Case Study 5 - Harmonic Distortion of Current
Office building with personal computers, terminals, copiers and other electric office
equipment supplied by three-phase wye service.
Facility engineers at this site experienced repeated problems with the failure of
electrical distribution equipment. A distribution transformer overheated and failed,
circuit breakers were tripping and electrical connectors were burning out. These problems
are all symptomatic of overload conditions.
However, initial measurements of phase currents using a true RMS ammeter showed current
readings of 257 to 298 amps. These values did not exceed equipment ratings.
The real problem started to become apparent when readings were taken of the current in
the common neutral conductor. The neutral was carrying 229 amps, nearly equal to the phase
currents, even though the phase loads were well balanced.
Further analysis was performed using a power monitor. Figure 1 shows the wave
form of Phase A current. The non-sinusoidal shape is due to harmonic currents typical of
switching mode power supplies which are used in the majority of modern office automation
equipment. These are nonlinear loads. The peak current shown here is 475 amps. If the wave
form was sinusoidal, its peak current would be only 363 amps. As shown in the plot of Figure
2 total harmonic distortion is about 32 percent, of which the third harmonic
contributes about 31 percent.
Figure 1. Non-sinusoidal Phase A current wave form caused by nonlinear loads.
Figure 2. Plot of Phase A current harmonics showing high third harmonic content.
When phase currents are distorted to this extent, the normal three-phase cancellation,
which results in near zero neutral current, does not take place. The odd harmonics
produced at 180 Hz, 300 Hz and higher frequencies in the phase conductors result in large
currents being carried by the neutral at predominantly 180 Hz. This is shown in Figure 3.
The net effect on facility wiring is that the common neutral conductor will frequently
be carrying current beyond its rated capacity. In severe cases this can well exceed phase
currents. These high frequency currents can be damaging to transformers and other devices
designed to operate at 60 Hz.
Figure 3. Common neutral current wave form showing predominance of 180 Hz,
modulated by 60 Hz fundamental.
In the short term these problems can be addressed by over-sizing neutral conductors and
de-rating transformers to a more conservative value of 60 percent.
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