulting horizontal force is distributed proportional to the arrester’s total mass distribution and hence applied at the designated centre-of-gravity. Only the cantilever loading in the horizontal direction need to be considered since arresters are relatively stiff and strong in the vertical direction and vertical ground acceleration is hence of little consequence to their seismic performance.
Static analysis, especially with good input data for stiffness and damping, has been shown to mirror shake-table test results very well for a surge arrester; both in terms of eigen frequency and mechanical loading. The Force vs Deflection characteristic is available from measurements made as part of standardized mechanical bending type tests, and the result can readily be used to derive the stiffness of the arrester. A generic value of 2% modal damping is suitable to use for a conservative result.
Results from seismic test reports and case studies have consistently shown that the simple static analysis method is sufficient to adequately estimate bending moments on standard surge arresters mounted on typical steel supports. This is especially the case when the analysis assumes a peak of response spectra acceleration is applied with 2% damping, and thus Newton’s second law of motion (F = ma) can justifiably be applied to give a conservative result for a surge arrester without additional improved damping. In case an improved damping coefficient is desired/required to uphold safety factors, either naturally or with seismic dampers fitted, this can easily be determined by performing a snap-back test on the complete assembly, and then the same simple static analysis method used with the RRS adjusted for the actual damping.
The following alternative procedure is proposed for arresters that are base-mounted on a pedestal/bracket in vertical installations. Designs with parallel columns may present extra challenges and require specific testing to define the relevant criteria, although the same methodology can generally be applied. In case of angle-mounting installation, having also evaluated the risk for dislodgment of the internal elements when non-vertical, it may be possible to assign a corresponding bending moment strength reduced proportionally by the arrester’s self-weight due to the gravitational force permanently applied at the centre-of-gravity.
Step 1
Define the seismic strength of the arrester (Md) in Nm, as determined by suitable bending moment testing, routine test, manufacturer-specific conservative value).
Step 2
Calculate the seismic bending moment (Ms) in Nm required to be withstood, by the equation:
Ms= m × as × g × k1× k2× h
where,
m = mass of arrester in kg
as = seismic acceleration at arrester centre-of-gravity, from peak of required response spectra assuming 2% damping (unless verified higher) in per-unit g.
g = gravitational constant, taken to be 10 m/s2 for simplicity.
k1 = Static coefficient factor, taken to be 1.0 for a surge arrester.
k2 = Structure amplification factor, which is either known from separate analysis or assigned a realistic value (as discussed). If necessary, the arrester manufacturer shall have the right to define the required factor that the support structure manufacturer must uphold, e.g. within the range 1.0 – 1.5.
h = height to centre of gravity in m.
Note that additional terminal load or conductor interaction effects have not been included since, as discussed, they need not be considered for a surge arrester installed with “seismically suitable” connections. If nonetheless they are to be included, the relevant value of bending moment can then simply be combined algebraically to give a total value for Ms.
Step 3
Calculate the ratio Md /Ms.
If the ratio is > 1.0 for Performance Level qualification or > 2.0 for Design Level qualification, the surge arrester can be considered seismically qualified. If not, then either a dynamic analysis or shake-table test can be used as the alternative qualification method at the arrester manufacturer’s discretion; provided this gives the required positive result. Otherwise, the design is deemed inadequate for the seismic load.
As illustration:
A Hitachi Energy polymer-housed arrester solid-design type PEXLIM with Uc = 167 kV for Us = 245 kV has been successfully shake-table tested to High Performance Level qualification (1.0 g), including a generic structure amplification factor k2 = 2.5x. Excluding added terminal mass and conductor interaction effects for the sake of discussion, the maximum seismic load at the base in any direction during the Time History test was determined to be 5.52 kNm. If a more seismically suitable structure had been considered instead, e.g. k2 = 1.5x, and presuming a proportional reduction, the seismic load would have been only 3.31 kNm. The arrester design has rated mechanical strength of Md = 8 kNm, SSL = 6 kNm, SLL = 3 kNm, and additionally verified (3 samples) MBL = 10.2 kNm, MBL-2σ = 9.7 kNm.
Using the above procedure, the results of the analysis method showed that the same arrester gives the expected conservative result, i.e. higher load calculated by analysis than found in test, but nevertheless representative of actual performance. See Table 2, maximum load 6.304 kNm by analysis vs 5.52 kNm in test with k2 = 2.5x, and 3.783 kNm by analysis vs. assumed 3.31 kNm in test with k2 = 1.5x.
This shows that the proposed alternative method would have been sufficient for qualification purposes without the need to perform a shake-table test. It is also noteworthy that the result by analysis when using a more seismically suitable structure gave a loading of only 63% of the SSL type test withstand value.
