SPD残压测量回路中差模干扰问题

为了解决限压型SPD残压测量波形严重畸变上凸、幅值偏大的现象,进行了理论计算、EMTP-ATP程序仿真、新旧同轴电缆对比试验和人为将新电缆屏蔽层末端强行接地的试验,结果表明:相同引线长度,相同8/20冲击电流幅值情况下,理论计算、EMTP-ATP程序仿真、新电缆屏蔽层不接地测量残压波形符合要求,引线在SPD上端电压降(U′AB)等于引线在SPD下端的(UAB);人为将新电缆屏蔽层末端强行接地或破损同轴电缆呈现SPD残压波形严重变形上凸,冲击电流幅值越大,上凸越严重,引线在SPD上端时的电压降(U′AB)大于下端的(UAB),且均大于新电缆屏蔽层不接地的测量结果。新电缆屏蔽层接地或破损电缆构成两端接地测量系统,两端接地点存在电位差,通过阻性耦合产生电磁干扰,在同轴电缆屏蔽层和大地之间产生共模干扰电压,芯线和屏蔽层之间产生差模干扰电压。

The Differential Mode Interference in the Residual Voltage Measuring Circuit of SPD

Through circuit calculation and ATP-EMTP simulation,the U′AB(voltage drop arising from the inductance of wire connecting SPD to phase conductor) should equal to UAB(voltage drop arising from the inductance of wire connecting SPD to ground conductor)under the same situation.Using new coaxial cable,two voltages are approximately equal.When coaxial cable with out layer damaged is used for measuring during the experiment,the U′AB is little higher than UAB,and the voltage measured by old cable is much higher than measured by new one.The result show that the higher the peak value of the 8/20 μs surge current is,and the residual voltage is more convex.The writer artificial damages the new cable outer layer insulation and connects the shielding layer to the ground.The measurement result is the same as the damaged cable.Therefore,when both the head and end of shielding layer connect to the ground,the diversity of two terminal voltages generates common mode interference by means of resistance coupling.