Negative corona shielding of line and station hardware

A conductor support member includes an external surface having a first portion thereon. The first portion is shaped to generate a localized increase in voltage gradient. Shielding means including surfaces intersecting to define a sharp edge are provided to promote a negative corona discharge adjacent the first portion. The said negative corona discharge produces a positive space charge to reduce the voltage gradient at the first portion. This inhibits the generation of positive corona discharge.

This invention relates to electrical power transmission and in particular 
to method and apparatus for preventing energy loss, radio interference and 
other undesirable characteristics associated with conventional electrical 
power transmission systems. 
It is common practice to transmit electrical power between a generator and 
consumer by means of aerially suspended conductors. It is preferable to 
transmit such power at as high a voltage as possible so that the resistive 
loss of the conductor is minimized. A conventional alternating voltage 
(AC) system therefore utilizes a transformer to increase the voltage 
generated by the power generating station and a number of aerially 
suspended conductors to transmit the power at high voltage to a number of 
substations where it is returned to a low voltage to be distributed to the 
consumers. 
With aerially suspended systems, it is conventional to utilize the 
insulating properties of air to isolate the conductors and associated 
hardware. 
In the design of high voltage (HV), extra high voltage (EHV) and ultra high 
voltage (UHV) transmission lines and stations, one of the controlling 
design parameters is the partial breakdown of the air surrounding the high 
voltage elements. This phenomenon is known as corona. Corona is audible, 
visible, it constitutes a power loss and is a source of radio interference 
(RI). 
Corona discharge is caused by a partial breakdown of the insulating medium, 
usually air, due to the voltage gradient in the field surrounding the 
conductor exceeding a critical value. 
The localized electric voltage gradient imparts sufficient kinetic energy 
to the free electrons to cause ionization of the neutral molecules by 
collison. 
In the divergent high voltage field associated with AC transmission 
systems, the corona initiated in the peak regions of the positive and 
negative half cycle of the voltage is known as positive and negative 
corona respectively. The two types of corona differ both physically and in 
their effects. 
With positive corona, the free electrons move towards the positive 
electrode and once they attain the required kinetic energy to ionize the 
neutral air molecules, an electron avalanche is unleashed towards the 
positive electrode. Once initiated, the discharge is sustained until it 
terminates on the positive electrode. This results in a relatively large 
current pulse during each positive half cycle of the applied voltage. 
With negative corona, the free electrons move away from their negative 
electrode and as they attain the required kinetic energy to ionize the 
neutral air molecules, the electron avalanche proceeding away from the 
negative electrodes leaves a positive space charge cloud between the 
electron avalanche and the electrode. The positive space charge so formed 
alters the electric field gradient, particularly at the tip of the 
electron avalanche, resulting in the suppression of the partial breakdown 
process. 
The magnitude and duration of the negative corona current pulses are too 
small to cause objectionable RI in fair weather. Therefore, RI from high 
voltage (AC) lines and stations under fair weather conditions is caused 
entirely by the positive corona discharge. 
When designing aerially suspended EHV and UHV systems, it is usual to 
provide a number of conductors in relatively close proximity 
interconnected by appropriate support members. This is known as a bundle 
of conductors and can comprise two, three, four or more individual 
conductors. Each conductor sets up its own electrical field. The potential 
or voltage gradient in the vicinity of conductors is inversely 
proportional to the distance from the surface of the conductor. With a 
bundle of conductors the potential and the voltage gradient at a given 
point is equal to the sum of the individual components from each 
conductor. Thus for each configuration of bundle, a field of the varying 
potential and voltage gradient is developed. 
Hardware located within the periphery defined by the conductors of a bundle 
where the voltage gradient is relatively low is not susceptible to corona 
discharge. However, it is necessary for certain hardware to extend beyond 
the periphery of the bundle where the voltage gradient is sufficiently 
high to induce positive corona discharge. 
Two different techniques have been developed for controlling positive 
corona discharge. The first technique is to design the hardware outside 
the periphery defined by the bundle with relatively large surface contours 
which results in a voltage gradient on the surface of the hardware below 
the corona inception voltage gradient for the respective operative 
conditions for the line or station. A problem with this arrangement, 
however, is that additional mass is frequently added over and above that 
required for the hardware to perform its purely mechanical function. This 
of course adds to the expense of the hardware and also increases the 
mechanical loading of the conductors. 
