High current ac metrology

using Rogowski coil

This work is partially funded by ERA-NET Plus programme, Grant Agreement No. 217257.

Jari Hällström, Esa-Pekka Suomalainen

Centre for Metrology and Accreditation (MIKES)

Rogowski coil

dt

tdiMtv

)()( =

J. Hällström

Split-core coil, ID 150 mm

Our tool: 2 DMMs (3458A) sampling synchronously

U1

U2

DMM

DMMSignal

generator

GPIB

Trigger

J. Hällström

Characteristics studied

1. Temperature dependence2. Linearity with applied current

3. Frequency dependence

4. Dependence on conductor location

1. Compensation of temperature dependence

• Two temperature dependent phenomena can be recognized:

1) Mechanical expansion of the core material, which causes mutual inductance M to

change.

2) Increase in the resistance of coil wire (RCOIL, copper)

• A resistor RCOMP in parallel with the output forms a resistive voltage divider with the

resistance of the coil wire.

• The value of RCOMP can be selected so that the change of the divider ratio

compensates the mechanical expansion effect.

• TC of the split Rogowski coil was reduced from c. 50 ppm/K to < 5 ppm/K by adding a

compensation resistor.

dt

diMu

o=

RCOMP

RCOIL

M uO

D.A. Ward: “Precision measurement of AC currents in the

range of 1 A to greater than 100 kA using Rogowski coils”,

British Electromagnetics Measurement Conference, NPL, October 1985.

• Rogowski coil was kept in thermal chamber;temperature was varied from 8 to 40 °C

• Test frequency was 60 Hz

• Test current of c. 0.7 A was fed through the Rogowski coil 39 times, producing an effective current of about 30 A

• Reading was compared with a reading from stable current shunt

• Reference temperature was 23 °C

Current shunt 0.1 ohmCurrent shunt 0.1 ohmCurrent shunt 0.1 ohmCurrent shunt 0.1 ohm

Coil under investigationCoil under investigationCoil under investigationCoil under investigation

Thermal chamberThermal chamberThermal chamberThermal chamber

c. 50 turnsc. 50 turnsc. 50 turnsc. 50 turns

8 8 8 8 –––– 40 °C40 °C40 °C40 °C

J. Hällström

Compensation of temperature dependence

J. Hällström

Compensation of temperature dependence

-0,006 %

-0,004 %

-0,002 %

0,000 %

0,002 %

0,004 %

0,006 %

0,008 %

0,010 %

0,012 %

0,014 %

5 10 15 20 25 30 35 40 45

Temperature (°C)

Re

lati

ve

ch

an

ge

of

ou

tpu

t v

olt

ag

e o

f th

e c

oil 10k

10,4k10,8k11,2k11,6k12k12,4k12,8k13,2k

Parallel resistor

1. Measurement

2. Measurement

Zero point = 23 °C measurement

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Performance of temperature compensation

-20

0

20

40

60

80

100

120

140

160

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00

t [h:min]

dM

/M[x

10

-6]

0

5

10

15

20

25

30

35

40

45

T[°

C]

dM/M

Temperature

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-100

-80

-60

-40

-20

0

20

40

60

80

100

0 50 100 150 200 250 300 350

Applied effective current [A]

dM

/M[x

10

-6]

Split-core Rogowski coil

Auxiliary Rogowski coil

Reference current shunt

Split-core Rogowski coilor Auxiliary Rogowski coil

39 turnsEffective current=> 12 - 350 A

0.3 - 9 A

2. Linearity with applied current

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Linearity from 200 A to 5600 A

-10

-8

-6

-4

-2

0

2

4

6

8

10

0 1000 2000 3000 4000 5000 6000

Applied effective current [A]

dM

/M[x

10 -6

]

Split-core Rogowski coil

Auxiliary coil

10 turnsEffective current=> 200 - 5600 A

Split-core Rogowski coil

20 - 560 A

-0.35 %

-0.30 %

-0.25 %

-0.20 %

-0.15 %

-0.10 %

-0.05 %

0.00 %

0.05 %

0 250 500 750 1000 1250 1500 1750 2000

with integrator

no integrator

Frequency [Hz]

dM

/M

J. Hällström

3. Frequency response

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4. Pick-up from external conductors

0.0000 %

0.0050 %

0.0100 %

0.0150 %

0.0200 %

0 0.1 0.2 0.3 0.4 0.5

d [m]

Co

up

led

sig

nal

1a

1b

2a

2b

3a

3b

1000 A, 55 Hz

d

1a, 1b

2a, 2b

3a, 3b

Uncertainty budget

260Overall uncertainty

20Linearity

230Effect of conductor position

60Effect of ext. magnetic field

70Temperature dependence

60Short term stability

50Calibration uncertainty

Uncertainty, x10-6

(k=2)Mutual inductance, M

Lower uncertainties can be reached if the coil is calibrated in the setup, relying

on the linearity and low temperature dependence of the coil.

On-site calibration of CTs using Rogowski coil

Self-checking setup

1. System ratio (M /RM) is calibrated on-site, using low (c. 10 A) current:

2. The CT is inserted into the circuit for calibration:

RM U1

U2RC

CT

RM

Burden

U1

U2RC

3. Absolute values of RM and M are not important, only their ratio matters.

M

M

On-site calibration on a 400 kV substation

• December 2007

• Temperature:(3 ± 3) °C

• External current feed up to 750 A,52 Hz or 60 Hz, from 4,5 kVAac power source

• Uncertainty: 300 ppm for the ratio error, and 1’ for phase displacement.

Suomalainen, Hällström, On-Site Calibration of a

Current Transformer using a Rogowski Coil.

IEEE Trans. Instr. Meas., April 2009.

On-site with split-core Rogowski coil...

Case 1, November 2010

• Calibration of reference CT in customers testing laboratory

• Power frequency, 50 Hz

• Current from 80 A to 1000 A

• Discrepancies from earlier calibrations in the order of

50 to 150 ppm and 0.1´ to 0.9´.

Case 2, March 2011

• Calibration of CTs in a very compact MV test field

• Power frequency, 50 Hz

• Current from 50 A to 200 A

• Systematic difference from reference 550 pmm and 0.3´.

Experience

• Power frequency pick-up during calibration of the setup is a problem;

output from the coil is c. 10 mV.

• It is difficult to find a busbar with large enough clearances for optimum

calibration setup

Conclusions

• Temperature coefficient compensated to < 5 ppm/K.

• No saturation effects, linearity with applied current verified to < 5 ppm/K.

• Frequency response is not flat, this is a side effect of the applied TC compensation scheme.

• Conductor position and pick-up related effects can be controlled to < 100 ppm in fixed setup.

• On-site conditions are usually more challenging.

• Rogowski coil is linear by nature: it is an excellent tool for extension of the ac current scale.

Thanks for your attention!