RASAT LEO SATELLITE POWER SYSTEM DESIGN AND OPERATION

Metin Akin Bulent DagSenior Research Engineer Senior Research Engineer

Space Technologies Research Institute, Space Technologies Research Institute,TUBITAK, Turkey TUBITAK, Turkey

Email. metin.akinna v. tr Email. bulent. uzap. tubitakw ov. tr

required because of temperature dependence ofAbstract: This paper presents the power system the EoC voltage.design and operation of RASAT micro satellite.The design of the power system reflects the need I. INTRODUCTIONfor autonomous operation, independent of all Satellites use photovoltaic (PV) cells asother systems, and therefore should require no their source of solar energy because they convertintervention from the ground station in the event sunlight directly into electricity, which isofan anomaly arising on the spacecraft. effective in space. Even though solar energy isAutonomy is obtained by having dual redundant present, it is still unusable. The energy has to beand multiple systems. There is a dedicated sent through a regulator (BCR) that converts it toBattery Charge Regulator (BCR) for each of the DC energy on a battery. The converted DCfour body mounted solar arrays and two energy needs to be reduced to more usefulidentical, cold redundant Power Conditioning voltage levels by a DC/DC Converter of a subModules (PCMs) with autonomous switching to module (PCM). And distribution of differentensure that a regulated 5V line is always voltages to the payloads and protection againstavailable to the subsystems. the faults in the subsystems are the other tasks to

be performed. The RASAT LEO satellite PowerThe hardware IS also designed to adapt to System consists of the following componentsenvironmental changes. The most obvious (Fig 1)example of this is the BCR's ability to monitor (Fig. 1):the temperature of its respective solar panelsand the battery. The BCR is able to estimate the a Solar ArraysMaximum Power Point of the solar panels using a Battery Charge Regulators (BCRs)the array temperature as a parameter. The a Batterybattery charge rate is also controlled by the a Power Conditioning Module (PCM)BCR, which taper charges the battery once the a Power Distribution Module (PDM)End of Charge (EoC) voltage is reached Thebattery temperature measurement is also

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Fig. 2. Block Diagram ofBCR

Tracking/Estimation and End of Charge (EoC) Fig. 3 shows how a typical solar aclayI-voltage regulation. The BCR estimates the V characteristic varies with temperature. TheMaximum Power Point of the solar panels and figure shows also where the MPP of the array isEoC voltage of the batteries using the array and on the characteristic. While configuring BCR forbattery temperatures as the parameters solar array there are several factors to berespectively. Fig. 2 is the block diagram of a considered. The panel power drops considerablyBCR unit. For DC-DC conversion basic step- as you move towards the right of the MP point.down (buck) configuration is used. The design of Therefore, set point errors should tend to stay inthe BCR is based around the Unitrode UC494A the left plane. Panels output voltage drops due toPulse Width Modulation control IC. This IC the resistance of cell interconnects and lifeincorporates a complete Pulse Width Modulator, degradations should also be taken into account.an oscillator, two error amplifiers, soft-start and Therefore, for a solar array with 48 series cellsa 5V reference. per string, the BCR set

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point for 25°C will be equal to: The initial start up power to the BCR isMPl-BO VMPi-BOhL,,-&.dz + %E.&-tzohVBCR-Arr.ySetpoint 48x(VMPPcell-BO [ EOLdegradahon X]) provided by solar panels throughashunt

(1) regulator and a diode (see Fig. 2).

Where;III. POWER CONDITIONING MODULE

VMppcell-BOL Maximum power point cell voltageat the beginning of life at 25 °C The function of the Power Conditioning

Module (PCM) is to convert the raw battery/ohmiclossesdegradation Percentage cell voltage voltage into a regulated 5V line and a -5V line.degradation due to ohmic losses The -5V line is low power, and is used only by

/OEOL-degradation Percentage cell voltage the power system electronics. The dualdegradation at the end of life redundant PCMs (PCMA and PCMB) are

virtually identical.The solar cells used for RASAT have a

MPP voltage temperature coefficient of Fig. 5 shows the PCM block diagram.-1.9mV/°C. With 48 cells in series, the overall Similar to BCR it has two mode of operation: 5VMPP voltage temperature coefficient of - output voltage regulation and battery under-91.2mV/°C voltage protection. The 5V reference voltage is

connected to the negative input of error opamp-The battery cells have a negative 1, and the feedback to the positive input. The

temperature coefficient of approximately- feedback is obtained from the output via a3s.3mV/C per cell (-72.6mV/°C per 22 cells in voltage divider. Considering the resistive voltageseries. Fig. 4 shows the temperature drops the output of PCMA is set tocompensation network used for both MPP approximately 5.15 volt while PCMB is set totracking of solar panels and BoG voltage setting 5.25 volt. This voltage is higher so that PCMBfor batteries. The effect of temperature on the will take over when switched in parallel withsettled voltage values is represented by a PCMA.temperature sensitive resistor comprising athermistor network.

