Download LEM`s new class of Rogowski coil split

LEM’s new class of Rogowski coil split-core
current transducers: THE RT SERIES
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By Pierre Turpin, Industry Project Manager, LEM
Introduction
The monitoring of electric power consumption
has become a key element for managing electrical
installations in industrial and commercial sectors, such
as manufacturing facilities, data centers, food processing
industries, retail outlets, hospitals or educational
establishments. Three years ago LEM introduced a
system called Wi-LEM onto the market which is based on
wireless sub-metering components, the EMN, enabling
measurement of electricity segmented by activity (lighting,
HVAC, motors, heating, etc.). Initially, the range seemed to
be sufficiently comprehensive, with a metering capacity of
up to 100 A. However, it was soon found to be too limited
for the industrial or for the heavy-duty service domains
and this was without taking into account the fact that
monitoring often begins by measuring global consumption
at the point of energy input — requiring a capability to
measure up to 2000 A.
LEM developed the RT current sensor adapted to these
EMN devices, which provides the same flexibility of
installation as split-core current transformers from the
lower range, but with the same class 1 precision required
for the sub-metering field. The Rogowski coil, which has
long been noted for its ease of installation, offered the
right solution provided that its major drawback could be
overcome — that of inaccuracy caused by the sensitivity
to the position of the conductor inside the loop.
From theory to practice
A simple explanation of the Rogowski coil theory (“Die
Messung der magnetischen Spannung,” Archiv für
Elektrotechnik, 1912), is that it is a coil-winding that closes
back on itself, wrapping the conductor to be measured
like any toroidal-type current intensity transformer, the only
— but major— difference being that there is no magnetic
core. While Ampère’s theorem still applies, the equations
are slightly different because at the sensor output we find
that the voltage is in proportion, not to the primary current,
but rather to its derivative: U = M*di/dt. M is the mutual
inductance between the primary conductor and the coil,
which to some extent represents the coupling between
the primary and secondary circuits. All the difficulty in
obtaining good accuracy from this principle derives from
the fact that the simplified analytical expression of this
equation implicitly supposes perfect symmetry of the coil
(M must be constant). However, this is never the case in
practice, and we shall illustrate this by looking at the three
critical points that cause M to be variable.
• The density of the turns: The coil-winding must be
perfectly regular to ensure that the winding density
is uniform throughout. Turns that are not equidistant
create asymmetry, the effect of which is to cause
the coefficient M to vary according to the position of
the primary conductor. This induces a de facto error
resulting from the position of the cable or the busbar
to be measured, an error which increases the closer
the conductor is located to the area where the density
differs from the average spread value.
• The coil cross-section: In the same way as the
turns density, if the cross-section is not uniform all
along the coil surrounding the conductor, term M
will not be constant and an error is produced due
to the positioning of the conductor. In this case too,
the closer the conductor is located to a zone that
significantly differs from the average spread value,
the greater is the error.
RT series of Rogowski coil current transducers
• The coil clasp: A major advantage of the flexible
Rogowski coil is that it provides an extremity with
no electrical connection, the return signal being
wired back inside the coil. Here is a major source
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of asymmetry caused by the discontinuity in the
coil-winding which affects the turns density, when
the theory requires that the coil should be perfectly
continuous and homogeneous. This is by far the most
critical point and generates the greatest errors.
The reality of the figures
Until now, Rogowski coils have delivered at best a
2% positioning error. Added to this there are often
restrictions, which exclude the conductor from certain
zones inside the loop, in particular at the closure of the
clasp head. In reality this could even be quite catastrophic,
resulting in errors close to the clasp head in the order
of 6%. For this reason it is easy to understand why
manufacturers of energy metering equipment have
always avoided employing this technology. However,
LEM recognized the viability of this technology for energy
measurement but it depended on whether they could
manufacture coils which delivered a positioning error of
less than 0.75% as a minimum. In fact, the objective of
developing class 1 energy meters is to obtain an overall
accuracy of better than 1% over the entire measurement
chain, including the current sensor, the voltage sensor and
the signal processing.
Sensor head clasp implementing a new “magnetic sleeve”
for these unsuccessful attempts. Thanks to their expertise
in magnetics they have been able to develop a very simple
but effective solution... a sleeve made of magnetic material,
making it possible to make an entire zone around the coil
invisible (magnetically), and thus to mask the imperfections
on the closing mechanism as well as the connections of the
sensor’s secondary wires. The sleeve acts as a magnetic
short-circuit (or more precisely a reluctance short-circuit),
“virtually” bringing together the two sections of the coil
located on each side. Their approach was a complete
success — the error
associated with the
coil clasp has become
almost negligible.
Naturally enough, the
idea was the subject
of a patent application
in 2007.
