# Smart Energy Meters for Smart Grids, an Internet of Things Perspective
## 2 Sensing Techniques
### Electricity Meters
#### 2.3.2 Grid Sensing  
Apart from basic sensing at the simple end nodes, typically home and office, the meters are required to perform advanced analysis at higher levels of integration and complex end nodes, such as factories or buildings. Such measurement takes into the account the continuous as well as the complex nature of the current and voltages (complex as in magnitude and angle) in the steady state. Transient responses are also measured and recorded for advanced metering systems, allowing the system-level implementation of fault identification and preventive maintenance, along with other higher level algorithms and statistical inferences. The oscillatory responses in the power systems are very important for the wealth of information they contain, especially about the stability of the system. Usually, modal analysis using frequency domain, time-frequency, or fundamental modes decomposition techniques is performed at the meter, and the data are used by a higher layer in the smart solution for event detection, or the meter can also be programmed remotely to report specific events using AI techniques, among others, such as statistical processing, for specific event detection and reporting. The current sensing techniques presented are summarized in Table 5.  
Phasor measurements are also an important aspect of modern metering systems that measure the phase of the current and voltage along with the magnitude using phase measurement units (PMU), which extract the fundamental frequency component using Fourier transform and extracting real and imaginary components for complete phasor description. Adding a time reference to different PMUs will give a synchro-phasor that can provide frequency measurements across the grid to detect the grid events that cause changes in frequency. Synchro-phasors are also used to measure relative phase angle difference (RAPD), which can be used to detect complex power flow, tripping, and inter-area oscillations at higher levels of data processing. When the phase difference is measured at the same PMU, a phasor differential (PD) is obtained that can be used to detect frequency change for local event detection and to calculate a differential synchro-phasor by combining measurements from different PMUs, which are used to detect the same event by measuring its effect at different locations in the interconnected power system. PMUs are also used to detect phase unbalance and related events. At higher levels of data processing, the state estimation algorithms on the PMU data can provide a thorough picture of the state of the smart power system, enabling the control and optimization of the complete system. The instantaneous measurements of voltage and current provide useful information about the state of the system, but a power system is inherently a continuous system, and waveform centric measurement can provide additional information, especially in the case of non-sinusoidal waveforms that arise due to distortions. Such instruments, aptly named waveform sensors, are of various types, such as power quality (PQ) meters and digital fault recorders (DFRs). Common measurements made by PQ meters include voltage harmonics, crest factor, total harmonic distortion, phase harmonic index, inter-harmonics, and notching. DFRs monitor continuous waveforms and capture an instantaneous image in the case of triggering, referred to as a waveform snapshot, which is set by comparing the current waveform (windowed signal) with a reference waveform, which is usually the previous waveform instance. An event is triggered when the comparison exceeds the set threshold for at least a minimum set period of time, with the popular comparison techniques being THD, RMS, or point-to-point comparison and sub-cycle comparison. Cycle boundary identification and frequency variation compensation are important aspects of point-to-point comparison, introducing complexities which lead to methods such as comparing sub-cycles.