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Distributed Power System for Large Orbital Infrastructures: Architecture, Analyses and Lesson Learned Antonio Ciccolella Electrical Engineering Department ESA-ESTEC D/TEC-EEE Noordwijk - NL. Outline of the Presentation.

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  1. Distributed Power System for Large Orbital Infrastructures: Architecture, Analyses and Lesson Learned Antonio Ciccolella Electrical Engineering Department ESA-ESTEC D/TEC-EEE Noordwijk - NL LECC 2005 – 12 September 2005

  2. Outline of the Presentation • Distributed Power System for Space Applications: The International Space Station and its peculiarity • Summary Description of the ISS Power Architecture • Requirements for users: The Power Stability issue. • Highlights of analyses and test performed by ESA, as the contribution to the ISS global stability • Lesson Learned from on ground test and in-situ events • Effects of Space Environment on Hardware • Conclusions LECC 2005 – 12 September 2005

  3. International Space Station Altitude: 220 n.m. (nominal) Inclination: 51.6° Dimension: 87 m (L) x 106 m (W) Weight: 425 000 Kg Pressurized Volume: 1160 m3 Power: 110 kW (total); 45kW (users) Final Configuration LECC 2005 – 12 September 2005

  4. Boundary Conditions • The ISS is an International undertaking led by the USA, with contribution of other Partners (Rus, EU, JP,I, CND) • Power generation is from US & Russian PV systems • Two independent DC power buses are available, which can exchange power each other: • 120 V for the US segment (DSPG) • 28 V in the Russian Segment (Floating Grounding) • Power System is assembled in various stages: • High flexibility to accommodate various cable lengths and changing architectures (load conditions) → modularity • It is not possible to test the entire system on ground in advance, as it is with conventional satellites • Manned spacecraft requires top reliability levels: • Architecture must address failure isolation, detection and recovery LECC 2005 – 12 September 2005

  5. Implementation • The Power System of the ISS has to be “distributed” • Power is processed by multiple cascaded stages of switching regulators (i.e. mainly DC/DC converters). • Namely, the canonical arrangement of this distributed power system consists mainly of: • A stage of regulators, acting as line conditioners, which take the unregulated input voltage and convert it to a regulated bus voltage. This bus distributes power to the system • Cascaded stages of parallel regulators, acting as load converters, which take the power from the above bus and generate the appropriate output voltage required by the loads LECC 2005 – 12 September 2005

  6. Distributed Power System Additional Motivations • Difficulty in maintaining regulation at the load location with a centralised power distribution, due to the possible long distance between load and source • Placing intermediate converters closer to the loads, the requested voltage is regulated with higher accuracy • Enhancing isolation between source and loads in large infrastructures, which DC/DC converters with galvanic isolation inherently do (network separation) • Modularity derived from the use of parallel load regulators allows the spacecraft design to flexibly manage both power reconfiguration and reliability by adding redundant modules. LECC 2005 – 12 September 2005

  7. Power System Architecture #1 DCSU=DC Switching Unit SSU=Sequential Shunt Unit BCDU=Battery Charge Discharge Unit MBSU=Main Bus Switching Unit LECC 2005 – 12 September 2005

  8. Power System Architecture #2 - US Power Channel Connected to Columbus APM APM LECC 2005 – 12 September 2005

  9. E l e c t r o n i c s C o n t r o l U n i t RPCM-External D C - D C C o n v e r t e r B a t t e r y C h a r g e - RPCM-Internal U n i t D i s c h a r g e U n i t ISS Power System Electronics Battery Subassembly ORU Sequential Shunt Unit Plasma Contactor Unit Remote Power Controller Module DDCU-External DC Switching Unit Main Bus Switching Unit DDCU-Internal LECC 2005 – 12 September 2005

  10. Flight 4A (Nov 2000) LECC 2005 – 12 September 2005

  11. Design Issue: Impedance Interaction • Higher system complexity, which may render the analysis and the design of the system as impracticable if it is considered as a whole • Potential instability of the Power System or of its sub-network, which is due to the negative resistance input characteristics of the converter and its impedance interaction with the EMI filter and the power source • Hence, detailed intra-system compatibility control was exercised, which focused on the impedance interaction (both amplitude and phase) of the sub-networks • This results in damping the potentially detrimental EMI effects of either switch on/off or failure clearance transients onto the bus LECC 2005 – 12 September 2005

