The relationship between currents of Filter Commutated Converter and the harmonic characteristics of multifrequency AC/DC interconnected systems based on New-Type converter transformer have been studied with ideal models by means of theoretical analysis and matlab simulation.

To give Pspice simulation and experiment results, this paper analyses the harmonic characteristics of three-level inverters using double-Fourier series, and compares it with that of two-level ones, based on the principal circuit topology and its operating principles of three-phase SHPWM three-level inverter with clamp diodes, which shows many merits of the three-level one.

Further more the switching loss and harmonic characteristics of these SVPWM strategies are compared,the optimal SVPWM of the noneswitching areas of phase voltage lagging behind sinusoid voltage peaks 30° is selected according to the load characteristic.

Tish Paper studies a few method of generating PWM pulse,Also analyse and discuss the harmonic characteristic of these modulation techniques,and provide a prof to select these method.

The application of PSCAD / EMTDC and Matlab simulation software is used to simulate each case,it also gives an analysis to the difference of harmonic characteristics between single and multi-DFWG setting into the state grid.

A new method for assessing harmonic impedance and the harmonic emission level is preseented, which is based on the partial least-squares regression.

For controlling the harmonics in public power grid better, it is very important to assess harmonic emission level of system and customers at PCC.