In the previous applets the impact that the bridge rectifier has on the current drawn from the mains and the amount of distortion it causes on the mains voltage has been considered. These bridge rectifiers all operate off a single phase voltage supply, which is common in most households. However, in office buildings and at industrial sites the power is distributed with three voltage phases. It is taught in the fundamental electro-technology courses that if a balanced three-phase load is connected to the three-phase mains then there is no neutral current flowing, since the currents in the three phases sums to zero. However in the case of unbalanced phase loads then there is a resulting neutral current. 

What about the case of three balanced rectifier loads, with each one connected to a different phase. Will there be zero neutral current? This is an important question since most computer power supplies use a bridge rectifier and many computers are used out in industry and the universities. It is also an important question since the neutral conductors are generally sized for current levels less than the expected phase currents (since traditionally we expect no neutral current to be flowing in the conductor for balanced loads. So why put in more copper wire than we need). 

In this applet the load currents for each rectifier are set to be equal. Note how each rectifier draws current at different times and that the return path for the current is via the neutral conductor. There is no summing of the currents to zero happening with these rectifier loads. From the harmonic spectrum it can be see that the neutral current is made of the triplen harmonic components ie 3rd, 9th, 15th, .. etc. This has a significant impact on the sizing of the neutral conductors as there can be higher rms levels of neutral current than there is phase current. This has caused significant overheating problems of the neutral conductor in high-rise buildings that have a large number of computer loads. Overheating can lead to serious damage such as fires. 

When the individual rectifier loads become unbalanced there is now a change in the distribution of the harmonics and fundamental component. The imbalance now causes a fundamental current to flow just as in the traditional case of unbalanced linear loads. The harmonic current components contain all the odd frequencies, not just the triplen components. Depending on the level of unbalance, the level of triplen components can actually decrease.

• Initially view the current waveforms and neutral current harmonic graph with out altering the load resistor levels. This is the case of balanced rectifier loads on all three-phases.
• Now start to adjust the individual load resistor values and see the effect this has on the neutral current waveform shapes and the harmonic spectrum.