GB1563217A - Superconducting dynamoelectric machines - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO SUPERCONDUCTING DYNAMO-ELECTRIC MACHINES (71) We, NORTHERN ENGINEER ING INDUSTRIES LIMITED, a British Company, of NEI House, Regent Centre, Newcastle upon Tyne, NE3 3SB, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a synchronous alternating current machine having a D.C. excited rotor field winding of superconducting material and a stator armature winding of non-superconducting material.

In British Patent Specification No.

1,315,302 an electrical machine was described comprising a direct current field winding of superconducting material carried by a rotor provided with means to maintain the superconducting winding at the cryogenic tem peratures required to permit superconducting operation of the winding, the rotor being surrounded by a stator carrying an armature winding of non-superconducting material. The armature winding in such a machine may be of the alternating current multi-phase doublelayer type common in conventional turbogenerators when the machine is designed as a power generator.

British Patent Specification No. 1,351,601 is concerned with the requirement in machines as described in the preceding paragraph to screen the superconducting winding on the rotor from time-varying magnetic fields arising due to unbalanced load conditions which may occur in the armature winding. In a preferred machine according to British Patent Specification No. 1,351,601 the rotor is constructed in two parts, an inner part carrying the superconducting winding and an outer part carrying the screen, a sealed vacuum space being formed between the inner and outer parts of the rotor and a thermal radiation shield being located in the vacuum space. The rotor screen on the outer rotor part in such an arrangement must absorb all short circuit forces arising due to faults in the armature circuit or changes in the machine load and must further act to damp rotor oscillations following a load disturbance in addition to its screening functions.

Fulfilment of all the rotor screen requirements by a single screen associated with the outer rotor part can impose undesirable design restrictions on the machine.

According to the present invention a dynamo-electric machine is provided having a stationary alternating current winding of non-superconducting material surrounding a rotor carrying a direct current field winding of superconducting material, the rotor comprising an inner part carrying the superconducting field winding and an outer part spaced from the inner part to form a sealed vacuum space in which a thermal radiation shield is disposed, wherein an electromagnetic ,flux screen for the superconducting field winding is formed of inner and outer parts, the inner screen part being adjacent the radiation shield and the outer screen being formed by the outer part of the rotor.

Preferably the outer part of the rotor has a body composed of a material, such as ferritic steel, which also performs the screening function. If the material of the body does not have a high enough electrical conductivity (being, for example, an austenitic stainless steel) or is otherwise insufficient to form the outer screen part, a layer of material of higher electrical conductivity, for example an aluminium layer, may be formed on the surface of the body.

The invention will now be described in more detail with the aid of examples illustrated in the drawings accompanying the provisional specification, in which: Figure 1 is a simplified sectional side elevation of a superconducting alternator in accordance with one form of the present invention, Figure 2 is a transverse section through the rotor of the alternator shown in Figure 1, and Figure 3 is a transverse section through the rotor of an alternator according to an alternative form of the invention.

Referring first to Figure 1, the machine shown comprises a stator 1 provided with a multi-phase alternating current winding 2 and a rotor 3 composed of an outer rotor part 4 and an inner rotor part 5, the latter part carrying a superconducting direct current field winding 6 the end turns only of which are visible.

The rotor winding 6 is cooled to the necessary cryogenic temperature for superconducting operation by means of liquid helium supplied by way of an inlet pipe 7. The entire inner rotor part 5 is maintained in the region of the temperature of winding 6 by providing cooled end supports 8 and 9 of small cross-sectional area and by maintaining a vacuum in the space 10 between outer rotor part 4 and inner rotor part 5. Heat in-leak to the inner rotor part 5 from the ambient temperature outer rotor part 4 is minimised by the provision of a thermal radiation shield 11. The thermal radiation shield is supported from positions intermediate the length of the end supports 8 and 9 to enable it to be conduction cooled to a temperature intermediate the operating temperatures of the inner and outer rotor parts.

