德国 380 kW synchrorotor

380 kW synchronous machine with HTS rotor windings––development at Siemens and first test results W. Nick a,*, G. Nerowski b, H.-W. Neum€uuller a, M. Frank a, P. van Hasselt a, J. Frauenhofer c, F. Steinmeyer d a Siemens Corporate Technology, Siemens AG, CT EN 4, P.O. Box 3220, D-91050 Erlangen, Germany b Siemens Automation & Drives, P.O. Box 4743, D-90025 Nuremberg, Germany c Siemens Automation & Drives, P.O. Box 3220, D-91050 Erlangen, Germany d OMT, Wharf Road, Eynsham, Oxon OX29 4BP, UK Abstract Applying HTS conductors in the rotor of synchronous machines allows the design of future motors or generators that are lighter, more compact and feature an improved coefficient of performance. To address these goals a project collaboration was installed within Siemens, including Automation & Drives, Large Drives as a leading supplier of electrical machines, Corporate Technology as a competence center for superconducting technology, and other partners. The main task of the project was to demonstrate the feasibility of basic concepts. The rotor was built from racetrack coils of Bi-2223 HTS tape conductor, these were assembled on a core and fixed by a bandage of glass-fibre composite. Rotor coil cooling is performed by thermal conduction, one end of the motor shaft is hollow to give access for the cooling system. Two cooling systems were designed and operated successfully: firstly an open circuit using cold gaseous helium from a storage vessel, but also a closed circuit system based on a cryogenerator. To take advantage of the increased rotor induction levels the stator winding was designed as an air gap winding. This was manufactured and fitted in a standard motor housing. After assembling of the whole system in a test facility with a DC machine load experiments have been started to prove the validity of our design, including operation with both cooling systems and driving the stator from the grid as well as by a power inverter. � 2002 Elsevier Science B.V. All rights reserved. Keywords: Superconducting motor; Superconducting generator; Superconducting electrical machine 1. Introduction When the well-known advantages of supercon- ductors, such as high current densities, the possi- bility of high magnetic fields and dc transport with negligible electrical losses are brought to bear in the field of electrical machines, it is straightfor- ward to choose the synchronous motor (or gen- erator). In this case the rotating field winding uses a dc electrical current to generate a rotating mag- netic vector and this interacts with the rotating ac fields of the stator or armature winding to gen- erate torque. With the selection of superconduc- tivity it is possible to generate higher induction Physica C 372–376 (2002) 1506–1512 www.elsevier.com/locate/physc *Corresponding author. Fax: +49-9131-721339. E-mail address: wolfgang.nick@erls.siemens.de (W. Nick). 0921-4534/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved. PII: S0921 -4534 (02 )01069 -9 levels even using a smaller rotor, and this with drastically reduced exciter losses. On the part of the stator the iron teeth have to be omitted–– otherwise their magnetic saturation would be limiting. Using this so-called air gap winding tech- nique it is possible to place more turns and thus increase the output torque. Using a supercon- ducting rotor and air gap stator windings thus promises to provide a new type of electrical ma- chine with high efficiency and higher power density than conventionally available, and in addition there are strong dynamic advantages that become relevant for transient situations. These prospects are not a new discovery. Al- ready more than 20 years ago big efforts were under way to develop (large) superconducting generators on the basis of ‘low temperature su- perconductors’ (LTS) [1], but only in Japan these projects were continued far enough to large scale tests [2]. Since the advent of ‘high temperature superconductors’ and the availability of robust HTS conductors in industrial lengths there was the chance to do the same on the basis of a more economic technology [3], also in terms of the re- frigeration system which promises to make smaller size industrial motors economically feasible. Thus the ‘Automation & Drives, Large Drives’ division of Siemens decided to explore the potential by developing and constructing an HTS demonstra- tion machine, with specific goals of testing the feasibility of concepts such as a rotating super- conducting field winding and a robust cooling system, but also as a starting point for a possible future product development (Table 1). This pro- ject was addressed in collaboration with Corpo- rate Research, which contributes experience in superconductivity and cryoengineering, and ex- ternal project partners [4]. These are developing a new generation of powerful and robust pulse tube coolers in order to improve the applicability and reliability of an industrial HTS cooling system. Efforts to create large HTS bearings might become important for the area of high speed HTS motors, the state of these works is presented elsewhere on this conference [5]. 