The growth in energy supply from renewable sources: solar, wind, and wave or tidal power, is also prompting an increased demand for more accurate high-current measurement solutions. The need for improved accuracy over more conventional electricity metering devices is to ensure the warranted performance of the various elements of a distributed power system. A return to the principles of traditional ammeters that employ shunt resistors for high-current handling appears to be the answer. As a professional shunt resistor manufacture, Microhm Electronics provides full size traditional shunts as well as larget current ammter shunt resistor FL-2 series.
 

 

The DC nature of renewable sources, particularly in distributed systems, demands the means to measure DC as well as AC power. Unfortunately, this rules out current transformers, which are extensively used for AC power measurement and also provide the benefit of galvanic isolation from the circuit being measured. Instruments based on Hall-Effect current transducers can measure DC as well as AC, and provide galvanic isolation, but do not typically provide the required measurement accuracy down to 0.1% or less - this is where ammeters using precision shunt resistors come into play even if they require a separate means of isolation.

 

Ammeter shunt resistors enable higher currents to be measured than are possible with a normal ammeter because they “shunt” the majority of the current around the meter. The principle involves the insertion of a very low resistance in series with the load, resulting in a small voltage drop across this shunt resistor, which can then be measured with a suitable voltmeter. This allows the current reading to be determined by the simple application of Ohm’s Law so that the meter’s scale can be directly calibrated in Amps.

 

Naturally there is a bit more to ammeter shunts than this. Firstly, shunt resistors are designed to work with voltmeters to provide a full-scale deflection at the shunt’s maximum rated current, usually with standardized voltage drops of 50, 75 or 100 mV. This means that a shunt designed for 100 A operation and a 100 mV drop will have a resistance of just 1 mΩ, with even lower values for higher currents and lower voltage drops. To achieve the necessary accuracy, not only does the resistance need to be precisely defined, but also potential measurement errors need to be eliminated. This is why a four-terminal construction is the norm for high-current shunt resistors, separating the load-current connections from the measurement terminals.