In accordance with the preferred form of the invention, which is applicable to machines of the order of 1300 MW generating capacity, the outer rotor part is formed of material enabling it to act as the outer part of a twopart electromagnetic flux screen arrangement for the inner rotor and superconducting winding whilst the inner part of the flux screen comprises a layer 12 of material of high electrical conductivity lining the cylindrical thermal shield 11. The disposition of the inner rotor part, outer rotor part, radiation shield and inner flux screen part is more clearly seen in Figure 2.

The outer rotor part 4 is preferably constructed of ferritic steel having typical dimensions of 707mm outer radius and 575mm inner radius whilst operating at a mean temperature of 325"K. The preferred material for layer 12 is aluminium whilst the radiation shield may be constructed of A286 stainless steel. Typical dimensions for the radiation shield are an outer radius of 565mm and thickness of 1Smm, whilst the layer 12 of aluminium may be 15mum thick. Suitable operating temperatures for the radiation shield are 65 K at each end and 850K in the central region.

The adoption of a two-part flux screen as described greatly eases the design criteria for machines of the type considered as compared with a single screen located at or formed by the outer rotor part. In the design described in the previous paragraph, for example, whilst the outer rotor part is arranged to withstand 95% of short circuit forces and only 5% of such forces are required to be met by the inner screen and the radiation shield combination, the outer rotor part maximises the rotor oscillation damping which may be achieved, yet is maintained within dimensions substantially less than would be required if a single screen at the outer rotor part were adopted. The design enables flux variations at the superconducting winding due to rotor swing frequency variations of flux density at the outer rotor to be limited to less than 0.3 Tesla peak to peak, a figure corresponding to acceptable total losses in superconducting composites presently available to form suitable windings.

The alternative form of the invention shown in Figure 3 is applicable to machines of substantially smaller ratings than the 1300 MW design previously considered, and includes a further electrically conducting screening layer 13 carried at the inner surface of the outer rotor part 4. The reason for the provision of this additional layer is that in a smaller machine the screening effect of the material in the outer rotor part may not of itself be sufficient to limit the short circuit force on the radiation screen to under 5% of the total short circuit forces, whilst sufficient material may be present in the outer rotor part to provide adequate mechanical strength to withstand the entire effect of short circuit forces. Layer 13 may suitably be formed of aluminium, whilst the outer rotor part 4 may be formed either of ferritic steel or of austenitic stainless steel of lower electrical conductivity.

WHAT WE CLAIM IS: 1. A dynamolectric machine having a stationary alternating current winding of nonsuperconducting material surrounding a rotor carrying a direct field winding of superconducting material, the rotor comprising an inner part carrying the superconducting field winding and an outer part spaced from the inner part to form a sealed vacuum space in which a thermal radiation shield is disposed, wherein an electromagnetic flux screen for the superconducting field winding is formed of inner and outer parts, the inner screen part being adjacent the radiation shield and the outer screen part being formed by the outer part of the rotor.

2. A machine as claimed in claim 1 wherein the outer part of the rotor comprises a layer of material of relatively high electrical conductivity on a body of lower electrical conductivity.

3. A machine as claimed in claim 1 or 2 in which the outer part of the rotor has a body composed of ferritic steel.

4. A machine as claimed in claim 1, 2 or 3 in which the inner screen part is a layer on the inner surface of the radiation shield.

