CHAPTERS
I...... Dinosaur ~Zebra -+-wolfl 120
5.3.1.3 Fire conditions
The results were less predictable but shows that for the same surface gradients the twin Dinosaur conductor bundle generates the highest level of corona noise.
Twin Conductor Bundles (Fire in dry conditions)
I...
Dinosaur~Zebra-+-wolfl5.3.2 Affects of changing the number of conductors in a given bundle
Of interest and direct relevance to Eskom was the affects of changing the number of conductors in a bundle to determine its effectiveness in minimising air insulation breakdown due to fires.
The bundles compared were the single, twin and triple Wolf conductor bundles.
5.3.2.1 Dryconditions
In the normal dry conditions shown in the figure below, the triple conductor bundle shows marginally lower levels of noise at the lower gradients and virtually the same levels as the twin conductor bundle at the higher gradients. As the dry conditions are not entirely reliable these results should not be scrutinised too enthusiastically.
Number of Conductors in Bundle (Dry conditions)
I---Single -<>-Twin -+-Triple I 80
70
>
~ 60e.
>50E
.a-'tl
'E 40
Gl
EGl
~ 30
"'
~~ 20 ii1
10
o
L.a. ~
-- -
l.,...r"i,...A"
..,
~
~...-r
---
"'J'...
~I-'Q" .-0-:::,
if'
rr
11 : jV
{
7
... [1 r
Region for comparisonI--
--,.:-
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
Surface Gradient (kV/cm)
Figure 5.12: Comparison: Number of conductors in dry conditions.
5.3.2.2 Rain conditions
The results obtained from the rain conditions as shown in the figure below indicated that these predictions were not totally inaccurate. At 19 kV/cm in rain conditions the triple conductor bundle generated 3 and 6dB1.1.V, less noise than the twin and single conductor bundles respectively.
Number of Conductors in Bundle (Rain@2mm/min)
[ Single -o-Twin~Triple
I
80rrmm~~m$
- L..a-~--:
~ 70
; ...-:~~.~
E 60 i---j--+-+~ol-Jt:Sf-o""F-=--+--H~-T-t---t---t---t--t--t-I-:---j
%. ~~~
:g 50 +-.-.j....-O~~+---l--t---l--+--HM-+-+---l-+-+--+---+-++-1 C~40+----I--+-+-+---+--+--+--~:-+-+-+--+--+-+--t--t--r-~
~~
~30+----jI---+--+--+--+--+--+--~:-+--+--+----i---t----t--t----t---t----;--j
4>
::E~ 20L~~-L.-J--+--I---1--+--WW-+--l-tRR;e;;gl~·on~f~or:l;;co;;;m;:;;:p;,ar:;;iso;;;n~--l---h~
10+--I---+--+--+--f--+---+--t--::-t--+--+----i---t----t--t----t---t---j
o+--f--+--+--+----1--+-+--j----+--+-+---I--+-+--+-~--+-__I
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
Surface Gradient (kV/cm)
Figure 5.13: Comparison: Number of conductors in rain conditions.
(24) Using the excitation function, otherwise known as the radio noise generation function,
r,
the results for different conductor bundles can be compared to each other. The results obtained above match favourably when using equation 24 below as described by Gary and Moreau [33]r
n~=~
The excitation function is related to the quasi-peak measurements done here by a constant which is dependent on the dimensions of the test environment.
r
=(RN) -25.87dB and n is the number of conductors in the bundle.Table 4: Excitation Function at 17kV/cm
@ 17 kV/cm (in dB) n
RN
r
nr
n-r
1 -2010g.j;1 70 44.13 0 0
2 67 41.13 -3.0 -3.01
3 65 39.13 -5.0 -4.77
Table 5: Excitation Function at 19kV/cm
@19 kV/cm (in dB) n
RN
r
nr
n-r
1 -2010g.j;1 74 48.13 0 0
2 71 45.13 -3.0 -3.01
3 68 42.13 -6.0 -4.77
5.3.2.3 Fire conditions
Number of Conductors in Bundle (Fire in dry conditions)
j ...Single -o-Twin -.-TripleI
100 90
:> 80 S.~ 70
:>E 60
a
'0'E 50
'"
Ee 40
:I III
..
30'"
:;;
>ii2 20
10 0
:... Region for comparison
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
Surface Gradient (kV/cm)
Figure 5.14: Comparison: Number of conductors over fire in normally in dry conditions.
Inthe fire conditions, the results again became more unpredictable due to the influence that the environmental factors have on the test procedure and the activity of the flames due to the
The data recorded for n=3 appeared to be very high. In the light of previous discussions, these results were considered to be the typical results when a fire and its affects due to the heat are very close to the conductors. It was therefore concluded that the tests performed for the single and twin conductor bundles were done in conditions which were less stable and "ideal" than that for the triple bundle. Under environmental conditions similar to that for the triple conductor bundle the corona noise for the single and twin bundles were expected to be above that of the triple conductor bundle.
This uncertainty indicates that the measurement of fire-induced corona by means of a quasi-peak or averaging mechanism was possibly not sufficiently adequate to positively identify a fire under a power line.
5.4 CONCLUSIONS: QUASI-PEAK EXPERIMENTS
5.4.1 The Quasi-Peak values as a Measure
A "quasi-p~ak" measurement measures the "annoyance" level of the induced corona noise0 r signals. That is, the level of the output from the quasi-peak detector depends not only on the peak values of the noise pulses but also on the repetition rate of those noise pulses. Therefore, factors to consider when reviewing the quasi-peak data are:
1) Where two signals exist with impulses of equal amplitude, the signal with pulses with a higher repetition rate will have a larger quasi-peak value;
2) Where two signals exist with impulses of equal repetition rate, the signal with pulses with a higher amplitude will have a larger quasi-peak value;
3) Where two signals have a large differential between their respective impulse amplitudes, the repetition rate of those impulses will determine whether the signal with the larger amplitude has the higher quasi-peak value; and
4) Where two signals have a large differential between their respective impulse repetition rates, the amplitude of those impulses will determine whether the signal with the higher repetition rate has the higher quasi-peak value.
Itis possible therefore, to have a signal with impulses at a lower amplitude than another but that occurs more frequently and therefore has a larger quasi-peak amplitude than the other signal.
5.4.2 Rain induced electrical discharges
For all five bundle configurations, the rain induced discharges quasi-peak values were higher than that observed in nonnally dry conditions as expected.
5.4.3 Fire induced electrical discharges
For all five bundle configurations, the quasi-peak values for fire induced corona is larger than the corona for heavy rain conditions at all surface gradients.
5.4.4 Fire detection using Quasi-Peak measurements
The extent to which the fire induced corona is larger is dependent on the amount of corona generated from the two sources. Larger lengths of conductor exposed to rain will increase the number0 f discharges generated due tor ain. Ani ncreased number0 f discharges, even a t low amplitude, will increase the quasi-peak readings measured on a power line.
More realistic fire conditions will reduce the sustained fire presence at the conductors. The
/
number of fire induced corona discharges will decrease although the presence of particles will in turn increase the number of localised discharges associated with the presence of the particles.
However, the tests completed here were focused on producing a flame bridge between the conductors and earth for 100% of the time during which the tests were being carried out. On this basis alone, the number of electrical discharges due to the presence of fires will be lower in the operational environment reducing the quasi-peak values recorded.
As a result the increased rain induced corona and the reduced fire induced corona conditions may combine to make it difficult to distinguish rain from fire with the quasi-peak measurement method.