The market has several instruments for accurately measuring small DC currents, say up to 3A. You can also find some devices that can measure DC currents that extend beyond 50A with good accuracy. Large currents are common in photovoltaic renewable energy installations, grid energy storage, electric vehicles, to name a few. Usually, it is a common necessity for such systems to be able to predict accurately the state of charge or SOC of the associated energy storage batteries.
Usually, systems for current or charge measurements are designed to include built-in data acquisition modules such as ADCs or analog to digital converters, filters and suitable amplifiers. The arrangement is typically that of a current sensor followed by a filter/amplifier and finally an ADC. The current sensor senses the current a circuit for converting the output into a usable form such as voltage, typically follows it. The signal requires filtering to reduce the radio frequency and electromagnetic interferences. The cleaned signal may have to be amplified before being digitized. Current data samples multiplied by the appropriate time interval are accumulated for charge values.
Two sensor technologies are commonly used for measurement of large currents. The first of these techniques measures the voltage drop across a resistor (also called a current shunt) that carries the current to be measured. The voltage drop follows Ohm’s law and equals the product of the current times the resistance.
Large DC currents may cause power bus bars and cables to dissipate significant amounts of heat. As a thumb rule, designers of power installations strive to achieve less than 1% power loss from the wiring, including bus bars and heavy cables. For example, an offline storage system of batteries with output of 1KV and 1KA supplies power at 1MW. Although the dissipation of a 50W shunt is insignificant at 0.005%, the power cables and bus bars may dissipate heat upwards of several KW.
To put things in perspective, designers go by 1W per µOhm at 1KA, therefore, for a shunt with 10 µOhm resistance, a continuous current of 1KA passing through it will heat it up to 10W. Alternately, copper wire, with a diameter of one-inch, will be dissipating 12-14W of heat at 1KA for each foot, since the resistance of the wire is about 10 µOhm per foot, after correcting for resistance increase due to heating.
The second technology senses the magnetic field encircling the current carrying conductor. The device for sensing the current is generally known as the Hall-Effect current sensor. Usually, the magnetic field around the current carrying conductor is concentrated in a magnetic core, which has a thin slot and the Hall element resides here. The magnetic field is thus perpendicular to the plane of the Hall element, while the magnetic core makes it nearly uniform. Energizing the Hall element with an exciting current makes it produce a voltage proportional to the magnetic field in the core and the exciting current. This voltage, suitably amplified and filtered, is presented to the ADC.
One advantage of the second technique using Hall elements is the isolation between the current carrying conductor and the measuring electronics. Since the coupling is only magnetic, the current carrying conductor may have very high voltage potentials, which do not affect the current measuring elements.
