Surge arresters are installed on a transformer to protect it from over-voltage transients. A surge arrester is connected to each phase conductor just before it enters the transformer. The surge arrester is grounded, thereby providing a low impedance path to ground for energy from an over-voltage transient if one occurs. At normal operating voltages, the surge arrester must behave like an insulator, isolating the phase conductor from ground. These opposing characteristics are typically achieved through the use of a Varistor (MOV arresters), which has different resistances at different voltages.
There are two principal styles of surge arresters. Today, only Metal Oxide (ZnO) Varistor (MOV) arresters are applied. The older type is the silicon carbide gapped arrester and there are many still in service today. Note, however, that gapped arresters more than 25 years old are recommended for replacement because of their age; some propose an even more stringent policy that replaces silicon carbide gapped arresters after 13 years due to their propensity for moisture ingress (Dr. M Darveniza, IEEE Transaction on Power Delivery, October 1996).
Surge Arresters diagnosis
- (Watts) loss & current measurement: The losses (in Watts) and current measured in a power factor/dissipation factor test on an arrester are reliable indicators of contamination (particularly moisture ingress) or deterioration and are useful in determining physical changes in an arrester. The test is intended to assess the arrester’s insulating characteristics since it will “see” operating voltages for the majority of its service life. While the measurement does not test an arrester’s fault-to-ground directing characteristics, it has statistically proven more times true that when a surge arrester’s ability to isolate phase voltage from ground becomes compromised, that its operating characteristics have failed as well. Higher than normal losses may indicate contamination (e.g., moisture) or corroded gaps (in silicon carbide or early design MOV arresters). Lower than normal losses may indicate poor contact or open circuits between elements, and broken shunting resistors in a silicon carbide arrester, and discontinuities in the internal electrical construct of an MOV arrester.
- Visual inspection; an arrester should be examined to detect cracks in the porcelain, staining, and any other abnormal physical condition
- Infrared Test; gaining in popularity as the arresters are scanned together with the rest of the substation apparatus.