This working group meeting was held "off site" from the regular Winter and Summer Power Meetings, in Tucson, AZ, on November 10, 2000. This was the same week and location as the International Lightning Detection Conference and FALLS User Group Meeting hosted by Global Atmospherics. Several working group members attended ILDC and stayed over for the WG meeting.
The morning portion was joint with the FALLS user group meeting, which included about 30 utility engineers who were not members of this working group. Charlie Williams and Nick Demitriades of FPC presented a case study of a storm with very severe lightning, but which did not formally meet the NWS "severe storm" criteria. Following that, Ken Cummins presented some preliminary results from CIGRE WG 33.01, "Characterization of Lightning for Applications in Electric Power Systems - Draft 6".
Tom McDermott briefly described how EPRI is using the NLDN "data scrub" project results in LPDW. Based on 10 years of data, users can choose an average, worst case, or yearly return period value for GFD. The lower peak current values in the scrubbed NLDN data set are also more compatible with stroke collection using an electrogeometric model, resulting in a distribution of peak first stroke currents to lines having a median in the low 30s of kA.
A presentation and discussion ensued with the FALLS users on the topic of benchmarking actual line performance, or calculated line performance, with lightning detection networks. Most are using FALLS for targeted maintenance and problem resolution. FPC reported that correlation of flashover rates with small-area GFD values worked best at distribution voltage levels, but was questionable on transmission lines. Only a few utilities represented were using line arresters on transmission or subtransmission lines. Most expressed a willingness to contribute benchmark cases as they arise from FALLS, if the data submission is streamlined and preferably Web-based.
John Anderson presented an update on the EPRI lightning work at the Lenox, MA facility. The TFlash computer program is at version 3. Future plans include a virtual line flyover feature, distribution underbuild for induced voltages, and possible sales to non-EPRI members. John's exploratory software for a leader progression model replacement to the electrogeometric model is now at version 8. One of the primary needs for more progress is validation data from FALLS or other sources. Based on some testing at Lenox, John feels the 400 kV/m breakdown gradient for soil is too low in the impulse grounding models. He recommends the Liew/Darveniza ground model. Parallel rods defeat the ionization benefit, and there is a time delay of 8 to 10 us in the ionization.
Bill Chisholm presented some results using an RC model for transient grounding. The best performance would be obtained by approximating a disk electrode, and a crowfoot counterpoise would therefore be preferred over a continuous counterpoise. The tower grounding should be designed to avoid ionization at levels of 100 kA or less, so that lightning performance would not rely on ionization. Bill noted that soil ionization was removed from the IEEE Flash software in moving from version 1.2 to 1.3.
Bill Chisholm presented some flash count maps from the NASA Optical Transient Detector (OTD) project, covering 1995 through March 2000, when data collection stopped. The resolution is approximately a 37-km grid. A GFD map based on this data may be superior to the presently available thunder-day or thunder-hour maps. The raw flash counts from OTD should be divided by 10 for an approximate GFD value.
Flash will be converted to C and provided with a Visual Basic front end through Microsoft Excel. Tom McDermott will have a trial version of this ready for the Winter Power Meeting in Columbus.
The materials collected by the task forces on nonstandard voltages and stroke parameters were discussed. At the Summer Power Meeting in Seattle, Prit Chowdhuri expressed a desire to resign from chairing these task forces, but agreed to wrap up the existing materials in one or more draft TF papers for the Winter Power Meeting. Prit sent the TF materials to Tom McDermott shortly after the Seattle meeting, and these were discussed at the Tucson WG meeting by Anderson, Chisholm, Diendorfer (guest), Ishii, and McDermott. The conclusions were:
After the WG meeting, Chowdhuri and Chisholm agreed that the return stroke velocity topic should be deferred to a future task force. Chowdhuri also drafted the paper on review of stroke parameters.