Advanced power supply and distribution systems for Columbus

Vol. 17, No. 1, pp. 99-114, 1988 Printed in Great Britain. All rights reserved

0094-5765/88 $3.00+0.00 © 1988 Pergamon Journals Ltd

Acta Astronautica

A D V A N C E D POWER SUPPLY A N D DISTRIBUTION SYSTEMS FOR COLUMBUSt GERT EGGERS AEG Aktiengesellschaft, A472V41-2000 Wedel, F.R.G.

(Received 15 December 1986) A~traet--The paper describes power supply and distribution systems to be used on unmanned/man-tended Columbus elements, capable of supplying l0 kW to 30 kW to a variety of users in low earth orbits (LEO's). For the definition of the Electrical Power System (EPS) challenging requirements as the provision of high power levels under hard LEO conditions, maintainability, commonality etc. are to be taken into account. These requirements are to be seen in conjunction with the Columbus IOC (initial operational capability) scenario stipulating that EPS hardware shall be used on the Polar Platform, the Pressurized Module attached to the U.S. Space Station and the Man-Tended Free Flier. According to the availability of European technologies, the baseline in the power generation area is a photovoltaic system which provides three regulated main buses (150V d.c.) to the users. In order to maintain power supply during eclipse phases, nickel hydrogen batteries will be used for energy storage purposes with nickel cadmium as back-up solution. The power distribution system needs special attention. Due to the elevated voltage levels mechanical switch gear cannot be used any longer. It is to be replaced by solid state power controllers (SSPC). Because these devices show a totally different behaviour with regard to conventional relay contacts, new approaches in the area of switching and protection are necessary. In view of the crucial role of this new technology for the realization of medium voltage d.c. systems, it is of great importance for Columbus and, hence will receive adequate consideration in the paper. In order to cater for effective management and control of the power supply and distribution hardware, a so called _power system internal data p_rocessing assembly (PINDAP) has been introduced in the EPS. PINDAP is the key to reduced dependence on ground stations (alleviated ground support requirements); it keeps crew involvement in the EPS control process to as minimum and provides comprehensive means for EPS check-out and verification which facilitates in-orbit maintenance. I. INTRODUCTION

The Columbus program opens up dimensions for future LEO missions never before possible. A major consequence of the resulting thrust in scientific and commercial space activities are rapidly escalating power demands. In order to cope with this situation, that means in order realize power systems which are larger in size and simpler in operation, a major step towards new technics and technologies is indispensable. The resulting advanced power supply and distribution hardware for Columbus elements is not designed and developed yet, but in a series of studies, A E G intensively investigated related technics and technologies. These activities were strongly supported by comprehensive hardware experience from Spacelab, Eureca, Space Telescope and other projects. Subject of this paper is to present the results obtained so far. 2. ARCHITECTURE RELATED TOPICS

2.1. Requirements and technical objectives When defining the Electrical Power System (EPS) tPaper IAF-86-1h6 presented at the 37th Congress of the International Astronautical Federation, Innsbruck, Austria, 4-11 October, 1986.

for Columbus, a series of stringent requirements has to be taken into account: • Increased power levels; growth potential; accommodation of changing technologies. • Low earth orbit environment. (Large n u m b e r of cycles; atomic oxygen, etc.) • Maintainability, i.e. implementation of orbit replaceable unit (ORU) concept. • Commonality • Autonomy, i.e. reduced dependence on ground stations, functional independence from the on-board data management system. These requirements are to be seen in conjunction with the Columbus IOC scenario (initial operational capability). According to the present IOC scenario, the EPS must be designed for three major applications: • Power generation and distribution for the Platform (PF) for polar- and co-orbiting missions. 8 kW shall be supplied continuously throughout sun and shadow phases to subsystems and payloads. • Power distribution for the Pressurized Module (PM) when it is integrated with or attached to the U.S. Space Station (USSS). The power distribution capability shall be 30 kW. • Power generation and distribution for the mantended free-flier Consisting of Resource Module (RM) 99

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GERT EGGERS

and a two segment PM. Considered power levels and appr. in the same range as for the PF. The particular goal for the designing engineers is to conceive the EPS in such a way that the same equipment and hardware base can be used for all Columbus elements. This is an important part of AEG's cross-element task. In addition to these basic requirements consideration is to be given to top-level operational objectives: • Astronaut involvement in the EPS control process shall be kept to an absolute minimum such that more time for utilization related tasks is available. • Dependence on ground stations is to be reduced in order to alleviate ground support requirements and thereby reduce overall operational costs. • In view of the important role which the power interface plays in the frame of the services offered to the user community, a high quality standard and as much flexibility as possible are to be provided in this area. Such features can be realized only when a certain potential of automation is introduced in the EPS. Evidently this potential grows with increasing size and complexity of the EPS. Finally some important interface requirements are imposed to the EPS. During Launch and deployment sequences, for servicing and operation, Columbus elements have interfaces to: Orbiter --Ariane Hermes U.S. Space Station (USSS) In view of the "integral-" and the "attached mode" of the Pressurized Module, the power interface to the USSS is of particular importance. A major goal of all Columbus related design activities is to have as far as possible a compatible user interface with the USSS. A decisive question in this context is, whether a.c. or d.c. shall be distributed on Columbus elements. Present NASA activities concentrate very much on the a.c. option (440V; 400Hz; 3 ~ ) , whereas in Europe d.c. systems are favoured. This primarily for the following reasons: • Regarding present state of the art of European technology, d.c. sources (solar arrays/batteries) are available for Columbus. Conversion of the d.c. power to e.g. 400 Hz a.c. power would require large static inverters. Consequence: Additional complexity, additional mass and increased costs. • Modern technologies favour d.c. interfaces. d.c./d.c, conversion can be performed with minimum mass and power losses when high frequency operated power F.E.T.'s are used. Induction motors will be replaced by brushless d.c. motors which are of smaller physical size and mass and provide better dynamic properties.

