AN3095 datasheet(41/55 Pages) STMICROELECTRONICS

41 / 55 page

AN3095

STM32F103xx-based current control strategy for inverter grid connection

Doc ID 16555 Rev 3

41/55

The grid voltage and the 90° phase-shifted voltage are used to perform the reference frame

change, or “park transformation”, and create two voltage components on the DQ reference

frame called Vd and Vq. One of the two components is controlled to zero with a PI regulator.

The output of the PI regulator is the grid frequency which may be integrated to obtain the

grid angle.

It is worth knowing that if the Vq component is controlled to zero then the Vd component

follows the grid-voltage rotation. In this case, the active power injected into the grid may be

controlled, transforming the current in the same reference frame and by acting on the

amplitude of the Id component. The Iq component must also be controlled in order to ensure

zero reactive power injection. On the contrary, if the Vq component is controlled to zero in

the PLL, the active power is controlled with the Iq current component and the Id current

component is used to control the reactive power to zero or to the desired value.

This said, the advantages of such a control structure are clear: first of all the current

components on the synchronous reference frame are constants and may then be controlled

with standard PI regulators ensuring zero steady-state error; the second advantage is a

decoupled control of active and reactive power.

The reference values for the active and reactive component of the current are set by two

additional PI regulators in the outer control loop. The active reference current component is

generated by confronting the DC bus voltage with the reference voltage value. The error

between the actual value and the reference DC bus voltage is sent to a PI regulator whose

output is the active current component value (Figure 26).

Similarly, the reactive current reference value is set by another PI regulator whose input is

the error generated between the reactive power command and the actual estimated value.

Figure 27.

DQ components of the current

The difference between reference components of the current and the actual DQ

components are the inputs of the PI regulators in the inner control loop. The outputs of the

PI regulators in the inner loop are two voltage components, Vd and Vq. By performing a

reverse park transformation, two AC voltages are generated back to the stationary reference

frame, and therefore the generation of the modulating signals of the DC-AC converter may

be executed by the microcontroller.

The amount of power injected into the grid depends on the power available from the PV

array. This power is then processed by the DC-DC converter which is controlled in order to

maximize the energy yield from the array, independent to temperature variations and

irradiation conditions, by controlling its input impedance. The control of the input impedance

requires both PV array current and voltage sensing and some simple calculations executed

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