cd00289924

AN3306
Application note
Current sensing in metering applications using a
Pulse current sensor and ST metering devices
Introduction
This application note describes the benefits of a current sensing system for metering
applications using STPMxx metering devices and a current sensor developed by Pulse
Engineering Inc. (hereafter referred to as “Pulse current sensor”), based on the Rogowski
coil principle. Following an overview of the Rogowski coil principle, the Pulse current sensor
is introduced along with a comparison to other current measuring devices. This is followed
by a presentation of the characteristics of the STPMxx family of metering devices, and the
results of accuracy testing conducted using a demonstration board with the STPM01 and
the Pulse current sensor.
The results obtained from the accuracy testing conducted with the STPM01 can be
considered valid for all devices in the STPMxx family that share the same architecture(a).
In Figure 1 below the measuring system block diagram is provided.
Figure 1.
Block diagram of a Rogowski coil-based application with the STPM01
AM00178v1
a. Excludes only the STPM10
November 2010
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www.st.com
Contents
AN3306
Contents
1
Overview of the Rogowski coil principle . . . . . . . . . . . . . . . . . . . . . . . . 4
2
The Pulse current sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
Comparison of current measuring devices . . . . . . . . . . . . . . . . . . . . . . 6
4
Overview of the STPMxx metering device family . . . . . . . . . . . . . . . . . . 8
5
Benefits of using STPMxx with Rogowski coil-based sensors . . . . . 10
5.1
Power calculation algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2
Mutual current compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6
Operation of the Pulse current sensor with the STPM01 . . . . . . . . . . 14
7
Accuracy results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8
Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
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AN3306
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Block diagram of a Rogowski coil-based application with the STPM01 . . . . . . . . . . . . . . . . 1
Rogowski coil principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
The PA2999.006NL current sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Pulse current sensor adapted for a flat buss bar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Traditional power calculation approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
STPMxx power calculation approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Test board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Test configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Accuracy results vs. current and accuracy limit standards . . . . . . . . . . . . . . . . . . . . . . . . . 17
Cable in a diagonal position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Accuracy results vs. current comparison between axial and diagonal position . . . . . . . . . 19
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Overview of the Rogowski coil principle
1
AN3306
Overview of the Rogowski coil principle
As illustrated in Figure 2, the Rogowski coil principle states that a conductor carrying an AC
current i(t) and passing through a helical coil, induces a voltage across the coil that is
proportional to the rate of change of the current (di/dt) in the inductor.
Figure 2.
Rogowski coil principle
!-V
The voltage v(t) is a function of winding factor (Kr) and the frequency (Fr) of the sinusoidal
waveform i(t).
Equation 1
Kr is determined by the winding characteristics such as cross-sectional area (s), the number
of turns per unit length (n) and the symmetry of the coil. An integration of v(t) gives a
measure, proportional to Kr, of the instantaneous RMS current in the conductor.
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2
The Pulse current sensor
The Pulse current sensor
While the Rogowski coil principle is well-known and has been widely implemented in various
current sensing devices, the engineering challenge has been to control the winding
characteristics to achieve the accurate current measurements required for metering
applications. Pulse Engineering Inc. has developed a precision winding technique that
controls the parameters which directly influence the output voltage. A patent-pending
segmented winding approach allows for a high number of winding turns per unit length to
provide a sufficiently large output voltage for detection and integration.
The Pulse current sensor is an air coil winding which has a highly linear output voltage over
a very wide dynamic current range, meeting the Class 0.2 S accuracy limits defined by the
IEC 62053-22 meter standard for currents from 0.1 A to 200 A. A specially designed winding
configuration meets Class 1 requirements for immunity to external magnetic fields. An
additional Faraday shield over the winding prevents electrostatic voltage coupling from the
AC voltage of the conductor. This also acts as an effective barrier against external electrical
fields associated with nearby current-carrying conductors and automatic meter-reading
radio signals.
Pulse has implemented this winding technique in a highly automatable, low-cost standard
product, the PA2999.006NL, shown in Figure 3. It is lightweight and, due to the Rogowski
coil as a voltage source, is a zero power-consumption device with a stable voltage over a
wide temperature range. Further information on this product is available at
www.pulseeng.com
Figure 3.
The PA2999.006NL current sensor
!-V
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Comparison of current measuring devices
3
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Comparison of current measuring devices
The PA2999.006NL was developed as an alternative to the current transformer (CT)
typically used in metering applications. The CT is wound on an amorphous metal core and
therefore the operating principle is different. Here, the AC primary current passing through
the center of the CT is closely coupled with the core and induces a current in the
transformer winding. This current is used to drop a voltage across a terminating resistor
which is a direct measure of the primary current. The PA2999.006NL can be used as an
alternative to the CT when used with a metering IC which supports di/dt current sensors,
such as a device from the STPMxx family.
