594 ELECTRICAL EQUIPMENT 



phase conductors or between the phase conductors and ground. 

 Second, that covering localized high voltages in which the excessive 

 potential difference exists between two points along the same con- 

 ductor. In these cases the " conductor " is supposed to include 

 the line wires as well as the generator and transformer windings. 



To the first class belong those disturbances which are caused 

 by overspeeds, poor regulation and resonance, while the nature 

 of disturbances caused by switching, arcing -grounds, and light- 

 ning may be such that they may belong to either class. Where 

 the impulses or traveling waves set up are of comparatively low 

 frequency and consequently of sloping wave front, the disturbance 

 can, however, generally be classed with the former, and when of 

 high frequency and steep wave front with the latter. 



Excessive over-voltages are very apt to occur when water- 

 wheel-driven generators run away, especially if they are provided 

 with direct-connected exciters. Actual experience has thus 

 demonstrated that under such conditions the generator and trans- 

 mission voltages may reach three times their normal value, which 

 of course subjects the apparatus to unreasonable strains. To 

 provide against this, automatic brake equipments are provided 

 or else high voltage cut-out relays which automatically insert 

 resistances in the exciter fields if the voltage exceeds a certain pre- 

 determined value. 



In the design of modern long-distance transmission lines it 

 is generally the regulation, or the variation in voltage which occurs 

 when the load is thrown on or off, that is the governing factor 

 rather than the energy loss. Not only may the voltage drop 

 under load be quite large, especially when the load has a low power- 

 factor, but with the high-transmission voltages now in use the 

 capacity effect of the lines becomes very high, which in turn may 

 result in a considerable voltage rise at the substation at light 

 loads. This is now one of the chief arguments against isolated 

 delta connection for long-distance high-tension lines. It was 

 formerly claimed that such a system could be temporarily 

 operated with one line grounded. Recent experiences on large 

 systems, however, indicate that this is not feasible, as in the 

 event of a ground the charging current, which is a function of 

 the voltage from wire to neutral, will be increased because the 

 natural is shifted from the center of the delta to one corner. 

 This increase will be about 73 per cent and will of course in 



