Case Study A: 700 MVA GSU Transformer (GIC) Related

Background:

A 700 MVA GSU transformer indicated increasing gassing trends before shut down in 2004 for an internal inspection. Prior to this incident the transformer was located with a different generating unit and was taken off line due to an increase in acetylene levels from June 2002.  The core and core clamps megger tested down to earth with the visual inspection revealing insulation damaged between the core and core clamp feet. The transformer was repaired and was commissioned in the new location (unit) on 23 May 2003. Shortly thereafter, the dissolved gas analysis of oil samples taken on 27 June 2003 indicated sharp movement towards elevated degradation energy levels as depicted on the Low-Energy Triangle plotted in figure 1. Subsequent oil samples indicated a worsening state for the transformer.

On 16 May 2004 the transformer was taken off line for an internal inspection. During the internal inspection it was found that the blue phase HV exit lead conductors had burnt away. Upon further inspection it was also found that the LV winding exit leads showed signs of severe overheating. 

LEDT:

From the LEDT in figure 1 it can be seen that the oil samples for the year 2000 and 2001 were within the normal region. The first trigger was received on 26 July 2002 where subsequent samples were focused on this region. With the onset of acetylene levels the transformer was taken out and then repaired. 

Figure 1: LEDT Case Study 1 [Moodley1]

After the repair, the reinstallation oil sample taken on the 23 May 2003 was within the normal region. Although apparently repaired after the 2002 incident, the LEDT once again indicated, within a few weeks of being returned to service on the 27 June 2003, that the transformer was again in a defective state. Samples taken on the 8 July 2003, 7 October 2003 and 5 November 2003 indicated a progressing defective state. Between December 2003 and May 2004 the LEDT indicated a steady increase in the level of degradation. The slightly erratic path to failure is probably related to the variability of the analysis of the dissolved gases under changing conditions of ambient temperature and transformer loading.

Influence of Geomagnetic Storms

During the investigative process the impact of geomagnetic storms on transformer condition was attempted due to the evidence that GIC induce circulating currents in power transformers giving rise to low energy thermal faults. The proposal that the LEDT was sensitive to the effects of low energy degradation was also tested in this regard. 

After reinstallation on 23 May 2003, the first solar storm was recorded on the 29/30 May 2003. The next oil sample recorded on the 27 June 2003 was in the defective region. Subsequent samples taken on the 8 July 2003, 7 October 2003 and 5 November 2003 were all preceded by solar storms as presented in figure 1. It is found that the solar storm events correlated very closely to the changes in dissolved gases as presented in figure 1.

Reference

[Moodley1] Moodley, N., Gaunt, C. T., “Developing a Power Transformer Low Energy Degradation Assessment Triangle,” IEEE PES Power Africa Conference and Exposition, Johannesburg, South Africa, 9-13 July 2012