The absence of an amorphous core in the Pulse current sensor offers several advantages.
Lighter in weight and lower in cost, it has exceptional linearity over a wide current range. The
upper current is limited only by the self-heating effects of the primary conductor. In
comparison, the core of the CT needs to be sized to avoid saturation at the maximum
current. For high current, this can be large and expensive. The amorphous material does not
provide the same linearity (with current, frequency and temperature) and can have a
remnant magnetic field that produces a DC offset. The core limits the frequency response of
a CT to less than 8 harmonics, while the current sensor can accurately detect up to 100
harmonics for detailed load fingerprinting. Without all the limitations of a CT, a Pulse current
sensor can simplify meter calibration.
In addition, the core determines the geometry of the CT. The Pulse current sensor
presented here is toroidal only for the benefit of a fit, form and function comparison to the
CT. The shape of the Pulse current sensor can be adapted to suit the typically flat conductor
buss bar used in a meter, as illustrated in Figure 4. It is also possible to make it open-ended
so it can be clamped directly to the buss bar rather than passing the buss bar through it.
Figure 4.
Pulse current sensor adapted for a flat buss bar
!-V
Another current measurement device is the low resistance current shunt, typically the
solution of choice for low current measurement. Its main advantage is cost. However, when
isolation from the current-carrying conductor is required, it should be noted that the addition
of an isolation transformer raises the cost of this solution to a level comparable to the Pulse
current sensor option. Limitations of the shunt include its continual power consumption,
temperature rise, and current limitation, as well as output variation over temperature.
Some metering applications favor the use of a Hall effect sensor as a low-cost integrated
solution. However, the time and cost to develop this can be extensive. As a discrete solution,
the Hall effect sensor is rather expensive.
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Comparison of current measuring devices
Table 1 summarizes the strengths and weakness of the different current measurement
techniques under consideration.
Table 1.
Comparison of current measurement devices
Pulse current
sensor
Current
transformer
(CT)
Hall effect
++
0
−
++
Wide range - 5 decades
0
0
+
++
Wide bandwidth
+
0
0
++
No DC saturation
++
−
−
++
Low temperature coefficient
0
+
−
++
High electrical isolation
−
++
0
++
Low power consumption
−
+
0
++
Output voltage
++
++
−
0
Low cost
++
0
−
+
Light weight
+
−
+
++
Flexible size & shape
−
−
+
++
Characteristic
Linear amplitude & phase
Shunt
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(di/dt)
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Overview of the STPMxx metering device family
4
AN3306
Overview of the STPMxx metering device family
The STPMxx is a family of energy metering ASSPs (application specific standard products)
designed to address a wide range of electricity metering requirements, thanks to built-in
features including signal conditioning, signal processing, data conversion, input/output
signals and voltage reference.
These devices are designed for effective measurement of active, reactive and apparent
energy in a single- or poly-phase system using Rogowski coil-based sensors, current
transformers or shunt sensors, and can be implemented as a single-chip energy meter or as
a peripheral measurement system in a microcontroller-based energy meter.
The STPMxx devices consist, essentially, of an analog part and a digital part. The analog
part is composed of preamplifier and 1st order Σ/Δ A/D converter blocks, band gap voltage
reference, and low drop voltage regulator. The digital part is made up of system control,
oscillator, hard-wired DSP and SPI interface.
A hard-wired DSP unit computes the amount of consummated active, reactive and apparent
energy, RMS, and instantaneous values of voltage and current. The results of the
computation are available as pulse frequency and states on the digital outputs of the device
or as data bits in a data stream, which can be read by means of the SPI interface.
It is also possible to generate an output signal with pulse frequency proportional to energy,
allowing for simpler calibration.
The STPMxx devices have been developed to address all the features and cost
requirements of any metering application. Their features are summarized in Table 2 below.
Table 2.
STPMxx device features
Device
Features
Current measuring devices
(C: current transformer, S: shunt,
R: Rogowski coil-based)
Pulsed output
SPI output
OTP memory
Current channel gain
Mutual current compensation
Serial port
STPMC1
STPM01
STPM11
STPM12
STPM13
STPM14
C,S,R
C,S,R
C,S,R
C,S,R
C,S,R
C,S,R
9
9
9
9
9
9
9
9
9
9
X
X
X
X
9
9
9
9
Set by
STPMSx 8-16-24-32 8-16-24-32 8-16-24-32 8-16-24-32 8-16-24-32
modulators
9
9
N/A
9
N/A
N/A
N/A
N/A
Used only to program and calibrate the device
Active energy
Total and
per-phase
9
9
9
9
9
Fundamental active energy
Total and
per-phase
9
9
9
9
9
Reactive energy
Total and
per-phase
9
X
X
X
X
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Table 2.
