Grid-Connected DCAC Inverter utilizing DQ Transformation

Grid Connected Inverter using Active Current Control that PI controls in the DQ reference frame using the Clarke and Park transformations

Design Specifications of this converter include:

1) 5000W power rating

Tuning the output voltage and current via the following control strategies allows for the inverter to reach this desired power level within a tolerable margin,

2) 400VDC to 230VAC, 50Hz voltage inversion

Output voltage is controlled by feeding existing VI measurements at the output into a PI Controller, that produces a reference voltage that allows the SPWM module to generate an modulated waveform with a duty cycle set that corresponds to the desired output voltage magnitude.

3) Harmonics below 5% of the Grid-side VI

An LCL filter is implemented at the briidge output with the inverter-side and grid-side inductors sized to reduce current harmonics to the desired extent at the expense of a cumulative voltage drop that does not exceed 10% of the output voltage. The shunt capacitor filter is sized so that the total amount of reactive power it absorbs does not exceed 5% of the rated power.

4) Active Current Control

Grid side voltage along with inverter output current measurements are taken and fed into a once-cascaded LPF set then amplified with a 2x gain to produce VI analogues in the stationary α and β reference frame. The 4 returned values Va, Ia, Vb, Ib, are then converted into their Direct and Quardrature counterparts in the rotating DQ reference frame before being fed into a PI controller simultaenously. Seperating the current and voltage measurements taken at the output into 2 distinct VI pairs that reference diferent axes allows for fine-control of the Active (d-axis) and Reactive (q-axis) power that makes up the power that is fed into the grid.

5) P &or I &or D Control

PI control is utilized when synchronization with the grid is paramount and, this method effectively reduces steady-state-error to minimal levels that don't cause grid-side tripping. This is preffered over a PID controller that is also capable of minimizing steady-state-error at the expense of a tendency to 'over-react' when external disturbances act on the output variables, that leads to momentary oscillations and the possibility of descynchronization between the inverter ouput variables and the grid characteristics.