Jim Sanford wrote:
The situation you describe regarding load sharing between multiple�
parallel sources, without oscillations and circulating currents, is no�
different than the situation where there are multiple AC generators�
operating in parallel.� In that case, the speed governor (which sets�
frequency) has a frequency vs load characteristic that is not flat. �
This forces stable real load sharing, even when generators of different�
capacity are paralleled.� Similarly, the voltage regulator has a�
non-flat voltage vs. reactive load characteristic.� This forces stable�
sharing of reactive load.
I'm not sure this is the exact same situation. A generator is not a�
stored energy device (ignoring flywheel effects). It can't produce more�
electrical power out than the mechanical power going in. A battery or�
cap, on the other hand, can produce almost arbitrarily large powers�
until it is depleted.
I agree, this is all manageable.
My thoughts on this comes from experience with related problems in home�
photovoltaic power systems.
In the late 1990s, the Trace SW4048/5548 inverter models were very�
popular. These 4/5.5 kVA inverters are bidirectional; power can flow�
from the AC side to the 48V DC side as well as from the DC side to the�
AC side. This is useful in maintaining a battery bank at a constant�
voltage despite varying PV production and DC load, but it also led to�
problems in systems with more than one unit.
Two inverters are commonly used in larger (e.g., 5kW) grid-tied systems.�
They were typically wired in parallel on the DC side and connected to�
opposite 120V phases on the AC side.
Even if you set both inverters to the same "sell" voltage on their DC�
side, small differences in their internal voltage references often�
result in very different inverter power levels. You could even see one�
inverter "buying" (converting AC to DC) while the other "sold"�
(converting DC to AC), obviously a wasteful situation. This problem�
could have been easily avoided with a small dead band around the DC�
operating setpoint. This could also shut down the inverter at night,�
avoiding its idle power consumption.
Another problem comes from the use of a PV charge controller with this�
inverter. In a grid tied PV system, the charge controller is basically a�
safety device that protects the battery against an inverter or grid�
failure. Normally you always want maximum power from the PV array�
because the grid is an infinite energy sink and we can sell whatever we�
produce. During an outage or failure, though, we have to back off PV�
production to match local consumption. It does this whenever the battery�
bus voltage rises above a set point.
But the inverter is also trying to control the DC bus voltage. If you�
don't set them both up carefully, the charge controller could shut down�
the array unnecessarily. Again, an inverter dead band would have been�
very useful.
The bottom line is that all the devices on the DC bus must somehow�
coordinate their operation. If they communicate implicitly through DC�
bus voltage, then they need to agree on just what is meant by every�
possible value of bus voltage.
--Phil
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