The Design of a Fully Superconducting Power Network for Future Aircraft Propulsion

American Institute of Aeronautics and Astronautics1

The Design of Fully Superconducting Power Networks forFuture Aircraft Propulsion

Peter Malkin1 and Meletios Pagonis2

Cranfield University, Bedfordshire, United Kingdom, MK43 0AL

In recent years a new approach for the propulsion system of an aircraft has beeninvestigated by several authors. This approach involves an electrical distributed propulsionsystem where the gas turbines are mainly used to produce electrical power which istransmitted via a distribution network to several electrically driven fans. The main enablerof such a concept will be the use of superconducting technologies. The latter will improve thepower density of the electric machines as well as the current density of the cables. However,this will result in a far more complex and high performance power network which we arenow referring to as a Fully Superconducting Power Network or FSN. This paper presentssome of the characteristics of this FSN and some of the performance improvements that sucha configuration could add.

Nomenclature

FSN = fully superconducting power networkK = kelvinHTS = high temperature superconductingܯ ଶܤ = magnesium diborideBSCCO = bismuth strontium calcium copper oxideYBCO = yttrium barium copper oxideSFCL = superconducting fault current limiterܬ = critical current density

= critical temperatureଶܪ = critical magnetic fieldIF = increasing fieldDF = decreasing fieldCIF = cooling in the fieldkA = kilo-amperesCu = copperܫ = fault currents

ܫ = normal currents.ி.ܫ = full-load secondary current of a transformer

௦௨ = transformer impedanceܨ = electro-magnetic forcesܭ = magnetic force constantr = separation of the wiresߤ = magnetic constantEM = electromagnetic

1 Professor of Electrical Power Systems, Department of Power and Propulsion, School of Engineering,[email protected], AIAA Member.2 Research Engineer, Department of Power and Propulsion, School of Engineering, [email protected].


I. Introduction

ANY authors1,2,3,5 have proposed a new aircraft design involving a distributed propulsion system. Such anapproach could give very significant gains in efficiency of future aircraft. It seems likely that the best way of

powering such aircraft is using a Hybrid-Electric propulsion system where gas turbines are used mainly to generateelectrical power which is transmitted via a distribution network to many electrically powered fans. These fans areoften assumed to be placed around the trailing edge area of a blended wing body airframe to allow further benefits inpropulsive efficiency.

It is widely assumed that the electrical machines required will use superconductors to obtain neededimprovements in power density. Some authors have also assumed that superconducting cables will be used toconnect these machines. This paper argues that this will also require a Fully Superconducting Power Network orFSN which will further add to the performance of this propulsion system.

II. History of Superconducting Materials

In 1911 Dutch physicist Heike Kamerlingh Onnes first observed the phenomenon of superconductivity when hecooled mercury to the temperature of liquid helium (4K) and its resistivity suddenly disappeared. In the next fewdecades several metals, alloys, and compounds were discovered to have the same behaviour under a specifictemperature. However, it was not until the 80’s when the first so-called High Temperature Superconducting (HTS)materials were found and attracted new interest in the field of superconductivity. A material remainssuperconducting as long as it does not exceed certain limits of current density, magnetic field and of coursetemperature (see figure 1).

Figure 1. Critical Surface of a Superconductor.4 Where: �the critical magnetic field, the critical currentdensity, the critical temperature, IF the increasing field, DF the decreasing field, and CIF the cooling in field.

For aerospace applications there are three main superconducting materials being investigated:ܯ� ,ଶܤ BSCCO,and YBCO. Although ܯ ଶܤ has the lowest operational (30K instead of approximately 50K and 70Krespectively) the authors believe that it seems to be the best option at the moment due to its simple robustmechanical properties. Superconducting machines require fine, twisted superconductor filaments in a high-resistancematrix to reduce AC losses5. This required filament size is achievable only with ܯ ଶܤ at least at present (see figure2).

M


Figure 2. Typical Multifilament wire.6

With its relatively low cost and its capability of very sharp transition for the superconducting to the normalstate,ܯ� ଶܤ also seems the most appropriate choice for devices like resistive Superconducting Fault CurrentLimiters (SFLC)7, elements that will play a key role in this new system. Furthermore, recent publications8 aboutܯ ଶܤ indicate that further developments of ܯ ଶܤ will be available with even higher critical current densities ܬ(figure below). However, a future development of ac-tolerant architecture of YBCO may change the map ofsuperconductors, since the latter has demonstrated good performance and its use will decrease the power demandedfrom the cooling system.