2. Design and construction of rotor The basic design of the exciter winding is the quadrupole. The windings are composed of flat pancake coils of Bi-2223 conductor with Mg- reinforced Ag sheath, about 9.5 km manufactured by NST (Nordic Superconductor Technologies) were used for 48 coils (see Fig. 1). To ensure the performance a substantative quality programme was carried out: continuous dimensional checks in all manufacturing stages of the conductor, deter- mination of local critical current, comparison of critical parameters at different operating condi- tions (temperature, B-field) on samples. We used a wet winding technique with carefully controlled tension to manufacture a set of coils that were uniform in dimensions. These coils were stacked on a pole former made from a magnetically con- ductive steel. This reduced the quantity of wind- ing turns needed. The rotor design generates a peak induction of about 1.1 T on the outside of the cryostat, this meant a maximum induction in the winding space of about 2.5 T. Fig. 2 presents the 2D FE calculation and gives an impression of the basic configuration. The operating temper- ature for the winding was set in the range from 25 Fig. 1. Pancake coils for the 380 kW model motor. Table 1 Model motor design parameters Rating/speed 380 kW/1500 rpm Armature 400 V/560 A/air cooling Housing dimensions Diameter 700 mm/ length 1100 mm Field winding 49 A/25–30 K/Ne-thermosiphon Maximum induction at winding/cryostat surface 2.5 T/1.1 T W. Nick et al. / Physica C 372–376 (2002) 1506–1512 1507 to 30 K, in order to have sufficient current capacity at these fields. Fig. 3 shows the completed cold mass with all coils and filler pieces assembled. The parameters for the following bandaging process (prestress and thickness of bandage) were deter- mined from numerical FE calculations, checking stresses in the composite structure and limiting the maximum deformation of the coils to about 5 lm. At this stage the cold mass was balanced, in order to limit the mechanical loads on the elements connecting the cold mass to the room temperature flanges of the shafts. On the drive-end side this is the so-called torque tube, a very critical component of the total system, which has to be designed to withstand torsional loads an order of magnitude larger than nominal, while at the same time minimizing heat conducted into the cold region. Solutions can be found using glass-fibre reinforced plastics (G-10, G-11, . . .), the critical problem being the connection between the metal flanges and the plastic which has to withstand a large number of load cycles without showing degradation. The non-drive end (sliding) suspension was designed to compensate for length contraction during cooldown. Mechanically the two shafts are connected by the cylindrical outer cryostat wall, which also serves as a dynamic elec- trical damper for the harmonics of the armature winding. Due to the previous balancing of the cold mass the completed rotor could be balanced easily to a good quality. 3. Stator As already indicated this is an air core winding, inserted into an iron yoke into a standard Siemens motor housing. The winding voltage is designed so that the motor could be operated on the normal grid of 400 V. As the iron ‘‘teeth’’ are not existent, the torque on the windings is supported by a non- magnetic GFRP structure. In order to limit eddy current losses in the winding this is wound from a transposed Litz conductor composed of thin single wires. The current density in the stator winding was selected so that an air cooling scheme could be used to remove resistive and eddy current losses from the winding, the iron losses of the yoke and the eddy current losses of the room temperature damper. This worked fine in our experiments, for future HTS motors which want to exploit the full potential of compactness offered by supercon- ducting current densities it will be necessary to design a high voltage winding with forced flow water cooling. 4. Rotor cooling system The cryogenic cooling system is one of the critical components of an HTS machine because it is basically a new and unknown kind of system for the user of a drive or generator. Obviously it must be as robust and reliable as possible, so that no practical limitation is derived from it. Fig. 2. 2D FE calculation results for rotor cross section, left: magnetic induction, right: deformation due to prestressed bandage, rotation and torque. Fig. 3. Completed cold mass of rotor (superconducting coils and filler pieces) on pole former before bandaging and appli- cation of superinsulation. 