SEVENTH LECTURE 


HIGH FREQUENCY OSCILLATIONS 
AND SURGES 


1"^ N an electric circuit, in addition to the power consump- 
tion by the resistance of the lines, an energy storage 
■^ occurs as electrostatic energy, or electrostatic charge 
due to the voltage on the line (capacity) ; and as electromag- 
netic energy, or magnetic field of the current in the line 
(inductance). In the long distance transmission line, both 
amounts of stored energy are very considerable, and of about 
equal magnitude; the former varying with the voltage, the 
latter with the current in the line. Any change of the voltage on 
the line, or the current in the line, or the relation between volt- 
age and current, therefore requires a corresponding change of 
the stored energy; that is, a readjustment of the stored energy 

e^C 
in the system, the electrostatic energy and the electro- 

i'L 
magnetic energy — — , from the previous to the changed cir- 
cuit conditions. This readjustment occurs by an oscillation, 
that is, a series of waves of voltage and of current, which 
gradually decreases in intensity, that is, dies out. ' 

These oscillating voltages and currents are the result of 
the readjustment of the stored energy of the circuit to a sudden 
change of conditions, and are dependant upon the stored energy 
of the circuit, but not upon the generator frequency or wave 
shape ; therefore they occur in the same manner, and are of the 
same frequency, in a 25 cycle system as in a 60 cycle system, or 
a high potential direct current transmission; and occur with 
sine waves of generator voltage equally as with distorted 


92 GENERAL LECTURES 

generator waves. While the power of these oscillations ulti- 
mately conies from the generators, it is not the generator 
wave nor one of its harmonics which builds up, as discussed in 
the previous lectures; but the generator merely supplies the 
energy, which is stored as electrostatic charge of the capacity 
and as magnetic field of the inductance, and the readjustment 
of this stored energy to the change of circuit conditions then 
gives the oscillation. 

These oscillating voltages and currents, adding to the 
generator voltage and current, thus increase the voltage and 
the current the more, the greater the intensity of the oscilla- 
tion, and so may lead to destructive voltages. 

Obviously, the intensity of the oscillation, that is, its 
voltage and current, are the greater, the greater or more abrupt 
the change was in the circuit, which caused the oscillation by 
requiring a readjustment of the energy storage. The greatest 
change in a circuit, however, is the change from short circuit 
to open circuit, and the instantaneous opening of a short circuit 
on a transmission line — as it occasionally occurs by the sudden 
rupture of a short circuiting arc — ^therefore gives rise to the 
most powerful, and thereby most destructive oscillation. 

The wave length of oscillation thus depends on the length 
of the circuit in which the stored energy readjusts itself. For 
instance, in the short circuit oscillation of the system, the wave 
extends over the entire circuit, including generators and trans- 
formers ; and the entire circuit so represents one wave, or one- 
half wave, that is, the wave length is very considerable. If 
the readjustment of stored energy takes place only over a 
section of the circuit, the wave length is shorter. For instance, 
if by a thunder cloud a static charge is induced on the trans- 
mission line, and by a lightning flash in the cloud, the cloud 
discharges, the electrostatic charge induced by it on the line 


HIGH FREQUENCY OSCILLATIONS 93 

is set free and dissipates by an oscillation. In this case, the 
length of section on which an abnormal charge existed — one 
mile for instance — is a half wave of the oscillation, and the 
complete wave length would thus be two miles. Or, if a 
momentary discharge occurs over a lightning arrester to 
ground, the wave length may be only a few feet. 

The velocity with which the electric wave travels in an 
overhead line is practically the velocity of light, or about 
188,000 miles per second: it would be exactly the velocity of 
light, except that by the resistance of the line conductor the 
velocity is very slightly reduced. In an underground cable, 
by the high capacity of the cable insulation, the velocity of 
wave travel is greatly reduced, to about 50 to 70% of that ol 
light. 

From the wave length and the velocity follows the dura- 
tion or time of one wave, and thereby the frequency of the 
oscillation. For instance, in the wave of two miles' length 
resulting from induction by a thunder cloud, as discussed 
above, the duration of the wave, or the time it takes to travel 
the wave length of two miles, at 188,000 miles per seconc 

2 1 

velocity, is iggooo ~ 94000 second, and thus, during on^ 

second, 94,000 waves would pass, that is, the frequency is 
94,000 cycles. Or, if a transmission line of 80 miles' length 
short circuits at one end, and then disconnects at the other 
end by the opening of the circuit breaker, in the oscillation pro- 
ducd thereby the circuit is one-half wave. As the length of the 
circuit is 2 X 80 = 160 miles — conductor and return conductor, 
— the half wave is 160 miles; the complete wave therefore is 
2 X 160 = 320 miles long, and the duration of the wave is 

2g3 QQQ = ^ seconds; the frequency 587 cycles, and if this 


94 GENERAL LECTURES 

short circuit oscillation extends into, and includes the generat- 
ing system, the frequency may be still lower. 

Again, an oscillation of a very short section of the line, 
as for instance, lOO feet = ^230 ~ 52^ miles wave length, 
would have a duration of the wave of 52X1" iM 660 ~ 
9 900 000 second, or a frequency of 9.9 millions of cycles per 
second. 

