HTS Fault Current Limiters By: Leanne Naidoo & Pranesh Sarjoodei

Introduction Fault Current,Circuit Breakers Fault currents are unanticipated power disturbances in the utility grid caused by lightning etc This causes circuit breakers to open and close. Circuit breaker will remain open in wait for repair crew. Until the power is restored, an outage occurs. In today’s electricity dependant economy, significant hardships and economic losses occur during the outages

What is a FCL? When lightening or some other event causes a fault condition, the circuit breakers shut down the system to protect it from severe damage. A large HTS FCL acts as an energy absorbing device, reducing the current surge and the duty on circuit breakers to isolate the fault. HTS fault current limiter based on melt-cast BSSCO, system design and laboratory test

How Does HTS FCL Work? Takes adv of superconducting phenomena. At low temp, resistance is negligible, allowing high current transfer. When low temp are allowed to rise the resistance increases and limits current flow, hence protecting the downstream equipment from high/fault currents. Basically a variant of a transformer, this would significantly reduce fault currents and lower switch or breaker ratings in multiple circuit systems to the offending source within a given circuit.

Benefits of a HTS FCL Reduces high currents. Fast Response with no active control. Invisible to grid during normal operation No need for expensive circuit breakers with high interrupting capability. FCL in transformers can run times the steady state design current. HTS FCL can reduce this to levels not exceeding 3-5 times the steady state design current

Comparison To Conventional Solutions System fault current level rise with added generators loads, etc.. Solutions? 1.Circuit breaker technology near 80 kA upper limit. Interrupt power. Electric service is shut down unless an alternate path is found Explosive fault limiting fuses require service call to be replaced. Series reactors lossy, bulky, contribute to grid, costly power systems. 2. HTS fault current limiters (FCLs) : Reduction of fault currents by a factor of Single shot protection for 5-10 cycles. Automatic resetting after a recovery time of sec to min. Distribution and transmission voltages up to ~138 kV HTS FCLs–an attractive solution to a critical utility problem

Types of HTS FCL Types of HTS FCL’s : Bridge FCL Inductive FCL. Resistive FCL 1 st generation (1G)HTS wire 2 nd generation (2G) HTS wire Resistive HTS FCL can address critical utility issues cost- effectively 2G HTS wire is optimum for a practical resistive FCL

MFCL-An Improved HTS FCL MFCL (Matrix FCL) MFCL Design- columns :current limiting impedance required. Benefits: Modularity and scalability-By way of the matrix configuration. Reliability-Redundancy can be easily built in Cost benefit –No conventional counterpart, more economical than most other solutions. Handle max of 800Arms.

MFCL Test Results 1. Good sharing among current limiting elements 2. No excessive stress on any element 3. Fast dynamic Res. development & 1 st peak limitation 4. Fast current transfer from HTS elements to shunt coils Reduces stress on HTS Voltage elements during fault

Inductive HTS FCL Limiter suitable for high-current circuits (IL > 1000 A) A copper winding WCu is inserted in the circuit and is coupled to an HTS winding WHTS Normal operation, a zero impedance is reflected to the primary Resistance developed in the HTS winding during a fault is reflected to the primary and limits the fault. The large current is induced in the secondary winding which loses superconductivity. Figure: Inductive fault-current limiter

Inductive HTS FCL Figure: Cross section and Photograph of the inductive FCL

Series Resistive HTS FCL Exploits the nonlinear resistance of superconductors in a direct way. A superconductor is inserted in the circuit & for a full-load current of IFL, the superconductor would be designed to have a critical current of 2IFL or 3IFL. Fault current pushes the superconductor into a resistive state & resistance R appears in the circuit. The superconductor in its resistive state can also be used as a trigger coil, pushing the bulk of the fault current through a resistor or inductor. Figure: Fault- current limiter with HTS trigger coil

Resistive HTS FCL

Swiss Hydro Power Plant Using Resistive HTS FCL Inductive Tubes Based on Bi-2212

The Market HTS Fault Current Limiters (FCLs) address the market pull to cost-effectively correct fault current over-duty problems at the transmission voltage level of 138kV and higher The HTS FCLs will reduce the available fault current to alower, safer level (~50%,possibly greater) so that existing switchgear can still protect the grid HTS FCLs are anatural compliment to AC HTS cable systems

Design of a SFCL Limits fault currents to <60% of the steady state prospective fault current

British FCL Project The new £2.5m FCL and intelligent grid management project is lead by Rolls Royce Electrical Systems and Strategic Research Centre, with the finished machine expected to be installed at Scottish Power's test facility; the project involves Diboride Conductors, the University of Manchester, Strathclyde University, Hyper Tech Research (Ohio) and Scientific Magnetics (Oxford). The successful project which finished earlier in 2007 developed a single-phase 220V/1kA FCL using an innovative solid-state and cryocooler cooling system, operating at 24-30K. It validated current sharing between parallel superconducting wires during FCL operation

Technology Status A majority of American Superconductor's second generation (2G) high-temperature superconductor (HTS) wire, known as 344 superconductors, is being used in the development of fault current limiters.American Superconductor's There has been strong demand for fault current limiters among domestic and overseas utilities. American Superconductor’s fault current limiter product development effort with Siemens, has at least seven other electrical equipment developers in four countries who are now utilizing 344 superconductors to develop fault current limiters. This is a vast new market opportunity for HTS that the U.S. Department of Energy forecasts to be in the billions of dollars.

Technology Status Zero resistance up to a “critical” current. Ultra-fast switching to high resistance at the critical current. Immediate limitation of fault currents. Rapid recovery to zero resistance. Prototype superconductor fault current limiters expected to be deployed in the grid by the end of 2007.

HTS FCL Develepmont

References To learn more about FCLs please see ers.cfm and Siemens' activities on superconducting components and equipment please see ms/innovations.htm and o ps4u&sdc_si d= & gy.cfm pdf