Abstract: Due to the high cable ratio of city grid，the harmonic pollution is aggravated. For the buses with high harmonic voltage content，it is necessary to evaluate the harmonic emission level of the utility side and customer side. Most of the existing evaluation methods are based on two assumptions：1）the harmonic impedance of the utility side is far smaller than that of the customer side；2）the background harmonic is relatively stable. However，the capacitance to earth of the cable is large，which is easy to cause inductance-capacitance coupling and increase

the harmonic impedance of the utility side. In addition，the high power electronization rate of city grid also aggravates the fluctuation of background harmonics，thus，the two assumptions required by traditional evaluation methods are no longer tenable. To solve these problems，the FastICA algorithm，a popular method in this researching field，was improved. First，a screening mechanism based on sparse component analysis（SCA）and comprehensive negative entropy maximization was introduced to search the local signal which is highly similar to original harmonic source. Then，the harmonic impedance and harmonic emission level of two sides can be calculated through this local signal. The proposed method is effective even when the harmonic impedance amplitudes of both sides are close and the fluctuation of background harmonic is large. Simulation and engineering

cases verified the validity of the proposed algorithm.

      Key words: city grid with high cable rate；harmonic emission level；harmonic impedance；improved fast independent component analysis method；sparse component analysis（SCA）；comprehensive negative entropy