Overview of the STPMxx metering device family
STPMxx device features (continued)
Device
Features
STPMC1
STPM01
STPM11
STPM12
STPM13
STPM14
Apparent energy
X
9
X
X
X
X
VRMS, IRMS
9
9
X
X
X
X
Frequency pulse selection
9
9
9
9
9
9
Digital calibration
9
9
9
9
9
9
Pulses/kWh selection
9
9
9
9
9
9
RC oscillator
X
9
9
X
9
X
Quartz oscillator
9
9
X
9
X
9
Tamper detection
9
9
X
X
9
9
Negative power, no load
9
9
9
9
9
9
Phase delay calculation
9
N/A
N/A
N/A
N/A
N/A
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Benefits of using STPMxx with Rogowski coil-based sensors
5
AN3306
Benefits of using STPMxx with Rogowski coil-based
sensors
Using a Rogowski coil-based sensor like the Pulse current sensor together with the STPMxx
presents multiple benefits when compared to competitors’ approaches. These benefits are a
result of:
5.1
●
a proprietary power calculation and digital signal processing algorithm developed
specifically for Rogowski coil-based sensors
●
the capability of mutual current compensation when multiple sensors are used
Power calculation algorithm
In the traditional power calculation approach shown in Figure 5, when a Rogowski coilbased current sensor is used, an additional analog integrator is required to transform the
di/dt signal from the transducer into a signal proportional to the measured current i.
Figure 5.
Traditional power calculation approach
VT
VT
0'!
VT
(0&
!$#
PT
,0&
PT
VCOIL T
/UTPUT0OWER
IT
6)COS ϕ
!$#
/UTPUTPOWER
IT
6)COS ϕ
!TTENUATED
!#
!-V
Equation 2
Equation 3
10/21
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Benefits of using STPMxx with Rogowski coil-based sensors
Equation 4
This leads to an offset error in power measurement and to a residual sinusoidal ripple at
twice the line frequency after the LP filtering. Real power should be averaged within a line
period.
The STPMxx family implements a new proprietary algorithm for power calculation that
removes this ripple and the offset by digital cancellation, providing an accurate and flat
power calculation without the need for an additional integrator in the analog section.
Figure 6.
STPMxx power calculation approach
AM00183v1
v(t)
v(t)
11-bit
ADC
v(t)
v dt
Signal
conditioning
16-bit
p2(t)
di/dt
di/dt
PGA
ADC
Signal
conditioning
i(t)
Subtractor &
:2 divider
p1(t)
p(t)
16-bit
Real power
V.I.cos ϕ/
Referring to Figure 6, the device calculates, then subtracts, and then divides by two the
following powers:
Equation 5
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Benefits of using STPMxx with Rogowski coil-based sensors
AN3306
Equation 6
The final result is:
Equation 7
This makes the use of STPMxx devices with Rogowski coil-based current sensors, such as
the Pulse current sensor, advantageous. When a Rogowski coil-based sensor is used, a
very high degree of accuracy is obtained by design due to the following:
5.2
●
DC offset cancellation and the power calculation algorithm produces a DC component,
proportional to the active power, without any offset or AC ripple.
●
Ripple-free power calculation, so integration of power over the line period is avoided,
and thus energy accumulation is not affected by fluctuation of the line frequency.
●
The architecture makes the STPMxx devices ready for Rogowski coil-based sensors
without the need for an additional integration block, which would increase system
complexity and overall application cost.
Mutual current compensation
For poly-phase systems, where galvanic isolation between phases is a must, and immunity
to DC magnetic fields is becoming a requirement in international standards, a Rogowski
coil-based sensor offers an interesting and inexpensive solution.
The drawback of this approach is cross influence between current channels.
The STPMC1, a dedicated device for poly-phase measurement, features embedded
functionality which allows error compensation from mutual currents.
For single-phase systems, two correction factors, α (alpha) and β (beta), produce a ±3.1%
correction factor in 512 steps. Asymmetrical compensation is implemented by multiplying
the phase current with α and neutral current with β, and these values are then subtracted
from neutral and phase currents, respectively, as shown in Table 3 and Equation 8 and
Equation 9.
Table 3.
Mutual current compensation matrix for single-phase systems
Phase
S
T
S
−
β
T
α
−
Equation 8
I#3
12/21
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I4
AN3306
Benefits of using STPMxx with Rogowski coil-based sensors
Equation 9
I#4
I3
For poly-phase systems, three correction factors, a 7-bit •, 6-bit • and 4-bit • (gamma)
respectively introduce a ±0.78%, ±0.39% and ±0.09% correction factor.
From these factors, a 4 x 4 matrix, shown in Table 4, implements symmetrical compensation
multiplying each phase and neutral current with its row, adding the products together and
subtracting them from the currents (Equation 10, Equation 11, Equation 12, and Equation
13).
