Three-phase voltage imbalance method - Database & Sql Blog Articles

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First, the disconnection failure

If one phase is disconnected but not grounded, or the circuit breaker and isolating switch are not fully connected, it can lead to a blown voltage transformer fuse and unbalanced three-phase parameters. When the previous voltage level line is disconnected, the voltage of the next level shows that two phases remain close in value while one drops slightly. If the line at this stage is disconnected, the voltage of the disconnected phase becomes zero, while the other two phases maintain their normal phase voltage.

Second, ground fault

When a line is disconnected and single-phase grounded, the three-phase voltage becomes unbalanced, but the voltage values may remain unchanged after grounding. Single-phase grounding can be either metallic or non-metallic. In metallic grounding, the voltage of the faulty phase drops to zero or near-zero, while the other two phases increase by approximately 1.732 times and remain constant. In non-metallic grounding, the faulty phase voltage decreases to a certain level, and the other two phases rise, but less than 1.732 times.

Causes of Resonance

With the rapid development of industry, the use of nonlinear power loads has increased significantly. These loads not only generate harmonics but also cause voltage fluctuations and flicker, potentially leading to three-phase voltage imbalance. There are two main types of resonance-induced three-phase voltage imbalance:

One is fundamental frequency resonance, which resembles single-phase grounding. In this case, one phase voltage drops, while the other two increase. It can be challenging to locate the fault during troubleshooting. At this point, checking with special users might help identify if the issue is due to resonance rather than grounding.

The second type is sub-harmonic or high-frequency resonance, characterized by a simultaneous increase in all three-phase voltages. Additionally, when a busbar cut-off or single-phase ground fault disappears, but a grounding signal appears alongside abnormal voltage levels—such as one, two, or all three phases exceeding line voltage, or the voltmeter pointer hitting the limit and moving slowly—it is usually an indication of resonance.