High current ac metrology using Rogowski coil
Transcript of High current ac metrology using Rogowski coil
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
)()( =
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Split-core coil, ID 150 mm
Our tool: 2 DMMs (3458A) sampling synchronously
U1
U2
DMM
DMMSignal
generator
GPIB
Trigger
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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
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Compensation of temperature dependence
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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
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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!