Table 4.
Mutual current compensation matrix for three-phase systems
Phase
R
S
T
N
R
−
α
β
γ
S
α
−
α
β
T
β
α
−
α
N
γ
β
α
−
Equation 10
I#2
I3
I4
I.
I#3
I2
I4
I.
I#4
I.
I3
I2
I#.
I4
I3
I2
Equation 11
Equation 12
Equation 13
Doc ID 18182 Rev 1
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Operation of the Pulse current sensor with the STPM01
6
AN3306
Operation of the Pulse current sensor with the
STPM01
Accuracy testing was conducted with the PA2999.006NL sensor using an STPM01
demonstration board. Its schematic diagram is shown in Figure 7 below.
Figure 7.
Test board schematic
6/40
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14/21
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AN3306
Operation of the Pulse current sensor with the STPM01
The sensor outputs are connected through a crosstalk network, which reduces the influence
of the voltage channel on the current channel, to input pins IIP1 and IIN1.
In order to operate with a Rogowski coil sensor and obtain x32 amplification on the current
channel, the following configuration bits in the STPM01 must be set:
–
Bit 5 • PST = 1 (Rogowski coil sensor)
–
Bit 52 • ADDG = 1 (additional gain)
The board was calibrated at IN = 5 A.
Doc ID 18182 Rev 1
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Accuracy results
7
AN3306
Accuracy results
Three different sensors were tested in the configuration shown in Figure 8. The nominal
current IN is 10 A, and the accuracy was tested down to 1% IN at PF=1.
Figure 8.
Test configuration
!-V
The results of the accuracy tests are shown in Table 5 below.
Table 5.
Accuracy results vs. current
I [A]
% IN
S1 error %
S2 error %
S3 error %
10
100%
0.0000%
0.000%
0.0000%
1
10%
-0.0640%
0.046%
-0.0100%
0.5
5%
-0.0040%
-0.034%
0.1568%
0.1
1%
-0.8436%
-0.951%
0.2211%
In Figure 9, the results of accuracy testing at the different currents are shown graphically for
the three sensors, together with the accuracy limits for Class 1 and 0.5 meters (in
accordance with international standards IEC 62053-21 and IEC 62053-22).
16/21
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Accuracy results
Figure 9.
Accuracy results vs. current and accuracy limit standards
%RROR
#LASS LIMITS
3ENSOR
#LASS LIMITS
3ENSOR
3ENSOR
OF)N
!-V
As can be noted from the results in the graph, there is a very high degree of accuracy for all
sensors, fulfilling class 0.5 specifications even at very low currents.
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Recommendations
8
AN3306
Recommendations
Because the current sensor is wound on a dielectric support, the positioning of the spires
with respect to the cable has a relevant impact on performance in terms accuracy. Placing
the spires of the sensor in an axial position with the cable in the middle, as shown in
Figure 8, produces a higher sensitivity. Additional tests were conducted for current sensor 1,
placing it in a diagonal position with respect to the current cable.
Figure 10. Cable in a diagonal position
!-V
The results are shown in column 4 of Table 6, and are compared to the accuracy error of the
same sensor with the cable in an axial position (column 3).
Table 6.
18/21
Accuracy results vs. current comparison between axial and diagonal
position
I [A]
% IN
Error % - axial
Error % - diagonal
10
100%
0.0000%
0.0000%
1
10%
-0.0640%
-0.0409%
0.5
5%
-0.0040%
-0.0214%
0.1
1%
-0.8436%
-1.0730%
Doc ID 18182 Rev 1
AN3306
Recommendations
Figure 11. Accuracy results vs. current comparison between axial and diagonal
position
%RROR
2,0%
Class 1 limits
Class 0.5 limits
1,0%
Axial position
0,0%
-1,0%
Diagonal position
-2,0%
1%
10%
% of In
100%
!-V
As illustrated in the graph above, changing the cable orientation from an axial to a non-axial
position increases the transfer sensitivity, especially when the cable contacts the sensor
body.
Moving the cable within the center-passing hole does not result in an appreciable lack of
performance.
Attention should also be given to proper shielding, which can prevent errors on the
measured current resulting from the high potential of the cable coupling with the sensor. It
must be connected to ground.
Connecting the shield wire of the sensor (the white wire, if using the PA2999.006NL sensor)
to the active signal input (hot), rather than a GND-referred input increases the error at low
currents, and could cause external noise, further affecting accuracy.
The accuracy tests were also conducted with the shield connected to hot. However, even
with this erroneous positioning, the lack of accuracy was very small and the error still far
below Class 1 requirements.
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Revision history
9
AN3306
Revision history
Table 7.
20/21
Document revision history
Date
Revision
05-Nov-2010
1
Changes
Initial release.
Doc ID 18182 Rev 1
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