3. 1. INTRODUCTION
In recent years the use of higher voltage levels for power transmission has
caused renewed interest by the electric power industry in the problems
associated with increased potentials.
The cause of many of these problems is the corona discharge occurring on
transmission line conductors and hardware.
Efforts to study the electrical characteristics of the phenomenon in the
laboratory have met with difficulties due to undesirable distortions of the
measured wave shapes.
A new circuit is developed which eliminates distortion of the observed
waveforms and thus enables meaningful, quantitative measurement of the
electrical characteristics of coronas active in the coaxial cylindrical
geometry.
4. 2. HISTORICAL RETROSPECT
With the advancement of EHV power transmission in the early
sixties, interest developed in the study of coronas generated on
transmission lines.
The coaxial cylindrical geometry was selected by the majority of
workers as being the most convenient for laboratory studies of
corona on transmission line conductors, because this configuration
re produces satisfactorily the electric field intensity in the vicinity of
the conductors on actual transmission lines.
6. High voltage is applied to some form of electrode through a current limiting
resistance and a radio frequency choke.
The arbitrarily-shaped low-voltage electrode is connected to ground through a
low impedance. An oscilloscope is used to display the wave shape of voltage
developed across this impedance.
Usually, in corona studies, one of the two electrodes is physically much smaller
than the other. A region of high electric field intensity will exist near the smaller
electrode.
Corona discharges will occur at high potential difference between electrodes.
The partial discharge process of high voltage corona includes avalanche
mechanisms in which rapidly moving electrons are involved in inelastic col-
lisions with gas molecules.
7. The movements of electrons and ions in the inter-electrode gap causes currents
to be induced in both electrodes according to the relationship.
𝐼𝐼=𝐸1.𝑉 (1)
𝐼𝐼= induced current in electrode i due to motion of a unit point charge
𝐸1= electric field at the charged particle due to an applied potential of 1 volt on
the 𝑖 𝑡ℎ
electrode, the other electrodes being grounded
𝑉 = velocity of particle
8. Two broad classifications of high-voltage coronas have been recognized.
The first group, positive corona, are partial-discharge processes active when the
highly stressed electrode is the positive electrode of the system.
The second, negative corona, are those processes occurring at the highly stressed
negative electrode.
Both positive and negative discharge events occur at random intervals of time.
But again with different mean repetition rates.
Since measurement of negative corona pulses constitutes the more severe
problem, the case of negative corona will be dis cussed primarily from this point
on.
9. 3.1 The Effect of Stray Capacitance
When the insignificant effect of the high voltage supply circuit is neglected and
when distributed stray capacitances are lumped, the approximate equivalent
circuit of Figure 2 results. From elementary circuit analysis, the following
relationships apply:
FIGURE 2
10. 𝑇𝑟 = 2.2𝑅𝐶 𝑇 (2)
𝐹1 =
1
2𝜋𝑅𝐶 𝑇
(3)
𝑇𝑟 =rise time for unit step of current
𝐹1= frequency bandwidth of the circuit given by 3db down frequency of the low-
pass filter
These two equations indicates that, for faithful reproduction of high-frequency
current pulses, stray capacitance must be minimized.
11. 3.2 Matching and Reflections
For faithful display on the oscilloscope, the currents induced in the measuring
electrode must be provided a reflection-free path to the recording instrument.
Reflections may be caused, by impedance discontinuities in the measuring
electrode itself, or at the connections of the measuring instrument.
The difficulty arises from the choice of electrode geometry. If the electrodes are
physic ally large, such that the corona-generated current pulses must be
transmitted along their length, the impedance of this path must be matched with
that of the measuring circuit.
If this is not accomplished then oscillation of the currents within the measuring
electrodes results and the measured waveforms are distorted.
12. 4. CONTINUED DEVELOPMENT
The circuit configuration shown in Figure 3 illustrates the form typical of the initial studies in technical
laboratories.
