**6. Discussion**

Vector control of grid connected voltage source inverter is implemented in MATLAB/simulink. Simulation results are obtained for different current controllers. The discussion of the results as follows: vector control in virtual grid flux oriented reference frame is having an capability to decouple the active and reactive powers flowing into grid, which we can seen from the waveforms of active and reactive powers for three current controllers. Reactive power flowing into grid is almost zero in all the controllers to ensure unity power factor operation of grid. Total harmonic distortion of three current controllers is as follows 1. For hysteresis current controller percentage of THD is 4.41 2. For current regulated delta modulator percentage of THD is 4.74 and 3. For modified ramp type current controller percentage of THD is 2.68. Among all the current controllers modified ramp type current controller is having fewer harmonics in grid current, which ensures fewer ripples in three phase active power following into grid, and current regulated delta modulator having more harmonics in grid current which leads to more ripple is three phase active power following into grid. The switching frequency of modified ramp type current controller is 2 KHz it is a constant value at its ramp generator frequency. In Current regulated delta modulator switching frequency is limited to 2 KHz, but the variation of frequency with in fundamental period cannot be controlled. In hysteresis current controller the switching frequency is varies with load parameters and hysteresis band, switching frequency when hysteresis band 20 is equal to 2.72 KHz. Form the above discussion modified ramp type current controller has constant switching frequency. The power factor by using modified ramp type current controller is very good then compared to other current controllers. The following table 1 shows the switching frequency, THD, power factor for three current controllers.


Table 1. Comparison of three current control techniques of grid connected VSI
