Title of Invention

"A DAMPERLESS SUPERCONDUCTING GENERATOR"

Abstract

The invention relates to a damperless superconducting generator wherein by developing a high short-circuit ratio eliminated the need of a damper and excitation system. Present liquid helium cooled superconducting generator requires a damper to protect field winding from quench. Using a damper needs an excitation system to counter rapid load change of the generator. Both the damper and the excitation system is very expensive. High short circuit ratio of 5 or more reduces quench and only 10% increase of no load field current will meet the full load requirement of the superconducting generator. A special manufacturing techniques of the winding ensured reliability and consistent high performance at speed of 3000 RPM or more. An air-gap type stator winding and a strong support system is developed to withstand electromagnetic torque and end wired forces.

Full Text

This invention relates to a super conducting generator. Liquid helium cooled superconducting generator has several advantages, such as/ double the output at half the electrical losses and higher short-circuit ratio (SCR)/ compared with a conventional generator of equal size. There are disadvantages associated with the liquid helium cooled superconducting generators. One of m ain disadvantages ia that there are sudden loss of superconductivity known as "quench" during the transient disturbances or unbalanced loading conditions of the generator. Such disturbances will cause alternating currents being induced in the superconducting field winding/ which will dissipiate AC loss in superconductors. If the loss is excessive it could lead to quench. Another disadvantage is that to overcome the above/ the super conducting winding are shielded by providing a rotating electromagnetic damper/ also known as warm damper or room temperature damper. This damper is a most expensive components in a superconducting generator. Yet another disadvantage with the electromagnetic damper is that the material having high mechanical strength to withstand strong electromagnetic and centrifugal forces and high electrical conductivity to damp AC fields is selected. The material should be non-magnetic to reduce field leakage flux. The damper slows down the response of, the excitation system during rapid load changes of the generation/ thereby affecting the dynamic stability of the power system. A further disadvantage is that in the superconducting field windings, the resistance R is zero and the inductanceL is large which requires long time to change the current/ unless high field forcing voltage ( ceiling voltage) is applied for the excitation system. Quick-response ( q-r) Excitation System with high ceiling voltage of the order 1 to 3 kV could over-stress the field winding leading to its eventual failure. A still further disadvantages is that when the current is rapidly changedin a superconductor/ AC loss is induced which could lead to quench. To overcome these problems/ a generator with q-r Excitation System should have high voltage insulation class-field winding and low AC loss superconductor with extra fine filaments of a few microns diameter for the field winding. Such superconducting generators with quick-response excitation system are very expensive. Therefore the main object of the present invention is to provide a simple/ reliable and economical superconducting generator/ wherein the electromagnetic damper is not required. An object of the present invention of an economical superconducting generator is to provide a device wherein the electromagnetic damper is not required. Another object of the present invention of an economical superconducting generator is to provide an innovative device where the quick-response Excitation System is not required. Yet another object of the present invention is to provide a reliable and consistently high performance of the superconducting field winding suitable for high speed application of rotational speed of 3000 rpm. Further object of the present invention of a super-conducting generator is to provide an air - gap type stator winding and a strong support system to withstand electr-magnetic torque and end winding forces. According to the present invention there is provided a damperless super conducting generator wherein high short-circuit ratio of 5 or more, and field winding laid in axial slots in a non - magnetic steel forgings, characterized in that the superconducting field winding (1) having an air-gap type armature winding (2) of special construction alongwith the extending lam inated iron core ( 3) for a negligible variation of current density from no load to full load which is quench - free to withstand accidental quench and short circuit forces. The nature of the invention, its objective and further advantages residing in the same will be apparent from the following description made with reference to non -limiting exemplary embodimentis of the invention represented in the accompanying drawing. Fig.1 - Cross section of the superconducting generator. Fig. 2 - Grid synchronisation of 200 KVA, 3000 RPM superconducting generator. Fig. 3 - Parallel operation of 200 KVA super-conducting generator with local grid (Reactive Power loading). Fig.4 - Direct-on-line starting of 20 HP Induction motor. Fig.5 - Sudden 3-phase short-circuit test on 200 KVA 3000 RPM superconducting generator. Fig. 6 - Quench test on 200 KVA, 300 RPM super-conducting generator. In accordance with the present invention of an economical superconducting generator, the requirement of the electromagnetic damper has been eliminated. This has has been achieved by designing the generator with high short-circuit ratio (SCR) of 5 or more, wherein the induced alternating currents and the AC loss in the superconducting field winding will be negligibly small to cause quench. High short-circuit ratio is achieved by the use of iron-free armature and field windings and optimising the spacing between