Sensing Techniques for Load Monitoring in Smart Energy and Automation

Sensing for smarter energy

Measuring energy is a key concern in today’s world, for conservation and billing purposes. Closures of fossil-fuel power stations under initiatives such as the EU’s Large Combustion Plant Directive are forcing grids to rely more on renewable sources. In turn, this raises the importance of demand management and encouraging changes in consumer behavior. Governments around the world are embarking on smart-meter rollouts that aim to give utilities and consumers the information needed to manage supply and cut demand to ensure grid stability while combating climate change.

Changing to smart meters will take time, but home energy-display devices (Figure 1) are already in the field. These present users with real-time consumption data to help identify opportunities to make savings. The power and energy calculations are dependent on accurate current-flow measurements gathered by a sensor unit installed in the meter cabinet that transmits data wirelessly to a receiver and display unit in the house. The current sensor must be easy to install with minimal disruption to the existing meter or the main supply cable entering the premises: simply clamping onto the supply cable, without needing to be connected in-circuit, is ideal.

Figure 1: A home energy display comprising clamp-on sensor and information display unit provides usage data without requiring a full smart-meter upgrade.

Sensing for equipment management and protection

Quite apart from the smart-energy applications now emerging, isolated current sensors have numerous roles in industrial automation, such as helping to ensure energy-efficient operation of equipment, rapidly detecting equipment failures, or coordinating safety interlocks. The currents to be detected can range from just a few milliamps up to several tens or hundreds of amps. Sending the information to a PLC enables the system to set an alert or take corrective action (Figure 2).

Figure 2: Current-dependent control of machines and external circuitry.

Power-factor correction (PFC) is widely used to help improve energy efficiency and prevent harmonic contamination of the AC line. Loads such as large motors, which are highly inductive, have low power factor unless banks of correction capacitors are connected at the input. The capacitance needed is highest at the heaviest load conditions, when the power factor is at its worst. At lighter loads, however, over-correction can occur if the capacitance is not reduced. Monitoring the motor input current enables the system to detect the load being applied. If the load is light, a current-operated switch can disconnect the PFC capacitor to prevent over correction.

In the event of failure of automated factory equipment, it is vital to detect the fault and take remedial action as quickly as possible. Examples include the control of industrial furnaces, or pharmaceutical production processes that require heating to a precisely controlled temperature. Any failure of a heater element must be detected quickly to prevent loss of productivity, but temperature monitoring can be slow to detect the fault. If no action is taken until the temperature has changed significantly, quality can be compromised and precious materials wasted. Detecting the sudden drop in the current that occurs as soon as the element fails provides an instantaneous indication that can be used to trigger a timely response such as turning on a backup heater.

Similarly, sensing the current input to a motor enables problems such as jamming of a conveyor belt to be detected straight away, and the current measurement to be sent to the PLC, so that the motor can be turned off quickly for safety.

Another application for current sensing in industrial equipment is in managing safety interlocks. These may be designed to protect operators by preventing guards from being opened while machinery is still running. Alternatively, interlocks can prevent damage to equipment or help coordinate processes by ensuring that various drives and actuators can only operate in the correct sequence. Since current consumption provides a reliable indication of whether a subsystem is turned on or off, current-operated switches provide an ideal means of coordinating these interlocks.

Finally, amid initiatives aiming to improve industrial safety generally, ground-fault protection is being implemented on a per-machine basis in addition to the protection circuitry typically installed at main breakers. Using a ground-fault sensor that monitors current in the power lines supplying the equipment enables tiny leakage currents, symptomatic of a fault in the ground circuit, to be detected quickly and safely.

Current-sensor choices

Important attributes of a suitable sensor for use in current-operated switches, fault detectors and metering circuitry include electrical isolation for optimum safety, minimal power draw from the circuit being monitored, ease of use, and low cost. Depending on the application, the measurement range and bandwidth, and the ability to withstand harsh environmental conditions can also be important criteria. Devices such as Hall sensors, current transformers and Rogowski coil sensors are the three key types of sensors meeting these requirements.

