A New Approach for On-Site Calibration ofVoltage Transformers Dr. Michael Krüger, Florian Predl, OMICON Austria Content > Measurement Methods • Conventional Measurement Method • Alternative Measurement Method (Using a Model) > Case Studies • Onsite Measurement on a 66kV Reference VT • Measurement on a 66kV CVT Page 2 ©OMICRON Conventional Measurement Method Source: IEC 61869-3 Page 3 ©OMICRON Measurement Procedure • 1. Measurement of short-circuit impedance • 2. Measurement of secondary winding resistance • 3. Measurement of secondary short-circuit impedance (only if more than one secondary winding) • 4. Measurement of initial magnetization curve • 5. Measurement of turns ratio correction • 6. Calculation of voltage ratio error and phase displacement based on the equivalent circuit diagram Page 4 ©OMICRON 1. Measurement of Short-Circuit Impedance Inductive Voltage Transformer Page 5 ©OMICRON 1. Measurement of Short-Circuit Impedance Capacitive Voltage Transformer Page 6 ©OMICRON 2. Measurement of Secondary Winding Resistance Page 7 ©OMICRON 4. Measurement of the Magnetisation Curve Page 9 ©OMICRON 4. Measurement of the Magnetization curve Page 10 ©OMICRON ↑ ↓ ↑ ↑ ⇒ ⋅ ⋅ = ⇒ ⋅ ⋅ ⋅ ⋅ = B f B V A f n V B A f B n V C C C or 2 ˆ ˆ 44 , 4 π H C B R F l u x d e n s i t y B [ T ] Magnetic Force H [A/m] 5. Measurement of Turns Ratio Correction Page 11 ©OMICRON 5. Measurement of Capacitive Ratio Page 12 ©OMICRON 1st step: 2nd step: 5. Measurement of Turns Ratio Correction • Measurement of capacitive ratio in two steps: • = = • Ratio of K total to K ind is equal to capacitive ratio: • = = 1 + • The total voltage ratio of a capacitive voltage transformer is equal to the product of capacitive ratio and inductive ratio obtained at higher voltages (3kV): • = ∗ @3 Page 13 ©OMICRON 5. Measurement of Turns Ratio Correction Example for turns ratio correction from a 10kV reference VT (inductive VT) Page 14 ©OMICRON -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 V o l t a g e r a t i o e r r o r i n % Primary voltage in V Ratio Error (V) [%] Reference Ratio Error without turns ratio correction correction Ratio error with turns ratio correction Case Study 66kV Reference VT Nameplate data Secondary terminal box Page 15 ©OMICRON Case Study 66kV Reference VT Obtained voltage ratio error of class ±0.03% ±1.5min reference VT Page 16 ©OMICRON Power Voltage ratio error in % of rated voltage VA VA [%] cos phi 80% 100% 120% 1,0000 100,0% 1,0000 -0,0064% -0,0038% -0,0019% 0,0000 0,0% 1,0000 -0,0052% -0,0025% -0,0006% Case Study 66kV reference VT Obtained phase displacement of class ±0.03% ±1.5min reference VT Page 17 ©OMICRON Power Phase displacement table (min) VA VA [%] cos phi 80% 100% 120% 1,0000 100,0% 1,0000 -2,5727 -2,5655 -2,5576 0,0000 0,0% 1,0000 -2,3296 -2,3225 -2,3146 Case Study 66kV Reference VT Conclusions: • Results have proven to be very stable over frequency range of 50Hz to 60Hz and for all successive measurement 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 • Voltage ratio error results were within the requirements of ±0.03% • Phase displacement results were off the specs by -1min (still acceptible for calibrating class 0.1 metering VTs) Page 18 ©OMICRON Case Study Capacitive VT 66kV Page 19 ©OMICRON Capacitive Voltage Transformer (CVT) VA fu [%] Reference fu [%] VOTANO phi [min] Reference phi [min] VOTANO 25 0,08 0,11 -0,50 -0,43 100 -0,22 -0,20 1,00 1,41 Conclusions: • Voltage ratio error results are within ±0.03% • Phase displacement results were ±1min
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