Submitted by:
Amitabha Saha

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cAnd Issues in PRACTICAL APPLICATION
IntroductionFault 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.
Properties of a superconductor
Superconductors have two outstanding features:
1). Zero electrical resistivity.
This means that an electrical current in a superconducting ring continues indefinitely until a force is applied to oppose the current.
2). The magnetic field inside a bulk sample is zero (the Meissner effect).
When a magnetic field is applied current flows in the outer skin of the material leading to an induced magnetic field that exactly opposes the applied field.
The material is strongly diamagnetic as a result.
In the Meissner effect experiment, a magnet floats above the surface of the superconductor
High Temperature Superconductor (HTS) Ceramics
Discovered in 1986, HTS ceramics are working at 77 K, saving a great deal of cost as compared to previously known superconductor alloys.
However, as has been noted in a Nobel Prize publication of Bednortz and Muller, these HTS ceramics have two technological disadvantages:
they are brittle and
they degrade under common environmental influences.
Applications of HTS
Magnetic Levitation- used in transport
MRI-Magnetic Resonance Imaging
Application in Electrical Engineering
HTS cables
Transformers to reduce resistive heating to near zero value and to increase life expextancy.
In Motor and Generators it can produce more torque than Cu cables because it can carry greater currents. And dissipation of Rotor coils is near zero.
Fault Current Limiters
Operating Principle of FCL
Under Normal Operation
Both cores operate in saturation
Low effective impedance of system
Low voltage drop
The direction of current and MMF in cores
Operating Principle of FCL
During fault operation
Cores comes out of saturation in alternative half cycle
2. Effective Impedance of the system increases
3. Limits the fault curren
- I Characteristics of FCL
PRACTICAL APPLICATION ISSUES
CoordinationIssues
Size
Cost
Maintenance Issues
Coordination issues for FCL application
The operation of relays with and without FCL was determined.
It was seen that when fault current is reduced below a certain limit, the relays could not determine the its operation point.
Coordination issues for FCL application
There was no serious coordination issues between the two relays.
Only the time of operation of the over-current relay was delayed due to less fault current value
Coordination time interval dt2 being greater than dt1 indicated better coordination between relays.
COST
Cryogenic system
Superconductors
High voltage insulation devices
SIZE
Volume of cryogenic system must be small to make the size of the installation viable.
Life and Maintenance
Increase in interruption capacity of circuit breakers becomes costly
To increase the operating time of CB more robust designs have to be implemented
A FCL can be a good choice to limit excessive fault current.
To construct a FCL for high voltage application many superconductors have to be arranged in series and parallel.
The capacity of cryogenic system has also to be increased.
TYPES of FCL
Inductive type
Resistive type
Hybrid Type
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.
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.
Hybrid SFCL
Under fault condition the electromagnetic repulsion plate moved up.
Fault current passes through resistor in parallel which limits the value of the current.
Hybrid SFCL