Fundamental coaxial cylindrical con figuration.
FIGURE 3
13. The outer cylinder is at essentially ground potential, while the inner highly
stressed electrode is energized through a current-limiting resistance.
It has the disadvantage of impedance discontinuities, and is characterized by
relatively high stray capacitance.
The current waveforms typical for this configuration are shown in the
oscilloscope of Figure 4.
Output waveform obtained from configuration of Figure 3.
The pulse rise time is approximately 40 ns.
FIGURE 4
14. According to equation (2), measured rise time can be improved by reducing the stray
capacitance between the measuring electrode and ground.
It can be seen by inspection of the configuration in Figure 3 , that stray capacitance in
this arrangement is larger than that present when the outer cylinder is energized and the
inner conductor used as the measuring electrode.
A further slight reduction in stray capacitance has been obtained by Denholm through
mounting the cylinder with its axis in a vertical position as shown in Figure 5.
FIGURE 5
15. The out put pulse shown in Figure 6 displays a reduced rise time of 14 ns,
however oscillation on the tail of the pulse persists.
Efforts were then made to improve the pulse shape by matching the impedance at
the end of the central conductor to the impedance of the measuring circuit. The
circuit shown in Figure 7 reported by Rakoshdas illustrates this development.
The waveforms obtained from this circuit, as seen in Figure 8, show
improvement, however, some undesirable oscillations are evident on the tail of
the corona pulses.
FIGURE 6
17. An attempt has been made in 1963 by Reinsborugh to terminate the cylinder at
both ends using the arrangement shown in Figure 9.
The circuit showed some promise, however, difficulties in achieving purely
resistive terminations with low inductance could not be entirely overcome and the
results were not completely satisfactory as seen in Figure 10.
FIGURE 9 FIGURE 10
18. 5. THE NEW CIRCUIT
The methods described in Section IV employed various forms of impedance
matching circuitry, however, the impedance discontinuity at the ends of the outer
cylinder could not be entirely overcome. The circuit technique illustrated in
Figures 11 and 12 provides the required reflection-free path.
FIGURE 11
FIGURE 12
19. As can be seen, the transmission line, connecting the corona-generating electrode to the
measuring instrument, is terminated in its characteristic impedance and connected
directly to the point in Figure 11 or the internal conductor in Figure 12, within the
cylindrical configuration.
The output of the coaxial point circuit is shown in the oscillogram of Figure 13. It can be
seen that the waveforms are essentially reflection free, and display a rise time of 5 ns.
This rise time is the lowest observed thus far for a negative corona pulse.
FIGURE 13
20. The developed system was also used to display the circuit current resulting from a
positive corona discharge. The result is shown in Figure 14.
The stray capacitance of the point to ground (geometry of Figure 11) was measured to be
0.5 pF . Using equations (2) and (3), the rise time and the frequency response are
calculated to be:
𝑇𝑟=.0275ns
𝐹1=12700MHz
FIGURE 14
21. For the measurements reported in Figures 13 and 14, a Tektronix type 585
oscilloscope, having a specified bandwidth of 85 MHz and a measured rise time
of 4 ns, as shown in Figure 15, was used.
The observation suggests that the actual rise time of negative corona pulses may
be below the observed value of 5 ns.
The measuring circuit itself is most satisfactory for the study of high voltage
corona pulses of both polarities.
FIGURE 15
22. 6. CONCLUSION
The developed pulse-measuring circuit has an estimated rise time of better than
30ps and the corresponding bandwidth of 12.7 GHz.
Consequently the display instrument used, a Tektronix 585 oscilloscope, type 82
plug-in, having a bandwidth of only 85 MHz, was the limiting element in the
developed system.
With the system limitation of 85 MHz, it was possible to faithfully observe the
current waveform associated with the discharge process of positive corona,
however, there is evidence that faster rise time oscilloscopes with millivolt
sensitivity, are required for correct reproduction of electrode currents caused by
negative corona discharges.
