By Paul Glendinning – Head of Power Networks and Renewables, WSP
What form of current does your computer use? Your phone? The LED lights in your house? They’re all DC (direct current), and yet the current in your home wiring and in the network to which it connects is AC (alternating current). There are historical reasons for this, but the world has changed and it’s time we took a fresh look at the advantages of DC.
One of the biggest merits of DC is that it enables you to get more power through the network because you can operate lines at a higher voltage without increasing the current. This means that more kWs of power can be transmitted. In the past, the limitations of AC in this regard didn’t matter because it suited both how we used our electricity and how it was generated. But times are changing.
Distributed generation and low-carbon technology
The UK is moving to distributed generation, with smaller, renewable generation systems spread across the country replacing the large centralised coal-fired power stations of the past. And renewable generation technology such as solar panels and wind turbines generate DC power, which has to be converted to AC before it can be distributed to our homes.
The way we use electricity is changing too, and demand is increasing as we switch to low-carbon technologies. More homes use heat pumps, for example, and more people are driving electric cars. These low-carbon technologies all require DC power.
This leads to a situation where, for example, a wind turbine generates power which is AC. This is then converted to DC and then converted again to AC for onward transmission before being converted yet again –this time back to DC when you refuel your electric car at an ultra-fast charge point. There are multiple conversions going on, and losses at each stage reduce efficiency.
Maybe we could live with this situation if networks weren’t now in need of substantial investment to meet future demands of increasingly distributed generation and the continued uptake of low-carbon technologies. Switching sections of the network to DC could be lower cost than these expensive upgrades which would require the cables to be excavated and new ones inserted.
Converting existing infrastructure
Imagine what would happen if, instead of rebuilding the AC network for a town, you switched the existing infrastructure to DC? Would everyone’s appliances stop working? No. The converters on electronic equipment such as computers can already cope with different voltages, and while some modifications would be required, it’s likely they would cost a lot less than rebuilding an electricity network. Typically, converters in the devices cost tens of pounds while investing in networks costs millions.
This is why work on DC networks is already underway and we’re delivering feasibility studies to enable DC network modelling and design, as well as DC control, protection and earthing studies. Our distribution team has been working with SP Energy Networks on its flagship £8.3m LV Engine innovation project. This aims to design a smart transformer which can provide a low-voltage DC supply to customers for the first time and maximise use of the existing AC network.
We previously supported SP Energy Networks on a smart and flexible method for reinforcing distribution networks by converting existing 33kV AC to DC using a Transformer replacement in the form of a power converter. The Angle-DC project is creating a controllable bidirectional Direct Current (DC) link between two sections of SP’s network, on the Isle of Anglesey and in North Wales.
DC smart infrastructure is a perfect technology for integration of EV changers and battery energy storage systems (BESS), in particular grid-scale BESS such as Brill Power’s intelligent life-extending battery management system, for which we are helping to develop a concept design and business case. These reduce the need for many converter stages to connect the BESS to the grid, reducing both power losses and the footprint of the battery.
Issues to overcome
It seems that the idea of the DC town is not that far-fetched after all, although there are technical issues that would need to be overcome.
The fact that the transformer and switch technology used in AC systems has been around for so long – since 1910 in the case of transformers – means it is well established and very reliable. The electronics that replaced them in a DC network would be inexpensive and capable of moving and sharing power efficiently. But they’re unlikely, at least initially, to be quite as reliable. And utilities engineers would need retraining to work on the new systems.
These are not insurmountable obstacles, though, and they shouldn’t prevent us capitalising on the potential of community-scale smart DC infrastructure.
The key to success is carrying out more trial projects.