RESULTS - REFERENCED TO SUGAR CANE FIRE

Time Domain Experiments

6.2 RESULTS - REFERENCED TO SUGAR CANE FIRE

drawing excessive currents from the power source supplying power to the corona cage. The excessive currents result in distortion in the power frequency supply signal in the last three cycles shown in figure 6.8.

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Figure 6. 9: Twin Zebra in normally dry conditions stressed to 15 kV/cm

During the heavy rain conditions also at 15kV/cm, the familiar envelope in the positive half cycles can be seen in figure 6.10 with marginal pulses in the negative half cycles.

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Figure 6.10: Twin Zebra in rain conditions stressed to 10 kV/cm

The high frequency signals shown in figure 6.11 are those generated due to the presence of a

sugar cane plantation, was ignited. Then the conductors in the corona cage were energised and gradually increased up to 10kV/cm.

The power supply transformers could not provide a stable voltage at the higher gradients, hence tests at the low gradient of only 10 kV/cm were performed. At 10 kV/cm the amplitude of the pulses exceeded the dry and rain condition pulses. Compared to the rain induced corona, the fire induced corona pulses were more than 6 times greater, when they occurred.

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Figure 6. 11: Twin Zebra over sugar cane fire conditions stressed to 10 kV/cm

The corona was visible in both half cycles as opposed to being primarily visible in the positive half cycle for gas fire induced corona. Based on the comparative sizes of the corona in the two half cycles, it appears as though negative and positive streamers were now present at the conductors.

The presence of corona in both half cycles at 10kV/cm may be indicative of the significant impact of particles about the conductors. In section 6.1.3.3 the gas fire (generating heat only) produced highly repetitive corona pulses in the positive half cycle with very little corona in the negative half cycle at a gradient 7kV/cm higher than the test reported in this section. Here the corona pulses in the positive half cycle and the negative half cycles have similar repetition rates

considerably lower than observed in either the rain induced corona tests or the gas fire positive half cycle test results.

6.2.3 Triple Pelican conductor bundle:

Figure 6.12 below reveals only positive half cycle corona pulses for the high surface gradient of 18kV/cm in normaldryconditions.

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Figure 6.12: Triple Pelican in normally dry conditions stressed to 18 kV/cm

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Figure 6. 13: Triple Pelican in rain conditions stressed to 18 kV/cm

At the same surface gradient under heavy rain conditions figure 6.13 reveals an increase in the corona pulses and amplitudes of up to twice the maximum amplitude observed in the dry

The sugar cane fire tests for the triple Pelican revealed a similar result to that observed for the twin Zebra conductor bundle. Itwas not possible to raise the gradient any higher than 10 kV/cm without deforming the 50 Hz cycle power signal which would introduce umealistic frequencies into the energising power supply signal.

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Figure 6.14: Triple Pelican over sugar cane fire conditions stressed to 10 kV/cm

The phenomena seen in the tests performed on the twin Zebra were duplicated in this conductor bundle as seen in figure 6.14. The corona in the positive half cycles were clipped by the measurement range selected on the instrument but it can still be seen that there is a high number of pulses across each half cycle. Despite the relatively low surface gradient, negative half cycle corona pulses are clearly observed of similar amplitudes in each half cycle and with a similar repetition rate across the three half cycles shown.

6.2.4 Twin Pelican conductor bundle:

Figure 6.15 reveals corona pulses at indryconditions at 15kVfcmin the positive half cycle only.

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Figure 6. 15: Twin Pelican in normally dry conditions stressed to 15 kV/cm

The corona produced at 15kVfcm under heavy rain conditions as shown in figure 6.16 below demonstrate the repeatable nature of rain induced corona conditions.

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Comparing figures 6.13 and 6.16 shows an increase in amplitudes for the higher surface gradients in figure 6.13 but the overall fingerprint is similar.

With the twin Pelican conductor bundle in the corona cage a gas fire test was also completed shown in figure 6.18 with the sugar cane test result shown in figure 6.17.

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Figure 6.17: Twin Pelican over sugar cane fire conditions stressed to 10 kV/cm

Infigure 6.17 negative glow appears to be present whilst streamers appear to be present in the positive half cycles.

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Figure 6.18: Twin Pelican over gas fire conditions stressed to 15 kV/cm

At 15kV/cm the negative glow is not visible. The streamers in the positive half cycle have a higher amplitude and may be the onset of breakdown streamers. The repetition rate of these streamers is also lower than that for the fire with particles.