# Smart Energy Meters for Smart Grids, an Internet of Things Perspective
## 2 Sensing Techniques
### Electricity Meters
#### 2.3.1 Instantaneous Measurements  
At the fundamental level, electricity measurement is about measuring the instantaneous values of currents and voltages. The voltage measurement is summarized in Table 4. The voltage is measured by bleeding a very small amount of current across the nodes, between which the potential difference is to be measured using a very high value of resistance in a series with the voltage sensor. Primitively, voltage sensors use the force experienced by a current conductor in the magnetic field and commonly include a permanent magnet moving coil, moving iron, an electrodynamometer, and an induction type of meters. The induction type of meter is most commonly employed in domestic and industrial electricity meters. The rectifier type of meter is another common voltage measuring technique which uses a diode bridge and a permanent magnet moving coil sensor to measure AC voltages. Electrostatic meters use capacitive coupling to induce a motion proportional to the voltage applied across the capacitor plate. Voltage measurement using an optical technique is reported in [98]; it uses the Pockels effect, which is a change in the refractive index of the material attributed to the applied voltage. The Pockels effect-based sensors have a wide bandwidth and are able to measure high voltages [99]. Finally, digital voltmeters use a comparator and a pulse train to measure the voltage level of the input signal and convert it into a stream of binary digits, with each digit indicating the result of the comparator for each pulse [100]. The design variation found in the digital voltage sensor is tremendous and is primarily divided between isolated and non-isolated techniques; both of these use high-performance, high-bandwidth, programmable-gain, and versatile and high-dynamic-range A/D converters for high-performing instruments. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Meter Type** & **Working Principle** & **Advantages/Disadvantages** \\ \hline \multirow{3}{*}{Induction [101]} & Alternating fluxes across the disk induces & **Advantages:** High torque/weight ratio, inexpensive, good damping, good dynamic range, reasonably accurate \\  & emf which, in turn, induces motion of disc & **Disadvantages:** power consumption, measures AC only, nonlinearity, requires compensation for accurate measurements \\ \hline \multirow{3}{*}{Electrostatic [102]} & \multirow{3}{*}{Deflecting torque due to electrical field} & **Advantages:** High-voltage measurements, lower power consumption, measures both AC and DC voltages \\  & & **Disadvantages:** large size, expensive, nonlinear, requires large forces for small output \\ \hline \multirow{3}{*}{Rectifier [103]} & \multirow{3}{*}{Diode rectification for AC-to-DC conversion} & **Advantages:** reasonably accurate, low-cost, lower power consumption, uniform scale \\  & & **Disadvantages:** calibration, measurements depend upon waveform shape, can introduce harmonics \\ \hline \multirow{3}{*}{Digital [104]} & \multirow{3}{*}{Pulse train-based comparator conversion to binary output} & **Advantages:** portable, lesser readout errors, easy to interface, low power, reasonably accurate \\  & & **Disadvantages:** complexity for greater accuracy, conversion speed, nonlinearities in conversion \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Different types of voltmeters and their working principles. Current measurement is performed either by a non-isolated technique involving placing a very low resistance in the series with the current-measuring circuitry or by an isolated technique involving sensing the electromagnetic field generated by the flowing current [105]. A basic technique is the shunt resistor technique, which measures the voltage drop across the resistor in the current path and uses an isolation amplifier to provide amplification whilst trying to minimize the effect of electrical contact, which introduces ohmic losses at the expense of a deterioration in bandwidth and accuracy along with the drift in the reported values [106].