Page 61 - Energize January 2022

TECHNICAL

large number of recent failures has convinced many that WTG The Duval pentagon showed a thermal fault (T3), (Figure 3), as
transformer designs must be substantially more durable. did the gas signatures (Figure 4), with significant rates of gas rise
Key characteristics of WTG step-up transformers that wind farm (Figure 5). Note that the abnormal gassing pattern was evident
owners and developers should pay attention to include transformer after energisation on load. The internal inspection revealed several
loading, harmonics and non-sinusoidal loads, transformer sizing, quality issues. Figure 6 shows one the findings.
voltage variations, and special requirements to withstand faults.
Partial discharge and harmonics
Case studies In this case study of 55 transformers at a wind farm, the DGA
The following case studies of transformer faults detected by DGA will showed a significant number with abnormal gas levels when
provide some insight to the challenges faced by wind farm owners. compared to sister units, as seen in Figure 6. The fault category in
most cases was partial discharge (PD) activity. According to CIGRE
Example of a unit affected by improper manufacturing from a 761 Assessment Index (TAI) Scoring Matrix (Risk), the results are
wind farm shown in Figure 6 (Transformer scored as Category D: E: F), while
Consider the case of a pad mount transformer for a 2,7 MVA, 33 kV Figure 8 shows the gas signatures.
WTG, without an on-load tap changer (OLTC). The transformer was
manufactured in 2014 according to SANS 780 and installed in 2016.
The nameplate information is given in Figure 2.

Figure 4

Figure 5

Figure 2

Figure 3 Figure 6

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