Judgment Methods and Countermeasures for Three-phase Unbalance (2)
(2) What are the causes of three-phase unbalance?
a. There are many reasons for the unbalanced three-phase voltage, such as single-phase grounding, disconnection resonance, etc. The operation and management personnel can quickly deal with them only if they are correctly distinguished.
b. Disconnection fault
If one phase is disconnected but not grounded, or if one phase of the circuit breaker and isolating switch is not connected, the fuse of the voltage transformer is blown, which will cause the three-phase parameters to be asymmetrical. 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 disconnected, the phase voltage of the disconnected line is zero, and the phase voltage of the unbroken line is still the phase voltage.
c. Ground fault
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. Metal grounding, the voltage of the faulty phase is zero or close to zero, and the voltage of the non-faulty phase is increased by 1.732 times and remains unchanged for a long time. Non-metallic grounding, the grounding phase voltage is not zero but reduced to a certain value, and the other two phases are raised less than 1.732 times.
d. Reasons for resonance
With the rapid development of the industry, the nonlinear power load has increased greatly. Some loads not only generate harmonics, but also cause power supply voltage fluctuations and flicker, and even cause three-phase voltage imbalance.
There are two types of three-phase voltage imbalance caused by resonance:
1) Fundamental resonance
Fundamental frequency resonance has characteristics 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 grounding cause, it may be caused by resonance.
2) Frequency division resonance
The other is the frequency division resonance or high frequency resonance, which is characterized by the simultaneous rise of the three-phase voltage.
In addition, it should also be noted that when the airdrop busbar cuts off part of the line or the single-phase grounding fault disappears, if a grounding signal occurs and the voltage of one, two or three phases exceeds the line voltage, the pointer of the voltmeter hits the head and moves slowly at the same time. Or the three-phase voltage rises more than the line voltage in turn. In this case, it is generally caused by resonance.
e. Unreasonable distribution of three-phase loads
Many staff 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 electricity. Just blindly and randomly connect the circuit and install the meter, which causes the imbalance of the three-phase load to a large extent. Secondly, most of the 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 unbalanced condition of the three-phase load of the distribution transformer.
f. Constant changes in electricity load
The reasons for the unstable electricity load include the frequent demolition of land II, the relocation of meters, or the increase in electricity users. Instability of temporary and seasonal electricity usage. In this way, the uncertainty and concentration in the total amount and time make a load of electricity consumption also have to change with the actual situation.
g. Weakening of monitoring efforts for distribution transformer loads
In the management of a distribution network, the management 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 the three-phase to unbalance, such as the influence of the line and the unphased load moment of the three-phase and so on.
