Thursday, September 12, 2013

Looking at Load Dump

Load Dump is a condition when a large DC load is suddenly removed.  A common example aboard Viking Star is when the Dryer is running, it consumes 1200watts to power the heating element.  This is supplied via the inverter, and it can consume upwards of 90-100Amps to meet the need.  But when all of a sudden the Dryer decides it is at temperature, it will turn off the heating element - with a very very sudden drop in a 90A+ load:   a Load Dump.

While the heating element is operating, the Alternator will work it hardest to meet the needs of the
inverter with the remaining energy being sent to the battery for its recharging needs.  However, once that Inverter load is suddenly removed, the Alternator will take some time to spool down - and there is a risk of overvolting the battery if not done quick enough.   BTW:  In the Automobile world, there are many recognized transient situations.  Most of which are described in the graph to the right.  And yes, I am using Load Dump a little different - removal of  large load, as opposed to disconnecting the battery . . . .

The Source code is configured to update the Alternator field drive every 100mS, or 10 times a second.   And any overvoltage situation takes absolute precedence in reducing the PWM drive.  With the DC generator, and its smallish 135A alternator, the normal PID logic was able to deal with any load-dump situations.  However, with using our Mains alternator (a 270A Leece Neville monster)  I found the PID was not able to respond quickly enough.  Example, on one run I noted a max Overvoltage of 450mV, almost half a volt!

So I added in some Load Dump code:  upon detecting as little as 40mV overvoltage, we now cut down the PWM by 50%. 80mV causes another 50% pull back, and 120mV over target just turns off the Alternator.  a Primary goal is to protect the Battery.  Using this approach I was able to reduce the maximum overvoltage to 80mV.  (As a side:  the regulator normally regulates within a 25mV range around the charge profile target).

After looking at the data dumps from todays trial run, I think I can do better - by looking not only at the absolute voltage to detect a Load Dump, but by also looking at the Rate of Change of voltage.  If I see a massive Rate of Change, and we are close to the target Charge Profile voltage, then we also trigger a Load Dump pull back of 50%.

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