For these reasons the experts from ESTeC, AEG and other involved companies decided that the "working horse" for all columbus elements will be a d.c. system.

2.2. Power system reference configuration The results of Section 2.1 have been synthesized into an EPS reference configuration which is shown in Fig. 1. This reference configuration consists of four major building blocks: • The Primary Power Assembly (Block I) where the power is generated, stored and conditioned. Because three independent regulated main-buses are provided, the EPS can be classified as fail-operational/fail-safe. In order to handle the high power levels within certain mass and volume constraints, the operation voltage is increased to 150 V d.c. • The Power Distribution Assembly (Block I1) which basically comprises two distribution levels and the power and signal harness. Each PDU on Level I splits one mainbus in a series of subsystems and payload dedicated subbuses. The Modular and Standardized Distribution Units on Level II offer versatile standard interfaces to the individual users. On both distribution Levels comprehensive monitoring, switching and protection functions are implemented. • The Power Interface Units (Block III) which feed power to Columbus elements --during launch and deployment sequences from Orbiter/Ariane, -~turing servicing periods from USSS/Hermes; - ~ u r i n g normal operation from the USSS (to the PM only which is either in the integral--or the attached mode). In order not to exhaust Hermes's fuel cells, power might also flow from serviced Columbus elements to Hermes, thus, bi-directional flow of power must be an essential property of the corresponding power interface unit. The USSS dedicated power interface unit has to convert U . S . a . c . power to regulated 150V d.c. power. This can be achieved either by applying conventional methods of phase controlled rectification or modern technics of resonant conversion which avoid bulky 400 Hz transformers. • The Power System Internal Data Processing (PINDAP) Assembly (Block IV) finally performs overall control and management of the above described building blocks. A central data link to the on-board Data Management System is provided for interactions between the EPS and other subsystems. Common to all described building blocks is a high degree of modular design (on equipment--and on bus level), such that the EPS can cope from the ORU concept and that it can grow in an orderly manner from IOC to AOC.

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Fig. 1. Reference configuration of the Electrical Power System (EPS) for Columbus. In order to sastisfy the needs of the individual Columbus elements, the described building blocks are combined in a varying fashion:

(b) Polar orbits (98 ° inclination) with altitudes ranging from 400 to 800 km and Sun- and shadow phases as under (a).

• Blocks II and IV and parts of block III for the PM. • Blocks I and IV; block II up to level I (level II is on the payload carrier) and parts of block III for PF and RM.

The short revolution time leads to a high number of Sun/eclipse-cycles (approx. 5600 per year) which primarily stress solar arrays and batteries. Modern solar arrays are very light configurations and represent, therefore, a low thermal mass. This has the effect that the temperature extremes will change from +80 to - 1 0 0 ° C during one cycle. For example, a 5 years mission will accumulate 28,000 thermal cycles which may cause material fatigue and mechanical stresses in compounds of materials which are not properly matched. Batteries are affected in a similar way. The high number of charge/discharge cycles has a limiting effect on battery life time and requires a low depth of discharge (DOD). This in combination with the relative long eclipse phase (approx. 63% of Sun phase) leads to very large batteries which seriously charge the elements mass budget. The fact that only very limited experience is available with respect to the long-term LEO behaviour of such large batteries, underlines the criticality of this area for Columbus. Another specific characteristic of the low earth orbit is the plasma environment which has a four orders of magnitude higher charged particle density than the geostationary orbit. This conductive plasma can act as a low-ohmic bypass to the solar array and

In many cases the physical interface between Columbus elements go right through the described building blocks. For the man-tended free-flier, for example, parts of the power distribution assembly are on the RM; the main portion of this assembly, however, is on the PM. Hence, power system engineers have to design across the element interfaces which underlines the overall system engineering related aspect being part of the EPS design activities.

3. T H E IMPACTS OF T H E LOW EARTH ORBIT ENVIRONMENT ON THE EPS

The operation of Columbus elements in grueling low earth orbit (LEO) environment causes a lot of head-ache to power system designers. The preferred operational orbits have the following characteristics: (a) 28.5 ° inclination, 400 km altitude with a sun phase of 0.95 h and a shadow phase of 0.6 h.

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may cause significant leakage currents and discharges under certain solar array operation conditions. Recent data from Shuttle flights indicate that the amount of atomic oxygen in low earth orbits has an eroding effect on several materials and may also change their thermo-optical data. Specifically organic materials and some metals seem to be affected most and will have to be reviewed for future long missions. Solar array blanket materials and solar cell interconnectors are of this category of materials and hence, need special attention. On the other hand some environmental characteristics for LEO are less severe than for geostationary orbits (GEO). The ionizing particle environment, for example, which is concentrated in the Van-Allen belts, is very moderate compared to GEO missions. Hence, array oversizing in order to cope with end of life power requirements is not so severe (approx. 10% degradation for a l0 years LEO mission). However, LEO radiation is high enough to affect power F.E.T.'s (as e.g. used in solid state switches) such, that it is uncertain whether they are in on- or in off-state. Hence, sufficient margins and measures are to be incorporated in the design in order to eliminate radiation effects.

4. THE SOLAR ARRAY

In view of its physical size and its complexity, the solar array is by far the most important element of the power system. The power demands in the multi-kW range can be satisfied best by a semi-rigid or a rigid array as it is used for example on Eureca. This array-type is composed of deployable rigid panels which carry the solar cell strings. Better properties with regard to growth capability offers the flexible foldable array type as used e.g. on the Olymus spacecraft. This solar array is characterized by a thin flexible blanket onto which the solar cells are bonded. In stowed configuration this electrical blanket is folded and stowed in a stowage box at the space craft walls. Once in orbit, special deployment mechanisms can deploy this blanket fold by fold to its full size and keep it under sufficient tension. The present Columbus baseline is a rigid array. However, a foldable array shall be studied as option. The solar array reference configuration for the Resource Module is shown in Fig. 2. The entire' array (S.A.) consists of two identical, deployable/i retractable wings, two bearing and power transfer assemblies (BAPTAs) including S.A. drive, the S.A. harness and electrical/mechanical interface devices. The S.A. output power shall be 16 kW (end of life). Deployment/retraction of the S.A. is performed by the extendable and retractable Mast (ERM) developed by Dornier System GmbH. The ERM is constructed such that partial deployment of the S,A. is possible.

