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The World Leader in High Performance Signal Processing Solutions
Current Sensing for EnergyMetering
By William Koon
Energy Measurement GroupPrecision Converters (PRC) Division
http://www.analog.com/energymeter
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Background
1. Solid-state energy meter have gained considerable market acceptance. In
some countries, solid-state meters have completely replaced electromechanical
Meters
2. Energy meters are often subject to hostile operating conditions. The
meters need to remain operational for a very long life-time
3. Most todays advanced solid-state meters adopt mixed-signal architecture,
with A/D converter front-end and DSP back-end. This architecture ensures
good accuracy and reliability for a meters long operating life
4. All solid-state meters contain voltage and current sensing elements.
Current sensing is a more difficult problem because of the wide load
variation and rich harmonics contents in the current
5. Maximum current consumption per household will continue to increase. It
is reaching the point where todays current sensing solution is no longer
Adequate
6. Rogowski coil has been traditionally used in many high current measuringapplications. Recently a digital solution has the promise to make this
current sensor feasible in energy meters
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Energy meter based on the energy
measurement ASIC
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Inside an Energy Measurement ASIC
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Todays Common Current Sensing
Solutions
1. Current shunt
2. Current Transformer
3. Hall-Effect Sensor
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Introduction to Current Shunt
Current Shunt is a resistor with resistance as low as 100
Parasitic inductance (typically in the order of a few nH) could
introduce significant phase shift to cause large measurement error
at low power factor
Current shunt should be made physically large to handle large
current (for heat dissipation)
Self-heating problem of this resistive element makes it unsuitable
for large current measurement
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Introduction to Current Transformer(CT)
The most common solutions for measuring high current
Typically have an intrinsic phase shift of 0.1 to 0.3
Iron core can saturate at current level beyond its rated current or at
a large DC. Once magnetized, its accuracy will degrade
The solution to combat saturation: use high permeability material
core but it leads to poor phase characteristics over current and
temperature
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Rogowski Coil
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Current Induces Magnetic Field
When current passes through a conductor, magnetic field is formed
around the conductor. The magnitude of magnetic field is directly
proportional to the current:
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Voltage induced in a wire loop by
Changing Magnetic FieldThe changes in magnetic field induces a electromotive force (EMF)
within a wire loop. The EMF is a voltage signal and is proportional to
the changes in the magnitude field inside the loop.
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Voltage Output Proportional to di/dt
Combining the two formulas:
The output voltage of the loop is therefore proportional
to the time-differentiation (di/dt) of the current
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An example of di/dt sensor
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Canceling External Interference 1:
Reducing undesirable loop area Rogowski coil relies on conductor loop to detect changing
magnetic field
Conductor loop will pick up both signals and interference
Look for and minimize undesirable loop in the sensor
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Canceling External Interference 2:
Design with Interference Cancellation Interference is usually far field in nature
Conductor loop will pick up both signals and interference
Design the Rogowski coil so that far field interference will
cancel while the near field signal will remain
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Canceling External Interference 3:
Shielding
Shielding can be used as the last resort
Shielding low frequency magnetic field is difficult; it
requires thick shielding or high permeability material
Shielding is not a must if design properly
Minimizing unwanted loop and far field interference
cancellation will provide much more benefits than
shielding along
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di/dt sensor has all advantages of CT
di/dt sensor has the same advantages of CT:
Electrical isolation
The di/dt sensor detects current without any contact to the conductor
Capable of handling high current
di/dt is capable of handling very high current
Low power consumption
No significant power consumption needed for current sensing
Low temperature shift
The output varies very little with changes in temperature
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But better
di/dt sensor outperforms CT in many ways:
No DC or high current saturation
The saturation point of air is extremely high comparing with ferrite core
Light weight
No need to use heavy ferrite core
Phase characteristics
Air core has linear phase response (90 phase shift for sinusoidal signal)
Low cost
Even a small di/dt sensor can accurately detect hundreds of Amperes of current
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Comparison between different current
sensing technologies
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Building an Integrator
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Why a digital integrator?
Advantages of digital implementation:
Stability of performance
The operating condition of an energy meter is very hostile. Digitalprocessing provides reliable performance over the long operating life
required. Analog components are susceptible to drift over time and
temperature which will result in large measurement error.
Outstanding phase characteristics
Analog components can introduce slight phase variation and would
require phase calibration. A digital integrator has consistent and accurate
phase response.
Reduced cost
No external component (high performance op-amp) is needed to
implement the integrator.
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Building an integrator: using op-amp
An analog integrator using an op-amp:
Error includes: offset error, leakage current, long-term
stability of op-amp and temperature drift
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Building an integrator: digital integrator
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Digital Integrator: an example
Digital Integrator in Analog Devices ADE7759 Energy Measurement ASIC:
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Performance of Rogowski coil with
digital integratorLess than 0.1% error over 1000:1 (60dB) measurement dynamic range
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Conclusion
Metering ASIC simplifies meter design tremendously and
new generation meters will incorporate more new
Functionalities
Energy consumption per household will continue to rise.
Increasing line voltage is not an option because of safety
concern. Therefore, maximum current consumption per
household will increase
Todays current sensing solution is not adequate for
measuring high current
Rogowski coil has been successful used in many high
current measuring applications outside of metering
Digital integrator removes the roadblock for more widelyadoption of Rogowski coil in residential energy meter