Electronic Based High Voltage Measuring Transformers Chen-Yu Hsieh /Xiao Nan Ma 19/11/2018

Introduction For electrical utilities in high-voltage networks, an important task is the voltage and energy measurement. Usually it is costly to get high accuracy transformers (up to 0.1%) together with high voltages. Without any auxiliary winding or double cores transformers, a compensation method is proposed to reduces the primary and secondary voltage-drops separately, the experimental result shows a reduction on the errors of a commercial voltage transformer. 19/11/2018

Overview The circuit consisting of 2 stages of compensating devices (primary + secondary). The main error source is the voltage drop in the series impedances of primary and secondary windings. 19/11/2018

Primary side At first, we look at the primary side and simulate the circuit to see that it actually compensate the voltage drop across the primary resistance and inductance. The primary compensation device consists of 1x high-pass filter +1x inverting amplifier. 19/11/2018

Equivalent circuit (block compensate1) The two OpAmps and the associated resistors is to compensate Rp, the capasitor C1 is to compensate Lp. The total series impedance now becomes zero and the error due to the voltage drop is cancelled. V1-V2 =( -R1·R3 /R2 – j ·1/C) -Rp -L 19/11/2018

Schematics (Primary side without compensation) 19/11/2018

Schematics (Primary side with compensation) The above 2 slides shows the voltage of the circuit with and without compensating device. And we see that the circuit with the compensating device actually compensates the voltages drop across the impedances, therefore its equivalent to almost no voltage lost in the primary impedance. 19/11/2018

So now we carry on to the secondary. Primary side So now we carry on to the secondary. 19/11/2018

Secondary side On the secondary side of the VT: Vm= Vi+ i(Rs- R4R6/R5) + di/dt (Ls-R4R6C2) To get null series impedances, we design the circuit according to the ratio R5/R4 =R6/Rs and C2=Ls/R4R6 19/11/2018

Schematics (Secondary side without compensation) 19/11/2018

Schematics (Secondary side with compensation) From left, we can see that the voltage across the VT nearly equal to voltage across the load, therefore we know that the compensating is returning the voltage loss at the series impedances 19/11/2018

Summary Concluding from the previous slides, shows that the voltage drop happens severely without the compensating device. Therefore we construct a table with different values of source voltage range :12kv~36kv to compare the voltage drop with and without the compensating device 19/11/2018

Conclusion (Phase error) Conditions Without Compensation With Compensation Voltage (KV) Load (VA) Phase Error (mrad) 12 -64.425 13.7413 24 -51.324 3.93341 36 -45.321 -4.51435 15 -31.312 -8.23141 Varying the input voltage between 40% and 120% of the nominal voltage ,the ratio decreases from more than 1200 (µV/v) to less than -12 (µV/v), the phase displacement is reduced under -8(mrad) 19/11/2018

Conclusion (Ratio error) Conditions Without Compensation With Compensation Voltage (KV) Load (VA) Ratio Error (µV/v) 12 1500 -12 24 1200 -1 36 900 10 15 -240 6 Errors in VT consist of the error under open circuit conditions when Z_load is infinite, and errors due to voltage drops as a result of the load current following through both windings. [N ratioxV(sec)-V(prime)]/V(prime) 19/11/2018

Application for the future device! The proposed method can be applied to improve measurement quality in a high-voltage network, instead of replacing it by a large and costly high accuracy transformer. Since the external electronic devices simulate negative impedances of the transformer, it can also be applied to in-service transformers. 19/11/2018

REFERENCE [1] G. Camilli, “New developments in potential-transformer design,” EE Trans., vol. 62, pp. 483–487, July 1943. [2] T. A. Deacon and J. J. Hill, “Two-stage inductive voltage dividers,” Proc. Inst. Elect. Eng., vol. 115, no. 6, pp. 888–892, June 1968. [3] D. Slomovitz, “Electronic compensation of voltage transformer,” IEEE Trans. Instrum. Meas., vol. 37, pp. 652–654, Dec. 1988. [4] , “Electronic compensation of inductive voltage dividers and standard voltage transformers,” IEEE Trans. Instrum. Meas., vol. 47, pp. [5] ,http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel5/6870/18483/00851061.pdf 19/11/2018