While external design is essential to how reliably a surge arrester performs in service, its electrical characteristics depend largely on the properties of the internal metal oxide resistors. These ceramic disks are sophisticated electronic devices manufactured using complex technologies and, despite the important role they play, their key properties are not always well understood by those who specify arresters.
Yet, much of the performance of an arrester is dictated by the disks. If they fail, so too will the arrester.
The following article from a past contribution by industry expert, Roger Perkins, reviews some of the main elements when it comes to the design and production of MOV disks.
MOV disks (the common terminology for zinc oxide varistors) were invented during the late 1960s by engineers in Japan. One can say that their development was mostly fortuitous since they were the unintended by-product of an attempt to create semiconductor metal surface barriers using polycrystalline zinc oxide and silver electrodes.
The disks were later adapted for use in HV surge arresters by several companies, notably General Electric, which conducted some of the landmark research to understand their behaviour.
The ’varistor effect’ is actually not a bulk effect but rather a grain boundary phenomenon associated with polycrystalline, semi-conducting ceramic materials. While many models have been developed to explain it, perhaps the most successful has been the Double Schottky Barrier (DSB) model. The huge influence on current flow of very small changes in voltage – the inverse of which represents the primary characteristic of an arrester – is due to a process called impact ionization
Optimization of Composition
The modern varistor block contains predominantly ZnO but also between a few percent and a few parts per million of at least 10 additional elements, each of which has been determined through experience to contribute to performance. Some of these dopants, which include elements such as bismuth and manganese, affect discharge voltage while others play a role in power loss, ageing behavior, energy absorption and so on.
For the development engineer in this field, the key challenge is to determine the optimal number, type and composition of these various elements. The task is not easy since the effect of one is not independent of the others and significant, often non-linear, interactions have to be considered. Moreover, there is not just a single varistor property to optimize but typically several, each of which will influence arrester performance.
Statistical techniques to cope with this problem have been developed and these enable production engineers to come up with an optimal formulation for their particular material system as well as the ideal related manufacturing process. Nonetheless, it is important to note that not all MOV discs offered in the marketplace and used in the production of surge arresters have the same optimal properties as well as varistor characteristics. Some of the reasons for this are discussed below.
Role of Microstructure
Optimal formulation does not in itself guarantee a good MOV disk. While formulation might assure intrinsically good electrical properties, there are other performance characteristics that depend more on production process than on material composition. This is because manufacturing involves ceramic technology and the essence of a ceramic material lies in its microstructure. The key properties of all ceramics – and this applies to MOV blocks – are determined by their microstructure.
Fig. 1 illustrates a typical varistor microstructure. To the untrained eye, it may not appear homogeneous, especially if one considers that: 1) grain boundaries are so important, 2) that the content of critical elements is measured in parts per million and 3) that current densities can exceed 90A/mm2 in arrester applications. Nevertheless, this device offers exceptional performance in spite of these constraints.
The area of the microstructure shown in Fig. 1 is only 0.02 mm2 and contains only 50 grains of ZnO. A key concern is that this same uniformity must persist over a volume of perhaps 105 mm3 as well as the huge quantity (300×106) of grains one finds in a typical MOV disk.
Fig. 2 shows a problem that can occur if the microstructure of a disc is not well-designed and controlled and in this case the defect resulted in failure under severe operating conditions. Ceramics tend to exhibit brittle fracture so that, if localized stress exceeds tensile strength, a crack will be initiated and expand catastrophically. Localized stress is influenced by defects, which, depending on their size and geometry, can magnify the impact of an externally applied stress.
For example, a high current discharge through an arrester (such as from a 100 kA lightning impulse) subjects the MOV disk to heating rates of as high as 2.5×107 °C per second. Resulting thermal expansion generates inertial stress waves within the material whose magntiude depends on the MOV’s physical dimensions as well as the rate of heating. Longer disks generate higher internal tensile stresses than shorter discs and, should the material contain any microstructural defects (e.g. of the type in Fig. 2), these stresses will lead to fracture. For a symmetrically constrained disk, reflections of the stress wave will maximize tensile stress at the center and a characteristic mid-plane crack will form. Clearly, the manufacturing process must avoid such defects if an MOV disk is to successfully withstand current impulses of this magnitude.
In addition to the above mechanical effects, a nonuniform current distribution through the MOV disk can lead to electrical failure. The current path is usually not uniform due to statistical variations in grain boundary properties coupled with the strongly non-linear conduction mechanism. Should the microstructure not be highly homogeneous, this effect is magnified and current paths can be formed which, due to the positive temperature coefficient of semiconducting materials, can lead to thermal instability and failure.
Microstructural homogeneity is therefore essential and the manufacturing process must aim to always achieve it.