This simplified procedure for evaluating seismic performance based on seismic size/vulnerability is considered to have the following advantages, along with potential disadvantages:
Advantages:
a) Does not require complicated math or specialized programs to make the evaluation. Based solely on mechanical and physical parameters that are already known, or easily obtained, and typically given on dimension drawings;
b) Extended length, creepage distance, parallel columns, heavy mass, and other physical attributes will inherently be captured, irrespective of the “electrical size”;
c) Arresters that have a low or no voltage rating will be evaluated equally to other arresters having the same physical dimensions.
Disadvantages:
a) Some designs, e.g. tall and/or heavy multi-column designs, may fall into the “test” category when they have not in the past;
b) Designs that used to be “inherently acceptable” will need an analysis to be made (albeit simple);
c) Additional mechanical tests may be needed to properly define seismic strength and damping, especially for parallel connected multi-column units.
It is proposed that this simple static method analysis would suffice for all pedestal- and bracket-mounted AIS surge arresters, irrespective of rating and physical size. Conversely, as is done today, requiring dynamic analysis or full-scale shake table testing on “too low” rated arresters is not value-adding nor practically needed.
A potential exception could be if significant uncertainty exists about the valid seismic amplification performance for applications on the upper range of EHV and UHV systems, e.g. Us 420 kV and greater. The system may be deemed more critical, the support structure of more intricate design, and the arrester may arguably be more complex due to the use of larger grading and corona rings. In such cases, verification by shake-table testing – preferably on the actual support structure to be used (or alternatively by Grouping based on valid previous testing) – may be beneficial to add further confidence about meeting the seismic qualification objective.
Conclusions
Surge arresters, while important for overvoltage protection, are not critical components integral to power system stability and restoration after an earthquake in the same way as for transformers, circuit breakers, etc. They can therefore justifiably have their seismic performance evaluated differently.
Instead of applying unnecessarily stringent generic seismic performance criteria, a more balanced approach would allow manufacturers and users to select and evaluate HV AIS surge arresters to meet reasonable seismic resilience without incurring technical and commercial disadvantages; to avoid e.g. rejecting the use of otherwise optimal standard designs, over-dimensioning non-standard types, unnecessarily performing expensive full-scale tests or time-consuming analyses, etc.
This approach includes giving due and reasonable consideration to factors applicable to surge arresters specifically with respect to:
• Seismic impact assessment, to understand risk and resultant cost vs. performance trade-off;
• Differentiated criteria and performance, as verified by relevant mechanical testing;
• Mechanical resilience of specific designs, including overload mode and consequences;
• Holistic approach, with manufacturer and user in partnership to devise a robust complete solution;
• Realistic post-earthquake expectations, especially when spares are available;
• Appropriate and relevant qualification method(s), with shake-table testing only when truly justified.
Applying a practical and cost-effective approach to defining seismic qualification requirements tailored specifically for HV AIS surge arresters will ensure they are sufficiently resilient against damage from an earthquake, while balancing performance, cost, and the functional importance of surge arresters in the substation for their primary purpose to protect against electrical surge overvoltages.
References
[1] IEEE 693, “IEEE Recommended Practice for Seismic Design of Substations”, 2018, including
Amendment 1, 2024
[2] IEC 60099-4, “Surge arresters – Part 4: Metal-oxide surge arresters without gaps for a.c. systems”,
Edition 3.0, 2014-06
[3] IEEE C62.11, “IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits (>1 kV)”, 2020
[4] IEC 62155, “Hollow pressurized and unpressurized ceramic and glass insulators for use in electrical
equipment with rated voltages greater than 1 000 V”, Edition 1.0, 2003-05
[5] IEC 61462, “Composite hollow insulators – Pressurized and unpressurized insulators for use in electrical
equipment with rated voltage greater than 1 000 V – Definitions, test methods, acceptance criteria and
design recommendations”, Edition 2.0, 2023-09
[6] IEC 60099-5, “Surge arresters – Part 5: Selection and application recommendations”, Edition 3.0,
2018-01
[7] M. Martin, J. Taylor, M. Popic, ” Optimisation of Air-Insulated Substations by consideration of surge
arrester protective characteristic”, 11th HRO CIGRÉ Session, Cavtat, November 10 – 13, 2013.
[8] IEEE 1527, “IEEE Recommended Practice for the Design of Buswork Located in Seismically Active
Areas”, 2018
[9] D. Backström, “Support amplification factor study”, Hitachi Energy, 1JNL2368271, 2025