Figure 3. 2nd Generation wires improved current density. (Courtesy of Hyper Tech Research Colum
bus)


III. Superconducting Fault Current Limiters (SFCL)

These new materials have permitted a development of new devices that are capable of improving theperformance of power networks. These devices are known as Superconducting Fault Current Limiters (SFCL) andhave currently been applied to energy networks.

Short circuits in electrical power networks result in abnormally high currents (fault currentsܫ�). More

specifically, the lower the system impedance the higher these fault currents. High fault levels are undesirable sincethey can cause stresses and can lead to high electrical and mechanical instabilities and even damage of electricnetworks. Conventional methods to reduce these fault currents usually include high power switchgear and/or devicessuch as transformers with high impedance, fault-limiting reactors, split bus-burs, and fuses. These methods althoughreliable, are heavy, bulky and difficult to install. An attractive alternative option is the aforementioned SFCLs.

A Fault Current Limiter (FCL) is a device that limits the perspective fault current of a network when a fault suchas a short circuit occurs. The current limiting behaviour of superconductors derives from their non-linear response tocurrent, temperature and magnetic field changes (figure 1). Exceeding a limit of any one of these three parameterscould lead the materials losing their superconductivity and behave as normal conductors. In a resistive SFCL, whichis the most common type of limiter, when a fault current occurs the superconductor quenches (loses itssuperconductivity) and the resistance rises sharply and quickly limiting the fault current. This superconductingdevice seems ideal because in the steady state has almost zero impedance while when a fault current occurs thisimpedance rises high enough to control the fault. After recovery of the fault, impedance again goes back to zero,making the device ‘invisible’ again. In addition, these devices will have intrinsically safe failure modes.

Several SFCLs have been tested and have worked successfully in recent years.9 The basic construction of aresistive SFCL can be seen in figure 4. The limiter’s components are dimensioned so that the reactance remains at alow level in the steady state and during a fault the required limiting characteristics could be achieved. Theaforementioned excellent current limiting behaviour ofܯ� ,ଶܤ due to its rapid transition from the superconducting tothe normal state, makes it the best candidate material for the future SFCLs.

Figure 4. Basic construction of a resistive type SFCL.10

IV. Conventional Power Networks Design Process

In order to develop a design process for FSNs we first need to understand this process for the design ofconventional power networks of similar power levels and complexity.


Figure 5. Design Process diagram of a conventional power network.

Figure 5 shows a process diagram to design electric power networks. Note that such designs are rarely carriedout currently as stand-alone networks are rarely used. However, more recently the majority of specialized shipdesigns (e.g. Naval, Cruise or Offshore support) use similar systems. As the authors have some experience in thisfield we have derived this design approach. For illustration a ship system could be as follows:

Figure 6. Electric/Hybrid Ship Propulsion System Diagram.


From figure 5 it can be seen that the key input that sizes the system is the maximum required power. Parameterselection is normally done by choosing normal currents that will allow a practical size of the electric bus-bar. Inaddition, the value of the fault currents depends on the normal currents as well as on the transient (short-circuit)impedance. For example the fault level of a transformer in a system depends on the full load secondary currentmultiplied by its impedance which is generally expressed as a voltage percentage. This is the percentage of normalrated primary voltage required to cause a full-load rated current to flow in the short-circuited secondary and itnormally gives a factor of 10-20 times (Eq. (1)). Similar equations apply to a short-circuit happening in other partsof an electrical network. These high levels ofܫ� must be controlled with switchgear, which has limits to its

interruption performance. This in turn limits the level ofܫ� that can be chosen (typically between 500 and 2000A).

)%/%100(*. subTLFf ZII (1)

In turns of frequency choice for ship applications, this is normally 50 or 60 Hz to match the availability ofconventional equipment (motors, transformers, switchgear etc.).

Once parameter selection is complete, we would normally begin a low-fidelity steady-state model that deliversthe load flows and fault currents associated with a presumed topology of network. This data can then be used to sizethe key elements of the network including:

1. Motors and generators

2. Switchgear for normal and fault current control

3. Transformers or drives

Note that it is usually the switchgear that is the critical factor as many panels are required and fault interruptioncapability is limited to around 40KA which for such closely coupled networks can be a constraint. Also the steadystate model would deliver the basic harmonic levels and protection coordination (required to ensure effective faultclearance).

Once the steady-state modelling is complete a more comprehensive dynamic model is established which iscapable of predicting the full dynamic behaviour of the system allowing a study of system stability and the effectsof transient disturbances, such as motor starting and transformer inrush transients.

V. Superconducting Power Network (FSN)

Whilst some of the basic principles of designing a power network still apply, a fully superconducting network(FSN) requires a totally different design process. Furthermore, it will not use the same components used in aconventional system.

