Ankerui Huanan
In daily work, we often encounter three-phase unbalanced phenomena, how can we analyze and solve them? Let's recharge and learn with the editor~
There are many reasons for three-phase voltage imbalance, such as: single-phase grounding, disconnected resonance, etc. Operation and management personnel can only quickly deal with it if they are correctly distinguished.
1. Disconnection fault
If one phase is disconnected but not grounded, or one phase of the circuit breaker and isolating switch is not connected, the fuse of the voltage transformer is blown, which will cause the asymmetry of the three-phase parameters. When one phase of the line at the previous voltage level is disconnected, the voltage of the next voltage level shows that the voltages of the three phases are all reduced, one of which is lower, and the other two are higher but the voltage values of the two are close. When the line of this level is broken, the phase voltage of the broken 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 broken and the single phase is grounded, although the three-phase voltage is unbalanced, the voltage value does not change after the grounding. Single-phase grounding is divided into two types: metallic grounding and non-metallic grounding. Metallic grounding, the faulty phase voltage is zero or close to zero, the non-faulty phase voltage increases by 1.732 times, and remains unchanged; non-metallic grounding, the grounding phase voltage is not zero but decreases to a certain value, and the other two phases rise Less than 1.732 times higher.
3. Causes of resonance
With the rapid development of industry, non-linear power loads have increased a lot, and some loads not only generate harmonics, but also cause fluctuations and flickering of supply voltage, and even cause unbalanced three-phase voltage.
1 There are two types of three-phase voltage unbalance caused by resonance:
One is fundamental frequency resonance, which is similar to single-phase grounding, that is, the voltage of one phase decreases, and the voltage of the other two phases increases. It is difficult to find the fault point when looking for the cause of the fault. At this time, special users can be checked. If it is not the reason for grounding, it may be caused by resonance.
The other is frequency division resonance or high frequency resonance, which is characterized by the simultaneous rise of the three-phase voltage.
2 Harm and influence of three-phase unbalance:
(1) Harm to the transformer.
(2) Impact on electrical equipment.
(3) The impact on line loss.
3 Harm and solution of three-phase unbalance:
The main hazards caused by factors such as three-phase voltage or current imbalance:
(1) The operation of the rotating motor in an asymmetric state will cause additional loss and heat generation of the rotor, which will cause the overall or local temperature of the motor to rise.
(2) It causes various protections with negative sequence components as starting components to malfunction, which directly threatens the operation of the power grid.
(3) Unbalanced voltage causes non-characteristic harmonics in silicon rectifier equipment.
(4) For generators and transformers, when the three-phase load is unbalanced, if the control current is at the rated value, the other two phases cannot be fully loaded, so the utilization rate of the equipment will decrease. On the contrary, if the rated capacity is to be maintained, it will cause One phase with a larger load is overloaded, and the unbalanced magnetic circuit will cause waveform distortion and increase the additional loss of equipment.
4 Solutions to the three-phase voltage imbalance of the power grid caused by unbalanced loads:
(1) Distribute asymmetrical loads to different power supply points to reduce the problem of serious imbalance caused by centralized connections.
(2) Use the method of cross-exchanging to make the asymmetric load reasonably distributed to each phase, and make it as balanced as possible.
(3) Increase the short-circuit capacity of the load access point, such as changing the network or increasing the power supply voltage level to improve the system's ability to withstand unbalanced loads.