I am out of my depth and am seeking some knowledgeable input please.
My grid tie setup takes the AC output of my single phase 230v 50 Hz ST alternator and full wave rectifies it in a bank of home made bridge rectifiers in parallel – all mounted on a massive heat sink. The resultant DC output is fed into the input stages of a 6 kW grid tie inverter, which houses something like 200,000 microfarad of electrolytic capacitors.
My query is about “power factor” and its correction. If I were to connect the alternator AC output directly to a capacitive load, I would expect to see the voltage and current waveform out of phase with each other. However, in my scenario the capacitors are effectively seeing only DC (albeit quite heavily rippled) so there is no phase to be out of alignment. However, current flow will be highest when capacitor voltage is lowest, and least when the capacitor voltage is highest – which is pretty close to what could be a DC equivalent of “power factor”.
So what is the generator actually seeing ? As I view it, the only thing the generator sees is the bank of diodes (a slight resistance) in the full wave rectifiers and what might be regarded as a variable load (dependent upon voltage).
So, is the generator seeing a capacitive load ? (I think so). If the generator IS seeing a capacitive load, is there any benefit in introducing inductors to make life a little easier for the generator and reduce losses ? If yes, where should the inductors go ? (On the AC side or on the rippled DC side)?
If the addition of inductors would be helpful, how would I go about calculating the inductance to add and what would the result be ? As I said, I am out of my depth, so if there is anyone out there who may be able to make any sense of these ramblings, and would feel able to point me in the right direction, then a bit more information will undoubtedly be necessary. I think I have covered the majority of the likely needed information below….
Alternator = ST 7.5 kVA wired as 230 volts 50 Hz at 1500 rpm. The windings have a static resistance of less than 1 ohm. The alternator is actually run at 1800 rpm with a measured AC rms figure of about 280 - 300 volts. The measured current is about 17 amps when the grid tie inverter is squirting about 4 kW into the grid continuously.
Rectification = 4 x 1000 volt 35 amp bridge rectifier diodes wired in parallel. In use, the heat sink gets only slightly warm. The unloaded voltage can go as high as about 430 volts DC. When under load, the measured DC voltage drops back to around 265 volts DC as measured by a digital multimeter; of course the voltage will be rippled but I do not have a scope to see what the ripple waveform looks like.
Grid tie inverter = 6 kW SMA Sunny Mini Central with about 20 x 10,000 microfard capacitors at the DC end. There appear to be some heavy current inductors on the motherboard input stages but I cannot tell what their inductance is; they consist of about 20 turns of very thick (4 -5 mm diameter) enamelled copper wire on an approx 50 mm diameter ferrite ring. Neither the inductors or the capacitors appear to get unduly warm in continuous use.
Any comments from anyone who understands what may be going on ?
Regards,
Ian
