Why Does Excessive Neutral Line Current Occur in Balanced Three-Phase Four-Wire Systems？

In a three-phase four-wire power distribution system, it is a consensus among industry insiders that the neutral line current should be very small when the three-phase loads are balanced. However, more and more phenomena are subverting this concept.

For example, the advertising light boxes around a building use fluorescent lighting with electronic ballasts. The loads on the three-phase lines are balanced, with each phase current being approximately 90A, but the neutral line current reaches 160A.

In fact, the phenomenon of excessive neutral line current is becoming increasingly common nowadays. Why does current still appear on the neutral line when the three-phase loads are balanced, and even reach more than 150% of the phase line current? This is caused by the rectifier circuit.

When the current waveform of the phase lines is a sine wave, if they are 120° out of phase and have the same amplitude, the result of their vector superposition on the neutral line is zero. This is what everyone is familiar with.

But if the currents on the phase lines are pulsed and 120° out of phase, their superposition result on the neutral line is as shown in Figure 2. It can be seen from Figure 3 that the pulsed currents on the neutral line are staggered and cannot cancel each other out. Counting the number of pulsed currents on the neutral line, there are three in one cycle, so the current on the neutral line is the sum of the currents of each phase line. According to the calculation method of effective current value, the current on the neutral line is 1.7 times that of the phase line current.

Since most modern electrical loads are rectifier circuit loads, even with balanced three-phase loads, a large neutral current may occur. Excessive neutral current is highly hazardous, mainly for two reasons: first, the neutral’s cross-sectional area is usually no larger than the phase line’s, so an overcurrent causes overheating; second, no protective devices are on the neutral, so it can’t disconnect like phase lines, creating a huge fire risk.

• For three-phase sinusoidal symmetric AC, with balanced loads, phase current vectors (equal magnitude, 120° phase difference) sum to zero, so zero-sequence current is zero.

• With unbalanced loads, unequal current vectors (phase differences not all 120°) give a non-zero sum; the zero-sequence current (unbalanced current) is smaller than any phase current.

• If three-phase loads have non-linear components (e.g., diodes), causing DC and 3rd/6th - order harmonics, zero-sequence current (arithmetic sum of these) may exceed phase current. For example, in a three-phase half-wave rectifier, any phase current is 1/3 of the load current (the zero-sequence current).

• In a three-phase bridge rectifier, current flows in both AC half-cycles (symmetric, balanced across phases), so no DC or 3rd - order harmonics; the three-phase current sum is zero (zero-sequence current = 0).

• In a single-phase bridge rectifier, current flows in both AC half-cycles (symmetric), so no DC or 3rd - order harmonics in the single-phase current.

• If all three-phase loads are single-phase bridge rectifiers, even with imbalance, the three-phase current sum is non-zero (zero-sequence current exists), but the neutral current won’t exceed the phase current.