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 if the circuit breaker and isolating switch are not properly connected, it may result in a blown voltage transformer fuse and asymmetrical three-phase parameters. When the previous voltage level line is disconnected, the voltage of the next level shows that two phases are higher while one is lower, with the values of the two higher phases being close. If the line at this stage is disconnected, the voltage of the disconnected phase drops to zero, while the unbroken phase continues to show the normal phase voltage.

Second, the ground fault

When a line is disconnected and single-phase grounded, the three-phase voltage becomes unbalanced, but the voltage levels 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 1.732 times and remain constant. In non-metallic grounding, the faulty phase voltage decreases to a certain level, while 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 significantly increased. These loads not only generate harmonics but also cause fluctuations and flicker in the supply voltage, potentially leading to three-phase voltage imbalance. Resonance can cause two types of three-phase voltage imbalance:

One is fundamental frequency resonance, which resembles single-phase grounding, where one phase’s voltage drops and the other two rise. This makes it difficult to locate the fault. At this point, checking for special users or conditions can help determine if resonance is the cause.

The other type is subharmonic or high-frequency resonance, characterized by all three phases experiencing a simultaneous voltage increase.

Additionally, when a line is disconnected and a single-phase ground fault disappears, if a grounding signal appears and one, two, or all three phases exceed the line voltage, with the voltmeter pointer hitting the limit and moving slowly, or if the three-phase voltage alternately rises above the line voltage, it is usually caused by resonance.