Energy Considerations of SVU
Shielded Systems Below 230 kV
For shielded systems below 230 kV that are not equipped with large capacitor banks, the energy handling requirements from a lightning surge typically exceed switching surge requirements. Distribution class arresters are therefore commonly used for this application.
Shielded Systems Above 230 kV
In the case of this type of system, switching surge energy handling capability needs to be considered. For longer lines, switching surge capability can exceed lightning charge transfer requirements. A basic system study or the rule of thumb equations from IEEE C62.11 can be used to make the necessary determinations. It is even possible that station class disks may need to be used for such applications.
For distribution systems that are typically unshielded, a standard distribution arrester will suffice for the SVU of an EGLA. For higher voltage systems where current sharing between arresters is an option, higher charge transfer disks may need to be used.
SVU Housing & Leakage Distance
Because the SVU does not have a voltage stress along its surface, it does not require the long-term power frequency withstand capability of a typical arrester. Normally, arresters have some voltage stress along the surface over their full service life. This stress demands that the arrester have a longer surface distance than would typically be needed for the internal components. The only time the SVU experiences a voltage stress is during a surge clamping event. At other times, the voltage difference between the two ends of the SVU is virtually zero.
Because there is no voltage stress along the length of the SVU over its lifetime, leakage distance can in most cases be equal to strike distance. For contaminated environments, leakage distance may need to be somewhat longer. Reduced material requirement is in fact an area of opportunity to make such arresters more sustainable from an environmental viewpoint.
Besides providing leakage distance, the housing is also the major environmental seal of the SVU. Moisture ingress in the presence of voltage stress is a major challenge for the housing of any SVU. Even though the AC stress is minimal, it is still critical that the SVU remain well sealed.
Guarding Against Wildlife Intrusion
For arresters applied above 46 kV, the SVU becomes long enough so as not to have any serious concerns about animals bridging the ground to phase electrodes. However, wildlife can still get across the gap. In such cases, the resulting leakage current will likely kill the creature but not cause an outage. It is unlikely the arrester will fail in such instances since the current is limited by the varistors.
EGLA Arrester Voltage Rating Considerations
The voltage rating of surge arresters can at times be confusing because the standards and data sheets for nongapped arresters routinely discuss two ratings: MCOV and Duty Cycle Rating. The only rating that really matters to users, however, is the MCOV, or Uc in the IEC area.
In the case of EGLAs, the voltage rating that really matters is also MCOV. This is the maximum voltage the arrester can withstand over its service life and still withstand any TOV events as these arise. This level is claimed by the manufacturer and has been verified as part of the operating duty test. An 8.4 kV MCOV of an EGLA is the equivalent of an 8.4 kV MCOV non-gapped arrester. Similarly, the 70 kV MCOV rating of an EGLA is exactly the same as the 70 kV MCOV rating of a non-gapped arrester.
The benefits of an EGLA are numerous. Because of the gap, losses are zero and there is no chance of a long-term fault due solely to the arrester. Construction is also more sustainable due to less material requirements. Moreover, the life of such arresters is virtually infinite because of the minimal AC voltage stress placed on the SVU. A more subtle benefit is that less rubber can be used on the housing because leakage distance is optional. Also, with no voltage applied to the varistor after an impulse, the MOV blocks do not need sophisticated formulation and processing as in some other arrester applications.
In the future, transmission lines everywhere will be better protected from momentary outages by using arresters. Based on its advantages, the EGLA will probably be the arrester of choice for this type of application.
Note: Mr. Woodworth wishes to acknowledge the contribution of NYSERDA for partially funding this project under Shawn Allen of Ceralink.