Latest Advances in Superconducting Power Apparatus for Electric Utilities
Dr. Stephen Kuznetsov
The Superconducting Synchronous Condenser (SSC) is a high- speed rotating machine with a superconducting field wound with either YBCO or MgB superconductor and cooled with liquid nitrogen or liquid neon. A rotating magnetic field is established at a high field ( 5-7 Tesla ) and the stator is an air-core helical wound unit with 5 kV to 15 kV, 3 phase output. The machine acts to compensate for lagging power factor on a long transmission line or large inductive loads and is commonly referred to as a “rotating capacitor” for it produces current which leads the voltage by 90 degrees. These machines at very high power levels e.g. 8-200 MVAR are much more compact than a static capacitor bank and offer infinite controllability of VARS from zero to the maximum rating of the machine. The SSC currently under development at PSA is an 8 MVAR machine at 5 kV with a 3600 rpm rotating field. The newest technology is geared to YBCO conductors at 77 K. When in use on systems up to 500 kV, the SSC allows greater flow of real power in a large system without the problems of sub-synchronous resonance and instabilities which have been found to exist with large utility capacitor banks on the West Coast. The SSC also increases the critical fault clearing time (CFCT) of both distribution and transmission systems.
The Superconducting Current Fault limiter (SCFL) is a compact device being developed for use in 5 kV to 115 kV power systems that rapidly impedes the flow of current upon sensing a high rate of rise of utility line current. The SCFL development at PSA has both copper and superconducting coils and automatically inserts inductance into the power line upon reaching a threshold level in dI/dt or absolute current. The response time is geared to produce maximum inductance in 4 ms, consistent with utility operating parameters. The new technology does not allow the superconductor to quench and consequently the device reset time is 10 ms rather than 3 minutes since there is no need to re-cool the cryostat. The advantage of the SCFL to the utility operator is the ability to eliminate lossy power station series air-core reactance that have both real and reactive power losses. Alternately the SCFL allows the utility to reduce the main transmission step-up transformer internal reactance to a minimum adding to the overall power system stability.