The second approach has been to utilize tubular auxiliary rings known as 
grading and/or shielding rings. This however requires additional support 
assemblies to mount the rings in the correct location and again increases 
the cost and adds unwanted weight to the system. 
A further disadvantage associated with both techniques is that the contours 
or shield rings require matching with the particular power transmission 
systems. Thus for each voltage range contemplated, it is necessary to 
redesign or modify existing components to ensure that the voltage gradient 
does not exceed the inception value for positive corona discharge. 
It is therefore an object of the present invention to provide a power 
transmission system in which the above disadvantages are obviated or 
mitigated. 
According to the present invention, there is provided a conductor support 
member comprising an external surface having a first portion thereon, the 
first portion being shaped to generate a localized increase in voltage 
gradient at the first portion of the external surface, and shielding means 
including convergent surfaces and intersecting to define a sharp edge to 
promote a negative corona discharge adjacent the first portion, whereby 
the negative corona discharge produces a positive space charge to reduce 
the voltage gradient at the first portion. 
The production of a positive space charge reduces the voltage gradient on 
the surface of the high voltage components so that the inception of 
positive corona is suppressed. 
Since the inception of negative corona occurs at a lower voltage than 
positive corona, a positive space charge is formed prior to the voltage 
gradient reaching the inception value for positive corona discharges. 
Further it has been found that an increase in voltage or a change in other 
conditions will enlarge the negative corona discharge in direct proportion 
to the increase in voltage. Thus the suppression of positive corona 
discharge is self regulating and it is not necessary to design each item 
of hardware for a particular voltage range. 
A further advantage with the present invention is that the hardware 
components may be designed primarily from a mechanical functional 
standpoint and the negative corona generating means will inhibit the 
formation of positive corona discharge. 
The negative corona inhibiting means can consist of sharp edged hardware 
components, sharp edged attachable metallic stampings, fine wire overlay, 
or other conductive elements that will produce sufficient amount of 
positive space charge during the negative half cycles of the applied 
voltage at all critical field locations.

Referring now to FIG. 1, an aerially supported electrical power 
transmission system includes a number of pylons 10 (only one of which is 
shown) which are spaced apart at appropriate intervals and support a 
number of conductors 12. The conductors are arranged in bundles and each 
bundle is suspended from the arms of the pylon 10. The conductors are 
connected to the apices of a suspension plate 16 which is in turn 
connected to the arm 14 by means of insulator strings 18. The conductors 
are connected to the plate 16 by clamps 20. 
The conductors 12 of the bundle are maintained in spaced relationship 
intermediate the pylons 10 by means of spacer brackets 22. Clamps 24, 
similar to the clamps 20, connect the conductors 12 to the apices of the 
spacer brackets 22. 
The spacer brackets 22 may include a damping mechanism of known 
construction to inhibit relative movement between the conductors. 
Each conductor carries an AC voltage which may be as high as 900,000 volts, 
root mean square (RMS), and thereby generates a localized high voltage 
gradient on the surface of the conductor. 
Referring now to FIG. 2 the clamp 24 comprises a body 26 which is adapted 
to be connected to the spacer brackets 22 by way of a throughbore 28. A 
semi-circular recess 30 is provided at the opposite end of the body 26 to 
the throughbore 28 to receive the conductor 12. A clamping member 32 is 
located above the body 26 and is formed with an arcuate portion 34 to 
co-operate with the recess 30 and partially encompass the conductor 12. A 
notch 33 is formed at the opposite end of the clamping member to the 
arcuate portion 34 and engages an upstanding rib 37 provided on the body 
26. 
A shielding plate 36 overlies the clamping member 32 and body 26 and is 
secured by a bolt 38 which extends through bores 40, 42, 44 in the shield, 
clamping member and body respectively. The bolt 38 is secured by a nut 46 
so that the bolt 38 interconnects the clamping member and body and ensures 
that the conductor is firmly gripped by the clamp 24. It will be apparent 
therefore that the extremity of the recess 30 and arcuate portion 34 lie 
outside the periphery of the bundle defined by the conductors 12 and are 
therefore susceptible to positive corona discharge. 
