Xiao-Ping Zhang using an industry-standard real time power grid simulator

New method for controlling DC faults on the cheap

A new way of managing faults in direct current power systems could drastically cut the cost of converting from alternating current grids, researchers say.

DC systems are widely accepted as faster, more efficient and more suited to renewable energy sources than conventional AC grids, but when faults occur they are extremely hard to control because the huge flow of direct current is very difficult to stop.

In AC systems, on the other hand, the current switches direction many times a second and each time the current passes through zero there is an opportunity to stop the current and repair any fault.

The current solution to repair faults on DC power lines is circuit breakers, which work by converting a DC electricity flow into AC and breaking the circuit as the current passes through zero, but these are large and extremely expensive. One device could cost as much as a DC terminal – the devices that control the flow of current in and out of the grid.

Now electrical engineers from the University of Birmingham, led by Professor Xiao-Ping Zhang have developed a solution that incorporates the technology needed to control the DC current into the grid’s AC/DC converter terminals, rather than building stand-alone circuit breakers.

“Nobody has yet thought of using the terminals themselves to control the DC current. It’s a very practical solution to the problem, which will make the DC current breakers much simpler and much cheaper,” said Prof Zhang.

“The need for DC current is such that huge, multi-billion pound infrastructure projects are already underway in anticipation of a solution to this problem. We believe we have the missing piece of the puzzle that will allow DC power supplies to be adopted worldwide.”

The innovation has been patented through Alta Innovations, the University’s technology commercialisation office, and partners are currently being sought to develop the work commercially.

The solution, which the team has validated on an industry-standard real-time power grid simulator, uses AC/DC converter terminals with a very simple set up known as the ‘half-bridge topology’, according to Prof Zhang.

“The DC current can be controlled and isolated from the AC side without the need for additional hardware investment. The DC fault current is converted into AC current and hence can be interrupted very easily even by a simple power switch or by a low-capacity DC circuit breaker,” he said.

“The key difference between our approach and a DC circuit breaker approach is the investment cost. Our approach will need very little investment while the cost of a DC circuit breaker means a significant portion of a high-voltage direct current (HVDC) project.”

The Birmingham team is now actively looking for industrial partners to explore commercial opportunities for the technology.

DC power cables can carry larger supplies of power, and over greater distances, which means the equipment required can be smaller and lighter making it more suitable to transport power from offshore renewable energy sources to onshore power systems.

A DC system is also better able to handle and control the fluctuating amounts of power which need to be fed into power grids from these renewable energy sources, making DC power technologies a major area of interest for electricity suppliers.

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