Check the failure methods of switching power supply varistor
Zinc oxide varistor is a kind of polycrystalline semiconductor ceramic element made by typical electronic ceramic process with zinc oxide as the main body, adding a variety of metal oxides. Due to the common grain boundary structure, under certain electric field, the grain boundary conduction changes from thermal electron emission conduction to electron tunnel conduction instantaneously, and the resistance value decreases sharply with the increase of voltage, which has excellent nonlinear voltammetry characteristics. Then, when there is an overvoltage, the grain boundary electron tunnel effect inhibits the increase of the peak value of the overvoltage, absorbs part of the overvoltage energy, and thus plays a protective role to the line or equipment. However, regardless of whether the varistor is used in power lines or electronic lines, if various types of overvoltage frequently appear, the varistor will frequently act to suppress the overvoltage amplitude and absorb the release of surge energy, and maintain electrical equipment and components, which will inevitably lead to deterioration or even failure of the varistor.
There are three ways to fail a varistor:
(1) deterioration, manifested as an increase in leakage current and a significant decrease in pressure-sensitive voltage until it reaches zero;
(2) Burst, if the surge energy caused by the overvoltage is too large, exceeding the ultimate bearing capacity of the selected varistor, the varistor will break out the ceramic phenomenon when suppressing the overvoltage;
(3) perforation, if the overvoltage peak is particularly high, resulting in instantaneous electrical breakdown of the varistor ceramic, which is manifested as perforation.
Among them, under the premise of hierarchical lightning protection maintenance, most of the failure forms of varistor are manifested as deterioration and perforation (that is, short circuit), therefore, when using varistor, it is necessary to series a suitable circuit breaker or fuse to avoid circuit short circuit accidents.
At present, the international popular switching power supply overvoltage maintenance device is the varistor and current limiting, overcurrent and deterioration alarm device organic combination, it in addition to the overvoltage maintenance function, but also to avoid its own deterioration, resulting in circuit short circuit function.
Lightning is a kind of strong electromagnetic wave, which can induce transient overvoltage in the current carrying conductor. In addition to protecting against direct lightning, it is more important to protect against lightning induced overvoltage. In fact, the damage of lightning to electrical equipment mainly comes from induced overvoltage. Therefore, the protection of induced overvoltage has become a research topic. For example, national standards and IEC standards have been published for lightning protection inside buildings and electrical equipment, i.e. inductive overvoltage protection. Generally, lightning waves are induced into lines and electrical equipment through power cables, signal transmission lines, and spatial alternating electromagnetic fields. The maintenance of lines and electrical equipment should be carried out from three aspects: lightning protection of the power system, lightning protection of the signal system, and space shielding.
