SUGAR CANE FIRE UNDER THE TRANSMISSION LINE

Operational Line - Controlled Fuel

9.4 SUGAR CANE FIRE UNDER THE TRANSMISSION LINE

Therefore

dB

=

2010 27.577

2 g 8.66

= 10.06 dB

dB(TOTAL)= 16.34+10.06

=26.40 dB

The additional frequency components visible in figure 9.9 between 52 kHz and 324 kHz were more than likely harmonics due to the clipping in the data. The components above 324 kHz were harmonics of the 52 kHz signal added onto the carrier signal.

The frequency spectrum calculation has an averaging affect, and hence an attenuation of the change in the increase in noise had to be considered. To understand the full extent to which the averaging affect attenuated the noise, a complete analysis of the time domain noise levels was necessary. A projected maximum peak in the waveform in figure 9.8 following the trigger (at

o

ms) was measured at approximately ±600 mV by graphical means. The background noise before the fire affected the noise levels, was ±25 mY. Therefore the increase in noise from the carrier signal leakage level to the fire-induced noise was approximated to:

dB=2010g-600 25

= 27.6 dB.

The affects of the carner leakage "voltage" then had to be added. However, the actual background noise in the time domain could not be measured. Therefore the loss through the frequency spectrum information was approximately 1 dB plus the actual increase in level from the background noise to the carrier leakage "voltage".

who then recorded any significant corona activity in accordance with the information supplied to him from the bum site.

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illS Figure 9.10: Sugar cane fire under the center phase (cold condition).

The large burst observed in figure 9.10 was recorded with the information of a "cold bum"

taking place. That is, the flames were low relative to the height of the conductors. A lot of smoke was present but due to the strong wind none of it came near the conductors and hence the probability of the corona activity being initiated by the presence of particles was excluded. The fire fluctuated over a reasonably long period and some thirty minutes later flames erupted from the smoke and the surge in figure 9.11 below was captured at a sampling rate of 10 MHz.

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illS

Figure 9.11: Sugar cane fire under the center phase (matched flames present).

When comparing these bursts of noise with the spurious burst observed in figure 9.6, it was obvious (from our earlier findings of the affects of corona activity on the background noise levels), that the increase in noise occurred rapidly in accordance with that0 f induced corona noise and was therefore not characteristic of the unexpected surges observed from time to time.

Positive recognition of these signatures would be possible.

. 9.5 GRASS FIRES UNDER THE TRANSMISSION LINE

Grass bales were used for this test. Failure to mount the bales for sufficient draft amplified the problems created by wind and only one set of data of any value was captured.

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illS Figure 9. 12: Grass fire under the center phase (poor fire condition).

The flames were again well below the conductors with no particles reaching the height of the conductors due to the high winds. Only an intense heat was generated which may have influenced the generation of corona about the conductors. The almost instantaneous rise in noise level was again an indication that the noise generated was as a result of induced corona activity at the conductors.

9.6 AFFECT OF THE FILTERING CmCUIT

The frequency spectrum of the background noise levels from the two BBC filters used were calculated from a segment ofthe data and is shown in the two graphs below.

600 800

200 400

j

I

o o

4 mV

16

8 12

kHz

Figure 9. 13: Frequency spectrum of noise on the transmission line via a 52kHz filter.

The remainder of the carrier signal not attenuated by the 52 kHz filter was clearly seen breaking through the background level in figure 9.13 whilst the true background level of noise at the filter centre frequency of 52 kHz was determined at approximately 2.2 mV for the sample used.

The remainder of the carrier signal not attenuated by the 428 kHz filter was clearly seen breaking through the background level in figure 9.14. Due to the proximity of the filter centre frequency to the carrier frequencies both on the CraighalVLepini line and the neighbouring lines, these signals leaked through. Higher levels were measured here (where the centre frequency is in fact at approximately 390 kHz) than at the 52 kHz centre frequency of the first filter. The carrier frequencies used on the Craighal1/Lepini line was 320 kHz and 324 kHz with the stronger 324 kHz component being transmitted from Lepini substation where the measurements were being made. Therefore the 52 kHz filter was preferred. Another reason for the selection of only one filter was the limited data memory capacity on the digital storage oscilloscope used, which at

the time was considered to have the highest memory capacity on the local market. Inaddition, on a national level most frequencies used for the carriers are higher than 100 kHz.

800

kHz

400 600 200

\ ,l j

o

o

4 8 12 mV

16

Figure 9. 14: Frequency spectrum of noise on the transmission line via a 428kHz filter.