1508 W. Nick et al. / Physica C 372–376 (2002) 1506–1512 Our first cooling system was not close to this goal, it used cold He vapour from a LHe storage vessel. The cold gas entered into the center of the rotor through the bore of the non-drive-end shaft, by a specially designed siphon, there it absorbed heat, then the gas was pumped out over a throt- tling valve. By measuring mass flow, inlet and outlet temperatures, the delivered cooling power can be determined. By varying the throttling, the cooling could be regulated. Another advantage is that large cooling power is easily available, how- ever operation of the rotor at constant operating conditions (temperature) is difficult to achieve. Better operating characteristics were demon- strated by cooling system II (see Fig. 4). In this case the source of the cooling power is a com- mercially available Gifford–McMahon cryocooler [6]. The cold head is in contact with the condensor of a Ne-filled thermosiphon (or heat pipe), that reaches straight into the inner space of the rotor forming the evaporator. Heat from the HTS coils is conducted across a Cu heat bus to this surface and evaporates Ne. The vapour moves through the same pipe back to the cryocooled condensor and is recondensed there. In this closed system there are coexisting liquid and gaseous Ne, with tempera- ture and pressure according to the vapour pressure line. This heat transfer mechanism allows a very efficient heat transport with small total tempera- ture difference. We actually operated with only a few Kelvins between the windings and the cryo- cooler. By controlling the temperature of the cold head one has a very direct and effective control of rotor temperatures. As a result we had constant oper- ating temperatures in the whole system even for drastic load changes. Care was taken to prevent the Ne from freezing at the condensor due to surplus cooling power of the cold head. This ad- ditional heating of the cryocooler thus could be used as a signal to monitor how much additional cooling power is still available in a specific situa- tion. 5. Testing After separately checking the HTS coils of the rotor and the magnetic induction generated at the surface of the damper the rotor was inserted into the stator and housing and the whole system in- stalled at the test facility (see Fig. 5). This included a DC load machine that could be used to drive the HTS machine as a generator but also to serve as controllable load in the motor mode. The instal- lation also included a synchronising device, vari- ous power feeds (different levels of grid harmonics) and a resistor load bank to operate in generator mode independent from grid. Tests with the motor Fig. 4. Closed cycle rotor cooling system, GM cryocooler in- serted from top, motor shaft on the left. Fig. 5. Project team from A&D Large Drives and Corporate Technology at completion of model motor. W. Nick et al. / Physica C 372–376 (2002) 1506–1512 1509 directly driven by an inverter are planned to be performed at a later stage. A first check referred to the cryogenic require- ments. At nominal speed with full current the total losses were between 20 and 30 W, depending on the amount of harmonics of the grid. This means that we were perfectly in the operating range of the cryocooler (40 W at 25 K) and our efforts to built a good rotating thermal insulation and thermally optimized warm-to-cold connections were success- full. Experiments with different filtering of the grid, in conjunction with eddy current calculations for the rotor structure using FE models, show how the losses depend on the harmonic content of the ar- mature current. This will lead to a specification for the inverter to be used with the motor. The voltage generated in the stator in the open loop is of a perfect sinusoidal shape, with negligi- ble harmonic content (total harmonic distortion THD < 0:15%). This is a consequence of the large air gap design, in spite of the fact, that the winding turns are not located close to a sinusoidal profile along the circumference. Fig. 6 presents the re- cording of experiments in the motor mode, with variation of delivered power. First it can be poin- ted out that the temperatures in the rotor do not seem to be influenced at all by the ongoing varia- tions, that means, the regulating mechanism of the thermosiphon is working automatically. The load angle found by using a strobe was very small, only about 8� for full power (for conventional machines this might be about 50�). This is one of the im- portant dynamic properties of an HTS machine. Consequently, the tilting moment is very large, 700% of nominal torque, while 130% is typical of conventional machines. Care has to be taken that the whole structure, especially the torque tube can tolerate such torques. The effect is that such an HTS motor is a very dynamic drive which can easily follow large variations of torque, even peaks an order