A. Basic Parameter SelectionHTS wire is now available with critical current density around 500-1000 times the one of a typical Cu wire. The

latter reduces significantly the cost and the size of the main bus-bar. Clearly, within the standard range of normalcurrents this would result in a very small diameter wire indeed. For example, if we were to choose a 2000A bus-barthis would give us a HTS wire of 2-3A Cu wire. In fact in this system we will not want a very fine wire as our maincable for practical reasons of making connections and mechanical support. In view of this, it will seem logical tosignificantly increase the normal current levels giving for example a range between 6 and 30 kA.

Normally this would result in major problems involving the generation of extremely high fault currents ܫ which

are related to normal currents (as discussed in section IV), however crucially, these will now be controlled throughthe use of superconducting fault current limiters as described in part III of this paper. Not only does this remove theconstraint of the high peak values of ܫ , but also the network will in fact see maximum currents of well below that

value.

In addition, such abnormally high levels of ܫ would also give problems in load-switching. Most power networksrequire frequent switching devices for circuit reconfiguration under no-fault conditions and even switching the usuallevels of normal currents could be problematic if DC is used to remove AC losses as it is described later. This couldbe resolved using power electronic converters as load switchers. These converters could gain significantly in powerdensity from being cryogenically cooled11, but nevertheless it seems a highly inefficient use of such complex


devices for this type of applications. However, new types of superconducting switching devices are underdevelopment which will address this problem without adding significant weight to the aircraft.One further issue to address is that of the electro-magnetic forces exerted between conductors at these current levels.This force derives from Amperes law (Eq. (2)), where the force per unit length is proportional to the product of thecurrents divided by the distance between wires. At these levels of normal current this must be taken account of in thesystem design.

r

IIKF am

21 ***2 (2)

, where

*40aK (3)

B. Design IssuesOne of the key differences between normal and superconducting networks is the main characteristic of

superconductivity: zero resistance. Note that this is a real issue with power network design as current distribution inmost networks relies on a resistive divider effect which is not present here. Clearly, this implies that current flowwill be dominated by the load impedances. However, the load flows through a complex multi-node network will bedifficult to predict by using “standard” models. Nevertheless, it has been reported recently that good currentdistribution has been obtained in multi-strand ܯ ଶܤ wire12. Although the arrangement of Ref. 12 is much simplerthan the one described here, it does suggest that successful current distribution can be achieved.

Fully superconducting networks will have AC losses, except if we choose a pure DC network for our system (butagain other difficulties will exist). These losses will be a function of the AC harmonics and their impact will bedifferent from a conventional network since they will affect the cooling requirements of the network.

These cooling requirements of the network also add an extra control factor for the FSN. The overall performanceof the network depends on the cooling system performance and reliability. Operational temperature will be anadditional dimension to the overall performance of any architecture.

As stated previously, very high prospective ܫ will exist, but they will be controlled by several SFCLs. These

devices could act so quickly that high fault levels will not even been seen by the network. However, a new strategywill have to be developed to allow for multiple operation of SFCLs and allowing the impact of any fault to beminimized. This would also require novel methods of circuit post-fault reconfiguration.

Potential differences will also exist in terms of system stability. The network stability of a lighter rotor istypically worse than a conventional one; however, with appropriately designed EM shielding on superconductingmachines the stability could be improved. Moreover, the behaviour of a number of interconnected superconductingmachines has not been studied yet and thus the dynamic stability and the architectural impact are still unknown.Clearly, this implies that the transient behaviour of the network and the dynamic modelling required cannot beachieved as yet without further experimental data.

C. Structure and BenefitsThese networks will be complex multi-node networks with novel, non-mechanical, load break switches or

“contactors” and specialised fault management devices. Whilst this adds complexity, it will allow maximumflexibility and minimise the impact of any failed component by allowing very rapid post fault reconfiguration forexample. This would also permit rapid and seamless transmission of power to any combination of loads such as fanmotors as well as optimising the use and recharging of energy storage systems.

VI. Conclusions

Recent progress in superconducting technology will allow the use of Fully Superconducting Networks whichwill have a significant gain in power density.

These networks will also allow a very flexible power system that can rapidly switch large amounts of powerbetween any aircraft loads, for example for rapid maneuvering of the aircraft as well as minimising the impacts ofany faults or failures in the system.

However, in order to fully model and design these networks it seems likely that novel techniques and processeswill have to be developed. This is likely to require additional experimental data and validation systems beforereliable modelling can be carried out.


Acknowledgments

The authors acknowledge the support from NASA, under their grant NNX13AI78G enabling the presentation ofthis paper.

References

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The Design of a Fully Superconducting Power Network for Future Aircraft Propulsion