ROGOWSKI COILS
David Ward, Rocoil Limited
General Description
Rogowski coils are used for measuring alternatingcurrent. They
work by sensing the magnetic fieldcaused by the current without the
need to make anelectrical contact with the conductor. These
coilshave been used in various forms for detecting andmeasuring
electric currents for decades but it isonly in recent years that
their potential is beingrealised on a commercial scale.
They operate on a simple principle. An 'air cored'coil is placed
round the conductor in a toroidalfashion so that the alternating
magnetic fieldproduced by the current induces a voltage in thecoil.
The coil is effectively a mutual inductancecoupled to the conductor
being measured and thevoltage output is proportional to the rate of
change ofcurrent. To complete the transducer this voltage
isintegrated electronically (figure 1)
to provide an output that reproduces the current
waveform. This combination of coil and integratorprovides a
system where the output is independentof frequency, which has an
accurate phaseresponse, and which can measure complex
currentwaveforms. The output from the integrator can beused with
any form of electronic indicating devicesuch as a voltmeter,
oscilloscope, protectionsystem or metering equipment.
The coils are wound either on a flexible former thatis
subsequently wrapped round the conductor to bemeasured or on a
rigid toroidal former.
Features
Other devices exist that measure electric currentwithout making
electrical contact with theconductor. Many of these, including
theconventional current transformer, use aferro-magnetic core and
are subject to magneticsaturation effects that limit the range of
currents that
they can measure. A Rogowski coil, on the otherhand, is
'linear'; it does not saturate and the mutualinductance between the
coil and the conductor isindependent of the current.
Many of the useful features of Rogowski coilsystems result from
their linearity.
(i) They have a wide dynamic range in that the samecoil can be
used to measure currents ranging from afew milliamperes to several
million amperes.
(ii) Calibration is easier because the coil may becalibrated at
any convenient current level and thecalibration will be accurate
for all currents includingvery large ones.
(iii) They respond accurately to transient currentswhich makes
them an excellent choice for use inprotection systems and for
measuring currentpulses.
(iv) They are useful in situations where theapproximate value of
the current to be measured isnot known beforehand.
Coils wound on flexible formers have the additionalunique
feature that they can be wrapped round theconductor being measured.
A long coil can be usedas a compact portable device to measure
thecurrent in large conductors. Flexible coils can bemanufactured
with a cross-section only a fewmillimetres (fraction of an inch)
across and can beused where there is limited space round
theconductor.
Development
In 1887 Professor Chattock of Bristol Universitydescribed the
use of a long, flexible coil of wirewound on a length of
india-rubber as a magneticpotentiometer. The output of such a coil
isproportional to the line integral of the magnetic fieldalong its
length ie. proportional to the'magnetomotive force' or the
'magnetic scalarpotential' between its ends. Chattock used his
coilfor measuring the magnetic reluctance in ironcircuits but he
calibrated his coil by bringing theends together to encircle an
electric current. Thiscalibration method depended on Ampre's
Law,which states that the value of the line integral ofmagnetic
field along a loop which completelyencircles a current is equal to
the current.
Rogowski and Steinhaus described the technique in1912. They were
also interested in measuringmagnetic potentials. Their paper
describes severalingenious experiments to test that their
coil was providing reliable measurements includingusing it to
measure electric currents.
For accurate measurements using a Rogowski coilit is essential
that the winding is extremely uniform.From Ampre's Law, with a
perfectly uniform coilencircling a current, the output does not
depend on
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1965, Rocoil Limited, 5, Almsford Avenue, Harrogate, North
Yorkshire, UK. 0 01423 871792
the path the coil takes round the current or on theposition of
the conductor within the loop. It is onlynecessary that the ends of
the coil are broughttogether accurately. Also, if the coil does
notencircle a current the output is zero even if the coilis
positioned near a current-carrying conductor.These features are
obviously highly desirable in aneffective current-measuring
transducer.
