04-01-2013, 10:37 AM
Superconducting Fault Current Limiter(SFCL)
Superconducting.ppt (Size: 576 KB / Downloads: 166)
Introduction
In highly interconnected and expanded power system ,
faults of increased magnitude start creeping frequently
into the system , so we have to look for a solution that
can help reduce these increased magnitudes of fault
currents.
Problems of Fault Control
Many approaches to limiting fault currents have been proposed in the past which include the use of circuit breakers with ultra-high fault current rating, high impedance transformers and current limiting fuses.
Circuit breakers are expensive, cannot interrupt fault currents until the first current zero comes and also they have limited life times.
The high impedance transformer with their high losses makes the system inefficient.
The fuses have a very low withstandable fault current and it has to be replaced manually.
High Temperature Superconductivity
The phenomenon which occurs at relatively higher temperature.
There transition temperature(Tc )is above 30K.
Cooled by relatively inexpensive liquid nitrogen(b.p. 77).
Examples:
1) Yttrium Barium Copper Oxide(YBCO) has Tc at 92 K
2) Bismuth Strontium Calcium Copper Oxide(BSCCO) has Tc at 110 K.
Principle of SFCL
During normal operation :
SFCL is superconducting => impedance of SFCL i.e. Z(SFCL) =0 , =>SFCL
conducts without losses.
In the event of fault:
electric current rises above the critical value => the superconductivity of current limiter shuts off =>resistance of current limiter rises instantly
=>thus it limits the fault current.
OPERATION
A resistive FCL limits the fault current by its increased resistance when the HTS wire
transitions to its normal state during a fault.
The key parameters impacting the resistive FCL design are fault current (Ilim), fault duration (Δt) and permissible temperature
rise (ΔT) of the HTS elements.
RESISTIVE FCL 3 PHASE ASSEMBLY
It has three HTS coil subassemblies,one for each phase. The HTS coils are submerged in liquid nitrogen maintained at 15 bar pressure.
Liquid nitrogen is contained in a double wall vessel. The outer vessel contains liquid nitrogen at atmospheric pressure, which boils at 77K. This nitrogen cools the pressurized nitrogen in the inner vessel through the wall. The top portion of the inner vessel is not insulated to permit pressurized gaseous nitrogen to separate 77K liquid from the top plate of the vessel at room temperature. Current leads connect HTS coils in liquid nitrogen to the room-temperature bushings. Standard 20kV bushings are suitable for outdoor operation.