The most important components of the solar array are the solar cells which convert the incident sun light directly to electrical power. From aspects such as efficiency, cost effectiveness, reliability and environmental resistivity silicon solar cells have prevailed upon various potential alternatives since about 20 years and their technical improvement is still progressing. A major goal in this area is to increase the solar cell efficiency. This can be achieved by two methods: • One method is to apply an optical reflector to the cell rear side to reflect the red portion of sun light which is not converted to electrical current. This method decreases the cell temperature in orbit by 10~C and has a corresponding efficiency increase of 5%. These cells are called BSR-cells (Back-Side-Reflector). • The second method is to apply a special doping procedure which generate an electric field in the silicon on the cells rear side. The effect of these BSF-cells (Back Side _Field) is that the life time of the minority carriers is increased which again improves the cell efficiency. Finally both effects can be utilized simultaneously in a BSFR-Cell. The efficiency of a BSFR-cell is approx. 14% (at 25'~C). Due to the reduced radiation in LEO environment (the back side field is radiation sensitive), the BSFRcell is a clear candidate for Columbus. A very recent innovationin solar array technology at AEG is the other candidate for Columbus. However, primarily for the option (foldable blanket). This innovation consists of a solar cell blanket which converts direct sun light on the blanket front side, but is also sensitive for reflected sun light from the Earth (albedo) which significantly contributes to the total LEO radiation. Hence, the power generating elements are "bifacial" solar cells that are photoelectricallky sensitive on front and rear side. These cells are bonded to an optically transparent solar array/blanket which allows the earth shine to penetrate into the solar cell from the blanket rear side. This increases the solar cell efficiency to approx. 16% (at 25°C). The final choice between these two cells types is subject to ongoing Columbus study activities. 5. POWER CONDITIONING AND ENERGY STORAGE

Subject of this section are basic functions of the Primary Power Assembly as power conditioning, energy storage and the associated control of currents and voltages. All this is indispensable in order to convert solar array raw power such, that it is provided in a high quality version (regulated buses) to the users. A block diagram representing the basic topology of the Primary Power Assembly and its modularity is shown in Fig. 3. In sun light the mainbus is regulated by a sequential switching shunt regulator (S3R) which con-

Solar Array

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Fig. 2. Solar array reference configuration for the Resource Module. (a) Deployed on-orbit configuration. (b) Stowed configuration.

ERM o . g

NECTORS TO PM

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o

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trois all solar array sections by means of dump circuits which are connected across each section. In parallel the battery is charged via a Battery Charge Regulator (BCR) from the regulated mainbus. The BCR is a modular buck regulator. The module dedicated current control loop is configured such that the battery is charged with constant power. For eclipse operation the battery is discharged via a modular Battery Discharge Regulator (BDR). The BDR works according to the SMART principle (series connection of buck- and push-pull stage). Again each module has its own current control loop in order to ensure well balanced current sharing. Integrated voltage control of S3R, BCR and BDR is achieved by a highly redundant mode controller housed within the S3R which also contain the mainbus tank as common output filter. The energy storage function is provided by a 60 cell 50 Ah nickel hydrogen battery which is operated with 50% DOD. Cell voltage and temperature/pressure monitoring is performed by the Battery Control Unit (BCU). Because the nickel hydrogen technology for LEO applications is presently not fully available, a 40 cell 40 Ah nickel cadmium battery is considered as back-up solution (due to the BCR, different battery types can be easily adopted). In order to meet the battery's three years life-time requirement, the NiCd battery is operated with a reduced DOD of approx. 25%. With regard to the size of BCR and BDR, two NiCd batteries will replace one NiH2 battery which, however, will halve the output power of the conditioning cell. Taking the below listed battery parameters and regulator efficiencies into account (NiCd values in brackets) K = 1.1 (1.15);

U,.h = 1.45 V (1.4 V);

Uo = 1.2 V (1.1 V); ncg = 0.94 (0.92); n~R = 0.91 (0.9);

nsh = 0.98, the power flow in the conditioning cell can be determined as follows: PDR = 2.73 kW (1.36 kW); PD = 3.0 kW (1.52 kW);

Pc = 2.52 kW (1.4 kW); Pc,~ = 2.68 kW (1.53 kW); Ps = 5.41 kW (2.9 kW); PA = 5.52 kW/2.96 kW), (End of life values). Hence, when the considered modularity is maintained, the Primary Power Assembly can grow in increments of 2.7 kW which stipulates that the solar array grows in steps of 5.5 kW. With respect to ORU packaging it is presently planned that the equipment of the power conditioning cell shall be integrated in one standard ORU. The solar array forms a separate non-standard ORU.

6. POWER DISTRIBUTION Columbus elements will see during their operational life a great variety of payloads and missions. Effective distribution of on-board electrical power in order to provide both, reliable/safe load supply and flexibility with respect to payload accommodation is, therefore, of primary importance.

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Fig. 3. Functional block diagram of the primary power assembly for one bus.

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Fig. 4. Principal block diagram of a solid state power controller.

The particular functions of the distribution assembly may be summarized as follows:

- - E a s y parallel operation due to inherent current control loops

• Activation/deactivation of subsystems • Switching of power to experiments according to predefined time lining • Connection of buses and loads to active alternate or back-up modes • Isolation of single point failures to prevent failure propagation.