5. A dynamo-electric machine substantially as described with reference to Figs. 1 and 2 or Figs. 1 and 3 of the drawings accompanying the provisional specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. shown comprises a stator 1 provided with a multi-phase alternating current winding 2 and a rotor 3 composed of an outer rotor part 4 and an inner rotor part 5, the latter part carrying a superconducting direct current field winding 6 the end turns only of which are visible. The rotor winding 6 is cooled to the necessary cryogenic temperature for superconducting operation by means of liquid helium supplied by way of an inlet pipe 7. The entire inner rotor part 5 is maintained in the region of the temperature of winding 6 by providing cooled end supports 8 and 9 of small cross-sectional area and by maintaining a vacuum in the space 10 between outer rotor part 4 and inner rotor part 5. Heat in-leak to the inner rotor part 5 from the ambient temperature outer rotor part 4 is minimised by the provision of a thermal radiation shield 11. The thermal radiation shield is supported from positions intermediate the length of the end supports 8 and 9 to enable it to be conduction cooled to a temperature intermediate the operating temperatures of the inner and outer rotor parts. In accordance with the preferred form of the invention, which is applicable to machines of the order of 1300 MW generating capacity, the outer rotor part is formed of material enabling it to act as the outer part of a twopart electromagnetic flux screen arrangement for the inner rotor and superconducting winding whilst the inner part of the flux screen comprises a layer 12 of material of high electrical conductivity lining the cylindrical thermal shield 11. The disposition of the inner rotor part, outer rotor part, radiation shield and inner flux screen part is more clearly seen in Figure 2. The outer rotor part 4 is preferably constructed of ferritic steel having typical dimensions of 707mm outer radius and 575mm inner radius whilst operating at a mean temperature of 325"K. The preferred material for layer 12 is aluminium whilst the radiation shield may be constructed of A286 stainless steel. Typical dimensions for the radiation shield are an outer radius of 565mm and thickness of 1Smm, whilst the layer 12 of aluminium may be 15mum thick. Suitable operating temperatures for the radiation shield are 65 K at each end and 850K in the central region. The adoption of a two-part flux screen as described greatly eases the design criteria for machines of the type considered as compared with a single screen located at or formed by the outer rotor part. In the design described in the previous paragraph, for example, whilst the outer rotor part is arranged to withstand 95% of short circuit forces and only 5% of such forces are required to be met by the inner screen and the radiation shield combination, the outer rotor part maximises the rotor oscillation damping which may be achieved, yet is maintained within dimensions substantially less than would be required if a single screen at the outer rotor part were adopted. The design enables flux variations at the superconducting winding due to rotor swing frequency variations of flux density at the outer rotor to be limited to less than 0.3 Tesla peak to peak, a figure corresponding to acceptable total losses in superconducting composites presently available to form suitable windings. The alternative form of the invention shown in Figure 3 is applicable to machines of substantially smaller ratings than the 1300 MW design previously considered, and includes a further electrically conducting screening layer 13 carried at the inner surface of the outer rotor part 4. The reason for the provision of this additional layer is that in a smaller machine the screening effect of the material in the outer rotor part may not of itself be sufficient to limit the short circuit force on the radiation screen to under 5% of the total short circuit forces, whilst sufficient material may be present in the outer rotor part to provide adequate mechanical strength to withstand the entire effect of short circuit forces. Layer 13 may suitably be formed of aluminium, whilst the outer rotor part 4 may be formed either of ferritic steel or of austenitic stainless steel of lower electrical conductivity. WHAT WE CLAIM IS:

1. A dynamolectric machine having a stationary alternating current winding of nonsuperconducting material surrounding a rotor carrying a direct field winding of superconducting material, the rotor comprising an inner part carrying the superconducting field winding and an outer part spaced from the inner part to form a sealed vacuum space in which a thermal radiation shield is disposed, wherein an electromagnetic flux screen for the superconducting field winding is formed of inner and outer parts, the inner screen part being adjacent the radiation shield and the outer screen part being formed by the outer part of the rotor.

2. A machine as claimed in claim 1 wherein the outer part of the rotor comprises a layer of material of relatively high electrical conductivity on a body of lower electrical conductivity.

3. A machine as claimed in claim 1 or 2 in which the outer part of the rotor has a body composed of ferritic steel.

4. A machine as claimed in claim 1, 2 or 3 in which the inner screen part is a layer on the inner surface of the radiation shield.

5. A dynamo-electric machine substantially as described with reference to Figs. 1 and 2 or Figs. 1 and 3 of the drawings accompanying the provisional specification.