By Paul Bearpark, Head of Electronics and Software at 42T
Q: What do an electricity substation, an electromechanical switch and power tools have in common?
A: They can be remotely monitored by harvesting power from their electrical supplies.
Making the world smarter and more connected is a valuable ambition – it allows companies to optimise their operational efficiency, consumers to maximise convenience and whole cities to flow. The end result is improved asset monitoring and improved control over those assets leading to enhanced services, higher efficiency and less waste.
Making electrical devices smarter will often be straightforward because the existing electrical supply can be utilised to provide a permanent supply to the monitoring, control and communications electronics. While the benefits are clear and the vision tangible, there are practical difficulties to make it a reality.
One of these challenges is often to provide a source of power, even at the low levels required for sensing and occasional low power communications. In many circumstances, access to the mains or primary power supply might not be available or may be impractical even for an electrical system. This is particularly the case when considering retrofit; batteries could be employed but they have many major disadvantages – limited life, transportation difficulties (particularly for Lithium), disposal and safety issues to name just a few.
However, at 42T we’ve found that remote monitoring and/or control capability is usually still possible where direct access to the mains or primary power is not available, and whilst avoiding using a battery.
Let’s have a look at 3 examples…
Most electricity substations have a very crude form of monitoring. The only device usually available is a ‘maximum demand indicator,’ which records the peak current flow in each phase since it was last reset. It’s an electromechanical device requiring an on-site visit to read and reset.
With the transition to electric vehicles, electric heating and renewable installations, electricity networks are becoming increasingly complex. Without understanding network demand patterns Distribution Network Operators may be forced to make unnecessary upgrades or make them too late.
Providing power to a retrofitted monitoring system requires strict safety precautions, often involving disrupting the supply to consumers. With careful design of the electronics such that the power consumption is low, a retrofittable monitoring and communications system can be developed that harvests power from the electrical field generated by the current flowing in the supply.
At 42T we have developed a concept for a low-cost non-contact system which is installed on each phase of the supplies without the need to disrupt them. It provides continuous measurement of voltage and current which can be communicated over a low power radio network such as NB-IoT. It is continually powered whilst there is current flow and easily fitted without interrupting the supply. If monitoring is required during periods when there is little or no current flow charge stored in a supercapacitor can provide a bridge. This technique could be applied to any single or multi-phase electricity supply where there is sufficient electric field to power the electronics and the phases are individually accessible.
Figure 1 provides a schematic of the system employed on each phase of the electricity supply. The device is attached to the cable or ‘bus bar’ providing the supply. The attached device contains all the energy harvesting, sensing and communications electronics required to measure the electricity supply and occasionally communicate some data. In a multiphase system the devices can be connected together to generate ‘pseudo-neutral’ making it possible to measure voltage in addition to current.
Electromechanical switches for powered devices are often the primary mechanism for switching them on and off. However, there are many occasions when it would be beneficial to switch them on or off remotely. This could be to replace the electromechanical switch where remote control operation is needed or in addition to the electromechanical switch itself.
Often there is a simple way to achieve this capability by leaking a small amount of current through the circuit to power the electronics when the main device is off. The power needed is a few tens of milliwatts or less, and leaking such a small amount of current through a device will not turn it on or result in any undesirable behaviour. It’s only when the device is switched on fully that it operates as normal.
At 42T we have investigated this current leakage scavenging method of powering circuits for remote control for several applications. It’s not suitable in every instance but often worthy of investigation because it can result in a simple often retrofittable, device upgrade without requiring a separate supply to the mains or a battery.
Remote control is one application, but the technique of current leakage scavenging could be employed to monitor a piece of electrical equipment by replacing the electromechanical switch with a smart electromechanical switch. For instance, the smart switch could report how often and how long the device is used for or its energy usage. With energy costs rising it can be useful to have a measure of energy usage of a piece of electrical equipment to determine whether there are energy saving measures that could be employed or the equipment replaced by something more efficient.
The third example of a novel method for making devices smarter is to do so even when they are switched off. Power tools could benefit from this; it would often be useful to know where a tool is or whether it has been dropped or mishandled during transport. One solution would be to provide a battery to power the electronics that monitors its state and communicates that state occasionally even when it’s off. However, as previously mentioned there are lot of downsides to adding a battery to a product.
An alternative is to use a super-capacitor. Super-capacitors do discharge over time but it is possible to design circuits that provide back-up power for a few hundred hours. The energy density is lower than a battery so the art is in designing the system to utilise the available charge carefully. For instance, the communications technology used needs to be suitable for a low energy supply. BLE (Bluetooth Low Energy) or NB-IoT could both be considered for this type of application. BLE requires that it is in range of a gateway device such as a mobile phone whereas NB-IoT is a cellular technology for very low power devices.
Enabling energy harvesting in the ways described requires two aspects to be considered and balanced; the power consumption of the system and the availability of harvestable power. Often, the available harvestable power is small if it relies on scavenging from the electromagnetic field, leaking current through the primary device or relying on the charge stored in a supercapacitor. The available energy then determines the requirement for very low power system design.
So really unleashing the power of connected everything should not be limited by the local availability of abundant, easily accessible energy. Often some lateral thinking about the energy sources that may be tapped into and the technology to do that combined with up-front design and engineering of the bespoke sensing and communications system can enable the value of the measurement and control to be realised. After all, free energy is the key to power freedom.
Paul Bearpark is an experienced engineer, engineering manager and product manager. He has spent over 25 years working in the field of electronics in both product development companies and technology consultancies. During his career he has led the development of products through their entire life-cycle from concept to volume manufacture and on-going support and upgrade. He specialises in radio communications, sensor systems and systems engineering. He has worked in the defence and security, telecommunications, and test and measurement industries.