The main disadvantage is that the "on-resistance" of presently available devices is approximately 10 times higher than that one of a mechanical relay contact which causes additional losses and deteriorates the power quality at the user interface. Consideration is to be given furthermore, to the radiation sensitivity of power FET's which means for example that a negative gate voltage is needed in order to bring a n-channel device really in the off-condition. SSPC's cover both, the switching and protection task and hence, represent the indispensable main elements of medium voltage distribution systems. Regarding the fact that back-up solutions are not available (as for example NiCd batteries for energy storage),

In order to perform these tasks, the distribution equipment consists primarily of switchgear and the associated monitoring and control hardware. Switchgear may respond to crew initiated signals, computer generated commands, or operate automatically when predetermined overload conditions exist. Several types of load control switches are available but they all can be classified as electromechanical relays or solid state power controllers (SSPC's). An immediate consequence of the increased operation voltage of 150 V D.C. is that mechanical relays can hardly be used for Columbus. This is due to the arcing problem. An arc always will form between metallic contacts as they separate, provided that the initial current exceeds about one ampere and there is sufficient voltage ( ~ 50 V) to establish the ionized plasma or to initiate transfer of conducting metal vapor. There are several methods available for arc interruption. However, the corresponding auxiliary equipment is so bulky and heavy that it is not suitable for space applications. Resulting thereof the application of mechanical relays for Columbus is limited to the special case of switching under zero load conditions only (in order to provide galvanic isolation for safety reasons). The availability of new technologies as power FET's resulted in the development of so called solid state power controllers (SSPC's) where a semiconductor performs current interruption and thus, eliminates the arcing problem. Such a solution provides the following advantages: --Increased reliability and cycle life - - I m m u n i t y to shock and vibration effects - - T h e FET inherent fast response time allows rapid failure isolation A.A.

17/I--H

switching and protection is considered as the most critical area of the Columbus EPS.

As shown in the block diagram of Fig. 4, SSPC's basically consist of the following main parts: • The power section which contains the FET and thus, performs on/off switching and current limitation in the overload case. • A thermal control circuit (self protection of the switch) which determines the FET's junction temperature and initiates switch-off when a critical value is exceeded. • The signal interface which receives on/off commands and provides signals with respect to status and current flow. • The internal power supply Regarding the power section, two different switches are presently under development (ESTeC contracts): L The "Linear Switch"

In the failure case the FET is driven in the "constant current mode". A current control loop is activated which limits the overcurrent to a fixed value such that the switch acts as constant current source. Because the dissipation of the switch is drastically increased in this limiting mode, the thermal control circuit will interrupt its operation after a short time interval (some milli-seconds).

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H. The " P W M switch"

In the order to handle the overload case without excessive dissipation, the FET is operated in the switched mode. When an overload occurs, a current detector switches off the FET which remains off until the output filter current drops to a preset value which results in switch-on, once again, of the FET. Hence, if the overload is maintained, the filter current will limit cycle between the high and lower current limits, resulting in an effective output current equal to their mean value. This is the so-called "Limit Cycling Conductance Control" (LC3) principle which is described in more detail in [1]. When comparing both "switches", it becomes evident that the PWM switch has the following additional feature: in the failure case an output current, close to the limitation value, is maintained at all output voltages, whilst the input current reduces as the output voltage drops (input current fold-back characteristic). The switch remains in this status until it is tripped off by an external signal. Hence, it can cope with short circuit-, overload- and inrush situations for an indefinite time without excessive dissipation. The disadvantage of the PWM switch resides in the larger volume and mass due to the needed in- and output filters. For reasons of miniaturization, all SSPC's must be built in modern hybrid technology. Hence, a lot of development effort is to be spent which again underlines the criticality of this area for Columbus. With regard to the described features, PWM- and linear switches will be used in different locations of the distribution system. The PWM-switch will be applied as primary switching element for overall bus management. The linear switch is located close to the loads (so called secondary switch). However, when a load draws a very high inrush current (motor load),

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the PWM-switch may be used as load dedicated switch too. It is, furthermore, important to recognize that the SSPC is able to distinguish very accurately between normal operation and faulty overload conditions. This is due to its fast response time and the fact that the limitation current is exactly set. This feature of the SSPC is in the following referred to as "high selectivity". In low voltage power systems applying mechanical switchgear (as e.g. Eureca), inrush current limits according to Fig. 5 hae been adopted. These relative high limits have been introduced because a mechanical relay contact can easily cope with the shown overcurrents and the power source (regulator) is able to kep resulting voltage transients low. In medium voltage systems with solid state power controllers it does not make such sense to specify such general inrush current limits because the SSPC will automatically limit the current as shown in Fig. 5 (dashed line). The question arising now is, how the limit I L shall be fixed. In order to answer this question we have to recall that the SSPC provides excellent properties with respect to selectivity and that it is rather expensive. Both features exclude any overdesign which means that the limitation current shall be set as close as possible to the steady state load current. This can be achieved only when the SSPC is "matched" to the load which requires a careful analysis of the interactions between SSPC and load during switch-on since this represents the most critical case. In order to give an example how such an analysis looks like, switch-on of a resistive load with input filter by means of a SSPC has been investigated in the Appendix. The constraints derived in the frame of the analysis are some equations which relate sizes of reactive filter elements to the limitation current of the switch. It is quite obvious that this load-switch dependence re-

Inrush current when Lood swilch-on is performed by means of o SSPC. 7"¢ (appr. 5~.s) is the response time of the current control loop.

\

\

it.

0

t 4

400~s

2 to 5 0 m s

Fig. 5. Typical inrush current imits for low voltage d.c. systems.

