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re: breeze in the trees
27 dec 2001
pioneer   wrote:

>whats the best way to achieve redundancy for inverters?

well, it wants to be simple. adding more parts to a non-redundant
system with equal capacity can raise the cost and failure rate...

and we want an alarm, so we know if a redundant unit fails, and
we can switch to another and send the failed one off for repair,
before anything else fails, ideally.

we could use 2 inverters with some additional alarm and switching
hardware, but that's expensive, since we are basically providing twice
the needed capacity, and if they are not synchronized, we might have to
interrupt load power while switching from one to another. it would be
nice to avoid that. 

using something like 5 500w inverters to supply 2,000 watts makes
sense to me, since a lot of the cost of an inverter is proportional
to its power output, eg transformers and switching transistors and
heat sinks. we might think of this as increasing the basic cost of a
system by 5/4, vs a factor of 2 for a dual-redundant system.

numbers can help with this kind of design, eg the costs and failure
rates of components and subsystems. one way is to begin with a non-
redundant system, and ask "if we only have one dollar to spend on
increasing reliability, what should we do next?" make a change, and
calculate the new cost and availability. ask that question over and
over, making a finer-grained system. if a certain diode fails once
every 10k hours vs once per million hours, that may bias the design. 

we might fuse the inputs of 5 parallel inverters, so any inverter
input can short-circuit without shorting the whole battery input bus.
we might synchronize them by using the sync signal from one to drive
the other four. we might have some way to switch to another sync source
if the first one goes bad. as an alternative, we might use some sort o
phase-locked loop, which might use analog majority voting, and flag
any inverter that refuses to cooperate.

we might fuse each output, then tie them together, so that any inverter
output that refuses to cooperate blows its output fuse and allows the
rest to continue their normal operation. if the inverters have a dc-dc
boost circuit that steps the battery voltage up to 300 or 400 vdc, we
might wire-or those high-voltage bus points together with series fuses
and diodes... 

nick




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