Researchers seek dc power grid circuit breakers
Direct current power transmission is more efficient than AC (see below), but high-power dc is far harder to interrupt than ac – there is no current zero crossing in which to switch and plenty f potential for arcing.
The university, working with Florida State University, is taking the ‘hybrid’ circuit breaker route, which parallels a mechanical circuit breaker with a semiconductor switch – for example a stack of series-parallel silicon carbide transistors.
While the mechanical switch will normally carry all current, the transistors are able to briefly short the switch, diverting current while it breaks.
However, the transistors will not be sized to handle the load for long, requiring the mechanical circuit to operate extremely quickly – because of this, piezoelectric actuators are being developed.
“We need to be extremely fast,” said Georgia researcher Lukas Graber. “We have to separate the contacts within 250μs and to completely break the current within 500μs. For that reason, we cannot use spring-loaded or hydraulic actuators common to AC breakers. Devices that rely on the piezoelectric effect can do that for us.”
GaTech is also looking for insulators that can handle extreme temperatures – the photo is of Graber (left ) and Graber and researcher Chanyeop Park studying the such materials in a plasma – a low-energy argon plasma in this case.
If all goes to plan, a prototype will be evaluated at FSU’s 5MW test facility within three years.
The research has industrial partners, plus $3.3m from the US government’s ARPA-E’s organisation, who’s BREAKERS (Building reliable electronics to achieve kilovolt effective ratings safely) programme which is also supporting other approaches to interrupting high-power dc.
≥99.97% efficiency and ≥60MW/m3 density (passive or forced air cooling) is the target for circuit breakers developed under BREAKERS, with lifetime of at least 30,000 cycles and 30 years. Nuisance tripping has to be <0.1%.
According to ARPA-E, over 1,000km dc links should loose only 3.5% of the power they are carrying, compared with 6.7% for ac. Dc also requires two rather than three conductors and, for the same conductors and insulators, can carry 1.4x the power.
Material being evaluated in low-pressure plasma
A material being evaluated for use in new DC breakers is assessed in a low-pressure plasma at the Georgia Institute of Technology. A Langmuir probe with a tungsten tip is introduced into the plasma. (Photo: Rob Felt, Georgia Tech)