Page 34 - Energize June 2021

TECHNICAL

Reducing the risk of failure of HV

instrument transformers

Information from Megger

The application of narrowband dielectric frequency response (NB DFR) on HV CVTs to reduce the risk of
failure of HV instrument transformers in the transmission system.

A case study: A utility’s SCADA system indicated a loss of
voltage in one phase. Since 765 kV CVTs provide the voltage
information to all control, metering, and protection systems at this
location, failure of the CVT (CVT-1) corresponding to that specific
phase was immediately suspected. Tests, which were conducted
on the capacitance and power factor (PF) at line frequency,
confirmed insulation failure of C1 sections B2 and B3 of CVT-1
(see Figures 1 and 2).
For additional insights, 250 V narrowband dielectric frequency
response (NB DFR) tests, comprising PF measurements between 1
and 500 Hz, were performed on all four sections of CVT-1 (Figure 3).
The customer reported this fairly obvious failure as an over-
pressure event and replaced CVT-1 with CVT-1R. CVT-1R was
similarly tested. Its acceptable capacitance and PF and NB DFR test
results serve as helpful, normative references for CVT test results Figure 3: NB DFR plot of failed CVT 1’s C1 sections B1 to B4 (Note the
(Figures 4 and 5). PF at 60 Hz in the 250 V NB DFR test results above. When compared
to the 10 kV PF test results in Figure 1 significant, abnormal voltage
dependencies become obvious.).

Figure 1: Megger delta control results of CVT-1’s C1 sections B1 to
B4 (Note elevated PF results, significant discrepancies in capacitance
between sections and high, non-zero VDFs, which indicate PF voltage
dependencies, in sections B2 and B3). Figure 4: Acceptable capacitance and PF test results for replacement
CVT-1R (Note that a ratio test performed at 10 kV and a digital
multimeter indicated acceptable ratio results).

Figure 5: Acceptable NB DFR test results for replacement CVT-1R C1
Figure 2: CVT simplified scheme. sections B1 to B4.

energize | June 2021 | 32