current sensing for meteringnew

7/30/2019 Current Sensing for MeteringNew

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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
http://www.analog.com/energymeterhttp://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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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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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

current sensing for meteringnew

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