of magnitude above nominal, without going out of synchronism as a conventional drive would do. Another aspect of this property can be visu- alized in generator mode in load switching ex- periments (see Fig. 7). Whereas a conventional generator shows a significant drop in armature voltage when full power is suddenly applied from zero, the HTS machine seems to be almost insen- sitive to this change. This is a critical property for conventional electrical machines, so there must be a fast controller to counteract the voltage change quickly, and the resulting total effect is used as a quality criterion for generators. The supercon- ducting generator however does not need a fast controller at all to demonstrate best performance. Although our demo machine was not optimized in any respect, the measurements of all loss con- tributions showed that its coefficient of perfor- mance (about 97%) was already better than that of Fig. 6. Experimental traces with load steps �400–50–100–200–300–400 kW power. 1510 W. Nick et al. / Physica C 372–376 (2002) 1506–1512 conventional machines (synchronous or asynchro- nous) of the same rating. This figure included the room temperature input power for the compressor of the cryocooler with its presently still limited thermodynamic performance. 6. Conclusions The positive results of the Siemens project to design and build a demonstration motor of 380 kW power have proven that our concepts for possible future HTS machines are viable. We can build a rotor with rotating HTS coils, we are able to provide cooling at the level of 25 K with com- mercially available cryocoolers, we designed and built the air-gap stator and we successfully oper- ated this system for a couple of months. It appears technically feasible to construct a superconducting synchronous machine with a power density that is raised by a factor of 2–4 relative to conventional technology, at the same time improving the effi- ciency. For large machines efficiency is expected to get above 99%. The door to a future with a new generation of very compact and efficient large electrical machines is slightly open now. What are the prospects? We are investigating for what kinds of customers the HTS technology could be advantageously applied. This is a quite broad range, from slow wind generators at one end (20 rpm), over ship propulsion (250 rpm) and in- dustrial drives to high speed generators (15,000 rpm) that can be directly flanged to small gas tur- bines. This spectrum requires a set of different specific solutions and it will be necessary to focus the effort. In order to make those HTS machines com- mercially competitive and attractive to the cus- tomers there is still some way to go. It will be necessary to push the performance of available HTS tape conductors, current densities of 300 A/ mm2 would be ideal, but also to reduce their spe- cific price (per kAm) by an order of magnitude. It is clear that a successfull YBCO conductor could easily step into the place of presently used BSCCO tapes. What about robustness and reliability of HTS electrical machines? Taking a look at the success of LTS MRI systems we think that superconducting coils, cryogenic and even vacuum systems with good long term performance should not be of concern. It should be remembered, however, that the operating environment of some of the HTS machine applications is a lot rougher than that of a modern hospital. An important component that needs further development is the cryocooler with its associated compressor. It requires reliability and servicability improvements in order to con- vince future customers that operating an HTS machine really pays out without adding risk. Fig. 7. Load switching experiment 0! 380 kW (speed decreases due to time constant of drive controller). W. Nick et al. / Physica C 372–376 (2002) 1506–1512 1511 Acknowledgements This work is funded by the German Ministry for Education and Research (BMBF). References [1] D. Lambrecht, Status of development of superconducting AC generators, IEEE Transactions on Magnetics MAG-17 (5) (1981). [2] T. Nitta et al., Review articles collected in: cryogenic engineering, Journal Cryogenic Society of Japan 36 (3) (2001). [3] B. Zhang, D. Driscoll, V. Dombrovski, Development status of a 1000 hp superconducting motor, paper presented at Applied Superconductivity Conference 2000, Virginia Beach, 17–22 September, 2000. [4] Leybold Vakuum GmbH, K€ooln, and TransMit GmbH, Giessen, Germany. [5] P. Kummeth, W. Nick, G. Ries, H.-W. Neum€uuller, Physica C 372–376 (2002), these Proceedings. [6] GM cooler type ‘‘RGS 120T’’, Leybold Vakuum GmbH. 1512 W. Nick et al. / Physica C 372–376 (2002) 1506–1512 380 kW synchronous machine with HTS rotor windings--development at Siemens and first test results Introduction Design and construction of rotor Stator Rotor cooling system Testing Conclusions Acknowledgements References