The appropriative test menu for "power frequency variation" is normally built in domestic relay protection test instruments, but it should be calculated and set beforehand for imported instruments.

国产继电保护试验仪一般都有针对工频变化量的专用测试菜单，而进口仪器则需预先计算和设置。

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For Shanghai power grid, which has large short circuit capacity and short electrical distance, a method using imported instrument to test power frequency variation distance protection is presented.

结合上海电网具有短路容量大、电气距离短的特点，提出了使用进口仪器测试工频变化量距离保护的具体方法。

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Orthogonal Multi-Carrier Modulation(OMCM) is adopted to overcome the frequency selectivity and time variation of low-voltage power line channel.

为克服低压电力线信道的频率选择性和时变性，系统采用正交多载波方式传输信息。

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