them. The present invention of a damperless superconducting generator with the innovative design of high short-circuit ratio has also eliminated the need of the quick-response excitation system. High SCR of 5 or more alongwith 10% increase of no load field current will meet the full load requirement of the superconducting generator. Hence, the quick response excitation system is not required, and any commercially available high current DC power supply unit meant for superconducting magnets is adequate. A simple automatic voltage regulator (AVR) with load compensating loop may be provided. In the present invention, the winding of the super-conducting field is laid in axial slots milled in the body of a non-magnetic steel forging.lt is held in slotted portion by metallic wedges and in the overhang position by resiglass bandage and non-magnetic steel retaining rings. A unique impregnation system using cryo-compatible epoxy resin through vacuum-pressure process was developed for consolidating the superconducting field winding. These special manufacturing techniques ensured the reliability and consistently high performance of the superconducting winding for high speed application at rotational speed of 3000 rpm. The present invention has developed an air-gap type stator winding and a strong support system to withstand electromagnetic torque and end winding forces. An air-gap winding configuration gives higher output and minimum iron losses. A new variant of air-gap winding has been developed. The phase coil has been separately wounded on a speciallly shaped mould and epoxy encapsulated. Six of these pre-consolidated coils forming three-phase winding, are arranged symmetrically over an insuating support cylinder and tightly fitted inside a profiled laminated iron core. The winding is circumferentially locked to contain electromagnetic torque. The overhang winding is fully supported by extending the iron core and support cylinder beyond the straight part. The constructive feature of a Damperless Superconducting Generator designed and manufactured with the following specifications is described hereinbelow. 200 kVA, 300V, 3000RPM,3-phase,50Hz,2-pole,0.85 PF Short-circuit Ratio : 8.75 Superconducting Field winding (item 1 of Fig.1) was wound on non-magnetic steel rotor and cooled by liquid helium. Rated field current : 350A Superconductor Nb-Ti wire of diamter 1.52 mm Number of filaments 402 Filament diameter 30 microns Copper Superconductor rtio 4:19 Air-gap armature winding (2) is on the stator and cooled by air. A laminated iron core (3) encloses the stator winding. The above generator was subjected to a variety of operating conditions such as : (a) Synchronization of the generator with local grid. The test record is shown in Fig.2. (b) Loading and unloading of the generator up to rated capacity with constant field current of 350A. The test results are shown graphically in Fig.3. (c) Tripping the generator from grid and reloading the generator with resistive and inductive loads. No significant voltage drop was observed. (d) Unbalanced load testing of generator with 30% negative-phase sequence current. (e) Direct-on-line starting of 20 HP Squireel-cage Motor. The test record is shown in Fig. 4. This type of load test is unique and new. (f) Sudden 3-phase short-circuit test at reduced voltage. The test record is shown in Fig.5. The recorded wave-forms of the rotor current during the above tests did not indicate any significant AC ripples. The field winding retained the state of Superconductivity throughout the above tests. To test the integrity of the superconducting field winding even during accidental quency conditions, a test was conducted to deliberately create quench by increasing the current upto its critical value. The oscillographic record is shown in Fig.6. The test results showed that the superconducting field winding is capable of enduring quench conditions without damage and regained Superconductivity in a short time by increasing the liquid helium flow rate. The invention described hereinabove is in relation to a non-limiting embodiment and as defined by accompanying claims. CLAIM : 1 . A damparless superconducting generator wherein high short-circuit ratio of 5 or more, and field winding laid in axial slots in a non-magnetic steel forging, characterised in that, the superconducting field winding (1), an air-gap type armature winding (2) of special construction alongwith the extending laminated iron core (3) for a negligible variation of current density from no load to full load which is quench-free to withstand accidental quench and short-circuit forces. 2. A damperless superconducting generator as claimed in claim 1 , wherein high short-circuit ratio of 5 or more eliminates need of a damper and quick-response excitation system. 3. A damperless superconducting generator as claimed in claim 1 , wherein the field winding provides excitation from no load to full load with negligible variation of current density. 4. A damperless superconducting generator as claimed in claim 1 , wherein the field winding is quench-free during normal operating mode of the generator. 5. A damperless superconducting generator as claimed in claim 2, wherein the rotor withstands accidental quench without damage. 6. A damperless superconducting generator as claimed in claim I/ wherein a sturdy air-gap armature winding support system withstands high short-circuit forces. 7. A damperless superconducting generator as claimed in claim 1 wherein extending laminated iron core and air-gap type armature winding specially constructed of a unique impregnature system provides for high speed application.

Documents:

2814-del-1996-abstract.pdf

2814-del-1996-claims.pdf

2814-del-1996-correspondence-others.pdf

2814-del-1996-correspondence-po.pdf

2814-del-1996-description (complete).pdf

2814-del-1996-drawings.pdf

2814-del-1996-form-1.pdf

2814-del-1996-form-19.pdf

2814-del-1996-form-2.pdf

2814-del-1996-form-3.pdf

2814-del-1996-gpa.pdf