MEASUREMENT OF AC CURRENT

WITH

COAXIAL CURRENT SHUNTS

Bostjan Voljc, Mag. Matjaz Lindic

VERIFICATION OF AC CURRENT MEASUREMENT USING COAXIAL

CURRENT SHUNTS

Bostjan Voljc1, Matjaz Lindic1, Rado Lapuh2

Bostjan Voljc, Mag. Matjaz Lindic

SIQ - Slovenian Institute of Quality and

Metrology, Slovenia

bostjan.voljc@siq.si, matjaz.lindic@siq.si

1 SIQ - Slovenian Institute of Quality and Metrology, Ljubljana, Slovenia

2 MIRS - Metrology Institute of Republic of Slovenia, Ljubljana, Slovenia

bostjan.voljc@siq.si, matjaz.lindic@siq.si, rado.lapuh@gov.si

Overview1

1. New direct ac current measurement method

2. Accuracy of the new method

3. Comparison of the new method

• comparison between different shunts

• comparison to ac-dc current transfer measurement method

4. Conclusion

New direct ac measurement technique2

• coaxial current shunts are suitable for ac-dc current transfer measurement and for direct measurement of ac current

• direct measurement of ac current: method is based on the measurement of the ac voltage drop (Us) on resistance (Rs) of the coaxial current shunt

• current shunts characterized for ac-dc current transfer difference, long term drift of its dc resistance and self heating effect

• 6 coaxial current shunts covering range from 10 mA to 20 A in frequency range from • 6 coaxial current shunts covering range from 10 mA to 20 A in frequency range from 10 Hz to 30 kHz

AC voltmeterAC current source

Current shunt resistor

3 New direct ac measurement technique

)1( pwcdcacdriftS

Sm

R

UI

δδδ +++⋅=

−

where

• I is the unknown ac current,• Im is the unknown ac current,

• Us is measured ac voltage across the shunt,

• Rs is dc resistance of the shunt,

• δdrift is correction of the dc resistance long term drift (from dc resistance calibration history),

• δac/dc is correction due to ac-dc difference of the shunt (from PTB calibration certificate),

• δpwc is correction due to self heating of the current shunt (from measurement of temperature and power coefficient).

4

Shunt 10 Hz 20 Hz 40 Hz 1 kHz 10 kHz

100 mA 54 44 29 29 29

300 mA 54 45 29 29 29

1 A 56 47 33 33 34

5 A 57 49 37 37 37

10 A 62 55 47 47 47

Accuracy of the new method

20 A 70 63 54 54 62

Best measurement capabilities

5

AC current source

• two coaxial current shunts were compared over the special current T-piece

• additional 30 cm high current coaxial cable was used for each shunt to minimize interference between shunts

• two shunts compared on the nominal current of the smaller shunt

• following combinations were compared:

Shunt 1 Shunt 2 source

current

Comparison between shunts

AC voltmeter

Shunt 1 Shunt 2

current

100 mA 300 mA 100 mA

300 mA 1 A 300 mA

1 A 5 A 1A

6 Comparison to ac-dc transfer technique

DC current source

• dc+, ac and dc- measurement sequence was used

• from the same measuring results the ac current was calculated from equations for both methods:

• ac-dc current transfer method

• direct method

+

+

−−+⋅= −

−+

−+dcac

dcdc

dcdcacdcac

UU

UUUII δ

21

AC/DC voltmeter

AC current source

Current

switch

• direct method

• common parameters:

• ac-dc transfer difference (δac-dc)

• ac voltmeter (Uac)

)1( pwcdcacdriftS

acm

R

UI

δδδ +++⋅=

−

Shunt 1 Shunt 2 source current 40 Hz 1 kHz 5 kHz 10 kHz

0,1 A 0,3 A 0,1 A -14,0 -13,7 -14,1 -14,4

0,3 A 1 A 0,3 A -12,1 -11,2 -11,5 -11,5

1 A 5 A 1 A -0,8 -2,6 -1,4 -2,8

Frequency

7 Results

Difference between measurement results obtained on two different coaxial current shuntsin µA/A

Shunt 40 Hz 1 kHz 5 kHz 10 kHz

0,1 A -6,6 -6,6 -7,5 -8,8

0,3 A 9,4 10,3 9,6 8,7

1 A 10,6 10,8 9,9 8,9

5 A 5,9 7,3 5,9 4,9

Frequency

in µA/A

Difference between the direct ac current measurement method and ac-dc transferdifference method in µA/A

Conclusion8

• results shows very well controlled behavior of both shunts and measurement method

• deviations typically less than 1/3 of the stated uncertainty

• measuring results of the comparison prove that the direct ac current measuring method is suitable and that the uncertainties stated are appropriate

• uncertainty of the ac voltmeter presents the major uncertainty contribution

• minimizing its uncertainty would decrease also the uncertainty of the direct acc current measurement method

• between direct ac current measurement and ac-dc transfer difference method • between direct ac current measurement and ac-dc transfer difference method systematic deviations are observed

• further investigation shall be made to identify and resolve these deviations