A DC breaker breakthrough - a cost effective solution?
Tim Probert explores the steps being taken by the world’s leading power grid equipment manufacturers to crack a particularly tough nut: the ability to create a cost-effective, efficient high voltage DC circuit breaker.
Imagine being on the M1 motorway, having joined at Junction 1 from London’s North Circular. Now imagine being unable to exit until having driven the full 318 kilometres to Junction 47, east of Leeds. This, in essence, is the nature of high voltage direct current (HVDC) power lines and why they have until now been restricted to point-to-point installations, such as underwater connections or linking a remote hydroelectric plant to a node in a national grid.
The lack of practical entries and exits that allow the power to flow to different load centres has dogged the implementation of DC since the Battle of the Currents at the dawn of the electric age in the late 19th Century. DC, promoted by Thomas Edison and General Electric, was pitted against alternating current (AC), supported by Nikola Tesla and Westinghouse.
At the time, AC won the battle because it was simpler to control and better suited to the applications of the day, but DC still has the advantages of higher capacity and lower transmission losses, particularly over long distances. However, the inability to put a viable circuit breaker on a DC line to enable the fault to be isolated is perhaps the single biggest obstacle to the development of a true DC grid.
A reason to revisit the problem
As the deployment of renewables in remote locations, such as offshore wind, proliferates so does the desire for a DC grid. Currently, voltage source converter (VSC) technology is used to connect offshore wind farms to the onshore network as point-to-point systems.
Here, DC breakers are not needed. If there is a problem, the link to the onshore network is simply disconnected. However, the situation becomes more complex when there are multiple offshore wind farms. It will become increasingly necessary to have a meshed DC grid that allows you to optimise generation and monitor wind turbines. As with an AC grid, it will be necessary to have the ability to isolate branches of the network to mitigate problems.
The DC circuit breaker is widely viewed as the starting point for the creation of a DC grid. However, it is not presently feasible with current to isolate a fault in a DC line, as can be done with an AC line – a fault in a DC line would require the entire line to shut down.
The reason it is so hard to break DC compared to AC is fundamental to each variant. As AC oscillates between positive and negative, the current passes through zero twice during a complete cycle, schematically depicted as sine waves. AC breakers are based on the ability to switch or break when the current passes through the zero point. This means it is possible to break the circuit when the energy level is at zero or at a minimum, thus avoiding a large arc. With DC, there is no zero crossing. This means any breaking has to be done at full load, resulting in undesirable arcing, which can cause extensive damage to equipment.
There have been many attempts to build DC breakers over the past 100 years, and although there are mechanical DC breakers that can break at full current, 3kV seems to be the upper limit. Such a low limit is totally impractical for HVDC connections, which typically operate at 400kV or more.
Furthermore, mechanical breakers interrupt the flow of current for so long – tens of milliseconds – that they would cause grid instability, while attempts to use pre-existing power electronics have resulted in large losses.
Hybrid breaker steps into the spotlight
What is needed is the innovative use of power electronics to force DC power to zero and then open a mechanical switch to isolate the circuit with minimal losses, all in just a few, rather than tens, of milliseconds. Following years of research, Swedish-Swiss firm ABB claims it has developed the world’s first low loss circuit breaker for HVDC.
The breaker is a hybrid breaker that, says ABB, combines the low losses of a mechanical breaker with the breaking capabilities of a power electronics breaker. The company claims the system resets a circuit in fewer than five milliseconds and with less than 0.01% energy loss – small enough not to cause problems for the network.
The hybrid essentially consists of three key elements: two power electronics breakers – one large and one small, and a mechanical breaker. The large electronic breaker sits in the lower current path while the smaller electronic breaker and the mechanical breaker are in the upper current path (see below, top). Under normal operating conditions, both paths are in operation and current attempts to flow through the path of least resistance – the upper path.
