1. Disconnection fault
The system fails to break the line but is not grounded, or a phase of the circuit breaker and isolation switch is not connected, and the voltage transformer fuse is blown, which will cause the three-phase parameter imbalance, the fault phase voltage is zero, and the non-fault phase voltage is normal. At this time, the three-phase electrical equipment is in the state of phase deficiency operation.
2. Ground fault
When the neutral point is not directly grounded and the system is broken and grounded, the three-phase voltage imbalance is caused, but the voltage value of the grounded line does not change.
The single-phase grounding is divided into two types: metallic grounding and non-metallic grounding. When metallic ground occurs, the fault phase voltage is zero or close to zero, and the non-fault phase voltage increases the line voltage, and it is constant. When non-metallic grounding occurs, there is an arc between the fault phase and the earth, the grounding phase voltage is not zero but decreases to a certain value, and the other two phase voltage increases less than √3 times, because of the existence of arc pulling phenomenon, there will be arc pulling overvoltage.
3. System resonance
With the rapid development of industry, nonlinear power load increases greatly, some loads not only produce harmonics, but also cause power supply voltage fluctuation and flicker, and even cause three-phase voltage unbalance.
4. Unreasonable distribution of three-phase load
Most of the power systems in our country are power systems and lighting mixed system shared transformers, the early design did not take into account the average distribution of load or in the equipment is not opened according to the design requirements of the three-phase average use, to a large extent caused by the three-phase load imbalance.
5, three-phase equipment without phase operation
The three-phase load stops working or works abnormally when the three-phase current is unbalanced and the noise of the device is abnormal.
6. Weakening of monitoring of load distribution
In the management of distribution network, the management problem of three-phase load distribution is often ignored. In the detection of distribution network, the three-phase load of distribution transformer has not been tested and adjusted regularly. In addition, there are many factors that cause the phenomenon of three-phase imbalance, such as the influence of the line and the difference of the three-phase load moment and so on.