Transmission Line Switching Surges D. F. SHANKLE

M E M B E R A I E E

RECENT IMPROVEMENTS in arrester impulse protective levels have permitted the application of

power apparatus having insulation reduced two volt­age classes on well-grounded systems where 75% or 80% arresters can be applied. Although the insulation of such apparatus can be adequately protected against impulse voltages, the adequacy of lightning arresters to protect the reduced insulation against switching surges is not so well established. Some of the areas re­ceiving increased attention by the electrical industry are:

1. Determination of magnitude, wave shape, and du­ration of switching surge voltages.

2. Establishment of the insulation strength of ap­paratus subject to switching surge voltages.

3. Determination of the duty that arresters will be subjected to in providing protection for apparatus when exposed to excessive switching surge voltages.

This digest is primarily concerned with areas 1 and 3, and shows the results of a study of a miniature system representation on the ANACOM (analogue computer). Saturated characteristics of transformers were provided by saturable cores in series with linear air core induc­tors, and the unsaturated characteristics by the combi­nation of miniature transformers and saturable cores in parallel. The specific types of switching surges investi­gated are those resulting from dropping a long, high-voltage transmission line with a modern circuit breaker, with a maximum of one restrike during interruption.

In order to evaluate the effects of generator and trans­former sizes and characteristics, line length, and arrester operations upon voltages that result from breaker re­striking when dropping a line, the investigation was di­vided into four separate studies.

Study 1—Dropping an Open-Ended Line. In this study, line lengths of 100, 200, and 300 miles at 345 kv were used. For each line length, the generator and send-ing-end transformer ratings were varied from 220 mva to 880 mva. This range of mva ratings shows the effects of varying the source inductance.

Study 2—Dropping an Open-Ended Line with Ar­rester Operation. With the previous conditions given in Study 1, 80% arrester operations were simulated at the ends of the line. Sparkover of each arrester was 2.7 per unit of crest normal line-to-neutral sending-end voltage.

Study 3—Dropping a Line with Receiving-End Trans­former Only. With the addition of transformers con­nected grounded Y-delta on the end of the line, a path is provided for the modification of the trapped voltage on the line following the initial breaker opening, and for the transfer of energy between phases. Voltages that

E. R. TAYLOR, J R . ASSOCIATE M E M B E R A I E E

result from dropping a 100- and 300-mile line with a receiving-end transformer were studied.

Study 4—Dropping a Line with Receiving-End Trans­former and Arrester Operation. For the 310-mva trans­former on the end of the 300-mile line and low-voltage level, the "C" phase was restruck to give the maximum voltage on the "A" phase at the sending end. A 75% arrester operation was simulated on the "A" phase at the sending end, sparking over at 1.5 times the crest of the maximum arrester a-c voltage rating.

A summary of the magnitude and duration of switch­ing surge voltages along with the arrester currents are given in the following paragraphs.

1. Switching surge voltages without modification by arrester operations or transformer characteristics were in the range of 3.1 to 3.4 per unit of crest normal line-to-neutral sending-end voltage. Times to crest increased with line length and ranged from 2,000 to 8,000 micro­seconds.

2. For the arresters assumed in this study, bus volt­ages were never high enough to cause arrester spark over. Arrester operations on the ends of the line with no receiving-end transformer resulted in crest arrester currents of 550 to 800 amperes and maximum discharge voltages of 1.8 to 2.1 per unit for the line lengths and source sizes studied. Duration of arrester currents in­creased with line length from approximately 1,100 to 3,300 microseconds.

3. When dropping a line with a transformer at the receiving end, maximum magnitudes of voltages on the line as a result of switching surges were lower, ranging up to 2.8 per unit of sending-end crest line-to-neutral voltage. Higher voltages resulted at the sending end than did at the receiving end. In general, receiving-end crest voltages were lower than sparkover for the ar­resters considered in this study. Lower voltages were ob­tained when working the transformer iron closer to the saturation point since lower breaker voltages were available for restrike.

4. Where arrester operation was simulated with re­duced sparkover and with the receiving-end transformer on the line, a lower crest magnitude of arrester current, 380 amperes, resulted, but the duration approached 10,000 microseconds. The receiving-end transformer provided a path for the transfer of switching surges between phases. As a result, the arrester discharge cur­rent could be extended.

Digest of paper 58-1216, "Transmission Line Switching Surges as Modified by Transformer Impedances and Arrester Operation," recommended by the A I E E Transmision and Distribution Committee and approved by the A I E E Technical Operations Department for presentation at the A I E E Fall General Meeting, Pitsburgh Pa. , Oct. 26-31, 1958. Published in A I E E Power Apparatus and Systems, Feb. 1959.

D. F. Shankle, and E . R . Taylor, J r . , are with Westinghouse Electric Corp., East Pittsburgh, Pa.

1012 Shankle, Taylor—Transmission Line Switching Surges ELECTRICAL ENGINEERING