Ring Main Units (RMU) are commonly used in the power system. At present, there is no example of partial discharge (PD) detection in RMU by optical method. In this article, the PD in RMU was detected by optical method. Meanwhile, the characteristics of the optical signals and the influential factors have been analyzed. The phase, amplitude and quantity characteristics of the PD optical signals in the positive and negative half cycle was analyzed and listed and we can find the PD optical signals under four typical insulating defects have their own characteristics; if the output of the PMT have not reach it saturation value, the maximum amplitude of the optical signals will increase with the increase of the applied voltage while the average amplitude of the optical signals did not always increase except the insulation gap defect; besides, the number of the optical signals also have a positive relationship with the applied voltage. When the optical receiving length quadruple, the initial PD voltage (IPDV) of optical method has decreased about 14.7% while the number of PD optical signals has increased about 1046.5%; when the pressure changing from 0.1 MPa to 0.14 MPa, the IPDV has increased about 83.65% while the signals number has decreased about 36.23%; when the distance between the optical fiber and the defect model changing from 2.5 cm to 7.5 cm, the IPDV has increased about 9.3% while the signals number has decreased about 65.7%.

环形主机（RMU）通常用于电力系统中。目前，尚无通过光学方法检测RMU中局部放电（PD）的示例。本文采用光学方法检测了RMU中的PD。同时，分析了光信号的特性和影响因素。分析并列出了PD光信号在正负半周期内的相位，幅度和数量特性，发现在四种典型的绝缘缺陷下的PD光信号各有特点；如果PMT的输出未达到饱和值，则光信号的最大幅度将随着施加电压的增加而增加，而光信号的平均幅度除绝缘间隙缺陷外不会一直增加。除了，光信号的数量也与施加的电压成正比关系。当光接收长度增加四倍时，光学方法的初始PD电压（IPDV）下降了约14.7％，而PD光信号的数量已增长了约1046.5％；当压力从0.1 MPa变化到0.14 MPa时，IPDV增加了约83.65％，信号数量减少了约36.23％。当光纤与缺陷模型之间的距离从2.5 cm更改为7.5 cm时，IPDV增加了约9.3％，而信号数量减少了约65.7％。PD光信号的数量增加了约1046.5％，为7％；当压力从0.1 MPa变化到0.14 MPa时，IPDV增加了约83.65％，信号数量减少了约36.23％。当光纤与缺陷模型之间的距离从2.5 cm更改为7.5 cm时，IPDV增加了约9.3％，而信号数量减少了约65.7％。PD光信号的数量增加了约1046.5％，为7％；当压力从0.1 MPa变化到0.14 MPa时，IPDV增加了约83.65％，信号数量减少了约36.23％。当光纤与缺陷模型之间的距离从2.5 cm更改为7.5 cm时，IPDV增加了约9.3％，而信号数量减少了约65.7％。