Power supply for Columbus

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LED DISPLAY

Fig. 6. The modular and standardized distribution box which can be used as versatile standard power interface on all Columbus elements. quires new approaches for the design of power distribution boxes because in most cases the SSPC is to be changed when the load changes. As response to this new situation A E G has conceived the Modular and Standardized Distribution Box (DB). An artist impression of the exterior layout is given in Fig. 6. The basic idea is that a carrier structure, the Branching Unit (BU) can accommodate up to six exchangeable Modular power Outlets (MO's). The BU is of relative simple design, it just provides the following functions: --Switching of power feeder in order to cut DB electrically from the distribution system (for safety reasons). --Branching of power feeder in order to supply individual MO's. --Sensing of voltage and current. - - D a t a interface to the STAU's (Standard Acquisition Unit). The BU shall become an integral part of all load centers where users install their equipment (e.g. racks, experimenter ORU's, etc). When the electrical properties of the user equipment are known, the individual MO containing the SSPC can be selected. The MO's are conceived as plug-in units (in-orbit replaceable) and will be available in different ratings (limitation current adjustable) such that user demands can exactly be matched. Such a design provides outstanding features with respect to maintainability and accommodation of different user. The DB can be applied as common

building block throughout the distribution systems of all Columbus elements. It shall be mentioned finally that in spite of the increased "on-resistance" of the SSPC's, the overall voltage drop of the power distribution assembly shall be kept to a minimum. Because the power quality at the user interface is strongly influenced by this overall voltage drop, it represents an important "figure of merit" of the distribution assembly. The present goal is to keep an overall value of approx. 2% which shall be achieved in the following way: • Distribution systems of Columbus elements shall comprise not more than two distribution levels. • SSPC's to be designed such that the individual device meets a voltage drop requirement of 0.75 V. • Cable cross sections to be selected such that overall harness voltage drop stays within a limit of 1.SV. Technical constraints and guidelines derived so far have been synthesized into a basic distribution design shown in Fig. 7. 7. POWER MANAGEMENT AND CONTROL

The area of EPS control, check-out and operation is strongly influenced by the new Columbus scenario. For the power system the following goals are of primary importance: • EPS autonomy to be enhanced. --Reduced dependence from ground stations - - H i g h degree of functional independence from the DHS to be achieved.

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• In-orbit maintenance shall be facilitated. --Selfcheck capability/failure diagnostics • Interface complexity between subsystems to be reduced. --Simplification of design/development/test/ integration • Decentralization. --Tasks shall be shared such that a high degree of functional modularity is obtained (increased reliability). By clearly recognizing the significance of these topics, much study effort has been spent in order to develop new ideas in the area of power management

and subsystem control. As result AEG has proposed the P system I__N_NternalDAta Processing (PINDCAP) assembly. PINDAP is a data acquisition and processing system taylored to the specific EPS needs. Hardwarewise it is in line with the present Columbus baseline, i.e. it makes use of presently defined building blocks as: • STAU's (Standard Acquisition Units) as data interface to the individual EPS equipment. • System LAN for data transfer between the equipment dedicated STAU's and the EPS subsystem processor. • Subsystem processor as PINDAP Kernel.

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Fig. 7. Distribution hierarchy of the Columbus power system.

Power supply for Columbus

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BT IE I

Power distribution

Fig. 8. PINDAP in the frame of the Columbus on-board intelligence hierarchy. Automated systems must be structured hierarchical. For Columbus the following three levels are to be distinguished with respect to the on-board intelligence hierarchy: • System Level (Level 1): on-board management • Subsystem Level (Level 2): subsystem management • Equipment Level (Level 3): Local management PINDAP's role and position in the frame of this hierarchy is shown in Fig. 8. It provides a centralized interface to the DMS such that interactions between both subsystems are performed at highest feasible level. Interactions between DMS and P I N D A P are required for activation/deactivation of the EPS or in the case of EPS reconfiguration (e.g. switch-over to back-up buses, load shedding, etc.). On the other hand P I N D A P coordinates and supervises all subsystem internal functions. Therefore, it performs thorough status monitoring, command generation and management of redundancies. In this context special attention is to be paid to the energy storage system. According to the present baseline, NiH2 batteries shall be used for the IOC version. Hence, hundreds of cells must be supervised individually which will result in a fairly high data stream to the central processor. The task of closed loop control of currents and voltages, as frequently encountered in all kind of power conditioning equipment, will not be transferred to PINDAP. It can be used, however, in order to modify reference values of the control loops such

that performances can be adjusted to changing internal or external conditions. In the power distribution area a great number of SSPC's and some mechanical switches are to be operated (on/off commands, status acquisition), which can be extremely simplified by PINDAP. In order to support in-orbit maintenance, checkout data from the built-in test equiment (BITE) are transferred to the subsystem processor. Evaluation of these data leads e.g. to indication of equipments/life limited items which need to be replaced. Due to PINDAP, failure cases can be handled now with a clear allocation of tasks: - - F a i l u r e identification and isolation is performed on equipment level. --Recovery actions, however, on subsystem and system level by giving highest priority to crew safety and mission objectives. A remarkable feature of the described hierarchy is that only processed results will routinely progress upward through the different management levels. Raw data will do this on request only. The immediate consequence is a reduced data stream to the ground station, representing an important step towards alleviated ground support requirements. It is of importance, however, that the mentioned automated control functions will be accessible to crew or ground in order to provide the possibility for manual override. The described properties clearly underline the benefits of this new power management and control

110

GERT EGGERS

approach, A chief a d v a n t a g e is t h a t the entire control procedure is less rigid. It allows a d a p t a t i o n to changing operational conditions. This is based on the fact that due to P I N D A P h a r d w a r e is replaced by software. 8. S U M M A R Y / C O N C L U S I O N S

