From Boeing:

Rocketdyne is responsible for the end-to-end electrical power system for the International Space Station.

The system consists of power generation, energy storage and power management and distribution. The system provides 78 kw to the users with the power being generated by four photovoltaic (PV) Array Modules. Each module consists of two flexible deployable array wings of silicon solar cells supported by an extendible mast.

The station orbits the earth every 95 min. spending approximately 2/3 of the time in sunlight 1/3 in eclipse. Power is provided during sunlight by the PV Arrays. During the eclipse the 78 kw of station power is provided by Nickel-Hydrogen batteries which are charged during the sunlit part of the orbit.

The power management and distribution subsystem distributes the power at 160 volts DC around the station through a series of continuous duty, DC contactor switchgear. The switchgear has built-in microprocessors, controlled by Ada software and connected to a data bus running throughout the station. DC-DC converter units stepdown and condition the voltage from 160 to 120 volts DC to form a secondary power system to service the loads through solid-state switchgear. The converters also provide isolation of the secondary from the primary system and maintain uniform power quality throughout the station.

Wow, that's quite a continuous load to supply from PV panels, although they have the advanatge of not having to worry about cloudy days!

It would be interesting to know the reasons behind the decision to use 160V as the level for primary distribution.

The 120VDC choice is interesting.

48VDC {24-cell battery set} is common in a lot of communications, with some 48- and a lot of 125VDC {60-cell battery string} use in electrical transmission and distribution for breaker tripping and other critical loads where there is a chance of AC-power nonavailability. That way everything can crash and burn outside [or otherwise not be 110% reliable] the switchyard breakers can still be operated. Arcing burndowns are bad enough on 480Y systems—it can be phenomenally worse at higher voltages.

Minor aside — On spacecraft voltage changes, such an oversight is what led to the Apollo 13 disaster. There were heaters for cryogenic oxygen tanks, with a simple thermostat control cycling the heaters to boil off gaseous oxygen to keep the crew breathing.

Over various stages of spacecraft development, the heater voltage {and battery set} went from 28 to 65VDC. [This might be roughly comparable to using a 250V fusible switch called on to interrupt a 600V motor circuit.] Unfortunately, the thermostat contact spacing remained at the 28V spec, passing all design testing but, while sensing contents temperature on the tank wall 250,000 miles away it failed catastrophically, rupturing the tank.


[This message has been edited by Bjarney (edited 09-19-2003).]

So what voltage do I have to set my radio cassette player before plugging it in?

Or do I have to dig out one of my DC radios?

They would supply the necessary switching DC-DC convertor, or a supply of rechargeable packs, charged by a DC charger. The laptop computers, since they probably have universal switcher supplies, witll most likely operate fine off of the DC power system. The lights also would have SS ballasts.

C-H,
Thanks for that!,
I've always wondered what sort of Electrical system that they used up there.
We get the odd Comms coming through on our 2-metre Ham band over here, it's good to hear them, but it's all in Russian!
That would look real good on the CV, "performed Electrical wiring out in Space!"

A service call might be a bit tricky, though.

And at the cost per mile travel they'd have to charge, the words you don't want to hear are "We don't carry them in the spaceship. We'll have to pop back to Earth to get one!"