Page 20 - Issue 3 2023

TECHNICAL

and bushing flange with no indication
of a resistance issue. Only with the use
of a four-terminal low resistance test
instrument was the specialist able to
Table 5: Post-installation C1 test results for H3 replacement bushing, 10 kV LF PF and 250 V NB DFR detect a ‘before and after’ difference.
This underscores the exceptional
sensitivity of a NB DFR test to insufficient
bushing grounding.
Figure 4 shows the NB DFR curves
of the original H3 bushing, the H3
replacement bushing before installation,
after installation with poor grounding,
Table 6: Post-installation C1 test results for H3 replacement bushing, before and after grounding and finally in its ‘as left’ condition after
correction, 10 kV LFPF and 250 V NB DFR restoring a good ground connection. This
provides a striking visual reminder of
same. The 505 Hz PF test result is both non-typical and notably higher than the 505 Hz how bad the original bushing was, how
PF test results for the H1 and H2 bushings. NB DFR tests for the installed H3 replacement practical NB DFR is for post-installation
bushing revealed a non-typical response, with uncharacteristically high losses in the high verification, and what the DFR curve for
frequency range (blue curve in Figure 3). a bushing in good condition should look
The data implicated the connection integrity of the bushing flange to the grounded like.
tank. To verify the suspected grounding problem, the testing specialist applied a ground
strap to the bushing flange and repeated the LF PF and NB DFR tests (Table 6). A Conclusion
significant improvement was observed in the LF PF test results and the dielectric response LF PF testing is the primary approach
with the ground strap in use (green curve in Figure 3). to evaluating HV insulation. Significant
It should be noted that the field test specialist had no prior experience with detecting differences between UST and GST
poor flange grounding. When presented with the probable cause of this 505 Hz anomaly, measurements may require further
the specialist used a multimeter and measured the resistance between the tank ground investigation. Bushings constitute part
of the overall GST measurement of a
transformer and therefore may be a
factor in an elevated winding-to-ground
LF PF test result. Megger recommends
always testing bushings equipped with a
test tap or potential tap.
Temperature-correction look-up
tables are not accurate for bushings
with compromised insulation. The
only reliable way to access the true,
equivalent value of LF PF at 20°C is to
determine the test specimen’s ITC factor.
NB DFR in the range from 1 Hz to 505
Hz confirms both early and advanced
Figure 3: Ground effect on the NB DFR test results of H3 replacement bushing
degradation in bushing insulation.
After a bushing installation, NB DFR
testing is recommended as a verification
procedure to detect poor grounding.
PF test results at 1 Hz and 505 Hz for
bushings in particular carry significant
meaning and are sometimes referred to
as the “microscope” of insulation testing.

Contact Marie-Claude Rasendra, Megger,
Marie-Claude.Rasendra@megger.com,
Figure 4: H3 bushing NB DFR test curves from ‘as found’ to ‘as left’ www.megger.com

20 | energize Issue 3 | 2023 SUBSCRIBE FREE