  12. Strategy toward the requirements • The intrinsic modularity suggests the use of an approach for both the design and the analysis, which foresees the complete system as partitioned in smaller power subsystems (sub-networks) that can be independently and individually considered • The system is then built through the appropriate interconnection of the previously mentioned subsystems, whose interaction needs to be carefully analysed eventually to generate compatible specifications • This approachis based on the underlying assumption that the interacting subsystems are linear or weakly non-linear, thus small signal stability is the objective to achieve LECC 2005 – 12 September 2005

  13. Basic Approach to Stability The term 1+Hm represent the loading effect caused by integrating the two subsystems.1+Hm=0 is the characteristic equation of the overall system.Hence, Hm can be viewed as the system equivalent loop gain.The integrated system stability is determined by applying the Nyquist criteria to Hm LECC 2005 – 12 September 2005

  14. Consideration on Hm • When |ZS| << |ZL| for all the frequencies of interest (e.g. below 100 kHz), the stability of both the individual source and load blocks guarantees stability for the integrated system. (|Hm|<< 1 or Middelbrook criterion) • In general, this condition is not realistic. In some frequency intervals we can expect |ZS| > |ZL| due to the presence of filters. Hence |Hm| will be equal to one 1 (|ZS| & |ZL| will intersect) • Although this case does not necessarily imply a stability problem, additional analysis is required for Hm (e.g. by applying the Nyquist or Bode criterion) to determine whether system and/or subsystem stability is achieved • Gain and phase separation come into picture and become the initial requirement (3 dB & 30°) LECC 2005 – 12 September 2005

  15. Initial Assessment for Known Networks • The following ingredients: • Impedance characterisation of the known power sources (output Z) and loads (input Z), in all their operational mode • Layout of the power sub-network under consideration • Harness length and inherent AWG (mainly R, L parameters) • Allow us to obtain a preliminary assessment of the impedances’ congruence within the considered sub-network by circuital simulation • For the frequencies of interest, DC/DC converter’s input impedances can be accurately modelled with their input filter loaded by a negative resistance, whose magnitude is |R|=V2/P. • This is not valid when the input filter is integral part of the power cell, which can occur for some topologies. LECC 2005 – 12 September 2005

  16. Toward an impedance requirement • The above impedance matching concept was applied to determine realistic boundaries to levy requirements on loads and subsystems in flight elements (i.e. APM, JEM) • It implied analysis of multiple loads with a known source • Requirements’ attribute (US Responsibility): not overly conservative or impractical to implement • Way to proceed → NASA initially identify two load’s category: • Complex (load assemblies or equipment racks): interface B • Simple (individual loads): interface C • Interface B category (required to have their own power distribution and protection) was subdivided in function of their feeder rating (10-12 A, 25-30 A, 50-60 A) • Interface C category was subdivided in function of their line branch rating (1.5-3.5 A, 10-12 A, 25-30 A) • Further category: harness length (four ranges) LECC 2005 – 12 September 2005

  17. Example of Impedance Requirement • Notes: • Limit when steady state EPCE loading is less than 800 watts (see note 3). • Limit when steady state EPCE loading is at least 800 watts (see note 3). • 800 watts if from two DDCUs operating in parallel, or 400 watts from a single DDCU. • 10-12 AMPERE INTERFACE C LOAD IMPEDANCE LIMITS LECC 2005 – 12 September 2005

  18. Impact of Impedance Requirements • Monte-Carlo simulation (NASA) with ad-hoc representative loads gave rise to impedance requirements for each category considered • Stability requirements were introduced at a late stage in the ISS programme: Industrial Contract & Specification were already frozen for Columbus between ESA and Industry • Changing the established baseline might have implied a severe cost impact: many units were being manufactured • To allow an industrial assessment of the problem, ESA developed a model of the power source, anchored by test data. On the other hand, Industry initiated a parallel activity on the loads. This built confidence to accept the requirements in ICD. • Later, ad-hoc BIVP tests were performed with flight H/W • Stability was finally verified by analysis supported by test data LECC 2005 – 12 September 2005

