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fault-arc paths such as are produced by lightning flashovers on high-voltage transmission lines and the subsequent power follow current. Since that time numerous tests have been conducted involving production of such arc-trails while photographic records of the phenomena were made. These motion pictures plus oscillographic records of electrical variables have enabled analysis of the phenomena and predictions of behavior which have been applied to system design and operation.
Timing Techniques
Inasmuch as the production of faults on large power systems cannot be made with impunity it is essential that satisfactory records of each test be obtained. In order to insure such results, accurate data on camera acceleration and frame speeds must be had and close control must be exercised over initiation of the arc and camera and oscillograph timing.
Figure 1 is typical of the type of camera data necessary. In addition the following information should be on hand:
1. Frame speed at which it is desired to record.
2. Arc duration.
3. Approximate duration of post-arc phenomena to be recorded.
4. Closing time of arc-initiating switch.
5. Interrupting time of fault clearing switch.
With this information and suitable timing equipment, satisfactory records may be obtained providing proper lens aperture is selected.
As an example to illustrate selection of timing sequences let it be assumed that it is desired to photograph an arc at the highest frame speed possible while recording the entire duration of the phenomena. Assume the following data are known:
1. Frame speed desired — maximum possible.
2. Arc duration — 0.1 sec.
3. Duration of post-arc phenomena to be recorded — 0.2 sec.
4. Closing time of arc-initiating switch — 0.1 sec.
5. Interrupting time of fault clearing switch — 0.05 sec.
From Fig. 1 the speed curve for 250 v represents the upper limits for this camera. In this case the running time for 100 ft of film is 0.90 sec. Total recording time (from the sum of (2) and (3) above) is 0.30 sec. This means that the arc must be initiated at 0.90 — 0.30 = 0.60 sec. As the closing time of the initiating switch is 0.10 sec, its initiating signal must occur at: 0.60 0.10 = 0.50 sec. As the arc duration is to be 0.10 sec it must be interrupted 0.10 sec after initiation or at 0.60 + 0.10 = 0.70 sec. The interrupting switch takes 0.05 sec to interrupt, consequently it must be tripped at 0.70 0.05 = 0.65 sec. Summarizing the above gives the following sequence of events:
Time in
Seconds Eotnt
0 Camera voltage applied
0 . 50 Arc initiating switch tripped
0 . 60 Arc established
0.65 Interrupting switch tripped
0 . 70 Arc interrupted
0 . 90 Camera voltage interrupted
From Fig. 1 it may be seen that this should result in a record approximately 50 ft in length on which the arc will be recorded for about 15 ft at a frame speed from 6100 per sec to 6600 per sec. Post-arc record will be approximately 35 ft in length at frame speeds from 6600 to 7000 per sec.
In actual practice it is advisable to move the events ahead in time somewhat in order to not work too near the end of the film. This allows a margin for variation in running time and flashing at the trailer end if unloading is done under high ambient light.
Harrington and Ramberg: Photography of Electrical Arcs
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