AN3095 datasheet(43/55 Pages) STMICROELECTRONICS

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AN3095

STM32F103xx-based current control strategy for inverter grid connection

Doc ID 16555 Rev 3

43/55

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Input current control

The same kind of strategy is used to detect any condition of overcurrent in the system. This

protection is enabled when the average input current is above 18 A.

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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 total harmonic distortion (THD) of the injected current.

This limit depends on grid-voltage fluctuations and may be calculated according to the

following equation:

Equation 59

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 IGBTs and the

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

of 480 V.

In case of bus DC under/overvoltage the system is disconnected from the grid according to

the strategy already described.

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Burst mode operation at startup

In the case of an overcurrent or overvoltage event, the DC-DC converter modulation is first

disabled by the control algorithm. Then, the DC-DC modulation is disabled and the interface

relays are disconnected, preventing any power flow from the system to the grid. After that,

the control algorithm performs some checks on the input voltage, bus DC voltage, grid-

voltage and grid-frequency value. If the sensed voltages are in the allowable range (200-400

V input, 380-450 V bus DC, 230 Vrms +/- 10 %, 49.7 Hz-50.3 Hz grid voltage) the DC-DC

converter is started in burst mode in order to charge the bus DC voltage at the reference

voltage level. This same check procedure is executed at startup, before the connection to

the grid is actually performed. During burst mode of operation the DC bus voltage is

regulated with hysteretic control. The boundary values of the hysteresis window Vb1, Vb2

are chosen to limit the DC bus voltage ripple to 5 % 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.

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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/overfrequency 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 according to the

strategy explained above. The current threshold value is set to 15 A.

⎟

⎠

⎞

⎜

⎝

⎛

max

grid

max

grid

min

ref

bus

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