Overvoltages in electrical supply networks result from the effects of lightning strokes and switching actions and cannot be avoided. These endanger electrical equipment and, for economic reasons, it is impossible to design voltage withstand capability to cover all such eventualities. This fact applies to all power network systems.
Until the mid-1980s, so-called ‘conventional’ surge arresters consisted of a series connection of SiC resistor elements (i.e. non-linear resistors) and spark gaps, placed inside a porcelain housing. But since design of power system insulation depends largely on the protective characteristics of arresters and with power grids expanding to higher transmission voltages, these arresters were no longer always able to meet the new requirements. For example, conventional gapped type arresters sometimes failed due to natural phenomena, such as multiple lightning strikes or from pollution deposited on their housings. Utilities needed high performance surge arresters to be used for the next-generation UHV power transmission systems and also compact high performance and high reliability arresters for such applications as gas-insulated switchgear.
Panasonic Corporation in Japan first discovered ZnO varistor as a surge absorber for electronic devices at low voltage and defined the basic principle. Meidensha Corporation then developed the gapless surge arrester for power systems based on Panasonic’s patent.
MOSA soon became the first gapless surge arrester that could meet all of the strict power system requirements of utilities and conventional gapped type surge arresters more or less disappeared, except in some special applications. MOSAs have since contributed to increased network reliability against multi-lightning strikes and pollution-derived problems. This also initiated birth of more economical designs for power systems and power equipment.
CIGRE WG A3.39
TB 060 (1991) described the effects on gapless MOSAs from various electrical stresses encountered in 3-phase AC systems. The main aim of this first TB was to understand the relationship between system parameters and a MOSA as well as the extent to which system parameters affect arrester performance and how the arrester affects system performance. The first part described the general properties of MOSAs, whose microstructure consists of a mixture of ZnO grains with granular layers of additives. The combination is pressed into a disc shape that has low resistivity thus making it more conductive. Choice of material by the arrester manufacturer is important since it will have a ‘knock-on’ effect on the MOSA’s power dissipation.
The second part of the Technical Brochure described the performance of MOSA under operating voltage. The design of the surge arrester has to be thermally stable and the current through the MO varistor block at the operating voltage has to stay well within the voltage-current characteristics.
The TB’s third part dealt with temporary overvoltages and their stresses on MOSAs. For example, a temporary overvoltage (TOV) is an oscillatory phase-to-ground or phase-to-phase condition that is of relatively long duration and is undamped or only weakly damped. TOVs are one of the most crucial stresses to an MOSA and are detrimental for its layout. In most cases, although TOVs do not cause conventional arresters to spark-over, they may result in MOSAs conducting sufficient current to cause considerable heating of their ZnO resistor block.
The fourth part of the TB described the stresses in metal oxide surge arresters due to temporary harmonic overvoltages. The main objective was to identify the system and transformer parameters affecting the prospective overvoltages, and the energy dissipated in the MOSA during transformer saturation effects caused by transformer energization and fault clearing at transformer terminals. Results showed that surge arresters applied in networks exhibiting parallel resonances at low harmonic frequencies can be subjected to severe energy stress.
Performance of MOSAs under different electrical stresses in AC systems was discussed in the fifth section. This included protection performance at switching (slow-transients) overvoltages and lightning (fast-transients) overvoltages. Performance of a MOSA can be described by its voltage-current characteristics.
The last part of the TB dealt with selection of metal oxide arrester characteristics using the standards. Selecting a surge arrester for a specific application is a compromise between its prospective levels, TOV capability and energy capability. An arrester with a higher energy capability reduces risk of failure but usually means increased costs.
Attend the 2022 INMR WORLD CONGRESS in Berlin for a lecture by internationally-known arrester expert, Dr. Robert LeRoux. He has served as Convenor of a CIGRE Working Group that evaluated different stresses on MOSAs and also reviewed relevant test procedures, including investigation into the state of present designs. His presentation will review development of modern MOSAs and describe their interaction with system conditions and ambient stresses. He will also explain the basics of the metal oxide material as well as of the different arrester designs on the market.