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Battery under voltage protection is very high short duration currents are necessaryprovided by the second op-amp in the controller, for a system such as boom pyros. On the otherWhen the PCM input (battery) voltage goes hand systems with redundancy can be poweredbelow 20 V the PCM switches off completely. via fuses. Redundancy is necessary because onceThe PCM will re-start once the battery voltage break fuses can not be reset. The most flexiblehas recovered to approximately 22.7 V. This way of distribution is via a power switch. Thehysteresis is achieved by the additional voltage power switch has two main functions:contribution at the negative input of the second

error mpliferby he ouput votage.* Firstly, it can switch the subsystem ON and

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OFF by means of a telecommand. This is veryIV. POWERDISTRIBUTIONMODULE useful when it is not necessary to have a

The tskofthe Pwer Dstribtionsubsystem powered up all the time. It can alsoModule~~(PM is todstiut hedffrn be a necessity to switch off a subsystem whenvoltageto thepayload. A secnd ver the power available can not support the power

important task is the protection of the power demand in such cases for example the highsystem against faults and short-circuits in the sub poe stg ftetasitrsystems and bus circuitry. The Power * The second function ofthe power switch isDistribution Module on RASAT is split into 2 protection. The switch is also an electronic fusesections, the main section is located inside the that automatically switches off when the currentPCMIPDM Module and the second part, the drawn by the subsystem becomes larger than aDaughter board, is mounted alongside the BCR pre-set value.electronics.

V. PRACTICAL RESULTSThe PDM has three different ways ofditiuin The system described here was

previously used in BILSAT Leo Satellite. Fig. 6* Hard Wired shows typical battery voltage and current

waveforms for several cycles of charge-* Fuses discharge. As seen from the figure, during* Power Switches sunlight operation battery current rapidly

increases under MPPT operation of the panelsand once the BOG voltage has been reached the

Hard wired means that the subsystem isdirectly connected to the power system (i.e. noprotection). Hard wired connection is used when

495

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Time

Fig. 6. BILSAT LEO Satellite Battery Voltage and Current Waveforms

charging continues almost at constant voltage chrigmto.Itisaeamuiobcivand cuffrent starts to decay until the satellite chagin meho inldn asth bter

current limiting in addition to EoC Voltageregulation and MPPT might be needed [4].

VI. COCLUSIONS AND FUTURE REFERENCESCONSIDERATIONS [1] E. Koutroulis, K. Kalaitzakis, "Novel Battery

Charging Regulation System for PhotovoltaicThe observed life time of the proposed Apiain,lEPo.Eet.PwrAp.

BCR and battery system is 2.5 years while the Voions, IEE Prch2004.expected minimum life time is 5 years. This Vol. 151, No.2, March 2004.discrepancy is most probably due to overcharge [2] Technical Marketing Staff of Gates Energyof the Ni-Cd batteries because of one of the Products, Inc, "Rechargeable Batteriesfollowing reasons: ineffective EoC voltage Application Handbook", (Butterworth-sensing, charge unbalance of the cells in the Heinemann, 1998).string or wrong battery charging method. In [1]some battery charging methods are described [3] "Harding Battery Handbook-LIthium-Ionincluding constant voltage charging of batteries. Batteries", Harding Energy, Inc., [online]As stated in [2], constant voltage charging is not http://www.hardingenergy.com/pdfs/LiIon.pdf.suitable for Ni-Cd batteries as the overcharge [4] Zhenhua Jiang, Roger A. Dougal,starts immediately after the reduction of the "Multiobjective MPPT/Charging Controller forcharging current below %0O of the cell capacity Standalone PV Power Systems under Different(C). This might be the main reason of the Insolation and Load Conditions, 0-7803-8486-overcharge observed. As stated in [3] Li-ion 5/04/$20.00, ©) 2004 IEEE.batteries well suits to the constant voltage

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