Hence the frequency of such oscillations, caused by the 
readjustment of the stored energy of the system, may vary 
from values as low as machine frequency, up to many millions 
of cycles per second. It is the higher, the shorter the section 
of the circuit is in which the readjustment of energy occurs. 
The higher the frequency, and therefore the shorter the section 
of the circuit in which energy readjustment occurs, obviously 
the less is the amount of energy which is available in the oscil- 
lation— the stored energy of this section — and the less destruc- 
tive therefore is the oscillation. That is, very high frequency 
oscillations are of very low energy and therefore of little 
destructiveness ; but the energy and thus the destructiveness 
of an oscillation increases with decreasing frequency, and con- 
sequent increasing extent of the oscillation. 

Such oscillations in a transmission line may result : 
a. From outside sources, atmospheric electric disturbances, 
as illustrated in the above instance. 

b. They occur during normal operation of the system: 
any change of load, or switching operation, as connecting or 
disconnecting circuits, etc., results in an oscillation, which 
usually is so small as to be harmless. 


HIGH FREQUENCY OSCILLATIONS. 95 

c. It may result from a defect or fault in the circuit, as 
an arcing ground or spark discharge, etc. 

One of the most serious and destructive oscillations or 
surges is that produced by a spark discharge to ground, or an 
arcing ground, in an overhead transmission line or an under- 
ground cable system. 

Assuming for instance a 44,ocx) volt transmission line of 
50 miles' length, which is insulated from ground, that is, in 
which the neutral is not grounded. At 44,000 volts between 
the line conductors, the voltage between each conductor and 
the ground, normally, that is, with all conductors insulated, is 

— ' — = 25,000. If now somewhere in the middle of this 

line an insulator breaks, and the conductor thus drops near the 
grounded insulator pin or cross arm to about 2"; with 25,000 
volts between conductor and ground, a spark would jump from 
the conductor to the ground, at the broken insulator, over the 
2" gap. This spark develops into an arc, over which the 
electrostatic charge of the conductor discharges to ground as 
current, and the voltage of this conductor against ground thus 
falls to zero, since it is grounded by the arc; the two other 
line conductors then have the full line voltage, of 44,000, 
against ground ; and their electrostatic charge against ground 
therefore increases, from that corresponding to their normal 
potential of 25,000, to that corresponding to 44,000 volts. As 
soon as the first conductor has discharged and fallen to ground 
potential, the current from this conductor to ground, over the 
gap, ceases, the arc goes out, and the conductor so is again 
disconnected from ground. It then begins to charge again to its 
normal potential of 25,000 volts against ground, while the 
other two conductors discharge, from 44,000 down to 25,000 


96 GENERAL LECTURES 

volts. As soon, however, as during the charge the voltage 
of the first conductor has risen to the voltage required to 
jump across a 2" gap, this conductor again discharges to 
ground by a spark, which develops into an arc and so on, the 
phenomena of discharge and charge of the conductor repeat- 
ing continuously. Such an oscillation, which continues in- 
definitely, that is, until the defect in the circuit is remedied, 
or the circuit has broken down and gone out of service, is 
usually called a surge. The duration of each oscillation of 
such an arcing ground is the time required : i. To develop the 
arc, 2. to discharge the line, 3. to extinguish the arc, 4. to charge 
the line. In the above instance, the time of charge or discharge 
of the 25 miles of line from the arcing ground to the terminal 

25 1 

station is : .^gg qqq = ^^ second. Assuming the velocity of 

the arc stream as about 2000 feet per second, the development 

2 1 

or extinction of a 2" arc would require 12 x 2000 ~ 12000 

second, and the total duration of one oscillation therefore is : 
i5ioo + ^ + n^ + Wo = ^ -ond, so giving a 
frequency of 2300 cycles. 

The two other lines therefore oscillate in voltage against 
ground, that is, charge and discharge also at a frequency of 
2300 cycles. They receive their charge, however, over the 
transformers at the two ends of the line, and their capacity 
therefore is in series with the self-inductance of these trans- 
formers in the circuit of the surge frequency of 2300 cycles; 
and the voltage of the other two lines thus may build up by the 
combination of capacity and inductance in series, to excessive 
values ; that is, a destructive breakdown occurs from the other 
lines to ground — or in the apparatus connected to them in the 
terminal stations of the line, as transformers, current trans- 
formers, etc. 


HIGH FREQUENCY OSCILLATIONS. 97 

A spark discharge or oscillating ground therefore is one of 
the most serious, as well as not infrequent disturbances on a 
long distance transmission line or underground cable circuit; 
and it is mainly as a protection against this surge that it is 
recommended by many transmission engineers to ground the 
neutral of the system and so immediately convert a spark dis- 
charge on one conductor into a short circuit of one phase of the 
system, and thereby automatically cut out the circuit; that is, 
rather shut down this circuit than continue operation with an 
arcing ground on the system. Where, as in underground cable 
systems, a number of cables are used in multiple, the immediate 
disconnection of an arcing cable undoubtedly is advisable. In 
a single overhead transmission line, where a shutdown means 
a discontinuity of service, the question, whether by grounding 
the neutral it is preferable to shut down immediately in the 
case of an arcing ground, or continue service with ungrounded 
neutral and try to find and eliminate the arcing ground on the 
conductor, depends upon the length of time which the surge 
would probably last before causing a break down; it thereby 
depends upon the character of the circuit; the margin of in- 
sulation in transformers and insulators ; and also on the value 
of continuity of service. The question of grounding or not 
grounding the neutral of a transmission line therefore requires 
investigation in each individual instance. 


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