Surge Current Protection using Superconductors
Published on Feb 12, 2016
Damage from a short circuit is a constant threat to any electric power system. Insulation damaged by aging an accident or lightning strike can unloose immense fault currents practically the only limit on their size being the impedance of the system between their location and power sources.
At their worst, faults can exceed the largest current expected under normal load - the nominal current by a factor of 100 producing mechanical and thermal stresses in proportion to the square of the current's value.
All power system components must be designed to withstand short circuit stresses for certain period determined by time needed for circuit breakers to activate (20-300 ms). The higher the fault currents anticipated the higher will be the equipment and also the maintenance cost. So there obviously is a big demand for devices that under normal operating conditions have negligible influence on power system but in case of fault will limit the prospective fault current. A device of this kind is called fault current limiter.
According to the accumulated intelligence of many utility experts, an ideal fault current limit would:
(i) Have zero impedance throughout normal operation
(ii) Provide sufficiently large impedance under fault conditions
(iii) Provide rapid detection and initiation of limiting action within less than one cycle or 16ms.
(iv) Provide immediate (half cycle or 8ms) recovery of normal operation after clearing of a fault.
(v) Be capable of addressing tow faults within a period of 15 seconds.
Ideal limiters would also have to be compact, light weight inexpensive, fully automatic, and highly reliable besides having long life.
In the past, the customary means of limiting fault current have included artificially raising impedance in the system with air-coil rectors or with high stray impedance of transformers and generators or splitting power-grids artificially to lower the number of power sources that could feed a fault current. Nut such measures are inconsistent with today's demand for higher power quality, which implies increased voltage stiffness and strongly interconnected grids with low impedance.
What is need is a device that normally would hardly affect a power system bit during a fault would hold surge current close to nominal value that is a fault current limiter. Until recently most fault current limiter concepts depend on mechanical means, on the detuning of L_C resonance circuit or use of strongly non-linear materials other than High Temperature super conditions (HTS). None is without drawbacks.
Super conductors because of their sharp transition from zero resistance at normal currents to finite resistance at higher current densities are tailor made for use in fault current limiters. Equipped with proper power controlled electronics, a super conducting limiter can rapidly detect a surge and taken and can also immediately recover to normal operation after a fault is cleared.
Superconductors lose their electrical resistance below certain critical values of temperature, magnetic field and current density. A simplified phase diagram of a super conductor defines three regions
Low-temperature superconducting (LTS) wire has been available for several decades. Its ac losses have been reduced by the development of multi filament wire. The diameter of the filament is of the order of 0.1µm and they are decouples by a highly resistive, normal conducting matrix which also serves as thermal stabilization. Since any magnetic field interacts only with the very thin and decoupled filaments, the ac losses in the materials are tolerable even at extremely low temperatures (for LTS application, usually 4.2 K, boiling point of liquid helium).
Kept this cold, the specific heat of LTS is very low, but the current carrying capacity is very high (greater than 105 ACm2). Consequently any conceivable SCFCL based on LTS would exceed its critical temperature within several hundred microseconds of a fault. By the same token the material is prone to hot spots, which some tiny disturbance can trigger even at sub critical current values.
Because of such properties LTS material is predestined for the fast heating resistor design. A fast homogenous transmission into the normal conducting state is supported by excellent thermal conductivity which together with the low specific heat, leads to rapid propagation of hot spots.
While there is only one large program left in the low temperature type of SCFL, more than 10 major projects are under way worldwide on high temperature type of device. The main reason in the lower HTS cooling cost.
Essentially just three types of HTS materials are available; all made from bismuth (BSCCO) or yttrium-cuprate (YBCO) compound. They are silver sheathed wire (based on Bi 2223), thin films (based on YBCO) and bulk material (based on Bi 2212, Bi 2223 or YBCO). Usable in varying degrees either resistive or shielded core SCFLs, these materials are very poor at conducting heat, unlike the LTS. In other words, hot spots don’t propagate fast in the HTS so that electrical stabilization becomes a major concern.
The HTS materials with the highest critical current are YBCO films. They are typically, 1µm thick and have a current criticality threshold at 77K or up to 2000KA cm-2. But it is very difficult produce YBCO films that are either long or extensive. Nevertheless, several groups are developing limiters based on these materials. Because of their high critical current and the need to conserve material, any economically justifiable design will perforce be of fast heating type. The huge electric field-current density product in a fault will heat the HTS to the point of normal resistance setting in within a few hundred microseconds.
SCFLs may be categorized as resistive or shield core.
In the resistive SCFCL, the super conductor is directly connected in series with the line to be protected. To keep it superconducting, it is usually immersed in a coolant that is chilled by a refrigerator. Current leads are designed to transfer as little heat as possible from the outside to the coolant.
In normal operation, the current and its magnetic field can vary but temperature is held constant. The cross section of super conductor is such as to let it stay below critical current density, since its receptivity is zero in this regime; the impedance of the SCFCL is negligible and does not interfere with the network. All the same the superconductor’s impedance is truly zero only for dc currents. The more common as applications are affected by two factors. First, the finite length of the conductor produces a finite reactance which however can be kept low by special conductor architecture.
Second a superconductor is not loss free in ac operation, the magnetic as field generated by the current produces so called ac losses basically, just eddy current losses. These are heavily influenced by the geometry of the conductor and can be reduced by decreasing the conductor dimension transverse to direction of local magnetic field. They barely contribute to total SCFCL impedance but dissipate energy in superconductor, thus raising cooling costs.
In case of a fault the inrush of current and magnetic field take the super conductor into the transition region, between zero resistance and normal receptivity. The fast rising resistance limits the fault current to a value some where between the nominal current and what ever fault current otherwise would ensure. After some time, perhaps a tenth of seconds, a breaker will interrupt the current.
The behavior of resistic fault current limiter is largely determined by the length of the superconductor and the type of material used for it.
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