Page 52 - Energize October 2021

TECHNICAL

Pinpointing concealed problems inside a

resistive-type OLTC before physical inspection

A case study by Megger

he following experience centred on a seventeen-position
resistive-type on-load tap changer (OLTC) which is installed
Ton the HV side of a three-phase YNd1, 25 MVA, 132/11,5 kV
transformer. This load tap-changing transformer is operated by Electric
Power Transmission Network in the City of Karbala, Iraq.
A systematic procedure of off-line testing techniques, including
dynamic resistance measurement (DRM), identified internal damage of Table 1
the OLTC. After repair, a verification testing protocol assured the OLTC’s
successful restoration to a normal and safe operating condition, as
evidenced by the DRM graphical and numerical response.
The maintenance team at the Electric Power Transmission
Network in the City of Karbala performed routine electrical tests on
the transformer including winding resistance measurements (WRM),
transformer turns ratio (TTR), short circuit impedance (SCI), excitation
current and line-frequency power factor (also known as dissipation
factor or tan delta).
The maintenance team’s TRAX instrument afforded the opportunity
to complete advanced testing also, including magnetic balance,
frequency response of stray losses (FRSL) and OLTC dynamic resistance
measurement (DRM) tests. Figure 1: WRM results for all 17 taps and all three phases
Only the DRM test results clearly flagged a deficiency in Phase B
of the OLTC, which was not observed in static measurements. Static
measurements are those that are performed while the OLTC (and
DETC) is stationary at each of a number of selected tap positions.
Examples of tests that provide static tap changer measurements
include excitation current, TTR, WRM and sweep frequency
response analysis (SFRA).
Dynamic tap changer measurements, such as provided by a DRM
test, are critical to assess a resistive-type OLTC’s bridging or ‘transition’
components, which only appear in a test circuit during the OLTC’s
transition from one tap position to the next. A resistive-type OLTC
does not use its bridging state as a viable tap service arrangement/
position, so these components never appear in the test circuit of a
static measurement.
Figure 2
Test results
In this case, the DC WRM on all tap positions (a series of static
measurements, Figure 1) shows a very small variation (around 0,2
%) between the three phases and for all taps. This is well within the
generally agreed, maximum allowable variation of 2 to 3%.
Looking at the TTR measured values (Figure 2), it was observed
that all phases and all taps have a measured ratio well within the
generally agreed acceptance value of 0,5%
The line-frequency (LF) power factor (PF), or dissipation factor
(DF), and NB DFR (narrowband dielectric frequency response) test
results (Table 1 and Figure 3) were acceptable. No notable insulation
degradation was indicated.
Advanced OLTC testing was carried out with the dynamic resistance
measurement application in the TRAX SW. During the transition from
one tap position to the next, the current drops and that is represented Figure 3

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