When the hybrid breaker is switched, the breaking process is carried out in three stages (see below, bottom). In the first stage the small electronic breaker in the upper current path is opened for a very short period of time. This forces current through the large power electronics breaker in the lower path.
Then during the few milliseconds when there is low or zero current in the upper path, the mechanical breaker is opened in the second stage to completely block the upper path. In the last stage, the large power electronics breaker opens to cut off the alternative current pathway as well.
Key to the design is an ultrafast disconnector on the main path driven by magnetic actuators that opens and closes to reset the voltage and shifts the current into a bank of arresters, which absorb the excess energy. Per Skytt, ABB’s DC Grid Programme Manager, explains, ‘The disconnector operates around 50 times faster than anything available today’.
Skytt likens breaking an HVDC circuit to stopping a high speed truck in a very short time – approximately 60–80 times faster than the blink of an eye – and taking care of the resulting dissipated energy. He continues, ‘We have a very short time period because the small electronic breaker can only withstand the voltage for a few milliseconds. The full sequence takes just five milliseconds.’
ABB began patent work on the new technology in 2007. This started with an overall design, followed by a programme to develop the components needed for mechanical switching and the power electronics. These components were verified individually before being put together as a complete hybrid breaker system.
A prototype has been built successfully and its functionality at 320kV was successfully verified in the laboratory in October 2012. Skytt says ABB is now in discussions with utilities such as Sweden State Grid to install a demonstration unit on an HVDC line. ‘We expect to have a pilot in 12–18 months from now, which will provide operational experience. And then we should hopefully see the first commercial hybrid breaker in operation in around five years.’
Despite the impressive progress, ABB acknowledges it will be some time before HVDC breakers will be widely adopted. The cost of a commercial hybrid breaker will be more than a corresponding AC breaker, running into several million pounds, but ABB says it should present a commercially viable solution when considering the overall scheme.
The company sees the breaker as a crucial part of its programme to develop HVDC grids. It is investing heavily in trying to understand the interaction between AC and DC and has built a special purpose simulation centre in Sweden to model the interaction between AC and DC grids in real time.
The record breaking contender
ABB is not alone in developing DC breakers. In February, as part of the EU TWENTIES project to fund research into technology to accelerate renewables deployment, Alstom claimed it had achieved the best performance ever seen in a HVDC circuit breaker.
In the presence of an independent expert, the French firm interrupted currents exceeding 3,000A in less than 2.5 milliseconds using a HVDC circuit breaker prototype at its testing facility in Villeurbanne near Lyon. Alstom is cagey about revealing too may details of the product, but it is believed to be fundamentally similar to ABB’s hybrid breaker of combining electronic and mechanical breakers in tandem with a fast disconnector to break the current.
It has been developed in collaboration with Réseau du Transport d’Électricité (RTE), the French electricity transmission system operator. RTE’s involvement was crucial in terms of providing information on current flow in a network and the rate of current rise in the network when a fault occurs, says Alstom.
Having successfully tested the breaker at 3,000A, the next step is to build a full-size prototype to carry out tests at 7,500A and 180kV in September this year. This is the level that has been defined under the scope of the TWENTIES project. Beyond that, the next step for Alstom is to scale up the breaker in order for it to be installed on 320kV connections currently used for connecting offshore wind farms to the grid.
Meanwhile, waiting in the background…
German engineering giant Siemens is also working on developing a DC breaker. Asking for Siemens to divulge information on their progress, however, is rather like trying to get blood out of a stone. A spokesman for Siemens told Materials World, ‘We are quite conservative in speaking about our developments in HVDC technology. Usually we only communicate after having developed products, just before market entry.
‘Siemens is – as are all relevant competitors – working on the topic of HVDC circuit breaker technology. Once the market demand for such technology has evolved, we plan to have adequate products and solutions in place.’
It may be some decades before the full value of the hybrid breaker and a DC grid are realised but now that utilities know it is possible to make a DC breaker, they can start planning for the future.