AN4070 datasheet(32/53 Pages) STMICROELECTRONICS

32 / 53 page

STM32F103xx based current control for inverter grid connection

AN4070

32/53

Doc ID 022934 Rev 1

Figure 21.

Block diagram of the control algorithm implemented on the 250 W MIC STM32F103xx

Apart from gird connection and MPPT, some other functions are implemented for the correct

operation of the conversion system. Here below a brief description of these functions:

●

Input voltage monitoring

Input voltage value is constantly monitored to ensure that the array voltage is always in

the correct operating range, between 18 V and 40 V.

●

Input current monitoring

The input current is also sampled to detect any condition of overcurrent in the system.

This protection is enabled when the average input current is above 12 A.

●

Bus DC voltage control

The output of the bus DC is controlled in order to stabilize the inverter input voltage to

the bus DC reference voltage. The minimum DC bus voltage is a function of the peak-

to-peak AC line voltage in order to minimize the total harmonic distortion (THD) of the

injected current. This limit depends on grid voltage fluctuations and can be calculated

according to the following equation:

Equation 44

where Pdc is the average power on the DC bus, Vgrid_max is the maximum RMS value of the

grid voltage and Zc is the output LCL filter impedance. In other words, the DC bus must

never decrease below the peak grid voltage value plus the drop across the MOSFETs and

the LCL filter. To ensure safe operation, this voltage must never surpass the protection

⎟

⎠

⎞

⎜

⎝

⎛

max

grid

max

grid

min

ref

bus

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