War Of Currents – Part II?

In the end of the XIX century was started the implementation of an electrical distribution system with arc lamps to lighting the streets and the interior of large buildings. This system was using high voltage (>3000V) and based in alternate current (AC). At that time, it main disadvantages were the safety issues and maintenance requirements, so it was only suitable for outdoor use.

Meanwhile Thomas Edison revolutionized it by inventing the incandescent lamp. With that would be possible to bring the light to the houses in a safer way. With the lamp he also created a new electrical distribution system based on a lower voltage (110V) and continuous current (DC). That system included the generators, distribution network and also consumption meters.

The advantages were significant: it used a safer relative lower voltage, the generators could be connected in parallel which allowed the use of smaller machines during low power consumption periods and the energy could be stored in batteries to provide energy during interruptions and improve the network stability.

But that system had a great limitation: due to it lower voltage to deliver the power at long distances the wires required were very thick to accommodate the resistive losses, making them very heavy and expensive. To become practical the electrical powerplants should be in the middle of the population centers and would be able to supply only customers up to 1 mile away (around 1.5km).

At that time George Westinghouse had information about a newer European transformer-based AC system. With that, the higher voltage line used for arc lighting could also be used to the electrical distribution at longer distances and, with a transformer that voltage would be reduced to a safer level for residential usage. He purchased the patent rights and created a company to improve and implement it, the Westinghouse Electric Company.

Meanwhile, Nikola Tesla was also working on the AC system and discovered the rotating magnetic fields which were essential to create AC motors and multiphase systems. He worked for some time with Thomas Edison, but how he was always against that technology he sold later it patents to Westinghouse which were the key to complete the new AC system.

The implementation of this system was much cheaper and it spread fast, including territories with DC system. By seeing that and some of their patents being infringed, Thomas Edison start claiming that the AC system was very dangerous and using controversial examples to prove that. Meanwhile the DC system was losing territory and with the Thomas Edison leaving the electric business his company also adopted the AC system.

With the technology evolution most of the traditional electrical equipment’s are being replaced by higher efficient versions: the incandescent lamps by LEDs, the resistive heating systems by heat pumps, the resistive cookers by induction cookers and the linear transformers used in some devices by high frequency switching converters.

Besides that, other things are also changing: the high-power motors are using electronic controllers, the electrical vehicles chargers are arising and microgeneration systems with solar panel and backup batteries are becoming part of the installation.

What all these devices have in common? All of them works with DC voltage. So, to supply them from the AC mains the first step is to rectifying it to a DC voltage, followed by a power factor correction (PFC) stage and a bulk capacitor to obtain a smooth DC voltage to power the next stages. By using DC directly these stages would be avoided, increasing the efficiency and reducing the size, price and complexity.

Other problem with AC voltage is that is not possible to store it. To use backup systems such as UPS it is required to convert it to a lower DC voltage to charge the batteries and then convert the battery voltage to a higher AC voltage. That, in tur will be converted again to a DC voltage by the equipment. Even with high efficiency converters, these conversion stages contribute to significative energy losses, reducing the overall autonomy.

So, the question is: why not using a hybrid system and take the advantage of both? AC voltage for distribution and DC voltage for consumption. The DC voltage could be obtained directly from the medium voltage distribution, there is already some work done about that: https://ieeexplore.ieee.org/document/8113593

One solution could be by using a SELV (safe extreme low voltage) in the level of 60V for lower power systems, like lighting, informatics and multimedia equipment’s. For higher power requirements could be used a higher voltage in the level of 400V. With that was also solved the problem of load balancing in triphasic systems.

In fact, the -48V DC system is already a standard for telecom and some companies are also using 380V DC as a standard for data centers. Would be the next step using it to residential and commercial buildings?