Power Factor and Grid-Tie Inverters

One reason engineers specify active rectifiers (a.k.a. active front-ends) for their systems is that they can operate with near unity power factor.  Being nearly the same system (see my recent post “Active Front-end or Grid Tie Inverter?“), grid tie inverters share this same beneficial characteristic.  However, this does not mean that an active rectifier or grid tie inverter must operate with near unity power factor, and in fact, we can use this to our advantage in certain applications.  This also implies that we may not want to specify power factor as a means of quantifying how well the system minimizes AC line current harmonics.

Let’s back up for a moment and first consider what we mean by Power Factor.  The purest definition is that power factor is the ratio of measured power to apparent power; that is PF = W/VA.  However, you may be more familiar with a couple of other equations that address two very different classes of applications.  PF = VA*COS(phi), where phi is the phase angle between voltage and current, is valid for linear loads that draw sinusoidal current and have negligible harmonic distortion.  This equation is well know by those working with AC induction motors, and until the last 20 or so years, is the one most used by electrical engineers.

Typical inductive (AC motor) current and voltage

More recently, PF = 1/SQRT(1+THD2), has come into common use, as it addresses nonlinear rectifier loads used in a wide range of electronic equipment.  This equation ignores phase shift, however, the error this introduces is small in typical cases where THD dominates.

Typical 3-phase current and voltage with bridge rectifier and LC filter

So how does this relate to power factor and grid tie inverters?  Consider first how the grid tie inverter’s power stage and modulator are configured.  Most circuits (OZGTI3000 and others used by Oztek) are designed to inherently produce sinusoidal voltages.  This means that even without control loop intervention, to a first order, the inverter emulates a linear system.  We can then vary the AC voltage amplitude and phase angle in relation to the line voltage to produce currents with any phase relationship to the AC line that we want.  In fact, it may be obvious at this point that the resulting currents are simply a result of the differential voltage impressed across the grid interface filter impedance.

With a properly designed filter, the differential voltage is relatively small in comparison the AC line voltage, making the precision afforded by closed loop control necessary for a practical implementation.  Closed loop control also helps compensate for non-ideal power stage behavior, and we determine how well this is all working by measuring THD.  Phase angle can actually be set to any practical angle.  If you want to move real power, it has to be in the vicinity of zero degrees, but it can be leading or lagging.  This can be used to advantage in larger installations to offset the lagging power factor produced by induction motor loads, or to compensate for transformer leakage and distribution inductances.

Typical end-systems usually require compliance with more complex distortion specifications like IEEE 519, which specify current harmonic limits in absolute terms, place limits and both individual harmonics and the total, and limit even harmonics to much lower levels than odd harmonics.  In specifying grid tie inverters and active front-ends, you should therefore be primarily concerned with how well current harmonic distortion is controlled and whether current phase angle is readily adjustable, not power factor.