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A New Approach for On-site Calibration of Voltage Transformers: Part II

A New Approach for On-site Calibration of Voltage Transformers: Part II

OMICRON has developed a new way of testing VTs. Part 2 looks at case studies. See Part 1 of A New Approach for On-site Calibration of Voltage Transformers

Case Studies

Measurement on a 66kV reference VT

The first case study is about an on-site measurement on a 66kV to 132kV reference VT. The secondary winding has two taps for adapting the voltage ratio between 132kV/√3:110V/√3 and 66kV/√3:110V/√3.  The nameplate information about the VT is indicated on figure 10. The accuracy of this reference VT is defined as ±0.03% in voltage ratio error and ±1.5min in phase displacement at a rated load of 1VA with a power factor of 1.0 and for a voltage range from 50% to 125% of rated primary voltage. The accuracy of the VT is specified for a frequency range of 50Hz to 60Hz.

Nameplate of reference VT


The picture below shows the terminal box of the secondary winding. The VT was tested several times in order to prove the stability of the test results for 50Hz, 60Hz and even for both possible voltage ratios.

Secondary terminal box


The test results below are an example for the full tap (66kV/√3:110V/√3) at 50Hz. It can be seen below that the voltage ratio error is within the required limits of ±0.03% from 80% up to 120% of rated primary voltage and from 0VA up to 1VA load condition.

Voltage ratio error table

Voltage ratio error diagram

The obtained phase displacement was slightly outside the required limits of ±1.5min, see below. The highest variance was at 80% rated primary voltage and 1VA load with an absolute value of -1.07min. This discrepancy would be acceptable for verification measurements on class 0.1 metering VTs.

Phase displacement table

Phase displacement diagram


The results itself have proven to be very stable over the frequency range of 50Hz to 60Hz and for all successive measurements conducted. The biggest absolute variance between all successive tests was ±0.001% in regards to the voltage ratio error and ±0.03min in regards to the phase displacement.

This proves that the modeling concept works in a reliable and repeatable way.

Measurement on a 4kV reference VT

The second case study is about a comparison measurement on a 4kV cast resin VT. The VT has been calibrated at an independent calibration laboratory in Austria which is traceable to national standards, which realize the physical units of measurement according to the International System of Units (SI). The extended measurement uncertainty UFu for the voltage ratio error is 0.006%, respectively Uδu for the phase displacement is 0.4min.

The figure below shows the nameplate information of the reference VT and under that illustrates the obtained voltage ratio error and phase displacement obtained at the laboratory.

Nameplate information
Voltage ratio error and phase displacement


The obtained test results by VOTANO 100 in regards to the voltage ratio error and phase displacement are shown in figure 17 to figure 20. The test had been conducted at 50Hz and 60Hz.

At 100% of rated primary voltage and 100% of rated load the absolute difference in regards to the reference is 0.0042% in voltage ratio error and 0.75min in phase displacement.

At 100% rated primary voltage and 0VA load the absolute difference is 0.0062% in regards to the voltage ratio error and 0.703min in phase displacement.

Voltage ratio error table

Voltage ratio error diagram

Phase displacement table

Phase displacement diagram

References

  1. IEC 61869-3:2011 Instrument transformers – Additional reqirements for inductive voltage transformers
  2.  Freiburg, Michael et al.: A New Approach For In-situ Calibration of Voltage Transformers. CMD Korea,  2014
  3. Bergman, Anders: In situ calibration of voltage transformers on the Swedish national grid. PhD Thesis; Upsala, 1994
  4. Raetzke, Stephanie et al.: Condition assessment of instrument transformers using Dielectric Response Analysis. Cigre 2012
  5. Azcarraga, C.G. et al.: On-site Testing of Instrument Transformers; 2006 Annual Report Conference on Electrical Insulation and Dielectric Phenomena
  6. VOTANO 100 user manual
  7. IEC 60044-2 Edition 1.2 / 2003-02 "Instrument Transformers, Part 2: Inductive voltage transformers", Reference number CEI/IEC 60044-2:1997+A1:2000+A2:2002
  8. IEC 60044-5 First Edition / 2004-04 "Instrument transformers, Part 5: Capacitor voltage transformers" Reference number CEI/IEC 60044-5:2004
  9. IEC 61869-3 Edition 1.0 / 2011-07 "Instrument transformers, Part 3: Additional requirements for inductive voltage transformers"
  10.  IEC 61869-5 Edition 1.0 / 2011-07 "Instrument transformers, Part 5: Additional requirements for capacitor voltage transformers"
  11.  IEEE "Standard Requirements for Instrument Transformers" IEEE Std C57.13TM-2008
  12. ANSI C93.1 – 1999 Requirements for Power-Line Carrier Coupling Capacitors and Coupling and Coupling Capacitor Voltage Transformers (CCVT)

About the Authors

Florian Predl works for OMICRON as an application engineer, mainly focusing on instrument transformer applications since September 2007. He graduated at the Federal Higher Technical Institute in Rankweil, Austria, with the focus on high-frequency technology.

Dr. Michael Freiburg is currently working as product manager at OMICRON electronics in Austria. He is responsible for instrument transformer test and diagnostic equipment. Previously, he worked as a research assistant at the Technical University of Dortmund, Germany. His research interests include diagnostics of high-voltage equipment and material science. He received the PhD degree in high-voltage engineering in 2014.

Dr. Michael Krüger is Head of Engineering Services “Test & Diagnostics Solutions for Primary Assets”, OMICRON, Austria. He studied electrical engineering at the University of Aachen (RWTH) and the University of Kaiserslautern (Germany) and graduated in 1976 (Dipl.-Ing.). In 1990, he received the Dr. techn. from the University of Vienna. Dr. Michael Krüger has more than 30 years of experience in high-voltage engineering and insulation diagnosis.

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