The three-phase voltage is the voltage between the phase and the phase. There are various voltage standards in various countries in the world. The three-phase voltage is 220V, the three-phase 380V is one kind, and there are other voltages. In addition, the frequency is also 50HZ and 60HZ. of. At present, China's three-phase voltage standard is 380V, the frequency between each phase is the same, the frequency is 50Hz. Because the star connection is used for residential electricity access, if China says three-phase 380V, it means the line voltage, and its phase voltage is 220V; if it is three-phase 220V, it means the line voltage, and its phase voltage is 127V. Taiwan and other places are such power sources. Japan is a special case of the world. The east and west parts of Japan are different power grids. The phase voltages are 120V/60HZ and 220V/50HZ, respectively. At the same time, they are connected to the triangular connection used by residents. Therefore, the line voltage and phase voltage in Japan are equivalent. of.
The three-phase electricity we usually use can be understood as follows: three different two-pole generators emit alternating current, and the current and voltage are sinusoidal, but the three generators have different rotational power generation sequences, and their respective sine waves are in The value of the same instant is also different. If it is a zero-degree sine wave, the second one is 120 degrees, and the third one is 240 degrees (-120 degrees). Of course, the third and the first one are also different by 120 degrees. The difference between them is fixed. The difference between the angles is called the phase difference. The so-called three-phase power is the power supply with three phase differences.
If it is unstable to install the voltage regulator, there is also an unbalanced current in the three-phase unbalanced power grid, which will increase the copper loss of the line and the transformer, and also increase the iron loss of the transformer, reduce the output of the transformer and even affect the safety of the transformer. Operation will eventually cause an imbalance in the three-phase voltage. Adjusting the unbalanced current reactive power compensation device-automatically adjusting the capacitor bank effectively solves this problem. The device has the function of adjusting the unbalanced active current while compensating the system for reactive power. The theoretical result can compensate the three-phase power factor.
1. Single-phase grounding causes three-phase voltage imbalance, but the voltage value does not change. Single-phase grounding is divided into metallic grounding and non-metallic grounding. The metal grounding phase voltage is zero, and the other two phase voltages are increased by 1.732 times; the non-metallic grounding phase voltage is not zero, but is reduced to a certain value, and the other two phases are raised by less than 1.732 times.
Second, the disconnection not only causes the three-phase voltage imbalance, but also causes the line voltage value to change. When the previous voltage level line is disconnected, the voltage of the next voltage level shows that the three phase voltages are reduced, one of which is lower, and the other two are higher but close. When the line of this stage is disconnected, the voltage of the disconnected phase is zero, and the voltage of the unbroken phase is still the phase voltage.
Third, there are two kinds of three-phase voltage imbalance caused by resonance: one is fundamental frequency resonance, the characteristics are similar to single-phase grounding, that is, one phase voltage is lowered, the other two phases are increased, and the other is frequency-dividing resonance or high. Frequency resonance, characterized by a simultaneous increase in three-phase voltage.
1. Distribute the asymmetric load to different power supply points to reduce the problem that the imbalance is seriously exceeded due to the concentrated connection.
2. Use the cross-change equal method to distribute the asymmetric load to each phase reasonably and try to balance it.
3. Increase the short-circuit capacity of the load access point, such as changing the network or increasing the supply voltage level to improve the system's ability to withstand unbalanced loads.
4. Install a balancing device. Briefly list the above technical measures to solve the three-phase voltage or current imbalance on the grid and power quality hazards.
Which kind of measures should be taken is more reasonable and effective, and it must be determined according to the actual situation and after technical and economic comparison. In the low-voltage three-phase four-wire urban residents and rural power supply systems: because the electricity users are mostly single-phase load or single-phase and three-phase load mixed, and the load size is different and the power consumption time is different. Therefore, the unbalanced current between the three phases in the power grid exists objectively, and this power imbalance is irregular and cannot be predicted in advance. This results in a long-term imbalance in the three-phase load of the low-voltage power supply system. For three-phase unbalanced currents, the power sector has few effective solutions in addition to distributing the load as reasonably as possible. The unbalanced current in the power grid will increase the copper loss of the line and the transformer, and will increase the iron loss of the transformer. Lowering the output of the transformer may even affect the safe operation of the transformer, which will eventually cause an imbalance of the three-phase voltage. Adjusting the unbalanced current reactive power compensation device-automatically adjusting the capacitor bank effectively solves this problem. The device has the function of adjusting the unbalanced active current while compensating the system for reactive power. The theoretical result can compensate the three-phase power factor to 1, and the three-phase current is adjusted to balance.
The practical application shows that the three-phase power factor can be compensated to above 0.95, and the unbalanced current is adjusted to within 10% of the rated current of the transformer. According to the wangs theorem (Wang's theorem), a capacitor that is connected across phases can transfer active current between phases. Adjusting the unbalanced current reactive power compensation device is designed by using the wangs theorem. Appropriately connecting different numbers of capacitors between each phase and phase and between each phase and the neutral line can not only make the phases get good. Compensation, and the unbalanced active current can be adjusted. The commutation switch automatically selects the power supply phase through intelligent logic to automatically adjust the imbalance of the three-phase load. Reduce the loss of electrical energy during transmission, maximize the utilization of electrical energy and enhance the reliability of power supply.
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