What Would Cause Three Phase Unbalance?
Three-phase unbalance is an important indicator of power quality. Although there are many factors affecting the power system, most of the normal unbalance is due to three-phase components, line parameters or load asymmetry. Because the factor of three-phase load is not certain, the three-phase voltage and current of the power supply point are prone to unbalance and line loss. Furthermore, it will adversely affect the motor on the power supply point and endanger the normal operation of the motor. Therefore, if the three-phase unbalance exceeds the range that the distribution network can bear, the safe operation of the overall power system will be affected. There are many reasons for three-phase voltage imbalance, such as single-phase grounding, disconnection resonance, etc. The operator should correctly distinguish the reasons and deal with them quickly.
Break Line Fault
One phase is disconnected but not grounded, or one phase of the circuit breaker and isolating switch is not connected, and the voltage transformer fuse is blown, all of which cause three-phase parameter asymmetry. When one phase of the line of the previous voltage level is disconnected, the voltage of the next voltage level shows that the voltages of the three phases are reduced, one of which is lower and the other two phases are higher but the voltage values of the two are close. When the line at 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.
When one phase of the line is disconnected and the single phase is grounded, although the three-phase voltage is unbalanced, the voltage value does not change after grounding. Single-phase grounding is divided into metallic grounding and non-metallic grounding. For metallic grounding, the voltage of the faulty phase is zero or close to zero, and the voltage of the non-faulty phase increases by 1.732 times and remains unchanged; for non-metallic grounding, the voltage of the grounding phase is not zero but reduced to a certain value, and the other two phases increase less than 1.732 times higher.
With the rapid development of the industry, the nonlinear power load increases greatly. Some loads not only generate harmonics, but also cause power supply voltage fluctuations and flicker, and even cause three-phase voltage imbalance. The causes of resonance are further divided into fundamental frequency resonance, frequency division resonance and high frequency resonance. The fundamental frequency resonance 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 searching for the cause of the fault. At this time, special users can be checked. If it is not the grounding cause, it may be caused by resonance; Both the frequency resonance and the high frequency resonance are characterized by the simultaneous rise of the three-phase voltage.
Unreasonable Distribution of Three-phase Loads
Many workers who install meters and connect electricity do not have professional knowledge and concept of three-phase load balance, so they do not pay attention to controlling three-phase load balance when connecting the power, but just blindly and randomly connect circuit, which to a large extent caused the unbalance of the three-phase load. Secondly, most circuits in our country are combined with power and lighting, so when single-phase electrical equipment is used, the efficiency of electricity consumption will be reduced. This difference further exacerbates the inconsistency of the three-phase load of distribution transformers balance condition.
Constant Changing Electrical Load
The reasons for the unstable electricity load include frequent demolition, relocation of meters or increase of electricity users; temporary electricity consumption and seasonal electricity consumption. In this way, the uncertainty and inconcentration in the total amount and time make the load of electricity consumption also have to change with the actual situation.
Poor Monitoring of Distribution Load
In the management of distribution network, problems in three-phase load distribution are often ignored. In the detection of the distribution network, the three-phase load of the distribution transformer has not been regularly detected and adjusted. In addition, there are many factors that cause the phenomenon of three-phase unbalance, such as the influence of the line and the unphased load moment of the three-phase and so on.