Distinguishing, judging and solving methods of three-phase voltage imbalance

There are many reasons for three-phase voltage imbalance, such as single-phase grounding, broken line resonance, etc. Only by correctly distinguishing them can the operation management personnel quickly deal with them.

1、 Disconnection fault If one phase is disconnected but not grounded, or one phase of the circuit breaker and disconnector is not connected, the fuse of the Voltage transformer is blown, which will cause three-phase parameter asymmetry. When one phase of the line at the previous voltage level is disconnected, the voltage of the next voltage level shows that the voltage of three phases is reduced, one phase is low, and the other two phases are high, but their voltage values are close. When this level of line is disconnected, the phase voltage of the disconnected line is zero, and the phase voltage of the unbroken line is still the phase voltage.

2、 Ground fault When one phase of the line is disconnected and the single phase is grounded, although it causes three-phase voltage imbalance, the voltage value does not change after grounding. Single phase grounding is divided into two types: metallic grounding and non-metallic grounding. Metal grounding, with zero or near zero fault phase voltage, and a 1.732 fold increase in non fault phase voltage, which remains unchanged for a long time; Non metallic grounding, the voltage of the grounding phase is not zero but decreases to a certain value, and the other two phases increase by less than 1.732 times.

3、 Resonance causes With the rapid development of industry, nonlinear power loads have increased significantly. Some loads not only produce harmonics, but also cause supply voltage fluctuations and flickers, and even cause three-phase voltage imbalance.

There are two types of three-phase voltage imbalance caused by resonance

One type is fundamental frequency resonance, which is similar to single-phase grounding, where one phase voltage decreases and the other two phases voltage increases. It is difficult to find the fault point when searching for the cause of the fault. At this time, special users can be checked. If it is not due to grounding, it may be caused by resonance.

The other is frequency division resonance or high-frequency resonance, which is characterized by simultaneous rise of three-phase voltage.

In addition, it should be noted that when the air drop bus cuts off part of the line or the single-phase grounding fault disappears, if there is a grounding signal and the voltage of one phase, two phases or three phases exceeds the line voltage, the pointer of the Voltmeter will reach the end and move slowly at the same time, or the three-phase voltage will rise in turn to exceed the line voltage, which is generally caused by resonance.

The harm and impact of three-phase imbalance

1. Hazards to transformers. In production and daily electricity consumption, when the three-phase load is unbalanced, the transformer is in an asymmetric operating state. Causes an increase in transformer losses (including no-load losses and load losses). According to the transformer operation regulations, the neutral line current of the transformer during operation shall not exceed 25% of the rated current on the low-voltage side of the transformer. In addition, unbalanced operation of three-phase loads can cause excessive zero sequence current in transformers, increase the temperature of local metal parts, and even lead to transformer burnout.

2. Impact on electrical equipment. The occurrence of three-phase voltage imbalance will lead to several times of current imbalance. Induce an increase in reverse torque in the electric motor, resulting in an increase in temperature, a decrease in efficiency, an increase in energy consumption, vibration, and output losses. The imbalance between different phases can shorten the service life of electrical equipment, accelerate the frequency of component replacement, and increase the cost of equipment maintenance. The circuit breaker allows for a reduction in the residual current, which can easily cause overloading and short circuits when the load changes or alternates. Excessive unbalanced current flows into the neutral wire, causing it to become thicker.