The p a p e r describes a d v a n c e d power supply a n d distribution systems to be used on u n m a n n e d / m a n tended C o l u m b u s elements, capable of supplying 8 to 30 k W to a variety of users in low earth orbits. E m p h a s i s has been p u t on new technics a n d technologies which are indispensable in order to cope with the stringent C o l u m b u s requirements. In this context it is to be recognized t h a t a subsystem as the EPS which shall deliver electrical power for large scale space activities can only use proven technologies. The challenge to the designing engineers is to m a k e the described i n n o v a t i o n s so reliable that they can be used w i t h o u t any restrictions o n all C o l u m b u s elements. In order to achieve this, the related research a n d d e v e l o p m e n t activities have been initiated already (ESA's PSTP program). Areas receiving special a t t e n t i o n are: • Photovoltaic power generation in low earth orbit environment • Energy storage devices with i m p r o v e d cycle-life a n d d e p t h of discharge properties (NiH 2 technology). • M e d i u m voltage d.c. distribution systems (switching, protection). • New m e t h o d s for power m a n a g e m e n t a n d control. A E G is confident t h a t these R & D activities in conj u n c t i o n with the technical competence o f all parties involved, will bridge the gap between escalating d e m a n d s on technical p e r f o r m a n c e and state of the a r t - - s u c h t h a t the described new approaches will be realized successfully.

APPENDIX Analysis o f the Switching-Behaviour o f Solid State Power Controllers (SSPC's )

The purpose of the following investigations is to study transient responses when a simple eletrical circuit consisting of input filter and resistive load is switched on by means of a linear SSPC. The linear SSPC is an electronic switch with current limitation feature. Due to its thermal self-protection it can maintain overload situations for a limited time T F (trip-time) only. When using such "switches" it is of interest to know up to which extent they can cope with inrush currents as they will occur for example when a.m. loads shall be connected to the bus. By establishing the energy balance of the SSPC during switch-on, constraints will be derived which relate sizes of reactive filter elements to the limitation current of the SSPC. In order to have a reference against which SSPC performance can be measured, switch-on of the considered electrical circuit by means of a mechanical relay contact will be analysed first. l. The relay contact as switching element The corresponding circuit diagram reflecting this case is shown in Fig. A-l. At the time t = 0 the switch S is closed and a unit step of the bus voltage is applied to the input filter. The task is to determine input current i(t) and load voltage u(t) as response to this unit step. The corresponding circuit diagram reflecting this case is shown in Fig. A-l. At the time t = 0 the switch S is closed and a unit step of the bus voltage is applied to the input filter. The task is to determine input current i(t) and load voltage u(t) as response to this unit step. The Laplace transforms of i(t) and u(t) are given by: I(s) -

U,,

I + R'C'S

s R.L.C.(s2+s/R.C+I/L.C) u,, 1 U(s) = • . s L'C'(se+s/R'C+I/L'C)

(O0

x / l II.~ ' . .

... Resonance frequency of the circuit 2.,5 . t o 0 = l / R . C :

Zo =

1. D. O'Sullivan and A. Weinberg, LC3: application to voltage regulation; LC3: applications in power switching and protection. Proc. Third ESTeC Spacecraft Power Conditioning Seminar, Noordwijk, The Netherlands, 21 23 September (1977). 2. C. Cazaoulou and F. Fachinetti, medium voltage power switch (preliminary specification). Ref.: C.Se.MVPS.ST.017. Crouzet, Valence, France.

1

/L

,5=2~.x/C

/

~

(1.3)

Zo

C " " " Characteristic impedance

(1.4) (1.5)

it is obtained: Uo (00/6 + ¢00/'26 l(s) = R "S 2 -}- 26 "O)~)'S + ¢O0

~0. ,o ~,,,'s U(s) -,s.2 + 26~0s + ~J0

Initial

conditions :

A t t=O

no s t o r e d

energy in the reactive elements

•.,,m--B u s

(1.2)

By introducing the abbreviations

... Damping coefficient

REFERENCES

([.1)

,,L,er--------.oad----i

Fig. A-I. Circuit topology for switch-on with a relay contact.

(1.6) (1.7)

Power supply for Columbus /it)

111

By using (1.9) and the identity: arc cos x + arc cos y = arc cos(x .y - x / l - x2"x/l - y 2 ) ( x + y >/0)

(1.17)

(1.16) can be remodelled: flo, = n - (arc cos 23 "~fl - 3: + arc cos x / l - 32) = x - arc cos 6

2??

(1.18)

It is important to note that the voltage u(t) reaches for t = tim the first time the U0 (steady state) value (as shown in Fig. A-2). With (1.18) the current maximum becomes:

Fig. A-2. Normalized input current and load voltage as damped harmonic oscillations when circuit switch-on is performed by means of a mechanical relay contact.

I,~ = I,,.[1 + 1/26 .e -~(. . . . . . . . at]

(1.19)

with ,~

6 -

(1.20)

x/1 -62

The voltage maximum is described by: Retransformation of (1.6) and (1.7) in the time domaine yields for 6 ~< 1: - ~ " " c ° s ( t o ° ' x / 1 - - 3 2 ' t +~b); i ( t ) = l ~ " I 1-- eCOS~

(1.8)

with cos ~b = 2"6 "x/1 - 62

(1.9)

and u ( t ) = U0.[l - e-'~°t~ .sin(to0.x/l - 62.t + ~ ) ]

(1.10)

with sin $ = x / l - 62

(1.11)

I,~ = Uo/R denotes the steady state load current. Equations (I.18) and (1.10) are valid for 0 ~< 6 ~< 1. The special case 6 = l is called "critical damping". It can be derived from (1.8) and (1.10): i(t)]a=t = I~.[I --(1 + 1/2.too.t).e -°'°'t]

(1.12)

u(t)la=l = U0'[l - ( 1 + c%.t).e ,~t]

(1.13)

The undamped case (6 = 0) reflects the condition R--* oo (no load) which as well can be derived from (1.8) and (I.10):

v0.

i(t) 1~= 0 = 7~,' sin too• t

(1.4)

u(t) = U0(1 - cos too' t)

