The monopolar ground circuit operation mode of DC transmission system gives rise to excessive vibration and noise of some transformers in the power grid, which impacts the safety of equipment.

直流输电系统单极大地回路运行方式导致电网部分变压器振动加剧，噪声增大，影响了设备安全。

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For the parallel inverter system without output isolation transformers, the difference of DC components of the output voltage can cause large DC loop-current among modular inverters.

对于无输出隔离变压器的逆变器并联系统，各模块输出电压直流分量不一致会在模块间产生较大的直流环流。

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For a parallel inverter system without output isolating transformers, discrepancy in DC biases contained in the output voltage of each module can cause large DC circulating current among inverters.

对于无输出隔离变压器的逆变器并联系统，各模块输出电压直流分量不一致会在模块间产生较大的直流环流。

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Transformers for DC, however, did not exist.

FORBES: Magazine Article

High-voltage DC can carry power over long distances at lower cost than alternating current, but the transformers still cost too much to make the idea widespread.

FORBES: Magazine Article