504 ALTERNATING CURRENTS 



18. Show that a three-phase line may be split into three single lines, any 

 one of which may be used for purposes of calculation. Why can the ground 

 be considered as having zero resistance and zero reactance under these 

 conditions? 



19. How is the capacitance of a line actually distributed? For purposes 

 of calculation how may this total capacitance be distributed? What effect 

 does the current taken by each condenser have on the line behavior? The 

 generator load? 



20. What is the general nature of "corona"? Upon what factors does its 

 appearance depend? Upon what parts of a conductor does it first appear? 

 How may corona loss on transmission lines be minimized? 



21. What factors may cause abnormal voltage rise in a power system? 

 What is the purpose of a lightning arrester? What four properties should 

 lighting arresters possess? 



22. Describe the multigap arrester, discussing its operation. What is 

 the weak point in this type of arrester? 



23. On what principle does the horngap operate? Why is it necessary 

 to use resistance or reactance in series with such a gap? What are the 

 disadvantages of the horngap arrester? 



24. What is the underlying principle of the aluminum cell arrester? 

 Why can an alternating current flow into such an arrester when a direct 

 current cannot, even though the film is intact in both cases? 



25. Describe the characteristics which make such a device an excellent 

 lightning arrester. What is the general arrangement of the individual cells 

 in the actual arrester? Why is oil used? 



26. Why is there a short horngap in series with each arrester? Why 

 is it necessary to "charge" such arresters at intervals? Sketch the con- 

 nections of an arrester designed to protect a three-phase ungrounded system. 



27. Sketch the connections showing the position of the arrester, the 

 choke coil with relation to the incoming (or outgoing) line, and of the 

 apparatus which it is designed to protect. 



28. State the advantages of pin-type insulators for low and moderate 

 voltages. What are their limitations at the higher voltages? What 

 materials are used for these insulators, and what are their relative advantages 

 and disadvantages? Why are the larger units made up in sections? 



29. In what manner does the suspension-type of insulator support the 

 line conductors? What are the advantages of this type of insulator over 

 the pin-type? 



30. Under what conditions are wooden poles employed as line supports? 

 Steel poles? Steel towers? Compare steel towers and steel poles. 



31. What is meant by "flexible tower" construction? Under what con- 

 ditions are flexible towers used and what are their advantages? 



32. What is the function of the sub-station? Sketch roughly the con- 

 nections of a transformer sub-station. 



33. By what types of apparatus is direct current obtained from alter- 

 nating-current supply? Compare the advantages and disadvantages of 

 these different types. 



