Difference Between Grid Connected and Standalone Wind Energy Conversion Systems

There are a few key differences between Grid-connected (paralleled) and Stand-alone (isolated) operation of any alternating current generation. The two primary variables that you can control from any unloaded ac source are its frequency and output voltage. When you are grid-connected the power system that you are connected to is many times larger (known as an "infinite bus" in utility talk) than your small system meaning that the frequency and the voltage at your connection point are at the mercy of the utility. Just prior to synchronization with the utility you must have your frequency and voltage identical to that of the utility, and the voltage phasors exactly in sync; i.e., the peak or zero crossing of your voltage sinewave has to occur at exactly the same time as the utility's peak or zero crossing. Your generator now pushes power out to the grid because your generator is running slightly faster than the utility (in the case of an induction generator), the voltage phasor is leading the system voltage phasor, known as the power angle (in the case of a synchronous generator), or your inverter control system voltage firing angle is leading the utility voltage phasor (just like a synchronous generator). Please note that the power angle is not to be confused with the power factor, more on this next. The other variable is the terminal voltage, which does affect the power factor; simply put as your control system attempts to raise the voltage above the utility voltage you start supplying VARs (Volt-Amp-Reactive) to the system. Conversely if it allows the voltage to fall below the utility then you are absorbing VARs from the grid.

All this discussion about generator voltage being "above" or "below" the utility voltage refers to the excitation level of the generator; another way to think about it is to imagine opening the connection between the generator and grid and comparing the the terminal voltage to the utility voltage- generator voltage higher = VARs out, lower generator voltage = VARs in, matched voltages = no VARs (unity power factor). By now you should be saying "...but wait, there is no excitation system inside my inverter, since there's no field, rotating mass, or iron to energize, how do I even measure or control the power angle and/or the power factor?" That's where your d-q control comes in, d stands for direct (real) power, q for quadrature (imaginary) power; d-q control affects the magnitude/timing of the firing pulses to the converter circuitry. Magnitude affects the height/width of the pulses and hence the output voltage, and timing affects where the peak/zero crossing occurs and therefore the power angle.

For Stand-alone systems there are generally two very similar modes of operation, isolated or isochronous. They both refer to the type of frequency control exhibited by the generator, the main difference being that an isolated generator tends to have poorer frequency control so that its frequency falls and rises as the load rises and falls whereas an isochronous generator has a tighter frequency control since it has a timebase controller that forces (up to its rated power) a relatively constant frequency.

The last piece of the puzzle is the nature of the loads. When grid-connected, the grid sets the frequency and voltage that your system feeds, and the constant minute variations in load, frequency and voltaqe are shared proportionately by the size of the generation. In stand-alone mode the generator/inverter must respond to every change in the nature of the load; i.e., if additional load comes on then your control system has to increase the real and/or reactive power while attempting to hold the frequency relatively constant. It's a real interesting balancing act, one that utilities are quite adept at, that when coupled with the widely varying input of the wind, results in a control strategy that requires quite a bit of sophistication.

So to answer your question, unless your loads are constant, real power, and frequency/voltage insensitive; i.e., resistive only, you're going to need some form of variable d-q control. There's plenty of stuff in Google about how to connect the various types of WECS.