The shielding plate 36 is defined by upper and lower surfaces 48, 50, 
respectively. The upper and lower surfaces converge at the periphery of 
the shielding plate to define a sharp edge 52 which extends completely 
around the periphery. 
In operation, the sharp edge 52 increases the voltage gradient at the 
periphery of the shielding plate 36 and thereby generates negative corona 
discharge at a lower potential than the inception of positive corona 
discharge. A positive space charge is thus generated about the shielding 
plate and envelopes the extremities of the recess 30 and arcuate portion 
34. The positive space charge reduces the potential gradient at the 
surface of the clamp 24 so that during positive half cycles the potential 
gradient necessary to generate localized positive corona discharge is not 
attained. 
It is believed that the negative corona acts to modify the electric field 
produced by the conductor and provides an electrostatic equipotential 
surface that is equivalent to a large curved surface previously utilized 
to inhibit positive corona discharge. It has been found that the extent of 
the positive space charge increases with increased voltage and thereby 
automatically regulates the negative corona to prevent positive corona 
discharge. A further advantage is that the negative corona is generated at 
the point where positive corona is likely to occur. Thus the most 
susceptible point of positive corona discharge is shielded. 
The utilization of the shield 36 enables the clamp 24 to be designed 
according to parameters dictated by its mechanical function and avoids the 
necessity to provide additional mass to enable contouring of the surface. 
In comparative tests between a conventional clamp and a similar clamp 
fitted with a shield 36 it was found that positive corona occurred with 
the conventional clamp at an applied voltage of 290 kV RMS. The clamp 
fitted with the shield 36 was tested up to 367 kV RMS at which time no 
positive corona was observed from the clamp. However corona had occurred 
on the bus of the test apparatus. It will be appreciated that corona 
discharge on the bus represents the upper limit of the test apparatus so 
that the full capability of the shielded clamp could not be utilized. 
In a further test, positive corona was observed at 310 kV RMS with a 
conventional clamp whereas an applied voltage of 376 kV RMS failed to 
produce similar positive corona discharge from the shielded clamp. 
A further embodiment of the clamp is shown in FIG. 4. This clamp 60 
includes a body 61 formed in an arcuate recess 62 at one end thereof to 
embrace a conductor 12. A throughbore 64 is provided to permit connection 
of the body to a bracket. A keeper 66 is provided with an arcuate portion 
68 which cooperates with the body portion to define an aperture to 
accommodate a conductor 12. The clamping plate 66 is connected to the body 
61 by means of a bolt 70 which passes through bores 72, 74 in the plate 
and body, respectively. A nut 76 secures the bolt in position. 
The clamping plate 66 is provided with a raised edged portion 78 extending 
along the periphery of the arcuate portion 68. The raised portion 78 has 
an upper surface 80 and a side surface 82 which converge to define a sharp 
edge 84 shown in FIG. 5. 
In operation the sharp edge 84 generates a positive space charge during 
negative half cycles which modifies the electric field surrounding the 
extremities of the clamp and inhibits positive corona discharges during 
positive half cycles. 
FIG. 6 shows an arrangement of a portion 89 of an insulating string which 
comprises a series of generally conical insulators 90. The string is 
formed from a number of integrally molded synthetic plastics components 
which are joined to one another to form a string. Each molding defines 
three or four conical insulators. The string is arranged to be suspended 
from the arm 14 to the support frame 16 of FIG. 1. At the lower end of the 
lowermost molding the insulator 90 is provided with a connecting member 92 
to facilitate connection of the string to the frame 16. The connecting 
member 92 has a throughbore 94 which accommodates a bolted connection with 
the support frame 16. 
The connecting member 92 has an upper planar surface 96 and an outer 
peripheral surface 98. The upper end 99 of the peripheral surface 98 
extends radially outwardly and converges towards the upper surface 96. A 
sharp edge 100 is defined at the junction between the peripheral surface 
98 and the upper planar surface 96. 
