AN4070
STM32F103xx based current control for inverter grid connection
Doc ID 022934 Rev 1
33/53
threshold of 440 V. In the case of bus DC under/overvoltage the system is disconnected
from the grid according to the strategy already described. During normal operation the DC
bus is regulated at 380 V.
●
Burst mode operation at startup
Burst mode operation is active during system startup and can also be activated for low
load operation.
During burst mode operation the DC bus voltage is regulated with hysteresis control.
The boundary values of the hysteresis window Vb
1
, Vb
2
are chosen to limit the DC bus
voltage ripple to 2% of the reference value. Once the bus capacitor is charged, the
control loop mode of operation is enabled and the connection to the grid performed.
●
Line voltage and frequency detection and anti-islanding
The PLL continuously measures the line voltage and frequency in all operating states.
If the voltage or frequency exceeds the high or low limits, the inverter ceases to deliver
power to the grid. These conditions are also used to implement a passive method for
island operation detection. An island operation occurs when the utility power is
disconnected for maintenance or fault reasons while the inverter is still delivering
power. With a passive method, detection of islanding from the utility grid is achieved via
AC under/overvoltage and under/over-frequency detection functions.
●
Output overcurrent
Due to fault conditions or AC line transient conditions, the maximum current may be
exceeded. In this case, the inverter ceases to deliver power to the grid. The current
threshold value is set to 1.2 A.
The demonstration board is provided with the components required to implement
hardware short-circuit protection. However, this protection is disabled and may be
activated by properly sizing the R11 and R14 which currently have a zero Ohm value.
●
Open loop operation
This mode of operation has been implemented to allow system debug independently
from grid operation. This mode, used for maintenance, test and debug, allows system
operation only with manual control by acting on a set of pushbuttons on the board. The
DC-DC converter power transfer can be adjusted by acting on the duty cycle parameter
through the pushbutton placed on the microcontroller board. In the same way, the
power transfer of the DC-AC converter can be modified acting on the modulating index.
●
LCD display
The microcontroller board is equipped with a graphic LCD display. The selectable functions
are:
1.
open loop mode;
2.
closed loop mode;
3. calibration;
4.
DC-DC converter manual control;
5.
DC-AC converter manual control;
6.
PI parameters manual regulation.
It is important to note that the calibration function must be performed by the operator when
the system is first connected to the grid. In this way, any offset affecting the feedback signals
used for control mode operation is compensated via firmware. When the calibration function
is executed the display shows a grid current offset of about 1.7 V and a grid voltage offset of
about 2.2 V.
