From AC to DC: powering intelligent buildings

By Philip Lechner   |   September 21, 2021

Most electronics require DC at board level and even motor-driven AC machines such as washers and dryers are giving way to intelligent, inverter-driven versions that operate internally from a high voltage DC bus. In parallel, buildings are becoming ‘smarter’ for added convenience, productivity and reduced energy consumption. This only means yet more DC-powered electronics, widely distributed in sensors, actuators and controls, interconnected in the IoT.

All this begs the question – “Why not supply buildings with DC, not AC?”

AC v DC – “The war of the currents”

Let’s backtrack a little and look at why historically AC is used. DC was originally favoured in the US, as championed by Edison, but in the ‘War of the Currents’ Tesla’s AC generators, with their ability to easily convert up and down voltage through transformers, outweighed the advantage of DC – no transmission losses from dielectric and skin effects. As AC could be transformed up to hundreds of kV with consequent low current, losses became less of a problem anyway. So, AC won out and has dominated for the last 150 years, with a vast infrastructure built around it.

Grids of the future will be localised

Click to enlarge

Using the existing infrastructure becomes less of a factor as nano, micro and mini grids are considered for the future with local renewable energy sources - typically DC-output solar arrays coupled with storage for continuity of supply. This might be a wall-mounted lithium-ion battery or even the household EV, with a bi-directional charger (Figure 1). The advantage of the arrangement is less reliance on the utility supply, lower bills, potential feed-in of excess energy to the grid and a smaller environmental footprint.

In the DC-powered home, voltages needed range from less than 0.6VDC for the CPU in your PC to perhaps 385VDC to drive a dishwasher motor inverter effectively and retain some backward compatibility with AC input versions.

A standard bus voltage or perhaps two in tandem, needs to be chosen. Then DC-DC converters will be required at many loads and at sources such as solar panels and wind turbines with their variable output voltage, or the EV battery which could be 800VDC. Ironically, DC-DC converters operate by generating AC to feed a transformer to change voltage and provide isolation. Calculations have shown however, that overall efficiency with DC input equipment can be between 5% and 20% better [1].

Voltages being considered for building DC buses are 385VDC as in data centres and 24VDC. The former because this is the peak value of rectified and power factor-corrected AC, at high tolerance, present in ‘universal’ AC input products. In many cases, 385VDC applied to the AC input will effectively disable the PFC stage and pass straight through, with the equipment operating as normal. Products operating from 385VDC will take approximately the same current as from 240VAC. So, although existing cabling would be of adequate rating, it can’t be used as it is colour-coded for AC and standards for DC, when they exist, will mandate different colours to avoid confusion. Legacy universal AC input products without power factor correction (such as lighting power supplies below 25W and other equipment below about 75W) will usually operate satisfactorily with 385VDC, at the top end of their rated input, but in the DC home new low power products are likely to be rated for operation off a lower voltage bus for convenience.

DC input equipment requires special fusing and switching design

Fusing for DC input products is an important consideration. With an overload on an AC power line, a fuse will open and the momentary arc will quickly extinguish as it does so, as AC crosses zero volts every 10 milliseconds for a 50Hz supply. In a DC line, the arc can continue for much longer and even be self-sustaining, depending on the separations and fuse type. To avoid this and consequent equipment stress and safety concerns, DC and AC fuses have different construction. In in the extreme, circuit breakers are used, sometimes using magnetic or compressed air deflection of the arc to extinguish it more quickly. Solid state circuit breakers are also an option and are reducing in price. A longer arc is also drawn when mating or un-mating DC power connectors in normal operation, risking burning or even welding of the contacts closed. The same situation occurs with switch contacts.

The solution, which is now standard in EV charging connectors, is to incorporate an additional control connection. This forms an interlock to ensure that high current is disabled while mating or un-mating. If existing products are marketed as AC or DC compatible, a practical solution will be to use separate inlet connectors.

Figure 2: rectifiers on AC input equipment isolate energy storage components

With a direct DC connection to equipment, there will often be an energy storage element on its internal DC line, such as a bulk ‘reservoir’ capacitor, which can store many joules of energy. In AC-input products, this capacitor is isolated from the input by the line rectifier. Safety standards can be easily met to reduce the residual voltage on the AC terminals to a safe value within one second (Figure 2). With DC input and no rectifier, the input connection could be at full 385VDC and take minutes or longer to discharge internally, posing a safety concern, especially as the equipment connection is likely to have exposed ‘male’ contacts. A diode fitted in line internally is a solution, also protecting against reverse polarity, but it reduces efficiency.

A MOSFET however can be used as an isolator with low loss.

24VDC is another option as a building power bus, with some history in control electronics and sensors in industry. This is only suitable for low power, however, as currents are over 15 times higher than 385VDC systems for the same power rating. An advantage though is that 24V is ‘Extra Low Voltage’ (ELV) which means it is deemed inherently low risk for electric shock, wiring does not have to be installed by certified electricians and can be re-located quickly and cheaply. Depending on the power source, however, 24VDC could still supply dangerously high currents leading to overheating, injury and fire, so standards will be imposed for installation practice and protection methods [2]. 24VDC is useful for LED lighting, IoT nodes, sensors, controllers and other low power electronics with internal DC-DC converters. It has been written that a voltage such as USB 5V for portable appliance charging could be generated centrally from 385VDC or 24VDC and distributed out across a building. However this will not be viable, as the voltage must be quite accurate at the load, and droops, spikes, and surges on a 5V bus would be unacceptable. Devices also often need individual intelligent control of voltage and current sourcing as in the USB-C standard, so, in practice, we will still see individual USB chargers, but fed perhaps from 24VDC rather than AC mains.

This will make them cheaper, smaller without the worry of safety clearances and easier to integrate into connector wall plates.

Figure 3: DC supply grounding options. Image reproduced with permission from G Kenyon Technology Ltd and The Institution of Engineering and Technology

A grounding scheme must be chosen

DC buses in buildings, like AC systems, will need a defined relation to ground for safety and functional reasons; un-grounded arrangements could ‘float’ up to damaging voltages and produce indeterminate electromagnetic interference levels from the array of switching DC-DC converters now connected. Some equipment may have an internal connection to ground already; for example, solar panels can have grounded outputs, so it is likely that a firm connection to a central ground will be defined in a building electrical system, similar to AC neutral connecting to ground at a central point. In IT DC systems, the positive of a 48V bus is grounded, producing -48V as distributed power. This is to prevent galvanic corrosion of connections to ground in moist atmospheres, but in controlled industrial and domestic environments, this is less likely to be a concern and a positive bus with negative grounded is likely to be the preferred solution, though there are other options (Figure 3). Non-isolated DC-DCs off a positive bus are also preferred as they can be simple buckconverters, rather than less common ‘buck-boost’ types, which can operate off a negative input for positive output.

DC-connected buildings do therefore have their challenges, with embryonic standards and a complex payback calculation. This is heavily dependent on cost and availability of DC-powered equipment, especially in the transition period when volumes are low and a premium is imposed for dual-input products. As we move to carbon-free economies and energy efficiency must be maximised, however, Edison’s dreams are more likely to become reality.

_References:

[1] https://www.ase.org/blog/direct-current-power-systems-can-save-energy-so-building-developers-are-getting-new-incentive
[2] https://www.theiet.org/media/2734/practical-considerations-for-dc-installations.pdf

This article was taken from the latest edition of Focus magazine. Click below to read the magazine in full, or alternatively, if you have a question about sensors for building automation you can get in touch with our team of technical specialists.

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Philip Lechner

Philip Lechner studied electronics and telecommunications in Amsterdam before beginning his career i...

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