AN4070 datasheet(33/53 Pages) STMICROELECTRONICS

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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 Vb1, Vb2 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:

open loop mode;

closed loop mode;

calibration;

DC-DC converter manual control;

DC-AC converter manual control;

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.

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