  19. Main Characteristics of the Power Source • The power source of the Columbus APM is the DC/DC Converter Unit (DDCU), provided by the US and installed on the ISS. • DDCU can be in either standalone or parallel configuration • Each DDCU delivers up to 6.25 kW with 150% peak power • Downstream (output) voltage is nominally 124.5 V • Input voltage ranges from 115 V to 173 V • Switching frequency: 80 kHz (40 kHz x 2) • DDCU has Weinberg topology and peak current control. • Its operation scenario involves both DCM and CCM • Current limitation (80 A), under-voltage protection available • Capability of programming the current share ratio when two DDCU are configured in parallel (50:50, 60:40, 70:30) LECC 2005 – 12 September 2005

  20. Objectives • Model a Weinberg Converter with automatic detection of the DCM or CCM operational mode, including peak current control capabilities and current limitation features. • Model the parallel configuration of the above converters • Tailor the parameter of the model to the DDCU ones • Model the parallel configuration of two DDCU • Verify coherence with experimental data Attributes of the Model • Convergence to the right mode in a DC analysis • Correct small signal response in an AC analysis • Capability of changing operation modes during a transient analysis (large signal stability) LECC 2005 – 12 September 2005

  21. Sequence of tasks towards the objective • Analyse separately and sequentially the DCM, the boundary between DCM and CCM and the CCM with the state average approach of the power cell • From the resulting equations, derive a PSPICE circuital realisation of both DCM and CCM operational modes • Introduce the current programming control feature and the stabilisation ramp, which gives the converter’s duty cycle • Derive a automatic duty cycle generation for both DCM and CCM and integrate it in the unified model of the power cell • Introduce the DDCU parameters and the true circuits of input and output filters, error amplifiers, reference voltages from the design schematics • Implement connection diagram and control circuitry for the two DDCU in parallel • Compare the simulation results with test data LECC 2005 – 12 September 2005

  22. The Weinberg Converter CCM DCM LECC 2005 – 12 September 2005

  23. Single DDCU – Validation #1 Vin=138V, Iload=1 A LECC 2005 – 12 September 2005

  24. Single DDCU – Validation #2 Vin=138V, Iload=26 A LECC 2005 – 12 September 2005

  25. Phase and Magnitude of the Output ImpedanceSingle DDCU – Load Sweep LECC 2005 – 12 September 2005

  26. Magnitude of the Outer Open Loop Gain LECC 2005 – 12 September 2005

  27. Parallel DDCU - Connection Diagram LECC 2005 – 12 September 2005

  28. Parallel DDCU – Validation #1 Vin=138V, Iload=1 A, 50:50 LECC 2005 – 12 September 2005

  29. Parallel DDCU – Validation #2 Vin=138V, Iload=52 A, 50:50 LECC 2005 – 12 September 2005

  30. Phase and Magnitude of the Output ImpedanceParallel DDCU, 50:50 current sharing – Load Sweep LECC 2005 – 12 September 2005

  31. Parallel DDCU – Validation #3 – TransientCurrent step down from 80 A to 12 A LECC 2005 – 12 September 2005

  32. The ATV case LECC 2005 – 12 September 2005

  33. ATV: Statement of the Problem • ATV will dock to the Russian Segment of the International Space Station. • ATV will draw part of its required power from the Russian Service Module, which is floating. • ATV processes this power via four independent parallel DC/DC converters, each acting as a PCDU. • Crucial information were needed on the power source to ensure by design that the interaction between the Russian Service Module and the ATV load leads to a compatible and stable system • The design of the Electrical Ground Support Equipment for test and verification had to represent the flight hardware LECC 2005 – 12 September 2005

  34. Significance to ESA • ESA had to define unequivocal power interface specifications to consolidate the preliminary Interface Control Document, which constitutes the baseline for the Industrial design • The availability on ground of the Russian Service Module hardware emulator suggested to characterise the impedance of the power source experimentally. • ESA and the Russian Partner agreed a common test campaign aiming to characterise the Service Module-ATV electrical interface • Russian Bus is floating → Synthesise an equivalent network for common and differential mode as specification for Power Control & Distribution Unit LECC 2005 – 12 September 2005

