AN2167
Application note
Using the STPM01 with a shunt current sensor
Introduction
This document describes how a shunt current sensor can be used with the STPM01
metering device in single-phase metering applications without tamper detection.
Special considerations must be taken into account when the shunt is designed:
■
■
The current density must be kept constant
The power dissipation must be reduced, as it can influence the ambient temperature
within the meter housing. In the event of large current overloads, this can become an
explosion hazard.
Note:
Special safety precautions must be followed while the meter is connected to the power line
and opened. The ground plane of the meter is at a high voltage level because the shunt has
to measure the current flow through a hot conductor without any galvanic isolation. This is a
safety hazard for the operators and can destroy the connected equipment.
For more information on the STPM01, refer to the full datasheet.
October 2008
Rev 2
1/5
www.st.com
Theory of operation
AN2167
1
Theory of operation
A shunt is a resistor intended for relatively high current levels. This means that Ohm's law
may be applied to it as a first-order approximation model and measure its response to the
primary current: (see
Equation 1).
Equation 1
u
S
=
i
p
⋅
R
S
The first problem is that R
S
is not constant, stable, or perfectly linear. It is affected by
changes in temperature and current density, so a special shape and material must be used
to:
1.
2.
keep all of the coefficients as low as possible
maintain low levels of mechanical expansion
The second problem is self-heating due to power loss in the conductive material: (see
Equation 2)
Equation 2
P
S
=
u
S
⋅
i
p
=
i
2
p
⋅
R
S
The second equation represents the prime limiting factor to the usage of a shunt sensor. For
example, if the user wants to use a 500 µΩ shunt sensor to build a class 0.5 direct meter
with I
NOM
/I
MAX
= 10/80A
RMS
, the voltage output level would be 40 mV
RMS
, and the power
dissipated in the shunt at I
MAX
would be 3.2 W.
However, this meter must be able to withstand an overcurrent of 5000 A
RMS
for 60 ms,
which yields 750 J of energy. This would melt the shunt and, consequently, generate an arc
and overheat the air in the sealed meter housing, causing the meter to explode.
To keep the shunt from melting, a higher volume of material must be used and power
dissipation needs to be reduced. Power dissipation can be reduced by decreasing the
resistance of the shunt. For a 210 µΩ shunt, the output voltage would be 16.8 mV
RMS
, the
power dissipated would be 1.344 W, and the energy level would be 315 J.
The third problem is the meter accuracy. For a class 0.5 direct meter, the accuracy of the
power and energy values produced by the shunt must be within 1% or better, at 5% of
nominal current (I
NOM
). For a 210 µΩ shunt, the sensitivity of the meter must be
±1.05 µV
RMS
.
This makes it more difficult to deal with electromagnetic interference (EMI) and new error
sources (e.g., Kelvin and Peltier effects) which become important, and may include stray
capacitances and mutual inductances, as well as noise generated by the preamplifier. The
shunt connection to the line and the STPM01 must be assembled very carefully in order to
minimize error source contributions.
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AN2167
Examples of sensor shunt usage in energy meters
2
2.1
Examples of sensor shunt usage in energy meters
Single-phase application without tamper
A shunt measures the current flow through the hot conductor of the line, so pin F becomes
the local ground of the meter and the larger resistor of the voltage divider is connected to pin
N of the meter. The voltage divider drop must be connected to the anti-aliasing filter input for
the voltage channel of the STPM01. Both voltage pins of the shunt should be connected to
the anti-aliasing filter input for the primary current channel of the STPM01, while the
secondary current channel may be connected to local ground.
All of the connections to the anti-aliasing filter inputs should be implemented with the
shortest wires possible, and with the smallest area of loop possible (without any other signal
in the vicinity), formed by the following:
●
●
●
source impedance element
both wires
input impedance of the filter
The STPM01 device should be configured with PST = 3 and ADDG = 1 (settings for the
shunt current sensor, current amplification factor 32, and no tamper detection).
This type of metering application can operate as a standalone with a stepper display or a
microcontroller may be implemented in order to access more measured results in the
STPM01 using the LCD, automatic meter reading (AMR), or tariffing functions.
2.2
Single-phase application with tamper
It is not possible to connect two shunts to the STPM01 because one would generate its
output at the high voltage level. Therefore, a current sensor with integral galvanic isolation
must be used for the primary current channel, while another shunt is used for the secondary
channel or vice-versa.
Although a shunt produces an output voltage which is proportional to the primary current
like a current transformer (CT) does, it is difficult to combine a CT with a shunt in single
phase metering applications with tamper detection due to differences in phase error
indicators and output voltage levels. These differences must be resolved with components
outside of the STPM01.
2.3
Multi-phase application with or without tamper
It would be cost-prohibitive to use three or four shunts with three or four STPM01 devices,
plus the necessary three galvanic isolations and additional power supplies, as well as a
microcontroller to achieve the objectives of a three-phase meter design. The other problem
would be the excessive levels of power dissipation produced by three shunts operating at
maximum phase currents.
3/5
Revision history
AN2167
3
Revision history
Table 1.
Date
15-Feb-2006
27-Oct-2008
Document revision history
Revision
1
2
Initial release
Document reformatted. No content change.
Changes
4/5
AN2167
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