A CIRCUIT FOR MEASUREMENT OF HIGH VOLTAGE

CORONA PUISES IN COAXIAL CYLINDRICAL

GEOMETRY

CONTENT

INTRODUCTION

HISTORICAL RETROSPECT

THE BASIC CIRCUIT

CONTINUED DEVELOPMENT

THE NEW CIRCUIT

CONCLUSION

1. INTRODUCTION

In recent years the use of higher voltage levels for power transmission hascaused renewed interest by the electric power industry in the problemsassociated with increased potentials.

The cause of many of these problems is the corona discharge occurring ontransmission line conductors and hardware.

Efforts to study the electrical characteristics of the phenomenon in thelaboratory have met with difficulties due to undesirable distortions of themeasured wave shapes.

A new circuit is developed which eliminates distortion of the observedwaveforms and thus enables meaningful, quantitative measurement of theelectrical characteristics of coronas active in the coaxial cylindricalgeometry.

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.

3. THE BASIC CIRCUIT

FIGURE 1

High voltage is applied to some form of electrode through a current limitingresistance and a radio frequency choke.

The arbitrarily-shaped low-voltage electrode is connected to ground through alow impedance. An oscilloscope is used to display the wave shape of voltagedeveloped across this impedance.

Usually, in corona studies, one of the two electrodes is physically much smallerthan the other. A region of high electric field intensity will exist near the smallerelectrode.

Corona discharges will occur at high potential difference between electrodes.

The partial discharge process of high voltage corona includes avalanchemechanisms in which rapidly moving electrons are involved in inelastic col-lisions with gas molecules.

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 𝑖𝑡ℎ electrode, the other electrodes being grounded

𝑉 = velocity of particle

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.

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

𝑇𝑟 = 2.2𝑅𝐶𝑇 (2)

𝐹1 =1

2𝜋𝑅𝐶𝑇(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.

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.

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

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

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

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

FIGURE 7

FIGURE 8

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

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 11FIGURE 12

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

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

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

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.