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Content of the Open DC Grid standard (work in progress)
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48V max normal voltage #2

Open jlgula opened 4 years ago

jlgula commented 4 years ago

We need to make a decision on the the maximum voltage that will be consider a normal operating voltage. ISO 21780 considers that to be 52V. For 2030.10 we raised that to 58V to accommodate the charging voltage of lead-acid type batteries. I'm not sure that's the right decision for ODG.

In a little more detail, 21780 permits devices to go into Functional State 2 at any voltage above 52 and below 58 where FS2 defined as: "The function shall meet a specified performance with a specified variation below or above the specified performance of FS1. Derating is allowed, switch-off is not allowed." The derating amount is not defined and is up to the manufacturer. I think we would not lose too much compatibility to require devices to operate a full performance at 58V. That was the basis of the P2030.10 decision.

There is another test in which a device is supplying power at its maximum load and 52V and is then detached from the load. The test requires the overshoot to not exceed 58V and then drop to below 54V in 300ms. What sort of overshoot are we seeing on the current DC-DC converters? If we allow the bus to operate at 58V, won't we violate this test? I think this is the more problematic concern.

Going forward, I think the need for direct connected lead acid batteries is likely to dwindle. There are a variety of combinations serial connected lithium batteries that would keep us under 52V but if we use, for example 14 lifepo4, that gives us a charging voltage of 51.1. I'm not sure what the maximum permitted voltage on these cells is but one data sheet I saw had a test requirement of no fire at 4.2V which would be 58.8 @ 14S.

martinjaeger commented 4 years ago

For efficiency reasons I would try to get as close to 60V as possible, so I agree that we should not start derating at 52V. And I'm not sure what derating at high voltage in ISO 21780 means. I assume that it's because the power of regenerative braking needs to be reduced at high state of charge. But that's not relevant for us.

The load dump test could indeed be a critical test for the grid aswell. The overshoot depends on several factors, especially the capacitance at the grid side and the control algorithm of the converter. I don't have an estimation yet, which maximum overshoot is achievable under typical boundary conditions.

LiFePO4 batteries are typically charged to something around 3.6V. See also here for an overview of different battery voltage ranges.