This second part of an edited contribution to INMR by Prof. Volker Hinrichsen of the High Voltage Laboratories at the Technische Universität Darmstadt in Germany addresses key challenges still to be faced as part of further development of gapless MO arrester technology. Part 1, which appeared last week in INMR WEEKLY TECHNICAL REVIEW, focused on energy handling capability. Here, Part 2 focuses on external grading systems that mitigate the effects of uneven electric field distribution and also reviews developments in international arrester standards.
Simulating Design of External Field Grading
Due to the impact of stray capacitances to earth, there is uneven potential distribution along AC arresters more than about 2 m high. This means that top units are exposed to higher voltage stress than are the bottom units, resulting in higher power losses and therefore higher temperatures. Voltage stress across the top units is partly reduced by the self-grading effect that becomes effective when MO resistors are operated closer to the breakdown region of their U-I-characteristic (termed above reference voltage in arrester standards), thereby becoming more conductive. But the cost is heat generation due to increased power losses. To limit temperature increase to acceptable limits, external grading rings (and sometimes internal grading capacitors as well) are typically applied to compensate for the effects of stray capacitances to earth.
Increased operating voltage stress of upper MO resistors is already considered in arrester test standards, e.g. in the specification of the “Test to verify long term stability under continuous operating voltage” (colloquially: “accelerated aging test”) in Clause 8.4 of (IEC 60099-4:2014, 2014). Moreover, Annex F of IEC 60099-4:2014 provides a “Guide for the determination of the voltage distribution along metal-oxide surge arresters”. However, for a variety of reasons, this only partly covers such problems.
First of all, only voltage increase is currently considered. But since arrester operation is ultimately also affected by increased temperatures, their equalization or reduction must be a goal in any optimization effort. Secondly, the Guide for determination of voltage distribution addresses non-linear electrical effects but not resulting thermal effects. It also provides sparse information on how to model a real, non-axisymmetric 3D arrester configuration by an equivalent 2D arrangement that can be simulated at lower cost. Moreover, while application of virtual grading rings is recommended, detailed information on their dimensioning and positioning is not provided. Finally, the impact of uneven temperature distribution along the arrester axis on its thermal stability limit is, in principle, unknown.
The arrester community has long had to deal with these issues yet still gained experience in dimensioning external grading systems of EHV arresters up to 800 kV system voltage level. But now, UHV arresters are emerging where dimensions of external grading rings have to be reduced compared to the optimum case so as to be able to dielectrically withstand the arrester’s own switching impulse protection level. A range of questions has therefore come up: to what degree can temperature imbalance caused by suboptimal grading be accepted? What are the highest acceptable temperatures on top arrester units? How is thermal stability limit affected by increased temperature?
Unfortunately, these questions have never and still cannot be answered by full size laboratory tests since injection of rated impulse energy into full-scale EHV and UHV arresters is not possible. As such, tests on thermal stability are currently performed on small thermally equivalent prorated sections and assumed to yield conservative results, i.e. that err on the side of caution. But even experimental verification of this assumption is not possible.
Now, the time has come to make increased use of simulation approaches. Today’s commercially available FEM simulation software tools offer a solution to non-linear, fully electro-thermal coupled problems. But work remains to be done. For example, 3D structures have to be converted into 2D to reasonably reduce calculation efforts. Also, complex heat transfer mechanisms, i.e. radiation, convection and conduction, must be simplified to equivalent heat transfer numbers. Correct experimental determination and theoretical modeling of the non-linear electric characteristics of MO resistors (electric field and temperature dependent conductivity and permittivity) is challenging. Even minor details of the laboratory environment cannot be overlooked in the simulation if good agreement between simulation and experimental results is to be achieved.
Two recently published collaborative Ph.D. theses, indicated in the References section at the end, have dealt with this complex matter and below is a brief summary of findings:
1. Impact of Grading System on Thermal Stability & Thermal Energy Rating
The thermal stability limit of an EHV or a UHV arrester can be experimentally estimated by slow energy injection at applied power-frequency voltage some 30% above the arrester’s continuous operating voltage. This takes time – up to 30 minutes – and does not yield the exact stability limit because the heat capacitances of all passive arrester components (flanges, internal metallic spacers, housing, etc.) are also thermally ‘charged’ during this time. But once a model has been established that gives simulation results that agree with experimental results in the case of AC heating, the real thermal stability limits under the impact of impulse energy injection, can be determined using only simulation, as would be the case under real service conditions. Fig. 1 shows such a comparison for a typical 550 kV arrester (h = 4.2 m, Uc = 300 kV, Ur = 375 kV, U10kA = 960 kV).
