04-07-2012, 03:56 PM
Super Generators using high temperature superconductors
Super Generators using high temperature superconductors.ppt (Size: 4.9 MB / Downloads: 65)
Introduction
Superconductor
example: mercury 5K
Ni-Ti 4.2K
High temperature superconductor
examples: helium 4.2K
hydrogen 20K
Super generator
Generator concepts
Copper
Permanent magnet
Superconductor
Permanent magnet Synchronous generator
Field excitation obtained by mounting permanent magnets on the rotor.
It eliminates dc source and losses associated with field winding.
Power factor cannot be controlled because field excitation cannot be changed.
Why Supergenerator?
Conventional generator :
Expensive
Heavy and occupy large space
Less superior in maintaining power system stability
Less reliable
Less efficient
Prone to faults
Three Main Subsystems
Rotor
Rotor cooling
Stator
Generator is about half the length and two thirds the diameter of a conventional machine
Major components of the generator
Stator assembly:
AC stator winding
Back iron
Winding support structure
External field cooler module for cooling the field winding
2. Rotor assembly:
HTS field winding
Winding support structure
Cryostat
Torque tubes
EM shield
Unique stator windings
Employ no magnetic iron teeth
Magnetic field experienced is 1.6T (2 times that in conventional machines)
This will saturate stator iron teeth.
Refrigeration system uses cold circulating helium/neon
Helium/neon flows in a closed loop from rotor body to GM cold heads.
The boiling point of helium/neon is near the HTS operation point
The HTS wire material used in super machine is BSCCO or Bi-2223 material which has a critical temperature of 35K.
The Bi-2223 is a high strength reinforced wire which is able to withstand close to 300 MPa tensile stress and 0.4% tensile strain at 77K.
This reinforced wires provide a mechanically robust and reliable product, which are suitable for making high performance propulsion motors and generators.
EM shield
Protects the field winding by attenuating asynchronous fields produced by the stator windings.
Carries high transient torque during a fault.
Withstands large forces during a 3-phase short circuit faults.
Absorbs steady state negative sequence heating.
Performance characteristics
Super generators produces nearly clean AC voltage in the stator voltage.
Harmonic components are very minimal.
The negative sequence currents induce a 1000W loss in the EM shield.
These losses can be easily removed by the cooling system employed in the machine.
The machine has a very low synchronous reactance of 0.28pu with a load angle of 10.4 degrees at full load.
The low reactance allows their operation over whole load spectrum without requiring variation of the field excitation.
The HTS generator has a field excitation voltage under full load of 1.2pu, whereas a conventional machine has 2.7pu.
Current at short circuit will be lesser for a HTS generator compared to a conventional machine.
Super generator operates at peak torque of 4.3pu and because of very small load angel it is a stiff device.
It will not go unstable during a transient operation.
It would contribute to better power system stability.
This generator could be overloaded within stator winding loss limits.
SC and OC characteristics
In case of the conventional iron-cored synchronous machine, the open circuit voltage is saturated due to magnetic saturation of iron-core as the field current increases.
In HTS generator no Ferro-magnetic material (iron) is used for the rotor and the armature teeth.
Advantages
Compared to equivalent rating of conventional machine these Super Machines are expected to be :-
Increased efficiency.
More compact and lighter.
Better stability, high overload capacity.
Voltage quality.
Reactive power capability.
Less noise and vibration(no armature teeth).
Applications
Wind power generators.
High torque ship propulsion.
High speed generators.
Industrial generators and motors.
Utility generators.
Conclusions
Given the current HTS technology and the cost of the HTS wire, the cost disadvantage of a 100 MVA HTS generator, combined with its relatively infrequent operation, more than offsets any efficiency benefits.
Larger generators with ratings greater than 500 MW may be suitable candidates for the HTS technology.
They offer greater efficiency benefits and are more likely to be operated as base-loaded units.
References
A small size superconducting generator conceptKalsi, S.; American Superconductor Corp., Westborough, MA
This paper appears in: Electric Machines and Drives Conference, 2001. IEMDC 2001. IEEE International