(1.15)

It is to be noted, however, that 6 = 0 can also be achieved by putting L = 0. The state variables reflecting this case cannot be derived from (1.8) and (1.I0). One has to go back to (1.1) and (1.2) in order to analyse this case. (1.8) and (I. 10) represent damped harmonic oscillations. The graph in Fig. A-2 shows that both state variables go through a series of relative maxima and minima before approaching their steady state values. The input current i(t) runs through its greatest value for t = t~, to0.~/l--62"li~=~irn=X -- ~b -- arc sin 6

(1.16)

to0"~/1 - 62'tu,n = flum= x - ~

+ a r c c o s 6 =x

U,,= U0'(1 + e -~~)

0.05 0.1005 0.204 0.436 1

1.621 1.671 1.772 1.982 2.356

(1.22)

Conclusions o f Section 1. When considering Table 1 it can be easily recognized that input current and output voltage show high overshoots for small values of the damping coefficient. Since overshooting of the voltage may damage the load, it is important to select filter components such that 0.4 ~< 6 ~< 1 which keeps the voltage overshoot smaller than 25%. When the bus provides a sufficient inrush capability, the current overshoot does not represent a problem because mechanical relay contacts can easily cope with such a situation. Hence, the only design condition to be observed when the considered load shall be switched on by means o f a relay contact is the above restriction for 6. However, such a simple design procedure leads to undesired consequences when the mechanical switch is combined with a protection function. The trip threshold is to be adjusted such that it can tolerate the inrush peak. Especially for small damping coefficients the trip threshold may be much higher than the steady state load current. The result is a reduced detection capability of faulty overload conditions (low selectivity). 2. The SSPC as Switching Element After switch-on by the SSPC, i(t) and u(t) will rise in the same manner as described in Section I (the on-resistance of the SSPC is assumed to be small in comparison to the load R). When the input current exceeds the limitation value IL of the SSPC, input filter and load will see a constant current source delivering 1L. This new situation is characterized by: di uL=L'~t =0 (2.1) du=tC.dt= c

U

-~

'dt

(foru/R
Table 1. First maxima of current i(t) and voltage u(t) in dependence on the damping coefficient 6 a i,, u~ 6 x/1-32 fl~,=n - a r c c o s 6 e -~#" m I, Uo 0.05 0. I 0.2 0.4 0.707

(1.21)

0.922 0.845 0.700 0.421 0.09

10.22 5.23 2.75 1.53 1.06

1.85 1.73 1.53 1.25 1.04

(2.2)

112

GERT EGGERS

I i Uo

i(r)

L

Initiot

l-v-

5_ T

I

SSPC------~

I

•-Input

filter----=-

conditions :

At t = O no stored e n e r g y in the reactive

etements

='

•m--Load

Fig. A-3. Circuit topology for switch-on with a SSPC.

(2.2) establishes a relative simple relation between differentials of dependent and independent variables. The rate of energy dissipated by the SSPC in the constant current mode is given by:

dEe = Ur'lL'dt = ( Uo - u)" IL'dt

linear approximation will be used:

Ur~ IL.R.K

(2.3)

dE,,=C.R.IL.(I

D u e to (2.2), dt in (2.3) can be replaced by du w h i c h yields:

Uo- u dE e = C. R -I L du I#R -u

1L'R-Uo~ ~--R-~)

EF= C'R'IL'[u +(IKR -- Uo)ln(l#R -u)]~E°

The total energy dissipated by the SSPC is obtained by integrating (2.4) within the boundaries Ur and U0" UT is the voltage which has been built up in the capacitor when the SSPC goes in the constant current mode. In order to avoid complicated procedures for calculating Ur, the following

~ :0.2 K = 036

x InIL'R--Uz l I L' R 2 U0

~ =0.1 K = 0.t 9

~ =0.05 K : 0.098

B =O K=O

14 13 12 41 10 9 8

6

5 4

3 2 1 0

I

4

1.2

'1.4

1.6 4.8

2

I 3

t

I 4

I

I 5

I

I 6

I

I 7

X

Fig. A-4. Maximum size of filter capacitor in dependence on x (quotient of limitation and steady state current) and the damping coefficient &

/it)

- -

IL

/ I's

u(t)

/Uo

1

tiM

--

(2.7)

(2.8)

15

4,

(2.6)

which is a convenient form for integration

(2.4)

~=0.4 K = 0.65

(2.5)

K will be determined later on as a function of 6. Remodelling of (2.4) leads to:

l

Fig. A-5. Input current and load voltage response when switch-on is performed by means of a SSPC.

Power supply for Columbus By using (2.5) and introducing

IL'R=It.

t2o

I~,

= x

(2.13)

When the input filter shall have a damping coefficient of 6 = 0, 1, the maximum value o f the capacitor is according to Fig. A-4; C = 2.3. C0 = 384/~F. This, however, stipulates that according to (1.4) the inductor must have a value of at least L = 230/~H. Table 1 exhibits for 6 = 0, 1 a voltage overshoot of 73% (for a mechanical relay contact) which is unacceptable high. It is therfore important to verify the SSPC dedicated voltage overshoot which can be performed by means o f (2.32). The result is: 0.0473.73% = 3.45% which is extremely low and underlines the excellent properties o f the linear SSPC. Although the "no load case" was excluded from the previous considerations, it shall be mentioned that (2.17) approaches for R--, oo (i.e. K = 0) the limit value of CL = 2. T F. IL/U o (apply Bernoulli/L'Hospital). This means that the considered 5 A switch is able to charge a single capacitor of CL = 4 0 0 # F without tripping. Finally it is of interest to determine the shape o f input current and output voltage when the SSPC leaves the limitation mode and goes back in the saturation mode. In order to investigate this case, Fig. A-3 is considered again, however, with the following initial conditions:

(2.14)

Application of Kirchhoff's laws leads to the following differential equations:

