Why does the three-phase variable frequency power supply appear unbalanced:
1.On the distribution network side, there are a large number of unbalanced single-phase loads, which leads to the distribution station area. In most areas, the degree of three-phase imbalance is different.
2. The randomness and uncertainty of the user's power consumption process, as well as the increasing use of high-power loads, increase the load of the single-phase power grid and cause three-phase imbalance. The imbalance of the three-phase variable frequency power supply will seriously affect the distribution network and the user end.
This is mainly reflected in the following aspects:
(1) Increase the power loss of the line. On the four-wire distribution network side of the three-phase variable frequency power supply, the current flows through the neutral line, not only on the phase line, but also on the neutral line when the three-phase load is unbalanced, and the power loss increases the power grid. Performance loss;
(2) Increase the power loss of the distribution transformer. As an important configuration on the low-voltage distribution network side, when using a single-ended, three-phase, variable frequency power supply, the distribution transformer will cause an increase in distribution loss.
(3) The zero-sequence current generated by the distribution transformer will increase with the increase of three-phase imbalance. Due to eddy current loss, the local temperature of the distribution transformer increases, which leads to a shortened service life of the equipment.
(4) It affects the normal operation of electrical appliances, and the three-phase imbalance leads to a decline in power quality, which affects the operation of electrical appliances. Reducing three-phase imbalance can not only stabilize the power quality of the power grid, but also reduce the power loss of the power grid and save energy.
How to deal with the imbalance of three-phase variable frequency power supply?
In order to fundamentally solve the three-phase imbalance problem based on the idea of load balancing, a control method with variable frequency power supply as the core is being developed. This method can not only adjust the load to the maximum balance within a certain range, but also ensure the stability of the network power quality during the adjustment process, realize phase change while continuously supplying power to the load, and achieve overall balance.
Since the three-phase imbalance is a real-time dynamic variable, it must also meet linear requirements. In view of the above requirements, the overall idea of studying the three-phase imbalance management based on variable frequency power supply is to establish an intelligent management terminal on the low-voltage user side in the distribution station area to perceive and analyze the degree of three-phase imbalance in the area in real time, and obtain load balance by analyzing the commutation instructions in the distribution process;
As the core component of the commutation device, the variable frequency power supply is installed between the three-phase grid side and the load side, and commutates by receiving the instructions issued by the intelligent management terminal, playing an important role in the entire commutation. In addition, the network control center corresponds more closely to the information flow of the ubiquitous Internet of Things and plays an important role in building a smart network [7].
With the growth of the economy and the rapid increase in power load on the power grid, the three-phase imbalance problem in the power grid has become more and more serious.
At present, there are three main solutions for three-phase imbalance.
1.This method must use manual commutation to record the three-phase imbalance in the area for a long time, and balance the load by analysis and connect it to each phase. However, due to power outages during the commutation process, it cannot meet safety requirements.
2. Connect a reactive power compensation device on the distribution side to control the three-phase imbalance of the power grid. The reactive power compensation device adjusts the current of the lower phase by absorbing the current of the higher phase among the three phases. However, the device has limited effect on the overall regulation of the production line and cannot guarantee the overall balance of the production line.
3. The unbalanced grid of the three-phase variable frequency power supply is controlled by the commutation technology of the intelligent mechanical switch. This technology can maintain the overall balance through real-time detection and control of the intelligent mechanical switch, but it will have an impact on the grid and load during the commutation process.
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