Renewable energy travels a more complex route to the power grid. Photovoltaic panels generate pure dc energy, which must use an inverter to be converted to ac to feed into the grid. Some wind turbines also generate dc, especially the smaller installations seen near homes, farms, and industrial units.
In larger wind-farm installations, it’s possible for the turbines to produce ac power. However, connecting the power to the grid requires the voltage and frequency to be compatible with the grid. In many cases, though, this isn’t the case. The voltage and frequency of the output power is correlated with the rotational speed of the wind turbine, which depends on the wind speed.
One solution is to use a gearbox to sync the turbine speed with mains frequency. The drawback is that this adds complexity to the system, along with extra cost and weight to the overall design. In most cases, it’s preferable to convert the turbine output to dc and then use an inverter to convert it back to ac at the correct frequency and voltage for the grid.
Since almost every renewable power source uses both ac and dc at some point in the power-distribution system, there’s a need to accurately measure both types of power. For ac measurements, current transformers are popular, given that they provide the additional benefit of having galvanic isolation from the measured circuit. But transformers can’t be used for dc power. There are some measurement tools based on Hall-effect current transducers that can measure both ac and dc current, and they too provide galvanic isolation. The main problem with this type of measurement is the accuracy—it’s normally impossible to get the required accuracy of 0.1% (or better) using Hall-effect current transducers. A third option is to use ammeters with precision shunt resistors. Users can find proper shunt resistors, such as FL-2 type from Microhm Electronics for applications.
