Part of the intelligence in today’s smart grid is being able to monitor the grid accurately. What this means in real terms is measuring several parameters in the grid, including currents and voltages. Current measurement is a critical requirement in protection, substation automation and power quality. Different types of current sensors are used to measure current. The most common sensor technologies today are the current transformer (CT) and the low resistance current shunt.
A current transformer (CT) is a transformer which converts the primary current into a smaller secondary current. The CT is the most common sensor used for measurement and can measure up to a very high current while consuming very little power.
The low resistance current shunt offers good accuracy at a lower cost, and the current measurement is simple. When performing a high-precision current measurement, one must consider the parasitic inductance of the shunt.
When multiple channels need measurement, some of the benefits of replacing a CT-based solution with a shunt-based solution include a reduction in solution size, a reduction in solution weight, mitigation of cross talk, and potential increase in product life through lower mechanical issues by replacing the CT with a shunt.
One challenge with using a shunt for current measurement is that the isolation that a CT provides is not available anymore when replacing it with a shunt. Another challenge when using shunts is the ability to get very accurate (below 1%) current measurements across a wide dynamic range.
TI’s reference design on Isolated Shunt based Current and Voltage Sensing for Protection Relays (TIDA-00080) shows how the above mentioned challenges are overcome. To do this, the reference design uses the AMC1304 which is a 16-bit delta-sigma (ΔΣ) modulator with the output separated from the input circuitry by a capacitive isolation barrier that is highly resistant to magnetic interference. This barrier is certified to provide reinforced isolation of up to 7000 VPEAK according to the UL1577 and VDE V-0884-10 standards. The output of the modulator is passed to a C2000™ Delfino™ TMS320F28377 microcontroller (MCU).
This solution enables an accuracy of 0.25% for current ranges from 0.5A to 10A and an accuracy of 1% from 10A to 200A. To achieve an extended range of current measurement, a provision to cascade multiple current channels is provided. The reinforced isolator is capable of providing up to 10 kV of surge related isolation. Voltages from 90V to 270V can also be measured with this solution.
More information can be found at www.ti.com/tool/TIDA-00080.