The junction between the connecting member 92 and the insulators 90 in 
conventional insulator strings causes a high voltage gradient which can 
lead to positive corona discharge. However in the connecting member shown 
in FIG. 6 the sharp edge 100 induces a negative corona discharge which 
modifies the electric field at the junction of the insulator body and 
connecting member and thereby reduces the potential gradient. Since this 
appears at a lower potential than the formation of a positive corona 
discharge, the formation of positive corona discharge is inhibited. Hence 
it is not necessary to use the shielding rings normally associated with 
the connection between the insulator strings and the support members. 
The connecting member 92 may be made from a metal shell with an insulating 
material cast within the shell. In this case the upper edge 99 of the 
shell will constitute a sharp edge to generate negative corona discharge. 
FIG. 7 shows a similar arrangement to FIG. 6 in use on a bushing top 
connector. Insulating members 102 are connected to a support member 104 
which has an outer circumferential surface 106. A planar surface 108 
merges with the circumferential surface 106 to define a sharp edge 110. 
This sharp edge induces negative corona discharge which modifies the 
electropotential field. This reduces the potential gradient and inhibits 
the generation of positive corona discharges. 
FIG. 8 shows a live line tool end fitting. This fitting is placed at an end 
of an insulating rod which may be used to support a conductor temporarily 
to enable maintenance and repair work to be undertaken without 
interrupting the power transmission. 
An insulating rod 116 is received in an end fitting 118 which has an outer 
circumferential surface 120. The rod 116 is received in an end face 122 
which is dished so that the radially outer portion of the face extends 
further along the surface of the rod 116 than the radially innermost 
portion of the face 122. The circumferential surface 120 is flared at the 
end adjacent the end face 122 and merges with the end face 122 to define a 
sharp edge 124. 
The junction between the insulating rod and the end fitting results in a 
high voltage gradient which renders the junction susceptible to positive 
corona discharge. This large voltage gradient may also result in a 
breakdown of the insulating properties of the rod adjacent to the junction 
which results in flashover or complete failure of the insulating rod. 
However, the provision of the sharp edge 124 generates negative corona 
which modifies the electric field surrounding the junction of the end 
fitting 118 and insulator rod 116 and reduces the potential gradient at 
the junction. A similar arrangement may also be provided on end fittings 
of insulating rods which are used to support bus bars in power stations. 
In this case, the use of shielding rings normally associated with such 
installations is obviated. 
FIG. 9 shows an end fitting for a bus bar which reduces the high voltage 
gradient that occurs at the end of a bus bar. The end fitting 130 
comprises a tubular body portion 132 and a conical flange 134. The flange 
terminates in a sharp edge 136. The tubular portion 132 fits over the end 
of the bus bar and the sharp edge 136 generates a negative corona 
discharge which reduces the potential gradient at the surface of the 
conical portion 134. 
It is possible to manufacture the end fitting 130 with a radially extending 
flange. However the use of the conical portion 134 permits the body 
portion and flange to be welded and for the weld to be located within the 
flange portion. 
FIG. 10 shows a T-connector for connecting two bus bars 140, 142 at right 
angles. The T-connector 144 has three branches 146, 148 and 150 
respectively. A throughbore 152 is provided between the branches 146 and 
150 to accommodate the bus bar 140. A throughbore 154 is provided in the 
branch 148 to accommodate the bus bar 142. 
Each branch terminates in a sharp edge 156 which provides negative corona 
discharges prior to the inception of positive corona discharges. The 
negative corona discharges emanating from each sharp edge 156 modify the 
electric force field surrounding the T-connector and reduce the potential 
gradient on the surface of the connector. 
Similar arrangements may be effected with L-connectors and X-connectors to 
permit the connection of the bus bars in any desired configuration without 
the necessity for additional shielding rings. 
From the above it will be apparent that the present invention provides a 
unique method of inhibiting positive corona discharge by the generation of 
a negative corona shield at critical locations of the hardware associated 
with electrical power transmission systems. Moreover the shielding effect 
is self regulating in that high potentials carried by the conductors 
result in increased shielding against positive corona discharge. This is 
achieved without the requirement for conventional auxiliary hardware such 
as shielding rings and clearly results in economy of manufacture and 
improved performance of the electrical power transmission system.