(2.9)

it is obtained:

Er=C'R'lL'Uo'[l-K'x-(x-1)lnX(Ix~K) ]

(2.10,

x represents a new independent variable which indicates the ratio between limitation current o f the SSPC and steady state load current I~. Evidently the SSPC will dissipate energy only when it is actually driven in the constant current mode, i.e. when IL < Ira- Where I m is the current maximum according to (1.17). Thus, (2.10) is valid for: l~
x,~=--

Im

(2.12)

i,,

For x = x,, the dissipated energy, i.e. (2.10) becomes zero which furnishes a condition in order to determine K.

l -x,,.K

lnXm(l-K)

x,,--I (2.13) is solved for

x,,--1

i(t)l,=o=IL; K = l[x m

The x~, values in dependence on the damping coefficient can be extracted from Table I. Because the x,, values in Table AI are defined for 0 < 6 ~< 1, this applies for the K values as well. In order to extend this definition to 6 = 0, we make the allocation: 6=0

corresponds to

K=0

(2.15)

and look for the consequences. According to (2.5)we get for K =0; UT=O. This, however, can only be obtained when the inductance of the considered circuit approaches zero which represents the most critical case for switch-on. It is furthermore important to recognize that the above allocation (2.15) excludes the case R--* ~ (no load). The switch does not trip when its energy balance is positive, i.e, when

E r ~ UO.IL.Tr (TF: trip time)

(2.16)

c~<

(2.20)

Transformation in the s-domain under consideration of (2.18) yields:

1 )(.)

(s'L

(,o,+L,.)

c's+I/RJ'\U]=\

c.U o

J

(2.21)

with

D is the determinate of the matrix in (2.21). Resolution of (2.21) with respect to the state variables gives:

1~,.c°~ + IL(2.6.ego + S) S

l(s) = s2 + 2.f.coo.s +w02

(2.17)

(2.23)

U(s)=Uo(~+s)+2"6"°°o'R'IL s 2 + 2.6 "COOS+ a~02

x--I

with

1 ~
i(t)=I,s,

(2.17) is plotted in Fig. A-4 for different K values. The curves of Fig. A-4 allow to match input filter design to SSPC rating. As example a linear SSPC with technical data extracted from [2] is considered now. Rated current:

0 to 5 A

Limitation current:

I, = 6 A

Trip time:

T F = 5 ms

Rated Voltage:

U0 = 150 V

Note: The switch can withstand a short circuit without current interruption for a time t = TF. It is assumed now, that this switch shall be dedicated to a 5 A load, which means: R=30Ohm;

(2.24)

In (2.23) and (2.24) the same abbreviations as in Section 1 have been used. Retransformation of (2.23) and (2.24) in the time domain leads to the following results:

K

K according to (2.14) and (2.15); and

x=l,2;

(2.19)

L .d~:)" + u(t) = Uo

1).ln x ( l - K )

Co= rr/n;

(2.18)

(O2

Co 1 - x .K - (x -

u(t)l,=o= Uo

i(t) = C . du(t) + 1/R.u(t) dt

is valid. By combining (2.10) and (2.16), the following condition is established for the capacitor:

I~,= 5A;

113

C0=167/~ F

I

e~.t

l+(x--l),

fi

x sin(a~0x/~ -- 62. t + ~ ) ]

[

26

u ( t ) = U 0. l + ( x - l ) .

(2.25)

../

/~

v× e ~ " . s i n co0.l x f ~ - 6 2 ,

t] J

with a=arccosr;

I,

x =--;i,,

(2.26)

i _ Uo "-R

It is clear that (2.25) and (2.26) are valid for 0 < 6 ~< 1 and 1 ~
GERT EGGERS

114

In a similar manner it is derived:

i(t) Runs through its first relative m a x i m u m for c%.x/1 _ ~5. tiM = 2=

(2.27)

which gives a current m a x i m u m of

l u = I~,[1 + (x - 1).e :~]

(2.28)

It can be easily verified that I M < I t for 0 < 6 ~< 1, which indicates that the SSPC does not go a second time in the limitation mode. The voltage m a x i m u m is described by:

%" ~/1 - 6LtuM = arc cos 6 U M = U0[l + (x - 1).26 .e ~. . . . . ~]

(2.29) (2.30)

with 2 according to (1.20). (2.30) dearly shows that UM reaches its greatest value for x = x,,. This becomes even more evident when (2.30) is combined with (1.19) in such a manner that 2 .arccos 6 is eliminated.

Remodelling of (2.31) under consideration of (1.22) yields:

uM-Uo x - l u~-Uo _ Uo x~- I Uo

(2.32)

IM--l~s-2.6.(x

L~

-- l).e

.~ ~

~~

+ ......

~.lm -- I~.., __ (2.33)

L,

By relating relative overshoot of input current and load voltage for switch-on of the considered electrical circuit by either a relay contact or a SSPC, (2.32) and (2.33) represent a very important result of the investigations. The "damping properties" of the SSPC are put into evidence. Input current and load voltage have been plotted for the entire switch-on process in Fig. A-5. The shape of the load voltage for the time interval t r has been obtained by integrating (2.2). Conclusions o f Section 2. Due to its current limitation feature and its fast reaction, the SSPC shows a completely different switching behaviour than a mechanical relay contact. Electrical circuits with a very small damping coefficient can be switched on by keeping voltage and current overshoots to a minimum. Particularily eqns (2.32) and (2.33) reveal this property. The damping behaviour is improved when the difference between limitation current of the switch and steady state load current is reduced, i.e. when the selectivity is increased. Even a high selectivity (i.e. when the switch is really matched to the load) does not lead to dramatic restrictions regarding sizes of reactive filter elements. Capacitors up to several I00 itF can be charged by a 5 A device without tripping. In order to facilitate the matching process between load and SSPC, its limitation current should be adjustable (up to 35%).