Metal Oxide Technology
Whether we talk about Zinc Oxide (ZnO) Blocks, Metal Oxide Varistors (MOV), or Metal Oxide (MO) resistors, we are discussing the backbone of the surge arresters manufactured today for the electrical industry. This MOV technology became prevalent in the 1980s and is now completely dominant because it is unique in protecting electrical networks against transient surges. Surge arresters are primarily composed of stacked MO Resistors; the rest is simply the structure to provide mechanical strength, electrical insulation, and a sealed system to prevent moisture penetration.
Through the historical development, distinct eras reflect evolving challenges and innovations. Pre-1930s solutions relied on rudimentary spark-gap arresters. Post-WWII, Silicon Carbide (SiC) arresters gained favor but proved insufficient for addressing complex issues like switching surges and AC follow current. The rise of advanced power systems in the 1970s led to the adoption of MO surge arresters. The Japanese industry pioneered this technology, overcoming initial limitations related to lifespan and power loss.
Today, MO arresters are industry standard, highlighting the continuous evolution in this specialized field.
Credit Photo: TGE, China
Credit Photo: Arresterworks, USA
Fundamentals of MO Resistor Technology
In medium-voltage and high-voltage electrical grids, surge arresters equipped with MO Resistors have become the international standard for safeguarding against overvoltages. MO Resistors function on the principle of a strongly non-linear voltage-current (V-I) characteristic, which enables them to absorb and dissipate excessive voltage spikes. This non-linear characteristic is crucial as it allows MO Resistors to accommodate a wide application range without the need for additional components like serial spark gaps.
Below diagram was taken from "Experimental Investigations of the Multiple Impulse Energy Handling Capability of Metal-Oxide Varistors for Applications in Electrical Power Engineering - Dipl.-Ing. Maximilian Nikolaus Tuczek"
Assessing the Handling Capabilities and Performance Intricacies of MO Resistors
Overvoltages can be induced by various factors, including switching operations, operational errors, and atmospheric influences like lightning strikes. MO Resistors are designed to repeatedly handle both single and multiple overvoltages. However, their handling capabilities are influenced by several variables. One critical factor is heat generation during the discharge process, which can impair the MO Resistors’ subsequent performance until dissipated.
Moreover, the non-linear nature of MOVs, while beneficial, has drawbacks. Specifically, uneven distribution of electric current can occur inside the MOV when subjected to certain energy or charge injections, thereby diminishing its handling capability. Additionally, the V-I characteristic of the MOV is temperature-dependent and can be altered by microscopic changes in the distribution of charge carriers, impacting its performance even before any macroscopic mechanical defects appear.
Understanding the conditions under which various electrical stresses cause reversible or irreversible changes in MOVs is vital. This knowledge informs not only the design and operational limits of surge arresters but also the quality control processes, such as end-of-line destructive tests aimed at identifying defective MOVs without damaging the functional ones.
Overall, while MOVs are instrumental in surge protection, the intricacies of their behavior are subject to ongoing research and analysis.
A high-magnification image, captured at 5,000x using a Scanning Electron Microscope (SEM), reveals the intricate microstructure comprising the core of a standard MO Resistor
Factors in Performance of Metal Oxide Varistors – INMR Contribution – Jonathan Woodworth, Arresterworks
Manufacturing Process of MO Resistors
The primary constituent of MO Resistors is zinc oxide (ZnO), making up roughly 90% of the total mass. The residual 10% comprises oxide additives, which serve multiple functions: they fine-tune the non-linear V-I characteristics, control grain growth, maintain the electrical stability of the MO Resistors, and facilitate the manufacturing process.
The manufacturing process can be described in a seven-step process, geared towards achieving precise electrical characteristics and high-quality standards:
Weighing and Mixing of Additives: The initial step involves weighing and mixing these additives according to the manufacturer's formula. To achieve homogeneous mixing, the additives are sometimes ground and often mixed wetly with solvents and binders.
Dehumidification / Spray Drying: Post-mixing, the mass is dehumidified to reduce liquid content, commonly through spray drying, preparing it for the subsequent stages.
Dry Pressing: The dehumidified powder is then pressed into a "green block", reaching 50-60% of its final density. This is where the block takes shape.
Pyrolyzing and Sintering: Organic solvents and binders are removed through pyrolyzing, followed by sintering in air at temperatures that can peak up to 1450°C. This forms the grains and grain boundaries, resulting in grain sizes of 10-20µm, especially significant for high-voltage applications.
Surface Preparation and Electrode Application: After cooling, the front surfaces are smoothed and metal electrodes are applied. Aluminum is often preferred over silver for this purpose. The aluminum layer typically has a thickness of about 0.1 mm.
Passivation: A passivation layer, generally made from glasses, is applied to the lateral surface of the MOV to protect it against environmental factors and to improve flashover behavior. The layer is sensitive to temperature effects and can sometimes include lead to achieve a lower melting point, although this is less environmentally friendly.
Quality Control and Testing: The final step involves rigorous quality checks. Electrical characteristics such as AC/DC reference voltages and residual voltage are measured. Long-duration current impulses and high-current impulses are applied to test the quality defined by ceramic homogeneity. Defective MOVs are expected to fail these tests, often by mechanical cracking due to energy/charge injections.
Each step in this complex manufacturing process is meticulously developed to ensure that the MO Resistors meet stringent electrical and quality standards.
"Green Block" before sintering process, photo TGE China
Passivation/Glass Coating, photo TGE China
Electrode Application/Aluminum Spraying, photo TGE China