To achieve these ideal properties the coil must bewound with a
constant number of turns per unitlength on a former of uniform
cross section. With aflexible coil the winding must remain uniform
whenthe coil is bent. The more uniform the winding thebetter the
coil will approximate to the ideal. BothChattock and Rogowski were
aware of theimportance of good coil geometry and bothremarked that
their coils left room for improvement!Rogowski only managed to wind
one coil anddescribed how the wire broke three times in
theprocess.
Practical Systems
By using the right technique it is now possible towind both
flexible and solid coils with sufficientuniformity for them to be
used in a wide range ofapplications including those demanding
precisionmeasurements. The sensitivity of a completesystem
comprising a coil and integrator is the ratiobetween the voltage
output and the current beingmeasured. Referring to figure 1 the
sensitivity isgiven by
Where I is the current and M is the mutualinductance between the
coil and the conductor. Fora given coil the sensitivity is
adjustable over anenormous range by choosing suitable values of
Cand R. For example, with a typical flexible coil thesensitivity
can be varied over a range greater than1V/A to 1mV/A . With the
coils themselves there isalso plenty of scope for modifying
theircharacteristics by altering the turns density
andcross-sectional area. The full range of permutationsof coils and
integrators provides an exceptionallyversatile measuring
system.
Rogowski coils are not suitable for measuring directcurrents but
by careful design, systems can be builtthat measure at frequencies
as low as 0.1Hz. Thehigh-frequency limit is determined by
theself-resonance of the coil and depends on the coildesign.
High-frequency limits in the range 20kHz to1MHz are typical.
Very high frequency measurements can be madeusing a Rogowski
coil by terminating the coil with alow impedance and using the
self-inductance of thecoil to perform the integration. The output
signal is
then a current rather than a voltage. Coils operatingon this
principle have been used to measurecurrents up to 100MHz.
Applications
An area of applications where Rogowski coils havebeen
particularly valuable is in the measurement ofcurrent transients.
Conventional currenttransformers can become 'confused' during the
initialstages of a transient especially if the transientcontains an
asymmetric component (sometimesreferred to as a DC offset).
Examples of transient measurements whereRogowski coils have been
used are: (i) Monitoringthe current in precision welding systems.
(ii)Measuring the plasma current in a fusionexperiment such as the
JET experiment at CulhamLaboratory. (iii) Current measurement in
arc meltingfurnaces: Arc furnaces use very large
fluctuatingcurrents and they can be made more efficient
bymonitoring the current and appropriately regulatingthe arc. (iv)
Monitoring electrical plant for protectionpurposes: Rogowski coils
give a more accuratemeasurement particularly of the early stages of
afault current and are suitable for interfacing withmodern,
all-electronic protection relays. (v)Measuring the current pulse in
an electromagneticlauncher (rail gun): The current can be
severalmillion amperes lasting a few milliseconds. (vi)Sudden
short-circuit testing of generators.
Rogowski coils have also been used to advantagefor the
measurement of steady currents. Energymanagement systems that
monitor the currentconsumption patterns of large buildings
andindustrial plant are becoming increasingly important.Some
systems use Rogowski coils because of theirversatility. They are
useful for measurement of theharmonic components in electric
currents because,being exceptionally linear, they faithfully
reproducethe harmonic content. Rogowski coils are alsoused to
measure currents with complex waveformssuch as in thyristor
circuits. They are used in theRailway Industry to monitor the
signalling currents inrailway lines. Flexible coils have been used
to tracethe currents induced in metal structures exposed tomagnetic
fields, for example near a largetransformer. The flexible coil has
an educationalvalue as an excellent practical demonstration
ofAmpre's Law.
Reference
'Using Rogowski coils for transient currentmeasurements', David
A. Ward and John La T. Exon, IEE Engineering and Science Journal,
June 1993, pp105 - 113.
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1965, Rocoil Limited, 5, Almsford Avenue, Harrogate, North
Yorkshire, UK. 0 01423 871792
VoutI =
MCR
(volts per Ampere)
