The utility model patent discloses an Enhanced Active Power Filter with a three-phase unbalance adjustment function. The IGBT converter of the Enhanced Active Power Filter is a three-level three-phase four-wire four-arm PWM transformer. The fourth bridge arm is connected in parallel with the remaining three bridge arms, and the midpoint of the fourth bridge arm is connected to the neutral line of the three-phase nonlinear load through an inductor to realize the control decoupling of the neutral line current and the three-phase current. The utility model has the advantages of making the output current waveform smoother and the high-order harmonic component smaller; the introduction of the fourth bridge arm not only increases the control accuracy of the unbalanced current, but also increases the control ability of the unbalanced current.
Most of the low-voltage power grids are stepped down by 10kV/0.4KV transformers, and then supply power to users in a three-phase four-wire system. It is a power supply network that mixes three-phase production power and single-phase load power. When designing low-voltage power grid loads, each single-phase load should be evenly distributed on the three phases A, B, and C. However, in actual operation, due to factors such as uncontrollable increase of single-phase users, access of high-power single-phase loads, and asynchronous power consumption of single-phase loads, the three-phase loads will be unbalanced.If the low-voltage power grid operates under the condition of large three-phase load imbalance, the efficiency of the line and distribution transformer will be reduced at the slightest level, and serious consequences such as overheating and burning of a certain phase wire, switch burnout, and even distribution transformer burnout will result. . The relevant regulations of the State Grid stipulate that the load current imbalance at the outlet of the distribution transformer should be less than 10%, the neutral line current should not exceed 25% of the rated current of the low-voltage side, and the head-end current imbalance of the low-voltage main line and the main branch line should be less than 10%. should be less than 20%.
At present, the main methods to solve the unbalanced three-phase load of the low-voltage power grid are:
Aiming at the deficiency of the existing three-phase unbalance control means, the prior patent document (CN106356887A) discloses a three-level distributed energy grid-connected and reactive power compensation composite control system, which can effectively adjust the three-phase power grid low-voltage side Unbalanced current, two-way fast adjustment of reactive power but has the following disadvantages:
1) The output current has high-order harmonics exceeding the Active Power Filter APF compensation cut-off frequency, especially the high-order harmonics equal to the IGBT switching frequency are too large, the main loop inductance is too large, and the cost is high
2) The unbalanced current compensation capability is insufficient, and the compensation accuracy is not high.
The above is just the place where this patent needs to focus on improving.
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