  35. Russian Segment Simulator LECC 2005 – 12 September 2005

  36. Impedance Measurement Test Set Up Differential Mode Common Mode LECC 2005 – 12 September 2005

  37. Test Results vs. Synthesised Network LECC 2005 – 12 September 2005

  38. Network reproducing both the Common Mode and the Differential Mode impedance LECC 2005 – 12 September 2005

  39. Example of Troubleshooting – Case #1 • The Columbus APM, provided by ESA, takes power from two parallel DC/DC converters (DDCU) which are provided by NASA. • The PDU distributes power lines inside the Columbus Module. Solid State Power Controllers (SSPC) are in place for overload protection purposes. • NASA and ESA agreed a sequence of bilateral tests to verify the compatibility of their respective hardware, including critical operational scenarios (such as faults). • The management of power faults is related to safety and it is essential for a manned orbital infrastructure. Transients generated in fault conditions shall also be considered in the EMC specification. LECC 2005 – 12 September 2005

  40. SSPC simplified block diagram LECC 2005 – 12 September 2005

  41. Description of the problem:Simultaneous SSPC trips during fault test • During faults on a power line, only the affected SSPC should trip, so isolating the faulted channel (Pass/Fail criterion). All the other channels should be ON • The test in matter consisted of characterising the behaviour of the EMC and power quality of the bus in presence of a representative SSPC breadboard. • Faults consisted of both hot-to-return and hot-to-chassis short circuits on various channels, induced through a FET box at the switch output. • During the fault hot-to chassis on the channel A, all the SSPC hybrids tripped at once, shutting down the power to the all the resistive loads in the test set up. LECC 2005 – 12 September 2005

  42. Test Configuration LECC 2005 – 12 September 2005

  43. Troubleshooting • This anomaly is typical of common mode phenomena • We postulated that any individual circuitry of the SSPCs was susceptible to a common triggering event. • Subsequent measurements of the bus common mode transient voltage at various SSPC inputs, while the hot-to-chassis fault was reproduced, proved this assertion. • We detected negative transient’s peaks as high as –140 V between the bus return line and the chassis at the input of adjacent channels. LECC 2005 – 12 September 2005

  44. Common Mode on two adjacent channels LECC 2005 – 12 September 2005

  45. Explanation of the Anomaly • The common mode voltage transient is generated by the reaction of the return power cable’s inductance to the sudden current interruption. • The common mode voltage transient is seen by the auxiliary circuitry of the SSPCs, which are connected in parallel on the power bus. • This specific type of fault, associated to long cables, generates transient levels that exceed the susceptibility threshold of the auxiliary circuitry of the SSPCs. • Hence, the electronics of the SSPCs switches off and all the SSPCs go in OFF status as well • This was confirmed by modelling LECC 2005 – 12 September 2005

  46. The Fixing • The insertion of a tranzorb clamping the voltage between return line and chassis. • The tranzorb used was previously designed and manufactured by MICROSEMI on specification written ad hoc by NASA/BOEING • It was subjected to a flight qualification campaign. • The tranzorb in matter is a combination of three groups of two unidirectional Transient Voltage Suppressor diodes mounted back to back, in parallel. This ensures at least one failure tolerance • The tranzorb clamping voltage is between 20 and 30 V and it can withstand very high currents for short times. • A commercial equivalent exists since: MY056 LECC 2005 – 12 September 2005

  47. Result of the fixing LECC 2005 – 12 September 2005

  48. Lesson learned • In case of fault, the performances of the SSPCs depend greatly on: - The nature of the overload. - The inductance of the operational environment (both upstream source and downstream). - The interaction of the auxiliary circuitry with the transient phenomena associated to the fault. • Hence, design requirements must account for the expected system environment rather than rely on general practice rules LECC 2005 – 12 September 2005

  49. Troubleshooting Case #2 In orbit failure of DC/DC Converter • After eight months of nominal functioning, a local assembly of four Power Supplies, feeding a ISS payload, suddenly exhibited anomalies in orbit and caused protection tripping • Attempts to recover manually (by the astronaut) were not successful • There was a clear indication that at least one power supply was permanently damaged • The assembly was dismounted and brought back to the Earth for repair • Troubleshooting immediately started and, in parallel, an investigation board was established LECC 2005 – 12 September 2005

  50. Topology of the Assembly PS 1 PS 2 EMI FILTER PS 3 PS 4 LECC 2005 – 12 September 2005

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