800 kHz
6.4 MEASUREMENTS COMPILED FROM POLLUTED INSULATOR TESTS
6.4.1 Overview
Two pollution procedures were attempted in the corona cage to measure the induced corona noise levels generated from polluted insulators. The first was the salt fog test and the second was the clean fog test as laid out in the
IEe
publication 507 [34].6.4.2 Polluted insulator test assembly
A string of insulators were assembled vertically at one end of the cage on the outer edge of one of the earthen outer cylinders with all the necessary metal fittings. This ensured that the fields generated by the stressed conductor bundle were generating minimal corona under the normally dry conditions. The number of insulators in the string were selected with an appropriate creepage length to ensure that the insulators would experience stresses very near the limit, thereby generating corona as soon as any form of increased stress took place on the insulators. A pipeline was strung along the perimeter of the cage cylinder about the insulators and jets were appropriately located to ensure the finest of sprays reaching the insulators. Via an electric pump, the applicable liquid mixture was pumped through the pipes and onto the stressed insulators. It was absolutely essential that the spray did not generate a flowing stream of liquid over the surfaces of the insulators, thereby allowing a path for current flow. The flow of current would reduce the surface tension on the insulators and prevent the generation of corona. A current measurement was made by bypassing the last insulator to earth through the digital storage oscilloscope, thus making it possible to ensure that a leakage current was flowing. From the description of the leakage current, it would then also be possible to establish whether or not the basic simulation of insulator pollution-induced corona activity had been successful.
6.4.3 Salt fog test
The salt fog test involves the spraying of a salt solution onto the insulators which are stressed to the normal operating gradient of the conductor bundle with the particular number of insulators in the string. The voltage was held constant for several minutes before the spray was applied. The insulators were then subjected to a defined ambient of pollution. Level of ambient pollution was measured by the salinity, in kg/m3, which is the concentration of salt (NaCl of commercial purity) in tap water.
Four levels of ambient pollution were selected for the tests. Very light pollution (2.5 to la kg/m3), light pollution (14 to 28 kg/m3), medium pollution (28 to 56 kg/m3) and heavy pollution (112 to 224 kg/m3). The temperature of the water was between 5 and 30 QC.
Following the aforementioned procedures and starting with very light pollution, as high levels of corona activity were expected, the pollution mixtures were applied to the conductor bundle. The results were an anti-climax as no' corona activity was observed above the normal amounts observed for the conductor bundle under normally dry conditions. One of two options could have occurred. Either the liquid mixture was arriving at the insulators in a droplet form instead of a mist or an insufficient amount of moisture was reaching the insulators. The former was considered more likely.
6.4.4 Clean fog test
In the clean fog test, the insulators were coated with an even layer of a composition of kaolin, tap water and a suitable amount of salt measured in mg/cm2 as the salt deposit density (SDD). The insulators were then baked to dry the deposit and assembled in position for the test. The insulators were then wetted with the insulators stressed to the normal operating gradient of the conductor bundle with the required number of insulators in the string. A finer set of jet nozzles were selected to minimise the size of the droplets arriving on the insulators.
The induced corona noise measurements were recorded (600 000 points per waveform) with the Nicolet 500 series digital storage oscilloscope. Each sample contained two bytes of information.
At a sampling rate of 10 MHz, each channel had the capability to capture a total 60 ms of data.
Sufficient channels were available to capture both the induced corona noise and the 50 Hz signal
simultaneously and to cascade the information over two sets of channels. The two sets were combined ("joined") in the figure below.
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Figure 6. 68: Polluted insulator attached to the twin Zebra conductor bundle.
Figure 6.68 showed a very low overall level of polluted insulator-induced corona noise activity (less than impressive) when compared to figure 6.5 on page 89. Therethe stress on the conductor was 17 kV/cm and the noise was only marginally different in pattern. The expected pattern was one similar to the fire-induced corona noise activity but with a higher degree of predictability.
That is, the pulses should have been larger and reasonably proportional to the applied voltage at that instant, more so than was observed in the fire tests.
In addition to the corona noise, the leakage current as measured as a voltage across a
Ion
high-power resistance, was recorded. The leakage current in figure 6.69 indicates that insulator corona losses were present on the system.
The noise levels however, were much lower than expected. In earlier work [u], indicated that the polluted insulator-induced noise levels should have been on a par with the fire-induced noise.
[u) See p14 [1)
In sub-contacted work performed at the University of Stellenbosch [v], the polluted insulator- induced corona noise pulses generated were significantly more severe than those measured here.
However, in the earlier field work, the data was not recorded in detail and several projected deductions were necessary.
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Figure 6. 69: Leakage current from the twin Zebra conductor bundle.
In the work performed by the University of Stellenbosch, the affects of the severe scaling of the laboratory cage were not fully evaluated and the polluted insulator measurement method was not consistent with that of the other induced corona noise measurements, making a comparison as attempted in the document ambivalent.