Hall sensors

The Hall-Effect current sensor responds to the magnetic field generated around the current-carrying conductor, and produces an output voltage proportional to the current flowing in the conductor. A typical linear current sensor combines an IC containing the Hall element with a magnetic core, which is designed to concentrate the magnetic flux on the Hall-Effect IC. The IC and the core are designed-into a plastic housing that ensures accurate positioning of both components relative to each other.

The Infineon TLI4970 Hall sensor contains integral differential Hall elements and does not require a concentrator. Hysteresis effects are eliminated since no concentrator is required, and the differential sensing principle prevents external magnetic fields from interfering with current measurements. The TLI4970 integrates the Hall sensors alongside analog and digital signal-conditioning circuitry (Figure 3), and occupies around one-sixth of the board space needed by comparable sensors. It is able to measure alternating and direct currents up to ±50 A. High current-measuring capability is a known strength of Hall sensors, although other sensors such as Rogowski coils and current transformers offer generally greater linearity over their measuring range.

Figure 3: The TLI4970 eliminates hysteresis effects and saves PCB space.

Current transformers

Current transformers have been used for a long time for control, circuit protection and monitoring in equipment such as switched-mode power supplies, and also for performing precision current measurements in instrumentation applications. These devices are able to measure alternating currents, and provide electrical isolation between the primary and secondary windings.

The current rating of the primary winding effectively governs the measurement range, and a high turns ratio allows high measurement resolution. Depending on the current transformer and the application, this ratio may be between 1:20 and 1:1000. An excessively high ratio can accentuate capacitive and inductive effects in the transformer, leading to inaccurate measurements. On the other hand, selecting a turns ratio that is too low can also result in inaccuracies due to distortion of the output signal.

One disadvantage of current transformers is that devices suitable for measuring high currents can be physically large. On the other hand, small surface-mounted current transformers, such as the Murata 5300 series, are suitable for use in equipment such as motor controls, switched-mode power supplies and electronic lighting ballasts and can measure currents up to around 10 A with a maximum bandwidth of 500 kHz.

Typical current transformers are wound around a toroidal metal core, through which the current-carrying cable must be passed. Alternatively, a split-core design allows the current transformer to be clamped around the cable. This allows the sensor to be installed more easily, such as in the home data display example. CR Magnetics CR4100 series true RMS AC current transducers are able to measure sinusoidal or non-sinusoidal current waveforms accurately, and can be ordered in toroidal or split-core configuration.

Rogowski-coil sensors

Current sensors that use the Rogowski-coil principle claim to offer a number of advantages over Hall-type sensors or current transformers. These include the ability to measure large currents without saturating, greater bandwidth than other sensor types, and the ability to measure currents that are changing rapidly at up to several thousand amps per microsecond. They can also measure small AC currents that have a large DC offset.

A Rogowski-coil current sensor is positioned around the current carrying conductor, as shown in Figure 4. The current flowing in the conductor induces a voltage in the coil, which is proportional to the rate of change of the current. The instantaneous current flowing is then calculated by integrating this voltage. The integrating circuitry may be implemented externally, or may be built into the sensor to produce a voltage at the output terminals that is proportional to the current. Since the coil is not electrically connected to the current-carrying circuit, electrical isolation is implicit.

Figure 4: The Rogowski coil is positioned around the cable carrying the current to be measured.

Rogowski-coil current sensors can be designed for measuring currents ranging from a few hundred milliamps to hundreds of kiloamps. Pulse Electronics has a large selection of sensors including the PA320 series, which has dynamic range from 0.1 A to 1000 A, bandwidth of 500 kHz, and very high accuracy meeting the ANSI C12.20 Accuracy Class 0.2 and IEC 62053-21 class 1 specifications. This allows the sensors to be used for precision current measurement in smart meters.

Conclusion

From high-accuracy current measurement in metering applications, to taking advantage of the high speed of current monitoring to help manage industrial machinery and detect critical failures immediately, Hall-Effect current sensors, current transformers and Rogowski coil current sensors give designers flexible choices to achieve a solution that meets important targets such as performance, reliability and cost.