New Developments in Current Sensors in Solid State Meters
In recent years, there has been a rapid incr-ease in the market share for the solid state energy meter. Tens of millions of solid state meters are deployed annually worldwide.
Modern solid state electric meters contain both voltage and current sensing elements. Voltage sensing is typically achieved by dividing down the line voltage using a resistor divider or a potential transformer when isolation from the line is required. However, current sensing is a much more difficult problem. Not only does the current sensor require a much wider measurement dynamic range, but it also needs to handle a much wider frequency range, because of the rich harmonic content in the current waveform.
Next generation energy meters must have high current handling capability. In the US residential market, for example, the maximum current in many houses is becoming as high as 200 A. Today’s popular sensing technologies are not well-suited for the next generation meters. Meter manufacturers need to find a new current sensor that can satisfy all the requirements and incur little or no cost penalty.
The Rogowski coil relies on measuring the magnetic field changes around the current carrying wire to produce a voltage signal, which is proportional to the derivative of current (di/dt), and an integrator is needed to convert the signal to the appropriate signal. The task of creating an integrator that is stable and accurate over a long metering lifetime used to be daunting. However, recent digital implementation of the integrator has the promise of making this technology a reality in electric meters.
For comparison purpose, we first summarise the three most common sensor technologies: the low resistance current shunt, the current transformer (CT), and the Hall Effect sensor.
Low Resistance Current Shunt
The low resistance current shunt is the lowest cost solution available today and it offers simple current measurement and superb accuracy. A model for this device is a resistor. When performing high precision current measurement, one must consider the parasitic inductance of the shunt, and although this affects the magnitude of the impedance at relatively high frequency, its affect on phase at line frequency is enough to cause noticeable error at low power factor. A phase mismatch of 0.1° will lead to about 0.3% error at power factor 0.5.
The low cost and high reliability make the low resistance current shunt a very popular choice for energy metering. However, because the shunt is fundamentally a resistive element, the power loss is proportional to the square of the current passing through it and consequently it is a rarity among high current energy meters.
The current transformer (CT) uses the principle of a transformer to convert the high primary current into a smaller secondary current.
The CT is common among high current solid state energy meters. It is a passive device, with no extra driving circuitry needed to use it. Furthermore, it can measure up to very high current and consumes little power. However, saturation of the ferrite material used in the core can occur when the primary current is very high or when there is a substantial DC component in the current. Once magnetised the core will contain hysteresis and the accuracy will degrade unless it is demagnetised again.
Hall Effect Sensor
There are two main types of Hall Effect sensors, open-loop and closed-loop. The latter offers better accuracy and wider dynamic ranges but at higher cost, and most Hall Effect sensors found in energy meters use open-loop design for lower system cost. The Hall Effect sensor has outstanding frequency response and is capable of measuring very large current. However, the drawbacks include the output having a large temperature drift and the requirement for external driving circuitry. These, coupled with the relatively high cost, make Hall Effect sensors somewhat rare compared with the CT.
A simple model of the Rogowski coil is an inductor with mutual inductance with the primary current (Figure 1).
If current i(t) passes through a long straight wire on the z-axis, the magnetic field at a random point P which has coordinates (r, q, z) in cylindrical coordinates is:
The electromotive force (EMF) generated by the magnetic field in any area in space can be calculated using Maxwell’s equation:
The constant term M is called the mutual inductance of the Rogowski coil, and has a unit Henry (H). It indicates the signal level from the output of the coil per unit di/dt. The voltage output of the coil relies only on the changes in primary current. When interfacing with an IC that has on-chip digital integrator, building a meter with a Rogowski coil is just as simple as using current sensors such as the CT or current shunt. The air-core coil has no hysteresis, saturation, or non-linearity problem. In addition, it has outstanding ability to handle large current where the theoretical upper limit of the coil is the breakdown voltage of air.
As the output from the Rogowski coil is proportional to the time derivative of the current, an integrator is needed to convert this back to the format i(t). In the frequency domain, this is equivalent to a –20 dB/dec attenuation and a constant
–90° phase shift. Figures 2a and b are the frequency response and phase response of the digital integrator implemented in Analog Devices’ ADE7759 energy measurement IC.
As can be seen, the phase and magnitude responses of a digital integrator are very close to ideal. The added benefit of the digital implementation is that it is more stable over time and environmental changes. These are very important for energy metering applications because of the hostile operating condition and long operation life of the meter. In-house experimental results show outstanding accuracy over a very wide dynamic range.
The popularity of the solid state meter has stimulated much interest in finding a new current sensing technology which can measure very large current without DC saturation. The Rogowski coil combined with a digital integrator offers a cost-competitive solution and could become the technology of choice for the next generation electric energy meters.