AN4070
STM32F103xx based current control for inverter grid connection
Doc ID 022934 Rev 1
31/53
controlled transforming the current in the same reference frame and by acting on the
amplitude of the I
d
component. The I
q
component must, also, be controlled in order to
ensure zero reactive power injection. On the contrary, if the V
q
component is controlled to
zero in the PLL, the active power is controlled with the Id current component and the I
q
current component is used to control the reactive power to zero or to the desired value.
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.
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.
The difference between the reference components of the current and the actual d-q
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, V
d
and V
q
. By performing a
reverse Park transformation two AC voltages are generated back on the stationary reference
frame, and so the generation of the modulating signals of the DC-AC converter can be
executed by the microcontroller. The block diagram of the control strategy described above
is shown in
Figure 21
. The amount of power injected into the grid depends on the power
available from the PV panel. This power is then processed by the DC-DC converter which is
controlled in order to maximize the energy yield from the array, independently from
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 by the well known maximum power point algorithm. The
perturb & observe (P&O) method is a very common and easy way to implement an MPPT
technique. The DC-DC converter duty cycle is incremented or decremented according to
both array power and voltage change.