From Fig. 1, it can be concluded that:
1. Measurement and simulation for the AC heating case are in excellent agreement (left: comparison of the two bars for each configuration). This demonstrates the power of modern simulation approaches;
2. Spread in axial temperature distribution is wide in the ungraded case (ung.: average = 54°C, max. = 131°C) and extremely narrow in the optimally graded case (“StR4”: average = 21°C, max. = 28°C);
3. Thermal stability limits in terms of mean temperatures, averaged over the full arrester height, are virtually unaffected by the grading configuration. This means that an ungraded arrester has the same thermal stability limit in terms of average temperature as an optimally graded one, independent of the excessively high temperatures in the top units, as was also published in (Hinrichsen, Giessel, & Tuczek, Thermal Stability of HV and UHV Arresters with Reduced Grading Systems, 2015);
4. Thermal stability limits in case of impulse energy injection are (25…33)°C higher than after AC energy injection (compare left and right of Fig. 1). This demonstrates the drawback of purely experimental approaches, which are overly conservative though at least on the side of safety.
Due to resulting higher average steady state temperatures, an arrester can handle less energy if less effective grading measures are being implemented. However, simulations have shown that the difference in thermal energy handling capability between a totally ungraded and an optimally graded arrester is only 17% (see Fig. 2).
One of the lessons from these investigations using modern simulation approaches is that aiming at optimal potential grading of EHV and UHV arresters is not as important as has been assumed. Any axial temperature imbalance along the arrester axis has little impact on thermal stability limit. Rather, only average temperature along the arrester axis is important. While reduction in thermal energy rating due to non-uniform temperature distribution must certainly be regarded, there is no real risk that under-graded UHV arresters will be less reliable or suffer thermal runaway due to temperatures at their top being so high. Furthermore, use of internal grading capacitors has to be questioned since these constitute additional active elements that might suffer electrical ageing. They may help increase thermal energy handling capability but are not necessary for thermal stability. Finally, these investigations also suggest that experimental approaches in arrester standards to verify thermal stability yield results on the ‘safe’ side.
This is reassuring but may result in safety margins that are unnecessarily large. For instance, actual limits of thermal stability are typically under estimated by type test procedures. At the same time, it is difficult to challenge such established approaches that have presumably contributed to the excellent service history of arresters.
2. Optimizing External Grading & Prediction of Thermal Stability Limit
Two additional examples of the benefits of present simulation approaches are shown below: the first is with respect to automated procedures, as demonstrated in the case of grading ring optimization; the other is prediction of thermal stability limits once the electro-thermal model of an arrester has been established.
It has been demonstrated that the virtual grading electrode proposed in IEC 60099-4:2014 for 2D simulations of arresters can be improved by assuming a cone shaped screen rather than the proposed ring electrode. It was also shown that its dimensions and position can be optimized by specially developed algorithms so as to perfectly model the real 3D arrester configuration. Moreover, an automated procedure has been developed to optimize an arrester’s grading ring to ensure that axial distribution of electric field, power loss and temperature become as uniform as possible. For the 550 kV arrester example, a global optimum (i.e. the minimum of the normalized goal function in Fig. 3) could be found, in this case for a ring installation height of 4.4 m and ring diameter of 1.2 m (see Fig. 3 top). As further seen in Fig. 3 middle, such a ring might be unrealistically large. While such optimization might not actually be the most important, this example nonetheless demonstrates the potential for automating arrester optimization using simulation approaches. The result is impressive since the outcome is a highly uniform steady state temperature distribution along the arrester’s axis (see Fig. 3 bottom).
As discussed, the thermal stability limit of an arrester is not easy to assess, even if simulation approaches are applied. Each variation in a design parameter requires several simulation runs to determine the maximum amount of energy injection under which thermal runaway will not occur. A procedure has been developed (see Ref. 27 & 28) that allows, using only two short simulation runs, prediction of the thermal stability limit of an arrester configuration for which a general electro-thermal model has been established. This is subsequently optimized by varying parameters such as grading system or U-I-characteristic of the MO resistors selected. For this purpose, a cooling rate is introduced that it averaged over the arrester’s height. So long as it has positive values after an energy injection (i.e. > 0), thermal stability is achieved. Thermal stability limit is determined for the amount of energy injection (or temperature increase, respectively) that results in a cooling rate of ≤ 0.
Fig. 4 shows an example, in which the non-linearity exponent, α, of the switching impulse current region of the U-I-characteristic has been varied from 18 to 22. As seen, the α-value has a strong impact on thermal stability limit, which is found to be in a range of the temperature step (due to energy injection) from 220 K to 300 K. It is important to note that each of the dashed curves is extrapolated based on only two simulations, indicated by red markers at the beginning of the curves. The other color markers stand for full simulations that were performed only to compare the prediction with an actual full simulation result. The excellent agreement between extrapolated curves and calculated points demonstrates the effectiveness of this approach. Only two simulations have to be performed for each parameter variation.
The basic message here is that future arrester development can, should and will be increasingly based on such approaches whereby modern tools allow simulation of non-linear, fully coupled electro-thermal problems. Given that construction and operation of high-voltage test halls becomes increasingly difficult at UHV voltage levels up to 1200 kV (and one day maybe higher), this option should increasingly be applied and further developed. While some special algorithms are beyond commercially available software tools and still need to be designed, this will be worthwhile.
Developments in Arrester Standards
There has been progress in further developing international arrester standards. For example, the concept of energy handling specification and testing was introduced into IEC standards in 2014. Subsequently, all other IEC arrester standards were re-worked to be in line with these new approaches, including IEC 60099-8:2017, 60099-5:2018 and 60099-6:2019. The most recent standard was published in 2019. Another requirement in regard to new standards, namely providing requirements and test procedures for UHV arresters, has been covered by IEC 60099-4:2014.
