Case Study 5 - Harmonic Distortion of Current

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Environment

Office building with personal computers, terminals, copiers and other electric office equipment supplied by three-phase wye service.

Problem

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.

Measurements

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.

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.

Solution

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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