The hidden
challenge
While the major
problem with the
split-core Rogowski
coil had finally been
solved, other problems
became apparent
Measurement error according to the position of the conductor within the loop: traditional Rogowski coil
which diminished
compared to the LEM RT
the success of the
magnetic sleeve. The
error
associated
with
the
design
of
the
coil clasp system
The challenge met by LEM
had previously been so important that it had, to some
Multiple solutions based on electrical or mechanical
extent, masked the other causes of asymmetry. LEM
concepts have been developed for nearly 100 years in
continued to work to improve this current sensor and after
order to resolve, albeit with very limited success, the main
a total of 2 years has been able to develop the processes
problem of the Rogowski coil current sensor, i.e. the error
and machinery that significantly reduce the symmetry faults,
caused by the imperfect sensor closure. Taking this into
both with regard to the regularity of the coil-winding and
consideration LEM engineers decided to revisit the theory
creating a uniform section over the entire length of the loop.
in greater depth in order to better understand the reason
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The results
The graph below illustrates the improvements that LEM
has been able to produce in the split-core Rogowski coil,
compared with the level of accuracy of the other products
on the market that are based on this technology.
Measurement error due to the conductor position within the loop: the new LEM RT
sensor compared to the traditional Rogowski coils
Today the error due to the positioning of the conductor is
specified at a maximum of 0.65% of the measured value
for a 15 mm diameter conductor irrespective of where it
is positioned, even if it is adjacent to the coil clasp. For a
better appreciation of the results that have been obtained,
here is another graph showing the maximum value of the
error over a sample of 210 RT Rogowski coils. Typical
value of the error due to the positioning is 0.31% of the
measurement for the new LEM sensor.
Distribution of the max. positioning error for a sample of 210 RT transducers
What we should also know about
Rogowski coil sensors
• External conductors
Performance of the Rogowski coil is
generally expressed in terms of error
associated with the positioning of the
conductor to be measured, but a good
sensor must also remain insensitive to
all other external conductors located
nearby. It happens that a relation exists
between these two characteristics a
perfect loop is perfect for both, and a
bad loop will be bad for both. This is
a result of Ampère’s theorem, which
states that any error associated with
any form of asymmetry is valid both
inside and outside the loop. Let us take,
for example, a conductor on which a
100 A current is circulating, situated
inside the Rogowski coil and which
is in contact with a section of the
loop inducing an error of +0.5%. A
measurement of 100.5 A is therefore
obtained. This same conductor at the
contact of the same section, but outside
the loop, will likewise cause an error of
0.5 A, but this is added to the current
measured inside the loop which results
from the principle of rejection of the
external magnetic fields.
• Absolute accuracy
In general, the absolute accuracy of
Rogowski coil sensors is low because
their gain (expressed by the term M)
depends on physical parameters that
are hard to control when it comes to
mass production. To summarize, it is
not realistic to try to manufacture this
type of sensor with a gain dispersion
of less than several percentage points
(say between 2 and 5% depending
on the technology used). This would
mean designing coil-winding machines
in which the pitch would be regulated
to an accuracy in the order of microns,
as well as being able to produce the
coil-winding base with the same level
of accuracy. It is therefore customary to
connect the Rogowski coil to an active
or passive electric circuit so that it can
be calibrated, and a good absolute
accuracy can be obtained.
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On the other hand it is essential to guarantee excellent
stability of the sensor characteristics, in particular with
regard to temperature, to prevent any drift from having to
be corrected by recalibration to compensate for changed
conditions during use. For example, LEM’s RT range has
proved to be excellent in this respect at 30 ppm/°C.
• Will not saturate
It is a recurring issue when specifying measuring
systems: will the sensor saturate if the current goes
above its nominal value? Of course in the case of the
Rogowski coil the answer to this is “no” since it does
not have a magnetic core, and as a result runs no risk
of saturation. In theory the limit of the current to be
measured is infinite! In practice it is the diameter of
the closed loop that establishes the nominal value of
the current, not in relation to the measuring capacity,
but rather in relation to the dimensions of the primary
conductor. In the specific case of high di/dt (impulsion)
the limit will be fixed by the voltage developed at the
coil terminals instead.
• Perfectly linear
Of course, linearity is important when a sensor is
intended for precision measurements. For the same
reason that no saturation takes place on the Rogowski
coil, it is not possible for there to be a lack of linearity,
since the coil is intrinsically perfect in this respect. If
differences were nevertheless observed, it would be
necessary to question the measurement methods and
not the Rogowski coil!
• No phase-shift
Phase-shift is an extremely important parameter
with respect to energy that is calculated from
measurements of currents and of voltages. In the
same way as for saturation and linearity, the Rogowski
coil is perfect with regard to the phase — which
means it induces no phase-shift. However it is worth
bearing in mind that it is necessarily associated with
an amplification stage (as described in the following
Application Note) which itself will generate a phaseshift. In conclusion, the phase error is intrinsically
zero when the coil is not connected, but it can reach
high values as soon as a load is connected. However
this error can be easily quantified by calculation
or by simulation of the equivalent RLC circuit, and
compensated for by an ad hoc method.
