Application Of Low Voltage Protection Coordination In Power Supply System

For domestic project in China,the relay settings are determined by convention and GB or DL standard,the calibration of protective selectivity is poorly-conceived and not very accurate.The concept of selectivity and selectivity coordination curves and tolerance curves are represented in this paper.According to project application,a graphic method of relay coordination is introduced,which is more intuitive and effective to illustrate the relay operating curves for protective devices and the tolerance curves for electrical equipment,such as,the transformers,motors and cables,etc.Based on above method,by analysis of ground fault and neutral protection configurations on power plant project application,correspondingly,the causes of the problem are found out and solutions are put forwarded.In domestic,the power of equipment usually represents the necessary of ground fault protection configurations.For instance,ground protection will be used for the motors,whose powers are more than 50 kW,however,this method turns out to be not scientific and economic.Actually,whether reliable movement can realize by instantaneous over-current protection is the correct way to judge,it is unnecessary to configure ground fault protection if the sensitivity can meet requirements.The method of calculating earth fault current of cable termination is introduced,which determine the maximum permissible length of the cable without ground fault protection.Based on above,by allocations with ground fault protection of the incoming line,the configuration criteria of feeder line is determined.According to the domestic design habits,neutral conductor protection is unnecessary to install.It is represented in this paper that whether neutral conductor protection should be configured alone and its relation with neutral conductor section.Finally,by illustrated technical characteristics of the mainstream switch equipment manufacturers,equipment selection criteria and relay settings are suggested to realize the reliable and safe operation of the LV power distribution system.