IEC standardization is therefore able to support manufacturers as well as users in both existing and novel arrester applications. For the next edition of IEC 60099-4:2014, apart from fixing some editorial bugs, improvements have been discussed within IEC TC37 MT4 to complex items such as short-circuit testing in general or test requirements for separable and dead-front arresters. These are based on feedback from manufacturers, users and test laboratories.
The situation is similar when it comes to IEEE arrester standards. Final publication of the next revision to IEEE C62.11-2012, the main standard, took place late in 2018. At the same time, great effort has been made toward harmonizing IEC and IEEE arrester standards. The respective working groups have held joint meetings at least once a year and it is expected that the most important test requirements will be unified within the coming several years.
An even more consistent approach has been chosen for future standardization of line arresters, which are rapidly gaining importance worldwide. Today, these arresters are internationally covered by IEC 60099-4:2014 and 60099-8:2017 as well as in IEEE C62.11-2012. The plan is to develop a new IEC/IEEE dual logo standard that will be applicable to all types of line arresters, without exception i.e. distribution, station, gapless or externally gapped, AC and DC applications (IEC TC37, PT60099-11). This will be challenging since, for instance there is virtually no experience with DC applications while the approaches in Japanese standards are somewhat different from those in IEC or IEEE standards. However, line arresters were ‘invented’ in Japan and that country has the most operational experience, which must be recognized and reflected in the future standard.
Yet another challenging project for the responsible IEC Working Group is development of ‘Test rationales’, an approach adopted from the respective IEEE WG (IEEE SPDC WG 3.3.11). The idea is to assemble and publish past motivation and justifications behind existing test requirements and specifications (this has turned out to be difficult, even for experienced arrester specialists) and also to write rationales for all future new test requirements and procedures. The intention is to publish “Test rationales” as an IEC Technical Report (TR) within the 60099 series of standards.
Summary & Conclusions
Gapless MO arresters are relatively new devices in electrical power systems, first brought to the market in 1975 and standardized from around 1990 onwards. They have undergone impressive development and currently are among the most reliable types of equipment. Exceptions exist, but these often relate to special factors, such as underestimated system conditions, insufficient dimensioning, lack of expertise or simply quality problems, with mechanical sealing deficiencies being most common.
New arrester applications have become possible thanks only to this novel technology. These include UHV arresters, HVDC arresters, line arresters (with and without series gap), huge arrester banks in FACTS and, most recently, as energy absorbers in HVDC circuit breakers. But these applications require approaches in specifying and verifying energy handling capability other than just by line discharges, as was the case at the start of arrester standardization. Supported by Cigré, test standards have been improved to cover all aspects of present arrester applications, though work remains to be done. In addition, more so than for any other apparatus, test standards of IEC and IEEE are now being harmonized. Finally, as one of the most exciting developments, simulation tools have become so powerful that it has become possible to optimize EHV and UHV arrester designs with respect to external grading systems and thermal stability limits based on non-linear, fully coupled electro-thermal simulations.
This contribution has focused on aspects of energy handling and electrical ageing, where lessons still had to be learned. Related findings have now found their way into the arrester standards. Possibilities to simulate operating behavior with the help of simulation tools have opened the door to designing UHV arresters with considerably reduced need for full size testing in the laboratory. It is recommended to make increasing use of these approaches. For example, many operational aspects of UHV arresters are extremely difficult to assess experimentally simply because the required large high-voltage test halls are not easily available.
An emerging MO arrester application will be their use as the indispensable energy absorbers in HVDC circuit breakers. It has been shown above that this application is certainly feasible. But further investigation of different makes of MO resistors will be necessary to yield general recommendations and to eventually introduce these into arrester standards. Energy absorbers are typically arrester banks, often made up of large numbers of MO columns in parallel. Such configurations can also be found in EHV or UHV AC long-distance transmission systems, for which FACTS are becoming increasingly important.
Arrester banks will also be required for protection of HVDC long-distance transmission cable systems, as now being discussed in places such as Germany. Proper dimensioning of huge arrester banks will therefore become a standard design task rather than an exception, as the case today. Since in such arrester banks thousands of MO resistors are energetically stressed at the same time, risk of failure of individual MO resistors and consequently columns is increased compared to standard substation arresters. This requires not only ever better MO resistor quality (in terms of material homogeneity, reproducibility, energy handling capability and electrical degradation) but also cost-effective strategies to replace individual MO columns versus exchanging complete arrester banks once the spare columns have been used.
Awareness of increasing use of line arresters has been the reason to develop a dual logo IEC/IEEE standard for all types of line arresters. This is in underway and might soon be published.
In summary, MO surge arresters are mature devices with an excellent service record. Major challenges have been solved in recent years and only a few items remain open for future work. The major concern is to maintain a high technical and quality level. This applies not only for the manufacturers and standardization bodies but also for users who should not underestimate the expertise required for correct arrester application.
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