The choices made by LEM
Today, the Rogowski coil sensors can compete against
the best current intensity transformers in the energy
measurement sector. It became very clear that LEM would
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Installation of an EMN energy meter with 3 RT Rogowski coils in
an electrical cabinet
need to exploit the properties of this technology to the
maximum which could create a net benefit when measuring
high currents, i.e. the weight, overall dimensions, flexibility
and manageability. With a cross-section measuring 5 mm,
which could almost be classed as a “universal” size, the
sensors in the RT range are among the most slender
Rogowski coil sensors on the market.
The (patented) coil clasp device is also small (28 x 30 x
16 mm), and provides a reliable connection of the loop to
its coaxial signal cable. Here, the choice of a coaxial-type
cable is directly associated with the low cross-section
of the coil. In fact, since the gain is proportional to the
cross-section, a fine coil develops little voltage, and it
is appropriate to control the signal-over-noise ratio by
starting to eliminate all risks of interference between the
loop and the amplification stage.
Finally, to guarantee stability in time and temperature,
the RT coil is molded integrally into a PU resin, using
an original process developed by LEM engineers. This
wrapping technique also helps to maintain the different
sections firmly and imparts a robustness to the assembly,
as required for places where it is difficult to install.
So, current transformer (CT) or Rogowski coil (RT)? LEM
has already made its choice, but is prepared to share it
with you!
APPLICATION NOTE: Design of an integrator for the Rogowski coil
The Rogowski coil supplies a voltage in proportion to the derivative of the primary current at its terminals. An electrical
integrator circuit is therefore necessary to convert this signal into a signal that is proportional to the value of the primary current.
The integrator is an essential component in current measurement with the Rogowski coil, and the way this amplification
stage is implemented will have a major impact on the sensor’s electrical performance (linearity, phase-shift and frequency
bandwidth). A list of the various critical aspects of such an integrator, with some possible solutions, is given below:
• Very low signal level (for example
20 mV/kA for sensors in LEM’s RT range)
• The use of very low noise OpAmps is
recommended to optimize the signal/
noise ratio.
• It is necessary to try to minimize the
surface of the PCB or possibly to
shield the amplification stage to reduce
sensitivity to external fields.
• Low cut-off frequency
When an integrator is connected to a
Rogowski coil the two form a high-pass filter.
Since it will reject very low frequencies it is
necessary to define the cut-off frequency
in order to optimize performance at the
nominal operating frequencies, while still
obtaining the best possible response time.
• Rejection of offset
The main problem of a pure integrator lies
in the fact that it will integrate the slightest parasitic offset (e.g. due to the AmpOp), with the effect that the output will
always be unstable and will drift sooner or later to saturate at the upper or lower level. Consequently it is essential to
limit this drift, using a static gain or an active compensation stage:
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• Total offset rejection
It is possible to completely eliminate the residual offset by adding a capacitive coupling device between the integrator
and the measuring stage:
• Phase-shift
The offset rejection circuits described above will generate several degrees of phase error which poses a major
problem for the measurement of power. In this type of application, it is therefore necessary to add a phase-shift
compensation stage, which generally consists of a low-pass filter. Unfortunately, the correction will not be constant,
but will depend on the frequency, meaning it will be necessary to optimize the design to minimize the phase difference
at the fundamental frequency, typically 16-2/3, 50, 60 or 400 Hz.
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• Calibration: active adjustment of gain
A Rogowski coil requires calibration against a reference signal in order to fine-tune its gain which is never exact by
construction, due to inevitable imperfections in the manufacturing process. In general, engineers use the integrator
stage to which an analogue device, such as a potentiometer, is attached. The most recent digital calibration
solutions are more likely to use a microcontroller or the combination of a microcontroller and a PGA (programmable
gain amplifier). In all cases calibration is specific to each individual Rogowski coil which must always use the same
circuit with which it has been calibrated.
• Calibration: passive adjustment of gain
Historically, the Rogowski coil was used simply for measurement of the current effective value (rms) without phase
constraint. Many loops offered factory calibration based on a purely resistive or a resistive/capacitive circuit (RC
circuit). While this method continues to be straightforward and economical, unfortunately it does not lend itself
to power measurements due to the strong phase error that it generates, and its possible dependence on the
frequency (if an RC circuit is used).
When developing the new Rogowski coils, LEM decided to offer a simple and generic product, keeping in mind that the
integrator technology leads to the best performances and is a well known method. Therefore the transducers of the RT
family are not calibrated in the factory, do not use any additional electronic components or housings and do not require
power supply. Using an integrator specific to the device connected to the Rogowski such as energy, power quality or
pulse power monitor, is a cost effective and high performance solution.
For more information please contact Erik Lange, Marketing and Applications Engineer, at 414-577-4125 or [email protected].
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