DATOS ARCHIVOS_MAESTROS
6. INFORME DE RESULTADOS DEL ANÁLISIS DE SEGURIDAD DE LA BASE DE DATOS DEL ASEGURAMIENTO
Although the currents measured by the AECP appear to be free from noticeable leakage currents as shown by Figure 4.18 and Figure 4.19, significant leakage currents (enough to
saturate the output) are often present during times of high relative humidity. The connection between insulator degradation and water absorption has been investigated, the results of which are described in this section.
An example of leakage currents on both collecting plates of the AECP is shown in Figure 4.23. Leakage currents are seen to saturate the border plate (BP) during morning, and cause the pyramid plate to also have spuriously high values. A possible reason why the BP insulation allows a greater leakage current than the P insulation is that there are twice as many insulators supporting the BP than the P, and the contact area of the four P insulators is kept to an absolute minimum by allowing the pyramid’s edges to rest on the cylindrical insulators.
When the relative humidity is low in the afternoon (between approx. 12-17UT in Figure 4.23) the leakage current is rapidly diminished, with current of the expected magnitude observed for both collectors. BP is approximately twice as large as P during this time owing to the larger cross sectional area. There is no reason to suspect strong correlation of leakage currents between the BP and P collectors as both have different insulators, so the close correlation between both collectors during the afternoon also suggests that JC
and JD variation (common to both collectors) are the dominant source of variability during
0 10 20 30 40 50 60 70 0 3 6 9 12 15 18 21 24
Time (UT) 9 October 2005
C u rr e n t (p A ) 30 40 50 60 70 80 90 100 R e la ti v e H u m id it y ( % ) BP P RH
Figure 4.23 An example of leakage currents resulting from insulation water absorption on the border (BP) and pyramid (P) collecting plates of the AECP during times of high atmospheric relative humidity (RH). All measurements displayed as five-minute means.
The source of this leakage current was investigated, with a hypothesis that it was due to water absorption by the insulators and associated decrease of surface resistivity during times of high relative humidity. Despite having many beneficial properties the PTFE (like most other plastics) appears to absorb water. Once water is absorbed into the surface it is suggested to lower the resistivity and greatly degrade the quality of the insulator, allowing leakage currents to pass between the collector and ground. This was discovered by observing the coincidence between anomalous currents and high relative humidity, such as in Figure 4.23. In order to test whether sufficient water absorption by the PTFE can occur in a suitable timescale (i.e. hours) the rate of water absorption was investigated by completely immersing an initially dry PTFE cylinder of known weight and similar dimensions to the insulators in a beaker of water. The PTFE cylinder was weighed over several days and the calculated increase of mass plotted against time, shown in Figure 4.24.
y = 0.0152x0.4331 R2 = 0.9929 0.01 0.10 1.00 1 10 100 1000
Time since immersion (hrs)
% i n c re a s e i n m a s s Obs Power (Obs)
Figure 4.24 The percentage increase in mass of a PTFE cylinder of length 159mm and diameter 26mm completely immersed in water (at room temperature). The observed increase (Obs) has been modelled using a power law trendline, indicting that the percentage mass increase is approximately related to the square root of immersion time. The starting mass was 117.40g.
The mass increase with time was approximately proportional to the square root of immersion time, implying that water was rapidly absorbed in the first few hours before increasing at a slower rate after this initial absorption when the concentration gradient between the water and PTFE was reduced. Although the percentage increase is small, only a thin film of conducting medium would be required to allow a leakage current comparable or exceeding the magnitude of the picoamp signal. The results support the hypothesis that sufficient water absorption occurs to degrade the insulation. The timescale of absorption has implications for the expected nature of insulator degradation when exposed to the environment, with rapid degradation during a high moisture event such as rainfall or heavy dew, followed by a similarly fast recovery as the PTFE dries out again afterwards, as shown in Figure 4.23 at approximately 11:00UT. In the long term however, a slow overall degradation of the PTFE insulation would occur on longer timescales as more gradual yet persistent water absorption penetrates deeper into the insulator whenever there is a net absorption with time. This situation is likely to occur during the winter half of the year in mid-latitudes such as the RUAO, where relatively high rainfall and dew occurrence combined with weaker sunshine is likely to allow a daily net absorption of water.
The problem of leakage currents due to water absorption at times of high relative humidity persisted throughout the winter and even nocturnally in the summer of 2006. An additional insulator component that was highly resistive but did not absorb water was considered to reinforce the AECP insulators. After consideration, Sapphire was the chosen material, based on its high resistivity (of order 1x1014 Ohm cm) and immunity to water absorption. However, Sapphire is an expensive material, so must be used sparingly. Heated Sapphire was used by Gunn (1964) for successful insulation of a marine air conductivity instrument, although the material has not been used to insulate air-Earth current instruments before.
The Sapphire reinforcements (installed from June 2006) are shown in Figure 4.25. A Sapphire ball of 3mm diameter was inserted in the end of the BP insulators to act as an impervious and resistive barrier between the BP and PTFE in the event of water absorption. The spherical shape allowed the smallest contact area to be used to support the collector and the least amount of (expensive) Sapphire. For the P insulators, a 3mm diameter Sapphire rod was positioned in between the PTFE insulator and supporting edge of the collector. Like the sphere, a rod shape allowed minimum supporting contact area.
Figure 4.25 Use of additional Sapphire ball and rod to reinforce the BP and P insulators respectively, whilst retaining minimal contact area with the collector surface.
An overall improvement in BP and P reliability was seen after the additional Sapphire reinforcements were added, with longer periods of non-saturated values, although it was unclear how much of this improvement was due to the increasing amount of drier days as the summer progressed. However, despite many hours of good quality data the problem of leakage currents during the night time was still apparent. It was concluded that water absorption by the PTFE insulators that was not the only mode of degradation, but also the rapid accumulation (despite regular cleaning) of hygroscopic particles on the outside of the insulator (both PTFE and Sapphire) also a likely source of insulator degradation. This coating of foreign material such as from blowing soil, dust etc, is suspected to be
hygroscopic due to insulator breakdown during high, but not necessarily 100%, relative humidity (Figure 4.23).