This
is provided by four batteries storing 40 ampère-hours each, each weighing almost
45 kg. SPOT 4's average power consumption is 1 kW. The batteries are
manufactured by SAFT and each one consists of twenty-four Nickel-Cadmium accumulators.Generation of electrical power:
the solar array
Power storage: batteries
Control
and regulation of electrical power: the RSJ (shunt junction regulator)
Distribution of electrical power:
the BD (power distribution unit)
ancillary functions
The power supply system produces, stores, controls, regulates and distributes electrical power onboard the satellite.
As it would be too dangerous and restrictive to carry either a nuclear generator or a fuel cell which would require a quantity of reagent incompatible with the nominal satellite lifetime of 5 years, a natural choice was to use a renewable, available and ecological source, the sun.
Solar array design takes into account variations in solar flux, electron and proton fluxes, meteor showers and ultraviolet radiation which degrade performance by about 2% per year.
The SPOT 4 solar array consists of 5 panels covered with solar cells..
The panels have a rigid, honeycomb structure with a carbon fibre skin; the boom between the panels and the satellite body is made of carbon composite tubes developed by the Aerospatiale company. Once the panels have been deployed, the total area is 25 m2. 8640 silicon cells each 24 cm2 in size, manufactured by ASE in Germany, are bonded on one side and provide more than 2200 W of power.
A device known as SADM (solar array drive mechanism), developed by SEP, enables the
array to track the sun by rotating the boom
throughout the orbit.
The electrical power from the solar array is transported to the satellite via ten cable
pairs (corresponding to ten sections), routed through the boom. The electrical link
between the panels and the satellite uses slip-rings on collector brushes.
Due to the satellite's orbit, the solar array is in the Earth's shadow for an eclipse
period of about 35 min. and in sunlight for 65 min during each orbit. Current is
only generated by the cells during periods of sunshine. This means that power must be
stored during the day and restored during the eclipse so that the satellite is
continuously powered.
Also, the satellite needs power the during launch phase, before the solar array has been
deployed.
This
is provided by four batteries storing 40 ampère-hours each, each weighing almost
45 kg. SPOT 4's average power consumption is 1 kW. The batteries are
manufactured by SAFT and each one consists of twenty-four Nickel-Cadmium accumulators.
These are charged on the ground before launch and can provide power for about three
orbits should the satellite not be able to generate its own power with the solar array
before.
The batteries undergo a complete discharge/recharge cycle during each orbit. In order to
last for the nominal five years of life, which means more than 25 000 cycles, the
batteries have to be used very carefully; the following control laws were defined after
lengthy, joint experimentation by SAFT, CNES and the European Space Agency.
Finally, the battery temperatures must be controlled. These are nominally maintained between - 5°C and 0°C.
How can we carry out the battery management functions while limiting the surplus power from the solar array relative to that actually used by the satellite?
A large electronics package weighing almost 30 kg, called the RSJ (shunt junction
regulator), as its name indicates, acts as a regulating junction between the solar array,
the batteries and the satellite power bus. It is made by ETCA in Belgium. The four
batteries are connected directly, in parallel, to the power bus.
The advantage of this design is that it guarantees a bus impedance equivalent to that of
the batteries, approximately 25 mohms DC.
The power regulation function works by shunting sections of the solar array when necessary. When it becomes necessary to decrease power, the RSJ "shunts " a section of the solar array, in other words by short-circuiting it. On the other hand, to increase power, it connects the array directly to the power bus. This type of regulation, known as decadic since it is done in steps equal to one tenth of the total power of the solar array (10 sections), is fairly crude and is only done for 7 sections. The three remaining sections are controlled linearly and very precisely by regulating the power; this is done by the same kind of shunt regulator but with a switching frequency of approximately 40 kHz. So, every time a decadic section is switched, the linear sections are automatically adjusted so that the regulator's overall transfer function is linear.
The satellite's nominal operating mode uses the following procedure :
Nominally, the power supply voltage varies as a function of the charging/discharging cycles, between 27 Volts and 37 Volts.
In addition the RSJ supplies electrical power for heating the batteries. Driven by the onboard computer, it controls the temperature by switching between the heaters' power lines.
In the safe-hold mode, when the onboard computer's resources are no longer available, the RSJ manages the batteries and controls the power from the solar array, autonomously. The operating principles are identical to the nominal mode, but the functions usually handled by the computer are now done by dedicated electronic circuits. Some values of the management parameters are different. For instance, the maximum voltage of the batteries is reduced by 1 Volt and the maximum charging current is 36 Ampères.
Power from the RSJ now has to be distributed to the satellite's subassemblies. This is done by the power distribution unit (BD), also made by ETCA. Via the main power bus, it supplies the dedicated buses for the platform, payload, passengers and platform and payload heating systems. It also generates regulated 50 Volt bus power to supply the onboard computer and priority items of platform equipment. It further controls a "switch-off line" which is used to put the satellite into safe-hold mode.
The satellite power system described, includes several ancillary functions.
Protective and monitoring devices have been included, either directly in the RSJ and BD
through electronic functions, or controlled by the onboard computer. Monitoring which does
not require immediate action is done by the ground segment. This involves routine analyses
and is capable of detecting degraded performance due to ageing (particularly for the
batteries and the solar array).
Voltages, currents, temperatures, relay states, etc. are acquired and sent to the onboard computer. The computer sends commands to the RSJ and the BD. This input data and the commands pass through an interface in the RSJ called the bus coupler, developed by Sextant for dialogue with the computer via the communication bus.
page updated on the 00-06-06