# Smart Energy Meters for Smart Grids, an Internet of Things Perspective
## 2 Sensing Techniques
### Electricity Meters
#### 2.3.1 Instantaneous Measurements
Basically, shunt current sensors are usually coaxial shunts [107] or a series of surface-mounted shunts [108], where the former are used for high-magnitude pulses with stringent transient responses such as fast rise time [109], whereas the latter are used in power electronics and industrial, consumer, and electronics systems [110]. Instead of SMD shunts, copper trace can also be used as a series shunt [111]; it is inexpensive but also has large thermal drift and low dynamic range, limiting its applications [112]. Isolated current measurement enables non-contact floating voltage measurement using the magnetic field generated by the current flow, allowing greater dynamic ranges and electrical safety [105]. The magnetic field can be sensed using induction or using magnetic field sensors. Popular induction-based techniques for isolated current include sensing using the Rogowski coil [113] and current transformers [114; 115; 116], the basic limitation being the inability to measure DC current, as DC current cannot produce alternating magnetic fields that can induce current in the sensing side circuit. The Rogowski coil has the advantages of linear response without saturation [117], less insertion impedance, easier integration into the PCB, and allowance of flexible coils for ease of use, whereas the disadvantages include the need for output signal integration [118] and thermal drift [119]. Rogowski coils are used in industrial instrumentation and high-power scenarios, such as distribution systems, short circuit testing, fault detection, induction motors, etc. [120]. Current transformers use simple transformer action to induce current at the secondary/sensing side using an alternating magnetic field the produced by primary current and are arguably the most widely used current-sensing devices in power conversion [121] due to their low cost [122], ADC compatible signal [123], proportional output that does not need integration, and robust low-frequency application. The primary disadvantages [124] of current transformers are saturation, droop, and hysteresis due to magnetizing inductance, core losses, especially at high frequencies, and the requirements of core resetting in pulsed operation. Magnetic field sensors are used to sense both static and magnetic fields and, hence, are applicable in more general scenarios. The two fundamental aspects of magnetic field sensors are the connection topology and the sensing mechanism itself, where the former includes open loop, closed loop, and connection with either a current transformer or a Rogowski coil, whereas the latter includes the Hall effect, fluxgate, and magneto resistance mechanisms [125]. Open loop topology directly places the sensor in the magnetic field, which can be \"focused\" by placing the sensor in a gap of highly permeable core, which can induce a permanent offset requiring degaussing of the core [126] and parasitic voltages due to field fringing. In closed loop sensing, the magnetic sensor is used as an error-correcting device, whose signal compensates the magnetizing current by forcing a proportional current through the secondary winding [127]. Closed loop sensing minimizes drift and the heat losses due to reduction in the eddy currents and greater bandwidth, whereas its main disadvantage is that it is not suitable for all types of sensing mechanisms, such as anisotropic magneto resistance [128]. The combination of the magnetic field sensor with either CT or Rogowski sensors produces a sensor that can use the best of both worlds: the DC measurement capability of the former with the accuracy and the bandwidth of the latter, resulting in active current probes [129], an eta current sensing principle [126], and a combination of the open loop Hall sensor with the Rogowski coil to measure the high frequency part of the current [130]. Such a combination of sensing topologies and techniques is usually expensive and not suitable for mass production, and hence, it is used for specialty applications.