Page 120 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
118 Chapter Four

Harmonic number 1 2 3 4 5 6 7 8 9 10 11 12
Phase sequence 0 0 0 0

The concept of harmonic phase sequence may not be familiar to read-
ers and Table 4.1 may offer an explanation.
It will be observed that the harmonic orders 3, 6, 9, 12, etc. have zero
phase sequence, they are divisible by three and are known as triplen har-
monics or triplens. They are of particular importance because in a three-
phase system the phase currents become additive in the neutral. In a
balanced system the triplen current flow in the neutral will be three times
what is in each phase conductor. If there is no neutral conductor (a three-
wire system) three-phase triplen harmonic currents cannot flow.
The rectifiers normally included in UPS equipment are of the three-
phase bridge type; as there is no neutral connection there will be no sig-
nificant triplen harmonic currents and as positive and negative half
cycles should be balanced there will be no significant even harmonic
currents. Any that are present will be due to a lack of symmetry
between the firing of pairs of thyristors. The half-controlled bridge gen-
erates even harmonics but is not usually found in UPS equipment and
will not be discussed further.
The “power factor” of a nonlinear load can have several meanings;
whenever the term is used it should be defined. Assuming a pure sine
wave of voltage, only the fundamental current component can supply
power, any harmonic components of current are parasitic. Industrial
ammeters may indicate the rms or the rectified average value of either
the fundamental or of the total current, and it is not always clear what is
being indicated. To calculate power from current and voltage readings it
is necessary to include a distortion factor in addition to the conventional
power factor, cos of the fundamental current and voltage components.

TABLE 4.1 Explanation of Harmonic Phase Sequence

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