6.7.1 Evaluation of neutral loading and transformer derating

Loads in a data center facility are dominated by hundreds of single-phase computer servers and networking equipment. The phase currents in the low-voltage circuits have the harmonic characteristics shown in Fig. 6.27. Since these loads are rich in the third harmonic, there is a good likelihood the neutral conductor may be overloaded. The problem is to estimate the neutral conductor loading in amperes and in percent of the rms phase current. In addition, the amount that the transformer supplying this load must be derated is to be determined assuming the eddy-current loss factor under rated load P _EC-R is 8 percent.

Figure 6.27: Phase current and its harmonic characteristics. Fundamental amps: 285.5 A. Phase angles are in degrees.

The system is assumed to be balanced. Therefore, the sum of all phase currents results in mostly third-harmonic current in the neutral conductor. The rms phase current is

The third-harmonic current is 65.7 percent, giving a neutral ccurrent of

Based on this estimate, the neutral conductor will be loaded to approximately 156 percent of the phase conductor. This phenomenon has been responsible for neutral overloading. Common solutions are to use a

• Separate neutral conductor for each phase

• Double neutral conductor size

• Zigzag transformer close to the loads to shorten the return path for the third-harmonic...