PROBING MYSTERIES OF THE INVERTER: PART I

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As I'm sure you're aware, most alternative energy sources, particularly the wind and the sun, are by nature intermittent. To get through the calm or dark times, therefore, people often use storage batteries. Unfortunately, batteries are expensive and messy, and they require maintenance.

The synchronous inverter eliminates the problems of chemical storage by allowing owners of alternative energy systems to use utilities for backup power. Though different utilities have different ways of making the necessary connection, the essential elements of the arrangement are that you can buy power from the grid when you run short, and sell it to the utility when you've got an excess. An interesting note is that the utility is required-by law-both to make this connection and to pay you a fair (but, of course, wholesale) price for the power you deliver to it.

SELF-COMMUTATION: This sort of synchronous inverter is quite similar to the static inverter we talked about earlier ... with one important exception: Utility power controls the unit's output waveform and frequency.

Therefore, since the utility's waveform is purely sinusoidal, neither square- nor modifiedwaveform inverters can be used for interfacing. They can be adapted, however, with the proper filtering to remove unwanted harmonic distortion. The basis for the synchronous inverter is usually a stairstep waveform unit, though, since output from such a device requires far less filtering and is thus more efficient. The frequency of a self-commutated inverter is locked onto the line frequency by phasing the operation of the electronic switches with the AC input.

LINE COMMUTATION: Another way to handle current switching in a synchro nous inverter is to rely entirely on the sine wave from the utility for commutation. With this approach, the inverter won't function if the power lines go dead. Consequently, an independent power system connected to the utility by a linecommutated inverter won't be of use in the event of a public power failure.

POWER FACTOR: One major shortcoming of many synchronous inverters is their low power factors. Because of present design practices and intrinsic characteristics, it's not uncommon to find units with ratings of 0. 5 or less!

Now power factor correction for synchronous inverters can be accomplished with capacitors, as it is for the static inverters we discussed earlier. However, an inverter's power factor-and specific demand for capacitance—is influenced by changing loads, grid inductance, phase angle, etc., and as a result, it changes constantly. To date, inverter manufacturers have relied on the sheer size of the utility network to absorb these defects. But as more and more independent producers are placed on line, additional efforts will likely be necessary.

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