This paper discusses the ability of avoiding maximal power flow of quadrilateral impedance relay and round impedance relay in detail. It puts forward the setting principle and method of load limiting resistor of backup distance protection and gives the propositional setting value of load limiting resistor. The setting method is applied universally in North China Power Systems， which guarantees that distance protection will not mal-operation and enlarges the fault’s range when the power flow of electricity grid diverts in a wide scope.