2 Comparing current-sensor elements used for energy metering

Table 1 summarizes the performance benefits and challenges vs. cost for three current-sensing technologies used in energy metering applications. Current transformers are the most popular sensors given their wide dynamic range, durability and low insertion impedance (noninvasive current measurement) [4]. Their cost is most likely the highest of all current-sensing technologies, however. Shunt resistors (shunts) are very attractive given their magnetic immunity, smaller size and reduced cost, but lack isolation and provide less accuracy at higher currents because of thermal self-heating [5].

Rogowski coils are an interesting alternative to the other two sensors and are the lowest-cost option, especially when considering printed circuit board (PCB) coils vs. bulk Rogowski coils [6].

Table 1 Current-sensor comparison.

Sensor Type	Current Transformer	Rogowski Coil	Shunt
Block diagram
Transfer function	${\mathrm{V}}_{\mathrm{o}\mathrm{u}\mathrm{t}}=\frac{{\mathrm{I}}_{\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{e}}{\mathrm{R}}_{\mathrm{b}\mathrm{u}\mathrm{r}\mathrm{d}\mathrm{e}\mathrm{n}}}{{\mathrm{N}}_{\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{n}\mathrm{s}}}$	${\mathrm{V}}_{\mathrm{o}\mathrm{u}\mathrm{t}}\propto \frac{\mathrm{d}{\mathrm{I}}_{\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{e}}}{\mathrm{d}\mathrm{t}}$	${\mathrm{V}}_{\mathrm{o}\mathrm{u}\mathrm{t}}={\mathrm{I}}_{\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{e}}{\mathrm{R}}_{\mathrm{s}\mathrm{h}\mathrm{u}\mathrm{n}\mathrm{t}}$
Benefits	Provides isolation High linearity High durability High accuracy	Provides isolation Saturation not possible Small form factor (PCB) High linearity Fast response time Low power-loss	Not isolated Antimagnetic Small form factor
Challenges	Saturation possible Power loss in burden resistors Phase calibration required Size and weight	Integration required Cannot measure DC PCB coils can have lower sensitivity	Resistance may vary when self-heated Less reliability for overload signals
Cost	≥$0.21 (no shield) ≥$0.26 (with shield)	<$0.10 for some PCB coils ≥$0.10 for bulk coils	≥$0.10 (manganese)

Because of their low cost and flexibility of installation, PCB Rogowski coils are attractive for low-cost energy metering applications [7], [8]. Let’s analyze the benefits and challenges of a PCB Rogowski-based metering design and how to optimize the signal chain for the lowest possible cost while complying with regional metering standards.